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.
