Before surgery, 277 menisci in 144 knees were examined with magnetic resonance (MR) imaging. They were then examined directly with arthroscopy or arthrotomy. Menisci were graded on a scale of 1-3 according to the character of the intrameniscal MR imaging signal. At surgery, 137 of 154 (89%) menisci exhibiting only grade 1 or grade 2 signal were found to be normal. One hundred sixteen of 123 (94%) menisci exhibiting intrameniscal signal communicating with a meniscal articular surface (grade 3 signal) had tears. If only a grade 3 signal is considered consistent with meniscal tears, then MR findings and surgical findings agreed in 91.3% of menisci. MR imaging can separate surgically significant from nonsignificant meniscal lesions and is useful in the noninvasive preoperative screening of suspected meniscal tears.

Menisci from 12 autopsies and above-knee amputations were imaged with magnetic resonance (MR) at 1.5 T and then sectioned for gross and histologic examination. A histologic staging system was developed and showed a one-to-one correlation with corresponding grades of MR signal intensities. Histologic stages 1 and 2 represented a continuum of degeneration culminating in stage 3 fibrocartilaginous tears, seen most frequently in posterior-horn segments of the medial meniscus. Correlation of histologic stages with MR signal intensity allows for an improved diagnostic reading of MR images.

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Follow-up knee magnetic resonance (MR) examinations were performed on 17 patients (18 menisci) with arthroscopically proved tears of the outer third of the meniscus who were treated either conservatively (six patients) or with surgical repair (11 patients). All patients satisfied accepted clinical orthopedic criteria for meniscal healing. MR examinations obtained 3-27 months after injury revealed persistent signal intensity (grade 3), unchanged from that seen on the preoperative study, in all 15 patients in whom both pre- and postoperative studies were obtained and in three of four menisci that were proved to be healed at second-look arthroscopy. It appears that grade 3 signal from both conservatively treated and repaired menisci may persist long after the tear has become asymptomatic and has presumably healed. The presence of such signal should not be interpreted as necessarily indicating meniscal retear in these patients. Persistent signal intensity at the site of previous injuries may account for some reported cases of disagreement between MR and arthroscopic findings.

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