Abutment screw loosening and bending resistance of external hexagon implant system after lateral cyclic loading

Rigorous efforts to reduce the recurrence of abutment screw loosening in single-tooth implant restorations have recently been made. However, the behavior of the implant/abutment joint components with respect to critical bending force is still unclear.

This study investigated the effect of different cyclic loading periods on abutment screw loosening and bending resistance of a single-tooth external hexagon implant system.

Fifteen Brånemark implant assemblies were divided equally into groups A, B, and C. Each assembly consisted of a Brånemark System Mk IV 4 x 10 mm implant (Nobel Biocare AB, Gothenburg, Sweden) mounted in a brass block, a CeraOne 3 mm abutment (Nobel Biocare AB), and an experimental cement-retained superstructure. For groups A and B a cyclic load of 50 N was applied centrally and perpendicular to the long axis of the implant. Targets of 1.0 x 10(6) cycles (40 months of simulated function) and 0.5 x 10(6) cycles (20 months of simulated function) were defined for groups A and B, respectively. Group C (control) was left unloaded for the same loading time period as was group B. Reverse torque was recorded before and after loading, and the difference was calculated. After cyclic loading, specimens were mounted in a testing machine, and the yielding and bending strengths were measured. The data were analyzed with one-way analysis of variance and were compared by means of the Tukey test (p < .05).

There were statistically significant differences (p < .001) in the reverse torque difference values of group A ([-5.6 to -3.4] +/- 0.86 Ncm) as compared to those of group B ([-2.4 to -1.6] +/- 0.32 Ncm) and group C ([-0.7 to 0.0] +/- 0.26 Ncm). Likewise, group B showed a significant difference compared to group C (p = .002). On the other hand there was no statistically significant difference in the mean values among the test groups in regard to the yielding and bending strengths (p >.050).

Within the limitations of this study, long-term fatigue significantly affected the reverse torque values under centric lateral load (p <.001) whereas it had no significant effect on the resistance of the implant/abutment joint to static bending.