Preheating and "snow-plow" composite application technique affect double bond conversion but not bond strength to dentine

Objective: To measure degree of conversion (DC) of a flowable composite, microtensile bond strength (MTBS) to dentine in the snow-plow technique with/without preheating and temperature in the preheated composite. Materials and Methods: For DC, snow-plow specimens of Filtek Ultimate Flowable (Flow) and Filtek Bulk Fill (Bulk) (3M) were prepared by light-curing composites simultaneously in standardized molds while in control groups light-curing was separate. DC of Flow was measured using micro-Raman spectroscopy. For MTBS, specimens were prepared on dentine of human extracted molars by simultaneous (snow-plow) or separate (control) light-curing. MTBS was measured using a universal testing machine after 24 h and 6 months. Data were analyzed using ANOVA with Tukey's post hoc (alpha = 0.05). Results: Increased curing time significantly increased DC in snow-plow from 13.4% +/- 11.6% (10 s) to 31.8% +/- 4.4% (40 s) albeit significantly lower than controls (p < 0.05). Preheated Bulk improved conversion in snow-plow (44.3% +/- 1.7%) and control (50.5% +/- 2.6%) (p < 0.05). No significant differences occurred in MTBS between groups (p > 0.05). MTBS values ranged between 66.7 +/- 8.4 MPa (snow-plow group_21 degrees C_baseline) and 54.1 +/- 15.8 MPa (control_21 degrees C_long-term). Temperature in the preheated Bulk dropped to similar to 38 degrees C after 30 s. Conclusions: Snow-plow technique, irrespective of preheating, resulted in lower DC of Flow than separate light-curing of composite increments. Snow-plow, irrespective of preheating, resulted in similar initial and long-term MTBS to dentine.