Influences of configuration changes of the patella on the knee extensor mechanism.

Biomechanical analysis of the configuration changes of the patella was studied. Normal patellar tracking in the sagittal plane was obtained by recording and digitizing knee extension of six healthy adults with fluoroscopy. Based on the digitized data, the changes of patellar thickness were simulated by translating its axis and its forward and backward tilting by rotation in the sagittal plane. We assumed that the extensor moment remained constant before and after the patellar configuration was changed. Therefore, using a balance beam, model equilibrium was reached, after each simulated change of patellar configuration, of the forces involved in the extensor mechanism: the quadriceps muscle force, tension of patella tendon and patellofemoral joint reaction force. The results revealed that when the patellar thickness decreased, quadriceps force increased but patellofemoral joint reaction force decreased and the reverse was seen when the patella thickened. Backward tilting of the patella decreased quadriceps force and patellofemoral joint reaction force and increased the patella tendon/quadriceps force ratio, which produced a better mechanical advantage of force transmission of patella. The reverse was seen in forward tilting. These results suggest that the patella could be tilted backward for the older patient undergoing total knee replacement. For younger patients with relatively strong quadriceps musculature, we suggest that the patellar thickness could be decreased to reduce the patellofemoral joint reaction force so as to reduce the chance of failure of the patella prosthesis.