Comparing the Pivot Shift Test to the Lachman Test for Knee StabilityOrthopedic surgeons rely on clinical tests to help them diagnose knee problems. A common knee injury among athletes is an anterior cruciate ligament (ACL) tear. The most commonly used test for this problem is called the Lachman test. This test is routinely used despite the fact that the Pivot-shift test may be more reliable. In this article, the advantages and disadvantages of each test are presented.
Lachman test for knee instability is positive (indicating a torn ACL) when the lower leg slides farther forward from the femur (thighbone) than it is supposed to. Two test positions are used: one with the knee in neutral alignment and the second with the lower leg rotated slightly outwards (15 degrees). The test is positive if/when the amount it slides (called translation) is greater when the leg is rotated than when the leg (foot) is straight during the testing.
There are various ways to perform the pivot-shift test. The basic test applies a crosswise force of the tibia against the femur while rotating the lower leg. If it shifts enough to clunk (can be felt and sometimes heard as an audible clunk), the test is positive for instability.
Differences in the specific pivot-shift test techniques occur based on individual theories about what is causing the instability and how to test for it. In fact, the authors of this article put together a table of 13 studies using the pivot-shift technique. The type of pivot-shift test done is listed along with a description of the technique. The underlying problem for which the test has been designed was described. These are based on studies by well-known orthopedic surgeons such as Losee, Slocum, Galway, and Noyes to name just a few.
The very nature of how many different ways this test can be performed is the first strike against it. It is a complex test that is easy to interpret differently by different examiners. And it can be positive in people who are naturally overly flexible or who have joint laxity. The amount of force used and/or degree of rotation is left up to each individual doing the test. And if there are other soft tissues damaged, it can cloud up the results.
The authors list a few other disadvantages of the pivot-shift test. First, it isn't always positive even when there is a deficient ACL. Second, it can be a painful or unpleasant test for the patient. If they involuntarily tense their muscles, it reduces the reliability of the test. Sensitivity (ability of the test to determine a true positive) is much lower for the pivot-shift test compared with the Lachman test.
On the other hand, the Pivot-shift test is highly specific (able to accurately reflect a true negative -- showing that the patient does not have the problem). If the pivot-shift is negative or there is a low grade (mild injury), patient outcomes are better with faster return to sports. Patients tend to be more satisfied with treatment outcomes if the pivot-shift test suggests a low-grade injury.
The reason these two tests are being compared to one another is that over time, surgeons have become aware that just using Lachman's test to diagnose the problem and then repairing the anterior-posterior motion of the knee often left the knee with rotational instability. The pivot shift test is better able to identify the presence of a rotational instability, which must also be corrected during ACL reconstruction.
What hasn't been cleared up yet is whether it's more reliable and accurate to keep the lower leg in the neutral position or to internally or externally rotate the lower leg during the pivot shift test. It's possible that the answers really lies in what type and degree of injury are present.
Some experts have even suggested that the patients' unique anatomy is what makes the difference in testing and test results. Others believe that the presence of other soft tissue injuries (in addition to the ACL tear) contribute to a variety of patient results to the same test.
Scientists studying this problem have new tools and new technology to explore the normal and pathologic biomechanics of the knee and tests for knee instability. The authors of this article propose that in the future, computer systems with the ability to measure and analyze movement will make it possible to plan ACL reconstruction surgery with each individual patient in mind.
The authors of this article offer their favorite pivot-shift test. The hip is abducted (leg is moved away from the body). This position relaxes the iliotibial band (ITB) along the outside of the leg. With a relaxed ITB, the tibia can rotate. In this way, the results of the test are not influenced by how tight the ITB is (possibly limiting rotation).
A valgus force (from outside of the knee toward the body) is applied to the upper part of the tibia (lower leg). This movement is done while simultaneously moving the leg from flexion (bent) to extension (straight) and internally rotating the lower leg. If the tibia subluxes (jumps or clunks), the test is positive for knee instability. A positive test means that the knee is unstable in forward and backward translation as well as in rotation. This test is a modification of the well-known Losee technique.
The fact that the pivot-shift test provides information on the amount and direction of excess rotational movements (not just forward and back instability) is significant. These findings are important for the athlete who needs to jump, stop suddenly, pivot, shift, and cut quickly. Just repairing or reconstructing the torn ACL will not restore stability in the movements that require rotation. All other soft tissue injuries and imbalances must be identified and corrected.
The examiner must keep in mind that in acute cases (recent injury), swelling may prevent the clunk. Likewise, injury to the medial collateral ligament (MCL) or lack of knee extension for any reason can also prevent subluxation and mask the true results.
Clayton G. Lane, MD, et al. The Pivot Shift. In Journal of the American Academy of Orthopaedic Surgeons. December 2008. Vol. 16. No. 12. Pp. 679-688.
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