Collateral ligament

The one on the inner aspect is the 'medial collateral ligament' and the one on the outer aspect is the 'lateral collateral ligament'.

They are important in preserving the integrity of the joint, although of lesser importance than the cruciate ligaments. Injuries which stress the knee sideways may cause the collateral ligament on the stressed side to tear partially (sprain) or fully (rupture). As the ligament ruptures the person may hear a loud 'pop!' and experience severe pain over the site of rupture, with later bruising. Swelling is usually limited. Tears usually heal well with rest and support without needing surgical intervention.

This picture shows a 'valgus' force on the lateral (outer) side, but it is the collateral ligament on the medial (inner) side which would most likely tear. (A valgus forces bends the knee sideways into a knock-knee position). A direct blow to the ligament can also tear it on the same side as the blow. A 'varus' force may cause a lateral collateral tear, but this is less common. (A varus forces bends the knee sideways into a bow-leg position).

Stress tests (on the examination table and during X-ray) confirm the diagnosis when the joint on that side can be 'opened' more than on the good side.

Relevant resources on this site:

• Collateral overview
• Knee anatomy - the collateral ligaments