The influence of cross-sectional shape and surface area on the microtensile bond test

Objectives. The purpose of this study was to determine the effect of the cross-sectional area shape (cylindrical vs, rectangular) and the bonding surface area on the microtensile bond strengths and stress distribution of four dentin adhesive systems (Scotchbond MP Plus, OptiBond FL, OptiBond Solo, One-Step). In addition, finite element analysis (FEA) models were developed to investigate stress distributions. Methods. Extracted human molars were cut vertically and the occlusal enamel removed; one-half of the tooth was used for rectangular specimens, the other half for cylindrical specimens. The occlusal dentin was bonded according to the manufacturers' directions and covered with a block of resin composite. For the cylindrical specimens, the bonded dentin was shaped with a diamond bur on a lathe to produce specimens of area 1.1, 1.5 or 3.1 mm(2) at the bonded interface. The rectangular specimens were sectioned to obtain bar-shaped specimens, which were shaped to produce hourglass shaped specimens with the same area as the round specimens. Bonds were stressed in tension at a speed of 1 mm min(-1). The mean bond strengths were compared using two-way ANOVA, one-way ANOVA, LSD and Student's t tests. The fractured surfaces were examined by scanning electron microscopy, and the frequencies of the fracture modes were compared using the Kruskal-Wallis and Mann-Whitney U tests. FEA models were created simulating the cross-sectional areas for bonding to determine the stress distribution. Results. The 3.1 mm2 bonding area groups showed significantly lower bond strengths than the 1.1 mm(2) bonding area groups (p < 0.05), except for the rectangular specimens using Scotchbond MP Plus and One-Step. Most cylindrical specimens of bonding area 1.1 or 1.5 mm exhibited adhesive failure at the interface between the dentin and the adhesive resin. No differences were determined between cylindrical and rectangular specimens. The fracture mode matched the stress distribution patterns calculated from the FEA modeling. Significance. The results indicate that the test methods using small surface areas produce higher bond strengths than those using larger surface areas, and that cross-sectional shape has little effect. This is probably a result of fewer defects occurring in the small-area specimens. (C) 1998 Published by Elsevier Science Ltd on behalf of the Academy of Denial Materials.