Ligaments are the structures that ensure that the knee only flexes and extends (bends and straightens). The knee is not supposed to open up on the medial side (inside) or lateral side (outside). Neither is it supposed to slide forwards or backwards, or rotate. Ligaments prevent these abnormal motions.
The ACL is one of the most important ligaments in the knee . It has 2 main functions:
- prevents the tibia (leg bone) from sliding forwards.
- prevents abnormal rotation between femur (thigh bone) and tibia (leg bone)
Preventing abnormal motion is critical in allowing for a stable knee during cutting and pivoting sports such as soccer, basketball and other field sports. It is also important to allow for a stable knee for during landing from a jump, such as sports like wakeboarding, snowboarding, netball, etc. In fact, you even need your ACL for normal daily activities, such as turning corners, or jumping out of the way of an oncoming car when you cross the road!
The ACL is a very commonly injured ligament, and it is estimated that about 100,000 ACL tears occur each year in the USA. Often the injuries occur when the person is changing direction quickly, or landing awkwardly after a jump. There may be a "pop" sound, and usually the athlete is unable to continue playing. The knee usually swells up in the next 1-2 days, and this swelling may last several weeks.
The diagnosis is made by a good history taking, clinical examination, and usually an MRI (Magnetic Resonance Imaging) scan of the knee.
Unfortunately, most ACL injuries do not heal naturally on their own, and most patients require surgery to restore stability to their knees, to allow a return to sports and normal activities, as well as prevent future damage to other structures in the knee such as the meniscii (cartilage).