Can BioFlx Crowns Bridge the Gap Between Zirconia and Stainless Steel? An In-Vitro Comparative Analysis

Can BioFlx Crowns Bridge the Gap Between Zirconia and Stainless Steel? 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An In-Vitro Comparative Analysis Alp Abidin Ateşçi, Dilek Özge Yılmaz, Berk Gergit, Munevver Coruh Kilic, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8026149/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 12 You are reading this latest preprint version Abstract Background: Esthetic and durable crowns are essential for restoring extensively decayed primary molars. Zirconia and polymer-based alternatives to stainless steel crowns (SSCs) have been introduced, but comparative evidence on their mechanical behavior after aging is limited. Aim: To evaluate and compare the wear resistance and fracture strength of zirconia, polymer-based BioFlx, and SSCs for mandibular second primary molars after standardized thermomechanical aging. Methods: Eighty prefabricated crowns (NuSmile zirconia, ProfZr zirconia, BioFlx polymer, and SSC; n = 10 per group for wear and fracture testing) were cemented on standardized 3D-printed resin dies. All specimens underwent 10,000 thermal cycles between 5°C and 55°C; half were additionally subjected to 150,000 chewing cycles under 50 N before testing. Results: Zirconia crowns exhibited the lowest wear scores (p < 0.05), while BioFlx and SSCs showed significantly higher fracture resistance than zirconia (p < 0.001). No significant difference occurred between BioFlx and SSCs (p = 0.093). All materials exceeded physiological bite-force thresholds for children. Conclusions: Zirconia crowns provide superior wear resistance, whereas BioFlx and SSCs exhibit higher fracture resistance and stress absorption. BioFlx may represent a promising alternative for pediatric full-coverage restorations. prefabricated crowns pediatric zirconia crown BioFlx crown stainless steel crown occlusal wear fracture resistance thermomechanical aging Figures Figure 1 Figure 2 Figure 3 Figure 4 Background Dental caries remains the most prevalent chronic disease of childhood, disproportionately affecting primary molars due to their anatomical vulnerabilities and prolonged exposure in the oral cavity [ 1 ]. When conventional restorative methods fall short in managing extensive decay, full-coverage crowns offer a reliable treatment modality. Stainless steel crowns (SSCs) have long been considered the gold standard in pediatric dentistry due to their high success rates and durability in high-risk populations [ 2 , 3 ]. Despite their functional excellence, the metallic appearance of SSCs has generated increasing parental demand for esthetically superior alternatives [ 4 , 5 ]. To address this aesthetic shortcoming, prefabricated zirconia crowns (ZCs) have been introduced, offering natural tooth-like translucency and excellent biocompatibility. Clinical studies have demonstrated their favorable gingival response, high parental satisfaction, and excellent longevity—retention rates ranging from 76% to 94% over 3 years have been reported [ 6 – 8 ]. However, the use of ZCs often requires more aggressive tooth preparation and presents challenges in terms of marginal adaptability, chairside adjustability, and may compromise pulp vitality [ 9 , 10 ]. In response to the limitations of both SSCs and ZCs, polymer-based esthetic crowns such as BioFlx have emerged as a novel alternative. BioFlx crowns are fabricated from a high-impact hybrid resin polymer that is metal-free and Bis-GMA-free, providing enhanced flexibility, shock absorption, and color stability. These innovative crowns aim to combine aesthetic appeal with mechanical resilience and minimal invasiveness. Designed using flexible high-performance polymers, BioFlx crowns are characterized by their elasticity, conservative preparation requirements, and easy intraoral manipulation. Emerging in-vitro data indicate that high-performance polymer crowns can withstand clinically relevant occlusal loads while preserving tooth structure, due to their elastic modulus and simplified preparation requirements [ 11 , 12 ]. Evaluation of restorative materials under simulated intraoral conditions remains a cornerstone of preclinical validation. Chewing simulators, in conjunction with thermocycling protocols, are frequently employed to replicate one year or more of oral aging. These methodologies are critical in assessing long-term behavior, particularly in pediatric populations where mechanical loads and thermal fluctuations impact restoration integrity [ 13 , 14 ]. Kessler et al. demonstrated that crown material and cementation type significantly influence wear and fracture resistance, further emphasizing the importance of holistic evaluation [ 15 ]. In this context, BioFlx crowns warrant rigorous investigation to elucidate their durability, fracture behavior and performance under standardized thermomechanical aging. While studies on zirconia crowns have validated their mechanical strength and clinical longevity, comparative data on BioFlx and other prefabricated esthetic crowns remain limited [ 13 ]. For instance, an in-vitro finite element analysis comparing BioFlx crowns, prefabricated zirconia crowns and stainless steel crowns found that the BioFlx models exhibited notably lower stress values under axial and angled loading scenarios, suggesting potential biomechanical advantage, yet the authors highlighted the absence of long-term empirical data[ 16 ]. Moreover, a recent in-vitro study directly comparing BioFlx, zirconia and SSCs reported that while BioFlx crowns achieved intermediate fracture resistance and performed favourably in surface wear and retentive strength assessments, the overall dataset remains small and brand-dependent [ 17 ]. Given this, there is a clear need for further standardized preclinical and clinical investigations of BioFlx crowns to determine whether their aesthetic and procedural advantages are matched by equivalent or superior mechanical performance in the paediatric restorative context. To date, no published research has concurrently examined BioFlx, zirconia, and stainless steel crowns under a unified thermomechanical ageing protocol employing independent specimen cohorts for wear and fracture analyses. Such a dual-parameter in-vitro framework provides a more clinically meaningful assessment of material-dependent fatigue behaviour by eliminating cumulative degradation bias and closely approximating the functional stresses encountered in paediatric mastication. This in-vitro study evaluated and compared the wear behavior and fracture resistance of four prefabricated crown types for primary mandibular second molars—two zirconia systems (NuSmile, ProfZr), a polymer-based crown (BioFlx), and stainless-steel crowns (SSC)—after standardized thermomechanical aging. The null hypothesis was that no statistically significant differences would exist among the crown types for either outcome. Materials and Methods Sample Size Calculation and Grouping The sample size was determined in accordance with previous in-vitro study that employed comparable thermocycling aging protocols in the evaluation of pediatric crowns [ 15 ]. A priori power analysis was performed using G*Power software (version 3.1; Düsseldorf, Germany), with an effect size (f) = 0.55, a significance level (α) = 0.05, and a statistical power (1 – β) = 0.80. This calculation indicated a minimum of ten specimens per group, yielding a total of eighty prefabricated crowns for mandibular second primary molars, distributed evenly among four material categories (n = 10 per group for each testing modality). To preserve methodological independence, the specimens were divided into two separate cohorts (n = 40 each). Both cohorts underwent identical thermocycling to simulate intraoral temperature fluctuations. In the cohort assigned for wear evaluation, chewing simulation was additionally performed in the same device. The crowns were divided into four groups according to material type: Group 1 : NuSmile® zirconia crowns (NuSmile, Houston, TX, USA) Group 2 : ProfZr® zirconia crowns (Prof Teknoloji, Erzurum, Türkiye) Group 3 : BioFlx® polymer crowns (Kids-e-dental, Mumbai, India) Group 4 : Kids Crown® stainless steel crowns (Shinhung Company Ltd., Seoul, South Korea) Crown Preparation and Standardization To ensure standardization across all samples, a mandibular second primary molar was digitally designed using Exocad software (Exocad GmbH, Germany). A single calibrated pediatric dentist performed all digital preparations on the master design; STL files were locked prior to printing to prevent unintended parameter drift. For the pediatric zirconia groups, the digital preparation included 2.0 mm occlusal reduction, 1.5 mm proximal clearance, and 1.0 mm circumferential preparation with rounded internal angles and complete edge bevelling. For BioFlx and stainless steel crowns, the occlusal reduction was limited to 1.5 mm, with identical proximal and circumferential parameters. Die Fabrication and Crown Cementation Eighty standardized resin dies were fabricated using a 3D printer with NextDent Model 2.0 resin (Vertex-Dental B.V., Soesterberg, The Netherlands), a material with mechanical properties resembling natural dentin. During cementation, a constant load of 5 kg was applied for 3 minutes to ensure uniform seating pressure, following the methodology described by Kessler et al. (2021) [ 15 ]. A conventional glass ionomer luting cement (Fuji I®, GC Corporation, Tokyo, Japan) was used for crown cementation following the manufacturer’s protocol. Excess cement was removed, and specimens were stored in distilled water at 37°C for 24 hours to allow for complete setting of the cement [ 13 ]. All specimens were embedded in cylindrical blocks of methyl methacrylate resin (Technovit 4000, Heraeus Kulzer, Germany). This embedding method was chosen because of its elastic modulus, which closely approximates that of human alveolar bone, and is recommended for simulating periodontal support in in vitro loading conditions [ 18 ]. Thermocycling, Chewing Simulation, and Wear Assessment Thermocycling was performed in accordance with ISO 11405 recommendations to simulate intraoral temperature fluctuations. Each specimen underwent 10,000 cycles between 5°C and 55°C (dwell time: 25 s; transfer time: 10 s) using a dual-function thermocycling and chewing simulation unit (Model ACS-8, Analitik Medikal Ltd., Gaziantep, Türkiye). This protocol represents approximately one year of thermomechanical aging in vivo (Fig. 1 ) [ 19 ]. Simultaneously, the same unit performed a standardized two-body wear protocol using a chewing simulator (Model ACS-8, Analitik Medikal Ltd.,Gaziantep, Türkiye) for 150,000 loading cycles under 50 N vertical force at 1.2 Hz. The samples were mounted opposing a 4 mm steatite sphere antagonist attached to a vertically moving bar. Each loading stroke started from the functional cusp and slid approximately 0.3 mm toward the central fossa, simulating pediatric masticatory movement. The loading and lifting speeds were 20 mm/s and 60 mm/s, respectively, with continuous water rinsing at 37°C throughout the test. This setup followed previously validated protocols for two-body wear simulation in primary molar crowns [ 12 , 20 ]. After chewing simulation, each specimen was examined under a stereomicroscope (×20 magnification) to assess occlusal wear. The amount of visible wear was scored using a 4-point wear index (0 = no visible wear, 1 = slight polishing marks, 2 = moderate wear facets, 3 = severe material loss) previously described in the studies by Krejci and Metwally [ 21 , 22 ]. Two independent blinded evaluators performed all assessments, and inter-observer reliability was calculated using Cohen’s kappa coefficient (κ = 0.87, indicating excellent agreement). Mean wear scores were used for statistical analysis. Fracture Resistance Testing After completion of the thermocycling protocol of the second set of crowns, specimens were subjected to fracture testing. Each crown was embedded in self-cured acrylic resin blocks, leaving 1 mm of the cervical margin exposed to simulate clinical support, and positioned vertically in a universal testing machine (Instron 2710-003, Instron Corp., Norwood, MA, USA) equipped with a 5-kN load cell (Fig. 2 .). Compressive loading was applied at the center of the occlusal fossa using a 7-mm diameter stainless-steel indenter oriented perpendicular to the crown’s long axis. A 0.5-mm thick tin foil sheet was interposed between the indenter and the occlusal surface to ensure uniform stress distribution and prevent localized stress peaks. The test was performed under dry conditions at room temperature (approximately 23°C). The load was applied at a crosshead speed of 1 mm/min until fracture occurred. Fracture was defined as a sudden drop in the load–displacement curve, often accompanied by an audible crack or visible segment separation, ensuring an objective and reproducible failure criterion. The maximum load at fracture (in Newtons) was automatically recorded by the testing software. Each fractured specimen was subsequently inspected under a stereomicroscope (×20 magnification) to categorize the failure pattern as cohesive (within the crown), adhesive (at the cement–crown interface), or mixed. Statistical Analysis Statistical analyses were performed using IBM SPSS Statistics for Windows, version 26.0 (IBM Corp., Armonk, NY, USA). Descriptive results were expressed as mean ± standard deviation (SD). Data normality was assessed with the Shapiro–Wilk test and homogeneity of variance with Levene’s test. Since both assumptions were met (p > 0.05), fracture resistance values were analyzed using one-way analysis of variance (ANOVA) followed by Tukey’s honestly significant difference (HSD) post-hoc test for pairwise comparisons. The effect size (η²) and 95% confidence intervals (CIs) were calculated to indicate the magnitude and precision of differences. As wear data did not follow a normal distribution, non-parametric tests were applied. Inter-group comparisons were performed using the Kruskal–Wallis test, and post-hoc pairwise comparisons were adjusted with the Dunn–Bonferroni correction. Effect size for the Kruskal–Wallis test was expressed as epsilon-squared (ε²). The level of statistical significance was set at α = 0.05 for all analyses. Results Wear Assessment After one year of simulated thermomechanical aging and chewing cycles, all crowns exhibited observable occlusal wear, with statistically significant differences among the groups (Kruskal–Wallis, p < 0.05). The mean wear scores (0–3 scale) were as follows: NuSmile zirconia = 1.1 ± 0.3, ProfZr zirconia = 1.2 ± 0.4, SSC = 1.7 ± 0.4, and BioFlx = 2.2 ± 0.4 (Table 1 ). Table 1 Visual wear scores (0–3 scale) of the tested crowns after thermomechanical aging and chewing simulation Crown type Mean wear score ± SD Minimum Maximum p vs NuSmile p vs ProfZr p vs SSC p vs BioFlx NuSmile 1.1 ± 0.3 0.8 1.6 — 0.68 0.04* 0.01** ProfZr 1.2 ± 0.4 0.7 1.8 0.68 — 0.04* 0.01** SSC 1.7 ± 0.4 1.1 2.3 0.04* 0.04* — 0.03* BioFlx 2.2 ± 0.4 1.5 2.8 0.01** 0.01** 0.03* — Data are expressed as mean ± standard deviation (SD). Pairwise comparisons were performed using the Dunn–Bonferroni post-hoc test following the Kruskal–Wallis analysis. *p < 0.05; **p < 0.01 indicate statistically significant and highly significant differences, respectively. Lower scores correspond to higher wear resistance. Pairwise comparisons using the Dunn–Bonferroni test showed no significant difference between the two zirconia groups (NuSmile–ProfZr, p = 0.68), indicating comparable wear resistance. Both zirconia crowns exhibited significantly lower wear scores than SSC (p = 0.04) and BioFlx (p = 0.01). SSC crowns showed intermediate wear resistance, presenting significantly lower wear than BioFlx (p = 0.03) but higher wear than both zirconia crowns (p = 0.04). Microscopic examination revealed smoother, polished surfaces with minimal abrasion in zirconia crowns, shallow wear facets in SSC, and more distinct material loss in BioFlx specimens (Fig. 3 ). Fracture Resistance The Shapiro–Wilk test confirmed that all data sets followed a normal distribution (p > 0.05), and Levene’s test indicated homogeneity of variances among groups (p > 0.05). Therefore, a one-way analysis of variance (ANOVA) was performed to compare the mean fracture resistance values of the tested crowns. The ANOVA revealed statistically significant differences among the four groups (p < 0.001). The mean fracture loads (± SD) were BioFlx: 3148.4 ± 643.4 N, SSC (stainless steel crowns): 2703.4 ± 243.1 N, NuSmile zirconia crowns: 1076.1 ± 186.9 N, and ProfZr zirconia crowns: 565.5 ± 159.6 N (Table 2 ). Table 2 Mean fracture resistance values (N) of the tested crowns and pairwise intergroup comparisons according to Tukey’s HSD test (α = 0.05) Crown type Mean ± SD (N) Minimum Maximum p-values (Tukey HSD) BioFlx SSC NuSmile ProfZr BioFlx 3148.4 ± 643.4 1835.30 4070.36 — 0.093 < 0.001*** < 0.001*** SSC 2703.4 ± 243.1 2445.01 3210.31 0.093 — < 0.001*** < 0.001*** NuSmile 1076.1 ± 186.9 747.46 1325.89 < 0.001*** < 0.001*** — 0.043* ProfZr 565.5 ± 159.6 367.24 808.32 < 0.001*** < 0.001*** 0.043* — Comparison of mean fracture resistance (N) values among different prefabricated crown types. Data are presented as mean ± standard deviation (SD). Statistical comparisons were performed using one-way ANOVA followed by Tukey’s HSD post-hoc test (α = 0.05). p < 0.05; **p < 0.01; ***p < 0.001 indicate statistically significant differences. According to Tukey’s HSD post-hoc test, BioFlx exhibited significantly higher fracture resistance than both NuSmile (p < 0.001) and ProfZr (p < 0.001) crowns, while no statistically significant difference was found between BioFlx and SSC ( p = 0.093 ) . SSC crowns showed significantly higher values than NuSmile (p < 0.001) and ProfZr (p < 0.001). Among the zirconia groups, NuSmile demonstrated significantly greater strength than ProfZr (p = 0.043). Overall, BioFlx and SSC crowns exhibited the highest fracture resistance, with no significant difference between them, whereas both zirconia crowns—particularly ProfZr—showed considerably lower performance (Fig. 4 ). Failure patterns differed across materials and followed the expected trend for their mechanical behavior. Zirconia crowns predominantly exhibited cohesive failures within the crown material (NuSmile: 5/10, ProfZr: 6/10), reflecting their brittle nature. SSCs showed mainly mixed failures (8/10), consistent with their ductile, energy-absorbing behavior. BioFlx crowns demonstrated a higher proportion of mixed (5/10 ) and adhesive (4/10) failures, indicating stress dissipation through the crown–cement interface rather than catastrophic fracture (Table 3 ). Group distributions were compared using Fisher’s exact test due to small cell counts (α = 0.05). Table 3 Failure mode distribution among the tested crown types after fracture testing Crown type Cohesive n (%) Adhesive n (%) Mixed n (%) n BioFlx 1 (10%) 4 (40%) 5 (50%) 10 SSC 0 (0%) 2 (20%) 8 (80%) 10 NuSmile 5 (50%) 2 (20%) 3 (30%) 10 ProfZr 6 (60%) 2 (20%) 2 (20%) 10 Cohesive failure = fracture within the crown material; Adhesive failure = separation at the cement–crown interface; Mixed failure = combination of cohesive and adhesive patterns observed under stereomicroscopic evaluation (×20). Discussion This in-vitro investigation evaluated the wear behavior and fracture resistance of four prefabricated crown systems for primary mandibular second molars—two zirconia crowns (NuSmile and ProfZr), a polymer-based crown (BioFlx), and a stainless-steel crown (SSC)—following standardized thermomechanical aging. Statistically significant differences were identified among the tested materials, resulting in the rejection of the null hypothesis. Zirconia crowns exhibited the lowest wear scores, reflecting their superior surface hardness and abrasion resistance, whereas BioFlx crowns demonstrated the highest fracture resistance, comparable to SSCs. These findings highlight a fundamental mechanical balance: materials with higher surface hardness tend to exhibit increased brittleness, while those with greater elasticity can better absorb masticatory loads but are more susceptible to surface degradation. Collectively, the long-term performance of pediatric crowns appears to depend not solely on absolute strength, but on achieving an optimal equilibrium between rigidity and resilience under functional stresses. Zirconia crowns demonstrated the lowest occlusal wear scores among the tested materials, reflecting their superior hardness, surface stability, and resistance to micro-abrasion. This observation aligns with previous in-vitro reports showing that monolithic zirconia ceramics resist volumetric material loss under cyclic loading better than polymeric or resin-based materials [ 23 ]. The glazed and polished surfaces of zirconia act as a barrier to micro-scratch formation and crack initiation, reducing progressive wear over time [ 24 ]. However, the price of this high wear resistance is brittleness: under repeated loading, microcracks can propagate, but until catastrophic failure, the surface remains stable. In contrast, BioFlx crowns in this study showed the highest wear scores, indicating more pronounced material removal or deformation at occlusal surfaces. This result is consistent with findings from studies on BioFlx crowns, which report higher average material wear compared to zirconia while sometimes causing less wear on the antagonistic tooth [ 11 ]. The mechanism behind this pattern may be the elastic and viscoelastic nature of the polymeric matrix: under load, the material may undergo slight plastic deformation or creep, absorbing energy and redistributing stresses rather than resisting wear completely. In effect, the crown surface may gradually wear, but the bulk structure avoids fracture by spreading the load [ 25 ]. Stainless-steel crowns (SSCs) occupied an intermediate position in wear performance: more wear than zirconia but less than BioFlx. Their metallic structure provides toughness and resistance against abrupt failure, but due to the harder opposing surfaces and potential micro-roughness, mild abrasion can occur over long cycles. Clinical studies have observed that SSCs may cause less wear on opposing enamel compared to zirconia crowns in vivo, likely because of their lower hardness contrast with natural teeth [ 26 ]. Therefore, SSCs may offer a compromise: moderate surface stability combined with ductile resilience. Overall, the three crown systems illustrate how enhancing hardness often compromises toughness—zirconia resists wear, BioFlx absorbs stress, and SSCs maintain an intermediate performance. BioFlx and SSC exhibited the highest fracture resistance, with mean loads far exceeding physiologic pediatric bite forces reported across primary and mixed dentitions, supporting a comfortable mechanical safety margin [ 27 ]. The superior performance of BioFlx is consistent with in-vitro data showing that BioFlx crowns withstand higher loads than zirconia after thermomechanical aging; this behavior is attributable to their viscoelastic polymer network, which redistributes stress through elastic/plastic deformation rather than brittle crack propagation [ 28 , 29 ]. In contrast, zirconia’s lower fracture loads in our study align with prior work demonstrating zirconia’s sensitivity to stress concentration and microcrack growth despite excellent surface hardness; inter-brand differences can also reflect compositional and processing variables (e.g., translucency grade, grain size, sintering), which modulate strength and toughness [ 30 , 31 ]. Notably, multiple investigations on pediatric crowns converge on the same pattern: polymer-based or metal crowns tend to absorb functional loads more effectively, whereas zirconia prioritizes surface stability over energy absorption. Clinically, all tested materials withstood forces well above the maximum bite loads reported for children, which typically range from several dozen to a few hundred newtons depending on dentition stage. Therefore, catastrophic failure under normal function appears unlikely, and material selection should focus on balancing toughness and stress absorption with surface durability and esthetics [ 32 , 33 ]. The experimental design of this study was structured to provide a realistic simulation of intraoral conditions with high reproducibility. Standardized thermomechanical aging—combining 10,000 thermal cycles between 5°C and 55°C with 150,000 chewing cycles at 50 N—approximates about one year of clinical function for primary molars, as supported by validated laboratory protocols [ 19 ]. All crowns were cemented on 3D-printed resin dies with mechanical properties resembling dentin, a method shown to provide reliable load distribution compared with natural tooth substrates. The use of separate specimen sets for wear and fracture testing prevented cumulative fatigue effects, ensuring that mechanical degradation did not bias fracture outcomes—a refinement rarely addressed in earlier pediatric crown studies [ 20 ]. Although such in-vitro simulations cannot fully reproduce the complex biological environment of the oral cavity, their high level of standardization provides valuable insight into the comparative mechanical behavior of emerging materials under clinically relevant stresses. From a clinical standpoint, all tested crowns demonstrated fracture resistance values far exceeding pediatric masticatory forces, indicating sufficient durability for routine use [ 32 ]. Therefore, clinical selection should be based not solely on strength but on the balance between esthetics, resilience, and biological compatibility. BioFlx crowns may be advantageous in situations requiring conservative preparation or enhanced shock absorption, as their viscoelastic behaviour dissipates stress and minimises tooth reduction, potentially improving comfort and long-term restoration performance. Nevertheless, considering their relatively higher surface wear under chewing forces, clinicians should exercise caution when using BioFlx crowns in cases involving high occlusal load—such as the Hall technique—or in children presenting with parafunctional habits like bruxism, where increased material wear may occur [ 17 ]. Zirconia crowns remain preferable when esthetics and color stability are prioritized, though their brittleness and limited adjustability must be considered [ 30 ]. SSCs continue to serve as a durable, cost-effective benchmark for high-caries-risk children and heavy occlusal loading scenarios [ 2 ]. Despite strict standardization, this in-vitro study has inherent constraints. First, the chewing simulation applied a constant, predominantly vertical load, which cannot fully reproduce the complex, multiaxial and time-dependent fatigue that dental materials experience intraorally; fatigue mechanisms and failure modes can evolve under different cyclic conditions and geometries [ 34 ]. Second, the aging regimen approximates about one year of service; however, longer-term degradation pathways—hydrolytic softening, water sorption, temperature-pH fluctuations and enzymatic effects—were not captured. Third, crowns were tested on standardized resin/epoxy analogs rather than natural teeth; while common in laboratory studies, such substrates differ from dentin in moisture and structure and can influence stress distribution and fracture behavior [ 35 ]. Fourth, only one loading configuration and one laboratory protocol were evaluated; prior work shows that fabrication/processing variables and cyclic loading history can materially alter fracture resistance outcomes [ 36 ]. Finally, we focused on mechanical endpoints (wear, fracture) and did not assess other clinically relevant outcomes (e.g., marginal adaptation, color stability, plaque accumulation, gingival response); future long-term clinical studies are needed to validate laboratory rankings under real oral conditions [ 37 ]. Building upon previous investigations that evaluated individual or pairwise comparisons under separate aging conditions, the present study implemented a fully standardized dual thermomechanical protocol on independently aged specimen sets to eliminate cumulative fatigue bias. Furthermore, this research concurrently compared two zirconia systems, stainless steel, and the novel polymer-based BioFlx crowns within the same experimental framework using 3D-printed dentin-analog dies and uniform cementation parameters. This comprehensive design enables a more valid assessment of material-dependent mechanical behavior, highlighting BioFlx as a promising intermediate option bridging the rigidity of zirconia and the ductility of stainless steel crowns. Within the limitations of this in-vitro study, all tested crown systems exhibited adequate mechanical performance for primary molars. Zirconia crowns demonstrated superior wear resistance but lower fracture strength, whereas BioFlx crowns showed the opposite trend due to their viscoelastic nature. Stainless steel crowns provided a balanced performance between strength and resilience. Therefore, clinical selection should consider each material’s inherent balance of esthetics, strength, and resilience rather than relying solely on mechanical performance outcomes. Clinical Significance • BioFlx crowns demonstrated fracture resistance comparable to stainless steel crowns while requiring less tooth reduction, offering a conservative and stress-absorbing alternative that may enhance comfort and clinical efficiency in paediatric full-coverage restorations. • Zirconia crowns exhibited the lowest surface wear but significantly lower fracture strength than both BioFlx and stainless steel crowns, underscoring the importance of balancing esthetics with mechanical durability in clinical selection. • Stainless steel crowns remain a reliable benchmark for high-caries-risk children and heavy occlusal loading conditions; however, when high esthetic demand or minimally invasive preparation is desired, BioFlx crowns may provide a viable intermediate option, though attention to occlusal wear is advised in Hall-technique and bruxism cases. Declarations Funding: No specific grant from any funding agency in the public, commercial, or not-for-profit sectors was received. Conflict of interest: The authors declare no potential conflicts of interest. Ethical approval Not applicable. This in-vitro study did not involve human participants or animals. However, all procedures were performed in accordance with institutional guidelines and the principles of the Declaration of Helsinki. Consent for publication: Not applicable. Clinical trial number: Not applicable. Consent to Participate: Not applicable. Authors’ contributions: Conceptualization – A.A.A., M.C.K., H.S.; Methodology – A.A.A., D.Ö.Y., H.S.; Data Collection – D.Ö.Y., B.G.; Statistical Analysis – M.C.K.; Writing – Original Draft – A.A.A.; Writing – Review & Editing – A.A.A., M.C.K. All authors have read and approved the final manuscript. References Chi DL, Scott JM. Added sugar and dental caries in children: a scientific update and future steps. 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Two-body wear and fracture behaviour of an experimental paediatric composite crown in comparison to zirconia and stainless steel crowns dependent on the cementation mode. Dent Mater. 2021;37(2):264–71. 10.1016/j.dental.2020.11.010 . Lath T, Rathi N, Mehta V, Mopagar VP, Patil RU, Hugar S et al. Evaluation of stress generation in core build up-material of mutilated primary teeth: a comparative finite element analysis between BioFlx, stainless steel and zirconia crowns. J Clin Pediatr Dentistry. 2024;48(6). Al-Haj Ali SN. BioFlx Pediatric Crowns: Current Evidence on Clinical Outcomes and Material Properties. Children. 2025;12(10):1281. Kongkiatkamon S, Booranasophone K, Tongtaksin A, Kiatthanakorn V, Rokaya D. Comparison of Fracture Load of the Four Translucent Zirconia Crowns. Molecules. 2021;26(17). 10.3390/molecules26175308 . Morresi AL, D'Amario M, Capogreco M, Gatto R, Marzo G, D'Arcangelo C, et al. Thermal cycling for restorative materials: Does a standardized protocol exist in laboratory testing? A literature review. J Mech Behav Biomed Mater. 2014;29:295–308. https://doi.org/10.1016/j.jmbbm.2013.09.013 . Kale YJ, Deshmukh NN, Dadpe MV, Dahake PT, Kendre SB. Evaluation of color stability, wear, and fracture resistance of preformed Bioflx and zirconia crowns: An in vitro study. J Indian Soc Pedod Prev Dentistry. 2025;43(1):143–51. 10.4103/jisppd.jisppd_485_24 . Krejci I, Lutz F, Reimer M. Wear of CAD/CAM ceramic inlays: restorations, opposing cusps, and luting cements. Quintessence Int. 1994;25(3):199–207. Metwally NM, Elshenawy EA, Elghareb LA. Surface roughness and wear performance of Bioflx versus stainless-steel primary crowns (an in-vitro study). BMC Oral Health. 2025;25(1):343. 10.1186/s12903-025-05655-6 . Ghabchi B, Mavi F, Çömlekoğlu E, Saklakoğlu IE, Uzel I. Wear behavior of CAD-CAM zirconia, ceramic, and 3D printed nano-hybrid resin crowns for the restoration of primary and permanent molars: An in vitro study. J Prosthet Dent. 2025;134(1):177. https://doi.org/10.1016/j.prosdent.2025.02.040 . .e1-.e8 . Alzanbaqi SD, Alogaiel RM, Alasmari MA, Al Essa AM, Khogeer LN, Alanazi BS, et al. Zirconia Crowns for Primary Teeth: A Systematic Review and Meta-Analyses. Int J Environ Res Public Health. 2022;19(5). 10.3390/ijerph19052838 . Abhay SS, Ganapathy D, Veeraiyan DN, Ariga P, Heboyan A, Amornvit P, et al. Wear Resistance, Color Stability and Displacement Resistance of Milled PEEK Crowns Compared to Zirconia Crowns under Stimulated Chewing and High-Performance Aging. Polym (Basel). 2021;13(21). 10.3390/polym13213761 . Amer DM, Abdellatif AM. In vivo evaluation of the enamel wear of primary molar against four types of crowns using the intra-oral scanner. BMC Oral Health. 2024;24(1):1438. 10.1186/s12903-024-05206-5 . El Hayek JE, Tohme H, Nasr L, El Hachem R, Kabbani N, McHayleh NF. Fracture strength of preformed zirconia crown and CAD-CAM zirconia, ceramic, and hybrid composite crowns for the restoration of primary molars: An in vitro study. Int J Paediatr Dent. 2024;34(6):721–8. 10.1111/ipd.13171 . Çakmak G, Donmez MB, Molinero-Mourelle P, Kahveci Ç, Abou-Ayash S, Peutzfeldt A, et al. Fracture resistance of additively or subtractively manufactured resin-based definitive crowns: Effect of restorative material, resin cement, and cyclic loading. Dent Mater. 2024;40(7):1072–7. https://doi.org/10.1016/j.dental.2024.05.020 . Pareek P, Tirupathi S, Kumari K, Afnan L. Antagonistic Primary Tooth Wear Caused by Opposing BioFLX(®), Zirconia, and Stainless Steel Crowns: Chewing Cycle Simulation Study. Int J Clin Pediatr Dent. 2025;18(7):784–91. 10.5005/jp-journals-10005-3161 . Abushanan A, Sharanesha RB, Aljuaid B, Alfaifi T, Aldurayhim A. Fracture Resistance of Primary Zirconia Crowns: An In Vitro Study. Child (Basel). 2022;9(1). 10.3390/children9010077 . Zhang F, Reveron H, Spies BC, Van Meerbeek B, Chevalier J. Trade-off between fracture resistance and translucency of zirconia and lithium-disilicate glass ceramics for monolithic restorations. Acta Biomater. 2019;91:24–34. 10.1016/j.actbio.2019.04.043 . Owais AI, Shaweesh M, Abu Alhaija ES. Maximum occusal bite force for children in different dentition stages. Eur J Orthod. 2013;35(4):427–33. 10.1093/ejo/cjs021 . Jayakumar P, FelsyPremila G, Muthu MS, Kirubakaran R, Panchanadikar N, Al-Qassar SS. Bite force of children and adolescents: a systematic review and meta-analysis. J Clin Pediatr Dent. 2023;47(3):39–53. 10.22514/jocpd.2023.022 . Zhang Y, Sailer I, Lawn BR. Fatigue of dental ceramics. J Dent. 2013;41(12):1135–47. 10.1016/j.jdent.2013.10.007 . Szczesio-Wlodarczyk A, Sokolowski J, Kleczewska J, Bociong K. Ageing of Dental Composites Based on Methacrylate Resins-A Critical Review of the Causes and Method of Assessment. Polym (Basel). 2020;12(4). 10.3390/polym12040882 . Refaie A, Bourauel C, Fouda AM, Keilig L, Singer L. The effect of cyclic loading on the fracture resistance of 3D-printed and CAD/CAM milled zirconia crowns-an in vitro study. Clin Oral Investig. 2023;27(10):6125–33. 10.1007/s00784-023-05229-2 . Kruzic JJ, Arsecularatne JA, Tanaka CB, Hoffman MJ, Cesar PF. Recent advances in understanding the fatigue and wear behavior of dental composites and ceramics. J Mech Behav Biomed Mater. 2018;88:504–33. https://doi.org/10.1016/j.jmbbm.2018.08.008 . Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 08 May, 2026 Reviews received at journal 04 May, 2026 Reviewers agreed at journal 29 Apr, 2026 Reviews received at journal 01 Mar, 2026 Reviewers agreed at journal 31 Jan, 2026 Reviews received at journal 24 Jan, 2026 Reviewers agreed at journal 23 Jan, 2026 Reviewers invited by journal 23 Jan, 2026 Editor invited by journal 02 Jan, 2026 Editor assigned by journal 21 Nov, 2025 Submission checks completed at journal 21 Nov, 2025 First submitted to journal 04 Nov, 2025 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. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8026149","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":579755399,"identity":"4a7cb7c7-87e1-45d0-bb3b-bd2e9452ee40","order_by":0,"name":"Alp Abidin Ateşçi","email":"data:image/png;base64,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","orcid":"","institution":"Beykent University","correspondingAuthor":true,"prefix":"","firstName":"Alp","middleName":"Abidin","lastName":"Ateşçi","suffix":""},{"id":579755400,"identity":"7ebff6e9-e103-4b4d-83fa-cee4e79b4b38","order_by":1,"name":"Dilek Özge Yılmaz","email":"","orcid":"","institution":"Beykent University","correspondingAuthor":false,"prefix":"","firstName":"Dilek","middleName":"Özge","lastName":"Yılmaz","suffix":""},{"id":579755403,"identity":"422f6a67-3bf7-4acf-9d24-60dc07294647","order_by":2,"name":"Berk Gergit","email":"","orcid":"","institution":"Atatürk University","correspondingAuthor":false,"prefix":"","firstName":"Berk","middleName":"","lastName":"Gergit","suffix":""},{"id":579755404,"identity":"a23c4ad3-b7dd-4340-aa3a-646c3eecc38a","order_by":3,"name":"Munevver Coruh Kilic","email":"","orcid":"","institution":"Biruni University","correspondingAuthor":false,"prefix":"","firstName":"Munevver","middleName":"Coruh","lastName":"Kilic","suffix":""},{"id":579755406,"identity":"1d0443d5-4740-490e-8689-702b86efd5ad","order_by":4,"name":"Huseyin Simsek","email":"","orcid":"","institution":"Ordu University","correspondingAuthor":false,"prefix":"","firstName":"Huseyin","middleName":"","lastName":"Simsek","suffix":""}],"badges":[],"createdAt":"2025-11-04 08:08:09","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8026149/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8026149/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":101363055,"identity":"2ecba8a7-ca57-444e-9ce1-fad94f633ae0","added_by":"auto","created_at":"2026-01-29 00:33:14","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":3719225,"visible":true,"origin":"","legend":"\u003cp\u003eDual-function thermocycling and chewing simulation unit\u003c/p\u003e\n\u003cp\u003eThermocycling and chewing simulation were simultaneously performed using a dual-function thermocycling and chewing simulation unit to replicate intraoral conditions. Specimens underwent 10,000 thermal cycles between 5 °C and 55 °C and 150,000 chewing cycles at a 50 N load, corresponding to approximately one year of clinical function for primary molars.\u003c/p\u003e","description":"","filename":"Figure1.tiff.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8026149/v1/1c33c0a39e3a29c2326addb5.jpg"},{"id":101398409,"identity":"0fce857e-739f-4474-87e1-e7d2d8d7340f","added_by":"auto","created_at":"2026-01-29 09:41:19","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":766275,"visible":true,"origin":"","legend":"\u003cp\u003eFracture resistance testing setup\u003c/p\u003e\n\u003cp\u003eFracture resistance was evaluated using a universal testing machine (Instron, Model 2710-003, USA).\u003c/p\u003e","description":"","filename":"Figure2.tiff.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8026149/v1/b5101f7a480feff91793cb3e.jpg"},{"id":101363052,"identity":"728ec58d-0fd5-4f93-a1ee-4080aa11ac1b","added_by":"auto","created_at":"2026-01-29 00:33:14","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":415568,"visible":true,"origin":"","legend":"\u003cp\u003eRepresentative Occlusal Surface Wear Patterns of Pediatric Crown Materials after Thermomechanical Aging and Chewing Simulation\u003c/p\u003e\n\u003cp\u003eRepresentative microscopic images illustrating occlusal surface wear after 10,000 thermal cycles and 150,000 chewing cycles under 50 N load.\u003cbr\u003e\nFrom left to right:\u003cbr\u003e\n(A) \u003cstrong\u003eStainless steel crown (SSC)\u003c/strong\u003e showing moderate polishing marks with limited material loss;\u003cbr\u003e\n(B) \u003cstrong\u003eBioFlx crown\u003c/strong\u003e displaying pronounced wear facets, surface deformation and perforation consistent with viscoelastic plasticity;\u003cbr\u003e\n(C) \u003cstrong\u003eProfZr zirconia crown\u003c/strong\u003e exhibiting shallow wear traces and smooth polished areas indicating high surface hardness;\u003cbr\u003e\n(D) \u003cstrong\u003eNuSmile zirconia crown\u003c/strong\u003e revealing minimal wear and intact glaze layer.\u003cbr\u003e\nZirconia crowns demonstrated the lowest wear scores (p \u0026lt; 0.05), while BioFlx exhibited the highest, consistent with quantitative wear results.\u003c/p\u003e","description":"","filename":"Figure3.tiff.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8026149/v1/25f1b7b90fae83e288d2d33c.jpg"},{"id":101363053,"identity":"9e55f001-2fbe-4e09-9071-7fca66a7529e","added_by":"auto","created_at":"2026-01-29 00:33:14","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":404113,"visible":true,"origin":"","legend":"\u003cp\u003eRepresentative post-fracture appearances of tested pediatric crown types after thermomechanical aging and compressive loading\u003c/p\u003e\n\u003cp\u003eRepresentative occlusal views illustrating typical fracture and deformation patterns of the tested prefabricated crown systems after thermomechanical aging and fracture resistance testing.\u003cbr\u003e\n(A) \u003cem\u003eStainless steel crown (SSC)\u003c/em\u003e showing ductile surface deformation without complete structural failure.\u003cbr\u003e\n(B) \u003cem\u003eBioFlx polymer crown\u003c/em\u003e exhibiting surface abrasion and localized plastic deformation consistent with its viscoelastic nature.\u003c/p\u003e\n\u003cp\u003e(C) \u003cem\u003eProfZr zirconia crown\u003c/em\u003e revealing marginal and cohesive cracks within the crown body, reflecting lower fracture toughness compared with the other groups.\u003cbr\u003e\n(D) \u003cem\u003eNuSmile zirconia crown\u003c/em\u003e demonstrating brittle fracture with radial crack propagation originating from the occlusal fossa.\u003c/p\u003e","description":"","filename":"Figure4.tiff.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8026149/v1/fa9ba959d8abd67ce9fe3ba2.jpg"},{"id":101751479,"identity":"8410ec63-3cde-484e-9781-823cfc6af7e1","added_by":"auto","created_at":"2026-02-03 10:20:39","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6110843,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8026149/v1/78a24acf-a5ff-4cf4-9d0e-255b1eff2174.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Can BioFlx Crowns Bridge the Gap Between Zirconia and Stainless Steel? An In-Vitro Comparative Analysis","fulltext":[{"header":"Background","content":"\u003cp\u003eDental caries remains the most prevalent chronic disease of childhood, disproportionately affecting primary molars due to their anatomical vulnerabilities and prolonged exposure in the oral cavity [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. When conventional restorative methods fall short in managing extensive decay, full-coverage crowns offer a reliable treatment modality. Stainless steel crowns (SSCs) have long been considered the gold standard in pediatric dentistry due to their high success rates and durability in high-risk populations [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Despite their functional excellence, the metallic appearance of SSCs has generated increasing parental demand for esthetically superior alternatives [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTo address this aesthetic shortcoming, prefabricated zirconia crowns (ZCs) have been introduced, offering natural tooth-like translucency and excellent biocompatibility. Clinical studies have demonstrated their favorable gingival response, high parental satisfaction, and excellent longevity\u0026mdash;retention rates ranging from 76% to 94% over 3 years have been reported [\u003cspan additionalcitationids=\"CR7\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. However, the use of ZCs often requires more aggressive tooth preparation and presents challenges in terms of marginal adaptability, chairside adjustability, and may compromise pulp vitality [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn response to the limitations of both SSCs and ZCs, polymer-based esthetic crowns such as BioFlx have emerged as a novel alternative. BioFlx crowns are fabricated from a high-impact hybrid resin polymer that is metal-free and Bis-GMA-free, providing enhanced flexibility, shock absorption, and color stability. These innovative crowns aim to combine aesthetic appeal with mechanical resilience and minimal invasiveness. Designed using flexible high-performance polymers, BioFlx crowns are characterized by their elasticity, conservative preparation requirements, and easy intraoral manipulation. Emerging in-vitro data indicate that high-performance polymer crowns can withstand clinically relevant occlusal loads while preserving tooth structure, due to their elastic modulus and simplified preparation requirements [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eEvaluation of restorative materials under simulated intraoral conditions remains a cornerstone of preclinical validation. Chewing simulators, in conjunction with thermocycling protocols, are frequently employed to replicate one year or more of oral aging. These methodologies are critical in assessing long-term behavior, particularly in pediatric populations where mechanical loads and thermal fluctuations impact restoration integrity [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Kessler et al. demonstrated that crown material and cementation type significantly influence wear and fracture resistance, further emphasizing the importance of holistic evaluation [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn this context, BioFlx crowns warrant rigorous investigation to elucidate their durability, fracture behavior and performance under standardized thermomechanical aging. While studies on zirconia crowns have validated their mechanical strength and clinical longevity, comparative data on BioFlx and other prefabricated esthetic crowns remain limited [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. For instance, an in-vitro finite element analysis comparing BioFlx crowns, prefabricated zirconia crowns and stainless steel crowns found that the BioFlx models exhibited notably lower stress values under axial and angled loading scenarios, suggesting potential biomechanical advantage, yet the authors highlighted the absence of long-term empirical data[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Moreover, a recent in-vitro study directly comparing BioFlx, zirconia and SSCs reported that while BioFlx crowns achieved intermediate fracture resistance and performed favourably in surface wear and retentive strength assessments, the overall dataset remains small and brand-dependent [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eGiven this, there is a clear need for further standardized preclinical and clinical investigations of BioFlx crowns to determine whether their aesthetic and procedural advantages are matched by equivalent or superior mechanical performance in the paediatric restorative context. To date, no published research has concurrently examined BioFlx, zirconia, and stainless steel crowns under a unified thermomechanical ageing protocol employing independent specimen cohorts for wear and fracture analyses. Such a dual-parameter in-vitro framework provides a more clinically meaningful assessment of material-dependent fatigue behaviour by eliminating cumulative degradation bias and closely approximating the functional stresses encountered in paediatric mastication.\u003c/p\u003e \u003cp\u003eThis in-vitro study evaluated and compared the wear behavior and fracture resistance of four prefabricated crown types for primary mandibular second molars\u0026mdash;two zirconia systems (NuSmile, ProfZr), a polymer-based crown (BioFlx), and stainless-steel crowns (SSC)\u0026mdash;after standardized thermomechanical aging. The null hypothesis was that no statistically significant differences would exist among the crown types for either outcome.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSample Size Calculation and Grouping\u003c/h2\u003e \u003cp\u003eThe sample size was determined in accordance with previous in-vitro study that employed comparable thermocycling aging protocols in the evaluation of pediatric crowns [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. A priori power analysis was performed using G*Power software (version 3.1; D\u0026uuml;sseldorf, Germany), with an effect size (f)\u0026thinsp;=\u0026thinsp;0.55, a significance level (α)\u0026thinsp;=\u0026thinsp;0.05, and a statistical power (1 \u0026ndash; β)\u0026thinsp;=\u0026thinsp;0.80. This calculation indicated a minimum of ten specimens per group, yielding a total of eighty prefabricated crowns for mandibular second primary molars, distributed evenly among four material categories (n\u0026thinsp;=\u0026thinsp;10 per group for each testing modality).\u003c/p\u003e \u003cp\u003eTo preserve methodological independence, the specimens were divided into two separate cohorts (n\u0026thinsp;=\u0026thinsp;40 each). Both cohorts underwent identical thermocycling to simulate intraoral temperature fluctuations. In the cohort assigned for wear evaluation, chewing simulation was additionally performed in the same device. The crowns were divided into four groups according to material type:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eGroup 1\u003c/b\u003e: NuSmile\u0026reg; zirconia crowns (NuSmile, Houston, TX, USA)\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eGroup 2\u003c/b\u003e: ProfZr\u0026reg; zirconia crowns (Prof Teknoloji, Erzurum, T\u0026uuml;rkiye)\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eGroup 3\u003c/b\u003e: BioFlx\u0026reg; polymer crowns (Kids-e-dental, Mumbai, India)\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e \u003cb\u003eGroup 4\u003c/b\u003e: Kids Crown\u0026reg; stainless steel crowns (Shinhung Company Ltd., Seoul, South Korea)\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eCrown Preparation and Standardization\u003c/h3\u003e\n\u003cp\u003eTo ensure standardization across all samples, a mandibular second primary molar was digitally designed using Exocad software (Exocad GmbH, Germany). A single calibrated pediatric dentist performed all digital preparations on the master design; STL files were locked prior to printing to prevent unintended parameter drift. For the pediatric zirconia groups, the digital preparation included 2.0 mm occlusal reduction, 1.5 mm proximal clearance, and 1.0 mm circumferential preparation with rounded internal angles and complete edge bevelling. For BioFlx and stainless steel crowns, the occlusal reduction was limited to 1.5 mm, with identical proximal and circumferential parameters.\u003c/p\u003e\n\u003ch3\u003eDie Fabrication and Crown Cementation\u003c/h3\u003e\n\u003cp\u003eEighty standardized resin dies were fabricated using a 3D printer with NextDent Model 2.0 resin (Vertex-Dental B.V., Soesterberg, The Netherlands), a material with mechanical properties resembling natural dentin. During cementation, a constant load of 5 kg was applied for 3 minutes to ensure uniform seating pressure, following the methodology described by Kessler et al. (2021) [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. A conventional glass ionomer luting cement (Fuji I\u0026reg;, GC Corporation, Tokyo, Japan) was used for crown cementation following the manufacturer\u0026rsquo;s protocol. Excess cement was removed, and specimens were stored in distilled water at 37\u0026deg;C for 24 hours to allow for complete setting of the cement [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAll specimens were embedded in cylindrical blocks of methyl methacrylate resin (Technovit 4000, Heraeus Kulzer, Germany). This embedding method was chosen because of its elastic modulus, which closely approximates that of human alveolar bone, and is recommended for simulating periodontal support in in vitro loading conditions [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e\n\u003ch3\u003eThermocycling, Chewing Simulation, and Wear Assessment\u003c/h3\u003e\n\u003cp\u003eThermocycling was performed in accordance with ISO 11405 recommendations to simulate intraoral temperature fluctuations. Each specimen underwent 10,000 cycles between 5\u0026deg;C and 55\u0026deg;C (dwell time: 25 s; transfer time: 10 s) using a dual-function thermocycling and chewing simulation unit (Model ACS-8, Analitik Medikal Ltd., Gaziantep, T\u0026uuml;rkiye). This protocol represents approximately one year of thermomechanical aging in vivo (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e1\u003c/span\u003e) [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSimultaneously, the same unit performed a standardized two-body wear protocol using a chewing simulator (Model ACS-8, Analitik Medikal Ltd.,Gaziantep, T\u0026uuml;rkiye) for 150,000 loading cycles under 50 N vertical force at 1.2 Hz. The samples were mounted opposing a 4 mm steatite sphere antagonist attached to a vertically moving bar. Each loading stroke started from the functional cusp and slid approximately 0.3 mm toward the central fossa, simulating pediatric masticatory movement. The loading and lifting speeds were 20 mm/s and 60 mm/s, respectively, with continuous water rinsing at 37\u0026deg;C throughout the test. This setup followed previously validated protocols for two-body wear simulation in primary molar crowns [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAfter chewing simulation, each specimen was examined under a stereomicroscope (\u0026times;20 magnification) to assess occlusal wear. The amount of visible wear was scored using a 4-point wear index (0\u0026thinsp;=\u0026thinsp;no visible wear, 1\u0026thinsp;=\u0026thinsp;slight polishing marks, 2\u0026thinsp;=\u0026thinsp;moderate wear facets, 3\u0026thinsp;=\u0026thinsp;severe material loss) previously described in the studies by Krejci and Metwally [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e Two independent blinded evaluators performed all assessments, and inter-observer reliability was calculated using Cohen\u0026rsquo;s kappa coefficient (κ\u0026thinsp;=\u0026thinsp;0.87, indicating excellent agreement). Mean wear scores were used for statistical analysis.\u003c/p\u003e\n\u003ch3\u003eFracture Resistance Testing\u003c/h3\u003e\n\u003cp\u003eAfter completion of the thermocycling protocol of the second set of crowns, specimens were subjected to fracture testing. Each crown was embedded in self-cured acrylic resin blocks, leaving 1 mm of the cervical margin exposed to simulate clinical support, and positioned vertically in a universal testing machine (Instron 2710-003, Instron Corp., Norwood, MA, USA) equipped with a 5-kN load cell (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e2\u003c/span\u003e.). Compressive loading was applied at the center of the occlusal fossa using a 7-mm diameter stainless-steel indenter oriented perpendicular to the crown\u0026rsquo;s long axis. A 0.5-mm thick tin foil sheet was interposed between the indenter and the occlusal surface to ensure uniform stress distribution and prevent localized stress peaks. The test was performed under dry conditions at room temperature (approximately 23\u0026deg;C). The load was applied at a crosshead speed of 1 mm/min until fracture occurred. Fracture was defined as a sudden drop in the load\u0026ndash;displacement curve, often accompanied by an audible crack or visible segment separation, ensuring an objective and reproducible failure criterion. The maximum load at fracture (in Newtons) was automatically recorded by the testing software. Each fractured specimen was subsequently inspected under a stereomicroscope (\u0026times;20 magnification) to categorize the failure pattern as cohesive (within the crown), adhesive (at the cement\u0026ndash;crown interface), or mixed.\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eStatistical Analysis\u003c/h2\u003e \u003cp\u003eStatistical analyses were performed using IBM SPSS Statistics for Windows, version 26.0 (IBM Corp., Armonk, NY, USA). Descriptive results were expressed as mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation (SD). Data normality was assessed with the Shapiro\u0026ndash;Wilk test and homogeneity of variance with Levene\u0026rsquo;s test. Since both assumptions were met (p\u0026thinsp;\u0026gt;\u0026thinsp;0.05), fracture resistance values were analyzed using one-way analysis of variance (ANOVA) followed by Tukey\u0026rsquo;s honestly significant difference (HSD) post-hoc test for pairwise comparisons. The effect size (η\u0026sup2;) and 95% confidence intervals (CIs) were calculated to indicate the magnitude and precision of differences.\u003c/p\u003e \u003cp\u003eAs wear data did not follow a normal distribution, non-parametric tests were applied. Inter-group comparisons were performed using the Kruskal\u0026ndash;Wallis test, and post-hoc pairwise comparisons were adjusted with the Dunn\u0026ndash;Bonferroni correction. Effect size for the Kruskal\u0026ndash;Wallis test was expressed as epsilon-squared (ε\u0026sup2;). The level of statistical significance was set at α\u0026thinsp;=\u0026thinsp;0.05 for all analyses.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eWear Assessment\u003c/h2\u003e \u003cp\u003eAfter one year of simulated thermomechanical aging and chewing cycles, all crowns exhibited observable occlusal wear, with statistically significant differences among the groups (Kruskal\u0026ndash;Wallis, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05). The mean wear scores (0\u0026ndash;3 scale) were as follows: NuSmile zirconia\u0026thinsp;=\u0026thinsp;1.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3, ProfZr zirconia\u0026thinsp;=\u0026thinsp;1.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4, SSC\u0026thinsp;=\u0026thinsp;1.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4, and BioFlx\u0026thinsp;=\u0026thinsp;2.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eVisual wear scores (0\u0026ndash;3 scale) of the tested crowns after thermomechanical aging and chewing simulation\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" 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 \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCrown type\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMean wear score\u0026thinsp;\u0026plusmn;\u0026thinsp;SD\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMinimum\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMaximum\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ep vs NuSmile\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003ep vs ProfZr\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003ep vs SSC\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003ep vs BioFlx\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNuSmile\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e1.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.68\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.04*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.01**\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eProfZr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e1.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.68\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.04*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.01**\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSSC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e1.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.04*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.04*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.03*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBioFlx\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e2.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.01**\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.01**\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.03*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"8\"\u003eData are expressed as mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation (SD). Pairwise comparisons were performed using the Dunn\u0026ndash;Bonferroni post-hoc test following the Kruskal\u0026ndash;Wallis analysis. *p\u0026thinsp;\u0026lt;\u0026thinsp;0.05; **p\u0026thinsp;\u0026lt;\u0026thinsp;0.01 indicate statistically significant and highly significant differences, respectively. Lower scores correspond to higher wear resistance.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003ePairwise comparisons using the Dunn\u0026ndash;Bonferroni test showed no significant difference between the two zirconia groups (NuSmile\u0026ndash;ProfZr, p\u0026thinsp;=\u0026thinsp;0.68), indicating comparable wear resistance. Both zirconia crowns exhibited significantly lower wear scores than SSC (p\u0026thinsp;=\u0026thinsp;0.04) and BioFlx (p\u0026thinsp;=\u0026thinsp;0.01). SSC crowns showed intermediate wear resistance, presenting significantly lower wear than BioFlx (p\u0026thinsp;=\u0026thinsp;0.03) but higher wear than both zirconia crowns (p\u0026thinsp;=\u0026thinsp;0.04).\u003c/p\u003e \u003cp\u003eMicroscopic examination revealed smoother, polished surfaces with minimal abrasion in zirconia crowns, shallow wear facets in SSC, and more distinct material loss in BioFlx specimens (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eFracture Resistance\u003c/h2\u003e \u003cp\u003eThe Shapiro\u0026ndash;Wilk test confirmed that all data sets followed a normal distribution (p\u0026thinsp;\u0026gt;\u0026thinsp;0.05), and Levene\u0026rsquo;s test indicated homogeneity of variances among groups (p\u0026thinsp;\u0026gt;\u0026thinsp;0.05). Therefore, a one-way analysis of variance (ANOVA) was performed to compare the mean fracture resistance values of the tested crowns. The ANOVA revealed statistically significant differences among the four groups (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001).\u003c/p\u003e \u003cp\u003eThe mean fracture loads (\u0026plusmn;\u0026thinsp;SD) were BioFlx: 3148.4\u0026thinsp;\u0026plusmn;\u0026thinsp;643.4 N, SSC (stainless steel crowns): 2703.4\u0026thinsp;\u0026plusmn;\u0026thinsp;243.1 N, NuSmile zirconia crowns: 1076.1\u0026thinsp;\u0026plusmn;\u0026thinsp;186.9 N, and ProfZr zirconia crowns: 565.5\u0026thinsp;\u0026plusmn;\u0026thinsp;159.6 N (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\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\u003eMean fracture resistance values (N) of the tested crowns and pairwise intergroup comparisons according to Tukey\u0026rsquo;s HSD test (α\u0026thinsp;=\u0026thinsp;0.05)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" 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 \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCrown type\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD (N)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMinimum\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMaximum\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c8\" namest=\"c5\"\u003e \u003cp\u003ep-values (Tukey HSD)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eBioFlx\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eSSC\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eNuSmile\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eProfZr\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eBioFlx\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e3148.4\u0026thinsp;\u0026plusmn;\u0026thinsp;643.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1835.30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4070.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.093\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001***\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\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\u003e\u003cb\u003eSSC\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e2703.4\u0026thinsp;\u0026plusmn;\u0026thinsp;243.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2445.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3210.31\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.093\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001***\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\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\u003e\u003cb\u003eNuSmile\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e1076.1\u0026thinsp;\u0026plusmn;\u0026thinsp;186.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e747.46\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1325.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001***\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001***\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e0.043*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eProfZr\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e565.5\u0026thinsp;\u0026plusmn;\u0026thinsp;159.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e367.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e808.32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001***\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.001***\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.043*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u0026mdash;\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"8\"\u003eComparison of mean fracture resistance (N) values among different prefabricated crown types. Data are presented as mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation (SD). Statistical comparisons were performed using one-way ANOVA followed by Tukey\u0026rsquo;s HSD post-hoc test (α\u0026thinsp;=\u0026thinsp;0.05). p\u0026thinsp;\u0026lt;\u0026thinsp;0.05; **p\u0026thinsp;\u0026lt;\u0026thinsp;0.01; ***p\u0026thinsp;\u0026lt;\u0026thinsp;0.001 indicate statistically significant differences.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eAccording to Tukey\u0026rsquo;s HSD post-hoc test, BioFlx exhibited significantly higher fracture resistance than both NuSmile (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) and ProfZr (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) crowns, while no statistically significant difference was found between BioFlx and SSC \u003cb\u003e(\u003c/b\u003ep\u0026thinsp;=\u0026thinsp;0.093\u003cb\u003e)\u003c/b\u003e. SSC crowns showed significantly higher values than NuSmile (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) and ProfZr (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001). Among the zirconia groups, NuSmile demonstrated significantly greater strength than ProfZr (p\u0026thinsp;=\u0026thinsp;0.043).\u003c/p\u003e \u003cp\u003eOverall, BioFlx and SSC crowns exhibited the highest fracture resistance, with no significant difference between them, whereas both zirconia crowns\u0026mdash;particularly ProfZr\u0026mdash;showed considerably lower performance (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eFailure patterns differed across materials and followed the expected trend for their mechanical behavior. Zirconia crowns predominantly exhibited cohesive failures within the crown material (NuSmile: 5/10, ProfZr: 6/10), reflecting their brittle nature. SSCs showed mainly mixed failures (8/10), consistent with their ductile, energy-absorbing behavior. BioFlx crowns demonstrated a higher proportion of mixed (5/10\u003cb\u003e)\u003c/b\u003e and adhesive (4/10) failures, indicating stress dissipation through the crown\u0026ndash;cement interface rather than catastrophic fracture (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Group distributions were compared using Fisher\u0026rsquo;s exact test due to small cell counts (α\u0026thinsp;=\u0026thinsp;0.05).\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\u003eFailure mode distribution among the tested crown types after fracture testing\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=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCrown type\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCohesive n (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAdhesive n (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMixed n (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003en\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eBioFlx\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1 (10%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4 (40%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5 (50%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eSSC\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0 (0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2 (20%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e8 (80%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eNuSmile\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5 (50%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2 (20%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3 (30%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eProfZr\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6 (60%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2 (20%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2 (20%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003eCohesive failure\u0026thinsp;=\u0026thinsp;fracture within the crown material; Adhesive failure\u0026thinsp;=\u0026thinsp;separation at the cement\u0026ndash;crown interface; Mixed failure\u0026thinsp;=\u0026thinsp;combination of cohesive and adhesive patterns observed under stereomicroscopic evaluation (\u0026times;20).\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eThis in-vitro investigation evaluated the wear behavior and fracture resistance of four prefabricated crown systems for primary mandibular second molars\u0026mdash;two zirconia crowns (NuSmile and ProfZr), a polymer-based crown (BioFlx), and a stainless-steel crown (SSC)\u0026mdash;following standardized thermomechanical aging. Statistically significant differences were identified among the tested materials, resulting in the rejection of the null hypothesis. Zirconia crowns exhibited the lowest wear scores, reflecting their superior surface hardness and abrasion resistance, whereas BioFlx crowns demonstrated the highest fracture resistance, comparable to SSCs. These findings highlight a fundamental mechanical balance: materials with higher surface hardness tend to exhibit increased brittleness, while those with greater elasticity can better absorb masticatory loads but are more susceptible to surface degradation. Collectively, the long-term performance of pediatric crowns appears to depend not solely on absolute strength, but on achieving an optimal equilibrium between rigidity and resilience under functional stresses.\u003c/p\u003e \u003cp\u003eZirconia crowns demonstrated the lowest occlusal wear scores among the tested materials, reflecting their superior hardness, surface stability, and resistance to micro-abrasion. This observation aligns with previous in-vitro reports showing that monolithic zirconia ceramics resist volumetric material loss under cyclic loading better than polymeric or resin-based materials [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. The glazed and polished surfaces of zirconia act as a barrier to micro-scratch formation and crack initiation, reducing progressive wear over time [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. However, the price of this high wear resistance is brittleness: under repeated loading, microcracks can propagate, but until catastrophic failure, the surface remains stable.\u003c/p\u003e \u003cp\u003eIn contrast, BioFlx crowns in this study showed the highest wear scores, indicating more pronounced material removal or deformation at occlusal surfaces. This result is consistent with findings from studies on BioFlx crowns, which report higher average material wear compared to zirconia while sometimes causing less wear on the antagonistic tooth [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. The mechanism behind this pattern may be the elastic and viscoelastic nature of the polymeric matrix: under load, the material may undergo slight plastic deformation or creep, absorbing energy and redistributing stresses rather than resisting wear completely. In effect, the crown surface may gradually wear, but the bulk structure avoids fracture by spreading the load [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eStainless-steel crowns (SSCs) occupied an intermediate position in wear performance: more wear than zirconia but less than BioFlx. Their metallic structure provides toughness and resistance against abrupt failure, but due to the harder opposing surfaces and potential micro-roughness, mild abrasion can occur over long cycles. Clinical studies have observed that SSCs may cause less wear on opposing enamel compared to zirconia crowns in vivo, likely because of their lower hardness contrast with natural teeth [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Therefore, SSCs may offer a compromise: moderate surface stability combined with ductile resilience. Overall, the three crown systems illustrate how enhancing hardness often compromises toughness\u0026mdash;zirconia resists wear, BioFlx absorbs stress, and SSCs maintain an intermediate performance.\u003c/p\u003e \u003cp\u003eBioFlx and SSC exhibited the highest fracture resistance, with mean loads far exceeding physiologic pediatric bite forces reported across primary and mixed dentitions, supporting a comfortable mechanical safety margin [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. The superior performance of BioFlx is consistent with in-vitro data showing that BioFlx crowns withstand higher loads than zirconia after thermomechanical aging; this behavior is attributable to their viscoelastic polymer network, which redistributes stress through elastic/plastic deformation rather than brittle crack propagation [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. In contrast, zirconia\u0026rsquo;s lower fracture loads in our study align with prior work demonstrating zirconia\u0026rsquo;s sensitivity to stress concentration and microcrack growth despite excellent surface hardness; inter-brand differences can also reflect compositional and processing variables (e.g., translucency grade, grain size, sintering), which modulate strength and toughness [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. Notably, multiple investigations on pediatric crowns converge on the same pattern: polymer-based or metal crowns tend to absorb functional loads more effectively, whereas zirconia prioritizes surface stability over energy absorption. Clinically, all tested materials withstood forces well above the maximum bite loads reported for children, which typically range from several dozen to a few hundred newtons depending on dentition stage. Therefore, catastrophic failure under normal function appears unlikely, and material selection should focus on balancing toughness and stress absorption with surface durability and esthetics [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe experimental design of this study was structured to provide a realistic simulation of intraoral conditions with high reproducibility. Standardized thermomechanical aging\u0026mdash;combining 10,000 thermal cycles between 5\u0026deg;C and 55\u0026deg;C with 150,000 chewing cycles at 50 N\u0026mdash;approximates about one year of clinical function for primary molars, as supported by validated laboratory protocols [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. All crowns were cemented on 3D-printed resin dies with mechanical properties resembling dentin, a method shown to provide reliable load distribution compared with natural tooth substrates. The use of separate specimen sets for wear and fracture testing prevented cumulative fatigue effects, ensuring that mechanical degradation did not bias fracture outcomes\u0026mdash;a refinement rarely addressed in earlier pediatric crown studies [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Although such in-vitro simulations cannot fully reproduce the complex biological environment of the oral cavity, their high level of standardization provides valuable insight into the comparative mechanical behavior of emerging materials under clinically relevant stresses.\u003c/p\u003e \u003cp\u003eFrom a clinical standpoint, all tested crowns demonstrated fracture resistance values far exceeding pediatric masticatory forces, indicating sufficient durability for routine use [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. Therefore, clinical selection should be based not solely on strength but on the balance between esthetics, resilience, and biological compatibility. BioFlx crowns may be advantageous in situations requiring conservative preparation or enhanced shock absorption, as their viscoelastic behaviour dissipates stress and minimises tooth reduction, potentially improving comfort and long-term restoration performance. Nevertheless, considering their relatively higher surface wear under chewing forces, clinicians should exercise caution when using BioFlx crowns in cases involving high occlusal load\u0026mdash;such as the Hall technique\u0026mdash;or in children presenting with parafunctional habits like bruxism, where increased material wear may occur [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Zirconia crowns remain preferable when esthetics and color stability are prioritized, though their brittleness and limited adjustability must be considered [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. SSCs continue to serve as a durable, cost-effective benchmark for high-caries-risk children and heavy occlusal loading scenarios [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDespite strict standardization, this in-vitro study has inherent constraints. First, the chewing simulation applied a constant, predominantly vertical load, which cannot fully reproduce the complex, multiaxial and time-dependent fatigue that dental materials experience intraorally; fatigue mechanisms and failure modes can evolve under different cyclic conditions and geometries [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. Second, the aging regimen approximates about one year of service; however, longer-term degradation pathways\u0026mdash;hydrolytic softening, water sorption, temperature-pH fluctuations and enzymatic effects\u0026mdash;were not captured. Third, crowns were tested on standardized resin/epoxy analogs rather than natural teeth; while common in laboratory studies, such substrates differ from dentin in moisture and structure and can influence stress distribution and fracture behavior [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. Fourth, only one loading configuration and one laboratory protocol were evaluated; prior work shows that fabrication/processing variables and cyclic loading history can materially alter fracture resistance outcomes [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. Finally, we focused on mechanical endpoints (wear, fracture) and did not assess other clinically relevant outcomes (e.g., marginal adaptation, color stability, plaque accumulation, gingival response); future long-term clinical studies are needed to validate laboratory rankings under real oral conditions [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eBuilding upon previous investigations that evaluated individual or pairwise comparisons under separate aging conditions, the present study implemented a fully standardized dual thermomechanical protocol on independently aged specimen sets to eliminate cumulative fatigue bias. Furthermore, this research concurrently compared two zirconia systems, stainless steel, and the novel polymer-based BioFlx crowns within the same experimental framework using 3D-printed dentin-analog dies and uniform cementation parameters. This comprehensive design enables a more valid assessment of material-dependent mechanical behavior, highlighting BioFlx as a promising intermediate option bridging the rigidity of zirconia and the ductility of stainless steel crowns.\u003c/p\u003e \u003cp\u003eWithin the limitations of this in-vitro study, all tested crown systems exhibited adequate mechanical performance for primary molars. Zirconia crowns demonstrated superior wear resistance but lower fracture strength, whereas BioFlx crowns showed the opposite trend due to their viscoelastic nature. Stainless steel crowns provided a balanced performance between strength and resilience. Therefore, clinical selection should consider each material\u0026rsquo;s inherent balance of esthetics, strength, and resilience rather than relying solely on mechanical performance outcomes.\u003c/p\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eClinical Significance\u003c/h2\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003e\u0026bull; BioFlx crowns demonstrated fracture resistance comparable to stainless steel crowns while requiring less tooth reduction, offering a conservative and stress-absorbing alternative that may enhance comfort and clinical efficiency in paediatric full-coverage restorations.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e\u0026bull; Zirconia crowns exhibited the lowest surface wear but significantly lower fracture strength than both BioFlx and stainless steel crowns, underscoring the importance of balancing esthetics with mechanical durability in clinical selection.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003e\u0026bull; Stainless steel crowns remain a reliable benchmark for high-caries-risk children and heavy occlusal loading conditions; however, when high esthetic demand or minimally invasive preparation is desired, BioFlx crowns may provide a viable intermediate option, though attention to occlusal wear is advised in Hall-technique and bruxism cases.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding:\u003c/strong\u003e\u003cbr\u003e\u0026nbsp;No specific grant from any funding agency in the public, commercial, or not-for-profit sectors was received.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest:\u003c/strong\u003e\u003cbr\u003e\u0026nbsp;The authors declare no potential conflicts of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical approval\u003c/strong\u003e\u003cbr\u003e\u0026nbsp;Not applicable. This in-vitro study did not involve human participants or animals. However, all procedures were performed in accordance with institutional guidelines and the principles of the Declaration of Helsinki.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication:\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eClinical trial number:\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Participate:\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors’ contributions:\u003c/strong\u003e\u003cbr\u003e\u0026nbsp;Conceptualization – A.A.A., M.C.K., H.S.;\u003cbr\u003e\u0026nbsp;Methodology – A.A.A., D.Ö.Y., H.S.;\u003cbr\u003e\u0026nbsp;Data Collection – D.Ö.Y., B.G.;\u003cbr\u003e\u0026nbsp;Statistical Analysis – M.C.K.;\u003cbr\u003e\u0026nbsp;Writing – Original Draft – A.A.A.;\u003cbr\u003e\u0026nbsp;Writing – Review \u0026amp; Editing – A.A.A., M.C.K.\u003cbr\u003e\u0026nbsp;All authors have read and approved the final manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eChi DL, Scott JM. 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The effect of cyclic loading on the fracture resistance of 3D-printed and CAD/CAM milled zirconia crowns-an in vitro study. Clin Oral Investig. 2023;27(10):6125\u0026ndash;33. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1007/s00784-023-05229-2\u003c/span\u003e\u003cspan address=\"10.1007/s00784-023-05229-2\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKruzic JJ, Arsecularatne JA, Tanaka CB, Hoffman MJ, Cesar PF. Recent advances in understanding the fatigue and wear behavior of dental composites and ceramics. J Mech Behav Biomed Mater. 2018;88:504\u0026ndash;33. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jmbbm.2018.08.008\u003c/span\u003e\u003cspan address=\"10.1016/j.jmbbm.2018.08.008\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":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":"prefabricated crowns, pediatric zirconia crown, BioFlx crown, stainless steel crown, occlusal wear, fracture resistance, thermomechanical aging","lastPublishedDoi":"10.21203/rs.3.rs-8026149/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8026149/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eBackground:\u003c/h2\u003e \u003cp\u003eEsthetic and durable crowns are essential for restoring extensively decayed primary molars. Zirconia and polymer-based alternatives to stainless steel crowns (SSCs) have been introduced, but comparative evidence on their mechanical behavior after aging is limited.\u003c/p\u003e\u003ch2\u003eAim:\u003c/h2\u003e \u003cp\u003eTo evaluate and compare the wear resistance and fracture strength of zirconia, polymer-based BioFlx, and SSCs for mandibular second primary molars after standardized thermomechanical aging.\u003c/p\u003e\u003ch2\u003eMethods:\u003c/h2\u003e \u003cp\u003eEighty prefabricated crowns (NuSmile zirconia, ProfZr zirconia, BioFlx polymer, and SSC; n\u0026thinsp;=\u0026thinsp;10 per group for wear and fracture testing) were cemented on standardized 3D-printed resin dies. All specimens underwent 10,000 thermal cycles between 5\u0026deg;C and 55\u0026deg;C; half were additionally subjected to 150,000 chewing cycles under 50 N before testing.\u003c/p\u003e\u003ch2\u003eResults:\u003c/h2\u003e \u003cp\u003eZirconia crowns exhibited the lowest wear scores (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05), while BioFlx and SSCs showed significantly higher fracture resistance than zirconia (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001). No significant difference occurred between BioFlx and SSCs (p\u0026thinsp;=\u0026thinsp;0.093). All materials exceeded physiological bite-force thresholds for children.\u003c/p\u003e\u003ch2\u003eConclusions:\u003c/h2\u003e \u003cp\u003eZirconia crowns provide superior wear resistance, whereas BioFlx and SSCs exhibit higher fracture resistance and stress absorption. BioFlx may represent a promising alternative for pediatric full-coverage restorations.\u003c/p\u003e","manuscriptTitle":"Can BioFlx Crowns Bridge the Gap Between Zirconia and Stainless Steel? An In-Vitro Comparative Analysis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-29 00:33:09","doi":"10.21203/rs.3.rs-8026149/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-05-08T06:08:25+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-04T11:10:25+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"180587170332839588641944006782516036976","date":"2026-04-29T09:42:07+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-01T23:54:53+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"252792133822809076889854793195830418363","date":"2026-02-01T01:08:01+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-24T10:09:36+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"135255231386534544804807127183256675010","date":"2026-01-23T09:46:28+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-01-23T07:08:56+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2026-01-02T07:07:16+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-21T10:52:20+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-11-21T10:52:05+00:00","index":"","fulltext":""},{"type":"submitted","content":"BMC Oral Health","date":"2025-11-04T07:52:52+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":"929cf99d-4cc5-494a-a4ee-c7ac5ef045f6","owner":[],"postedDate":"January 29th, 2026","published":true,"recentEditorialEvents":[{"type":"decision","content":"Revision requested","date":"2026-05-08T06:08:25+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-04T11:10:25+00:00","index":84,"fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-14T14:08:44+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-29 00:33:09","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8026149","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8026149","identity":"rs-8026149","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}