The anterior cruciate ligament and the posterior cruciate ligament connect the femur (thighbone) to the tibia (shinbone) inside the joint of the knee. The ACL and PCL allow the knee to move forward and backward without permitting it to move from one side to the other. When an ACL/PCL sprain or strain occurs, the ligament may separate from the bone, rupture or be torn.

Injuries to knee ligaments often occur while an individual is participating in sports, or while he or she is at work and the knee is twisted, bent backwards or forcibly hit. Other common causes of injury to the ligaments in the knee include stopping suddenly or overextending the knee. During especially traumatic events, multiple ligaments can sustain injury.

Mild injuries to the ligament, such as a sprain, involve minor tearing or stretching of ligament fibers. A severe injury to the ACL/PCL may include completely torn ligament fibers. Symptoms of a severe ligament injury to the knee include extreme pain, inflammation and a snapping or popping sound at the moment the injury occurred.

While pain is usually present with a ligament injury even while the knee is at rest, putting weight on or bending the knee typically causes the intensity of the pain to increase. The ligament that has sustained damage determines which portion of the knee in which an individual will experience pain. If the pain is deep within the knee, the cruciate (ACL/PCL) ligaments may be damaged, whereas pain on the inner or outer portions of the knee may indicate that the medial or lateral collateral ligaments (MCL/LCL) have been damaged.