Head-locking durability of fixed and variable angle locking screws under repetitive loading

摘要

Polyaxial locking screws are increasingly applied in fracture fixation. To investigate the durability of the head-locking mechanism, the removal torque of variable angle (VA) and fixed angle (FA) stainless steel and titanium locking screws was investigated without and after a cyclic loading test. Stainless steel (St) and titanium (Ti) 2.4mm orthogonally inserted FA screws and 2.4mm VA screws inserted in different inclinations (0 degrees-15 degrees) (n=6 per group) were locked at 0.8Nm. Removal torque was determined without (W) and after (A) cyclic loading (sinusoidal load, 5Hz, constant amplitude of 25N, up to 10000 cycles, or failure). Significant differences in-between the groups were detected by Student's t-test (p<0.05). Except VA Ti in 0deg and FA, all groups exhibited a drop in removal torque below the insertion torque without and after cyclic testing. The removal torque was (St: FA W:0.81Nm +/- 0.04 A:0.72Nm +/- 0.04; VA0deg W:0.73Nm +/- 0.04 A:0.65Nm +/- 0.05; VA15deg W:0.51Nm +/- 0.05 A:0.50Nm +/- 0.08; Ti: FA W:0.82Nm +/- 0.03 A:0.70Nm +/- 0.04; VA0deg W:0.80Nm +/- 0.02 A:0.72Nm +/- 0.05; VA15deg W:0.55Nm +/- 0.03 A:0.54Nm +/- 0.06). In all groups, the removal torque after cyclic testing did not drop below 16% of the removal torque without cyclic testing. No head loosening was observed after cyclic testing. Stainless steel and titanium 2.4mm fixed and variable angle locking screws provide a stable and lasting head-locking mechanism. (c) 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:949-952, 2016.