Statement of problem: Fracture of all-ceramic fixed partial dentures (FPDs) tends to occur in the connector area.
Purpose: The objective of this study was to test the hypothesis that the radii of curvature at the connector affects the fracture resistance of 3-unit FPDs.
Material and methods: With the use of a standardized silicone mold, 40 three-unit FPD wax patterns were fabricated with the same dimensions and divided into 4 groups of 10 specimens per group. Each pattern was modified at the connector areas of the occlusal embrasure (OE) and the gingival embrasure (GE); 2 wax carvers with radii of curvature at their tips of 0.90 mm and 0.25 mm were used. The dimensions of the connectors were standardized with an electronic caliper to 4 +/- 0.12 mm in height and 5 +/- 0.13 mm in width. Connector designs were as follows: Design I: OE and GE 0.90 mm; Design II: OE 0.90 mm and GE 0.25 mm; Design III: OE 0.25 mm and GE 0.90 mm; and Design IV (control): OE and GE 0.25 mm. An experimental hot-pressed core ceramic was used to make the FPD frameworks, which were consequently cemented on epoxy dies with dual-polymerizing composite (Variolink II) and loaded to fracture in a universal testing machine at a crosshead speed of 0.5 mm/min. The failure load data were analyzed with analysis of variance (ANOVA; P=.05) and Duncan's test (alpha=.01).
Results: The mean failure loads and standard deviations were as follows: 943 +/- 151 N for Design I; 746 +/- 106 N for Design II; 944 +/- 144 N for Design III; and 673 +/- 55 N for Design IV. ANOVA revealed a significant difference (P< or = .0001) between the mean failure loads of different connector designs. The mean loads to failure for Designs I and III were significantly higher than those for Designs II and IV (Duncan's test).
Conclusion: Within the limitations of this study and for the experimental ceramic tested, as the radius at the gingival embrasure increased from 0.25 to 0.90 mm, the mean failure load increased by 140%. The radius of curvature at the occlusal embrasure had only a minor effect on the fracture susceptibility of 3-unit FPDs.