FRACTURE FORCE OF CANTILEVERED ZIRCONIA FRAMEWORKS: AN IN VITRO STUDY

Statement of problem. Little evidence is available showing the effect of connector dimension and cantilever length on the ultimate fracture force of computer-aided design/computer-aided manufacturing (CAD/CAM) zirconia implant frameworks. Purpose. The purpose of the study was to determine the impact of variations in the cross-sectional dimension of connector sites and variations in the effective cantilever length (load point) on zirconia implant frameworks. This would allow verification of the fracture force with 2 proposed mathematical models. Material and methods. Forty zirconia implant-supported frameworks with 12-mm distal cantilevers were divided into 4 equal test groups (n=10). Connector dimensions (3 x 5 mm, 3 x 4 mm) and cantilever loading distance (7 mm, 10 mm) were tested for ultimate fracture force. A 2-way analysis of variance (ANOVA) was used to examine the ultimate fracture force and examine the relationship between connector dimension and ultimate fracture force. The data obtained from all 4 groups were compared and verified with calculations from 2 theoretical mathematical models. Results. Two-way ANOVA revealed significant effects for cross-sectional area connector dimension on fracture force (P<.001) and cantilever length (P=.009). No statistically significant interaction was observed between the 2 factors (P=.229). The observed data were consistent with the data from the proposed mathematical models, with group comparisons showing no statistical significance. The largest difference between the mathematical results and mathematical models was in the 7 mm 3 x 5 mm group of the fixed cantilever bending model (P=.032). The predominant mode of failure was fracture of the zirconia framework, without damage or plastic deformation of the abutment screws or implant analogs. The 10 mm 3x5 mm specimens fractured at a mean load of 923.7 +/- 234.5 N; the 10 mm 3 x 4 mm specimens at a mean load of 474.8 +/- 122.9 N; the 7 mm 3x5 mm specimens at a mean load of 1011.7 +/- 185.3 N; and the 7 mm 3 x 4 mm specimens at a mean load of 700.9 +/- 152.4 N. Conclusions. Zirconia implant frameworks loaded 7 mm from the distal abutment failed at higher fracture loads than specimens loaded 10 mm from the distal abutment. Zirconia implant frameworks with cross-sectional area connector dimensions of 3x5 mm failed at higher fracture loads than specimens with cross-sectional area connector dimensions of 3 x 4 mm. No statistically significant interaction was observed between the cross-sectional connector area dimension and cantilever length. Calculations from the mathematical models closely approximated the observed data, which supports the use of the mathematical models as a predictor of fracture force.