A comparison of four functional methods to determine centers and axes of rotations.

Several functional methods to determine joint centers of rotation (CoR) and axes of rotation (AoR) have been reported in the literature. These have previously been compared using noise introduced to marker positions. The current study compares four of these methods under both rigid body and deformable body conditions using a mechanical analog of the lower extremity. In the latter condition, significant inter-marker distance changes were present in patterns directly related to the joint motion. Both hip CoR location and knee AoR location and direction are determined. Results indicate that while all methods produced accurate measures under rigid body conditions (<5mm location and <2 degrees direction), there were many differences between methods in deformable conditions. Under these conditions, the method described by Gamage and Lasenby performed better than the other methods examined. Maximum mean errors for this technique were approximately 1cm for CoR and AoR locations and 1 degrees for AoR direction.