Aim: to evaluate the effect of cavity design and cusp inclination on fracture resistance of indirect overlay restorations in maxillary 1st premolars. Subjects and Methods: Hundred extracted human maxillary 1st premolars were selected and divided into ten groups; Gr1: control group (CG), Gr2: shoulder design with normal cusp inclination, Gr3 & Gr4: shoulder design with 33° & 22° cusp inclinations (respectively, Gr5: butt joint design with normal cusp inclination, Gr6 & Gr7: butt joint design with 33° & 22° cusp inclinations respectively, Gr8: Anatomical design with normal cusp inclination, Gr9 & Gr10: Anatomical design with 33° & 22° cusp inclinations respectively. All restorations were fabricated indirectly and cemented by RelyX Ultimate adhesive resin cement. Samples were stored for 24 hrs in distilled water at 37°C and then thermocycled for 5000 cycles. Universal testing machine was used to measure fracture loads. Samples were examined for determination of failure mode using a magnifying lens.
Results: ANOVA test revealed that the difference between groups was statistically significant (p=0.00). Tukey's post hoc test revealed no significant difference between Sh/33°CI, Sh/22°CI, B/33°CI, B/22°CI, A/NCI and A/33°CI. Conclusions: The teeth restored with cusp shoulder designs exhibited the highest percentages of restorable fractures (70-90%). Anatomical design presents the highest fracture resistance. However, 70-100% catastrophic failure is suspected. Fracture resistance increases significantly by decreasing cusp inclination of overlay restorations.