Effect of different modeling agents on the shear bond strength between incremental layers of resin composite: an in vitro study

Modeling liquids are widely used in restorative dentistry to facilitate the adaptation and contouring of resin composites during incremental layering. Because no standardized, manufacturer-recommended protocol exists for their type, application time, or polymerization sequence, their influence on the bond strength between composite increments remains unclear. This in vitro study evaluated the effect of three modeling liquids, two application times, and two polymerization protocols on the interlayer shear bond strength of a resin composite. Shear bond strength specimens were prepared using a single anterior resin composite (Estelite Sigma Quick 2, A2; Tokuyama Dental, Japan). A 2-mm composite substrate was built in two 1-mm increments, and a 1-mm-high, 2.60-mm-diameter composite cylinder was bonded to it through a silicone guide. Three modeling liquids (GC Modeling Liquid, Signum Liquid, and Composite Wetting Resin) were applied to the substrate surface for 5–10 s under two polymerization protocols: protocol C1, in which the modeling-liquid-coated surface was light-cured before the cylinder was placed, and protocol C2, in which the modeling-liquid-coated surface and the cylinder were co-cured in a single step. A control group received no modeling liquid. This yielded a 3 × 2 × 2 factorial design (12 experimental groups) plus the control group ( n = 12 per group). Specimens were stored in distilled water at 37 °C for 24 h and tested in shear at 1 mm/min according to ISO 29,022. Main effects and interactions were analyzed by three-way ANOVA; the control group was compared with the experimental groups by one-way ANOVA (α = 0.05). Modeling liquid type, application time, and polymerization protocol had no statistically significant effect on shear bond strength, and no significant two- or three-way interactions were detected (all p > 0.05). Mean (± SD) values were 21.84 ± 8.79 MPa for GC Modeling Liquid, 21.70 ± 8.56 MPa for Signum Liquid, and 22.81 ± 10.43 MPa for Composite Wetting Resin; the control group was 17.73 ± 3.51 MPa (liquid type p = 0.815, application time p = 0.071, polymerization protocol p = 0.960; all interactions p > 0.05). Failures in the experimental groups were mainly mixed and cohesive, whereas the control failed predominantly at the interface (adhesive). Within the limitations of this in vitro study, neither the type of modeling liquid nor the application time or polymerization protocol significantly affected the shear bond strength between resin composite increments under the tested conditions.