Tensioning the PCL Graft
ABSTRACT & COMMENTARY
Synopsis: The posterior cruciate ligament graft should be pretensioned and fixed with an anterior load placed on the knee at 90° knee flexion.
Source: Harner CD, et al. The effect of knee flexion angle and application of an anterior tibial load at the time of graft fixation on the biomechanics of a posterior cruciate ligament reconstructed knee. Am J Sports Med 2000;28(4):460-465.
This is a basic science cadaver study using a well-documented robotic/universal force moment sensor to test the biomechanics of the posterior cruciate ligament (PCL). This study addresses the effect of knee flexion angle and the application of an anterior tibia load applied to the knee during PCL reconstruction.
Ten fresh frozen cadavers aged 36-65 were tested. The intact knee, PCL deficient knee, and PCL reconstructed knee were compared. Testing was performed at full extension, 60°, 90°, and 120°. The PCL reconstructions were performed arthroscopically using 11 mm achilles allograft tendons.
The results demonstrated that if the PCL graft fixation was performed at full extension and a 134 N posterior load applied to the knee, then the resulting tibia translations were significantly decreased from normal at 30°, 60°, 90°, and 120°. Additionally, significantly increased in situ forces were reported at full extension, 60° and 120°. When the PCL fixation was performed at 90° with a concomitant anterior tibia force and the same 134 N posterior loads applied to the knee, the resulting tibia translation at all recorded flexion angles was not different than the normal knee. The in situ forces were similar to the intact knee at full extension, 60°, and 120°.
Comment by James R. Slauterbeck, MD
Controversies exist on what knee flexion angle one should pretension and fix the graft. PCL surgery is a complicated procedure and many different techniques are available to assist the surgeon. Many studies reflect patient satisfaction after PCL reconstruction but objective measurements demonstrate persistent laxity. This study addresses where and how to pretension and fix the PCL graft. Proper knee placement and pretensioning procedures may lead to knees that are more stable post-operatively.
The PCL is the primary restraint to posterior tibia translation at 90° of knee flexion, and the ACL is the primary restraint to anterior tibia translation at 30°. If the ACL graft is pretensioned and graft fixation performed at 30° it would stand to reason that the PCL should be pretensioned and fixed at 90°.
It is not commonly discussed where to set the tibia when pretensioning. If the tibia is not reduced at the time of graft fixation, the knee will have excessive AP excursion. If the graft is tensioned in full extension where the posterior capsule is the primary restraint to posterior tibia translation, the graft could be overly tensioned. Over-tensioning could lead to early graft failure from excessive forces or decrease the knee range of motion. If the knee is placed at 90° of flexion where the PCL is the primary restraint to posterior tibia translation, one could gently set the knee tension against the ACL and remove the posterior translation within the knee. Pretensioning in this position will remove any tunnel-bone plug graft friction and stretch the collagen crimp at the position where the graft is most functional to resist posterior tibia translation.
This study shows that with the knee positioned at 90° of knee flexion and a gentle anterior drawer placed on the knee the kinematics of the knee are restored to nearly normal. However, at full extension excessive graft forces and decreased tibia translations are noted. The increased forces and decreased tibia translation essentially over-constrains the knee and could lead to premature rupture or even DJD from the increased forces in the knee. This is another nice study from a respected biomechanical lab, which helps to answer some clinically pertinent questions.
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