The stability of the knee joint is dependent upon static and dynamic factors. The static stabiliser includes passive structures such as the knee joint capsule and the various ligaments and other associated structures such as the menisci, the coronary ligaments, the menisco-patella and patello-femoral ligaments. The ligaments which all act as static stabilisers include the medial collateral ligament, the lateral collateral ligament, the ACL, PCL, the oblique popliteal and arcuate ligaments. The ilio-tibial band is also considered a static stabiliser in spite of its muscular connections.
The bony geometry also contributes to the static stability of the knee. The contribution is variable but can be made worse by certain anatomic variants such as a flat lateral femoral trochlea which will predispose to lateral instability of the patella.
The dynamic stabilisers of the knee are all the muscles and their aponeuroses including:
– quadriceps femoris and extensor retinaculum
– pes anserinus
– biceps femoris
The structures on the medial, antero-medial and postero-medial side of the knee are medial compartment structures and stabilisers and structures on the respective lateral side are lateral compartment stabilisers.
The contribution that both muscles and ligaments make to stability is dependent on joint position of the knee and the surrounding joints, the magnitude and direction of the force and the availability of reinforcing structures to resist forces if the primary restraints become incompetent.
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.
Antero-posterior stability of the knee is provided by static and dynamic stabilisers and lateral and medial compartment structures. The anterior cruciate ligament provides the majority of the resistance to anterior tibial translation and likewise the posterior cruciate ligament provides the major resisting factor to posterior tibial translation.
The extensor retinaculum which is composed of fibres from the quadriceps femoris, fuses with fibres of the joint capsule to provide critical dynamic support for the antero-medial, antero-lateral aspects of the knee. The medial and lateral heads of the gastrocnemius reinforce the medial and lateral aspects of the posterior capsule. The popliteus is considered to be a particularly important postero-lateral stabiliser complimenting the function of the PCL.
The ACL and the hamstrings work in a complimentary manner to resist forces that are attempting to displace the tibia anteriorily or sheer the femur posteriorily. Such forces are exemplified by the pull of the quadriceps and by the effect of the ground reaction force on the tibia when the heel hits the ground.
The role of the patella itself can not be ignored when assessing antero-posterior stability of the knee. The patella prevents the femur from sliding forward off the tibia actually serving as an extension of the tibia, connected by an elastic tendon. This combination of patella and tibia creates a cradling effect on the femur.
Medial/lateral stability of the knee is provided for, once again, by static and dynamic soft tissue structures and by the tibial eminences and the menisci when the knee is in full extension. The knee is reinforced on its medial and lateral aspects by collateral ligaments and the collateral ligaments play a critical role in resisting varus/valgus stresses, especially in the more extended knee.
The tibial collateral ligament is a strong flat band that extends from the medial epicondyle of the femur to the medial condyle of the tibia and to the medial surface of the shaft of the tibia. It is a primary restraint to valgus angulation at the knee (resistance to knee going inwards or foot going outwards).
The lateral collateral ligament is a rounded cord approximately 5cm long and is the primary restraint to varus angulation at the knee (knee going outwards or foot going inwards). Superiorly it is attached to the lateral epicondyle of the femur, above and behind the groove of the popliteus muscle. It ends inferiorly 1cm below the apex at the head of the fibula on the lateral side.
Both cruciates also contribute to medial/lateral stability. As the knee flexes (bends) the dynamic stability provided by the musculature such as the muscles of the pes anserinus on the medial aspect of the knee become increasingly important. Naturally the ilio-tibial band, the lateral collateral ligament, the popliteus tendon, the biceps tendon, the postero-lateral capsule and the lateral head of gastrocnemius are all important factors contributing to stability. The postero-lateral capsule is particularly important to varus stability in extension whereas popliteus is the major stabiliser in the 0 to 90° of flexion range.
The menisci are also important to medial/lateral stability because the knee remains stable in full extension regardless of sectioning of ligamentous structures. Removing both menisci would appear to have its greatest effect in stabilisation during varus and valgus stresses (inward and outward stresses on the foot respectively).
This is a complex issue. It would appear however that the passive structures predominate over dynamic mechanisms in rotational stability. The cruciates are most often credited with rotational stability of the joint, especially in extension. Other structures that are important to rotational stability include the collateral ligaments, the postero-medial and postero-lateral capsule and the popliteus tendon.
As can be seen from the above discussion, the knee joint function is dependent upon static and dynamic factors. In general terms, the static factors can not be influenced in the normal knee, eg the bony geometry or ligament size and position. The dynamic structures however can be influenced and are of vital importance in normal knee joint function. The muscles can be strengthened with appropriate training regimes. Certain static deficiencies can be overcome by appropriate strengthening and training of the dynamic component of the knee. For example, in an anterior cruciate deficient knee, strengthening of the quadriceps and hamstring mechanism along with appropriate co-ordination training can control instability to allow the person to undertake various low risk sports and certainly will control antero-posterior instability. However, even with dynamic muscular training with an absent ACL, rotation stability cannot be offset by muscular training and so the knee will still be prone to giving way with twisting on the weight bearing ACL deficient knee.
The majority of rehabilitation regimes following injury and surgery to the knee emphasise the importance of training and strengthening the dynamic stabilisers to the knee and this is particularly important in patello-femoral pain syndromes where the muscular control of the patella is vital.