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Partial ACL Tears: A Diagnostic Dilemma

You may have heard the old expression Looks can be deceiving and that's the case with partial tears of the anterior cruciate ligament (ACL). The ACL is one of two very strong ligaments that hold the knee together and provide stability as the tibia (lower leg bone) moves under the femur (thighbone). Damage to either of the two bundles that make up this ligament can result in loss of stability and function of the knee.

In this article, two orthopedic surgeons review partial ACL tears. They explain why it's so difficult for surgeons to tell if the ligament is intact (okay) or not. Sometimes, a partially torn ligament looks perfectly fine. But it's really damaged and over time, it starts to lengthen or stretch out. Ligaments don't really stretch and bounce back like a rubber band. They are more likely to stretch and stay stretched out. And without a strong connection to hold the bones together, the tibia slides around too much under the femur.

Sometimes a partially torn ACL can be treated conservatively with nonoperative methods such as antiinflammatories and an exercise or rehab program. The surgeon's task is to determine which patients can be treated this way and who needs surgery to repair or reconstruct the ligament.

Making the right decision is important because ligaments don't have much of a blood supply. That means self-repair of a partial tear is not possible. Under the right conditions, it will eventually tear completely. Avoiding such an injury is often the goal, especially with athletes who are trying to stay in the game despite a partial tear.

How does the surgeon accurately diagnose the problem? That can be a problem in itself. When clinical tests commonly used by examiners are positive (e.g., Lachman test, pivot-shift test), then it's clear that there is an ACL tear. But studies show that these hands-on tests can appear normal when up to 75 per cent of the ligament is torn.

Several other diagnostic tests are available when the surgeon suspects a partial ACL tear. The first is the KT-1000 arthrometer test. This test provides a measure of joint laxity or looseness. Some people have naturally loose ligaments, so the injured knee is always compared to the uninjured side. More than three millimeters of difference from side-to-side is a red flag sign of pathologic injury. But once again, this test can fool the clinician. It simply isn't always a sensitive enough test. Results of the test can appear normal when there is a partial ACL tear.

Some suggest relying on MRIs for the diagnosis. But even with today's more advanced MRI systems, up to half (or more) of the partial ruptures will be missed using MRIs. A more reliable (but still not always 100 per cent accurate) method of identifying partial ACL tears is the arthroscopic exam.

Even with a scope inside the joint, if the outer covering of the ligament is intact, the surgeon won't see the torn fibers inside the sheath. And sometimes scar tissue mimics a normal appearing ligament support structure. Surgeons are advised to perform the pivot-shift test under anesthesia when the ligament appears to be torn.

In this way, the effects of muscle spasm and guarding are eliminated and the results are more accurate. Studies show that the pivot-shift test is only 24 per cent sensitive when the patient is awake compared to 92 per cent sensitive when under anesthesia. Results of the pivot-shift test while in the operating room can be misleading if there is other soft tissue damage inside the joint. The surgeon must take the opportunity to double-check for problems such as cartilage, bone, or meniscal fragments in the joint.

If the pivot-shift test is truly negative, the ligament is not functionally deficient. In other words, even though the ligament is partially torn, it can still function to some extent. Those are the patients who can do well with a rehab program instead of surgery. A nonoperative course of treatment is especially indicated if the individual isn't very active and/or doesn't have much in the way of symptoms.

Surgery is advised for anyone with high levels of activity that place heavy demands on the knee. There are several surgical options for managing partial ACL tears. These include various methods of repair or reconstruction of the ligament. Reconstruction is favored over simply repairing the tear. This assures the patient will have a stable knee that will support him or her in returning to sports activity at a preinjury level.

The use of thermal energy such as lasers and radiofrequency is not advised at this time. There's some concern that heat treatment of this type decreases the stiffness of the collagen cells that make up the ligament fibers.

The authors comment that every case requires an individual approach. There are so many possible factors that can interfere with an accurate diagnosis and/or with selecting the best treatment. How long has it been since the injury? What are the patient's expectations about recovery and return to sports participation? Is there any damage to the other soft tissue structures of the knee to consider? Has the patient already tried (and failed) nonoperative care?

No one really knows the natural history of partial ACL tears. Natural history refers to what happens if the injury isn't treated directly in any way. That's a tough one to figure out because some patients don't give the injury time to recover. They end up reinjurying the same knee with continued high-level sports participation.

Maybe it's the case that partial tears will always progress to complete tears. Maybe partial tears only become complete tears when more than 50 per cent or 75 per cent of the ligament is torn. Perhaps the patient's age makes a difference. These are all the factors we just don't know a lot about. There have been some studies in this area, but the results varied enough that a conclusion could not be reached.

For now, the authors provide an in-depth and insightful review of every aspect of the partial ACL tear. Anyone interested in understanding more about this condition is advised to read the details in this article.

Michael J. DeFranco, MD, and Bernard R. Bach, Jr, MD. A Comprehensive Review of Partial Anterior Cruciate Ligament Tears. In The Journal of Bone and Joint Surgery. January 2009. Vol. 91A. No. 1. Pp. 198-208.


*Disclaimer:* The information contained herein is compiled from a variety of sources. It may not be complete or timely. It does not cover all diseases, physical conditions, ailments or treatments. The information should NOT be used in place of visit with your healthcare provider, nor should you disregard the advice of your health care provider because of any information you read in this topic.
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