A new method to compare planned and achieved position of an orthopaedic implant.

The present study describes an automatic method to evaluate the efficacy of a computer aided orthopaedic surgery system by comparing the position of the joint implant, as derived from post-operative computed tomography (CT) scans, to that planned by the surgeon before the operation. The method relies on two spatial registrations, one to align the post-operative femur with the pre-operative femur, the second to compute the planned versus achieved (PVA) accuracy as the roto-translation that registers the pre-operative implant position with the post-operative position. Two surface registration algorithms (a generic average distance minimisation and the specialised iterative closest point (ICP) method) were comparatively evaluated first on a set of test cases to measure the absolute accuracy and robustness with respect to peculiar situations such as a distant starting point. The average distance method failed the registration of one test case and showed peak errors of 0.97 degrees on the rotations and 3.09 mm on the translations. The ICP method was found much more efficient and was able to register all test cases. The peak error was 0.44 degrees on the rotations and 0.67 mm on the translations. The ICP method was then used to compute the PVA accuracy on six clinical cases treated with a CT-based planning system in combination with conventional surgical procedures. The method successfully processed all cases demonstrating the efficacy of the proposed procedure in the specific application.