Accurately Testing for Meniscal Tears in the KneeWith the use of arthroscopic examination, surgeons can prove and refine clinical tests for joint damage. In the case of the meniscus, a C- or horseshoe-shaped piece of cartilage in the knee, the McMurray test is used most often to diagnose posterior (along the back of the knee joint) meniscal tears. Results of the McMurray can be verified during the arthroscopic exam when the surgeon can perform the same test and see what happens to the meniscus.
There are two McMurray tests: the standard or conventional McMurray and the paradoxical McMurray. Both tests are performed in the same way. The knee is bent; the examiner holds the heel and twists the lower leg in and out. If there is a torn or loose meniscus, there will be a painful, snapping or clicking sensation as the movement shifts or traps the meniscal flap or fragment between the tibia (lower leg bone) and the femur (thigh bone).
The results are what make the test conventional or paradoxical. For example, the test is considered "paradoxical" when the leg is rotated internally (inward) and the medial side of the knee (side closest to the other leg) clicks. There can also be a paradoxical McMurray when the leg is rotated externally (outward) and the clicking occurs in the lateral compartment (or side) of the knee. Lateral refers to the side of the knee away from the other knee.
The results just described are considered paradoxical, meaning the opposite of what is expected. In the conventional test, the clicking occurs on the opposite side of the knee from the rotation. When the leg is rotated inward, the clicking occurs on the outside of the knee. When the knee is rotated outward, the clicking occurs on the inside of the knee.
In this study, one surgeon compared each test before and during arthroscopy in two groups of patients. Group one had a confirmed meniscal tear. Group two did NOT have a meniscal tear. There were a total of 1,015 patients who had knee arthroscopy. Two thirds of the group had a meniscal tear. The remaining one-third did not have a meniscal tear.
This type of study gives us a better idea of how accurate the tests are. In statistical terms, sensitivity and specificity of each test is determined. Sensitivity refers to a "true positive" test -- in other words, when the test is positive, it is a true indication that the person has a meniscal tear. Specificity refers to a "true negative" test -- when the test is negative, the person really doesn't have a meniscal tear.
The more sensitive a test is, the fewer false positives are found. A false positive is when the test is positive suggesting a meniscal tear is present when, in fact, there is no tear. The more specific a test is, the fewer the false negatives. A false negative refers to the patient who has a negative McMurray test as if nothing is wrong when there really is a meniscal tear present.
So, how did the two tests compare in terms of sensitivity and specificity? The paradoxical McMurray was much less sensitive (less able to find patients who really have a torn meniscus) compared with the conventional McMurray test. But the paradoxical test was more specific (when the test was negative, the person did not have a meniscal tear).
The surgeon also looked at types of meniscal tears to see if some types are more likely to show up on one test versus the other. Types of tears are categorized based on the location and/or shape of the tear (e.g., lengthwise versus crosswise or tear versus detachment). He was able to see why sometimes the paradoxical test was accurate and sometimes inaccurate. If the tear was not long enough or located in the middle of the meniscus, the paradoxical test would be negative.
The author suggests an MRI should be ordered when the paradoxical McMurray test is negative but the patient has pain and other clues pointing to a meniscus problem. A positive paradoxical test is a sign that there is a large meniscal tear -- long enough to shift the loose or torn meniscus during the test. But a positive paradoxical McMurray test does not mean the patient should have surgery immediately. The surgeon must evaluate the patient history and clinical findings and find out the patient's goals and activity level.
Sung-Jae Kim, MD, PhD, et al. The Paradoxical McMurray Test for the Detection of Meniscal Tears. An Arthroscopic Study of Mechanisms, Types, and Accuracy. In The Journal of Bone and Joint Surgery. August 15, 2012. Vol. 94A. No. 16. Pp. e118(1)-(7).
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