Effect of filler contents on the bond strength of CAD/CAM resin crowns: New resin primer versus conventional silane agents

Purpose: This study aims to evaluate the effects of resin primers containing methyl methacrylate (MMA) and silane agent on the bonding effectiveness of indirect resin composite blocks with three different filler contents.Methods: A commercially available computer-aided design/computer-aided manufacturing (CAD/CAM) resin composite block and two experimental resin composite blocks with different filler contents were alumina-blasted and two surface treatments (primer and silane agent) were applied. The resin cement was built up, and the micro-tensile bond strength (mu TBS) was measured after 24 hours, 1 month or 3 months of water storage (n = 24 per group). The fracture surfaces after mu TBS measurements and resin block/cement interface were observed by scanning electron microscopy (SEM).Results: The primer treatment group showed a significantly higher bond strength than the silane group only in F0 (filler content 0 wt%) group (P < 0.001). In the primer group, F0 and F41 (filler content 41 wt%) groups showed significantly higher bond strengths than F82 (filler content 82 wt%) group (P < 0.001). In contrast, in the silane group, F41 group showed significantly higher bond strength than F0 and F82 groups (P < 0.001), and F82 group showed significantly higher bond strength than F0 group (P < 0.001). SEM revealed that the matrix resin was partially destroyed on the fracture surface of the primer group, and an uneven interface surface was observed compared with that of the silane group.Conclusions: MMA-containing primers showed higher bonding effectiveness to CAD/CAM resin composite blocks than the silane treatment.