5. Reducing set screw loosening failures: an analysis of final tightening technique

Abstract

<h3>BACKGROUND CONTEXT</h3> A 5% rate of S2AI screw failure consisting of rod slippage/set screw displacement has been reported. Fixation construct strength is largely dependent on the set screw's locking torque. Both the magnitude (how much torque is applied) and the rate (how fast the torque is applied) can impact set screw loosening failure. <h3>PURPOSE</h3> To evaluate the effect of set screw locking torque on the strength of the fixation construct. <h3>STUDY DESIGN/SETTING</h3> Benchtop biomechanical testing. <h3>METHODS</h3> Commercially available, Ti alloy 5.5mm polyaxial pedicle screws and set screws were used. To test the effect of the locking torque magnitude on construct strength, the set screws were tightened onto a Ti alloy rod at 85 and 105 in-lbs (N=8 each). A torque meter (TM) [Transducer Techniques, SWS-20] was attached to the driver to record both the locking and loosening torque. To test the effect of the rod alloy, the same implants were tested with 5.5 CoCr and Ti alloy rods (N=8 each). The set screws' locking and loosening torque were measured. Also, six torque limiting handles from five suppliers were attached to the same TM. To test the effect of torque application rate, an experienced user torqued the handle until the device's internal limit was reached. The resulting torque was measured. Each handle was subjected to 11 actuations at two rates fast (defined by taking less than one second to click over after resistance is felt) and slow (defined by taking 1-2 seconds to click over after resistance is felt). <h3>RESULTS</h3> Set screws tightened to 105.8(±.54)in-lbs retained 39.6% more torque than those tightened to 85.4(±.57)in-lbs(p <0.001), demonstrating that more torque applied results in more torque retained. Set screws tightened onto a Ti alloy rod retained 15.0% more torque than those tightened onto a CoCr rod (p < .003). When torque limiting handles are actuated at a slow rate, the resulting torque output produced is 22.6% higher than when actuated at a fast rate, demonstrating the magnitude of the torque applied is dependent on the rate at which the handle is rotated (p <0.05). <h3>CONCLUSIONS</h3> The choice of spinal rod alloy used may affect set screw security. More torque applied results in more torque retained; therefore, we recommend the final locking torque be applied in a slow and controlled manner to ensure the intended locking torque magnitude is applied. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.