Influence of Proximal Box Design on Primary Stability and Strength of Indirect Partial Bonded Restorations (IPBR) in Posterior Teeth

Background and Objectives: To evaluate the influence of proximal box design on primary stability and adhesive bond strength of indirect partial bonded restorations (IPBR) in posterior teeth. Materials and Methods: Thirty extracted human third upper molars were randomly assigned to three groups (n = 10 each): Group A, butt joint overlay-type restorations with proximal box (height-to-depth ratio 1:2–1:3); Group B, proximal box with height-to-depth ratio 1:1; and Group C, flat bevel preparation as control. Specimens underwent standardized preparation: immediate dentin sealing, restoration with overlays, and cementation with Panavia V5 adhesive resin cement. Mechanical measurements were performed to assess the tensile bond strength 48 h post-cementation using a universal testing machine. Finite element analysis (FEA) was also carried out to evaluate stress distribution at the adhesive interface. Data were analyzed by ANOVA with the Bonferroni post hoc tests. Results: Group A exhibited significantly higher bond strength values (403.7 ± 37.5 N) compared with Group B (340.4 ± 32.7 N; p < 0.01) and Group C (324.8 ± 33.3 N; p < 0.01). No significant difference was observed between Groups B and C. FEA corroborated the experimental findings, showing more homogeneous stress distribution for Group A and higher stress concentrations in Groups B and C. Conclusions: Proximal box design significantly affects the primary stability and bond strength of IPBR in vitro. A deeper proximal box with favourable geometric features enhances adhesive performance and stress distribution. Within the limitations of this study, these findings suggest potential clinical benefits, although further in vivo studies under dynamic conditions are needed to confirm long-term restoration performance.