Regional push-out bond strength of resin composite to dentin

Objectives: To compare regional bond strength of resin composite in different thirds of dentin, using the micropush-out test. Methods: Occlusal dentin surfaces of extracted human mandibular molars were exposed and flattened. Three disc specimens of 1.25 mm thickness (1 superficial, 1 middle and 1 deep dentin) were prepared from each tooth. Sixty standardized occlusal cavities of 2.20 mm occlusal diameter, 1.25 mm cervical diameter were prepared using conical-shaped diamond rotary cutting instrument. The adhesive (3M ESPE Adper Scotchbond Multi-Purpose Dental Adhesive) was applied according to the manufacturer's instructions and a composite material (3M ESPE Filtek Z250 Universal Restorative) was applied and polymerized with LED light curing unit (3M ESPE Elipar FreeLight 2) for 20 seconds each from occlusal, lingual, and buccal surfaces. After 24 h storage in distilled water, the discs (n=20/per group) were tested in a universal testing machine and pushed out with a cross-head speed of 1.0 mm/minute. Push-out force was applied from the bottom of the discs with the 1.25 mm diameter because of the convergence of the cavity. The force was recorded using software and the values were used to calculate push-out strength. Failure modes were evaluated using a stereomicroscope and SEM. The data were analyzed with analysis of variance (ANOVA) followed by Tukey's post-hoc test. (p=.05). Results: Push-out bond strength mean and standard deviation values of resin composite were: superficial dentin: 18.5 MPa ± 2.2, middle dentin: 11.9 MPa ± 1.8, deep dentin: 5.5 MPa ± 1.8. Push-out bond strengths were significantly different between superficial, middle and deep dentin locations (p<0.001). Conclusions: The superficial dentin region presented higher bond strength than middle and apical. Regional variations of dentin may affect the push-out bond strength of resin composites.