Effects of the light tip position on the degree of conversion and dentin bond strength of a universal adhesive

Introduction/Objectives To measure the degree of conversion (DC), immediate and long-term microshear bond strength (mu SBS) to dentin of a universal adhesive relative to the light tip position and adhesive application protocol. Methods Mid-coronal flat dentin of 48 human third molars was exposed and split in halves. Single Bond Universal (SBU; 3M) adhesive was applied to each half following 'total-etch' (TE) or 'self-etch' (SE) approach. Depending on the light tip (Bluephase G2, Ivoclar Vivadent, Schaan, Liechtenstein) angle and distance from adhesive surface, three groups were compared: "1 mm_90 degrees"(control); "8 mm_90 degrees"and "8 mm_60 degrees". Cylindrical composite build-ups ((empty set)1.7mm, Filtek Z250, 3M) were prepared in each half. DC was measured using Raman spectroscopy. mu SBS was measured after 24 hours and six months storage in distilled water at 37 degrees C. Fracture types were analyzed. Results No significant difference in DC was detected between groups "1 mm_90 degrees" (89.1 +/- 6.2%) and "8 mm_90 degrees"(94.6 +/- 1.2%) (p > 0.05), both showing significantly higher DC (p < 0.05) than "8 mm_60 degrees" group (74.9 +/- 9.5%) (p < 0.05). Initially, there were no significant differences in mu SBS between groups (p > 0.05). Group "1 mm_90 degrees" TE ( 12.8 +/- 4.3 MPa) and group "8 mm_60 degrees" TE (14.7 +/- 5.7 MPa) showed significantly lower mu SBS after aging (8.4 +/- 4.3 MPa and 9.2 +/- 2.6 MPa, respectively) (p < 0.05). Adhesive fractures were predomintantly detected. Conclusion Initially, both application protocols resulted in similar bond strength to dentin of a universal adhesive in suboptimal curing conditions. In the long-term, SE showed greater adhesive resistance to degradation resulting in smaller decrease in bond strength compared to TE. Light tip angulation affected DC and mu SBS more than tip-to-surface distance.