Microtensile bond strength testing and failure analysis of two dentin adhesives

This investigation was conducted to determine the tensile bond strength of two dental adhesive using a recently introduced "microtensile" bond strength testing design and to verify the failure mode for each test specimen with scanning electron microscopy (SEM).

Extracted human molars were mounted in stone and the enamel was removed the occlusal surface perpendicular to the long axis of the tooth. A composite resin crown was formed on this flat dentin surface utilizing each dental adhesive system according to manufacturer's instructions. Twenty-four hours later the bonded test specimens were sectioned perpendicular to the adhesive joint, producing six to seven thin slabs per tooth. These dentin/adhesive/composite resin slabs were sectioned free from the stone block and mounted into custom Plexiglas fixtures for trimming and subsequent tensile bond strength testing at 7 d post-bonding. The bond strength of the two adhesives was statistically compared with the t test. The broken specimens were examined with SEM to determine the fracture location or failure mode. Failures for each adhesive system were categorized as either interfacial (joint or mixed) or substrate (dentin and composite) and evaluated by Fisher's exact test.

The tensile bond strength and failure modes of All-Bond 2 (Bisco) and Optibond FL (Kerr) were not significantly different. Sixty per cent (12/20) of fractures involving All Bond 2 occurred at the interface, with seven being entirely maintained within the joint, whereas Optibond FL had 35% (7/20) involving some portion of the interface, two totally within the joint. Cohesive fractures of either dentin or composite accounted for 55% of the total failure modes (21/40). The remaining dentin thickness did not affect the measured tensile bond strength.

This versatile new method permits multiple measurements from a single tooth or small surface areas within a restoration but careful interpretation of the failure mode is required to prevent inappropriate conclusions about the utility of the test.