Effect of finish line variants on marginal accuracy and fracture strength of ceramic optimized polymer/fiber-reinforced composite crowns

Statement of problem. Ceramic optimized polymer (Ceromer)/fiber-reinforced composite (FRC) crowns have been promoted as alternatives to conventional crowns. However, little is known regarding the ideal tooth preparation for this type of crown. Purpose. This in vitro study evaluated the marginal adaptation and fracture strength of ceromer/FRC crowns with respect to the various types of finish lines. Material and methods. Four metal dies with different finish lines (0.9-mm chamfer, 1.2-mm chamfer, 1.2-mm rounded shoulder, and 1.2-mm shoulder) were prepared. Forty (10 for each finish line) Targis/Vectris crowns were fabricated on duplicated base metal alloy dies. The restorations were stereoscopically evaluated at 56 points along the entire circumferential margin for measuring the margin adaptation before and after cementation with a resin luting agent. The specimens were then compressively loaded to failure using a universal testing machine. The marginal adaptation (mum) was analyzed with the Kruskal-Wallis test and post-hoc Dunnett test (alpha = .05). The fracture load (N) was analyzed with a 1-way analysis of the variance and the Scheffe adjustment (alpha = .05). The fractured surfaces of the crowns were examined with a scanning electron microscope to determine the mode of fracture. Results. The marginal adaptation of crowns with a shoulder finish line was significantly better than crowns with a chamfer finish line before and after cementation (P < .001). The increased marginal gap after cementation was the lowest in the 1.2-mm rounded shoulder group. The fracture strength of the crowns with the 0.9-mm chamfer and crowns with 1.2-mm chamfer was significantly greater than those of the crowns with the 1.2-mm shoulder or rounded shoulder (P = .011, P = .049, respectively). The mean fracture load of all crowns, regardless of the finish line design, was 1646 N. The fractured surface of the crown revealed adhesive failure and 3 types of cohesive failure (fracture of the Targis and Vectris, Targis fracture with a crack in the Vectris layer, and crushing without fracture). Conclusion. The marginal gaps were greater for the chamfer finish line specimens than in the shoulder finish line specimens. However, the fracture strength of the chamfer finish line specimens was greater than that of the shoulder finish line specimens.