ACL (Anterior Cruciate Ligament) is one of the four ligaments that hold the bones of the knee together. It attaches the shin bone (tibia) to the thigh bone (femur). The thigh bone rests on the shinbone and the knee joint allows forward and backward motion of the shin bone.

The ACL functions as a restraint stopping the shin from moving too far forward. It also prevents excessive internal rotation of the shin bone and protects it from excessive sideways force. This provides stability to the joint, avoids dislocation and ensures normal range of motion.

The other ligaments of the knee are posterior cruciate ligament (PCL), and the medial collateral ligaments (MCL) and lateral collateral ligaments (LCL).