Discoid lateral meniscus and the frequency of meniscal tears

Objective. To use MRI to determine the incidence of discoid lateral menisci in a large study population, and to compare those patients with those with out a discoid meniscus in order to assess the impact of a discoid lateral m eniscus on the frequency of meniscal tears. Design and patients. Results of 1250 knee MRI studies were retrospectively reviewed. Using the criterion of three or more meniscal body segments on se quential sagittal images, 56 patients were found to have a discoid lateral meniscus. After exclusion of patients with prior knee surgery, 49 patients with a discoid lateral meniscus were compared with 1146 patients without a discoid meniscus. Patients were categorized as having tears of the medial m eniscus, lateral meniscus, or both menisci. Results. In our study population, there was a 4.5% incidence of discoid lat eral meniscus. Seventy-one percent of patients with a discoid lateral menis cus had one or more meniscal tears, compared with 54% of the comparison gro up (P=0.01). The frequency of solitary lateral meniscal tears in the discoi d group was also higher than in the comparison group: 20%. versus 11% (P=0. 03). The frequency of solitary medial meniscal tears and concomitant tears of both menisci were not significantly different between the two groups. Conclusions. The discoid lateral meniscus is an uncommon variant, but not a s rare as once believed. Compared with the normal semilunar meniscus, the d iscoid lateral meniscus has a higher frequency of meniscal tears, and solit ary tears of the lateral meniscus are more common in the discoid variant. T he frequency of medial meniscal tears is not altered by the presence of a d iscoid lateral meniscus.