MR imaging of the knee: expanded classification and pitfalls to interpretation of meniscal tears.

Findings from magnetic resonance (MR) imaging examinations of 194 knees were interpreted retrospectively with a new classification, including not only signal intensity but morphologic abnormalities. All findings were correlated with prospectively recorded results from arthroscopic examinations performed by a knee subspecialist. Meniscal MR imaging findings were expanded to eight types in this study. Positive predictive values for each type of finding and overall accuracy were calculated. Several potential sources of error that could be differentiated through pathologic change were recognized. Sensitivity and specificity of MR imaging findings were, respectively, 95% and 74% for the medial meniscus and 86% and 90% for the lateral meniscus. As the classification grade increased, the likelihood of meniscal tear also increased. Medial meniscal tears tended to occur in a predictable pattern, extending from the posterior to the anterior horn. Thus, when a definite tear of the body or anterior horn of the medial meniscus is present, a tear of the posterior horn should be suspected, even if the findings are not convincing. This expanded classification may be useful for radiologists in expressing their level of confidence in a particular MR imaging finding suggestive of meniscal injury.