Influence of Specimen Dimensions on Nominal Polymerization Contraction Stress of a Dental Composite

The objective of the present study was verify the influence of specimen dimensions on polymerization contraction stress of a self-cure dental composite and investigate the influence of confinement (expressed by the ratio between the bonded and unbonded area of the composite, or ‘C factor’) and volume of the specimen on stress values. The composite was inserted between the flat surfaces two glass rods attached to a universal testing machine. Specimen dimensions were defined using glass rods with different diameters (2.5, 5, or 8 mm) and adjusting the distance between them (0.63, 0.83, 1.25 or 2.5 mm). An extensometer was used to keep specimen height constant. Force development was monitored for 30 min and the maximum value was used to calculate nominal stress (MPa). System deformation (compliance) was estimated in order to calculate stress values on an ideally rigid situation. Data were analyzed by ANOVA/Tukey test (α=0.05) and regression analysis. The interaction was significant (p<0.001). Differences in nominal stress for different heights were verified only for 5-mm and 8-mm diameter specimens. In general, lower heights produced higher stress values. Regression analysis using all the collected data showed a linear correlation between stress and ‘C factor’. However, non-linear relationships were found when stress was plotted against ‘C factor’ or volume selecting specimens with similar same diameter. It was concluded that specimen dimensions influenced test results. However, neither ‘C factor’ nor volume can be considered reliable predictors of contraction stress values.