Interdental design of porcelain veneers in the presence of composite fillings: finite element analysis of composite shrinkage and thermal stresses.

PURPOSE This study was conducted to optimize the interdental design (wraparound) of porcelain laminates bonded to teeth in the presence of preexisting composite fillings. MATERIALS AND METHODS A finite element model (2-dimensional mesh generated from a horizontal cross section of a maxillary incisor) was used to evaluate the effects of luting composite shrinkage and thermal changes on the stress distribution within the ceramic. The mesh included 3 restorative designs (3 degrees of interdental wrapping) and a Class III composite filling. Curing contraction of the luting composite was simulated at baseline temperature (37 degrees C). Thermal loads from 37 to 60 degrees C and from 37 to 5 degrees C were assessed with and without preexisting composite shrinkage. Surface tangential stresses were calculated at the ceramic surface and interface. RESULTS Curing contraction alone generated mostly compressive stresses (peaks at 15 MPa) at both the ceramic surface and interface. Stresses remained compressive (peaks at 20 MPa) when thermal changes were added, except for the conservative veneer with minimum wraparound, the margins of which showed potentially harmful tensile stress peaks (approximately 7 MPa). Deformation of the tooth-restoration complex tended to be more uniform for veneers with maximum wraparound. In the presence of thermal loads alone, opposite effects were observed on the restoration surface and the interface, with compressive stresses on one side (up to 13 MPa) and tensile stresses on the other (up to 9 MPa). This effect of proximal bending (explained by the nearby expansion/contraction of the composite filling) was attenuated by the significant reduction of the bulk of the preexisting interdental composite by the overlapping veneer. CONCLUSION In the finite element environment, the negative effect of the nearby expanding/contracting composite fillings can be minimized by extending the veneer over the preexisting interdental restoration. Because of the precompressed state resulting from composite shrinkage, ceramics showed lower temperature-induced tensile stresses.