When to restore: Critical crack size thresholds in human first premolars

Objectives: This study aimed to investigate the effects of crack depth and width on the ultimate strength and stress distribution of human first premolars to identify critical crack sizes that can guide clinical decision-making for restorative intervention. Methods: Forty extracted, sound human first premolars were allocated into five groups: one control (no crack) and four experimental groups with mesio–occluso–distal cracks of varying depth (2–4 mm) and width (0.5–1 mm). Compression tests were performed to measure ultimate strength, fracture origin, and propagation patterns. Statistical analysis used one-way ANOVA and multiple linear regression with a significance level of p < 0.05. Finite Element Analysis (FEA) simulated stress distribution using principal stress and energy-based (Von Mises) failure theories, validated against experimental results. Additional FEA models (depth 1–3 mm; width 0.1–0.5 mm) were developed to estimate critical crack thresholds that could not be tested experimentally. Results: Crack depth significantly influenced ultimate strength reduction compared to width, which became negligible at a 4 mm depth. In 91% of cracked specimens, fractures originated at crack tips, whereas in intact teeth, fractures began at the palatal cusp. Von Mises stress analysis accurately represented experimental fracture behavior, unlike principal stress theory. Critical thresholds were established: 15% strength reduction at D1W0.5 and D2W0.1, and 67% reduction when cracks extended to the pulp (D3W1). Conclusions: Crack size, particularly depth, critically affects the structural integrity of human premolars. The Von Mises stress theory is the most reliable model for predicting fracture behavior in cracked teeth. Clinically, restorative treatment should be considered when cracks reach the 15% strength reduction threshold to prevent pulpal involvement and potential tooth failure.