Optimizing preparation design for metal-free composite resin crowns

Statement of problem. Although composite resin materials are used for posterior crown restorations, the influence of preparation design, material thickness, convergence angle, and method of cementation on fracture resistance remains unclear. Purpose. The purpose of this in vitro study was to test the hypothesis that minimal preparation designs provide an acceptable level of fracture resistance for posterior composite resin crowns. Material and methods. Nonreinforced Artglass composite resin crowns (n=128) were fabricated on human molars in 16 test groups (n=8). Axial tooth preparation included a 1-mm-deep shoulder or a 0.5-mm chamfer preparation, whereas occlusal reduction was either 0.5 mm or 1.3 mm. The total angle of convergence was 4 or 11 degrees and the crowns were cemented either with glass ionomer cement (Ketac Cem) or resin cement (2bond2). After 10,000 thermal cycles, crowns were vertically loaded until failure occurred; load was measured in newtons. Statistical analysis was performed by a 4-way ANOVA (alpha=.05). Results. Statistical analysis revealed significant effects of occlusal thickness (P<.001), cement (P<.001), preparation design (P=.011), and convergence angle (P=.001) on the fracture resistance of the composite crowns. For composite resin crowns with an occlusal thickness of 0.5 mm, the resistance to fracture was lower than for crowns with a 1.3-mm thickness. Fracture resistance was greater when resin cement was used, and greater for the chamfer finish line than for the shoulder finish line. Use of a greater total convergence angle reduced fracture resistance. Conclusions. Fracture resistance of composite resin crowns was significantly improved by increasing the occlusal thickness of the crowns, by using resin cement, and by reducing the total convergence angle.