Pathomechanics of posterior sag of the tibia in posterior cruciate deficient knees

Abstract

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 rela tionship of the posterior joint capsule and the medial and lateral collateral ligaments (MCL, LCL) with poste rior knee instability in the PCL deficient knee. Thirty newtons of posterior stress were applied to the knees to simulate postoperative conditions. Roentgeno graphic 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 meas ure the actual strain of the PCL and the collateral ligaments. The PCL, the posterior capsuie, and the medial and lateral collateral ligaments were sequentially divided and the above measurements were then re peated 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. Strain measurements on the mid- substance of the PCL were at a minimum at full exten sion and at a maximum at 90° of flexion with a passive range of motion from 0° to 90°. Both the MCL and LCL demonstrated an inverse relationship to the PCL in the intact knee: maximum strain on the MCL and LCL were at full extension. This study suggested that in cases with a PCL injury, postoperative immobilization in full extension appears to be beneficial if MCL and LCL stability can be achieved, as these ligaments will prevent sagging in full extension, when the PCL is under the least strain. Our study also suggested that a significant posterior sag in full extension is indicative of an associated collateral ligament injury and can be partially assessed by rotation of the tibia.