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IN BRIEF: The knee joint anterior cruciate ligament (ACL) guides the tibia (shin bone) through a normal stable range of motion. When the ACL is torn the joint loses its stability.  Loss of the anterior cruciate ligament leads to destruction of the articular and meniscal cartilage over time. New surgical approaches can promptly treat this injury with an early return to sports. This article describes the mechanism of injury to the anterior cruciate and the The Stone Clinic’s preferred treatment.

"Doc, I fell and twisted my knee. I heard a pop. It hurt briefly. When I stood up, the knee buckled, and my knee gave way. It swelled up by the next day and ever since feels as though it would pop out when I twist or even cross the street quickly." This common complaint describes the traumatic rupture of the anterior cruciate ligament of the knee joint. The injury is serious and usually requires surgical repair or reconstruction in an athletic person. Fortunately, the state-of-the-art in the 21st century is that an athlete, after a properly done cruciate repair or reconstruction, can return to sports as soon as they have a full range of motion and full strength.

The anterior cruciate ligament is the most commonly injured ligament in the knee. Over the last 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. 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 anterior cruciate ligament originates from the back of the femur (thigh bone) and inserts on the top of the tibia (shin bone). (See diagram on page 49.) The ligament is a broad, thick cord the size of a person's index finger. It has long collagen stands 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 why a swollen knee occurs 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 further damage to one or more of the important cartilage structures within the joint and progressive arthritis.

The advent of stiffer boots and improved ski bindings during the 1970s shifted the incidence of skiing injuries from fractures of the lower leg to rotational injuries of the knee joint. Specifically, this occurs because the bindings were designed to read tension generated in the lower extremity and transferred to the boot binding interface. Release occurred prior to fracture of the bone, as determined by estimates of the skier's height, weight, and level of ability. But no binding measured the forces generated by the femur (the thigh bone) on the ligamentous attachments to the tibia. As the rotational forces increased through the knee, or as hyper-extension or flexion occurred (essentially amputating the ligament), increased numbers of cruciate ligament injuries occurred, even as tibial fractures were declining. In other sports, as the athletes became bigger the forces from twisting and contact increased but the ligaments did not proportionately increase in strength.  Sports participation in younger and older athletes also increased spiking the incidence of ACL tears.

Treatment
Newer surgical techniques have evolved to repair and reconstruct the torn cruciate ligament, both immediately after rupture and even years later. Early examination and magnetic resonance imaging (special techniques for producing pictures of the interior structure of the body) produce an accurate diagnosis.

Recent data on early return to sports after ligament reconstruction has strengthened the conviction that a carefully guided rehabilitation program can include bicycling and swimming within two weeks of surgery and sports as soon as a full range of motion and strength are achieved.

Prevention
While protection against ligament injury and early return to athletics after ligament surgery can best be achieved by strengthening the muscles around the knee that act as shock absorbers and joint stabilizers, no program definitely reduces the rupture rate.  Hamstring strength protects the tibia from the anterior translocation that can rupture the cruciate. For instance, when a skier “catches an edge," the muscular ability to recall the deviant ski is based on the strength of the medial hamstrings. If they are weak, the knee goes into hyperextension, the skier falls, and the "pop and swell" scenario can begin. On the other leg, when the skier "catches an edge," stability is often dependent on the quadriceps and hamstring power for balance on one ski while attempting to reign in the wayward leg. If the quadriceps muscles are weak, the leg wobbles under the unexpected load, twists as the skier falls, and "pop and swell" occurs.

Decreases in knee injuries have occurred after specific knee strengthening exercises have been initiated.  For full details of knee rehabilitation and strengthening programs see the links below. A program of 20 minutes a day concentrating on the knee musculature can dramatically increase strength, improve performance, and diminish injuries.

Summary
The anterior cruciate ligament is the key stabilizer of the knee.  When ruptured repair and reconstruction are successful if combined with excellent surgical technique and rehabilitation.