MR imaging of the knee: clarification of its role.

In an attempt to further delineate the role of magnetic resonance (MR) imaging in evaluating knee pathology, a retrospective study of 60 patients who underwent MR scanning before arthroscopy was undertaken. For medial meniscus tears, the sensitivity of MR imaging was 97% whereas its specificity was 77%. For lateral meniscus tears, MR imaging was 90% sensitive and 87% specific. The positive predictive value of MR imaging was 85% for medial meniscus tears and only 79% for lateral meniscus tears. The negative predictive value was 95% for the medial meniscus and 94% for the lateral meniscus. MR imaging had an accuracy of 93% in assessing anterior cruciate ligament pathology also. These results begin to clarify the role of MR imaging in evaluating intraarticular knee pathology. Grade III signals represented tears at arthroscopy more than 90% of the time. False-positive results occurred more frequently than false negative results did. The high negative predictive value of MR imaging indicates that with a negative MR image, the orthopaedist can reliably conclude that no meniscal pathology is present. MR imaging is a useful diagnostic tool; however, it should be used selectively and in conjunction with the clinical examination in evaluating internal derangements of the knee.