Page updated July 2023 by Dr Sheila Strover (Clinical Editor)

The collateral ligaments are the 'guy ropes' on the sides of the knee, tethering the femur to the tibia.

Collateral ligaments
A ligament is a fibrous structure which connects from bone to bone (in contrast to a tendon which connects from bone to muscle).  The collateral ligament on the inner aspect is the 'medial collateral ligament' and the one on the outer aspect is the 'lateral collateral ligament', and they both connect the femur to the tibia. The medial collateral is the one that is most often injured.


Anatomy of the collateral ligaments

The medial and lateral collateral ligaments differ in size, and in their relationship to neighbouring structures. The lateral collateral ligament is a pencil-like cord and stretches from the femur to the fibula. The medial collateral, in contrast, is flattened into a band with two layers - deep and superficial - the deep layer is attached to the medial meniscus, while the superficial layer attaches to the tibia, and these layers contribute to the joint capsule in this area. It contributes largely to stabilising the bent knee against against any force tending to force the joint open on the inner aspect (valgus force).

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What do the collateral ligaments do?

The collateral ligaments restrain the joint from forces that would open it up on the side, ie varus or valgus forces.

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Collateral ligament injury and the posterolateral corner

rugby injury to knee

Excessive force from one side may stretch and tear the collateral ligament on the other side of the joint, such as might occur in a poor tackle. If one looks at the back of the knee on the lateral side, the anatomy is complex. The lateral collateral, unlike the medial collateral, does not form part of the capsule. This is because the popliteus tendon passes under it and separates it from the capsule. At this point the capsule is open, and this area is weak and prone to injury. This is a real problem as these structures contribute considerably to what is called the 'postero-lateral corner' - a number of tricky structures stabilising the knee on the outer side towards the back and postero-lateral corner deficiency is a common cause of continued instability after ACL reconstruction.

Both sets of ligaments - collaterals and cruciates - are necessary to maintain full stability of the knee joint.

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