Even more important, the A.C.L. is one of the only structures that controls rotation in the knee (5). When an A.C.L. is torn or damaged, it is usually a non-contact injury. Sports with frequent situations of planting the foot, pivoting and quick direction changes are the sports with the highest number of A.C.L. injuries. The force of a persons body weight through the femur, keeps the A.C.L. in tight quarters at all times. When the tibia rotates, it causes the A.C.L. to be stretched slightly. This small amount of stretch is necessary or the A.C.L. will tear the first time the knee is rotated. When an athlete plants their foot and pivots in their one leg quickly in a direction change, the A.C.L. is stretched out to near its limit. There is added pressure to the area, as the knee will take the weight of your body as well as any force caused by the direction change. When the A.C.L. is stretched, it becomes longer, but weaker like an elastic, it will break easier at this point. The added pressure causes the femur and the tibia to be pressed very tightly together putting pressure on the already weak A.C.L. When and athlete turns the tibia and the femur can work like scissors grinding against one another and shearing the A.C.L. Although this is one of the most common ways to tear the A.C.L., this injury can happen in many other ways (hyper-extension, second to be torn with an M.C.L. etc.). The most common sports this injury occurs in are soccer, football, basketball, and skiing, because they all involve a lot of twisting at the knees. .
When the A.C.L. is stretched, there may be few to no signs that there is a problem. If the A.C.L. is partially torn the knee will usually feel slightly unstable, there will be a moderate amount of pain and substantial swelling. When an A.C.L. completely tears there is normally a very loud "pop", the athlete will be unable to return to play, and there will be a constant throbbing pain that increases with movement.
