Assessing screw tightness achieved by orthopaedic surgeons and researchers

Screws are the most commonly inserted orthopaedic implants. Yet despite their use, nearly one in four non-locking screws irreparably damage the surrounding bone due to excessing tightening, reducing construct strength by more than 80%. Screw tightness is controlled manually based on a surgeon’s subjective assessment of the force required. Limited data exist on the tightness commonly achieved by surgeons’ and how it is affected by different variables such as screw diameter. Additionally, there are no existing studies specifically assessing the skills of orthopaedic researchers despite the numerous experiments into screw fixation performed by them. This study aims to identify the achieved tightness for orthopaedic surgeons and researchers and to measure the effect on tightness when using different screw diameters and when used in plate fixation or individually placed with a washer. Method Eight participants (four visiting surgeons and four researchers) gave informed consent for assessment of their surgical techniques. Each participant inserted 60 cortical screws for each of three screw diameters (2.7, 3.5 and 4.5 mm) into 20 PCF Sawbone sheets of 4 mm thickness. Half of screws were inserted into the correspondingly sized plate and half as individual screws with a washer. Participants were instructed to tighten each screw to what they determined to be optimal tightness. Each time, a digital torque screwdriver was used to record the stopping torque, with the participant blinded to its display. Once inserted, they reported the confidence in achieving their target tightness from 1-10 (one being very poor and 10 being optimal tightness) and whether they felt the screw had stripped the material. Screws were then inserted by a faculty member until the stripping torque (defined as the maximum recorded value) was achieved, allowing rationing of the stopping/stripping torque to calculate tightness. Tests of normality were performed, with comparisons made using Mann-Whitney U tests. Results Combining all screw diameters and insertion conditions, the median stopping/stripping ratios for unstripped screws inserted by researchers and orthopaedic surgeons were 78.1% (IQR 9.7) and 75.6% (IQR 7.4) respectively (p=0.67) (n=333 and 683). The former stripped 27.2% of insertions, whilst surgeons stripped 53.8% (p=0.02). Achieved tightness was less for screws inserted into washers compared to those inserted into plates; surgeons (p=0.04) and researchers (p=0.12). The average reported confidences in screw purchase when the materials were unstripped and stripped were 7.3 and 7.2 for researchers (p=0.59) and 6.7 and 5.3 for surgeons (p<0.001) respectively. Conclusion Surgeons and researchers generally applied the same tightness to screws when the material was not stripped. However, in the surgeons’ tests, they stripped the material twice as often. Using subjective assessment, surgeons were more likely to detect when overtightening had occurred. Further work with surgeons and researchers of a broad range of experiences will increase the validity of this work, alongside developing clinical methods for reducing both stripping rates and variation in achieved tightness.