Effect of patellar thickness on kinematics of the knee joint.

Patellofemoral problems are common complications after total knee replacement. In order to investigate the effect of an increase in the patellar thickness on the kinematics of the knee joint, four intact embalmed cadaveric knee specimens were prepared to control the thickness of the patella and to measure: 1) the excursion lengths of the quadriceps muscles during knee flexion using a linear voltage displacement transducer; 2) patellar tracking and patellofemoral contact pressure using pressure sensitive Fuji prescale film; and 3) patellar movement during knee flexion using an image processing system. As the patellar thickness increased from the original dimension to a 7-mm increment, the excursion lengths of the quadriceps muscle during knee flexion did not vary significantly, but lateral patellar subluxation was clearly demonstrated by an increase in the patellofemoral contact pressure on the lateral femoral condyle and by lateral displacement of the patella. Increased patellar thickness in a total knee replacement is considered to be one of the causes of lateral subluxation. The importance of monitoring the patellar thickness using a patellar cutting jig is stressed.