Stripping Torque Model for Bone Screws

Abstract

Abstract Correct torquing of bone screws is critical for positive patient outcomes in orthopaedic surgery. Under-or over-tightening screws can lead to thread stripping or screw loosening, leading to implant/fixation failure and potential tissue damage or disability. It has been proposed that an automated torque-limiting smart-screwdriver may be able to use model-based methods to determine the properties of bone as screws are inserted, and then use these to determine the optimal tightening torque and provide a torque-indication or-limitation to enforce this limit. Previous work focused on identifying the material properties from sensor data, but this paper will address the unanswered question of torque-limit prediction. Here we have developed a simple model of screw thread stripping. This model is based on the assumption that overtightening the screw will shear a cylindrical section of the underlying material. This simple assumption is augmented with a stress concentration factor dependant on the screw geometry. This model was tested against experimental stripping-torque data. We found that without the stress-concentration factor the model produced predicted torques with a strong linear relationship to the experimental values (R2 = 0.98), however the magnitude of the predictions was 2-3 times too high. Including the stress concentration factor brought these predictions into the range of the experimental values, but the strong linear relationship from before was disrupted (R2 = 0.80). Overall, this approach is promising for optimal torque prediction, but needs more thorough testing with a range of materials and screws, and has room for improvement with the stress-concentration factor.