In Vitro Fracture Resistance of Adhesively Restored Molar Teeth with Different MOD Cavity Dimensions

PURPOSE: Molar MOD (mesial-occlusal-distal) cavity preparation weakens relative cuspal stiffness by up to 63%, often resulting in cuspal fracture. This investigation inspects fracture resistance of MOD cavities restored using direct composite restoration. MATERIALS AND METHODS: 120 extracted, intact mandibular molars were selected. MOD cavities with different depth/wall thickness were prepared in 9 groups (n = 12): A: 3 mm/3.5 mm, B: 3 mm/2.5 mm, C: 3 mm/1.5 mm, D: 5 mm/3.5 mm, E: 5 mm/2.5 mm, F: 5 mm/1.5 mm, G: 7 mm/3.5 mm, H: 7 mm/2.5 mm, I: 7 mm/1.5 mm. Specimens with 7 mm deep cavities received root canal treatment. The teeth were restored with dental composite. Maximal fracture strength test was conducted. Intact natural teeth were used as control. For statistical analysis Kruskal-Wallis ANOVA with post-hoc pairwise comparisons was used (α = 0.05). RESULTS: Significant difference was indicated between the control and groups D, E, F, G, H, and I. No significant differences were found between the A, B, C groups and the control. Comparing the 5 and 7 mm cavity depth groups, there was no statistical difference between any of them. CONCLUSIONS: Within the limitations of this investigation, the following conclusions can be drawn regarding molar teeth with a MOD cavity: 3 mm or shallower cavities can be restored to the physiological fracture strength with direct composite restorations; 5 mm or deeper cavities cannot be restored to the physiological fracture strength with direct composite restorations. Cusp thickness does not significantly influence fracture strength in molar MOD cavities with a direct composite restoration.