What Is It?

The posterior cruciate ligament (PCL) and the anterior cruciate ligament (ACL) are two tough bands of fibrous tissue inside the knee joint. These ligaments connect the thighbone (femur) and the large bone of the lower leg (tibia) at the knee joint. Together, the ACL and PCL bridge the inside of the knee joint, forming an "X" pattern that stabilizes the knee against front-to-back or back-to-front forces. In particular, the PCL prevents the lower leg from slipping too far back in relation to the upper leg, especially when the knee is flexed (bent).

A PCL injury is a sprain (stretch or tear of a ligament). In daily life, the PCL most often is sprained when the front of the knee hits the dashboard during an automobile accident. During sports activities, the PCL also can tear when an athlete falls forward and lands hard on a bent knee, which is common in football, basketball, soccer and especially rugby.

Like other types of sprains, PCL injuries are classified according to a traditional grading system.

• Grade I - A mild injury that causes only microscopic tears in the ligament. Although these tiny tears can stretch the PCL out of shape, they do not significantly affect the knee's ability to support your weight.

• Grade II (moderate) - The PCL is partially torn, and the knee is somewhat unstable, meaning it gives way periodically when you stand, walk or undergo diagnostic tests.

• Grade III (severe) - The PCL is either completely torn or is separated at its end from the bone that it normally anchors, and the knee is more unstabile. Because it usually takes a large amount of force to cause a severe PCL injury, patients with Grade III PCL sprains often also have sprains of the ACL or collateral ligaments or other significant knee injuries.

Overall, some degree of PCL damage occurs in up to 37% of all people who are treated for knee injuries in U.S. emergency rooms. Athletes seem to have more PCL injuries than any other group, with football players and rugby players having the most, and basketball players close behind. Because a mild PCL sprain may not cause pain or movement problems at first, many athletes finish a game after their injury. Some have such mild symptoms that they never seek medical care, and the torn PCL is discovered only when they have diagnostic tests for some other type of knee injury.