Effect of delay in layering on the incremental adhesion of indirect dental composite resins

Adhesion of the new layer of indirect composite resin (ICR) to the already polymerized one may be affected when the time between the applications of subsequent layers is prolonged. The aim of this study was to compare the shear bond strength and degree of conversion (DC) of two ICR systems with different compositions and adhesive promoters, relayered after four time points. Disk shaped ICR materials (ICR1=Sinfony and ICR2=Targis) (N=96, n=48 per material) were fabricated and processed according to each manufacturer's instructions. They were then randomly assigned to 4 groups. While immediate layering acted as the control group, after 5 min, 24 h and 1-week delay, a new layer of the ICR of the same kind with the substrate was adhered to the substrate in polyethylene molds. The bonded specimens were loaded under shear (1 mm/min) and bond strength was calculated. DC of ICR1 and ICR2 were similar (75±1, 75±2, respectively). Delay in relayering at different timepoints did not significantly affect the adhesion between the incremental layers of ICR1 (32–34 MPa) compared to the control group (34 MPa), but the adhesion between the layers of ICR2 showed decreased bond strength after 24 h (30.9) and especially more after 1 week (25 MPa) compared to immediate layering (38.9 MPa). The effects of ageing before bonding additional layers of ICRs is variable, and depends on the chemical formulation of the ICR as reflected in the different brands.