Polymerization stress of resin composites as a function of system compliance

Objectives Evaluate the effect of testing system compliance on polymerization stress and stress distribution of composites. Methods Composites tested were Filtek Z250 (FZ), Herculite (HL), Tetric Ceram (TC), Helio Fill-AP (HF) and Heliomolar (HM). Stress was determined in 1-mm thick specimens, inserted between two rods of either poly(methyl methacrylate), PMMA, or glass. Experimental nominal stress (σexp) was calculated by dividing the maximum force recorded 5 min after photoactivation by the cross-sectional area of the rod. Composites’ elastic modulus (E) was obtained by three-point bending. Data were submitted to one-way ANOVA/Tukeyʹs test (α = 0.05). Stress distribution on longitudinal (σy) and transverse (σx) axes of models representing the composites with the highest and lowest E (FZ and HM, respectively) were evaluated by finite element analysis (FEA). Results σexp ranged from 5.5 to 8.8 MPa in glass and from 2.6 to 3.4 MPa in PMMA. Composite ranking was not identical in both substrates, since FZ showed σexp statistically higher than HM in glass, while in PMMA FZ showed values similar to the other composites. A strong correlation was found between stress reduction (%) from glass to PMMA and compositeʹs E (r2 = 0.946). FEA revealed that system compliance was influenced by the composite (FZ led to higher compliance than HM). σx distribution was similar in both substrates, while σy distribution showed larger areas of compressive stresses in specimens built on PMMA.