Force analysis of the patellar mechanism.

The aim of this study is an experimental evaluation of a force analysis of the patellar mechanism based on the assumption that patellofemoral contact is frictionless. At first, the geometric characteristics of contact surfaces, a prior knowledge of which is necessary for quantitative analysis, were measured from radiographs of 42 fresh-frozen knee specimens in the flexion range 0-120 degrees. The results were then used in the analysis to predict the relations between the forces acting on the patella. For the evaluation of the analysis, the ratio of the tension in the ligamentum patellae and the rectus femoris was measured in ten specimens during simulation of two knee functions: (1) "leg raising" against a resistance; and (2) "static lifting". The effect of flexion angle on the ratio is found to be rather complex. With increasing flexion, the ratio increases initially up to 30 degrees, then decreases up to 90 degrees, and finally increases again beyond 90 degrees. The ratio is above unity up to around 45 degrees and below that in the remaining flexion range. The analysis has been found to predict not only the characteristic variation of the ratio but also its magnitude with reasonable accuracy. It has been concluded that for an accurate prediction of the patellofemoral joint reaction, the force analysis needs to be based on the geometry of the contact surfaces. This implies that the mechanical consequences of surgical procedures involving tibial tubercle relocation cannot be inferred simply on the basis of their effect on the patellar mechanism angle, but that they also require consideration of their effect on the contact geometry.