Incremental filling technique and composite material--part II: shrinkage and shrinkage stresses.

OBJECTIVES: Finite element analysis (FEA) was used to study polymerization shrinkage stress in molars restored with composites and to correlate those stresses with experimentally measured tooth deformation.
METHODS: Three composites (Filtek LS, Aelite LS Posterior, Filtek Supreme) and three filling techniques (bulk, 2.0-mm increments, and 1.0-mm increments) for restoring a molar were simulated in a two-dimensional FEA. Polymerization shrinkage was modeled using post-gel shrinkage, which was measured using the strain gauge technique (n=10). Cuspal tooth deformation, measured at the buccal and lingual surfaces with strain gauges in a laboratory study, was used to validate the analysis. Residual shrinkage stresses were expressed in modified von Mises equivalent stresses. Linear Pearson correlations were determined between the laboratory and FEA results.
RESULTS: Post-gel shrinkage values (in volume %) were: Filtek LS (0.11 ± 0.03) < Aelite LS Posterior (0.51 ± 0.02) < Filtek Supreme (0.62 ± 0.09). The 1.0-mm increment filling caused substantially higher stresses and strains in the cervical enamel region. Significant correlations were found between: elastic modulus and FEA strain, elastic modulus and FEA stress, post-gel shrinkage and FEA strain, post-gel shrinkage and FEA stress, FEA strain and cuspal deformation by strain gauge, and FEA stress and cuspal deformation by strain gauge (p<0.05).
CONCLUSIONS: Increasing the number of increments and high post-gel shrinkage and/or elastic modulus values caused higher stresses in the remaining tooth structure and tooth/restoration interface. Cuspal deformation measured with the strain gauge method validated the finite element analyses.