Influence of specimen dimensions and their derivatives (C-factor and volume) on polymerization stress determined in a high compliance testing system

Objective. To verify the null hypothesis that in a high compliance testing system stress magnitude was not influenced by specimen dimensions and, therefore, by its cavity configuration factor (C-factor) and volume. Methods. Twelve experimental groups were defined according to the specimen height (0.5, 1, 2 or 4 mm) and diameter (4, 6 or 8 mm). A selfcure composite (Bisfil 2B, Bisco) was inserted between the flat surfaces of two acrylic rods attached to the opposite ends of a universal testing machine. An extensometer with a gauge length of 10 mm was attached to both rods to monitor specimen height. Force development was monitored for 30 min and nominal stress was calculated dividing the maximum force value by the crossection of the rod. A second set of data was obtained dividing nominal stress by the corresponding longitudinal compliance (LC) of the system for that particular specimen size. Data was analyzed using two-way ANOVA/Tukey test (nominal stress), Kruskal/Wallis (normalized stress), both at alpha = 5%, and regression analysis having either C-factor or volume as independent variable. Results. Regression analysis involving nominal stress did not reveal strong relationships with the independent variables (C-factor: 0.437, volume: 0.662). A strong relationship was found between normalized stress and specimen volume (R-adj(2) = 0.886). Normalized stress showed no relationship with specimens' C-factor. Pairwise comparisons between groups with similar volumes revealed that normalized stress increased at higher C-factors. Significance. By eliminating the influence of longitudinal compliance, a strong relationship between polymerization stress and specimen volume was revealed in a high compliance testing system. (C) 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.