Symptoms and Tests Used for the Diagnosis
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 an Audible “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.
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.
Knee 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.When the ACL is torn, vessels that feed the ligament can fill the joint space with blood, a condition known as hemarthrosis. Symptoms of hemarthrosis include swelling, redness, warmth, bruising, and a bubbling sensation in the joint.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.
Abnormal Physical Exam
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:
- Lachman test: 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. The knee is held slightly bent (about 20-30 degrees) and the femur is stabilized in one hand while shifting the tibia in the other hand.
- 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.
- Drawer test: The Drawer test is performed with the knee held with a 90-degree bend. The tibia is shifted forward and back to assess for an intact ACL (pulling forward) and an intact PCL (pushing back).
In addition to performing these specific tests, your surgeon will also examine your knee for swelling, mobility, and strength. The other major knee ligaments can also be assessed.
Your physician will also evaluate X-rays of the knee to assess for any possible fractures, and an 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 an MRI in diagnosing an ACL tear. An MRI can be particularly useful to diagnose associated injuries such as meniscus tears and cartilage damage.
When to See a Doctor
The signs and symptoms of an ACL injury are not always the same, so it is important to see a doctor if you experience any of the following:
- Knee pain or swelling that lasts more than 48 hours
- Trouble standing or walking on the affected knee
- Inability to support your weight on the affected knee
- A deformed or odd appearance of one side of the knee compared to the pain-free side