Evaluating the Screw Fastening Torque Required to Generate Appropriate Preload in Bone Fracture Fixation

Surgeons generally use subjective feel to tighten bone screws as much as possible without stripping the threads; however, subsequent fixation failures are common, particularly in osteoporotic bone. We hypothesise that if the preload generated in reduced bone quality and it’s dissipation over time was better understood, then some of these failures could be avoided through the use of more appropriate fastening torques or a different number of screws. This ongoing study aims to develop a validated finite element model capable of predicting the preload generated for different bone densities and cortical thicknesses.
To date, experimental tests (n=10) were conducted using bovine tibiae (4-5 months) to evaluate the bone-implant pressure (LLW Prescale Fuji Film) and surface strains (Ncorr v1.2, G.I.T.) for a range of tightening torques. The bone density (assessed using CT-images) and cortical thickness of each specimen were recorded. An axisymmetric finite element model was developed based on the experimental tests (Ansys 15.0).
Our early simulations predicted the equivalent strain at the edge of the implant-bone contact region to be 0.062%, which was within the range of the experimentally observed values (0.04-0.07%). The predicted bone-implant pressure also compared favorably with a pressure varying between 2.5  and 0 MPa. Using an experimentally derived relationship between fastening torque and bone-implant pressure, the model has the potential to predict preload for any bone quality.
Our intention is to develop this model, in combination with established formulas to calculate stripping torque, into a patient-specific tool providing surgeons with guidance regarding appropriate screw fastening torques for different bone qualities.