Knee Joint Stabilisation Knee Stability
Knee Stability
The front portion of the superficial MCL remains taut throughout flexion whereas the LCL is taut only in extension and relaxes as soon as the knee is flexed.
The superficial MCL is the most important medial stabiliser. With the knee in extension the posterior fibres are taut and the anterior fibres relaxed. With flexion of the knee the anterior fibres move proximally and become tight and are subject to an increase in tension as the joint is flexed. This action is partly due to the oval shape of the femoral origin of the MCL, as the anterior border becomes tight the posterior fibres slacken as the knee flexes and remains relaxed throughout flexion.
With an intact MCL there is approximately 1 to 2 mm of medial opening to valgus stress. The joint is slightly tighter in full extension with the greatest degree of medial opening occurring at 45°. The parallel fibres of the superficial MCL also control rotation and sectioning these fibres not only increases the amount of medial opening to valgus stress but also causes a significant increase in external rotation. In distinction, sectioning the capsule, the deep medial collateral ligament or the oblique fibres of the superficial MCL cause little or no increase in rotation.
The lateral ligament is also taut in extension but is relaxed throughout flexion. This is also true of the arcuate ligament. Thus, in flexion, a much greater degree of rotation is possible laterally than medially. This rotation is permitted by the attachment of the lateral meniscus and by a relaxation of the supporting ligaments in flexion. There is also a greater degree of rolling of the femur on the tibia laterally, whereas medially the motion is minimal. The attachment of the popliteus tendon to the lateral meniscus draws the meniscus posteriorily and prevents entrapment of the knee as it is flexed.
The ACL consists of two functional bundles, the antero-medial bundle and a stronger thicker postero-lateral part. In extension the postero-lateral bundle is taut. In flexion the antero-medial band becomes tight and the bulk of the ligament slackens. In flexion it is the antero-medial band that provides the primary strength against anterior displacement of the tibia.
The PCL consists of two inseparable parts. An anterior part forms the bulk of the ligament and a small posterior part runs obliquely to the back of the tibia. In extension, the bulk of the ligament is relaxed and only the posterior band is tight. In flexion a major part of the ligament becomes tight and a small posterior band is loose.
The ACL is a check against both hyperextension and internal and external rotation. The PCL is a check against posterior instability in the flexed knee but not against hyperextension, provided that the anterior cruciate ligament is intact.
