The static stabilizing function of the popliteal tendon in the knee. An experimental study.

We investigated the importance of the anterior cruciate ligament (ACL), its two components (the AMP and PLP), and the lateral compartment ligaments with special attention to the popliteal tendon (PT) in relation to valgus-varus, axial rotation, and anterior-posterior instability. Mobility patterns were drawn from 15 osteoligamentous knee preparations after successive transection of the structures. Even when combined lesion involved the lateral collateral ligament (LCL), the PT, and the posterolateral capsule (PLC), anterior tibial displacement was not effected until the ACL was cut. The lateral structures acted as secondary restraints to anterior displacement, and prevented simultaneous anterolateral rotation during application of the anterior force. LCL and PLC transection affected varus stability in full extension, and instability improved on additional section of the PT, even if the ACL remained intact. Maximum instability after combined lateral lesion occurred at 40 degrees of flexion, whether the ACL was transected or not. Further flexion caused a rapid decrease of instability. The PT was shown to effectively restrain varus instability from 0 to 90 degrees of flexion. It was experimentally established with regard to axial rotation that marked posterolateral instability was impossible with an intact PT. Maximum instability was recorded at 30 degrees of flexion after combined lesion to the lateral structures. Even in the extension, considerable posterolateral instability was observed. The restraining effect of the PLC was prominent in extension.