A knee sprain is an injury of the ligaments, tough bands of fibrous tissue that connect the bones of the upper and lower leg at the knee joint. The knee joint has four major ligaments.
Anterior cruciate ligament (ACL) — The ACL and the posterior cruciate ligament (PCL) bridge the inside of the knee joint, forming an "X" pattern that stabilizes the knee against front-to-back and back-to-front forces. The ACL typically sprains during one of the following knee movements: a sudden stop; a twist, pivot or change in direction at the joint; extreme overstraightening (hyperextension); or a direct impact to the outside of the knee or lower leg. These injuries are seen among athletes in football, basketball, soccer, rugby, wrestling, gymnastics and skiing.
Posterior cruciate ligament (PCL) — The PCL works with the ACL to stabilize the knee. It most often sprains because of a direct impact to the front of the knee, such as hitting the knee on the dashboard in a car crash or landing hard on a bent knee during sports. In athletes, PCL injuries are most common among those who play football, basketball, soccer and rugby.
Medial collateral ligament (MCL) — The MCL supports the knee along the inner side of the leg. Like the ACL, the MCL can be torn by a direct sideways blow to the outside of the knee or lower leg, the kind of blow that can happen in football, soccer, hockey and rugby. The MCL can be injured by a severe knee twist during skiing or wrestling, particularly when a fall twists the lower leg outwards, away from the upper leg.
Lateral collateral ligament (LCL) — The LCL supports the outer side of the knee. It is the least likely knee ligament to be sprained because most LCL injuries are caused by a blow to the inside of the knee, and that area usually is shielded by the opposite leg.
Like other types of sprains, knee sprains are classified according to a grading system:
Grade I (mild) — This injury stretches the ligament, which causes microscopic tears in the ligament. These tiny tears don't significantly affect the overall ability of the knee joint to support your weight.
Grade II (moderate) — The ligament is partially torn, and there is some mild to moderate instability (or periodic giving out) of the knee while standing or walking.
Grade III (severe) — The ligament is torn completely or separated at its end from the bone, and the knee is more unstable.
When one knee ligament suffers a serious sprain, there is a good chance that other parts of the knee may also be injured. For example, because the MCL helps to protect the ACL from certain types of extreme knee forces, the ACL can become vulnerable to injury when the MCL is torn. In more than half of moderate or severe MCL sprains, the ACL also is sprained.
Knee sprains are very common. ACL sprains tend to cause more significant symptoms compared to MCL injuries. Many MCL sprains are so mild that they don't result in a visit to a doctor.
More than any other group, competitive athletes have a very high risk of knee sprains and other types of knee problems. In U.S. high schools, the knee is the most frequently injured joint among athletes who compete in football, soccer or wrestling.