The effect of depth of dentin demineralization on bond strengths and morphology of the hybrid layer

Previous studies have shown that different phosphoric acid-based etchants d o not penetrate intertubular dentin to the same depth. The purpose of this study was to determine the effect of different phosphoric acid-based condit ioners on dentin shear bond strengths of three one-bottle bonding systems a nd to evaluate the corresponding interfacial ultramorphology. The null hypo thesis to be tested was that no correlation could be established between th e depth of intertubular demineralization and dentin shear bond strengths. T he labial surface of 90 bovine incisors was polished to expose middle denti n. The specimens were randomly assigned to three one-bottle adhesive system s (n=30): OptiBond SOLO, Permaquick PQ1, and Single Bond. For each adhesive system the specimens were divided into three subgroups of different silica -thickened etching gels (n=10): 37.5% phosphoric acid gel (Kerr Gel Etchant ), 35% phosphoric acid gel (Ultraetch), and 35% phosphoric acid gel (Scotch bond Etching Gel). After 24 hours in water at 37 degrees C, the specimens w ere thermocycled for 500 cycles in baths kept at 5 degrees C and 55 degrees C and the shear bond strengths measured. The data were analyzed with one-w ay and two-way ANOVA. Further, the adhesives were applied to 800 mu m-thick bovine dentin disks (two per subgroup), which were restored with a low-vis cosity composite resin. Six small dentin/resin sticks with a cross-section of 1.0 mm x 1.0 mm were obtained from each bonded disk. They were then deca lcified in a buffered solution of EDTA, fixed, stained, and sectioned in 90 nanometer-thick slices to observe under the Transmission Electron Microsco pe (TEM). The mean shear bond strengths were not statistically different at a confidence level of 95%. When the means were pooled for dentin adhesive and for etching gel, the number of cohesive failures was greater for Permaq uick PQ1 and for Ultraetch, respectively. Pearson's correlation coefficient showed no correlation between hybrid layer thickness and bond strengths. T he ultramorphological observation showed that all materials penetrated the dentin and formed a hybrid layer, regardless of the etching gel used.