Magnetic resonance imaging has emerged as an important modality in the non-invasive evaluation of osseous and soft-tissue structures in the post-traumatic knee. However, it is sometimes radiologically impossible to determine with confidence if a focus of high signal intensity in the meniscus is confined to the substance of the meniscus or if it extends to involve the joint surface. This is a critical differentiation because the latter represents menisci tears that can be found and treated arthroscopically, whereas the former represents degradation, intrasubstance tears or perhaps normal variants that are not amenable to arthroscopic intervention.

The aim of this study was to investigate the occurrence of altered signal intensity in the posterior horn of the medial meniscus in correlation with arthroscopic findings.

Sixty-four patients with suspected post-traumatic internal derangement of the knee who underwent magnetic resonance imaging prior to arthroscopy were evaluated retrospectively. There were 48 males and 16 females. Mean age was 28.2 years. Tears of the posterior horn of the medial meniscus were diagnosed unequivocally (Grade 3 signal) in 18 patients and equivocally (Grade 2/3 signal) in 10 patients. Arthroscopic correlation revealed 16 tears (89%) in the unequivocal group and only one tear (10%) in the equivocal group).

A meniscal tear is unlikely when magnetic resonance imaging shows a focus of high signal intensity in the posterior horn of the medial meniscus that does not unequivocally extend to involve the inferior or superior joint surface. An appropriate trial of non-operative treatment is recommended in such questionable cases. Magnetic resonance imaging is a useful diagnostic tool, however, it should be used selectively, and in conjunction with history and clinical examination in evaluating internal derangement of the knee.