Marginal accuracy and fracture strength of ceromer/fiber-reinforced composite crowns: Effect of variations in preparation design

Statement of problem. Targis/Vectris restorations provide excellent esthetics and clinical success; however, the relationship of their marginal accuracy and fracture strength to the tooth preparation design requires further investigation. Purpose. The aim of this study was to evaluate the effect of variations in tooth preparation design on the marginal accuracy before and after cementation and on the fracture strength of the ceromer/fiber reinforced composite crown. Material and methods. Three metal dies with varying total occlusal convergence angles (6 degrees, 10 degrees, 15 degrees) were prepared. A total of 30 (10 for each angle) Targis/Vectris crowns were fabricated. The restorations were evaluated at 48 points on the entire circumferential margin with a stereomicroscope measuring in micrometers for margin adaptation before and after cementation. The specimens then were compressively loaded to failure in a universal testing machine. Marginal adaptation was analyzed with Kruskal-Wallis test and post-hoc Dunnett test (alpha=0.05). The fracture strength was analyzed with analysis of variance and the Scheffe adjustment at the 95% significance level. Fracture surfaces of the crowns were examined with a scanning electron microscope to determine the mode of fracture. Results. The smallest marginal gap was recorded in angled crowns with a 6-degree convergence (47 Am mean). The marginal gap of most (95.6%) of the crowns was within a clinically acceptable level (established as less than or equal to100 mum). The mean marginal gap increased significantly after cementation, with the largest increase in the 6 degree group (76 Am mean). The 6-degree angled crowns had a significantly higher fracture strength (1543 N) than the more convergent crowns (1366 N) (P<.001). Mean fracture strength of all the crowns was 1390 N. Scanning electron microscope observation showed that all crowns exhibited a 2-phase fracture pattern: a crack and chipping of the Targis layer followed by adhesive failure. Conclusion. Within the limitations of this experimental design, decreasing the axial convergence angle of the ceromer/fiber-reinforced composite crowns diminished their marginal gap and increased their fracture strength.