The effect of grip configuration on achieved non-locking screw tightness

BackgroundScrews are the most commonly used implant in orthopaedics. However, nearly a quarter of screws strip the surrounding material on insertion. This damage reduces pullout strength by over 80%. Screw insertion is subjectively controlled by the surgeon, often using their experience to predict the optimal tightness for that screw. Previous work has shown that measures such as ‘two-finger tightness’ are too unpredictable for a specific torque level. However, there is limited assessment of different grip configurations in achieving consistent differences in torque levels and the screw tightness achieved.ObjectivesThe aim of this study is to measure the achieved tightness and maximum torque using different grip configurations when inserting non-locking screws.Study Design & MethodsFour visiting surgeons tightened 40 self-tapping, non-locking cortical screws of 3.5 mm and 4.5 mm into 40 PCF sawbone sheets of 4 mm thickness, until they had reached what they determined to be optimal tightness. Four different dominant hand grips were assess; thumb and index, thumb and middle, thumb and ring, and finally thumb, index and middle. Screws were then tightened until the maximum torque was reached. If the stopping torque was greater than the stripping torque, the screw was defined as having stripped the material on insertion. The tightness was calculated by measuring the ratio between the stopping:stripping torques. Finally, five screws for each grip and screw diameter were inserted into a block of 40 PCF sawbone of 16 mm thickness to measure the maximum force that could be generated using that grip. Following tests of normality, comparisons were performed between difference variables, with Bonferroni corrections for multiple comparisons.ResultsCombining the results of all surgeons, minimal differences in the achieved tightness with different grips were seen. However, with 3.5 mm screws, using three fingers generated tightness of 77±13%, compared to 70±13% for thumb and index (p=0.03). For 4.5 mm screws, the only difference was between index and middle finger grip, 63±15% and three finger grip 73±11% (p=0.003).With 3.5 mm screws, there were no differences in the maximum torque with any of the two finger grips, but all were less than when using a three finger grip (all p<0.001). Similar findings were seen with 4.5 mm screws for two of the grips compared to the three finger grip (p<0.001), except when using thumb and index grip. This generated higher values than thumb and middle (p<0.001) and thumb and ring (p<0.001) but not more than using three fingers (p=0.12).ConclusionsNo consistent differences in achieved tightness were seen with variations in the grip used. Higher torques were generally created using three fingers, or at least thumb and index. This could be used to moderate the force applied during insertion, using variations in two finger grips to limit the force that a surgeon can apply. Further work into the use of augmentation screwdrivers is being performed to assess alternative methods for repeatably controlling the applied torque and achieved tightness.