What Is It?

Ligaments are tough bands of fibrous tissue that connect two bones. The anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) are inside the knee joint. These ligaments connect the thigh bone (femur) and the large bone of the lower leg (tibia) at the knee joint. The ACL and PCL form an "X" inside the knee that stabilizes the knee against front-to-back or back-to-front forces.

An ACL injury is a sprain, which is a tear of a ligament. In almost all cases, the ACL is torn during at least one of the following patterns of injury:

• A sudden stop, twist, pivot or change in direction at the knee joint - These knee movements are a routine part of football, basketball, soccer, rugby, gymnastics and skiing. For this reason, athletes who participate in these sports have an especially high risk of ACL tears.

• Extreme hyperextension of the knee - Sometimes, during athletic jumps and landings, the knee straightens out more than it should and extends beyond its normal range of motion, causing an ACL tear. This type of ACL injury often occurs because of a missed dismount in gymnastics or an awkward landing in basketball.

• Direct contact - The ACL may be injured during contact sports, usually during direct impact to the outside of the knee or lower leg. Examples are a sideways football tackle, a misdirected soccer kick that strikes the knee or a sliding tackle in soccer.

Like other types of sprains, ACL injuries are classified by the following grading system:

• Grade I - A mild injury that causes only microscopic tears in the ACL. Although these tiny tears may stretch the ligament out of shape, they do not affect the overall ability of the knee joint to support your weight.

• Grade II - A moderate injury in which the ACL is partially torn. The knee can be somewhat unstable and can "give way" periodically when you stand or walk.

• Grade III - A severe injury in which the ACL is completely torn through and the knee feels very unstable.

Overall, most ACL injuries are severe Grade IIIs, with only 10% to 28% being either Grade I or Grade II. Currently, between 100,000 and 250,000 ACL injuries occur each year in the United States, affecting approximately one out of every 3,000 Americans. Although most of these injuries are related to athletic activities, especially contact sports, about 75% occur without any direct contact with another player.