Anatomy et physiology

The ACL is located in the center of the knee. Specifically, it is in the "notch" which is a large opening shaped like an inverted U. The ligament begins in the upper outer aspect of the knee and crosses obliquely across the knee (see figure). The posterior cruciate ligament (PCL) lies just behind the ACL. These two ligaments cross each other to form an X. When the lower leg turns inwards (or upper leg outwards), the two ligaments tend to twist around each other.

Right knee

line drawing of the cruciate ligaments

arthroscopic view of the normal ACL

Function of the ACL

As with all ligaments, the role of the ACL is to stabilize the knee joint. The ACL has two very specific functions:

The ACL prevents the tibia from sliding forward on the femur (it prevents an "anterior drawer" type of motion).

ACL intact

A torn ACL leading to an "anterior drawer" type of displacement

	The ACL also prevents undue internal rotation of the tibia relative to the femur. Its orientation from the top left to bottom right (on a right knee) can’t perform this function. This effect of limiting internal rotation is further enhanced by the twisting of two cruciate ligaments around each other during internal rotation.
		Excessive internal rotation of the tibia relative to the femur can lead to a tear of the ACL

ACL has no effects on simple straightening and bending of the knee. However, the knee may feel unstable when the knee is twisted as when the foot is firmly planted on the ground and the body twists to the right (on a right knee).

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