Methods: A 3-D FE model was created of an upper premolar with a buccal cusp-replacing direct resin composite restoration without (Geometry A) and with (Geometry B) palatal cuspal coverage. The model consisted of 7797 iso-parametric elements and 9500 nodes. Between the restoration and the tooth tissue adhesive contact was modeled. An occlusal load of 500N was applied for both geometries. Normal and shear stresses for the tooth-restoration interface and principal stresses for the composite material were calculated. Subsequently, strength/stress ratios were calculated. The lower this ratio, the higher the failure probability.
Results: In general, the strength/stress ratios for Geometry A were lower than for Geometry B. At the vertical interfaces of both geometries tensile and shear stresses were observed. At the horizontal interface of Geometry B compressive and shear stresses appeared. The lowest strength/stress ratio (0.38) was observed for the tensile stresses at the vertical interface of Geometry A. The strength/stress ratios for the composite material (minimum 3.0) were higher than for the tooth-restoration interface.
Conclusion: The results suggest that coverage of the remaining cusp increases the load resistance of this type of restorations. Adhesive failure of cusp-replacing resin composite restoration under load seems to be more likely than cohesive fracture of the composite material.