Analysis of the static function of the popliteus tendon and evaluation of an anatomic reconstruction: the "fifth ligament" of the knee.

BACKGROUND The popliteus tendon has important dynamic and static stabilizing functions at the knee. Evaluation of its static role as the "fifth ligament" of the knee and a subsequent analysis of a popliteus tendon reconstruction has not been performed. HYPOTHESIS In vitro knee stability can be restored to a popliteus tendon-deficient knee with an anatomic popliteus tendon reconstruction. STUDY DESIGN Controlled laboratory study. METHODS Eleven nonpaired cadaveric knees were tested under the following popliteus tendon states: intact, sectioned, and reconstructed using an autogenous semitendinosus graft. Each knee was subjected to 10-N.m varus moments, 5-N.m external and internal torques, and 88-N anterior and posterior loads at flexion angles of 0 degrees , 20 degrees , 30 degrees , 60 degrees , and 90 degrees . A 6 degrees of freedom electromagnetic motion tracking system was used to assess motion changes of the tibia with respect to the femur. RESULTS Significant increases in external rotation and small but significant increases in internal rotation, varus angulation, and anterior translation motion were found after sectioning the popliteus tendon compared to the intact state. Significant decreases in external rotation were found in the reconstructed state compared with the sectioned state at knee flexion angles of 20 degrees , 30 degrees , 60 degrees , and 90 degrees . Comparing the reconstructed state to the intact state, there were no significant differences at knee flexion angles of 0 degrees and 20 degrees , but significant decreases of external rotation were found at knee flexion angles of 30 degrees , 60 degrees , and 90 degrees . Additionally, there were small but significant differences between the reconstructed and intact state with respect to varus angulation at knee flexion angles of 20 degrees , 30 degrees , and 60 degrees ; anterior translation at 20 degrees and 30 degrees ; and internal rotation at all flexion angles. CONCLUSION The popliteus tendon has important primary stabilization roles at the knee. The authors also found that an anatomic popliteus tendon reconstruction significantly reduced the increase in external rotation that occurred with sectioning the popliteus tendon; however, differences seen with respect to internal rotation, varus angulation, and anterior translation were not restored. CLINICAL SIGNIFICANCE The popliteus tendon functions essentially as the fifth major ligament of the knee. An anatomic popliteus tendon reconstruction can restore external rotation stability to knees with popliteus tendon injury.