The effect of hybrid layer thickness on bond strength: demineralized dentin zone of the hybrid layer

Objectives: The purpose of this study was to evaluate the correlation between hybrid layer thickness and bond strength using specimens acid-conditioned for varying lengths of time. Methods: The dentin surfaces of human premolars, sectioned to remove the enamel from the labial surface, were conditioned with 35.0% phosphoric acid of an adhesive resin system (Scotchbond Multi-Purpose; 3M) for 15 (as directed by the manufacturer), 60, 120, or 180 s (experimental acid-conditioning times). The bonded specimens were then sectioned perpendicular to the adhesive interface to measure the hybrid layer thickness by SEM. The specimens for the micro-tensile test were sectioned perpendicular to the adhesive interface and trimmed to an hourglass-shape. Then, the micro-tensile test was performed at a crosshead speed of 1.0 mm/min. The bond strengths and hybrid layer thickness were statistically compared with Studentʹs t-test (p<0.05). All fractured surfaces were also observed by SEM. Results: Significant differences between the groups exposed to acid for 15 and 60 s, and those exposed for 120 and 180 s were observed in hybrid layer thickness and bond strength (p<0.05). SEM observation of the fractured surfaces revealed that a demineralized dentin zone without resin impregnation remained within the hybrid layer. Significance: A demineralized dentin zone was formed in the bond structures after prolonged acid-conditioning, resulting in low bond strength. The shrinkage of the hybrid layer due to desiccation during the SEM examination process provided evidence of the presence of the demineralized dentin zone within the hybrid layer.