Removal of the lateral or medial third of patellar tendon alters the patellofemoral contact pressure and area: an in vitro experimental study in dogs

Objectives. To compare patellofemoral contact pressure and areas after immediate removal of the lateral, central, or medial third of patellar tendon. Design. In vitro experiment in 12 dogs. Background. Alteration of the vector sum of the quadriceps muscle contraction after removal of partial patellar tendon might result in changes of patellar tracking in the trochlea. Methods. Patellofemoral contact pressure and areas were recorded using Fuji pressure-sensitive film at 45°, 60°, and 90° of knee flexion under an isometric quadriceps force of 100% body weight. The patellofemoral contact imprint from the intact knees were obtained as control, and the lateral (n=4), central (n=4), and medial (n=4) third of the patellar tendon were subsequently removed and the patellofemoral contact imprint was recorded. Results. The patellofemoral contact area was found to increase with increasing knee flexion angles. No change in patellofemoral contact pressure and areas was found after removal of the central third patellar tendon. However, after removal of either lateral or medial third of patellar tendon, the patellofemoral contact was rotated with increasing knee flexion angles. This was due to the altered vector sum of the quadriceps force, resulting in significantly decreased patellofemoral contact areas and simultaneously a significantly increased contact pressure, characterized with concentration of patellofemoral contact pressure on both lateral and medial facets of the patellofemoral joint. Conclusions. The results suggest that removal of the central third of patellar tendon may not alter the patellofemoral contact pressure and areas as compared with removal of either lateral or medial third of patellar tendon that may result in an altered postoperative tracking mechanism of the patellofemoral joint immediately after operation.