Collateral ligaments

1-We have two collateral ligaments, one on the medial side and the other on the lateral side.

2-Usually you can differentiate between them by fibula.

3-Lateral collateral ligament is attaching femur with fibula while medial ligament is attaching femur with tibia.

4-Lateral collateral ligament is passing away from the bone while medial collateral one is attached the bone.

Look to the image below.

5-If the fibula is not seen in the film, how can you differentiate between medial and lateral collateral ligament, look to the inter condylar notch to see cruciate ligaments, where the posterior cruciate ligament is attached to the medial condyle, so the collateral ligament on the side of posterior cruciate ligament is the medial collateral ligament and vice versa.

The posterior cruciate ligament appears as horizontal hypo intense rectangular area attached to medial condyle while the anterior cruciate ligament appears as vertical hypo intense area attached to lateral condyle.

6-If you go further lateral in the sagital plane, you can see the lateral collateral ligament attached to the head of the fibula near to the attachment of the biceps femoris tendon, both are seen as hypo intense linear structures.

FCL=Fibular collateral ligament.
BF=Biceps femoris.