Zirconium implant abutments: Fracture strength and influence of cyclic loading on retaining-screw loosening

Objective: The purpose of this study was to determine the fracture strength of zirconium implant abutments and the torque required to unfasten the retaining screw before and after applying cyclic loading to the implant-abutment assembly. The dynamic behavior and stress distribution pattern of zirconium abutments were also evaluated. Methods and materials: Static and cyclic loading of 7 XiVE implants with straight Cercon zirconium abutments were simulated under worst-case conditions. Cyclic loading tests were performed via a servohydraulic dynamic testing machine at loads between 100 and 450 N, for up to 5 million loading cycles, at 15 Hz. The dynamic behavior of the zirconium abutments was analyzed by finite element modeling and Pro/Mechanica software, comparing van-der-Mises and maximum stress levels. Results: Cercon zirconium-ceramic abutments exhibited a maximum fracture strength of 672 N during static loading and 269 N at 800,000 to 5 million cycles runout point, and 403 N at 10,000 cycles runout point during cyclic loading. The mean torque value required to unfasten the abutment retaining screws after (initial) tightening was 21 Ncm ± 1 and 20 Ncm ± 1 (measurement accuracy ± 2 Ncm) after loading with up to 5 million cycles respectively. Torque values decreased minimally and screw loosening did not occur. Within the limited number of test specimens (7), the difference was statistically significant (P = .015). FEM analysis displayed higher stress peaks up to 800 MPa at the cervical aspect of the zirconium abutment and at the apical third of its retaining screw at an external load of 250 N. Conclusion: Within the limitations of this study, zirconium implant abutments exceeded the established values for maximum incisal bite forces reported in the literature and tightly fit into the titanium implant after several millions of loading cycles. Keywords: cyclic loading, finite element analysis, fracture strength, screw loosening, zirconium implant abutments