MR of the knee: the significance of high signal in the meniscus that does not clearly extend to the surface.

On MR images of the knee it is sometimes impossible to determine with confidence if a focus of high signal in the meniscus is confined to the substance of the meniscus or if it extends to involve the surface. This is a critical differentiation because the latter represents meniscal tears that can be found and treated at arthroscopy, whereas the former represents degeneration, tears, or perhaps normal variants that cannot be detected or treated arthroscopically. We make an equivocal diagnosis of a tear when it is difficult to decide if signal in a meniscus involves the meniscal surface. We studied MR scans of the knee in 142 consecutive patients for the presence of such equivocal tears. Their prevalence was 14% (20/142); 17 were in the posterior horn of the lateral meniscus and three were in the posterior horn of the medial meniscus. In 13 cases with arthroscopy/arthrotomy correlation, no tears were found. In one of the 20 patients in whom the meniscus was removed during arthroplasty, histologic examinatio...