The anterior cruciate ligament (ACL) is the key stabilizer of the knee. The knee joint ACL guides the tibia (shin bone) through a normal, stable range of motion. When the ACL is torn, the joint loses its stability. Over time, loss of the ACL leads to destruction of the articular and meniscal cartilages. The ACL is the most commonly injured ligament in the knee. Over the past 15 years, ankle sprains have decreased by 86% and tibia fractures by 88%, but knee ligament injuries have increased by 172%. Twenty-five percent of all reported skiing injuries involve the knee.


Causes of an ACL Tear
People often tear the ACL by changing direction rapidly, slowing down from running, or landing from a jump. When you twist your knee or fall on it, you can tear a stabilizing ligament that connects your thighbone to the shinbone. An ACL unravels like a braided rope when it’s torn and does not heal on its own. Young people (ages 15 - 25) who participate in basketball and other sports that require pivoting are especially vulnerable. The injury usually occurs in either a slow twisting fall, a sudden hyperextension, or a sudden hyperflexion as when landing from a jump on flat terrain. The ACL originates from the back of the femur (thigh bone) and inserts on the top of the tibia (shin bone). The ligament is a broad, thick cord the size of a person's index finger. It has long collagen strands woven together in a fashion that permits forces of up to 500 pounds to be exerted prior to rupture. The ligament is crucial for guiding the tibia in a normal path along the end of the femur and maintains joint stability. The ligament has a relatively poor vascular supply and has limited ability to heal a complete tear. Complete ruptures of the ligament produce bleeding into the knee; the reason a knee swells after a rupture is that the swelling represents blood in the joint. Once torn, the knee usually becomes unstable. With increasing instability, the shear forces across the top of the tibia increase, the meniscal cartilages tear, and the articular cartilage erodes. This erosion is the degenerative arthritis felt as grinding and pain, particularly with stair climbing, running, or jumping. The patients who remain athletic have a 75% chance of progressive arthtritis and further damage to one or more of the important cartilage structures within the joint.