Wear resistance and fracture strength of BioFlx, zirconia and stainless steel crowns for primary molars after thermomechanical aging: an in vitro comparative study

BACKGROUND: Full-coverage restorations are often required for primary molars with extensive carious destruction. Stainless steel crowns remain the long-standing reference, while prefabricated zirconia and polymer-based crowns have become tooth-colored alternatives. Direct comparisons under a single thermomechanical protocol, with separate wear and fracture cohorts, remain limited. This in vitro study quantified post-aging occlusal wear and compressive fracture load in four prefabricated crown systems for mandibular second primary molars. METHODS: Eighty crowns were tested, with 10 specimens per material for each outcome: two prefabricated zirconia crowns, one polymer-based crown, and one stainless steel crown. Identical resin dies were 3D-printed and all crowns were luted with glass-ionomer cement. All specimens underwent 10,000 thermal cycles between 5 °C and 55 °C. The wear cohort underwent 150,000 chewing cycles at 50 N and 1.2 Hz, and wear was scored on a 0-3 scale by two blinded examiners. The fracture cohort was loaded to failure. Wear scores were compared using Kruskal-Wallis and Dunn-Bonferroni tests; fracture loads were analyzed using one-way ANOVA and Tukey HSD. RESULTS: Mean wear scores were 1.1 ± 0.3 for NuSmile, 1.2 ± 0.4 for ProfZr, 1.7 ± 0.4 for SSC, and 2.2 ± 0.4 for BioFlx. The zirconia groups did not differ significantly (p = 0.68), whereas both showed lower wear scores than SSC (p = 0.04) and BioFlx (p = 0.01). SSC also showed lower wear scores than BioFlx (p = 0.03). Mean fracture loads were 3148.4 ± 643.4 N for BioFlx, 2703.4 ± 243.1 N for SSC, 1076.1 ± 186.9 N for NuSmile, and 565.5 ± 159.6 N for ProfZr (p < 0.001). BioFlx and SSC did not differ significantly (p = 0.093), while both exceeded the zirconia groups (p < 0.001). NuSmile exceeded ProfZr (p = 0.043). CONCLUSIONS: Under this in vitro model, zirconia crowns showed the lowest wear scores, whereas BioFlx and SSC showed the highest fracture loads. Each material demonstrated a distinct mechanical profile. Laboratory rankings alone should not guide clinical recommendations; long-term clinical studies are needed.