Effect of Patellar Tendon Shortening on Tracking of the Patella

Background: Although 10% postoperative patellar tendon shortening after bone–patellar tendon–bone autograft reconstruction of the anterior cruciate ligament has been reported, there are no published studies assessing the effect of shortening on patellofemoral joint biomechanics under physiological loading conditions. Purpose: To investigate the influence of patellar tendon shortening on patellofemoral joint biomechanics. Study Design: Controlled laboratory study. Methods: The authors evaluated the patellofemoral contact area, the location of contact, and the patellofemoral joint reaction force and contact stresses in 7 cadaveric knees before and after 10% patellar tendon shortening. Shortening was achieved using a specially designed device. Experimental conditions simulating those occurring during level walking were employed: physiological quadriceps loads and corresponding angles of tibial rotation were applied at 15°, 30°, and 60° flexion of the knee. Patellofemoral joint contact areas were measured before and after shortening using the silicone oil–carbon black powder suspension squeeze technique. Results: After patellar tendon shortening, patellofemoral joint contact areas were displaced proximally on the patellar surface and distally on the femoral surface. Although the contact area increased by 18% at 15° of knee flexion ( P = .04), no significant change occurred at 30° or 60° of knee flexion ( P > .05). Patellofemoral contact stress remained unchanged after patellar tendon shortening ( P > .05) at each flexion angle. Conclusion: Our results suggest that a 10% shortening of the patellar tendon does not alter patellar contact stresses during locomotion. It is not clear whether apparent changes in contact location in all positions and contact area at 15° would have clinical consequences. Keywords: anterior cruciate ligament (ACL) reconstruction patellar tendon graft patellar tendon shortening patellar tracking