Evaluation of achievable registration accuracy of the femur during minimally invasive total hip replacement.

The aim of the paper was to investigate whether accurate, point-based registration of the intra-operative femur will be achieved within the context of minimally invasive surgery for total hip replacement. Computer tomography images, collected for pre-operative planning purposes, were used to simulate the intra-operative registration procedure using algorithms for various levels of measurement noise, different small areas of the femur available to the surgeon, and a limited number of collected data points (20-60). This helped with the choice of design variables to perform in vitro registration on a plastic bone model to validate the procedure, which included a multistart algorithm developed for intra-operative registration. The algorithm minimised the distance between the measured and image-derived surfaces and was able to cope with the presence of multiple local minima given sufficient computational effort, even with realistically large measurement noise. It was found that, if a small patch of the femur was used, accessible by a needle that could at times penetrate thin layers of soft tissue, errors in the order of 1.0 mm in translation and 0.5 degrees in rotation were achievable.