The main ligaments providing stability to the knee are the cruciate and the collateral ligaments.


Cruciate ligaments

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The cruciate ligaments are the important central 'stays' of the knee, contributing significantly to the stability of the joint.

cruciate ligaments

The word 'cruciate' means 'crossed', and the two ligaments are indeed crossed over one another, each attaching above to the femur and below to the tibia.

This illustration looks at the joint from the front when the knee is bent, displaying the cruciates deep within the joint. The cruciate ligament towards the front is the anterior cruciate ligament (ACL), and that towards the back is the posterior cruciate ligament (PCL).


oblique view of cruciates


This close-up on the left shows clearly the two ligaments crossing over. The word 'cruciate' means 'crossed'. The cruciate ligaments are also jokingly referred to as the 'crucial' ligaments, because they are so important in maintaining the stability of the knee (note that the bones are pulled apart a bit just to let you look inside).

The posterior cruciate ligament (PCL) attaches to the tibia at the back of the knee and the anterior cruciate ligament (ACL) attaches to the tibia at the front of the knee. Of the two, the anterior cruciate is the more important in this respect.

how the anterior cruciate twists

Note in the sketch on the left how the anterior cruciate ligament attaches, and the anatomy when the knee is bent and straightened. This is one of the reasons why ligament replacements do not do as good a job as the original ligament as this cannot be mimicked by the replacement.



Collateral ligaments

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The collateral ligaments function as guy ropes at the sides of the knee, protecting the knee against forces which would tend to prise it open sideways.

collateral ligaments

The collateral ligaments on either side of the knee differ in their anatomy. The lateral collateral ligament is a cord-like structure. The medial collateral ligament (MCL) is more of a band, and has both a deep and a superficial component. The MCL is the more commonly injured, although the injuy is more often a sprain than a complete rupture.



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.