Effect of polymerization contraction on interface µTBS in extended areas

Detrimental effects of polymerization shrinkage on µTBS of bonded interfaces at outer extremes of a longitudinal restorations have been recently described. This effect should be extremely relevant in peripheral parts of extended interfaces, resembling clinical restorations. OBJECTIVES: To test if microtensile bond strength decreases from center to periphery of extended restorations. METHODS: Labial dentin of bovine incisors (n=5) was exposed (500 grit grinding) and restored (etching, Scotch Bond1 XT, Filtek Z250) in an only increment of at least 2mm high and 10 x 10 mm in extension. Restorations were sectioned perpendicularly to interface, producing bar-shaped samples that were submitted to tension until adhesive separation (checked in an optical microscopy, 40x), its µTBS calculated and the percentage of µTBS values within each specimen transformed into PMPa dependent variable. Relationship of each sample with all others from same specimen was defined as its MODULE, the result of addition of all vectors with origin in the center of the specific sample and ending in the center of all other. MODULE was transformed to PModule: its percentage within each specimen, and distributed in ten percentile groups. Kruskal-Wallis test was applied to check if differences in PMPa among PModule percentage groups were significant. Jonckheere-Terpstra test was used to check if PMPa values were decreasingly arranged with increasing of PModule percentage group. RESULTS: K-W test showed that there were statistically significant differences between PMPa values in the different PMPa percentage groups (p = 5.01E-21), and J-T test showed that they were decreasingly arranged (p = 9.7E-31). CONCLUSIONS: In broad restorations, µTBS significantly decreased from the centre of the restoration towards its external borders.