Impact of Simulated Gastric Acid on the Physical and Mechanical Properties of Vacuum-Formed Retainers: An In Vitro Study

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Huda Alkawari, 2. Laila Baidas, 3. Areen Alyahya, 4. Renad Alotaibi, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4495989/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 01 Feb, 2025 Read the published version in BMC Oral Health → Version 1 posted 18 You are reading this latest preprint version Abstract Background A decrease in the oral pH levels may have detrimental effects on vacuum-formed retainers since an acidic oral environment could impact the mechanical and physical properties of these retainers. The present in vitro study aimed to evaluate the effect of simulated gastric acid at pH 4 and 1.2 on the surface microhardness, flexural strength, and color change of three different types of orthodontic retainer materials. Methods A total of 144 vacuum-formed commercially available retainer materials: copolyesters, polypropylene, and PET-G (n = 48/group) were evaluated. Each specimen was immersed in distilled water, and then the specimens were divided into two subgroups: those with a pH of 4 and those with a pH of 1.2. The microhardness test was performed using Innovatest, the flexural strength was evaluated by a universal testing machine, and the color change was analyzed using a spectrophotometer. The data were analyzed using One-Way ANOVA and paired t-tests at a significance value of P ≤ 0.05. Results Our findings indicated a significant decrease in flexural strength in all the experimental groups after immersion in pH 1.2 and 4 solutions (p-value = 0.000), with PET-G being the most affected material, followed by copolyester and polypropylene. Only two materials, copolyester, and PET-G, demonstrated a significant decrease in surface microhardness following immersion in a pH 4 solution (P = 0.001 and P = 0.025, respectively). The copolyester material exhibited the greatest degree of color alteration. Conclusion The study concluded that copolyester adversely affects flexural strength, surface microhardness, and color when subjected to immersion in simulated gastric acid. Therefore, care is recommended for patients diagnosed with gastroesophageal reflux disease. Retainer Simulated Gastric acid Vacuum formed Oral health Figures Figure 1 Figure 2 Figure 3 Figure 4 Background Retention is the final stage of orthodontic treatment, which attempts to maintain teeth in their corrected position after the completion of treatment by using various types of retainers [ 1 , 2 ]. Clinicians must approach the treatment of all patients with the assumption that there is a high risk of relapse due to the difficulty in properly predicting the level of relapse that may occur over a period of time [ 3 ]. Long-term posttreatment retention is advocated to lower this risk, with patient compliance playing a major role [ 4 ]. Both fixed and removable retainers are used to achieve this objective and reduce the chances of relapse. However, the choice of one approach over the other in terms of stability remains ambiguous owing to the absence of sufficient evidence. Consequently, many practitioners choose retainers on the basis of their own personal preferences [ 2 , 5 ]. At present, vacuum-formed retainers ​​(VFRs) are becoming more popular due to their ease of fabrication, esthetic appearance, and good patient compliance [ 6 , 7 ]. Various thermoplastic materials are used in manufacturing vacuum-formed retainers. Several materials, such as copolyester (CP), polyethylene terephthalate (PET), polyethylene terephthalate glycol-modified (PET-G), and thermoplastic polyurethane (PU), have been shown to have good properties. Consequently, these materials have been widely used in multiple current research studies [ 8 ]. However, their mechanical and physical properties may be impacted by the pH level of the oral cavity, which could result in chemical and physical changes that may affect the effectiveness of the retainer. DeNicola et al. (2015) reported a 35% prevalence of obesity in Saudi Arabia, and there is evidence to support the likelihood of a positive association between obesity and gastroesophageal reflux disorder (GERD) [ 9 , 10 ]. The increasing prevalence of sleeve gastrectomy operations may be attributed to its effectiveness in decreasing both body mass index and the risk of death. Although sleeve gastrectomy is widely regarded as a safe therapeutic technique, it is crucial to realize that problems may still occur, as with any surgical procedure [ 11 ]. The use of this particular approach has the potential to exacerbate pre-existing GERD or trigger its onset as a result of surgical intervention [ 12 ]. Obaidallah et al. (2018) concluded that the prevalence of GERD is slightly greater in Saudi Arabia than in Western countries and much greater in Saudi Arabia than in countries in East Asia [ 13 ]. Acid reflux decreases salivary pH from 6.7 to approximately 2–1.5, creating an acidic environment that may impact teeth, soft tissue, and intraoral appliances [ 14 , 15 ]. Hence, it is essential from a clinical perspective to assess the physical and mechanical characteristics of various materials after exposure to different pH values in the oral cavity. Previous studies on orthodontic wires have shown deterioration of both mechanical and physical properties after being subjected to an acidic environment [ 16 – 20 ]. Baidas et al. (2020) evaluated the surface roughness and frictional resistance of orthodontic archwires in simulated gastric acid and concluded that the surface roughness of beta-titanium and stainless-steel archwires was significantly affected when subjected to acidic conditions [ 16 ]. Chia-Tze Kao et al. (2010) studied the surface characteristics and corrosion behavior of metal brackets and wires in various electrolyte solutions after exposure to an acidic environment. The study concluded that most wire and bracket materials were easily corroded in a pH 4 medium of artificial saliva [ 20 ]. This assessment is crucial for determining the effectiveness and longevity of these materials throughout both the treatment and retention phases. Several studies have evaluated the mechanical, physical, and surface properties of retainers in general [ 21 – 30 ], including wear resistance [ 21 – 25 , 28 ], water absorption [ 22 , 23 ], transparency [ 22 , 26 ], surface hardness [ 22 – 25 , 28 , 30 ] and flexural strength [ 22 , 27 , 29 , 30 ]. However, none of the previous studies investigated the effect of acidic environments on the mechanical and physical properties of these materials. Therefore, this study aimed to investigate the effect of different pH levels on VFRs in terms of microhardness, flexural strength, and color change in consideration of the clinical environment in which they are used. Methods Materials and Study Design A total of 144 vacuum-formed retainers (VFRs) made of three thermoplastic materials were evaluated in this study: Clear Advantage Series I (Ortho Technology, Florida, U.S.), Clear Advantage Series II (Ortho Technology, Florida, U.S.), and DURAN®+ (Scheu Dental, Iserlohn, Germany) with a unified 0.040"/1 mm thickness, as preferred by most clinicians [ 31 ]. The VFRs were divided into three groups of 48 retainers according to material type: copolyester (CP) (Clear Advantage Series I), polypropylene (PP) (Clear Advantage Series II), and polyethylene terephthalate glycol (PET-G) (DURAN®+). Each group was further subdivided into three subgroups according to the pH of the immersed solution: control; distilled water at pH 6.7 (SG-A), simulated gastric acid at pH 4 (SG-B), and simulated gastric acid at pH 1.2 (SG-C) (Fig. 1 ). A sample power analysis indicated that each subgroup would result in an 80% chance of reaching significance at the 0.05 level. Specimen Preparation Thermoforming The sheets were vacuumed over a highly polished stainless-steel plate with a diameter of 110 mm and a height of 10 mm, which was placed in a thermoforming caster (Biostar®; Scheu Dental) for uniform vacuuming. Then, they were cut according to ISO 20795-2 (2013). The heating and cooling times applied during thermoforming are shown in Table 1 . Sample Treatment Protocol for Simulated Gastric Acid A simulated gastric acid solution with pH 1.2 (SG-B) was prepared by dissolving Na₂HPO₄ and NaH₂PO₄ in 1 L of distilled water, while a simulated gastric acid solution with pH 4 (SG-C) was prepared by dissolving NaCl and concentrated HCl in 1 L of water to simulate changes in the oral cavity. The procedure was repeated 6 times a day for 9 days with 24-hour intervals between each cycle, based on previous studies involving the simulation of gastric acid (Table 2 ) [ 16 , 32 ]. Table 1 Thermoforming conditions of the thermoplastic materials used in the study. Brand Component Temperature (°c) Pressure (bar) Heating time (s) Cooling time (s) Clear Advantage Series I Copolyester 220 6.4 35 60 Clear Advantage Series II Polypropylene DURAN PET-G Table 2 Sample treatment protocol of thermoplastic materials specimens. SG-A SG-B SG-C Stored in distilled water at 37°C 1. Immersion for 2 minutes in pH 1.2 1. Immersion for 2 minutes in pH 4 2. Rinsing with distilled water 2. Rinsing with distilled water 3. Storage for 30 minutes in 37°C distilled water 3. Storage for 30 minutes in 37°C distilled water Repeated 6 times per day for 9 days Three-Point Bending Test The flexural strength was measured using a universal testing machine (Instron 5965 series; North America). Twenty-four specimens of each material after thermoforming were prepared with a width of 5 mm and length of 40 mm, immersed in distilled water to keep the specimens wet, and stored at 37°C for 50 hours prior to flexural testing in accordance with ISO 20795-2 (2013). The specimens were removed from the water bath and immediately laid flat on the fixed lower jaw of the Instron machine. The free end of the upper jaw holding the blade was centered over the specimens to apply a load at a crosshead speed of 5 mm/min for 30 seconds. Time was fixed due to the nature of thermoplastic polymers, which can take considerable time to bend (Fig. 2 & Fig. 3 ). Twelve vacuum-formed retainers were utilized as a control group and were subsequently discarded. The other twelve samples were divided equally and immersed in the SG-B and SG-C solutions. Microhardness test Twelve specimens of each material were cut into 10×10 mm squares. Vickers microhardness was measured using a Nova 130 (Innovatest, Maastricht, The Netherlands). Three indentations were created by applying a load of 9.8 N for 10 seconds. After that, the size of the indentations was measured using the following formula: HV = 1.854 \(\frac{F}{{d}^{2}}\) , where F is the force applied on the surface and d is the average diameter of the indentations. After that, the mean of the three values was calculated for each specimen. Color Change Changes in color before and after immersion were measured according to the Commission Internationale de I’Eclairage L*a*b* color system. 33 The color parameter L* represents lightness (+ bright, − dark), while a* and b* are associated with the color scale, as a* represents the red (+) to green (−) and b* indicates the yellow (+) to blue (−) spectrum. The values of the three parameters were measured using a spectrophotometer (LabScan® XE, HunterLab Inc.) with a color data software program (EasyMatch QC ver 4.90) before and after immersion in the pH solution. Twelve square-shaped specimens were prepared with dimensions of 10 mm width and 10 mm length and numbered from 1 to 12 using a handpiece and small round bur. The numbers are shown on the top right of each specimen for future comparison. All specimens were washed in an ultrasonic bath for 5 minutes and dried using tissue papers. Before each measurement, the machine was calibrated using black and white glass plates for standardization. Each specimen was held by a specially designed holder for accurate fitting. After that, measurements were obtained by laying the specimens flat over the optical sensor tip and then covered with a black lid. Each specimen was scanned at three selected points. The color parameters were obtained, and ∆E, which represents the color change, was measured using the following formula: ∆E* = [(∆L*)2 + (∆a*)2 + (∆b*)2]1/2 The obtained values were converted into National Bureau of Standards (NBS) units using the formula NBS = ∆E* × 0.92 to describe color in clinical standards (Table 3 ). Table 3 Description of color changes using National Bureau of Standards (NBS) units. National Bureau of Standards units Descriptions of color changes 0.0–0.5 Trace: extremely slight change 0.5–1.5 Slight: slight change 1.5–3.0 Noticeable: perceivable 3.0–6.0 Appreciable: marked change 6.0–12.0 Much: extremely marked change 12.0 or more Very much: change to other color Statistical analysis The data collected were analyzed by using the IBM Statistical Package for the Social Sciences (SPSS) version 28 software (IBM Corp., Armonk, NY, USA). The level of significance for all tests was set at p < 0.05. The flexural strength, microhardness, and color change were tested for normality of distribution and homogeneity of variance using the Shapiro and Kolmogorov‒Smirnov tests. Two-way analysis of variance (ANOVA) was used for flexural strength if the normality of the data was satisfied. If the interaction was significant, Dunnett’s post hoc test was conducted to compare the effects of each pH on the three vacuum-formed retainer materials. Additionally, if the assumption of homogeneity of variance was not satisfied, a paired t-test was used to compare the means. However, the paired t-test will be utilized to analyze microhardness and color change, as it compares the means of two groups: the control group (SG-A) and the groups exposed to simulated gastric acid (SG-B and SG-C). Results The normality assumption of the data was assessed using the Shapiro‒Wilk test. The test indicated that the condition of normality was satisfied with a P > 0.05. Flexural Strength The comparison of flexural strength within each material using a two-way analysis of variance (ANOVA) revealed a statistically significant difference (P < 0.001). The test revealed that the flexural strength for all materials was greatly reduced in the SG-B and SG-C groups (p < 0.001). The flexural strength tested for the SG-A group was the highest for PET-G, followed by PP and CP (Table 4 ). The most affected material in SG-B and SG-C was PET-G, followed by PP and CP, respectively (Fig. 3 ). Table 4 Descriptive statistics and two-way ANOVA of flexural strength of the VFRs in various pH solutions Material N Mean SD p-value 95% Confidence Interval for Mean MCT (Dunnett’s test) MCT Lower Bound Upper Bound SG-A SG-B SG-C CP SG-A 12 37.123 0.000 < 0.001* 37.123 37.123 1 b SG-B 6 19.816 2.750 16.930 22.702 < 0.001* 1 a SG-C 6 19.307 1.344 17.897 20.718 < 0.001* 0.966 1 a PP SG-A 12 43.247 0.000 < 0.001* 43.247 43.247 1 b SG-B 6 25.088 2.147 22.835 27.341 < 0.001* 1 a SG-C 6 23.699 2.153 21.440 25.959 < 0.001* 0.256 1 a PET-G SG-A 12 61.976 0.000 < 0.001* 61.976 61.976 1 b SG-B 6 37.029 1.507 35.448 38.610 < 0.001* 1 a SG-C 6 38.861 2.905 35.813 41.909 < 0.001* 0.475 1 a CP: Copolyester; PP: Polypropylene; PET-G: Polyethylene terephthalate glycol; SG-A, control subgroup (distilled water; pH 6.7); SG-B, simulated gastric acid pH 1.2 subgroup; SG-C, simulated gastric acid pH 4 subgroup; SD: Standard deviation, MCT, multiple comparison test, * Significant at P < 0.05 Microhardness The surface microhardness of each VFR material was compared using a paired t-test following immersion in different SG solutions (Table 5 ). There were significant differences in CP and PET-G only when they were immersed in SG-C (P = 0.001 and P = 0.025, respectively). As shown in Fig. 4 , the CP surface microhardness decreased in the SG-C group, while the smallest reduction was found in the PP in the SG-B group. Color Change A paired t-test was used to compare the color difference of the VFR material between various simulated gastric acid solutions, as shown in Table 6 . The findings revealed a significant difference in color between SG-B and SG-C for different materials, namely, CP (P = 0.03), PP (P = 0.001), and PET-G (P = 0.003). Compared with other VFR materials, the CP material had a significant impact on the color change in both the SG-B solution (p = 0.003) and the SG-C solution (P = 0.025). Table 5 Descriptive statistics and paired t-test of surface microhardness of VFRs in various pH solutions (n = 6 per group). 95% Confidence Interval of the difference Material Mean N SD Std. Error Lower Upper t df p-value CP SG-A 10.683 6 0.1472 0.0601 0.222 0.745 0.745 5 0.490 SG-B 10.633 6 0.0817 0.0333 SG-A 10.600 6 0.1789 0.0730 0.208 6.708 6.708 5 0.001* SG-C 10.450 6 0.1643 0.0671 PP SG-A 8.967 6 0.1506 0.0615 0.139 0.349 0.349 5 0.741 SG-B 8.950 6 0.2168 0.0885 SG-A 9.017 6 0.1169 0.0477 0.152 2.000 2.000 5 0.102 SG-C 8.950 6 0.1517 0.0619 PET-G SG-A 9.788 6 0.0184 0.0075 0.087 1.034 1.034 5 0.349 SG-B 9.763 6 0.0497 0.0203 SG-A 10.000 6 0.1897 0.0775 0.121 3.162 3.162 5 0.025* SG-C 9.933 6 0.1861 0.0760 CP: Copolyester; PP: Polypropylene; PET-G: Polyethylene terephthalate glycol; SG-A, control subgroup (distilled water; pH 6.7); SG-B, simulated gastric acid pH 1.2 subgroup; SG-C, simulated gastric acid pH 4 subgroup; SD: Standard deviation, std Error: standard error of measurements, t: paired t-test, df: the difference between the means, * Significant at P < 0.05 Table 6 Comparison of color changes of VFRs in various simulated gastric acid solutions using a paired t-test. (n = 6 VFR per group). Material N Mean SD p-value CP SG-C 6 2.283 0.501 0.003* SG-B 6 4.499 1.281 PP SG-C 6 0.979 0.871 0.001* SG-B 6 3.601 1.059 PET-G SG-C 6 1.062 0.259 0.003* SG-B 6 2.544 0.914 CP: Copolyester; PP: Polypropylene; PET-G: Polyethylene terephthalate glycol; SG-B, simulated gastric acid pH 1.2 subgroup; SG-C, simulated gastric acid pH 4 subgroup; SD: Standard deviation *Significant at P < 0.05 Figure 1 . Flow chart showing groups distribution (CP: Copolyester; PP: Polypropylene; PET-G: Polyethylene terephthalate glycol; FS: Flexural strength; MH; Microhardness; CC: Color change; SG-A, control subgroup (distilled water; pH 6.7); SG-B, simulated gastric acid pH 4 subgroup; SG-C, simulated gastric acid pH 1.2 subgroup) Figure 2 . Three-point testing using the universal testing machine (Instron 5965 series; North America). Figure 3 . Specimens after performing a three-point bending test (A: Copolyester, B: Polypropylene, C: PET-G). Figure 4 . Microscopic images of the indentations made in all three materials before and after immersion in acidic solutions. A: Copolyester before immersion, B: Copolyester after immersion, C: Polypropylene before immersion, D: Polypropylene after immersion, E: PET-G before immersion, F: PET-G after immersion. Discussion With an increase in the demand for adult orthodontics, there is a corresponding increase in demand for esthetic orthodontic appliances to replace commonly used metal brackets [ 1 ]. A range of alternatives for fixed brackets and wires have been introduced [ 34 ]. Therefore, the mechanical and physical properties of these alternatives need to be evaluated in oral environments according to the material type. Several studies have investigated the effects of acidic environments on the properties, surface characteristics, and behavior of orthodontic wires and brackets [ 16 , 20 , 32 , 35 ]. These studies concluded that increased acidity of the oral environment significantly affects the surface roughness of wires and the surface properties of archwires and brackets. It is valuable to perform in vitro studies that examine the effects of exposure of VFRs to various pH levels on physical and mechanical properties. Therefore, the present study aimed to explore the effect of simulated gastric acid at pH 1.2 and pH 4 on the surface microhardness, flexural strength, and color change of three types of orthodontic retainer materials. Flexural Strength In the present study, the flexural strength was evaluated to investigate the durability and reliability of the VFRs. Based on the results of this study, it is evident that the flexural strength at the maximum load was significantly lowered after immersion in various concentrations of acidic pH compared to that of the control group. PET-G showed the greatest change in flexural strength, followed by CP and PP. This could be explained by the nature of the material, as PET-G and CP are amorphous polymers and lack a crystalline structure, which increases their susceptibility to water absorption [ 36 ]. In contrast, PP is a partially crystalline solid. To date, no studies have evaluated the effect of simulated gastric acid on vacuum-formed retainers. However, a few previous studies have assessed the effect of cleaning agents on flexural strength of vacuum-formed retainers [ 37 – 41 ]. The results showed that there was either no change or an increase in the flexural moduli of the thermoforming retainers after they were subjected to various cleaning agents. These results were not consistent with those of the present study, and this inconsistency may be attributable to several factors. First, the cleaning agents used have a different pH media than those used in the present study, as some of them are alkaline. Second, previous studies used hydrogen peroxide, which oxidizes polymers by releasing free oxygen radicals, resulting in an increase in the flexural modulus of the tested materials [ 42 , 43 ]. Finally, the soaking time might have influenced the results. Microhardness There was a significant difference in the microhardness loss between CP and PET-G after immersion at pH 4 (P = 0.001 and P = 0.025, respectively). Moreover, simulated gastric acid decreased the microhardness of PP, but the reduction was not statistically significant. This result is in line with Babanour et al.’s (2021) findings, which were based on PET-G material after exposure to 15% carbamide peroxide as a home bleaching agent, in which surface microhardness was significantly reduced [ 36 ]. Another study performed on Clear Aligner C, which is fabricated from PET-G material (and considering that the structure of a thermoplastic retainer is similar to that of a thermoplastic aligner), reported a significant reduction in the microhardness and elastic index after exposure to three mildly acidic cleaning agents (Retain Brite, Retain Cleaner and Steraligner), which implies a softening and brittleness effect [ 44 ]. A possible explanation is the induced hydrolytic degradation of amorphous polymers mentioned earlier. Color Change One of the major reasons for the popularity of VFR is aesthetics, and color change is, therefore, an important property to be tested because it may affect patient satisfaction and compliance [ 45 ]. In this study, spectrometer observation revealed a noticeable difference in color change before and after immersion at pH 1.2 and pH 4 in all three materials, with CP being the most affected (as its ∆ E was the highest) and PET-G the least affected, maintaining its optical properties. According to the spectrometer readings, the CP became lighter in color after immersion. This finding is consistent with the results of Hussein et al. (2022), who reported that CP showed less color stability in alcohol-based mouthwash than in other PP materials [ 41 ]. PET-G remained more transparent than the other two materials, in agreement with the findings of Liu et al. (2016), who compared the color stability of PET-G and PU and found that PET-G had the least effect on color change when exposed to staining agents [ 46 ]. Furthermore, a study by Daniele et al. (2021) evaluating the optical features of aligners used three different types of aligners composed of PET-G material and found that two of them were the least affected by red wine and coffee [ 47 ]. The difference in color change between materials might be attributed to their various polymer-based chemical structures and surface characteristics, as mentioned above. The penetration of water molecules can affect the physicochemical properties, including color. PP has a crystalline structure, which allows it to absorb less water than the amorphous structure of the other two materials [ 36 , 48 ]. Limitations The current study was subject to the following limitations. The clinical environment cannot be perfectly replicated in an in vitro study. An appropriately designed, randomized, controlled clinical trial may be helpful for determining whether these findings can be replicated in vivo. Furthermore, increasing the sample size is recommended to strengthen the reliability of the study. Conclusions The durability of VFRs is dependent on their mechanical and physical properties, as they are typically prescribed for lifetime retention after orthodontic treatment. The flexural strengths of CP, PP, and PET-G were significantly affected by immersion in simulated gastric acid at pH 4 and pH 1.2. In the control group, the flexural strength of PET-G was the highest, followed by those of PP and CP. The greatest microhardness value was observed for CP, followed by PET-G and PP in the control group. A significant difference was found only between CP and PET-G when they were exposed to simulated gastric acid at pH 4. There was a significant difference in color among the various materials when exposed to simulated gastric acid solutions with pH values of 4 and 1.2. Among all the materials considered, the CP material had the greatest color change. Abbreviations VFRs Vacuum-formed retainers CP Copolyester PET Polyethylene terephthalate PET-G Polyethylene terephthalate glycol-modified PU Polyurethane GERD Gastroesophageal reflux disorder PP Polypropylene SG Simulated gastric acid NBS National Bureau of Standards Declarations Acknowledgments The authors are indebted to Mr. Nasr Al-Maflehi for his help and advice on the statistical work. Author contributions Conceptualization, HK, LB, AY, and RO; methodology, HK and LB; formal analysis, KW, LB, and ES; investigation, HK; Resources and data processing, AY, RO, and EA; writing- original draft preparation, HK, and LB; Writing-review and editing, ES; visualization. All authors reviewed the final manuscript and gave their approval. Funding There was no funding provided to the authors by any organization. Data availability The data supporting the findings of this study are available upon request from the corresponding author. Ethics approval and consent to participate This study did not involve animal or human experiments. Consent for publication Not applicable. Competing interests The authors declare no competing interests References Meade MJ, Millett DT. Vacuum-formed retainers: an overview. Dent Update. 2015;42(1):24–34. 10.12968/denu.2015.42.1.24 . Al-Moghrabi D, Pandis N, Fleming PS. The effects of fixed and removable orthodontic retainers: a systematic review. Prog Orthod. 2016;17(1):24. 10.1186/s40510-016-0137-x . Johnston CD, Littlewood SJ. Retention in orthodontics. Br Dent J. 2015;218(3):119–22. 10.1038/sj.bdj.2015.47 . Lim ME, Dhaliwal JS, Wahab SWHA, Rahman HA. A survey of patient compliance with removable orthodontic retainer wear in Brunei Darussalam. BDJ Open . 2023;9(1):10. Published 2023 Mar 3. 10.1038/s41405-023-00138-8 . Littlewood SJ, Millett DT, Doubleday B, Bearn DR, Worthington HV. Retention procedures for stabilising tooth position after treatment with orthodontic braces. Cochrane Database Syst Rev . 2016;2016(1):CD002283. Published 2016 Jan 29. 10.1002/14651858.CD002283.pub4 . Hichens L, Rowland H, Williams A, et al. Cost-effectiveness and patient satisfaction: Hawley and vacuum-formed retainers. Eur J Orthod. 2007;29(4):372–8. 10.1093/ejo/cjm039 . Vagdouti G, Karvouni E, Bitsanis E, Koletsi D. Objective evaluation of compliance after orthodontic treatment using Hawley or vacuum-formed retainers: A 2-center randomized controlled trial over a 3-month period. Am J Orthod Dentofac Orthop. 2019;156(6):717–e7262. 10.1016/j.ajodo.2019.07.008 . Lawrence LMDdaC. Mechanical Properties of Orthodontic Aligners and Limitations of its Clinical Applicability: A Systematic Review. MSc Dissertation, CESPU, Porto, Portugal, 2021. https://repositorio.cespu.pt/handle/20.500.11816/3873 . DeNicola E, Aburizaiza OS, Siddique A, Khwaja H, Carpenter DO. Obesity and public health in the Kingdom of Saudi Arabia. Rev Environ Health. 2015;30(3):191–205. 10.1515/reveh-2015-0008 . Suter M, Dorta G, Giusti V, Calmes JM. Gastro-esophageal reflux and esophageal motility disorders in morbidly obese patients. Obes Surg. 2004;14(7):959–66. 10.1381/0960892041719581 . Popescu AL, Ioniţa-Radu F, Jinga M, Gavrilă AI, Săvulescu FA, Fierbinţeanu-Braticevici C. Laparoscopic sleeve gastrectomy and gastroesophageal reflux. Rom J Intern Med. 2018;56(4):227–32. 10.2478/rjim-2018-0019 . Guzman-Pruneda FA, Brethauer SA. Gastroesophageal Reflux After Sleeve Gastrectomy. J Gastrointest Surg. 2021;25(2):542–50. 10.1007/s11605-020-04786-1 . Alsuwat OB, Alzahrani AA, Alzhrani MA, Alkhathami AM, Mahfouz MEM. Prevalence of Gastroesophageal Reflux Disease in Saudi Arabia. J Clin Med Res. 2018;10(3):221–5. 10.14740/jocmr3292w . Fujimori S. Gastric acid level of humans must decrease in the future. World J Gastroenterol. 2020;26(43):6706–9. 10.3748/wjg.v26.i43.6706 . Ranjitkar S, Kaidonis JA, Smales RJ. Gastroesophageal reflux disease and tooth erosion. Int J Dent. 2012;2012:479850. 10.1155/2012/479850 . Baidas L, Alkawari H, Alshihah N, Almashaan G, Alwaalan H. Impact of Simulated Gastric Acid on Surface Roughness and Frictional Resistance of Orthodontic Archwires: An In Vitro Study. J Clin Diagn Res. 2020;14(12):10–5. https://doi.org/10.7860/jcdr/2020/45051.14297 . Parenti SI, Guicciardi S, Melandri C, et al. Effect of soft drinks on the physical and chemical features of nickel-titanium-based orthodontic wires. Acta Odontol Scand. 2012;70(1):49–55. 10.3109/00016357.2011.575083 . Stefański T, Kloc-Ptaszna A, Postek-Stefańska L. The effect of simulated erosive conditions on the frictional behavior of different orthodontic bracket-wire combinations. Dent Med Probl. 2019;56(2):173–7. 10.17219/dmp/105832 . Rajendran A, Sundareswaran S, Peediyekkal LV, Santhakumar P, Sathyanadhan S. Effect of oral environment and prescribed fluoride mouthwashes on different types of TMA wires - An in-vivo study. J Orthod Sci. 2019;8:8. 10.4103/jos.JOS_72_18 . Published 2019 May 23. Kao CT, Huang TH. Variations in surface characteristics and corrosion behaviour of metal brackets and wires in different electrolyte solutions. Eur J Orthod. 2010;32(5):555–60. 10.1093/ejo/cjp146 . Raja TA, Littlewood SJ, Munyombwe T, Bubb NL. Wear resistance of four types of vacuum-formed retainer materials: a laboratory study. Angle Orthod. 2014;84(4):656–64. 10.2319/061313-448.1 . Ryu JH, Kwon JS, Jiang HB, Cha JY, Kim KM. Effects of thermoforming on the physical and mechanical properties of thermoplastic materials for transparent orthodontic aligners. Korean J Orthod. 2018;48(5):316–25. 10.4041/kjod.2018.48.5.316 . Ryokawa H, Miyazaki Y, Fujishima A, Miyazaki T, Maki K. The Mechanical Properties of Dental Thermoplastic Materials in a Simulated Intraoral Environment. Orthodontic Waves. 2006;65(2):64–72. https://doi.org/10.1016/j.odw.2006.03.003 . Moshkelgosha V, Shomali M, Momeni M. Comparison of Wear Resistance of Hawley and Vacuum Formed Retainers: An In-Vitro Study. J Dent Biomater. 2016;3(2):248–53. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5608059/ . Gardner GD, Dunn WJ, Taloumis L. Wear comparison of thermoplastic materials used for orthodontic retainers. Am J Orthod Dentofac Orthop. 2003;124(3):294–7. 10.1016/s0889-5406(03)00502-x . Zafeiriadis AA, Karamouzos A, Athanasiou AE, Eliades T, Palaghias G. An In Vivo Spectrophotometric Evaluation of Vivera® and Essix® Clear Thermoplastic Retainer Discolouration. Aust Orthod J 2018 34,1,3–10. https://doi.org/10.21307/aoj-2020-053 . Azmuddin I, Mustapha NM, Khan HB, Sinniah SD. Physical Effects of Cleaning Agents on Orthodontic Thermoplastic Retainer Polymer: A Narrative Review. J Int Oral Health. 2022;14:4,349–56. http://dx.doi.org/10.4103/jioh.jioh_357_21 . Alexandropoulos A, Al Jabbari YS, Zinelis S, Eliades T. Chemical and mechanical characteristics of contemporary thermoplastic orthodontic materials. Aust Orthod J. 2015;31(2):165–70. Albilali AT, Baras BH, Aldosari MA. Evaluation of Mechanical Properties of Different Thermoplastic Orthodontic Retainer Materials after Thermoforming and Thermocycling. Polymers (Basel) . 2023;15(7):1610. Published 2023 Mar 23. 10.3390/polym15071610 . Kwon JS, Lee YK, Lim BS, Lim YK. Force delivery properties of thermoplastic orthodontic materials. Am J Orthod Dentofac Orthop. 2008;133(2):228–e3281. 10.1016/j.ajodo.2006.03.034 . Meade MJ, Millett D. Retention protocols and use of vacuum-formed retainers among specialist orthodontists. J Orthod. 2013;40(4):318–25. 10.1179/1465313313Y.0000000066 . Kulkarni A, Rothrock J, Thompson J. Impact of Gastric Acid Induced Surface Changes on Mechanical Behavior and Optical Characteristics of Dental Ceramics. J Prosthodont. 2020;29(3):207–18. 10.1111/jopr.12716 . Johnston WM. Color measurement in dentistry. J Dent. 2009;37(Suppl 1):e2–6. 10.1016/j.jdent.2009.03.011 . Kohda N, Iijima M, Muguruma T, Brantley WA, Ahluwalia KS, Mizoguchi I. Effects of mechanical properties of thermoplastic materials on the initial force of thermoplastic appliances. Angle Orthod. 2013;83(3):476–83. 10.2319/052512-432.1 . Thamer MR, Al-Joubori SK. The Effect of Acidity Level on Ions Released and Corrosion of Metal Orthodontic Appliances at Different Time Intervals: An In Vitro Study. J Bagh Coll Dentistry. 2015;27(4):168–74. https://doi.org/10.12816/0024957 . Babanouri N, Ahmadi N, Pakshir HR, Ajami S, Habibagahi R. Influence of a bleaching agent on surface and mechanical properties of orthodontic thermoplastic retainer materials: An in vitro study. Einfluss eines Bleaching-Präparats auf die Oberfläche und mechanische Eigenschaften von kieferorthopädischen thermoplastischen Retainermaterialien : Eine In-vitro-Studie. J Orofac Orthop. 2022;83(5):332–8. 10.1007/s00056-021-00312-3 . Wible E, Agarwal M, Altun S, et al. Long-term effects of various cleaning methods on polypropylene/ethylene copolymer retainer material. Angle Orthod. 2019;89(3):432–7. 10.2319/060818-429.1 . Wible E, Agarwal M, Altun S, et al. Long-term effects of different cleaning methods on copolyester retainer properties. Angle Orthod. 2019;89(2):221–7. 10.2319/010218-2.1 . Brehove J. Effects of Cleaning Agents on the Properties of Two Different Thermoplastic Retainer Materials. Doctoral dissertation, Marquette University, Milwaukee, WI (2021). https://epublications.marquette.edu/theses_open/669/ . Agarwal M, Wible E, Ramir T, et al. Long-term effects of seven cleaning methods on light transmittance, surface roughness, and flexural modulus of polyurethane retainer material. Angle Orthod. 2018;88(3):355–62. 10.2319/081517-551.1 . Hussein AM, Mohammed-Salih HS, Al-Sheakli II. Effects of various cleaning agents on polypropylene and copolyester thermoplastic orthodontic retainer materials. J Taibah Univ Med Sci. 2022;17(5):861–8. 10.1016/j.jtumed.2022.04.005 . Published 2022 May 6. Sepe M. The Mystery of Physical Aging, Part 1: Knowing the difference. Plastics Technology, 2019 March 19. https://www.ptonline.com/articles/the-mystery-of-physical-aging-part-1 . Sastri VR. Plastics in Medical Devices: Properties, Requirements, and Applications. William Andrew, 3rd Edition, 2021. Iliadi A, Enzler V, Polychronis G, Peltomäki T, Zinelis S, Eliades T. Εffect of Cleansers on the Composition and Mechanical Properties of Orthodontic Aligners in Vitro. Prog Orthodont. 2022;23(1). https://doi.org/10.1186/s40510-022-00449-w . Yazdi M, Daryanavard H, Ashtiani AH, Moradinejad M, Rakhshan V. A systematic review of biocompatibility and safety of orthodontic clear aligners and transparent vacuum-formed thermoplastic retainers: Bisphenol-A release, adverse effects, cytotoxicity, and estrogenic effects. Dent Res J (Isfahan). 2023;20:41. Published 2023 Mar 28. Liu CL, Sun WT, Liao W, et al. Colour stabilities of three types of orthodontic clear aligners exposed to staining agents. Int J Oral Sci. 2016;8(4):246–53. 10.1038/ijos.2016.25 . Published 2016 Dec 16. Daniele V, Macera L, Taglieri G, Spera L, Marzo G, Quinzi V. Color Stability, Chemico-Physical and Optical Features of the Most Common PETG and PU Based Orthodontic Aligners for Clear Aligner Therapy. Polym (Basel). 2021;14(1):14. 10.3390/polym14010014 . Published 2021 Dec 21. Inoue S, Yamaguchi S, Uyama H, Yamashiro T, Imazato S. Influence of constant strain on the elasticity of thermoplastic orthodontic materials. Dent Mater J. 2020;39(3):415–21. 10.4012/dmj.2019-104 . Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 01 Feb, 2025 Read the published version in BMC Oral Health → Version 1 posted Editorial decision: Revision requested 16 Dec, 2024 Reviews received at journal 16 Dec, 2024 Reviews received at journal 16 Dec, 2024 Reviewers agreed at journal 14 Dec, 2024 Reviewers agreed at journal 13 Dec, 2024 Reviewers agreed at journal 13 Dec, 2024 Reviewers agreed at journal 13 Dec, 2024 Reviewers agreed at journal 12 Dec, 2024 Reviewers agreed at journal 12 Dec, 2024 Reviewers agreed at journal 12 Dec, 2024 Reviewers agreed at journal 11 Dec, 2024 Reviewers agreed at journal 11 Dec, 2024 Reviewers agreed at journal 11 Dec, 2024 Reviewers invited by journal 08 Aug, 2024 Editor invited by journal 18 Jun, 2024 Editor assigned by journal 13 Jun, 2024 Submission checks completed at journal 13 Jun, 2024 First submitted to journal 29 May, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4495989","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":317004805,"identity":"e41291da-cca0-430b-aee3-aa0b41b59768","order_by":0,"name":"1.\tHuda Alkawari","email":"","orcid":"","institution":"King Saud University","correspondingAuthor":false,"prefix":"","firstName":"1.\tHuda","middleName":"","lastName":"Alkawari","suffix":""},{"id":317004806,"identity":"a5ff5fdc-f5d8-4845-ad2e-fc2eee7dfbc7","order_by":1,"name":"2.\tLaila Baidas","email":"","orcid":"","institution":"King Saud University","correspondingAuthor":false,"prefix":"","firstName":"2.\tLaila","middleName":"","lastName":"Baidas","suffix":""},{"id":317004807,"identity":"ff9b72c5-aca8-4809-ad30-b75bfd1e522a","order_by":2,"name":"3.\tAreen Alyahya","email":"","orcid":"","institution":"King Saud University","correspondingAuthor":false,"prefix":"","firstName":"3.\tAreen","middleName":"","lastName":"Alyahya","suffix":""},{"id":317004808,"identity":"91841d67-b091-4f73-89b4-e72f54d4482e","order_by":3,"name":"4.\tRenad Alotaibi","email":"","orcid":"","institution":"King Saud University","correspondingAuthor":false,"prefix":"","firstName":"4.\tRenad","middleName":"","lastName":"Alotaibi","suffix":""},{"id":317004809,"identity":"48edb365-5d6f-4c72-9776-a5be5320b786","order_by":4,"name":"5. Eman Alshayea","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7UlEQVRIiWNgGAWjYFAC5gZmMM3eBqYYGwhrYYRq4TlGshaJNCK18LMfbP5cUHNYjn/ms+TPPAw2shsOsD/8gE+LZE9im/SMY4eNJW6nHZPmYUgz3nCAx1gCnxaDA4ltzDxshxMbbqcDGQyHE4FaGPBqsT//sPkzz7/D9fNvHgcyGP4DtbA//oHXFonEBmnetsMJBjfYDgAddgCohcEMry0SNx62SfP2pRtuPJOWJjnHINl45mEeMwt8Wvj7kw9/5vlmLS93/JjxhzcVdrJ9x9sf38CnBQqaYe4EYmYi1ANBHXHKRsEoGAWjYGQCAEv7TPYMAZ17AAAAAElFTkSuQmCC","orcid":"","institution":"King Saud University","correspondingAuthor":true,"prefix":"","firstName":"5.","middleName":"Eman","lastName":"Alshayea","suffix":""}],"badges":[],"createdAt":"2024-05-29 09:38:09","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4495989/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4495989/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1186/s12903-025-05548-8","type":"published","date":"2025-02-01T15:57:45+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":60187795,"identity":"3491e7ec-e12d-42e1-abe5-89ec066b0fac","added_by":"auto","created_at":"2024-07-12 19:12:48","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":77386,"visible":true,"origin":"","legend":"\u003cp\u003eFlow chart showing groups distribution (CP: Copolyester; PP: Polypropylene; PET-G: Polyethylene terephthalate glycol; FS: Flexural strength; MH; Microhardness; CC: Color change; SG-A, control subgroup (distilled water; pH 6.7); SG-B, simulated gastric acid pH 4 subgroup; SG-C, simulated gastric acid pH 1.2 subgroup).\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-4495989/v1/170bf8131a77a1856ddf71a4.png"},{"id":60187797,"identity":"78480e51-14c1-4bfe-9410-0cfff3e53431","added_by":"auto","created_at":"2024-07-12 19:12:48","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":660129,"visible":true,"origin":"","legend":"\u003cp\u003eThree-point testing using the universal testing machine (Instron 5965 series; North America).\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-4495989/v1/baf6f342de7a89c708e23141.png"},{"id":60188915,"identity":"2cf41bba-d50f-4d94-a735-75bceb5bf5ba","added_by":"auto","created_at":"2024-07-12 19:20:48","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":488751,"visible":true,"origin":"","legend":"\u003cp\u003eSpecimens after performing a three-point bending test (A: Copolyester, B: Polypropylene, C: PET-G).\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-4495989/v1/96be4ee1670dafe1ec0bcc0f.png"},{"id":60187796,"identity":"fcadd71b-d886-43dc-9c2d-fcfec8ad8dd0","added_by":"auto","created_at":"2024-07-12 19:12:48","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":306301,"visible":true,"origin":"","legend":"\u003cp\u003eMicroscopic images of the indentations made in all three materials before and after immersion in acidic solutions. A: Copolyester before immersion, B: Copolyester after immersion, C: Polypropylene before immersion, D: Polypropylene after immersion, E: PET-G before immersion, F: PET-G after immersion.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-4495989/v1/421ddcacbab916e1c6dcb361.png"},{"id":75351876,"identity":"d10fbca2-d0ad-454c-8077-4121e72c7036","added_by":"auto","created_at":"2025-02-03 16:12:44","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2574017,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4495989/v1/444a47b5-2587-40e5-ad1a-634041737cce.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Impact of Simulated Gastric Acid on the Physical and Mechanical Properties of Vacuum-Formed Retainers: An In Vitro Study","fulltext":[{"header":"Background","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eRetention is the final stage of orthodontic treatment, which attempts to maintain teeth in their corrected position after the completion of treatment by using various types of retainers [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Clinicians must approach the treatment of all patients with the assumption that there is a high risk of relapse due to the difficulty in properly predicting the level of relapse that may occur over a period of time [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Long-term posttreatment retention is advocated to lower this risk, with patient compliance playing a major role [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Both fixed and removable retainers are used to achieve this objective and reduce the chances of relapse. However, the choice of one approach over the other in terms of stability remains ambiguous owing to the absence of sufficient evidence. Consequently, many practitioners choose retainers on the basis of their own personal preferences [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAt present, vacuum-formed retainers ​​(VFRs) are becoming more popular due to their ease of fabrication, esthetic appearance, and good patient compliance [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Various thermoplastic materials are used in manufacturing vacuum-formed retainers. Several materials, such as copolyester (CP), polyethylene terephthalate (PET), polyethylene terephthalate glycol-modified (PET-G), and thermoplastic polyurethane (PU), have been shown to have good properties. Consequently, these materials have been widely used in multiple current research studies [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. However, their mechanical and physical properties may be impacted by the pH level of the oral cavity, which could result in chemical and physical changes that may affect the effectiveness of the retainer.\u003c/p\u003e \u003cp\u003eDeNicola et al. (2015) reported a 35% prevalence of obesity in Saudi Arabia, and there is evidence to support the likelihood of a positive association between obesity and gastroesophageal reflux disorder (GERD) [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. The increasing prevalence of sleeve gastrectomy operations may be attributed to its effectiveness in decreasing both body mass index and the risk of death. Although sleeve gastrectomy is widely regarded as a safe therapeutic technique, it is crucial to realize that problems may still occur, as with any surgical procedure [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. The use of this particular approach has the potential to exacerbate pre-existing GERD or trigger its onset as a result of surgical intervention [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Obaidallah et al. (2018) concluded that the prevalence of GERD is slightly greater in Saudi Arabia than in Western countries and much greater in Saudi Arabia than in countries in East Asia [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Acid reflux decreases salivary pH from 6.7 to approximately 2\u0026ndash;1.5, creating an acidic environment that may impact teeth, soft tissue, and intraoral appliances [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Hence, it is essential from a clinical perspective to assess the physical and mechanical characteristics of various materials after exposure to different pH values in the oral cavity.\u003c/p\u003e \u003cp\u003ePrevious studies on orthodontic wires have shown deterioration of both mechanical and physical properties after being subjected to an acidic environment [\u003cspan additionalcitationids=\"CR17 CR18 CR19\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Baidas et al. (2020) evaluated the surface roughness and frictional resistance of orthodontic archwires in simulated gastric acid and concluded that the surface roughness of beta-titanium and stainless-steel archwires was significantly affected when subjected to acidic conditions [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Chia-Tze Kao et al. (2010) studied the surface characteristics and corrosion behavior of metal brackets and wires in various electrolyte solutions after exposure to an acidic environment. The study concluded that most wire and bracket materials were easily corroded in a pH 4 medium of artificial saliva [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. This assessment is crucial for determining the effectiveness and longevity of these materials throughout both the treatment and retention phases.\u003c/p\u003e \u003cp\u003eSeveral studies have evaluated the mechanical, physical, and surface properties of retainers in general [\u003cspan additionalcitationids=\"CR22 CR23 CR24 CR25 CR26 CR27 CR28 CR29\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e], including wear resistance [\u003cspan additionalcitationids=\"CR22 CR23 CR24\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e], water absorption [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], transparency [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e], surface hardness [\u003cspan additionalcitationids=\"CR23 CR24\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e] and flexural strength [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. However, none of the previous studies investigated the effect of acidic environments on the mechanical and physical properties of these materials. Therefore, this study aimed to investigate the effect of different pH levels on VFRs in terms of microhardness, flexural strength, and color change in consideration of the clinical environment in which they are used.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eMaterials and Study Design\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eA total of 144 vacuum-formed retainers (VFRs) made of three thermoplastic materials were evaluated in this study: Clear Advantage Series I (Ortho Technology, Florida, U.S.), Clear Advantage Series II (Ortho Technology, Florida, U.S.), and DURAN\u0026reg;+ (Scheu Dental, Iserlohn, Germany) with a unified 0.040\"/1 mm thickness, as preferred by most clinicians [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. The VFRs were divided into three groups of 48 retainers according to material type: copolyester (CP) (Clear Advantage Series I), polypropylene (PP) (Clear Advantage Series II), and polyethylene terephthalate glycol (PET-G) (DURAN\u0026reg;+). Each group was further subdivided into three subgroups according to the pH of the immersed solution: control; distilled water at pH 6.7 (SG-A), simulated gastric acid at pH 4 (SG-B), and simulated gastric acid at pH 1.2 (SG-C) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). A sample power analysis indicated that each subgroup would result in an 80% chance of reaching significance at the 0.05 level.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eSpecimen Preparation\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThermoforming\u003c/p\u003e \u003cp\u003eThe sheets were vacuumed over a highly polished stainless-steel plate with a diameter of 110 mm and a height of 10 mm, which was placed in a thermoforming caster (Biostar\u0026reg;; Scheu Dental) for uniform vacuuming. Then, they were cut according to ISO 20795-2 (2013). The heating and cooling times applied during thermoforming are shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eSample Treatment Protocol for Simulated Gastric Acid\u003c/p\u003e \u003cp\u003eA simulated gastric acid solution with pH 1.2 (SG-B) was prepared by dissolving Na₂HPO₄ and NaH₂PO₄ in 1 L of distilled water, while a simulated gastric acid solution with pH 4 (SG-C) was prepared by dissolving NaCl and concentrated HCl in 1 L of water to simulate changes in the oral cavity. The procedure was repeated 6 times a day for 9 days with 24-hour intervals between each cycle, based on previous studies involving the simulation of gastric acid (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eThermoforming conditions of the thermoplastic materials used in the study.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBrand\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eComponent\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTemperature (\u0026deg;c)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePressure (bar)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eHeating time (s)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCooling time (s)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eClear Advantage Series I\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCopolyester\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e220\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e6.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eClear Advantage Series II\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePolypropylene\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDURAN\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePET-G\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSample treatment protocol of thermoplastic materials specimens.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSG-A\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-B\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSG-C\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003eStored in distilled water at 37\u0026deg;C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1. Immersion for 2 minutes in pH 1.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1. Immersion for 2 minutes in pH 4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2. Rinsing with distilled water\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2. Rinsing with distilled water\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3. Storage for 30 minutes in 37\u0026deg;C distilled water\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3. Storage for 30 minutes in 37\u0026deg;C distilled water\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eRepeated 6 times per day for 9 days\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eThree-Point Bending Test\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe flexural strength was measured using a universal testing machine (Instron 5965 series; North America). Twenty-four specimens of each material after thermoforming were prepared with a width of 5 mm and length of 40 mm, immersed in distilled water to keep the specimens wet, and stored at 37\u0026deg;C for 50 hours prior to flexural testing in accordance with ISO 20795-2 (2013). The specimens were removed from the water bath and immediately laid flat on the fixed lower jaw of the Instron machine. The free end of the upper jaw holding the blade was centered over the specimens to apply a load at a crosshead speed of 5 mm/min for 30 seconds. Time was fixed due to the nature of thermoplastic polymers, which can take considerable time to bend (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e \u003cb\u003e\u0026amp;\u003c/b\u003e Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Twelve vacuum-formed retainers were utilized as a control group and were subsequently discarded. The other twelve samples were divided equally and immersed in the SG-B and SG-C solutions.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eMicrohardness test\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eTwelve specimens of each material were cut into 10\u0026times;10 mm squares. Vickers microhardness was measured using a Nova 130 (Innovatest, Maastricht, The Netherlands). Three indentations were created by applying a load of 9.8 N for 10 seconds. After that, the size of the indentations was measured using the following formula: HV\u0026thinsp;=\u0026thinsp;1.854 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\frac{F}{{d}^{2}}\\)\u003c/span\u003e\u003c/span\u003e, where F is the force applied on the surface and d is the average diameter of the indentations. After that, the mean of the three values was calculated for each specimen.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eColor Change\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eChanges in color before and after immersion were measured according to the Commission Internationale de I\u0026rsquo;Eclairage L*a*b* color system.\u003csup\u003e33\u003c/sup\u003e The color parameter L* represents lightness (+\u0026thinsp;bright, \u0026minus; dark), while a* and b* are associated with the color scale, as a* represents the red (+) to green (\u0026minus;) and b* indicates the yellow (+) to blue (\u0026minus;) spectrum. The values of the three parameters were measured using a spectrophotometer (LabScan\u0026reg; XE, HunterLab Inc.) with a color data software program (EasyMatch QC ver 4.90) before and after immersion in the pH solution. Twelve square-shaped specimens were prepared with dimensions of 10 mm width and 10 mm length and numbered from 1 to 12 using a handpiece and small round bur. The numbers are shown on the top right of each specimen for future comparison. All specimens were washed in an ultrasonic bath for 5 minutes and dried using tissue papers. Before each measurement, the machine was calibrated using black and white glass plates for standardization. Each specimen was held by a specially designed holder for accurate fitting. After that, measurements were obtained by laying the specimens flat over the optical sensor tip and then covered with a black lid. Each specimen was scanned at three selected points. The color parameters were obtained, and ∆E, which represents the color change, was measured using the following formula: ∆E* = [(∆L*)2 + (∆a*)2 + (∆b*)2]1/2\u003c/p\u003e \u003cp\u003eThe obtained values were converted into National Bureau of Standards (NBS) units using the formula NBS = ∆E* \u0026times; 0.92 to describe color in clinical standards (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDescription of color changes using National Bureau of Standards (NBS) units.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNational Bureau of Standards units\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDescriptions of color changes\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.0\u0026ndash;0.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTrace: extremely slight change\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0.5\u0026ndash;1.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSlight: slight change\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1.5\u0026ndash;3.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNoticeable: perceivable\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3.0\u0026ndash;6.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAppreciable: marked change\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6.0\u0026ndash;12.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMuch: extremely marked change\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e12.0 or more\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eVery much: change to other color\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eStatistical analysis\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe data collected were analyzed by using the IBM Statistical Package for the Social Sciences (SPSS) version 28 software (IBM Corp., Armonk, NY, USA). The level of significance for all tests was set at p\u0026thinsp;\u0026lt;\u0026thinsp;0.05. The flexural strength, microhardness, and color change were tested for normality of distribution and homogeneity of variance using the Shapiro and Kolmogorov‒Smirnov tests. Two-way analysis of variance (ANOVA) was used for flexural strength if the normality of the data was satisfied. If the interaction was significant, Dunnett\u0026rsquo;s post hoc test was conducted to compare the effects of each pH on the three vacuum-formed retainer materials. Additionally, if the assumption of homogeneity of variance was not satisfied, a paired t-test was used to compare the means. However, the paired t-test will be utilized to analyze microhardness and color change, as it compares the means of two groups: the control group (SG-A) and the groups exposed to simulated gastric acid (SG-B and SG-C).\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe normality assumption of the data was assessed using the Shapiro‒Wilk test. The test indicated that the condition of normality was satisfied with a P\u0026thinsp;\u0026gt;\u0026thinsp;0.05.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eFlexural Strength\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe comparison of flexural strength within each material using a two-way analysis of variance (ANOVA) revealed a statistically significant difference (P\u0026thinsp;\u0026lt;\u0026thinsp;0.001). The test revealed that the flexural strength for all materials was greatly reduced in the SG-B and SG-C groups (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001). The flexural strength tested for the SG-A group was the highest for PET-G, followed by PP and CP (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The most affected material in SG-B and SG-C was PET-G, followed by PP and CP, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDescriptive statistics and two-way ANOVA of flexural strength of the VFRs in various pH solutions\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"12\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c11\" colnum=\"11\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c12\" colnum=\"12\"\u003e\u003c/div\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" morerows=\"1\" nameend=\"c2\" namest=\"c1\" rowspan=\"2\"\u003e \u003cp\u003eMaterial\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eN\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eMean\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003ep-value\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c8\" namest=\"c7\"\u003e \u003cp\u003e95% Confidence Interval for Mean\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"3\" nameend=\"c11\" namest=\"c9\"\u003e \u003cp\u003eMCT (Dunnett\u0026rsquo;s test)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eMCT\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eLower Bound\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eUpper Bound\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003eSG-A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003eSG-B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003eSG-C\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003eCP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e37.123\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e37.123\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e37.123\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003eb\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e19.816\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.750\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e16.930\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e22.702\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003ea\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e19.307\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.344\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e17.897\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e20.718\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.966\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003ea\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003ePP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e43.247\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e43.247\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e43.247\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003eb\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e25.088\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.147\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e22.835\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e27.341\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003ea\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e23.699\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.153\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e21.440\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e25.959\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.256\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003ea\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003ePET-G\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e61.976\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e61.976\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e61.976\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003eb\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e37.029\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.507\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e35.448\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e38.610\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003ea\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e38.861\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.905\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e35.813\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e41.909\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003e0.475\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c12\"\u003e \u003cp\u003ea\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eCP: Copolyester; PP: Polypropylene; PET-G: Polyethylene terephthalate glycol; SG-A, control subgroup (distilled water; pH 6.7); SG-B, simulated gastric acid pH 1.2 subgroup; SG-C, simulated gastric acid pH 4 subgroup; SD: Standard deviation, MCT, multiple comparison test, * Significant at P\u0026thinsp;\u0026lt;\u0026thinsp;0.05\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eMicrohardness\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe surface microhardness of each VFR material was compared using a paired t-test following immersion in different SG solutions (Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). There were significant differences in CP and PET-G only when they were immersed in SG-C (P\u0026thinsp;=\u0026thinsp;0.001 and P\u0026thinsp;=\u0026thinsp;0.025, respectively). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the CP surface microhardness decreased in the SG-C group, while the smallest reduction was found in the PP in the SG-B group.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eColor Change\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eA paired t-test was used to compare the color difference of the VFR material between various simulated gastric acid solutions, as shown in Table\u0026nbsp;\u003cspan refid=\"Tab6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. The findings revealed a significant difference in color between SG-B and SG-C for different materials, namely, CP (P\u0026thinsp;=\u0026thinsp;0.03), PP (P\u0026thinsp;=\u0026thinsp;0.001), and PET-G (P\u0026thinsp;=\u0026thinsp;0.003). Compared with other VFR materials, the CP material had a significant impact on the color change in both the SG-B solution (p\u0026thinsp;=\u0026thinsp;0.003) and the SG-C solution (P\u0026thinsp;=\u0026thinsp;0.025).\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDescriptive statistics and paired t-test of surface microhardness of VFRs in various pH solutions (n\u0026thinsp;=\u0026thinsp;6 per group).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"11\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c11\" colnum=\"11\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colspan=\"6\" nameend=\"c6\" namest=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c8\" namest=\"c7\"\u003e \u003cp\u003e95% Confidence Interval of the difference\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c11\" namest=\"c9\"\u003e\u0026nbsp;\u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003eMaterial\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMean\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eN\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eStd. Error\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eLower\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eUpper\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003et\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\"\u003e \u003cp\u003edf\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\"\u003e \u003cp\u003ep-value\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003eCP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10.683\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.1472\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0601\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.222\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.745\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.745\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.490\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10.633\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.0817\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0333\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10.600\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.1789\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0730\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.208\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e6.708\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e6.708\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.001*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10.450\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.1643\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0671\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003ePP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e8.967\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.1506\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0615\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.139\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.349\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.349\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.741\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e8.950\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.2168\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0885\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e9.017\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.1169\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0477\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.152\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e2.000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e2.000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.102\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e8.950\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.1517\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0619\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003ePET-G\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e9.788\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.0184\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0075\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.087\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e1.034\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e1.034\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.349\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e9.763\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.0497\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0203\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10.000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.1897\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0775\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.121\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e3.162\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e3.162\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c11\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.025*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e9.933\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.1861\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0760\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eCP: Copolyester; PP: Polypropylene; PET-G: Polyethylene terephthalate glycol; SG-A, control subgroup (distilled water; pH 6.7); SG-B, simulated gastric acid pH 1.2 subgroup; SG-C, simulated gastric acid pH 4 subgroup; SD: Standard deviation, std Error: standard error of measurements, t: paired t-test, df: the difference between the means, * Significant at P\u0026thinsp;\u0026lt;\u0026thinsp;0.05\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab6\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 6\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComparison of color changes of VFRs in various simulated gastric acid solutions using a paired t-test. (n\u0026thinsp;=\u0026thinsp;6 VFR per group).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMaterial\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eN\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMean\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSD\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003ep-value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eCP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.283\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.501\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.003*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4.499\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.281\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003ePP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.979\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.871\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.001*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3.601\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.059\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003ePET-G\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.062\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.259\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.003*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSG-B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.544\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.914\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eCP: Copolyester; PP: Polypropylene; PET-G: Polyethylene terephthalate glycol; SG-B, simulated gastric acid pH 1.2 subgroup; SG-C, simulated gastric acid pH 4 subgroup; SD: Standard deviation *Significant at P\u0026thinsp;\u0026lt;\u0026thinsp;0.05\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Flow chart showing groups distribution (CP: Copolyester; PP: Polypropylene; PET-G: Polyethylene terephthalate glycol; FS: Flexural strength; MH; Microhardness; CC: Color change; SG-A, control subgroup (distilled water; pH 6.7); SG-B, simulated gastric acid pH 4 subgroup; SG-C, simulated gastric acid pH 1.2 subgroup)\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Three-point testing using the universal testing machine (Instron 5965 series; North America).\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Specimens after performing a three-point bending test (A: Copolyester, B: Polypropylene, C: PET-G).\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Microscopic images of the indentations made in all three materials before and after immersion in acidic solutions. A: Copolyester before immersion, B: Copolyester after immersion, C: Polypropylene before immersion, D: Polypropylene after immersion, E: PET-G before immersion, F: PET-G after immersion.\u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eWith an increase in the demand for adult orthodontics, there is a corresponding increase in demand for esthetic orthodontic appliances to replace commonly used metal brackets [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. A range of alternatives for fixed brackets and wires have been introduced [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. Therefore, the mechanical and physical properties of these alternatives need to be evaluated in oral environments according to the material type.\u003c/p\u003e \u003cp\u003eSeveral studies have investigated the effects of acidic environments on the properties, surface characteristics, and behavior of orthodontic wires and brackets [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. These studies concluded that increased acidity of the oral environment significantly affects the surface roughness of wires and the surface properties of archwires and brackets. It is valuable to perform in vitro studies that examine the effects of exposure of VFRs to various pH levels on physical and mechanical properties. Therefore, the present study aimed to explore the effect of simulated gastric acid at pH 1.2 and pH 4 on the surface microhardness, flexural strength, and color change of three types of orthodontic retainer materials.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eFlexural Strength\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eIn the present study, the flexural strength was evaluated to investigate the durability and reliability of the VFRs. Based on the results of this study, it is evident that the flexural strength at the maximum load was significantly lowered after immersion in various concentrations of acidic pH compared to that of the control group. PET-G showed the greatest change in flexural strength, followed by CP and PP. This could be explained by the nature of the material, as PET-G and CP are amorphous polymers and lack a crystalline structure, which increases their susceptibility to water absorption [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. In contrast, PP is a partially crystalline solid. To date, no studies have evaluated the effect of simulated gastric acid on vacuum-formed retainers. However, a few previous studies have assessed the effect of cleaning agents on flexural strength of vacuum-formed retainers [\u003cspan additionalcitationids=\"CR38 CR39 CR40\" citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. The results showed that there was either no change or an increase in the flexural moduli of the thermoforming retainers after they were subjected to various cleaning agents. These results were not consistent with those of the present study, and this inconsistency may be attributable to several factors. First, the cleaning agents used have a different pH media than those used in the present study, as some of them are alkaline. Second, previous studies used hydrogen peroxide, which oxidizes polymers by releasing free oxygen radicals, resulting in an increase in the flexural modulus of the tested materials [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. Finally, the soaking time might have influenced the results.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eMicrohardness\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThere was a significant difference in the microhardness loss between CP and PET-G after immersion at pH 4 (P\u0026thinsp;=\u0026thinsp;0.001 and P\u0026thinsp;=\u0026thinsp;0.025, respectively). Moreover, simulated gastric acid decreased the microhardness of PP, but the reduction was not statistically significant. This result is in line with Babanour et al.\u0026rsquo;s (2021) findings, which were based on PET-G material after exposure to 15% carbamide peroxide as a home bleaching agent, in which surface microhardness was significantly reduced [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. Another study performed on Clear Aligner C, which is fabricated from PET-G material (and considering that the structure of a thermoplastic retainer is similar to that of a thermoplastic aligner), reported a significant reduction in the microhardness and elastic index after exposure to three mildly acidic cleaning agents (Retain Brite, Retain Cleaner and Steraligner), which implies a softening and brittleness effect [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. A possible explanation is the induced hydrolytic degradation of amorphous polymers mentioned earlier.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003eColor Change\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eOne of the major reasons for the popularity of VFR is aesthetics, and color change is, therefore, an important property to be tested because it may affect patient satisfaction and compliance [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. In this study, spectrometer observation revealed a noticeable difference in color change before and after immersion at pH 1.2 and pH 4 in all three materials, with CP being the most affected (as its ∆ E was the highest) and PET-G the least affected, maintaining its optical properties. According to the spectrometer readings, the CP became lighter in color after immersion. This finding is consistent with the results of Hussein et al. (2022), who reported that CP showed less color stability in alcohol-based mouthwash than in other PP materials [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. PET-G remained more transparent than the other two materials, in agreement with the findings of Liu et al. (2016), who compared the color stability of PET-G and PU and found that PET-G had the least effect on color change when exposed to staining agents [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]. Furthermore, a study by Daniele et al. (2021) evaluating the optical features of aligners used three different types of aligners composed of PET-G material and found that two of them were the least affected by red wine and coffee [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. The difference in color change between materials might be attributed to their various polymer-based chemical structures and surface characteristics, as mentioned above. The penetration of water molecules can affect the physicochemical properties, including color. PP has a crystalline structure, which allows it to absorb less water than the amorphous structure of the other two materials [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003eLimitations\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe current study was subject to the following limitations. The clinical environment cannot be perfectly replicated in an in vitro study. An appropriately designed, randomized, controlled clinical trial may be helpful for determining whether these findings can be replicated in vivo. Furthermore, increasing the sample size is recommended to strengthen the reliability of the study.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe durability of VFRs is dependent on their mechanical and physical properties, as they are typically prescribed for lifetime retention after orthodontic treatment. The flexural strengths of CP, PP, and PET-G were significantly affected by immersion in simulated gastric acid at pH 4 and pH 1.2. In the control group, the flexural strength of PET-G was the highest, followed by those of PP and CP. The greatest microhardness value was observed for CP, followed by PET-G and PP in the control group. A significant difference was found only between CP and PET-G when they were exposed to simulated gastric acid at pH 4. There was a significant difference in color among the various materials when exposed to simulated gastric acid solutions with pH values of 4 and 1.2. Among all the materials considered, the CP material had the greatest color change.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003e\u003cstrong\u003eVFRs \u003c/strong\u003eVacuum-formed retainers \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCP\u003c/strong\u003e Copolyester \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePET\u003c/strong\u003e Polyethylene terephthalate \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePET-G\u003c/strong\u003e Polyethylene terephthalate glycol-modified \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePU\u003c/strong\u003e Polyurethane \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eGERD\u003c/strong\u003e Gastroesophageal reflux disorder\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePP\u003c/strong\u003e Polypropylene\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSG \u003c/strong\u003e Simulated gastric acid \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eNBS \u003c/strong\u003e National Bureau of Standards\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors are indebted to Mr. Nasr Al-Maflehi for his help and advice on the statistical work.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConceptualization, HK, LB, AY, and RO; methodology, HK and LB; formal analysis, KW, LB, and ES; investigation, HK; Resources and data processing, AY, RO, and EA; writing- original draft preparation, HK, and LB; Writing-review and editing, ES; visualization. All authors reviewed the final manuscript and gave their approval.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThere was no funding provided to the authors by any organization.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data supporting the findings of this study are available upon request from the corresponding author.\u003c/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study did not involve animal or human experiments.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eMeade MJ, Millett DT. Vacuum-formed retainers: an overview. Dent Update. 2015;42(1):24\u0026ndash;34. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.12968/denu.2015.42.1.24\u003c/span\u003e\u003cspan address=\"10.12968/denu.2015.42.1.24\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAl-Moghrabi D, Pandis N, Fleming PS. The effects of fixed and removable orthodontic retainers: a systematic review. Prog Orthod. 2016;17(1):24. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1186/s40510-016-0137-x\u003c/span\u003e\u003cspan address=\"10.1186/s40510-016-0137-x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJohnston CD, Littlewood SJ. Retention in orthodontics. Br Dent J. 2015;218(3):119\u0026ndash;22. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1038/sj.bdj.2015.47\u003c/span\u003e\u003cspan address=\"10.1038/sj.bdj.2015.47\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLim ME, Dhaliwal JS, Wahab SWHA, Rahman HA. A survey of patient compliance with removable orthodontic retainer wear in Brunei Darussalam. \u003cem\u003eBDJ Open\u003c/em\u003e. 2023;9(1):10. Published 2023 Mar 3. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1038/s41405-023-00138-8\u003c/span\u003e\u003cspan address=\"10.1038/s41405-023-00138-8\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLittlewood SJ, Millett DT, Doubleday B, Bearn DR, Worthington HV. Retention procedures for stabilising tooth position after treatment with orthodontic braces. \u003cem\u003eCochrane Database Syst Rev\u003c/em\u003e. 2016;2016(1):CD002283. Published 2016 Jan 29. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1002/14651858.CD002283.pub4\u003c/span\u003e\u003cspan address=\"10.1002/14651858.CD002283.pub4\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHichens L, Rowland H, Williams A, et al. Cost-effectiveness and patient satisfaction: Hawley and vacuum-formed retainers. Eur J Orthod. 2007;29(4):372\u0026ndash;8. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1093/ejo/cjm039\u003c/span\u003e\u003cspan address=\"10.1093/ejo/cjm039\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVagdouti G, Karvouni E, Bitsanis E, Koletsi D. Objective evaluation of compliance after orthodontic treatment using Hawley or vacuum-formed retainers: A 2-center randomized controlled trial over a 3-month period. Am J Orthod Dentofac Orthop. 2019;156(6):717\u0026ndash;e7262. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.ajodo.2019.07.008\u003c/span\u003e\u003cspan address=\"10.1016/j.ajodo.2019.07.008\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLawrence LMDdaC. Mechanical Properties of Orthodontic Aligners and Limitations of its Clinical Applicability: A Systematic Review. MSc Dissertation, CESPU, Porto, Portugal, 2021. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://repositorio.cespu.pt/handle/20.500.11816/3873\u003c/span\u003e\u003cspan address=\"https://repositorio.cespu.pt/handle/20.500.11816/3873\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDeNicola E, Aburizaiza OS, Siddique A, Khwaja H, Carpenter DO. Obesity and public health in the Kingdom of Saudi Arabia. Rev Environ Health. 2015;30(3):191\u0026ndash;205. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1515/reveh-2015-0008\u003c/span\u003e\u003cspan address=\"10.1515/reveh-2015-0008\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSuter M, Dorta G, Giusti V, Calmes JM. Gastro-esophageal reflux and esophageal motility disorders in morbidly obese patients. Obes Surg. 2004;14(7):959\u0026ndash;66. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1381/0960892041719581\u003c/span\u003e\u003cspan address=\"10.1381/0960892041719581\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePopescu AL, Ioniţa-Radu F, Jinga M, Gavrilă AI, Săvulescu FA, Fierbinţeanu-Braticevici C. Laparoscopic sleeve gastrectomy and gastroesophageal reflux. Rom J Intern Med. 2018;56(4):227\u0026ndash;32. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.2478/rjim-2018-0019\u003c/span\u003e\u003cspan address=\"10.2478/rjim-2018-0019\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGuzman-Pruneda FA, Brethauer SA. Gastroesophageal Reflux After Sleeve Gastrectomy. J Gastrointest Surg. 2021;25(2):542\u0026ndash;50. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1007/s11605-020-04786-1\u003c/span\u003e\u003cspan address=\"10.1007/s11605-020-04786-1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAlsuwat OB, Alzahrani AA, Alzhrani MA, Alkhathami AM, Mahfouz MEM. Prevalence of Gastroesophageal Reflux Disease in Saudi Arabia. J Clin Med Res. 2018;10(3):221\u0026ndash;5. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.14740/jocmr3292w\u003c/span\u003e\u003cspan address=\"10.14740/jocmr3292w\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFujimori S. Gastric acid level of humans must decrease in the future. World J Gastroenterol. 2020;26(43):6706\u0026ndash;9. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3748/wjg.v26.i43.6706\u003c/span\u003e\u003cspan address=\"10.3748/wjg.v26.i43.6706\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRanjitkar S, Kaidonis JA, Smales RJ. Gastroesophageal reflux disease and tooth erosion. Int J Dent. 2012;2012:479850. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1155/2012/479850\u003c/span\u003e\u003cspan address=\"10.1155/2012/479850\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBaidas L, Alkawari H, Alshihah N, Almashaan G, Alwaalan H. Impact of Simulated Gastric Acid on Surface Roughness and Frictional Resistance of Orthodontic Archwires: An In Vitro Study. J Clin Diagn Res. 2020;14(12):10\u0026ndash;5. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.7860/jcdr/2020/45051.14297\u003c/span\u003e\u003cspan address=\"10.7860/jcdr/2020/45051.14297\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eParenti SI, Guicciardi S, Melandri C, et al. Effect of soft drinks on the physical and chemical features of nickel-titanium-based orthodontic wires. Acta Odontol Scand. 2012;70(1):49\u0026ndash;55. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3109/00016357.2011.575083\u003c/span\u003e\u003cspan address=\"10.3109/00016357.2011.575083\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eStefański T, Kloc-Ptaszna A, Postek-Stefańska L. The effect of simulated erosive conditions on the frictional behavior of different orthodontic bracket-wire combinations. Dent Med Probl. 2019;56(2):173\u0026ndash;7. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.17219/dmp/105832\u003c/span\u003e\u003cspan address=\"10.17219/dmp/105832\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRajendran A, Sundareswaran S, Peediyekkal LV, Santhakumar P, Sathyanadhan S. Effect of oral environment and prescribed fluoride mouthwashes on different types of TMA wires - An \u003cem\u003ein-vivo\u003c/em\u003e study. J Orthod Sci. 2019;8:8. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.4103/jos.JOS_72_18\u003c/span\u003e\u003cspan address=\"10.4103/jos.JOS_72_18\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. Published 2019 May 23.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKao CT, Huang TH. Variations in surface characteristics and corrosion behaviour of metal brackets and wires in different electrolyte solutions. Eur J Orthod. 2010;32(5):555\u0026ndash;60. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1093/ejo/cjp146\u003c/span\u003e\u003cspan address=\"10.1093/ejo/cjp146\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRaja TA, Littlewood SJ, Munyombwe T, Bubb NL. Wear resistance of four types of vacuum-formed retainer materials: a laboratory study. Angle Orthod. 2014;84(4):656\u0026ndash;64. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.2319/061313-448.1\u003c/span\u003e\u003cspan address=\"10.2319/061313-448.1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRyu JH, Kwon JS, Jiang HB, Cha JY, Kim KM. Effects of thermoforming on the physical and mechanical properties of thermoplastic materials for transparent orthodontic aligners. Korean J Orthod. 2018;48(5):316\u0026ndash;25. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.4041/kjod.2018.48.5.316\u003c/span\u003e\u003cspan address=\"10.4041/kjod.2018.48.5.316\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRyokawa H, Miyazaki Y, Fujishima A, Miyazaki T, Maki K. The Mechanical Properties of Dental Thermoplastic Materials in a Simulated Intraoral Environment. Orthodontic Waves. 2006;65(2):64\u0026ndash;72. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.odw.2006.03.003\u003c/span\u003e\u003cspan address=\"10.1016/j.odw.2006.03.003\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMoshkelgosha V, Shomali M, Momeni M. Comparison of Wear Resistance of Hawley and Vacuum Formed Retainers: An In-Vitro Study. J Dent Biomater. 2016;3(2):248\u0026ndash;53. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.ncbi.nlm.nih.gov/pmc/articles/PMC5608059/\u003c/span\u003e\u003cspan address=\"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5608059/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGardner GD, Dunn WJ, Taloumis L. Wear comparison of thermoplastic materials used for orthodontic retainers. Am J Orthod Dentofac Orthop. 2003;124(3):294\u0026ndash;7. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/s0889-5406(03)00502-x\u003c/span\u003e\u003cspan address=\"10.1016/s0889-5406(03)00502-x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZafeiriadis AA, Karamouzos A, Athanasiou AE, Eliades T, Palaghias G. An In Vivo Spectrophotometric Evaluation of Vivera\u0026reg; and Essix\u0026reg; Clear Thermoplastic Retainer Discolouration. Aust Orthod J 2018 34,1,3\u0026ndash;10. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.21307/aoj-2020-053\u003c/span\u003e\u003cspan address=\"10.21307/aoj-2020-053\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAzmuddin I, Mustapha NM, Khan HB, Sinniah SD. Physical Effects of Cleaning Agents on Orthodontic Thermoplastic Retainer Polymer: A Narrative Review. J Int Oral Health. 2022;14:4,349\u0026ndash;56. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://dx.doi.org/10.4103/jioh.jioh_357_21\u003c/span\u003e\u003cspan address=\"10.4103/jioh.jioh_357_21\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAlexandropoulos A, Al Jabbari YS, Zinelis S, Eliades T. Chemical and mechanical characteristics of contemporary thermoplastic orthodontic materials. Aust Orthod J. 2015;31(2):165\u0026ndash;70.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAlbilali AT, Baras BH, Aldosari MA. Evaluation of Mechanical Properties of Different Thermoplastic Orthodontic Retainer Materials after Thermoforming and Thermocycling. \u003cem\u003ePolymers (Basel)\u003c/em\u003e. 2023;15(7):1610. Published 2023 Mar 23. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3390/polym15071610\u003c/span\u003e\u003cspan address=\"10.3390/polym15071610\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKwon JS, Lee YK, Lim BS, Lim YK. Force delivery properties of thermoplastic orthodontic materials. Am J Orthod Dentofac Orthop. 2008;133(2):228\u0026ndash;e3281. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.ajodo.2006.03.034\u003c/span\u003e\u003cspan address=\"10.1016/j.ajodo.2006.03.034\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMeade MJ, Millett D. Retention protocols and use of vacuum-formed retainers among specialist orthodontists. J Orthod. 2013;40(4):318\u0026ndash;25. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1179/1465313313Y.0000000066\u003c/span\u003e\u003cspan address=\"10.1179/1465313313Y.0000000066\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKulkarni A, Rothrock J, Thompson J. Impact of Gastric Acid Induced Surface Changes on Mechanical Behavior and Optical Characteristics of Dental Ceramics. J Prosthodont. 2020;29(3):207\u0026ndash;18. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1111/jopr.12716\u003c/span\u003e\u003cspan address=\"10.1111/jopr.12716\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJohnston WM. Color measurement in dentistry. J Dent. 2009;37(Suppl 1):e2\u0026ndash;6. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.jdent.2009.03.011\u003c/span\u003e\u003cspan address=\"10.1016/j.jdent.2009.03.011\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKohda N, Iijima M, Muguruma T, Brantley WA, Ahluwalia KS, Mizoguchi I. Effects of mechanical properties of thermoplastic materials on the initial force of thermoplastic appliances. Angle Orthod. 2013;83(3):476\u0026ndash;83. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.2319/052512-432.1\u003c/span\u003e\u003cspan address=\"10.2319/052512-432.1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eThamer MR, Al-Joubori SK. The Effect of Acidity Level on Ions Released and Corrosion of Metal Orthodontic Appliances at Different Time Intervals: An In Vitro Study. J Bagh Coll Dentistry. 2015;27(4):168\u0026ndash;74. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.12816/0024957\u003c/span\u003e\u003cspan address=\"10.12816/0024957\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBabanouri N, Ahmadi N, Pakshir HR, Ajami S, Habibagahi R. Influence of a bleaching agent on surface and mechanical properties of orthodontic thermoplastic retainer materials: An in vitro study. Einfluss eines Bleaching-Pr\u0026auml;parats auf die Oberfl\u0026auml;che und mechanische Eigenschaften von kieferorthop\u0026auml;dischen thermoplastischen Retainermaterialien : Eine In-vitro-Studie. J Orofac Orthop. 2022;83(5):332\u0026ndash;8. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1007/s00056-021-00312-3\u003c/span\u003e\u003cspan address=\"10.1007/s00056-021-00312-3\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWible E, Agarwal M, Altun S, et al. Long-term effects of various cleaning methods on polypropylene/ethylene copolymer retainer material. Angle Orthod. 2019;89(3):432\u0026ndash;7. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.2319/060818-429.1\u003c/span\u003e\u003cspan address=\"10.2319/060818-429.1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWible E, Agarwal M, Altun S, et al. Long-term effects of different cleaning methods on copolyester retainer properties. Angle Orthod. 2019;89(2):221\u0026ndash;7. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.2319/010218-2.1\u003c/span\u003e\u003cspan address=\"10.2319/010218-2.1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBrehove J. Effects of Cleaning Agents on the Properties of Two Different Thermoplastic Retainer Materials. Doctoral dissertation, Marquette University, Milwaukee, WI (2021). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://epublications.marquette.edu/theses_open/669/\u003c/span\u003e\u003cspan address=\"https://epublications.marquette.edu/theses_open/669/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAgarwal M, Wible E, Ramir T, et al. Long-term effects of seven cleaning methods on light transmittance, surface roughness, and flexural modulus of polyurethane retainer material. Angle Orthod. 2018;88(3):355\u0026ndash;62. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.2319/081517-551.1\u003c/span\u003e\u003cspan address=\"10.2319/081517-551.1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHussein AM, Mohammed-Salih HS, Al-Sheakli II. Effects of various cleaning agents on polypropylene and copolyester thermoplastic orthodontic retainer materials. J Taibah Univ Med Sci. 2022;17(5):861\u0026ndash;8. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.jtumed.2022.04.005\u003c/span\u003e\u003cspan address=\"10.1016/j.jtumed.2022.04.005\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. Published 2022 May 6.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSepe M. The Mystery of Physical Aging, Part 1: Knowing the difference. Plastics Technology, 2019 March 19. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.ptonline.com/articles/the-mystery-of-physical-aging-part-1\u003c/span\u003e\u003cspan address=\"https://www.ptonline.com/articles/the-mystery-of-physical-aging-part-1\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSastri VR. Plastics in Medical Devices: Properties, Requirements, and Applications. William Andrew, 3rd Edition, 2021.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eIliadi A, Enzler V, Polychronis G, Peltom\u0026auml;ki T, Zinelis S, Eliades T. Εffect of Cleansers on the Composition and Mechanical Properties of Orthodontic Aligners in Vitro. Prog Orthodont. 2022;23(1). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1186/s40510-022-00449-w\u003c/span\u003e\u003cspan address=\"10.1186/s40510-022-00449-w\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYazdi M, Daryanavard H, Ashtiani AH, Moradinejad M, Rakhshan V. A systematic review of biocompatibility and safety of orthodontic clear aligners and transparent vacuum-formed thermoplastic retainers: Bisphenol-A release, adverse effects, cytotoxicity, and estrogenic effects. Dent Res J (Isfahan). 2023;20:41. Published 2023 Mar 28.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLiu CL, Sun WT, Liao W, et al. Colour stabilities of three types of orthodontic clear aligners exposed to staining agents. Int J Oral Sci. 2016;8(4):246\u0026ndash;53. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1038/ijos.2016.25\u003c/span\u003e\u003cspan address=\"10.1038/ijos.2016.25\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. Published 2016 Dec 16.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDaniele V, Macera L, Taglieri G, Spera L, Marzo G, Quinzi V. Color Stability, Chemico-Physical and Optical Features of the Most Common PETG and PU Based Orthodontic Aligners for Clear Aligner Therapy. Polym (Basel). 2021;14(1):14. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3390/polym14010014\u003c/span\u003e\u003cspan address=\"10.3390/polym14010014\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. Published 2021 Dec 21.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eInoue S, Yamaguchi S, Uyama H, Yamashiro T, Imazato S. Influence of constant strain on the elasticity of thermoplastic orthodontic materials. Dent Mater J. 2020;39(3):415\u0026ndash;21. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.4012/dmj.2019-104\u003c/span\u003e\u003cspan address=\"10.4012/dmj.2019-104\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"bmc-oral-health","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"ohea","sideBox":"Learn more about [BMC Oral Health](http://bmcoralhealth.biomedcentral.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/ohea/default.aspx","title":"BMC Oral Health","twitterHandle":"BMC_series","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"em","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Retainer, Simulated, Gastric acid, Vacuum formed, Oral health","lastPublishedDoi":"10.21203/rs.3.rs-4495989/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4495989/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eBackground\u003c/h2\u003e \u003cp\u003eA decrease in the oral pH levels may have detrimental effects on vacuum-formed retainers since an acidic oral environment could impact the mechanical and physical properties of these retainers. The present in vitro study aimed to evaluate the effect of simulated gastric acid at pH 4 and 1.2 on the surface microhardness, flexural strength, and color change of three different types of orthodontic retainer materials.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e \u003cp\u003eA total of 144 vacuum-formed commercially available retainer materials: copolyesters, polypropylene, and PET-G (n\u0026thinsp;=\u0026thinsp;48/group) were evaluated. Each specimen was immersed in distilled water, and then the specimens were divided into two subgroups: those with a pH of 4 and those with a pH of 1.2. The microhardness test was performed using Innovatest, the flexural strength was evaluated by a universal testing machine, and the color change was analyzed using a spectrophotometer. The data were analyzed using One-Way ANOVA and paired t-tests at a significance value of P\u0026thinsp;\u0026le;\u0026thinsp;0.05.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e \u003cp\u003eOur findings indicated a significant decrease in flexural strength in all the experimental groups after immersion in pH 1.2 and 4 solutions (p-value\u0026thinsp;=\u0026thinsp;0.000), with PET-G being the most affected material, followed by copolyester and polypropylene. Only two materials, copolyester, and PET-G, demonstrated a significant decrease in surface microhardness following immersion in a pH 4 solution (P\u0026thinsp;=\u0026thinsp;0.001 and P\u0026thinsp;=\u0026thinsp;0.025, respectively). The copolyester material exhibited the greatest degree of color alteration.\u003c/p\u003e\u003ch2\u003eConclusion\u003c/h2\u003e \u003cp\u003eThe study concluded that copolyester adversely affects flexural strength, surface microhardness, and color when subjected to immersion in simulated gastric acid. Therefore, care is recommended for patients diagnosed with gastroesophageal reflux disease.\u003c/p\u003e","manuscriptTitle":"Impact of Simulated Gastric Acid on the Physical and Mechanical Properties of Vacuum-Formed Retainers: An In Vitro Study","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-12 19:12:44","doi":"10.21203/rs.3.rs-4495989/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-12-16T13:39:12+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-12-16T12:03:41+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-12-16T08:16:50+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"126280320874020028388853869752808404794","date":"2024-12-14T09:37:57+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"28605445547453540240129257280958090562","date":"2024-12-13T17:45:13+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"88234250676573425730169834083633418738","date":"2024-12-13T11:47:27+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"107886756011737924726095966107999320736","date":"2024-12-13T07:02:49+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"95532064983938475910862941191343638540","date":"2024-12-12T11:51:20+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"309482014990350579209450248464019749180","date":"2024-12-12T06:20:33+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"123504947372409909728620074964851888276","date":"2024-12-12T05:53:59+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"195412889931243377798747482450137769509","date":"2024-12-11T17:03:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"175596183811047644842021534750265029828","date":"2024-12-11T16:00:28+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"69302276034010861892434933576628317796","date":"2024-12-11T15:37:57+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-08-08T06:17:09+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2024-06-18T12:30:49+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-06-13T07:22:10+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-06-13T07:21:45+00:00","index":"","fulltext":""},{"type":"submitted","content":"BMC Oral Health","date":"2024-05-29T09:36:20+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"bmc-oral-health","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"ohea","sideBox":"Learn more about [BMC Oral Health](http://bmcoralhealth.biomedcentral.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/ohea/default.aspx","title":"BMC Oral Health","twitterHandle":"BMC_series","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"em","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"213dbb77-d4cb-47e3-9726-2c778bed055b","owner":[],"postedDate":"July 12th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-02-03T16:09:26+00:00","versionOfRecord":{"articleIdentity":"rs-4495989","link":"https://doi.org/10.1186/s12903-025-05548-8","journal":{"identity":"bmc-oral-health","isVorOnly":false,"title":"BMC Oral Health"},"publishedOn":"2025-02-01 15:57:45","publishedOnDateReadable":"February 1st, 2025"},"versionCreatedAt":"2024-07-12 19:12:44","video":"","vorDoi":"10.1186/s12903-025-05548-8","vorDoiUrl":"https://doi.org/10.1186/s12903-025-05548-8","workflowStages":[]},"version":"v1","identity":"rs-4495989","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4495989","identity":"rs-4495989","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}