Pathomechanics of posterior sag of the tibia in posterior cruciate deficient knees. An experimental study.

This study was an investigation of the pathomechanics of posterior sag of the tibia in knees with posterior knee instability caused by PCL deficiency. By using fresh cadaver knees, the authors hoped to define the relationship of the posterior joint capsule and the medial and lateral collateral ligaments (MCL, LCL) with posterior knee instability in the PCL deficient knee. Thirty newtons of posterior stress were applied to the knees to simulate postoperative conditions. Roentgenographic methods were then used to evaluate posterior sag and change in the distance between the origin and insertion of the PCL. Strain gauges were used to measure the actual strain of the PCL and the collateral ligaments. The PCL, the posterior capsule, and the medial and lateral collateral ligaments were sequentially divided and the above measurements were then repeated in the same way, using 30 N of applied posterior stress. When only the PCL was cut, posterior sag and medial rotation of the tibia occurred with increasing severity as flexion increased. No sagging or rotation of the tibia was observed at full extension in the knees that had isolated PCL "injury". When the posterior capsule was sectioned, no significant changes were noted in the severity of the sag or the rotation. When the MCL or LCL was divided in a PCL deficient knee, greater sag occurred with flexion and a significant sag was observed even at full extension. The MCL "injury" was associated with increased medial rotation, whereas LCL "injuries" were associated with lateral rotation of the tibia.(ABSTRACT TRUNCATED AT 250 WORDS)