Background: Connectors in zirconia fixed partial dentures (FPD) significantly affect fracture resistance especially for posterior 3-unit FPD where functional loads are focused within the pontic-retainer connectors. In posterior zirconia FPDs, a connector cross-section of 9 mm² is a standard recommendation, but the mechanical impact of connector contour at this standard connector cross-section clinically remains relevant. The aim of this in vitro study was to determine if elliptical connectors and rounded connectors differ in compressive strength for three-unit FPDs produced with CAD/CAM milled zirconia framework. Methods: 43 unit FPD frameworks made of CAD/CAM-milled zirconia were fabricated on a standardized mandibular posterior model with mandibular first premolar and first molar as abutment teeth and second premolar as the pontic. The specimens were divided into two groups: Group A (elliptical connector design, 20 specimens) and Group B (rounded connector design, 20 specimens). The designs were all of the same connector cross section of 9 mm². The same sintering and glazing procedure was performed for frames. Great reductions in crosshead speed were applied to the pontic area in a universal testing machine until failure. Parameters recorded were the failure load, initial crack load, stiffness, the displacement at fracture, the energy to fracture and the mode of failure. Independent t-tests, Fisher exact/χ² tests, non-parametric sensitivity analyses and effect sizes were utilized. Results: The peak fracture load for rounded connector was significantly higher than that of elliptical connector (1420 ± 160 N and 1250 ± 180 N respectively). The mean difference was 170 N [95% Confidence interval 61-279 N; p value 0.003]. The effect size was Cohen’s d = 1.00, considered to be large. Rounded connectors had statistically significantly higher initial crack load (930 ± 150 N) and the stiffness (560 ± 65 N/mm) compared to those of the square connectors (820 ± 140 N, 510 ± 70 N/mm), respectively, and the most common failure location was the connector region for both groups. Conclusion: Rounded geometry connectors of the CAD/CAM-milled zirconia three-unit FPDs significantly increased the fracture resistance of the units when compared to tetrahedron connectors of units with cross sectional area of 9 mm². A design development which could be feasible is connector rounding to enhance load tolerance without increasing connector bulk.
This work is licensed under a