Over-etching effects on micro-tensile bond strength and failure patterns for two dentin bonding systems

Objectives. The purpose of this study was to determine (1) the weakest zone of resin-dentin bonds and (2) the relation between bond strength and failure mode to clarify the effect of demineralized dentin. Methods. Human premolars were sectioned to expose the dentin surfaces, and the dentin surfaces were conditioned with phosphoric acid for 15, 60, 120, or 180 s. Resin-dentin bonded specimens were produced using two adhesives: One-Step (Bisco) and OptiBond Solo (Kerr). Each sample was sectioned to produce a beam (adhesive area: 0.9 mm ²). Microtensile bond tests were then conducted, and the mean bond strengths (n = 12 for each group) were statistically compared using two-way ANOVA and Duncan's multiple-range test (p < 0.05). The fractured surfaces of all specimens were examined using SEM, and the areas of failure were measured using an image analyzer. Results. For One-Step, the bond strength decreased with increase in acid-conditioning time (15 s: 50.7 ± 9.7, 60 s: 40.8 ± 11.0, 120 s: 23.6 ± 4.9 and 180 s: 12.1 ± 4.6 MPa) (p < 0.05). For OptiBond Solo, the bond strength in the case of 15 s acid-conditioning time (42.6 ± 7.9 MPa) was significantly greater than that for the other times (60 s: 31.9 ± 10.3, 120 s: 31.8 ± 14.4 and 180 s: 31.8 ± 7.4 MPa) (p < 0.05). Fractography showed that the area percentage of the hybrid layer increased with increase in etching time for both systems. Conclusions. The integrity of the hybrid layer, especially the top part, has an effect on bond strength.