Signs of an ACL Tear:
An ACL tear most often occurs during sports or athletic events. About 80% of ACL tears occur without contact with another athlete. The most typical story is an athlete suddenly changing direction (cutting or pivoting) and feeling their knee give out from under their body.
Hearing a "Pop":
People who suffer an ACL tear usually report hearing a "pop" at the time of the injury. Most people are surprised at how loud this can be, and many bystanders have heard this from the sideline of a football or soccer game. Even if you don't hear the pop, usually people will feel the sudden shift in the joint.
Knee Giving Out/Instability:
The ACL is critical to the stability of the knee joint, and when an ACL tear occurs, the joint is usually unstable. This means that the knee joint has a tendency to give out. Giving out or instability usually occurs with cutting or pivoting movements common in many sports. However, in some patients with an ACL tear, instability can occur with even simple movements while walking or getting into a car.
Swelling and Pain:
Swelling of the knee joint occurs in almost all patients with an ACL tear. This swelling is usually quite large, and occurs rapidly -- within minutes -- of the injury. The swelling that occurs with a torn ACL is actually a hemarthrosis, meaning the knee joint is filled with blood. The ACL has a blood vessel within the ligament that is torn at the time of injury, causing the knee to fill with blood.
Pain associated with an ACL tear is common, although can vary depending on associated damage in and around the knee joint. Much of the pain of an ACL tear is due to the swelling of the joint.
Your doctor can assess the ligaments of your knee with specific tests. The most commonly used tests to determine the presence of an ACL tear include:
The Lachman test is performed to evaluate abnormal forward movement of the tibia. By pulling the tibia forward, your surgeon can feel for an ACL tear.
Pivot Shift Maneuver
The pivot shift is difficult to perform in the office, it is usually more helpful in the operating room with a patient under anesthesia. The pivot shift maneuver detects abnormal motion of the knee joint when there is an ACL tear present.
Your physician will also evaluate x-rays of the knee to assess for any possible fractures, and a MRI may be ordered to evaluate for ligament or cartilage damage. However, MRI studies may not be needed to diagnose an ACL tear. In fact, the physical examination and history are just as good as a MRI in diagnosing an ACL tear!
RL Larson and M Tailon "Anterior Cruciate Ligament Insufficiency: Principles of Treatment" J. Am. Acad. Ortho. Surg., Jan 1994; 2: 26 - 35.