Effect of tightening locking screws torque on unscrewing ability and on the screw removal torque

Some manufacturers recommend using torque-controlled screwdrivers to tighten locking screws to prevent problems removing the screw due to seizing of the screw in the plate hole or stripped screw head recess. Alternatively, increasing the tightening of the screws has been recommended to reduce the risk of their unlocking. The aim of this study was to test the effect of screw insertion torques higher than the recommended torque on difficulty removing the screw and on the amount of unscrewing torque.

Stainless steel locking compression plates (LCP; Depuy Synthes) and 3.5 mm locking screws were implanted into synthetic bone models included in a customized 3D-printed template, allowing each screw tested to be aligned with a torquemeter. A total of 160 screws was used. Ten screws were tested at each torque of respectively, 1.5 Nm, 2.0 Nm, 2.5 Nm, 3.0 Nm, 3.5 Nm, 4.0 Nm, 4.5 Nm and 5.0 Nm. An additional screw was tightened and untightened in the same plate hole 10 times consecutively for each torque. Each screw tested was removed and the removal torque measured by the torque meter.

All locking screws were easily unscrewed even with an insertion torque of 5.0 Nm. The removal torque was slightly lower than the insertion torque in 58.8 % of the cases and slightly larger in 41.2 %.

To improve screw retention, the recommended torque of 1.5 Nm for the 3.5 mm stainless steel locking screws tested can be increased to 2.0 or 2.5 Nm, without affecting their unscrewing ability.