Effect of Connector Cross-Sectional Geometry on Fracture Resistance of Three-Unit Monolithic Zirconia Fixed Dental Prostheses Under Vertical and Oblique Loading: An In Vitro Study

Introduction: The long-term success of monolithic zirconia fixed dental prostheses depends not only on the mechanical properties of zirconia but also on the biomechanical design of the connector region. Although connector dimensions have been extensively investigated, limited evidence is available regarding the influence of connector cross-sectional geometry on fracture resistance in different loading directions. The aim of this study was to evaluate the influence of round, triangular, and square connector cross-sectional geometries on the fracture resistance of three-unit monolithic zirconia fixed dental prostheses under vertical and oblique loading conditions. Materials and methods: This in vitro study included 48 three-unit monolithic zirconia fixed dental prostheses fabricated using a standardized computer-aided design and computer-aided manufacturing workflow. The specimens were equally distributed into three connector design groups (round, triangular, and square), each further subdivided according to the vertical and oblique loading conditions (n = 8). The connector cross-sectional area was standardized at 9 mm² in all specimens, with the connector geometry serving as the only experimental variable. Following cementation onto the standardized resin dies, fracture resistance testing was performed using a universal testing machine at a crosshead speed of 1 mm/min until catastrophic failure. Data were analyzed using one-way analysis of variance followed by Tukey's post-hoc test, with statistical significance set at p < 0.05. Results: The round connector demonstrated the highest mean fracture resistance under both vertical (1451.50 ± 132.84 N) and oblique (1151.25 ± 166.70 N) loading conditions. One-way analysis of variance revealed statistically significant differences among the connector designs under vertical (F = 16.50, p < 0.001) and oblique loading (F = 3.82, p = 0.038). Post-hoc analysis showed that the round connector exhibited significantly greater fracture resistance than both the triangular and square connectors under vertical loading, whereas under oblique loading, a significant difference was observed only between the round and square connector designs. Conclusion: The connector cross-sectional geometry significantly affected the fracture resistance of monolithic zirconia fixed dental prostheses. The round connector demonstrated superior mechanical performance under both loading conditions, suggesting that rounded connector configurations may improve the structural reliability and durability of posterior zirconia fixed dental prostheses.