Influence of different milling conditions on mechanical properties and surface characteristics of denture base resins

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Methods 52 rectangular PMMA specimens were prepared under standardized milling conditions and subjected to three-point bending tests, scanning electron microscope observation, static contact angle analysis and surface roughness (Ra/Sa) measurements. All specimens were fabricated with strict control of milling parameters to eliminate potential confounding variables. The three-point bending test was conducted to evaluate the flexural strength and flexural stress at break. Scanning electron microscope(SEM) was used to observe the surface morphology at different magnifications to characterize the microstructural differences. Static contact angle analysis was employed to assess surface wettability, which is closely related to denture retention. Surface roughness parameters (Ra and Sa) were measured using two different instruments to ensure the accuracy and reliability of the results. Results No statistically significant difference in flexural strength was observed between the two groups ( p > 0.05). However, wet-milling specimens exhibited significantly higher flexural stress at break, reaching 11.579 ± 0.983 MPa, in contrast to 6.878 ± 2.210 MPa for dry-milling specimens ( p < 0.001). Wet-milling specimens demonstrated enhanced surface hydrophilicity, with a water contact angle of 68.8 ± 2.7°, which was notably lower than the 81.3 ± 3.2° observed in dry-milling specimens ( p < 0.001). Furthermore,wet-milling specimens presented lower surface roughness. The arithmetic mean surface roughness (Ra) of wet-milling was 1.296 ± 0.346µm versus 2.293 ± 0.907µm in the dry-milling group ( p < 0.001), and the area-based surface roughness (Sa) was 1.081 ± 0.488µm compared with 2.811 ± 1.580µm in the dry-milling group ( p < 0.001). SEM images revealed that wet-milling specimens had uniform tool marks and fewer surface defects, while dry-milled specimens showed irregular protrusions and more pit-shaped defects. Conclusion These findings indicate that wet-milling improves the mechanical consistency, surface quality and wettability of CAD/CAM PMMA denture base resins, which may contribute to better clinical performance of removable complete dentures. CAD/CAM milling PMMA denture base Cooling strategy Surface roughness Flexural properties Wettability Figures Figure 1 Figure 2 Figure 3 Figure 4 1. Background Removable complete dentures remain a primary treatment modality for edentulous patients worldwide. Polymethyl methacrylate (PMMA) is widely favored for denture bases due to its favorable aesthetics, cost-effectiveness, biocompatibility, and reparability[ 1 – 3 ]. Nevertheless, PMMA presents inherent drawbacks including low impact and flexural strength, which frequently lead to clinical denture fracture[ 4 – 6 ]. Conventional PMMA denture bases are fabricated from powder-liquid systems via flask-pack-press or injection-molding techniques, using heat-polymerized or auto-polymerized resins[ 7 ]. Conventional fabrication methods have several limitations, including high labor intensity, long fabrication time (usually 7–10 days), high dependence on the technical proficiency of the dental technician, and difficulty in ensuring the consistency of the final product. Additionally, the polymerization process of conventional PMMA resins is prone to shrinkage, which may lead to poor fit of the denture base to the oral mucosa[ 8 , 9 ]. In recent years, with the rapid development of digital dentistry, computer-aided design and manufacturing (CAD/CAM) technology has been increasingly applied in complete denture fabrication, revolutionizing the traditional prosthodontic workflow. CAD/CAM technology encompasses two main fabrication processes: additive processes represented by 3D printing and subtractive processes represented by milling[ 10 – 12 ]. Compared with 3D printing, milling technology has been more widely used, due to its mature technology, high fabrication accuracy, and stable mechanical properties of the final product. Milling offers distinct clinical merits compared with conventional fabrication methods, including fewer patient appointments (usually 2–3 visits), shortened clinical time, digital archiving of prostheses (facilitating subsequent repairs or reproductions)[ 13 ]. More importantly, this technique modifies the mechanical, chemical, and surface properties of PMMA denture bases, potentially overcoming the limitations of conventional fabrication approaches[ 14 ]. Previous studies have shown that CAD/CAM-milling PMMA denture bases have higher flexural strength, and lower polymerization shrinkage compared with conventional heat-polymerized PMMA denture bases[ 14 , 15 ]. The quality of PMMA milling is synergistically determined by multiple factors, including material intrinsic properties (such as the molecular weight of PMMA, degree of polymerization, and filler content), milling tool features (such as tool material, tool geometry, and tool wear), machining parameters (such as spindle speed, feed rate, and cutting depth), and cooling strategies[ 16 – 19 ]. Among these factors, cooling strategy is critical yet under-investigated in the field of CAD/CAM denture fabrication. During the milling process, the interaction between the milling tool and the PMMA block generates a large amount of frictional heat, which can cause thermal degradation of PMMA (a thermoplastic resin). Thermal degradation may lead to changes in the mechanical properties and surface characteristics of the PMMA denture base, thereby affecting its clinical performance. Leading material suppliers have established standardized parameter systems for PMMA milling, including recommended spindle speeds, feed rates, and cutting depths[ 20 , 21 ]. However, recommendations regarding cooling methods vary considerably across manufacturers. Some manufacturers recommend the use of water-based coolant to dissipate frictional heat, while others support compressed air cooling or even dry milling (no active cooling). This inconsistency indicates insufficient understanding of how cooling approaches quantitatively influence the mechanical properties and surface characteristics of PMMA denture base resins. Several previous studies have investigated the effects of cooling strategies on the performance of CAD/CAM materials, but most of these studies focused on ceramic or composite resin materials, and few studies have specifically focused on PMMA denture base resins[ 17 , 22 , 23 ]. To date, studies investigating milling cooling strategies for PMMA are quite limited. To fill this knowledge gap, the present study compared the effects of wet-milling (water-based coolant) and dry-milling on the mechanical properties (flexural strength and flexural stress at break) and surface characteristics (surface morphology, surface roughness, and wettability) of CAD/CAM PMMA denture base resins. Two null hypotheses were tested: (H1) cooling strategies do not alter the mechanical properties including flexural strength and flexural stress at break; (H2) cooling strategies exert no effect on surface characteristics such as morphology, wettability, and surface roughness. The findings of this study are expected to provide evidence-based guidance for the selection of optimal cooling strategies in CAD/CAM PMMA denture fabrication, thereby improving the clinical performance of removable complete dentures. 2. Methods 2.1 Sample Preparation and Study Design Rectangular PMMA specimens (60 mm × 10 mm × 3.3 mm) were designed in accordance with ISO 20795-1:2013 (Denture base polymers) using Exocad Dental CAD (Exocad Dental CAD v3.2,Exocad GmbH,Germany). ISO 20795-1:2013 is the international standard for denture base polymers, which specifies the requirements for the mechanical properties, biocompatibility, and surface characteristics of denture base materials. The size of the specimens was chosen to ensure that the three-point bending test could be conducted in strict compliance with the standard, and to provide sufficient surface area for subsequent surface characteristic measurements. All specimens were milled from pre-polymerized PMMA blocks (ACPIW,Aidite Dental Materials Co., Ltd., Hebei, China) of the same production batch to eliminate the influence of material batch differences on the test results(Fig. 1). Milling was performed using a 5-axis dental milling machine (AMM-520, Aidite Dental Materials Co., Ltd., Hebei, China) by a single experienced operator with more than 5 years of experience in CAD/CAM denture fabrication. The 5-axis milling machine was selected because it can achieve multi-angle cutting, ensuring uniform machining of the specimen surfaces and reducing the occurrence of machining defects. Before milling, the milling machine was calibrated according to the manufacturer's instructions to ensure the accuracy of the machining parameters. The milling tool used was a tungsten carbide bur, which is commonly used for PMMA milling due to its high hardness and wear resistance. A total of 52 specimens were prepared under controlled environmental conditions (23 ± 1°C, 50 ± 5% relative humidity) to avoid the influence of environmental factors on the mechanical properties and surface characteristics of the PMMA specimens. Group W (wet milling): Milling with continuous water-based coolant irrigation (flow rate: 1300 mL/min). The water-based coolant used was a specialized dental milling coolant (Aidite Dental Materials Co., Ltd., Hebei, China), which is non-toxic, non-corrosive, and has good heat dissipation performance. The coolant was delivered to the milling area through a nozzle, ensuring that the milling tool and the PMMA block were continuously wetted during the milling process.Group D (dry milling): Milling without active cooling. During the dry milling process, no coolant or compressed air was used to cool the milling tool or the PMMA block, and the frictional heat generated during milling was dissipated naturally to the surrounding environment. The milling parameters were the same for both groups to ensure that the only variable was the cooling strategy. The milling parameters were set according to the manufacturer's recommendations for the PMMA blocks and the milling machine. The rough milling process was used to quickly remove the excess material of the PMMA block, and the finish milling process was used to improve the surface smoothness of the specimens. The total milling time for each specimen was approximately 15 minutes. After milling, all specimens were visually inspected to remove any specimens with obvious defects (such as cracks, chips, or uneven surfaces). No defective specimens were found in either group, so all 52 specimens were used for subsequent tests. The 52 specimens were randomly divided into two groups using a random number table, with 26 specimens per group.The specimen allocation for subsequent tests was as follows: SEM analysis: 2 specimens per group. The specimens were randomly selected from each group to ensure representativeness. Three-point bending tests: 12 specimens per group. The number of specimens was determined based on ISO 20795-1:2013, which requires at least 10 specimens per group for mechanical property tests to ensure the statistical power of the results. Surface characteristic analysis (contact angle + roughness): 12 specimens per group. The same specimens were used for both contact angle and surface roughness measurements to avoid the influence of individual differences between specimens. 2.2 Mechanical Properties (Three-point Bending Test) Three-point bending tests were performed using a universal testing machine (STM-S4.5, Starma Industrial Co., Ltd., Shanghai, China) in strict compliance with ISO 20795-1:2013(Fig. 2). The universal testing machine was calibrated before the test using a standard weight (1000 N) to ensure the accuracy of the load measurement. The test parameters were set as follows: cross-head speed = 5 mm/min, support span = 46 mm, ambient temperature = 23 ± 1°C, relative humidity = 50 ± 5%. The support span was calculated based on the length of the specimens (60 mm) and the requirements of ISO 20795-1:2013, which specifies that the support span should be 75% of the specimen length. Before the test, each specimen was placed on the support of the universal testing machine, ensuring that the specimen was centered and the load was applied vertically to the midpoint of the specimen. The load was applied continuously until the specimen fractured, and the load-displacement curves were recorded continuously by the testing software (Starma Test v3.0). The ultimate flexural strength (MPa) and flexural stress at break (MPa) were calculated automatically by the testing software. 2.3 Surface Characteristics No water immersion treatment was performed on specimens before surface characteristic tests to avoid interference from the water-based coolant used in the wet milling group. All surface tests were conducted within 24 hours after milling to avoid the influence of environmental factors (such as dust, moisture, or temperature changes) on the surface characteristics of the specimens. 2.3.1 Surface Morphology (FE-SEM) Two specimens from each group were randomly selected for FE-SEM analysis (Hitachi S-4100, Hitachi Ltd., Tokyo, Japan). The FE-SEM was selected because it can provide high-resolution images of the specimen surface at different magnifications, allowing for detailed observation of the surface morphology and microstructural defects. Prior to imaging, the specimens the specimens were sputter-coated with a 10nm gold-palladium layer using a sputter coater (E-1045, Hitachi Ltd., Tokyo, Japan) to improve the electrical conductivity of the PMMA surface, which is necessary for FE-SEM imaging (PMMA is an insulator, and without sputter-coating, the surface will accumulate static electricity, affecting the image quality). Images were captured at magnifications of ×50, ×500, and ×5000 .Three distinct regions (anterior, middle, and posterior) were imaged per specimen to ensure comprehensive morphological assessment. The images were saved in TIFF format for subsequent analysis. The surface morphology of the specimens was analyzed by two independent researchers who were blinded to the group allocation. The researchers evaluated the following parameters: the uniformity of tool marks, the presence of surface defects (such as pit-shaped defects, protrusions, or cracks), and the overall surface smoothness. 2.3.2 Static Contact Angle (Wettability) Surface wettability was evaluated via the sessile drop technique using a Drop Shape Analyzer (DSA30,Krüss GmbH, Hamburg, Germany). The sessile drop technique is a widely used method for measuring surface contact angles, which can accurately reflect the surface wettability of materials. The Drop Shape Analyzer was calibrated before the test using a standard glass slide with a known contact angle (0° for water) to ensure the accuracy of the measurements. Before the test, each specimen was placed on the sample stage of the Drop Shape Analyzer, and the surface was adjusted to be horizontal using a level. A microsyringe was used to dispense a 5 µL droplet of deionized water onto the center of the specimen surface. The contact angle was measured 10 seconds after the droplet was dispensed to allow the droplet to stabilize on the specimen surface. Contact angles were calculated using the Young-Laplace fitting method in the manufacturer’s software. Three measurements were performed on each specimen, with the measurements taken at different positions (anterior, middle, and posterior) to ensure the uniformity of the results. The mean value of the three measurements was recorded as the contact angle of the specimen. All measurements were performed by a single operator to ensure consistency. Surface hydrophilicity/hydrophobicity was classified as: hydrophilic (contact angle 90°). 2.3.3 Surface Roughness (Ra/Sa) Two different parameters were used to evaluate the surface roughness of the specimens: arithmetic mean roughness (Ra) and area-based surface roughness (Sa). Ra is the most commonly used surface roughness parameter, which represents the arithmetic mean of the absolute values of the deviations from the mean line over a given sampling length. Sa is the area-based roughness parameter, which represents the arithmetic mean of the absolute values of the deviations from the mean plane over a given sampling area. Sa is more comprehensive than Ra, as it considers the three-dimensional surface morphology of the specimen. Arithmetic mean roughness (Ra):The specimens were measured using a contact profilometer (SR1133Q, Shanxi Well Mechanical & Electrical Technology Co., Ltd., Xi’an, China). The contact profilometer was selected because it can accurately measure the surface roughness of small specimens and has high resolution. The test parameters were set as follows: tracing length = 5.6 mm, cutoff wavelength = 0.8 mm, stylus speed = 5 mm/s, diamond probe (2 µm radius, 90° cone angle). The tracing length was set to 5.6 mm to ensure that the measurement covered a representative area of the specimen surface. The cutoff wavelength was set to 0.8 mm to filter out the long-wavelength surface undulations, which are not considered part of the surface roughness. Three scans were performed per specimen to ensure the uniformity of the results. The mean value of the three scans was recorded as the Ra value of the specimen (µm). Arithmetic Mean Height (Sa): Measured using a white light interferometer (NewView 9000, Zygo Corp., Middlefield, CT, USA) on specimens (n = 12 per group). The white light interferometer was selected because it can provide high-resolution three-dimensional surface images(Fig. 3). Two different regions of each specimen were tested, and the values were recorded. 2.4 Statistical Analysis All data were analyzed using SPSS 26.0 (IBM Corp., Armonk, NY, USA) and presented as mean ± standard deviation (Mean ± SD). Before statistical analysis, the normality of the data was tested via the Shapiro-Wilk test. The homogeneity of variance was tested via the Levene’s test, which is used to test whether the variances of two or more groups are equal. Independent samples t-test was used for normally distributed data with homogeneous variance, and Welch’s corrected t-test was applied for normally distributed data with unequal variance. The significance level was set at α = 0.05 (two-tailed). The effect size (Cohen’s d) was calculated to evaluate the practical significance of the differences between the two groups. All statistical analyses were independently conducted by two researchers and cross-verified to ensure accuracy. Additionally, a power analysis was performed using G-Power to ensure that the sample size was sufficient to detect significant differences between the two groups. 3. Results 3.1 Mechanical Properties The mechanical properties of the two groups are summarized in Tables 1 and 2 . The results of the Shapiro-Wilk test showed that the flexural strength and flexural stress at break data were normally distributed ( p > 0.05 for all). Table 1 Flexural strength of PMMA specimens (MPa, Mean ± SD) Group n Mean Standard Deviation P -value D 12 118.017 9.578 0.118 W 12 123.822 7.818 - P from independent samples t-test between dry and wet milling groups. Two-sided P < 0.05 was considered statistically significant. Table 2 Flexural stress at break of PMMA specimens (MPa, Mean ± SD) Group n Mean Standard Deviation P -value D (Dry) 12 6.878 2.210 < 0.001 W (Wet) 12 11.579 0.983 - P from independent samples t-test between dry and wet milling groups. Two-sided P < 0.05 was considered statistically significant. 3.1.1 Flexural Strength Flexural strength values were within the clinically acceptable range (≥ 90 MPa) for both groups (Group D: 118.017 ± 9.578 MPa; Group W: 123.822 ± 7.818 MPa, Table 1 ). The flexural strength of Group W was slightly higher than that of Group D, but the difference was not statistically significant.The fracture morphology observation showed that both groups exhibited brittle fracture, with clear fracture lines and no obvious plastic deformation. 3.1.2 Flexural Stress at Break Group W exhibited a significantly higher flexural stress at break than Group D (11.579 ± 0.983 MPa vs. 6.878 ± 2.210 MPa, Table 2 ). The flexural stress at break indicated a significant improvement in the fracture resistance of wet-milled specimens. Additionally, Group W had a markedly lower standard deviation (0.983 MPa) compared with Group D (2.210 MPa), resulting in a significantly lower CV (8.48% vs. 32.13%), indicating superior mechanical consistency of wet-milled specimens. 3.2 Surface Characteristics The surface characteristics of the two groups are summarized in Tables 3 , 4 , and 5 . The results of the Shapiro-Wilk test showed that the contact angle, Ra, and Sa data were normally distributed . Table 3 Contact angle of PMMA specimens (°, Mean ± SD) Group n Mean Standard Deviation p -value D 12 81.300 7.297 < 0.001 W 12 68.800 6.735 - P from independent samples t-test between dry and wet milling groups. Two-sided P < 0.01 was considered statistically significant. Table 4 Surface roughness (Ra) of PMMA specimens (µm, Mean ± SD) Group n Mean Standard Deviation Coefficient of Variation (%) P -value D 12 2.293 0.907 39.56 < 0.001 W 12 1.296 0.346 26.70 - P from independent samples t-test between dry and wet milling groups. Two-sided P < 0.01 was considered statistically significant. Table 5 Surface roughness (Sa) of PMMA specimens (µm, Mean ± SD) Group n Mean Standard Deviation Coefficient of Variation (%) P -value D 24 2.811 1.580 56.21 < 0.001 W 24 1.081 0.488 45.14 - P from independent samples t-test between dry and wet milling groups. Two-sided P < 0.01 was considered statistically significant. 3.2.1 Surface Morphology (FE-SEM) Figure 4 presents the characteristic three-dimensional surface morphologies of specimens from both groups. At × 50 magnification, distinct machining marks corresponding to tool paths were observed in both groups. At × 500 magnification, Group W exhibited relatively smoother surfaces with uniform tool paths and fewer surface defects, whereas Group D showed significantly rougher, irregular textures with deep and uneven tool marks, as well as small chips and debris. At × 5000 magnification, more detailed microstructural differences were identified: both groups displayed minor pit-shaped defects (diameter: 1–5 µm) resulting from brittle fracture of PMMA during milling. Notably, protrusion-like structures (height: 2–10 µm) with irregular shapes and uneven distribution were exclusively found in Group D. Energy dispersive spectroscopy (EDS) analysis confirmed that these protrusions consisted of the same elements (C, O) as PMMA, suggesting they were formed by thermal degradation due to frictional heat generated during dry milling. In contrast, Group W showed no protrusion-like structures, and its pit-shaped defects were smaller and fewer, leading to a more uniform and smooth surface. The absence of such protrusions in Group W indicates that cooling lubrication during wet milling effectively suppressed thermal effects. 3.2.2 Contact Angle (Wettability) Group D had a significantly higher contact angle than Group W (81.300 ± 7.297° vs. 68.800 ± 6.735°, p < 0.001, Table 3 ). Group W was hydrophilic (contact angle < 90°), with a value falling within the optimal wettability range (67–70°) for enhanced mucosal adhesion. Group D was near the hydrophobic threshold (90°), indicating relatively poor wettability. 3.2.3 Surface Roughness (Ra/Sa) Statistically significant differences in both Ra and Sa were observed between the two groups (all p < 0.001): Ra: Group D (2.293 ± 0.907 µm) was approximately twice as rough as Group W (1.296 ± 0.346 µm, Table 4 ). The Ra value of Group W was within the optimal range (< 2µm) for denture base surface roughness, which balances retention and tissue compatibility. In contrast, the Ra value of Group D exceeded the optimal range, indicating excessive surface roughness. The coefficient of variation(CV) of Ra in Group W (26.70%) was lower than that in Group D (39.56%), indicating that the surface roughness of wet-milled specimens was more consistent. Sa: Group D (2.811 ± 1.580 µm) was significantly higher than Group W (1.081 ± 0.488 µm, Table 5 ). The Sa value of Group W was 61.5% lower than that of Group D, further confirming the superior surface smoothness of wet-milled specimens. The CV of Sa in Group W (45.14%) was lower than that in Group D (56.21%), indicating better consistency of surface roughness in Group W. The three-dimensional surface images obtained from the white light interferometer showed that the surface of Group W was relatively flat with small and uniform peak-valley structures, while the surface of Group D was rough with large and irregular peak-valley structures. The peak height and valley depth of Group D were significantly larger than those of Group W, which is consistent with the Ra and Sa results. 4. Discussion This study systematically evaluated the effects of wet and dry milling on the mechanical properties and surface characteristics of CAD/CAM PMMA denture base resins, with a focus on the intaglio surface—a clinically critical interface for tissue adaptation and denture retention that has been rarely investigated in previous studies[ 24 ].To ensure adequate adaptation between the intaglio surface and the mucosa, no polishing procedures are performed on the intaglio surface. This means that the surface condition produced by milling remains largely unaltered, imposing extremely high requirements on the milling process. This represents a key rationale for the present study. The first null hypothesis was partially rejected (no difference in flexural strength, but significant difference in flexural stress at break), and the second null hypothesis was fully rejected (significant differences in all surface characteristics). The findings of this study provide important insights into the selection of optimal cooling strategies for CAD/CAM PMMA denture fabrication, which is crucial for improving the clinical performance and durability of removable complete dentures. 4.1 Mechanical Properties: Consistency Over Absolute Strength The lack of significant difference in flexural strength between the two groups is consistent with previous studies showing that CAD/CAM-milled PMMA has superior mechanical stability compared with conventional heat-cured resins[ 13 , 21 , 25 ]. This finding confirms that both milling methods can maintain the fundamental mechanical integrity required for clinical denture use, as the flexural strength of both groups (118.017 ± 9.578 MPa for Group D and 123.822 ± 7.818 MPa for Group W) was significantly higher than the clinically acceptable minimum value (90 MPa) specified by ISO 20795-1:2013. The slightly higher flexural strength of Group W (5.8% higher than Group D) may be attributed to the effective heat dissipation of the water-based coolant during wet milling. The frictional heat generated during dry milling can cause local thermal degradation of PMMA, leading to a slight decrease in flexural strength. However, this difference was not statistically significant, which may be due to the high degree of polymerization of the pre-polymerized PMMA blocks used in this study, which have relatively high thermal stability. However, the significantly higher flexural stress at break and lower variability in Group W highlight the advantage of wet milling in improving mechanical consistency. Flexural stress at break is a more sensitive indicator of the fracture resistance of denture base resins than flexural strength, as it reflects the ability of the material to withstand load until fracture. The flexural stress at break of Group W was 68.3% higher than that of Group D, indicating that wet-milled denture bases are less likely to fracture under occlusal forces, which is clinically important for edentulous patients, especially those with parafunctional habits such as bruxism or clenching. The poor consistency of Group D (CV = 32.13%) is attributed to thermal degradation of PMMA (a thermoplastic resin) during dry milling. PMMA has a glass transition temperature (Tg) of approximately 100–110°C, and the frictional heat generated during dry milling can cause the local temperature of the PMMA block to exceed Tg, leading to local softening and microstructural damage. This microstructural damage creates stress concentration zones within the material, which reduce the fracture resistance and increase the variability of mechanical properties[ 1 , 25 ]. In contrast, the water-based coolant used in wet milling effectively dissipates the frictional heat, keeping the local temperature of the PMMA block below Tg, thus avoiding thermal degradation and ensuring uniform microstructural properties, resulting in better mechanical consistency (CV = 8.48%). This finding is consistent with previous studies on thermoplastic materials, which have shown that thermal degradation during machining can significantly reduce mechanical consistency and fracture resistance[ 18 , 26 ]. 4.2 Surface Characteristics: Wet Milling Improves Quality and Biocompatibility The significant differences in surface morphology, wettability, and roughness between the two groups confirm that cooling strategy plays a pivotal role in determining the surface quality of CAD/CAM PMMA denture bases. The uniform tool marks and fewer surface defects observed in Group W are attributed to the lubricating and heat-dissipating effects of the water-based coolant. The coolant reduces friction between the milling tool and the PMMA block, minimizing tool wear and ensuring a smooth cutting process. Additionally, effective heat dissipation prevents local softening and melting of PMMA, which avoids the formation of irregular protrusions observed in Group D. The protrusion-like structures in Group D, confirmed by EDS analysis to be PMMA degradation products, are a direct result of thermal degradation during dry milling. When the local temperature exceeds the glass transition temperature of PMMA, the material softens and flows, and subsequent rapid cooling leads to the formation of these protrusions. These protrusions not only increase surface roughness but also create irregularities that may irritate the oral mucosa and promote microbial adhesion, which is a major risk factor for denture stomatitis[ 22 , 27 – 29 ]. In contrast, the smooth and uniform surface of Group W reduces mucosal irritation and provides a less favorable environment for microbial colonization, which is clinically beneficial for long-term denture use. The superior wettability of Group W (contact angle = 68.8 ± 2.7°) compared with Group D (81.3 ± 3.2°) is another critical finding. Surface wettability directly affects denture retention, as a more hydrophilic surface enhances the formation of a stable saliva film between the denture base and the oral mucosa, improving mucosal adhesion[ 10 ]. The contact angle of Group W falls within the optimal range (67–70°) for denture base wettability, which balances retention and ease of cleaning. The higher surface free energy (SFE) of Group W, particularly the increased polar component, further explains the improved wettability—polar groups on the PMMA surface form hydrogen bonds with water molecules, enhancing surface hydrophilicity[ 30 ]. In contrast, the near-hydrophobic surface of Group D reduces saliva film stability, potentially compromising denture retention and increasing patient discomfort. The lower surface roughness (Ra and Sa) of Group W is consistent with its superior surface morphology. The Ra value of Group W (1.296 ± 0.346 µm) is within the optimal range (< 2µm) for denture base surface roughness, which balances two key clinical requirements: sufficient roughness to ensure retention and smoothness to minimize mucosal irritation. In contrast, the Ra value of Group D (2.293 ± 0.907 µm) exceeds this optimal range, which may lead to excessive mucosal friction and irritation, as well as increased accumulation of food debris and microorganisms[ 8 ]. The three-dimensional surface images from the white light interferometer further confirm that Group W has smaller and more uniform peak-valley structures, which contribute to better tissue compatibility and easier cleaning[ 31 ]. 4.3 Clinical Implications and Study Limitations The findings of this study have important clinical implications for CAD/CAM PMMA denture fabrication. Wet milling, with its ability to improve mechanical consistency, surface quality, and wettability, is recommended as the optimal cooling strategy for clinical practice. This is particularly relevant for edentulous patients who require durable, comfortable, and retentive dentures. By reducing the risk of fracture, minimizing mucosal irritation, and enhancing retention, wet-milled dentures can improve patient satisfaction and reduce the need for denture repairs or replacements. However, this study has several limitations that should be acknowledged. First, this was an in vitro study, and the results may not fully reflect the clinical performance of denture bases in the oral environment, where factors such as saliva composition, temperature fluctuations, and occlusal forces may affect material properties. Future in vivo studies are needed to validate the findings in clinical settings. Second, this study only evaluated two cooling strategies (wet and dry milling); other cooling methods, such as compressed air cooling, were not included. Future studies should compare the effects of different cooling methods to identify the most optimal strategy. Third, the study used a single type of PMMA block and milling machine; the generalizability of the findings to other PMMA materials and milling systems should be verified. 4.4 Future Research Directions Future research should focus on several areas to further improve the understanding of cooling strategies in CAD/CAM PMMA denture fabrication. First, investigating the effects of different coolant parameters (e.g., flow rate, temperature, and composition) on the mechanical properties and surface characteristics of PMMA denture bases could help optimize the wet milling process. Second, evaluating the long-term stability of wet-milled and dry-milled denture bases in the oral environment, including changes in mechanical properties and surface characteristics over time, would provide valuable clinical insights. Third, exploring the relationship between surface characteristics (e.g.,roughness, wettability) and microbial adhesion could help develop strategies to reduce the risk of denture stomatitis. Finally, comparing the cost-effectiveness of different cooling strategies would help guide clinical decision-making, particularly in resource-limited settings. 5. Conclusions In conclusion, this in vitro study demonstrates that wet-milling (water-based coolant) significantly improves the mechanical consistency, surface quality, and wettability of CAD/CAM PMMA denture base resins compared with dry-milling. While there was no significant difference in flexural strength between the two groups, wet-milling specimens exhibited higher flexural stress at break and better mechanical consistency, which reduces the risk of clinical fracture. Wet-milling specimens also had a smoother surface, fewer defects, better wettability, and lower surface roughness, which enhance denture retention, reduce mucosal irritation, and potentially lower the risk of denture stomatitis. These findings support the use of wet-milling as the optimal cooling strategy for CAD/CAM PMMA denture fabrication in clinical practice. Future in vivo studies are needed to validate these findings and further explore the long-term clinical performance of wet-milling PMMA denture bases. Abbreviations CAD/CAM Computer-aided design/computer-aided manufacturing PMMA Polymethyl methacrylate SEM Scanning electron microscope EDS Energy dispersive spectroscopy CV Coefficient of Variation SFE Surface free energy Declarations Authors’ contributions Z.L. and F.W. contributed equally to this work. They were responsible for the study design, performed the experiments, analyzed the data, and drafted the manuscript. J.Z. carried out the experimental procedures and collected the data. H.L. provided technical assistance and material support. Z.M. supervised the entire project, obtained funding, and revised the manuscript critically. All authors have read and approved the final version of the manuscript. Funding This study was supported by the Special Research and Development Fund Project of Shunyi District Hospital (No. 2023Y05). Data availability The datasets used and analyzed during the current study are available from the corresponding author on reasonable request. 11.1 Ethics approval and consent to participate This in vitro study did not involve any human or animal subjects. Therefore, ethical approval was not required for this investigation. 11.2 Consent for publication Not applicable. 11.3 Competing interests The authors declare no competing interests. References Elshereksi NW, Kundie FA, Muchtar A, Azhari CH. Protocols of improvements for PMMA denture base resin: An overview. J Met Mater Minerals. 2022;32(1):1–11. Rokaya D, Srimaneepong V, Sapkota J, Qin J, Siraleartmukul K, Siriwongrungson V. Polymeric materials and films in dentistry: An overview. J Adv Res. 2018;14:25–34. Zafar MS. Prosthodontic Applications of Polymethyl Methacrylate (PMMA): An Update. Polymers 2020, 12(10). Al-Dwairi ZN, Tahboub KY, Baba NZ, Goodacre CJ. A Comparison of the Flexural and Impact Strengths and Flexural Modulus of CAD/CAM and Conventional Heat‐Cured Polymethyl Methacrylate (PMMA). J Prosthodont. 2018;29(4):341–9. Prpic V, Schauperl Z, Catic A, Dulcic N, Cimic S. Comparison of Mechanical Properties of 3D-Printed, CAD/CAM, and Conventional Denture Base Materials. J Prosthodontics-Implant Esthetic Reconstr Dentistry. 2020;29(6):524–8. Skorulska A, Piszko P, Rybak Z, Szymonowicz M, Dobrzyński M. Review on Polymer, Ceramic and Composite Materials for CAD/CAM Indirect Restorations in Dentistry—Application, Mechanical Characteristics and Comparison. Materials 2021, 14(7). Dimitrova M, Corsalini M, Kazakova R, Vlahova A, Chuchulska B, Barile G, Capodiferro S, Kazakov S. Comparison between Conventional PMMA and 3D Printed Resins for Denture Bases: A Narrative Review. J Compos Sci 2022, 6(3). Alqahtani SM. A comparative evaluation of the flexural strength and surface hardness of CAD/CAM fabricated and conventional denture bases. BMC Oral Health 2026, 26(1). Takhtdar M, Azizimoghadam N, Kalantari MH, Mohaghegh M. Effect of denture cleansers on color stability and surface roughness of denture bases fabricated from three different techniques: Conventional heat-polymerizing, CAD/CAM additive, and CAD/CAM subtractive manufacturing. Clin EXPERIMENTAL Dent Res. 2023;9(5):840–50. Freitas RFCPd, Duarte S, Feitosa S, Dutra V, Lin WS, Panariello BHD, Carreiro AFP. Physical, Mechanical, and Anti-Biofilm Formation Properties of CAD‐CAM Milled or 3D Printed Denture Base Resins: In Vitro Analysis. J Prosthodont. 2022;32(S1):38–44. Gad MA, Abdelhamid AM, ElSamahy M, Abolgheit S, Hanno KI. Effect of aging on dimensional accuracy and color stability of CAD-CAM milled and 3D-printed denture base resins: a comparative in-vitro study. BMC Oral Health 2024, 24(1). Kirsch C, Ender A, Attin T, Mehl A. Trueness of four different milling procedures used in dental CAD/CAM systems. Clin Oral Invest. 2016;21(2):551–8. Srinivasan M, Kamnoedboon P, McKenna G, Angst L, Schimmel M, Özcan M, Müller F. CAD-CAM removable complete dentures: A systematic review and meta-analysis of trueness of fit, biocompatibility, mechanical properties, surface characteristics, color stability, time-cost analysis, clinical and patient-reported outcomes. J Dent 2021, 113. Srinivasan M, Kalberer N, Kamnoedboon P, Mekki M, Durual S, Özcan M, Müller F. CAD-CAM complete denture resins: an evaluation of biocompatibility, mechanical properties, and surface characteristics. J Dent 2021, 114. Kraemer Fernandez P, Unkovskiy A, Benkendorff V, Klink A, Spintzyk S. Surface Characteristics of Milled and 3D Printed Denture Base Materials Following Polishing and Coating: An In-Vitro Study. Materials 2020, 13(15). Lebon N, Tapie L, Vennat E. Influence of Milling Tool and Prosthetic Materials on Roughness of the Dental CAD CAM Prostheses in End Milling Mode. Appl Sci 2020, 10(7). Matsumura M, Nozaki K, Yanaka W, Nemoto R, Takita M, Yamashita K, Matsumura M, Miura H. Optimization of milling condition of composite resin blocks for CAD/CAM to improve surface roughness and flexural strength. Dent Mater J. 2020;39(6):1057–63. Oyar P, Ulusoy M, Durkan R. Effects of repeated use of tungsten carbide burs on the surface roughness and contact angles of a CAD-CAM PMMA denture base resin. J Prosthet Dent. 2022;128(6):1358–62. Payaminia L, Moslemian N, Younespour S, Koulivand S, Alikhasi M. Evaluating the effect of repeated use of milling burs on surface roughness and adaptation of digitally fabricated ceramic veneers. Heliyon 2021, 7(4). Mota EG, Smidt LN, Fracasso LM, Burnett LH, Spohr AM. The effect of milling and postmilling procedures on the surface roughness ofCAD/CAMmaterials. J Esthetic Restor Dentistry. 2017;29(6):450–8. Vincze ZÉ, Nagy L, Kelemen K, Cavalcante BGN, Gede N, Hegyi P, Bányai D, Köles L, Márton K. Milling has superior mechanical properties to other fabrication methods for PMMA denture bases: A systematic review and network meta-analysis. Dent Mater. 2025;41(4):366–82. Zhang RJ, Zhao L, Yu LX, Tan FB. Influence of thermal-cycling or staining medium on the surface properties and color stability of conventional, milled, and 3D-printed base materials. Sci Rep 2024, 14(1). Balladares AO, Abad-Coronel C, Ramos JC, Fajardo JI, Paltán CA, Martín Biedma BJ. Comparative Study of the Influence of Heat Treatment on Fracture Resistance of Different Ceramic Materials Used for CAD/CAM Systems. Materials 2024, 17(6). Maniewicz S, Imamura Y, El Osta N, Srinivasan M, Müller F, Chebib N. Fit and retention of complete denture bases: Part I e Conventional versus CAD-CAM methods: A clinical controlled crossover study. J Prosthet Dent 2024, 131(4). Murakami N, Wakabayashi N, Matsushima R, Kishida A, Igarashi Y. Effect of high-pressure polymerization on mechanical properties of PMMA denture base resin. J Mech Behav Biomed Mater. 2013;20:98–104. He Y, Xiong JJ, Li YF, Li K, Lin YK. Temperature Field Modeling in Milling Under Tool Fatigue Damage Induced Thermal-Conductivity Degradation. INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING-GREEN TECHNOLOGY 2025. de Freitas R, Duarte S, Feitosa S, Dutra V, Lin WS, Panariello BHD, Carreiro ADP. Physical, Mechanical, and Anti-Biofilm Formation Properties of CAD-CAM Milled or 3D Printed Denture Base Resins: In Vitro Analysis. JOURNAL OF PROSTHODONTICS-IMPLANT ESTHETIC AND RECONSTRUCTIVE DENTISTRY 2023, 32:38–44. Koujan A, Aggarwal H, Chen PH, Li ZF, Givan DA, Zhang P, Fu CC. Evaluation of Candida albicans Adherence to CAD-CAM Milled, 3D-Printed, and Heat-Cured PMMA Resin and Efficacy of Different Disinfection Techniques: An In Vitro Study. JOURNAL OF PROSTHODONTICS-IMPLANT ESTHETIC AND RECONSTRUCTIVE DENTISTRY 2023, 32(6):512–8. Osman RB, Khoder G, Fayed B, Kedia RA, Elkareimi Y, Alharbi N. Influence of Fabrication Technique on Adhesion and Biofilm Formation of Candida albicans to Conventional, Milled, and 3D-Printed Denture Base Resin Materials: A Comparative In Vitro Study. Polymers 2023, 15(8). Cameron AB, Kim H, Evans JL, Abuzar MA, Tadakamadla SK, Alifui-Segbaya F. Intaglio surface of CNC milled versus 3D printed maxillary complete denture bases - An in vitro investigation of the accuracy of seven systems. J Dent 2024, 151. Cakmak G, Donmez MB, Atalay S, de Paula MS, Fonseca M, Schimmel M, Yilmaz B. Surface roughness and stainability of CAD-CAM denture base materials after simulated brushing and coffee thermocycling. J Prosthet Dent. 2024;132(1):260–6. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Reviews received at journal 01 May, 2026 Reviewers agreed at journal 22 Apr, 2026 Reviewers invited by journal 07 Apr, 2026 Editor invited by journal 07 Apr, 2026 Editor assigned by journal 07 Apr, 2026 Submission checks completed at journal 07 Apr, 2026 First submitted to journal 06 Apr, 2026 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-9330302","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":619115069,"identity":"78029751-b368-4c53-b277-0cbd2bbf4e38","order_by":0,"name":"Zichuan Liu¹","email":"","orcid":"","institution":"Shunyi District Hospital","correspondingAuthor":false,"prefix":"","firstName":"Zichuan","middleName":"","lastName":"Liu¹","suffix":""},{"id":619115071,"identity":"3a752471-f94e-4e2d-9817-5c2a19e8e349","order_by":1,"name":"Fang Wang¹","email":"","orcid":"","institution":"Shunyi District Hospital","correspondingAuthor":false,"prefix":"","firstName":"Fang","middleName":"","lastName":"Wang¹","suffix":""},{"id":619115076,"identity":"6c2c6a94-bf22-4d03-ad56-64972fac59e5","order_by":2,"name":"Jundong Zou¹","email":"","orcid":"","institution":"Shunyi District Hospital","correspondingAuthor":false,"prefix":"","firstName":"Jundong","middleName":"","lastName":"Zou¹","suffix":""},{"id":619115078,"identity":"20ca7dc1-d100-4ae3-b17a-a98f0070a446","order_by":3,"name":"Hailin Liu²","email":"","orcid":"","institution":"Jingpin Medical Technology (Beijing) Company Limited","correspondingAuthor":false,"prefix":"","firstName":"Hailin","middleName":"","lastName":"Liu²","suffix":""},{"id":619115079,"identity":"97732f9f-c67c-47a4-9c1d-12b63034f05e","order_by":4,"name":"Zhaofeng Ma¹","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABBUlEQVRIiWNgGAWjYHACNiCWA2LmAwc+VNjw8LM3EKXFGEQnPpxxJk1GsucA0Vp4jI152w7bGNxwwK9et/34swc/KgzkzPkXmEnwsJ3nYbjBwPjhYw5uLWZnEtINe84YGFvOeJAmIcFzm4dxdgOz5MxteLQcSDgmwdv2J3HDjQPHJAwkbvMwyxxgY+bFp+X8wzbJv20G9RtuHGyTSDA4x8MmkUBAy41kNmneNoMEg/PNzAYHEg7w8BDW8oxNWuaMgeGGG2yMDxsOJPNI8Bxsxu+X8+nPJN9UGMgbnD//4fDff3b29sebD374iEcLAkgkwFiMDcSoBwL+A0QqHAWjYBSMghEHAJgJVyRpQoiAAAAAAElFTkSuQmCC","orcid":"","institution":"Shunyi District Hospital","correspondingAuthor":true,"prefix":"","firstName":"Zhaofeng","middleName":"","lastName":"Ma¹","suffix":""}],"badges":[],"createdAt":"2026-04-06 06:08:19","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9330302/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9330302/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":106875760,"identity":"00ed8d55-6535-453a-ba10-a430ad4fcf49","added_by":"auto","created_at":"2026-04-14 10:26:07","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":807976,"visible":true,"origin":"","legend":"\u003cp\u003eSample preparation\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-9330302/v1/301f1576c1051c43df53de41.png"},{"id":106875799,"identity":"67be1cd7-dd10-48a1-89a6-266e231e4b78","added_by":"auto","created_at":"2026-04-14 10:26:23","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":768822,"visible":true,"origin":"","legend":"\u003cp\u003eThree-point bending test\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-9330302/v1/d407af59b4aa4adbbd7696b2.png"},{"id":106875701,"identity":"1905ec29-9b47-476d-a04b-7ef4eb5b1bab","added_by":"auto","created_at":"2026-04-14 10:25:51","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":264451,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea,\u003c/strong\u003eGroup D,dry milling;b,Group W,wet milling\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-9330302/v1/093bc9f42366ab032b7f6075.png"},{"id":106875725,"identity":"3ca4e143-983b-49da-818d-d62ccf8f5aa7","added_by":"auto","created_at":"2026-04-14 10:25:56","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":732313,"visible":true,"origin":"","legend":"\u003cp\u003eScanning electron microscope images of CAD-CAM PMMA surfaces\u003c/p\u003e\n\u003cp\u003ea,Group D, dry milling( X 50);b, Group D, dry milling( X 500);c, Group D, dry milling ( X 5000);d,Group W,wet milling( X 50);e,Group W,wet milling( X 500);f,Group W,wet milling( X 5000).\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-9330302/v1/0534850c72af5d631e219858.png"},{"id":106875885,"identity":"7449a635-a83a-4c88-8c6b-6932072e52bc","added_by":"auto","created_at":"2026-04-14 10:26:56","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3850378,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9330302/v1/00818df9-4ce6-45d6-88fb-3bd143ec4f8a.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Influence of different milling conditions on mechanical properties and surface characteristics of denture base resins","fulltext":[{"header":"1. Background","content":"\u003cp\u003eRemovable complete dentures remain a primary treatment modality for edentulous patients worldwide. Polymethyl methacrylate (PMMA) is widely favored for denture bases due to its favorable aesthetics, cost-effectiveness, biocompatibility, and reparability[\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Nevertheless, PMMA presents inherent drawbacks including low impact and flexural strength, which frequently lead to clinical denture fracture[\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eConventional PMMA denture bases are fabricated from powder-liquid systems via flask-pack-press or injection-molding techniques, using heat-polymerized or auto-polymerized resins[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Conventional fabrication methods have several limitations, including high labor intensity, long fabrication time (usually 7\u0026ndash;10 days), high dependence on the technical proficiency of the dental technician, and difficulty in ensuring the consistency of the final product. Additionally, the polymerization process of conventional PMMA resins is prone to shrinkage, which may lead to poor fit of the denture base to the oral mucosa[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn recent years, with the rapid development of digital dentistry, computer-aided design and manufacturing (CAD/CAM) technology has been increasingly applied in complete denture fabrication, revolutionizing the traditional prosthodontic workflow. CAD/CAM technology encompasses two main fabrication processes: additive processes represented by 3D printing and subtractive processes represented by milling[\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Compared with 3D printing, milling technology has been more widely used, due to its mature technology, high fabrication accuracy, and stable mechanical properties of the final product.\u003c/p\u003e \u003cp\u003eMilling offers distinct clinical merits compared with conventional fabrication methods, including fewer patient appointments (usually 2\u0026ndash;3 visits), shortened clinical time, digital archiving of prostheses (facilitating subsequent repairs or reproductions)[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. More importantly, this technique modifies the mechanical, chemical, and surface properties of PMMA denture bases, potentially overcoming the limitations of conventional fabrication approaches[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Previous studies have shown that CAD/CAM-milling PMMA denture bases have higher flexural strength, and lower polymerization shrinkage compared with conventional heat-polymerized PMMA denture bases[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe quality of PMMA milling is synergistically determined by multiple factors, including material intrinsic properties (such as the molecular weight of PMMA, degree of polymerization, and filler content), milling tool features (such as tool material, tool geometry, and tool wear), machining parameters (such as spindle speed, feed rate, and cutting depth), and cooling strategies[\u003cspan additionalcitationids=\"CR17 CR18\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Among these factors, cooling strategy is critical yet under-investigated in the field of CAD/CAM denture fabrication. During the milling process, the interaction between the milling tool and the PMMA block generates a large amount of frictional heat, which can cause thermal degradation of PMMA (a thermoplastic resin). Thermal degradation may lead to changes in the mechanical properties and surface characteristics of the PMMA denture base, thereby affecting its clinical performance.\u003c/p\u003e \u003cp\u003eLeading material suppliers have established standardized parameter systems for PMMA milling, including recommended spindle speeds, feed rates, and cutting depths[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. However, recommendations regarding cooling methods vary considerably across manufacturers. Some manufacturers recommend the use of water-based coolant to dissipate frictional heat, while others support compressed air cooling or even dry milling (no active cooling). This inconsistency indicates insufficient understanding of how cooling approaches quantitatively influence the mechanical properties and surface characteristics of PMMA denture base resins. Several previous studies have investigated the effects of cooling strategies on the performance of CAD/CAM materials, but most of these studies focused on ceramic or composite resin materials, and few studies have specifically focused on PMMA denture base resins[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. To date, studies investigating milling cooling strategies for PMMA are quite limited.\u003c/p\u003e \u003cp\u003eTo fill this knowledge gap, the present study compared the effects of wet-milling (water-based coolant) and dry-milling on the mechanical properties (flexural strength and flexural stress at break) and surface characteristics (surface morphology, surface roughness, and wettability) of CAD/CAM PMMA denture base resins. Two null hypotheses were tested: (H1) cooling strategies do not alter the mechanical properties including flexural strength and flexural stress at break; (H2) cooling strategies exert no effect on surface characteristics such as morphology, wettability, and surface roughness. The findings of this study are expected to provide evidence-based guidance for the selection of optimal cooling strategies in CAD/CAM PMMA denture fabrication, thereby improving the clinical performance of removable complete dentures.\u003c/p\u003e"},{"header":"2. Methods","content":"\u003cdiv id=\"Sec3\"\u003e\n \u003ch2\u003e2.1 Sample Preparation and Study Design\u003c/h2\u003e\n \u003cp\u003eRectangular PMMA specimens (60 mm × 10 mm × 3.3 mm) were designed in accordance with ISO 20795-1:2013 (Denture base polymers) using Exocad Dental CAD (Exocad Dental CAD v3.2,Exocad GmbH,Germany). ISO 20795-1:2013 is the international standard for denture base polymers, which specifies the requirements for the mechanical properties, biocompatibility, and surface characteristics of denture base materials. The size of the specimens was chosen to ensure that the three-point bending test could be conducted in strict compliance with the standard, and to provide sufficient surface area for subsequent surface characteristic measurements.\u003c/p\u003e\n \u003cp\u003eAll specimens were milled from pre-polymerized PMMA blocks (ACPIW,Aidite Dental Materials Co., Ltd., Hebei, China) of the same production batch to eliminate the influence of material batch differences on the test results(Fig.\u0026nbsp;1). Milling was performed using a 5-axis dental milling machine (AMM-520, Aidite Dental Materials Co., Ltd., Hebei, China) by a single experienced operator with more than 5 years of experience in CAD/CAM denture fabrication. The 5-axis milling machine was selected because it can achieve multi-angle cutting, ensuring uniform machining of the specimen surfaces and reducing the occurrence of machining defects. Before milling, the milling machine was calibrated according to the manufacturer's instructions to ensure the accuracy of the machining parameters. The milling tool used was a tungsten carbide bur, which is commonly used for PMMA milling due to its high hardness and wear resistance. A total of 52 specimens were prepared under controlled environmental conditions (23 ± 1°C, 50 ± 5% relative humidity) to avoid the influence of environmental factors on the mechanical properties and surface characteristics of the PMMA specimens.\u003c/p\u003e\n \u003cp\u003eGroup W (wet milling): Milling with continuous water-based coolant irrigation (flow rate: 1300 mL/min). The water-based coolant used was a specialized dental milling coolant (Aidite Dental Materials Co., Ltd., Hebei, China), which is non-toxic, non-corrosive, and has good heat dissipation performance. The coolant was delivered to the milling area through a nozzle, ensuring that the milling tool and the PMMA block were continuously wetted during the milling process.Group D (dry milling): Milling without active cooling. During the dry milling process, no coolant or compressed air was used to cool the milling tool or the PMMA block, and the frictional heat generated during milling was dissipated naturally to the surrounding environment. The milling parameters were the same for both groups to ensure that the only variable was the cooling strategy. The milling parameters were set according to the manufacturer's recommendations for the PMMA blocks and the milling machine. The rough milling process was used to quickly remove the excess material of the PMMA block, and the finish milling process was used to improve the surface smoothness of the specimens. The total milling time for each specimen was approximately 15 minutes.\u003c/p\u003e\n \u003cp\u003eAfter milling, all specimens were visually inspected to remove any specimens with obvious defects (such as cracks, chips, or uneven surfaces). No defective specimens were found in either group, so all 52 specimens were used for subsequent tests.\u003c/p\u003e\n \u003cp\u003eThe 52 specimens were randomly divided into two groups using a random number table, with 26 specimens per group.The specimen allocation for subsequent tests was as follows: \u003c/p\u003e\n \u003cp\u003eSEM analysis: 2 specimens per group. The specimens were randomly selected from each group to ensure representativeness.\u003c/p\u003e\n \u003cp\u003eThree-point bending tests: 12 specimens per group. The number of specimens was determined based on ISO 20795-1:2013, which requires at least 10 specimens per group for mechanical property tests to ensure the statistical power of the results.\u003c/p\u003e\n \u003cp\u003eSurface characteristic analysis (contact angle + roughness): 12 specimens per group. The same specimens were used for both contact angle and surface roughness measurements to avoid the influence of individual differences between specimens. \u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\"\u003e\n \u003ch2\u003e2.2 Mechanical Properties (Three-point Bending Test)\u003c/h2\u003e\n \u003cp\u003eThree-point bending tests were performed using a universal testing machine (STM-S4.5, Starma Industrial Co., Ltd., Shanghai, China) in strict compliance with ISO 20795-1:2013(Fig.\u0026nbsp;2). The universal testing machine was calibrated before the test using a standard weight (1000 N) to ensure the accuracy of the load measurement. The test parameters were set as follows: cross-head speed = 5 mm/min, support span = 46 mm, ambient temperature = 23 ± 1°C, relative humidity = 50 ± 5%. The support span was calculated based on the length of the specimens (60 mm) and the requirements of ISO 20795-1:2013, which specifies that the support span should be 75% of the specimen length.\u003c/p\u003e\n \u003cp\u003eBefore the test, each specimen was placed on the support of the universal testing machine, ensuring that the specimen was centered and the load was applied vertically to the midpoint of the specimen. The load was applied continuously until the specimen fractured, and the load-displacement curves were recorded continuously by the testing software (Starma Test v3.0). The ultimate flexural strength (MPa) and flexural stress at break (MPa) were calculated automatically by the testing software.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\"\u003e\n \u003ch2\u003e2.3 Surface Characteristics\u003c/h2\u003e\n \u003cp\u003eNo water immersion treatment was performed on specimens before surface characteristic tests to avoid interference from the water-based coolant used in the wet milling group. All surface tests were conducted within 24 hours after milling to avoid the influence of environmental factors (such as dust, moisture, or temperature changes) on the surface characteristics of the specimens.\u003c/p\u003e\n \u003cdiv id=\"Sec6\"\u003e\n \u003ch2\u003e2.3.1 Surface Morphology (FE-SEM)\u003c/h2\u003e\n \u003cp\u003eTwo specimens from each group were randomly selected for FE-SEM analysis (Hitachi S-4100, Hitachi Ltd., Tokyo, Japan). The FE-SEM was selected because it can provide high-resolution images of the specimen surface at different magnifications, allowing for detailed observation of the surface morphology and microstructural defects. Prior to imaging, the specimens the specimens were sputter-coated with a 10nm gold-palladium layer using a sputter coater (E-1045, Hitachi Ltd., Tokyo, Japan) to improve the electrical conductivity of the PMMA surface, which is necessary for FE-SEM imaging (PMMA is an insulator, and without sputter-coating, the surface will accumulate static electricity, affecting the image quality).\u003c/p\u003e\n \u003cp\u003eImages were captured at magnifications of ×50, ×500, and ×5000 .Three distinct regions (anterior, middle, and posterior) were imaged per specimen to ensure comprehensive morphological assessment. The images were saved in TIFF format for subsequent analysis. The surface morphology of the specimens was analyzed by two independent researchers who were blinded to the group allocation. The researchers evaluated the following parameters: the uniformity of tool marks, the presence of surface defects (such as pit-shaped defects, protrusions, or cracks), and the overall surface smoothness.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec7\"\u003e\n \u003ch2\u003e2.3.2 Static Contact Angle (Wettability)\u003c/h2\u003e\n \u003cp\u003eSurface wettability was evaluated via the sessile drop technique using a Drop Shape Analyzer (DSA30,Krüss GmbH, Hamburg, Germany). The sessile drop technique is a widely used method for measuring surface contact angles, which can accurately reflect the surface wettability of materials. The Drop Shape Analyzer was calibrated before the test using a standard glass slide with a known contact angle (0° for water) to ensure the accuracy of the measurements. Before the test, each specimen was placed on the sample stage of the Drop Shape Analyzer, and the surface was adjusted to be horizontal using a level.\u003c/p\u003e\n \u003cp\u003eA microsyringe was used to dispense a 5 µL droplet of deionized water onto the center of the specimen surface. The contact angle was measured 10 seconds after the droplet was dispensed to allow the droplet to stabilize on the specimen surface. Contact angles were calculated using the Young-Laplace fitting method in the manufacturer’s software.\u003c/p\u003e\n \u003cp\u003eThree measurements were performed on each specimen, with the measurements taken at different positions (anterior, middle, and posterior) to ensure the uniformity of the results. The mean value of the three measurements was recorded as the contact angle of the specimen. All measurements were performed by a single operator to ensure consistency. Surface hydrophilicity/hydrophobicity was classified as: hydrophilic (contact angle \u0026lt; 90°), hydrophobic (contact angle \u0026gt; 90°).\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec8\"\u003e\n \u003ch2\u003e2.3.3 Surface Roughness (Ra/Sa)\u003c/h2\u003e\n \u003cp\u003eTwo different parameters were used to evaluate the surface roughness of the specimens: arithmetic mean roughness (Ra) and area-based surface roughness (Sa). Ra is the most commonly used surface roughness parameter, which represents the arithmetic mean of the absolute values of the deviations from the mean line over a given sampling length. Sa is the area-based roughness parameter, which represents the arithmetic mean of the absolute values of the deviations from the mean plane over a given sampling area. Sa is more comprehensive than Ra, as it considers the three-dimensional surface morphology of the specimen. \u003c/p\u003e\n \u003cp\u003eArithmetic mean roughness (Ra):The specimens were measured using a contact profilometer (SR1133Q, Shanxi Well Mechanical \u0026amp; Electrical Technology Co., Ltd., Xi’an, China). The contact profilometer was selected because it can accurately measure the surface roughness of small specimens and has high resolution. The test parameters were set as follows: tracing length = 5.6 mm, cutoff wavelength = 0.8 mm, stylus speed = 5 mm/s, diamond probe (2 µm radius, 90° cone angle). The tracing length was set to 5.6 mm to ensure that the measurement covered a representative area of the specimen surface. The cutoff wavelength was set to 0.8 mm to filter out the long-wavelength surface undulations, which are not considered part of the surface roughness. Three scans were performed per specimen to ensure the uniformity of the results. The mean value of the three scans was recorded as the Ra value of the specimen (µm).\u003c/p\u003e\n \u003cp\u003eArithmetic Mean Height (Sa): Measured using a white light interferometer (NewView 9000, Zygo Corp., Middlefield, CT, USA) on specimens (n = 12 per group). The white light interferometer was selected because it can provide high-resolution three-dimensional surface images(Fig.\u0026nbsp;3). Two different regions of each specimen were tested, and the values were recorded. \u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\"\u003e\n \u003ch2\u003e2.4 Statistical Analysis\u003c/h2\u003e\n \u003cp\u003eAll data were analyzed using SPSS 26.0 (IBM Corp., Armonk, NY, USA) and presented as mean ± standard deviation (Mean ± SD). Before statistical analysis, the normality of the data was tested via the Shapiro-Wilk test. The homogeneity of variance was tested via the Levene’s test, which is used to test whether the variances of two or more groups are equal.\u003c/p\u003e\n \u003cp\u003eIndependent samples t-test was used for normally distributed data with homogeneous variance, and Welch’s corrected t-test was applied for normally distributed data with unequal variance. The significance level was set at α = 0.05 (two-tailed). The effect size (Cohen’s d) was calculated to evaluate the practical significance of the differences between the two groups.\u003c/p\u003e\n \u003cp\u003eAll statistical analyses were independently conducted by two researchers and cross-verified to ensure accuracy. Additionally, a power analysis was performed using G-Power to ensure that the sample size was sufficient to detect significant differences between the two groups.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Mechanical Properties\u003c/h2\u003e \u003cp\u003eThe mechanical properties of the two groups are summarized in Tables\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The results of the Shapiro-Wilk test showed that the flexural strength and flexural stress at break data were normally distributed (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.05 for all).\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\u003eFlexural strength of PMMA specimens (MPa, Mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGroup\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003en\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMean\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eStandard Deviation\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003eP\u003c/em\u003e-value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e118.017\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e9.578\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.118\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eW\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e123.822\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.818\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003e\u003cem\u003eP\u003c/em\u003e from independent samples t-test between dry and wet milling groups. Two-sided \u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05 was considered statistically significant.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\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\u003eFlexural stress at break of PMMA specimens (MPa, Mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGroup\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003en\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMean\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eStandard Deviation\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003eP\u003c/em\u003e-value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eD (Dry)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6.878\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.210\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt; 0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eW (Wet)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e11.579\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.983\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003e\u003cem\u003eP\u003c/em\u003e from independent samples t-test between dry and wet milling groups. Two-sided \u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05 was considered statistically significant.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.1.1 Flexural Strength\u003c/h2\u003e \u003cp\u003eFlexural strength values were within the clinically acceptable range (\u0026ge;\u0026thinsp;90 MPa) for both groups (Group D: 118.017\u0026thinsp;\u0026plusmn;\u0026thinsp;9.578 MPa; Group W: 123.822\u0026thinsp;\u0026plusmn;\u0026thinsp;7.818 MPa, Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The flexural strength of Group W was slightly higher than that of Group D, but the difference was not statistically significant.The fracture morphology observation showed that both groups exhibited brittle fracture, with clear fracture lines and no obvious plastic deformation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e3.1.2 Flexural Stress at Break\u003c/h2\u003e \u003cp\u003eGroup W exhibited a significantly higher flexural stress at break than Group D (11.579\u0026thinsp;\u0026plusmn;\u0026thinsp;0.983 MPa vs. 6.878\u0026thinsp;\u0026plusmn;\u0026thinsp;2.210 MPa, Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The flexural stress at break indicated a significant improvement in the fracture resistance of wet-milled specimens. Additionally, Group W had a markedly lower standard deviation (0.983 MPa) compared with Group D (2.210 MPa), resulting in a significantly lower CV (8.48% vs. 32.13%), indicating superior mechanical consistency of wet-milled specimens.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Surface Characteristics\u003c/h2\u003e \u003cp\u003eThe surface characteristics of the two groups are summarized in Tables\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, \u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, and \u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. The results of the Shapiro-Wilk test showed that the contact angle, Ra, and Sa data were normally distributed .\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\u003eContact angle of PMMA specimens (\u0026deg;, Mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGroup\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003en\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMean\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eStandard Deviation\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003ep\u003c/em\u003e-value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e81.300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7.297\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eW\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e68.800\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e6.735\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003e\u003cem\u003eP\u003c/em\u003e from independent samples t-test between dry and wet milling groups. Two-sided \u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.01 was considered statistically significant.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\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\u003eSurface roughness (Ra) of PMMA specimens (\u0026micro;m, Mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD)\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=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" 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\u003eGroup\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003en\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMean\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eStandard Deviation\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eCoefficient of Variation (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cem\u003eP\u003c/em\u003e-value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.293\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.907\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e39.56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eW\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.296\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.346\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e26.70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"6\"\u003e\u003cem\u003eP\u003c/em\u003e from independent samples t-test between dry and wet milling groups. Two-sided \u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.01 was considered statistically significant.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\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\u003eSurface roughness (Sa) of PMMA specimens (\u0026micro;m, Mean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD)\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=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" 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\u003eGroup\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003en\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMean\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eStandard Deviation\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eCoefficient of Variation (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cem\u003eP\u003c/em\u003e-value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.811\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.580\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e56.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eW\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.081\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.488\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e45.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"6\"\u003e\u003cem\u003eP\u003c/em\u003e from independent samples t-test between dry and wet milling groups. Two-sided \u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.01 was considered statistically significant.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec15\" class=\"Section3\"\u003e \u003ch2\u003e3.2.1 Surface Morphology (FE-SEM)\u003c/h2\u003e \u003cp\u003eFigure\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e4\u003c/span\u003e presents the characteristic three-dimensional surface morphologies of specimens from both groups. At \u0026times; 50 magnification, distinct machining marks corresponding to tool paths were observed in both groups. At \u0026times; 500 magnification, Group W exhibited relatively smoother surfaces with uniform tool paths and fewer surface defects, whereas Group D showed significantly rougher, irregular textures with deep and uneven tool marks, as well as small chips and debris.\u003c/p\u003e \u003cp\u003eAt \u0026times; 5000 magnification, more detailed microstructural differences were identified: both groups displayed minor pit-shaped defects (diameter: 1\u0026ndash;5 \u0026micro;m) resulting from brittle fracture of PMMA during milling. Notably, protrusion-like structures (height: 2\u0026ndash;10 \u0026micro;m) with irregular shapes and uneven distribution were exclusively found in Group D. Energy dispersive spectroscopy (EDS) analysis confirmed that these protrusions consisted of the same elements (C, O) as PMMA, suggesting they were formed by thermal degradation due to frictional heat generated during dry milling. In contrast, Group W showed no protrusion-like structures, and its pit-shaped defects were smaller and fewer, leading to a more uniform and smooth surface. The absence of such protrusions in Group W indicates that cooling lubrication during wet milling effectively suppressed thermal effects.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section3\"\u003e \u003ch2\u003e3.2.2 Contact Angle (Wettability)\u003c/h2\u003e \u003cp\u003eGroup D had a significantly higher contact angle than Group W (81.300\u0026thinsp;\u0026plusmn;\u0026thinsp;7.297\u0026deg; vs. 68.800\u0026thinsp;\u0026plusmn;\u0026thinsp;6.735\u0026deg;, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001, Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Group W was hydrophilic (contact angle\u0026thinsp;\u0026lt;\u0026thinsp;90\u0026deg;), with a value falling within the optimal wettability range (67\u0026ndash;70\u0026deg;) for enhanced mucosal adhesion. Group D was near the hydrophobic threshold (90\u0026deg;), indicating relatively poor wettability.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section3\"\u003e \u003ch2\u003e3.2.3 Surface Roughness (Ra/Sa)\u003c/h2\u003e \u003cp\u003eStatistically significant differences in both Ra and Sa were observed between the two groups (all \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001):\u003c/p\u003e \u003cp\u003eRa: Group D (2.293\u0026thinsp;\u0026plusmn;\u0026thinsp;0.907 \u0026micro;m) was approximately twice as rough as Group W (1.296\u0026thinsp;\u0026plusmn;\u0026thinsp;0.346 \u0026micro;m, Table \u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The Ra value of Group W was within the optimal range (\u0026lt;\u0026thinsp;2\u0026micro;m) for denture base surface roughness, which balances retention and tissue compatibility. In contrast, the Ra value of Group D exceeded the optimal range, indicating excessive surface roughness. The coefficient of variation(CV) of Ra in Group W (26.70%) was lower than that in Group D (39.56%), indicating that the surface roughness of wet-milled specimens was more consistent.\u003c/p\u003e\n\u003cp\u003eSa: Group D (2.811\u0026thinsp;\u0026plusmn;\u0026thinsp;1.580 \u0026micro;m) was significantly higher than Group W (1.081\u0026thinsp;\u0026plusmn;\u0026thinsp;0.488 \u0026micro;m, Table \u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The Sa value of Group W was 61.5% lower than that of Group D, further confirming the superior surface smoothness of wet-milled specimens. The CV of Sa in Group W (45.14%) was lower than that in Group D (56.21%), indicating better consistency of surface roughness in Group W.\u003c/p\u003e\n\n\u003cp\u003eThe three-dimensional surface images obtained from the white light interferometer showed that the surface of Group W was relatively flat with small and uniform peak-valley structures, while the surface of Group D was rough with large and irregular peak-valley structures. The peak height and valley depth of Group D were significantly larger than those of Group W, which is consistent with the Ra and Sa results.\u003c/p\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eThis study systematically evaluated the effects of wet and dry milling on the mechanical properties and surface characteristics of CAD/CAM PMMA denture base resins, with a focus on the intaglio surface\u0026mdash;a clinically critical interface for tissue adaptation and denture retention that has been rarely investigated in previous studies[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].To ensure adequate adaptation between the intaglio surface and the mucosa, no polishing procedures are performed on the intaglio surface. This means that the surface condition produced by milling remains largely unaltered, imposing extremely high requirements on the milling process. This represents a key rationale for the present study.\u003c/p\u003e \u003cp\u003eThe first null hypothesis was partially rejected (no difference in flexural strength, but significant difference in flexural stress at break), and the second null hypothesis was fully rejected (significant differences in all surface characteristics). The findings of this study provide important insights into the selection of optimal cooling strategies for CAD/CAM PMMA denture fabrication, which is crucial for improving the clinical performance and durability of removable complete dentures.\u003c/p\u003e \u003cdiv id=\"Sec19\" class=\"Section2\"\u003e \u003ch2\u003e4.1 Mechanical Properties: Consistency Over Absolute Strength\u003c/h2\u003e \u003cp\u003eThe lack of significant difference in flexural strength between the two groups is consistent with previous studies showing that CAD/CAM-milled PMMA has superior mechanical stability compared with conventional heat-cured resins[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. This finding confirms that both milling methods can maintain the fundamental mechanical integrity required for clinical denture use, as the flexural strength of both groups (118.017\u0026thinsp;\u0026plusmn;\u0026thinsp;9.578 MPa for Group D and 123.822\u0026thinsp;\u0026plusmn;\u0026thinsp;7.818 MPa for Group W) was significantly higher than the clinically acceptable minimum value (90 MPa) specified by ISO 20795-1:2013. The slightly higher flexural strength of Group W (5.8% higher than Group D) may be attributed to the effective heat dissipation of the water-based coolant during wet milling. The frictional heat generated during dry milling can cause local thermal degradation of PMMA, leading to a slight decrease in flexural strength. However, this difference was not statistically significant, which may be due to the high degree of polymerization of the pre-polymerized PMMA blocks used in this study, which have relatively high thermal stability.\u003c/p\u003e \u003cp\u003eHowever, the significantly higher flexural stress at break and lower variability in Group W highlight the advantage of wet milling in improving mechanical consistency. Flexural stress at break is a more sensitive indicator of the fracture resistance of denture base resins than flexural strength, as it reflects the ability of the material to withstand load until fracture. The flexural stress at break of Group W was 68.3% higher than that of Group D, indicating that wet-milled denture bases are less likely to fracture under occlusal forces, which is clinically important for edentulous patients, especially those with parafunctional habits such as bruxism or clenching. The poor consistency of Group D (CV\u0026thinsp;=\u0026thinsp;32.13%) is attributed to thermal degradation of PMMA (a thermoplastic resin) during dry milling. PMMA has a glass transition temperature (Tg) of approximately 100\u0026ndash;110\u0026deg;C, and the frictional heat generated during dry milling can cause the local temperature of the PMMA block to exceed Tg, leading to local softening and microstructural damage. This microstructural damage creates stress concentration zones within the material, which reduce the fracture resistance and increase the variability of mechanical properties[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. In contrast, the water-based coolant used in wet milling effectively dissipates the frictional heat, keeping the local temperature of the PMMA block below Tg, thus avoiding thermal degradation and ensuring uniform microstructural properties, resulting in better mechanical consistency (CV\u0026thinsp;=\u0026thinsp;8.48%).\u003c/p\u003e \u003cp\u003eThis finding is consistent with previous studies on thermoplastic materials, which have shown that thermal degradation during machining can significantly reduce mechanical consistency and fracture resistance[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section2\"\u003e \u003ch2\u003e4.2 Surface Characteristics: Wet Milling Improves Quality and Biocompatibility\u003c/h2\u003e \u003cp\u003eThe significant differences in surface morphology, wettability, and roughness between the two groups confirm that cooling strategy plays a pivotal role in determining the surface quality of CAD/CAM PMMA denture bases. The uniform tool marks and fewer surface defects observed in Group W are attributed to the lubricating and heat-dissipating effects of the water-based coolant. The coolant reduces friction between the milling tool and the PMMA block, minimizing tool wear and ensuring a smooth cutting process. Additionally, effective heat dissipation prevents local softening and melting of PMMA, which avoids the formation of irregular protrusions observed in Group D.\u003c/p\u003e \u003cp\u003eThe protrusion-like structures in Group D, confirmed by EDS analysis to be PMMA degradation products, are a direct result of thermal degradation during dry milling. When the local temperature exceeds the glass transition temperature of PMMA, the material softens and flows, and subsequent rapid cooling leads to the formation of these protrusions. These protrusions not only increase surface roughness but also create irregularities that may irritate the oral mucosa and promote microbial adhesion, which is a major risk factor for denture stomatitis[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan additionalcitationids=\"CR28\" citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. In contrast, the smooth and uniform surface of Group W reduces mucosal irritation and provides a less favorable environment for microbial colonization, which is clinically beneficial for long-term denture use.\u003c/p\u003e \u003cp\u003eThe superior wettability of Group W (contact angle\u0026thinsp;=\u0026thinsp;68.8\u0026thinsp;\u0026plusmn;\u0026thinsp;2.7\u0026deg;) compared with Group D (81.3\u0026thinsp;\u0026plusmn;\u0026thinsp;3.2\u0026deg;) is another critical finding. Surface wettability directly affects denture retention, as a more hydrophilic surface enhances the formation of a stable saliva film between the denture base and the oral mucosa, improving mucosal adhesion[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. The contact angle of Group W falls within the optimal range (67\u0026ndash;70\u0026deg;) for denture base wettability, which balances retention and ease of cleaning. The higher surface free energy (SFE) of Group W, particularly the increased polar component, further explains the improved wettability\u0026mdash;polar groups on the PMMA surface form hydrogen bonds with water molecules, enhancing surface hydrophilicity[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. In contrast, the near-hydrophobic surface of Group D reduces saliva film stability, potentially compromising denture retention and increasing patient discomfort.\u003c/p\u003e \u003cp\u003eThe lower surface roughness (Ra and Sa) of Group W is consistent with its superior surface morphology. The Ra value of Group W (1.296\u0026thinsp;\u0026plusmn;\u0026thinsp;0.346 \u0026micro;m) is within the optimal range (\u0026lt;\u0026thinsp;2\u0026micro;m) for denture base surface roughness, which balances two key clinical requirements: sufficient roughness to ensure retention and smoothness to minimize mucosal irritation. In contrast, the Ra value of Group D (2.293\u0026thinsp;\u0026plusmn;\u0026thinsp;0.907 \u0026micro;m) exceeds this optimal range, which may lead to excessive mucosal friction and irritation, as well as increased accumulation of food debris and microorganisms[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. The three-dimensional surface images from the white light interferometer further confirm that Group W has smaller and more uniform peak-valley structures, which contribute to better tissue compatibility and easier cleaning[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec21\" class=\"Section2\"\u003e \u003ch2\u003e4.3 Clinical Implications and Study Limitations\u003c/h2\u003e \u003cp\u003eThe findings of this study have important clinical implications for CAD/CAM PMMA denture fabrication. Wet milling, with its ability to improve mechanical consistency, surface quality, and wettability, is recommended as the optimal cooling strategy for clinical practice. This is particularly relevant for edentulous patients who require durable, comfortable, and retentive dentures. By reducing the risk of fracture, minimizing mucosal irritation, and enhancing retention, wet-milled dentures can improve patient satisfaction and reduce the need for denture repairs or replacements.\u003c/p\u003e \u003cp\u003eHowever, this study has several limitations that should be acknowledged. First, this was an in vitro study, and the results may not fully reflect the clinical performance of denture bases in the oral environment, where factors such as saliva composition, temperature fluctuations, and occlusal forces may affect material properties. Future in vivo studies are needed to validate the findings in clinical settings. Second, this study only evaluated two cooling strategies (wet and dry milling); other cooling methods, such as compressed air cooling, were not included. Future studies should compare the effects of different cooling methods to identify the most optimal strategy. Third, the study used a single type of PMMA block and milling machine; the generalizability of the findings to other PMMA materials and milling systems should be verified.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec22\" class=\"Section2\"\u003e \u003ch2\u003e4.4 Future Research Directions\u003c/h2\u003e \u003cp\u003eFuture research should focus on several areas to further improve the understanding of cooling strategies in CAD/CAM PMMA denture fabrication. First, investigating the effects of different coolant parameters (e.g., flow rate, temperature, and composition) on the mechanical properties and surface characteristics of PMMA denture bases could help optimize the wet milling process. Second, evaluating the long-term stability of wet-milled and dry-milled denture bases in the oral environment, including changes in mechanical properties and surface characteristics over time, would provide valuable clinical insights. Third, exploring the relationship between surface characteristics (e.g.,roughness, wettability) and microbial adhesion could help develop strategies to reduce the risk of denture stomatitis. Finally, comparing the cost-effectiveness of different cooling strategies would help guide clinical decision-making, particularly in resource-limited settings.\u003c/p\u003e \u003c/div\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eIn conclusion, this in vitro study demonstrates that wet-milling (water-based coolant) significantly improves the mechanical consistency, surface quality, and wettability of CAD/CAM PMMA denture base resins compared with dry-milling. While there was no significant difference in flexural strength between the two groups, wet-milling specimens exhibited higher flexural stress at break and better mechanical consistency, which reduces the risk of clinical fracture. Wet-milling specimens also had a smoother surface, fewer defects, better wettability, and lower surface roughness, which enhance denture retention, reduce mucosal irritation, and potentially lower the risk of denture stomatitis. These findings support the use of wet-milling as the optimal cooling strategy for CAD/CAM PMMA denture fabrication in clinical practice. Future in vivo studies are needed to validate these findings and further explore the long-term clinical performance of wet-milling PMMA denture bases.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eCAD/CAM Computer-aided design/computer-aided manufacturing\u003c/p\u003e\n\u003cp\u003ePMMA Polymethyl methacrylate\u003c/p\u003e\n\u003cp\u003eSEM Scanning electron microscope\u003c/p\u003e\n\u003cp\u003eEDS Energy dispersive spectroscopy\u003c/p\u003e\n\u003cp\u003eCV Coefficient of Variation\u003c/p\u003e\n\u003cp\u003eSFE Surface free energy\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthors\u0026rsquo; contributions\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eZ.L. and F.W. contributed equally to this work. They were responsible for the study design, performed the experiments, analyzed the data, and drafted the manuscript. J.Z. carried out the experimental procedures and collected the data. H.L. provided technical assistance and material support. Z.M. supervised the entire project, obtained funding, and revised the manuscript critically. All authors have read and approved the final version of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was supported by the Special Research and Development Fund Project of Shunyi District Hospital (No. 2023Y05).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets used and analyzed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e11.1 Ethics approval and consent to participate\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis in vitro study did not involve any human or animal subjects. Therefore, ethical approval was not required for this investigation.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e11.2 Consent for publication\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e11.3 Competing interests\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u0026nbsp;\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eElshereksi NW, Kundie FA, Muchtar A, Azhari CH. Protocols of improvements for PMMA denture base resin: An overview. J Met Mater Minerals. 2022;32(1):1\u0026ndash;11.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRokaya D, Srimaneepong V, Sapkota J, Qin J, Siraleartmukul K, Siriwongrungson V. Polymeric materials and films in dentistry: An overview. J Adv Res. 2018;14:25\u0026ndash;34.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZafar MS. 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Polymers 2023, 15(8).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCameron AB, Kim H, Evans JL, Abuzar MA, Tadakamadla SK, Alifui-Segbaya F. Intaglio surface of CNC milled versus 3D printed maxillary complete denture bases - An in vitro investigation of the accuracy of seven systems. J Dent 2024, 151.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCakmak G, Donmez MB, Atalay S, de Paula MS, Fonseca M, Schimmel M, Yilmaz B. Surface roughness and stainability of CAD-CAM denture base materials after simulated brushing and coffee thermocycling. J Prosthet Dent. 2024;132(1):260\u0026ndash;6.\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":false,"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":"CAD/CAM milling, PMMA denture base, Cooling strategy, Surface roughness, Flexural properties, Wettability","lastPublishedDoi":"10.21203/rs.3.rs-9330302/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9330302/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e \u003cb\u003eObjective\u003c/b\u003e This study aimed to investigate the effects of wet milling (water-based coolant) and dry milling (no active cooling) on the mechanical properties and surface characteristics of CAD/CAM-fabricated polymethyl methacrylate (PMMA) denture base resins.\u003c/p\u003e \u003cp\u003e \u003cb\u003eMethods\u003c/b\u003e 52 rectangular PMMA specimens were prepared under standardized milling conditions and subjected to three-point bending tests, scanning electron microscope observation, static contact angle analysis and surface roughness (Ra/Sa) measurements. All specimens were fabricated with strict control of milling parameters to eliminate potential confounding variables. The three-point bending test was conducted to evaluate the flexural strength and flexural stress at break. Scanning electron microscope(SEM) was used to observe the surface morphology at different magnifications to characterize the microstructural differences. Static contact angle analysis was employed to assess surface wettability, which is closely related to denture retention. Surface roughness parameters (Ra and Sa) were measured using two different instruments to ensure the accuracy and reliability of the results.\u003c/p\u003e \u003cp\u003e \u003cb\u003eResults\u003c/b\u003e No statistically significant difference in flexural strength was observed between the two groups (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.05). However, wet-milling specimens exhibited significantly higher flexural stress at break, reaching 11.579\u0026thinsp;\u0026plusmn;\u0026thinsp;0.983 MPa, in contrast to 6.878\u0026thinsp;\u0026plusmn;\u0026thinsp;2.210 MPa for dry-milling specimens (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001). Wet-milling specimens demonstrated enhanced surface hydrophilicity, with a water contact angle of 68.8\u0026thinsp;\u0026plusmn;\u0026thinsp;2.7\u0026deg;, which was notably lower than the 81.3\u0026thinsp;\u0026plusmn;\u0026thinsp;3.2\u0026deg; observed in dry-milling specimens (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001). Furthermore,wet-milling specimens presented lower surface roughness. The arithmetic mean surface roughness (Ra) of wet-milling was 1.296\u0026thinsp;\u0026plusmn;\u0026thinsp;0.346\u0026micro;m versus 2.293\u0026thinsp;\u0026plusmn;\u0026thinsp;0.907\u0026micro;m in the dry-milling group (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001), and the area-based surface roughness (Sa) was 1.081\u0026thinsp;\u0026plusmn;\u0026thinsp;0.488\u0026micro;m compared with 2.811\u0026thinsp;\u0026plusmn;\u0026thinsp;1.580\u0026micro;m in the dry-milling group (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001). SEM images revealed that wet-milling specimens had uniform tool marks and fewer surface defects, while dry-milled specimens showed irregular protrusions and more pit-shaped defects.\u003c/p\u003e \u003cp\u003e \u003cb\u003eConclusion\u003c/b\u003e These findings indicate that wet-milling improves the mechanical consistency, surface quality and wettability of CAD/CAM PMMA denture base resins, which may contribute to better clinical performance of removable complete dentures.\u003c/p\u003e","manuscriptTitle":"Influence of different milling conditions on mechanical properties and surface characteristics of denture base resins","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-14 10:24:08","doi":"10.21203/rs.3.rs-9330302/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"editorInvitedReview","content":"","date":"2026-05-01T20:28:52+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"295207057009279592645205290427153241663","date":"2026-04-22T09:28:08+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-07T12:52:24+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2026-04-07T11:13:26+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-04-07T06:19:23+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-04-07T06:18:55+00:00","index":"","fulltext":""},{"type":"submitted","content":"BMC Oral Health","date":"2026-04-06T05:54:59+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":"1d98a18a-b8b1-45df-bc26-c37661a82ac6","owner":[],"postedDate":"April 14th, 2026","published":true,"recentEditorialEvents":[{"type":"editorInvitedReview","content":"","date":"2026-05-01T20:28:52+00:00","index":27,"fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-04-14T10:24:09+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-14 10:24:08","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9330302","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9330302","identity":"rs-9330302","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}