Knee > ACL Tear

What is the ACL?

The ACL (anterior cruciate ligament) is the smallest of the four main ligaments in the knee but it is the primary stabilizer for rotational movement. It connects the thighbone (femur) to the shinbone (tibia) in the center of your knee, limiting rotation and forward movement of the shinbone. Without an ACL, the knee would be unstable and could dislocate during activities that involve twisting.

When the shinbone and thighbone rotate too far in opposite directions or the knee is bent in the wrong direction, the ACL can be torn, or, depending on the force of the injury, sprained. It is estimated that 50 percent of ACL tears are accompanied by cartilage tears, and 20 to 30 percent involve other ligament damage. The ACL and the MCL (medial collateral ligament) are often injured at the same time.

Quick directional changes while running cause most ACL injuries. When a basketball player, running down the court, plants his foot hard to change direction, his knee buckles as the thighbone and shinbone move in opposite directions, tearing the ACL. Sports like basketball, soccer, and skiing often cause these non-contact ACL injuries. Football players seem to be at the greatest risk for multiple knee injuries like combined ACL, MCL, and cartilage damage.

Some people with a completely torn ACL are able to build their muscle strength enough to resume normal activities without surgery. While activities of daily living may be possible without an ACL, it is less likely that patients will be able to return to cutting and twisting sports. Still, the non-operative option exists, and having surgery or forgoing it is partly a personal choice. Those who are not very active may choose a strengthening program instead of surgery, since the injury is not likely to interfere with their daily activities. Such a program takes about four months to complete. However, here is a statistic that should be considered: about 65 percent of all patients with a torn ACL will eventually develop a torn meniscus, which may predispose them to an early onset of arthritis. Active, athletic people are more likely to opt for surgery, so that the knee can be repaired and rehabilitated and they can return to their activities. Rehabilitation following surgery can take as little as three months but usually takes one year, and requires a commitment of at least 45 minutes three days a week. But the success rate is about 90 percent, meaning that most people will be able to return to their active lifestyles after an ACL reconstruction. An untreated ACL may leave you unable to trust the stability of your knee. It may continue to give way, putting you at risk for further injury to other knee structures. An untreated ACL injury often causes cartilage damage that can lead to the early onset of osteoarthritis.

There are three parts to an orthopedic evaluation: medical history, a physical exam, and tests that your physician may order.

Your doctor will likely ask you how you injured your knee, how it has been feeling since the injury, and if your knee has been previously injured. Physicians also typically ask about other conditions, such as diabetes and allergies, and medications currently being taken. The doctor also may ask about your physical and athletic goals ­ information that will help him decide what treatment might be best for you in achieving those goals.

A physician usually can make an early assessment of an injury by feeling around the area, but because the ACL is deep inside the knee, it is difficult to feel around in search of pain or tenderness. Instead, there are two common tests to determine the existence of an ACL injury and its severity that can be performed in your physician's office.

   The Lachman test, which determines instability of the ACL. In this, the leg is held slightly bent and the physician pulls on the lower leg (tibia). If the leg moves significantly more than the other, uninjured, knee, that may signify a torn ACL. A hand-held instrument called an arthrometer may also be used to measure the stability of the knee.

   With the knee bent 90 degrees, the doctor will pull the tibia forward in what is called an anterior drawer test. If the tibia moves excessively forward, that would strongly suggest a torn ACL.

TESTS

Should your physician require a closer look, these tests may be conducted:

   MRI (magnetic resonance imaging) has an accuracy rate of nearly 90 percent in determining whether an ACL has been torn and to what extent it has been damaged. It is not very good, however, at detailing a partial tear.

   Arthroscopy, in which the doctor inserts a small camera into your knee, is the best way to examine a partial tear. Arthroscopy allows the physician to examine the ACL to determine the extent of the tear and if reconstructive surgery is needed. Though less painful than some procedures, arthroscopy is not pain-free. If your ACL is torn, it is usually reconstructed at the same time as the arthroscopic exam, and you may be required to stay overnight.

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