Mechanical performance of conical implant-abutment connections under different cyclic loading conditions

Conical implant-abutment connections are popular for its anti-bending performance; on the other hand, the torsional and axial forces also play important roles in occlusion. However, so far there were scarce studies on their effects on connection stability. Therefore, this study seeks to investigate the mechanical performance of conical connections under different cyclic loading conditions.

15 conical implant-abutment assembles (Cowell Medi, Busan, South Korea) were divided into 3 groups according to different cyclic loadings. In group BTA, the loading condition of the posterior occlusion was simulated (20-200 N, 30° off-axis and 4 mm eccentric to implant axis), generating a bending moment, a torsional moment, and an axial loading. In group BT, a bending moment and a torsional moment of the posterior occlusion were applied (10-100 N, 90° off-axis and 4 mm eccentric to implant axis). In group B, only a bending moment was applied (10-100 N, 90° off-axis and through implant axis). The fatigue testing machine ran at 10 Hz until failure, or to the upper limit of 10⁶ cycles. The fatigue cycles and failure modes were recorded. Besides, the value of the torque loss of the abutment screw, the difference between initial torque and post-load reverse torque, was calculated. The data were statistically analyzed. Morphologies of the abutment conical surface were examined by scanning electron microscopy.

In group B and BTA, all samples passed the test (10⁶ cycles). While, in group BT, all abutments generated rotation within 140 cycles, showing significant differences compared to the other two groups (p < 0.001). However, from SEM observations, both group B and BT showed marked fretting wear, indicating obvious micromotion in the connection. Whereas group BTA showed indentation of tight contact, attributed to the axial loading. In terms of the torque loss of the abutment screw, the torque loss in group BT was much more than the other two groups with statistically significant differences (p < 0.05).

Owing to the effect of the bending moment, marked fretting wear was generated in the conical connections and further led to loss of the anti-torsional ability. However, adding an axial loading could improve their anti-torsional ability significantly.