Generation of Transient Deformations and Strains by Light Polymerization

Abstract

A significant factor in the long term viability of resin composite dental restorations is minimizing the stress development along the wall of the cavity as the material shrinks during polymerization. The stress of polymerization shrinkage can lead to gap formation between the restoration and the walls of the cavity (microleakage). Clinical manifestations associated with material shrinkage include tooth sensitivity, discoloration, loss of restoration, secondary decay and tooth fracture. Recent research in resin composite polymerization has incorporated Finite Element (FE) stress analysis. An FE model predicted initial extrusion of the resin composite surface during light curing in a bulk-filled Class V restoration. The purpose of this project was to measure transient events during the polymerization of a light-cured resin composite. The hypothesis tested was that during light-activated polymerization of a Class V type restoration, the resin composite surface initially extrudes and measurable stresses occur along the cavity walls. In Experiment 1, initial surface movements were measured. A specifically configured test apparatus enabled surface resin movements to be recorded by a profilometer. Eight identical Class V preparations were drilled into a black acrylic block. The preparations were filled with Herculite and cured. For each of the eight trials, the profilometer recorded an initial extrusion of the resin composite surface. This experiment proved that during light activate polymerization of the resin composite, the FEM predicted counterintuitive initial surface extrusion does take place. In Experiment 2, initial strains were measured. For this pilot study a thin walled black acrylic tube was used as the Class V type restoration. Miniature strain gauge strips were placed along the wall of the cavity at right angles to measure circumferential and longitudinal strains along the cavity surface as the polymerization front advances. The preliminary data suggests the largest circumferential and longitudinal strains initially recorded were closest to the resin surface. The results of these experiments demonstrated initial resin composite surface extrusion and measurable strains along the cavity wall. This experimental data served as a partial validation of the FEM approach.