Effect of luting composite shrinkage and thermal loads on the stress distribution in porcelain laminate veneers

Statement of problem. Cyclic thermal fatigue has demonstrated a significant influence of the thicknesses of luting composite and ceramic in crack prop ensity of porcelain laminates. Purpose. This study was conducted to define potentially involved parameters for crack development in porcelain laminates bonded to teeth. Finite eleme nt modeling was used to evaluate the respective effects of luting composite shrinkage and significant thermal changes. Material and methods. A buccolingual cross-section of a maxillary incisor w as digitized and used as a template to generate a single 2-dimensional mesh , including all the different restorative designs. Luting composite shrinka ge was simulated at a baseline temperature of 20 degrees C. The effect of t hermal loads from 20 degrees C to 5 degrees C and from 20 degrees C to 50 d egrees C was assessed with and without preexisting composite shrinkage. Results. Shrinkage of the luting composite alone generated important compre ssive forces on the ceramic, either at the restoration surface or interface . Compression intensity was related to geometry and ratio of thicknesses be tween the ceramic and luting composite (CER/CPR). Lower ratios produced hig her compression forces in the ceramic. When thermal loads were combined to the composite shrinking forces, the stress pattern was significantly change d only for the experimental conditions with the lowest CER/CPR ratio. Tempe rature increase reduced compressive stresses and exacerbated tensile stress es. Thermal loads were simulated alone (situation of an "ideal nonshrinking " luting composite) and generated mainly tensile stresses in the ceramic, w hich intensity was again modulated by the CER/CPR ratio and the local geome try of composite and ceramic. Because of ceramic brittleness, these tensile forces were more detrimental than the high compression created by composit e shrinkage alone. The stress pattern was not influenced by the incisal len gth of the veneer but rather by the facial thickness of ceramic. The worst record made with a shrinking luting agent (500 mu m of luting composite, lo west CER/CPR ratio, 5 degrees C) was much less harmful than the worst recor d made with a hypothetical "nonshrinking" luting material. Conclusions. The ratio of the thickness of cement and luting composite appe ars to have a relevant influence on the stress distribution in porcelain la minates. Restorations that are too thin, combined with poor internal fit, r esulted in higher stresses at both the surface and interface of the restora tion. Because of its precompressed state given by composite shrinkage, cera mics performed better with regard to temperature induced tensile forces.