Assessing screw tightness and stripping rates achieved by orthopaedic surgeons and researchers, including using an augmented screwdriver

Background
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.
Objectives
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. Finally, to identify the impact of using an augmented screwdriver to indicate optimum tightness.
Study Design & Methods
Twenty participants (10 visiting surgeons and 10 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. 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. Three participants from each group then used an augmented screwdriver that indicated when the predicted optimum tightness was reached.
Results
Combining all screw diameters and insertion conditions, the median stopping/stripping ratios for unstripped screws inserted by researchers and orthopaedic surgeons were 77.5% ±2.8 and 84.6% ±2.1 respectively (n=1406 and 928). The former stripped 21.9% of insertions, whilst surgeons stripped 48.1% (p<0.01). 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. Augmented screwdrivers did not affect screw tightness, but did reduce stripping rates.
Conclusions
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 more than twice as often. Using subjective assessment, surgeons were more likely to detect when overtightening had occurred. Using a screwdriver that indicates when optimum torque is reached reduces stripping rates whilst not affecting achieved tightness. 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.