Diagnosis of discoid lateral meniscus of the knee on MR imaging.

To establish the quantitative MR diagnostic criteria for discoid lateral meniscus, MR imaging of 60 knees with arthroscopically confirmed discoid lateral menisci and 134 knees with semilunar lateral menisci were analyzed. Following four quantitative parameters were measured. (1) Meniscal width (MW): The minimum meniscal width on the coronal slice. (2) Ratio of the meniscus to the tibia (RMT): The ratio of minimum meniscal width to maximum tibial width on the coronal slice. (3) Percent coverage of the meniscus (PCM): The ratio of the sum of the width of the anterior and posterior horns to the meniscal diameter on the sagittal slice showing the maximum meniscal diameter. (4) Continuity of the anterior and posterior horns (CAPH): The number of consecutive sagittal slices showing continuity between the anterior and posterior horns of the meniscus on the sagittal slice. The cut-off values of each parameter to discriminate between the discoid and semilunar meniscus were calculated based on a univariate logistic regression analysis. The sensitivity and specificity were also calculated for each of 4 parameters and combinations of these parameters. The cut-off value was 15 mm for the MW, 20% for the RMT, 75% for the PCM, and 3 slices for the CAPH. The sensitivity and specificity of the four parameters ranged between 87% and 50% and between 99% and 92%, respectively. The most accurate diagnostic criterion was either the RMT > or = 20% or the PCM > or = 75%, and the sensitivity and specificity were 95% and 97%, respectively, even when torn menisci were included.