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

Introduction/Objectives. To measure degree of conversion (DC), immediate and long-term microshear bond strength (μSBS) 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) angle and distance from adhesive surface, three groups were compared: "1 mm_90°" (control); "8 mm_90°" and "8 mm_60°". Cylindrical composite build-ups (o1.7mm, Filtek Z250, 3M) were prepared in each half. DC was measured using Raman spectroscopy. μSBS was measured after 24 hours and six months storage in distilled water at 37°C. Fracture types were analyzed. Results. No significant difference in DC was detected between groups "1 mm_90°" (89.1 ± 6.2%) and "8 mm_90°" (94.6 ± 1.2%) (p > 0.05), both showing significantly higher DC (p < 0.05) than "8 mm_60°" group (74.9 ± 9.5%) (p < 0.05). Initially, there were no significant differences in μSBS between groups (p > 0.05). Group "1 mm_90°" TE (12.8 ± 4.3MPa) and group "8 mm_60°" TE (14.7 ± 5.7MPa) showed significantly lower μSBS after aging (8.4 ± 4.3MPa and 9.2 ± 2.6MPa, respectively) (p < 0.05). Adhesive fractures were predominantly 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 μSBS more than tip-to-surface distance.