Abstract

Published data show that sensitivity and specificity of magnetic resonance imaging (MRI) for detecting meniscal pathology can approach 90% when compared to arthroscopic findings. The authors audited the MRI service in our unit, to judge our performance against this standard. The authors retrospectively analysed a single surgeon series of 240 scopes for all indications (meniscal pathology, osteochondral defects, ligament reconstruction). The arthroscopic reports included an outline diagram of the meniscus upon which the surgeon recorded operative findings. 112 of these patients had received a recent MRI. The authors looked at whether the MRI report showed a tear and compared this with findings detailed in the arthroscopic report. 66 patients had a positive scan. 64 of these were found to have a tear at surgery. 37 scans were reported as “no tear”, of which four were found to have a tear at surgery. Nine scans were not easy to classify as they were descriptive, for example, “signal change, possible tear” or “tear cannot be ruled out”. These tended to correspond with equivocal arthroscopic findings of “degeneration” or “fibrillation”, and were classified on balance of their subjective description. Of the nine equivocal scans, five were more positive for tear. Six out of nine equivocal scans had degenerate menisci at arthroscopy. In our series of 112 knees, MRI was 91% sensitive, 90% specific and 90% accurate, which is comparable to published data. When a definite diagnosis of “tear” or “no tear” was made at scan, there were two false positives and four false negatives. False positives may be unnecessarily exposed to the risks of surgery. Patients with negative scans had a mean delay to surgery of 33 weeks compared to 18 weeks for patients with positive scans. False negatives may therefore wait longer for surgery. Two of the false negative scans clearly showed meniscal tears which were missed by the reporting radiographer. In our series the scan itself was more accurate than the reporting. It is important to have an experienced musculoskeletal radiologist to minimise the number of missed tears. It is also important that surgeons check the scan, as well as the report, when making a decision to operate based on MRI findings.