A computer model of 23 knees was obtained by embedding, slicing and digitizing the bone outlines and ligament co-ordinates. Using co-ordinate transformations, various three-dimensional motions were imposed on the knees, and calculations made of femoral-tibial contact error, contact point locations and ligament lengths. Significant deviations in these parameters were noted for abnormal motions including the elimination of internal-external rotation and a-p displacement and the misplacement of a hinge producing correct motion. The resulting mismatch could result in shear in soft tissues, cuff-to-skin slippage and inaccurate ligament length patterns.