Effects of knee position, graft tension, and mode of fixation in posterior cruciate ligament reconstruction: A cadaveric knee study

Purpose: Many knees exhibit residual ligament laxity after posterior cruciate ligament (PCL) reconstruction, which is believed to be technique related. The purpose of this study was to investigate the optimal graft tension, the best angle of knee flexion, and the mode of fixation in PCL reconstruction. Type of Study: Anatomic biomechanical study. Methods: A testing apparatus with frictionless bearing that allows other degrees of freedom except for flexion and extension of the knee joint was designed. The normal PCL tension at different angles of knee flexion was measured with a force transducer, and the optimal tension of the PCL graft that allows full range of knee motion was studied with a tensiometer in 12 cadaver knees. The modes of fixation failure between interference screw fixation and post fixation were studied with an Instron (Canton, MA) machine in 8 cadaver knees. Results: The lowest PCL tension in normal knees was noted at 20° to 30° of knee flexion and the highest at 90°. The optimal tension of PCL graft, which allows full range of knee motion, was 15 lb (68 N). The average load of graft failure was 417 (179-730) N with interference screw fixation and 367 (149-701) N with post fixation when the patellar bone-tendon-bone graft was tested. There was no statistical difference in the failure load between interference screw fixation and post fixation (P = .753); however, the modes of failure differ. The sites of failure for interference screw fixation were 25% caused by rupture of ligament substance and 75% bone plug pullout; those of post fixation were 25% caused by rupture of ligament substance, 37.5% caused by fracture, and 37.5% as a result of suture breakage. Conclusions: The results of this study suggested that a 15-lb tension to the graft at 20° to 30° of knee flexion is optimal in PCL reconstruction. There was no statistical difference in the failure load between interference fixation and post fixation despite different modes of fixation failure.