Sports participation results in 70 percent of anterior cruciate ligament (ACL) tears and the majority of these occur in 15 to 45 year olds. Approximately 70 percent of ACL injuries are noncontact injuries that occur during a sudden change in direction with a planted foot (i.e., cutting) or stopping rapidly. In the United States there are between 100,000 to 250,000 ACL ruptures annually.
The ACL is one of the main stabilizing ligaments of the knee and helps provide the hinge that allows the knee to remain stable while moving. When an ACL tear occurs the athlete often reports hearing a pop and usually cannot walk on the injured limb. During the injury it is common for the knee to partially dislocate, resulting in bruising and sometimes a small fracture at the back of the tibia and on the femur. Also common is a sprain of the medial collateral ligament (MCL) which is located on the inside of the knee. These injuries result in a painful swollen knee that is tender outside and deep within the knee.