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

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.