Title: Discoid Meniscus

Imaging Findings:

Figure MRI of normal appearing lateral meniscus with 3 or less sagittal images showing bridging (arrows) of the anterior and posterior horns.

Figure MRI showing discoid meniscus with 3 or more continuous sagittal sections (5mm thick) revealing bridging of the anterior and posterior horns (1). Intrasubstance signal derangement (arrows) is indicative of tear (2).

Clinical:

Present at 15 – 35 years of age, more common in males. There is a 2.7% frequency in the general population. This entity can be discovered incidentally or associated with lateral knee pain and snapping (3).

Etiology:

One school of thought suggests embryologic aberration of meniscal canalization. Some studies have suggested a congenital / hereditary component. The most accepted theory is a developmental process due to deficiency of the lateral meniscal fasicles with resultant abnormal meniscal biomechanics and discoid meniscal remodeling (4).

References:

1. Silverman JM, Mink JH, Deutsch AL. Discoid menisci of the knee: MR imaging appearance. Radiology 1989; 173:351-354.
2. Hamada M, Shino K, Kawano K, Araki Y, Matsui Y, Doi T. Usefulness of magnetic resonance imaging for detecting intrasubstance tear and/or degeneration of lateral discoid meniscus. Arthroscopy 1994;10:645-653.
3. Woods GW, Whelan JM. Discoid meniscus. Clin Sports Med 1990; 9:695-706.
4. Resnick D, Kang HS. Internal derangement of joints. 1st ed. Philadelphia, Pa: WB Saunders, 1997; 625-628.