We reviewed the Magnetic Resonance Imagnings(MRI) of 41 knees in 41 patients who had confirmed discoid lateral menisci of the knee joint arthroscopically from Nov. 1991 to Jna. 1994. The purpose of this study is to demonstrate the diagnostic
value
of MR
imagings for discoid lateral meniscus and to describe the chraracteristic findings of MR imagings according the type of tears and to explain the causes of mis-diagnosis.
Type of tears were confirmed by either arthroscopic finding or gross specimen. Peripheral detachment were found in 13 discoid menisci(31.8%), complex tears in 16(39.0%), horizontal tears in 5(12.2%), longitudinal tears in 4(9.8%),. T4ransverse
tears in
2(4.8%) and intact in 1(2.4%) discoid meniscus. 36 out of 41 discoid lateral menisci(87.7%) were correctly diagnosed as discoid menisci preoperatively on MR imaging. However 5 were mis-diagnosed as lateral meniscal tear. It was very difficult to
diagnose discoid lateral meniscus which had detached from meniscosynovial junction and displaced into the center or had transverse tear in mid-horn. We also fund that MR imagings showed the correct torn shape of 27 out of 36(75%) discoid lateral
menisci
which were correctly diagnosed as discoid menisci, Accuracy of MR imagings for each type of tear were followings : 60% for horizontal tear, 100% for peripheral tear, 73.3% for compolex tear 33.3% for longitudinal tear, 0% for transverse tear.
On the MR imaging, capsular attachments of the popliteal muscle in knees with discoid menisci were compared to those of 100 knees with normal lateral menisci.
Capsular attachments of popliteal muscles were divided to three : type 1 is fibrous band, type 2 is thin muscle bundle, type 3 is thick muscle bundle. Capsular attachments of popliteus muscle in the knees with discoid menisci(68.3%) had
significantly
higher incidence of type 2 or 3 compare to the knees with normal lateral meniscus932%).
As conclusion, MR imagings for diagnosing of discoid lateral meniscus had a relatively accurate diagnostic value and also showed the its type of tear. It is very important to realize the type of discoid meniscal tear in order to obtain correct
diagnosis.
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