Virtual reality surgical simulators are showing good promise as a viable addition to surgical training. In orthopaedic surgery, drilling is one of the most common and critical skills performed. Although the technology to recreate the sights and feel exists, a suitable haptic feedback model of bone drilling does not exist. The aim of this research is to develop a suitable model for predicting the thrust force felt while drilling bone. The key factors affecting the thrust force are the drilling feed rate, drill bit geometry and bone mineral density (BMD). Experiments were performed to measure the thrust force while drilling 31 porcine vertebrae using drill bits of 2.0, 2.5 and 3.2 mm diameters and feed rates of 75, 100 and 125 mm/min. Quantitative computed tomography was used to document the BMD in the drilling trajectory. Maximum thrust force was recorded at 29 N and maximum bone mineral density was 1155 mg/cm³. Thrust force was found to correlate well with BMD with an exponential fit (F=a·e^b·BMD) for each drill bit diameter. Three models from the metals, composites and rock drilling literature were used to calculate an energy parameter, which was then correlated with BMD. Predicted thrust force was calculated using the average BMD profiles of the remaining data and compared to both the BMD based model and the experimental thrust force. All the theoretical models were better predictors than the BMD based model. The simplest model, F = k_s D/2 f/2 where k_s= 1.24x 10⁷ e^0.003·BMD, D is the drill bit diameter and f is the feed was the most accurate predictor. To further investigate the mechanics of drilling in bone, a finite element (FE) model of the chisel edge indenter was developed and validated. The model was validated with the experimental data for the 2.0 mm data but validation of the 3.2 mm data was not successful. A factorial design using the FE model determined that the trabecular bone density, the feed of the drill, and the base material supporting the bone are the significant factors in the thrust force.

A preliminary thrust force prediction model, suitable for the haptic portion of a virtual reality surgical simulator, has been successfully developed. Further experimental and theoretical work in expanding these results to more drill bits, human bone, anatomical sites and clinically relevant holes is recommended for a more complete model.