The meniscus is a cushion structure made of cartilage which
fits within the knee joint between the tibia and the femur. The medial meniscus
is C-shaped and the lateral meniscus in the more circular. The meniscus is made
up of type I collagen that provides shock absorption and stability to the knee
joint. The meniscus helps to protect the knee joint, allowing the bones to
slide freely on each other. Discoid meniscus is a rare variation of the
meniscus that usually affects the lateral meniscus of the knee in less than 5%
of the population and could be bilateral in about 25% of the cases.
Discoid
meniscus is a large meniscus with abnormal attachment causing increased
mobility of the meniscus. It causes a pop, click, or snapping with locking and
pain. There will be loss of full knee extension with tenderness on the lateral
joint space. Symptoms occur more during extension of the knee. The discoid
meniscus occurs due to the abnormal development and increase in size of the
meniscus. An x-ray could show increased widening of the joint space. An MRI
will show the “bow tie” sign in three or more sagittal continuous cuts. The
coronal MRI will show a thick and flat meniscus extending beyond the halfway
point of the condyle. Watanabe Classification of Discoid Lateral Meniscus
- Type I: Block-shaped stable
- Type II: Block-shaped, stable, partial meniscus (has good peripheral attachment)
- Type III: Unstable meniscus with stability arising only form the ligament of Wrisberg. (no posterior meniscal tibial attachment).
Treatment
An asymptomatic patient will be treated with observation. A
symptomatic patient may receive a partial meniscectomy and saucerization with
repair of type III (no posterior tibial meniscal attachment)
