Integration of ultrasound based registration with statistical shape models for computer assisted orthopaedic surgery

We present the first use of ultrasound to instantiate and register a statistical shape model of bony structures. Our aim is to provide accurate image-guided total hip replacement without the need for a preoperative computed tomography (CT) scan. We propose novel methods to determine the location of the bone surface intraoperatively using percutaneous ultrasound and, with the aid of a statistical shape model, reconstruct a complete three-dimensional (3D) model of relevant anatomy. The centre of the femoral head is used as a further constraint to improve accuracy in regions not accessible to Ultrasound. CT scans of the femur from a database were aligned to one target CT scan using a non-rigid registration algorithm. The femur surface from the target scan was then propagated to each of the subjects and used to produce a statistical shape model. A cadaveric femur not used in the shape model construction was scanned using freehand 3D ultrasound. The iterative closest point (ICP) algorithm was used to match points corresponding to the bone surface derived-from ultrasound with the statistical bone surface model. We used the mean shape and the first five modes of variation of the shape model. The resulting root mean square (RMS) point-to-surface distance from ICP was minimised to provide the best fit of the model to the ultrasound data.