Objectives: The currently used screw insertion techniques are reliant on a surgeon’s subjective feeling of the required and applied torques. In experimental testing, approximately 1 in 4 screws have irreparably damaged the surrounding bone whilst being inserted due to an inability to appreciate the stripping limits, increasing the risk of fixation failure . If maximum and optimum torques could be predicted prior to screw insertion, this could reduce failures, expedite operations and strengthen constructs. The aim of this study was to devise a method to quantify and optimise tightness for the insertion of cortical fracture-fixation screws, based on bone characterisation and screw geometry. Methods: Cortical human diaphyseal tibiae samples (n=20) underwent destructive testing to firstly establish the relationship between cortical thickness and experimental stripping torque (Tstr) and secondly to calibrate an equation to predict Tstr. Using the equation’s predictions, 3.5 mm screws were inserted (n=66) to targeted torques representing 40 to 100% of Tstr, with the compression generated during tightening being measured. Once the target torque had been achieved, immediate pullout testing was performed. Results: Cortical thickness predicted the stripping torque (R2=0.862, P<0.001) as did an equation based on tensile yield stress, bone-screw friction coefficient and screw geometries (R2=0.894, P<0.001). Compression increased with screw tightness – calculated as the ratio of targeted/stripping torque - up to 80% of the maximum (R2=0.495, P<0.001). Beyond 80%, no further tightness generated the same increase in compression. Pullout force did not change with variations in submaximal tightness beyond 40% of Tstr (R2=0.014, P=0.175). Conclusion: Screws tightened to 70 - 80% of the predicted maximum generated optimum compression and pullout forces. Further tightening did not significantly increase compression, did not improve resistance to pullout and increased the risk of the bone being stripped. Whilst intraoperative methods for accurately and reliably predicting the maximum tightness for a screw are needed, this work justifies controlled insertion considerably below the maximum torque and demonstrates methods to predict the required torque.
|Publication status||Published - Sep 2019|
|Event||Bone Research Society and British Orthopaedic Research Society 5th Joint Meeting - Sir Martin Evans Building, School of Biosciences, Cardiff University, Cardiff, UK United Kingdom|
Duration: 4 Sep 2019 → 6 Sep 2019
|Conference||Bone Research Society and British Orthopaedic Research Society 5th Joint Meeting|
|Abbreviated title||BRS/BORS 5th Joint Meeting|
|Country||UK United Kingdom|
|Period||4/09/19 → 6/09/19|
Fletcher, J., Zderic, I., Gueorguiev, B., Richards, R. G., Gill, R., Whitehouse, M., & Preatoni, E. (2019). What is the optimum tightness for nonlocking cortical screws, and how can this be predicted prior to insertion?. Poster session presented at Bone Research Society and British Orthopaedic Research Society 5th Joint Meeting, Cardiff, UK United Kingdom.