Effects of Velocity Profile and Plate Usage on Identified Bone Strength during Instrumented Screw Insertion

Abstract

Bone screws are important in orthopaedic surgery to treat fractures and secure implants. Over- or under-tightening these screws may lead to premature failure of the screw fixation, necessitating risky and costly revision surgery. Previously, a model-based method for automatically optimising bone screw insertion torque was developed. This paper expands on the prior testing of this method, to investigate how non-ideal conditions may affect the model accuracy/precision.A bone screw was inserted into pre-drilled holes in artificial bone made from PU foam. Three cases were tested: first the screw was inserted directly with a constant velocity, then it was inserted with a trapezoidal velocity profile, and lastly it was inserted with a constant velocity profile, but through a hole in a metal plate. Each case was repeated 20 times for 60 total insertions. Torque and angular displacement measurements were used with a previously-developed model to identify the foam material strength for each insertion. Summary statistics were calculated for each case and statistical tests were used to compare the means and variances for the identified values between each case.Comparing to base base case with constant velocity and no plate: we found a statistically significant (p < 0.05) difference in the mean identified strength for the trapezoidal velocity and the case with the plate. We found a statistically significant difference in the variance for the trapezoidal velocity profile (p < 0.05), but not for case with the plate present (p = 0.16).Future work should investigate these effects over a wider range of materials and screws.