To develop a laboratory technique for improving the retention of crowns, the present study investigated the effects of micro retentive grooves (MRGs) on the inner surface of CAD/CAM resin composite crowns on the pull-out bond strength. MRGs with four different depths were prepared for the inner surface of the CAD/CAM crown (0 μm: MRG0, 25 μm: MRG25, 50 μm: MRG50, 100 μm: MRG100). MRG0 was divided into two groups as controls by different surface treatments: air abraded with alumina, and not blasted (Con). A silane coupling agent was applied to all specimens. The crowns were then bonded to standard stainless-steel abutments using self-adhesive resin cement following the manufacturer’s instructions. Pull-out tests were conducted, and pull-out bond strength (N) was obtained. The number of repetitions was set to five with five conditions. It was found that the largest strength of 492.2 (79.9) N was obtained in MRG50, and the smallest of 55.1 (17.5) N in Con, indicating that MRG50 was about nine times stronger than Con. The strength of MRG was significantly larger than the others except for MRG100 (p<0.05). Thus, the bond strength of CAD/CAM resin composite crowns could be improved by preparing the inner surface of MRGs. MRG with a depth of 50 μm showed the highest bond strength.

View full abstract