Biomechanical evaluation of an adjustable patellar tendon bearing prosthesis

The purpose of this study was to determine the relation between geometry of the patellar tendon (PT) insertion in the socket and the dynamic load transmitted through the PT, in a patellar tendon bearing (PTB) prosthesis. The dynamic load was measured by a specially constructed two-component load cell, which was mounted on the experimental prosthesis. Geometry of the PT insertion relative to the socket was altered by adjusting the location of the mounting frame-load cell assembly. Evaluation of performance of the prosthesis in the range of geometries investigated was made objectively by gait analysis of the patients. The latter was performed on a 10 m walkpath, including at halfway two forceplates, from which the foot-ground forces were recorded. Simultaneously, the PT forces on the prosthesis, as sensed by the load cell were monitored. Computer processing of the data included full analysis of the force-trace and the determination of criteria for optimization of the PT insertion. The resulting criteria were those which expressed minimization of abnormalities in the foot-ground forces of each patient, such as vertical and braking forces and their timings. The complete optimization procedure, which was undertaken for two below-knee male amputees, indicated the correct positioning of the PT insertion, actually taken in the final fitting procedure of the socket of the prosthesis.