An anterior cruciate ligament injury occurs when the anterior cruciate ligament (ACL) is either stretched, partially torn, or completely torn. The most common injury is a complete tear. Symptoms include pain, an audible cracking sound during injury, instability of the knee, and joint swelling. Swelling generally appears within a couple of hours. In approximately 50% of cases, other structures of the knee such as surrounding ligaments, cartilage, or meniscus are damaged.
The underlying mechanism often involves a rapid change in direction, sudden stop, landing after a jump, or direct contact to the knee. It is more common in athletes, particularly those who participate in alpine skiing, football (soccer), netball, American football, or basketball. Diagnosis is typically made by physical examination and is sometimes supported by magnetic resonance imaging (MRI). Physical examination will often show tenderness around the knee joint, reduced range of motion of the knee, and increased looseness of the joint.
Prevention is by neuromuscular training and core strengthening. Treatment recommendations depend on desired level of activity. In those with low levels of future activity, nonsurgical management including bracing and physiotherapy may be sufficient. In those with high activity levels, surgical repair via arthroscopic anterior cruciate ligament reconstruction is often recommended. This involves replacement with a tendon taken from another area of the body or from a cadaver. Following surgery rehabilitation involves slowly expanding the range of motion of the joint, and strengthening the muscles around the knee. Surgery, if recommended, is generally not performed until the initial inflammation from the injury has resolved.
About 200,000 people are affected per year in the United States. In some sports, females have a higher risk of ACL injury, while in others, both sexes are equally affected. While adults with a complete tear have a higher rate of knee osteoarthritis, treatment strategy does not appear to change this risk. ACL tears can also occur in some animals, such as dogs. The most common knee injury in dogs is ACL or CCL rupture.
Signs and symptoms
When an individual has an ACL injury, they are likely to hear a "pop" in their knee followed by pain and swelling. They may also experience instability in the knee once they resume walking and other activities, as the ligament can no longer stabilize the knee joint and keep the tibia from sliding forward.
Reduced range of motion of the knee and tenderness along the joint line are also common signs of an acute ACL injury. The pain and swelling may resolve on its own; however, the knee will remain unstable and returning to sport without treatment may result in further damage to the knee.
Causes
Causes may include: Changing direction rapidly (also known as "cutting") Landing from a jump awkwardly Coming to a sudden stop when running A direct contact or collision to the knee (e.g. during a football tackle or a motor vehicle collision) These movements cause the tibia to shift away from the femur rapidly, placing strain on the knee joint and potentially leading to rupture of the ACL. About 80% of ACL injuries occur without direct trauma. Risk factors include female anatomy, specific sports, poor conditioning, fatigue, and playing on a turf field.
Female predominance
Female athletes are two to eight times more likely to strain their ACL in sports that involve cutting and jumping as compared to men who play the same particular sports. NCAA data has found relative rates of injury per 1000 athlete exposures as follows: Men's basketball 0.07, women's basketball 0.23 Men's lacrosse 0.12, women's lacrosse 0.17 Men's football 0.09, women's football 0.28 The highest rate of ACL injury in women occurred in gymnastics, with a rate of injury per 1000 athlete exposures of 0.33. Of the four sports with the highest ACL injury rates, three were women's – gymnastics, basketball and soccer.
Differences between males and females identified as potential causes are the active muscular protection of the knee joint, differences in leg/pelvis alignment, and relative ligament laxity caused by differences in hormonal activity from estrogen and relaxin. Birth control pills also appear to decrease the risk of ACL injury.