A Simple Model To Visualise Cracking Patterns Of Composite Resin

The contraction stress build-up of composite resin that occurs during polymerisation adversely affects the bond between composite and tooth structure. Previous studies done to study the effect of polymerisation shrinkage have on the bond at the interface involved sectioning the specimens to visualise what was happening at the interface, which has certain disadvantages. Objectives: To determine the effects of (i) different cavity sizes has on cracking patterns of composite and glass stubs; (ii) influence of incremental build-up on composite resin induced crack development and (iii) glass simulation of tooth cavity filling. Methods: Glass stubs of different diameter (2.2, 4, 5 and 5.6mm)(n=3) were used to visualise the failure modes associated with (i) different cavity configurations (C-factor) and (ii) effects incremental packing has on polymerisation shrinkage of composite resin (n=2). FEA was also used to provide some insights as to what happens at the resin-glass interface. Glass stubs were etched and pre-treated with silane-coupling agent. Composite resin was packed into the cavity either by single packing or incrementally and cured. Intact bond surfaces were evaluated and converted as a percentage of total surfaces for both experiments. Results: The smaller the internal diameter, the greater the % of intact bond surfaces, both for single packing and incremental packing. Incremental packing increases the intact surface adhesion but created cracking of the glass circumferentially at the level of each increment. This observation is in accordance the finite element analysis study that was done where it showed that the stress levels are highest at the free surface of the composite-glass interface. Conclusions: Incremental packing reduces polymerisation shrinkage force in a Class I cavity. However, it also caused a circumferential cracking pattern at the level of each increment which may have detrimental effects clinically.