Discoid Meniscus

Anatomy

A meniscus is a shock absorber cushion that sits between the cartilage of the femur (thighbone) and the cartilage of the tibia (shinbone). Each knee has two meniscia medial (inner) meniscus and a lateral (outer) meniscus. A discoid meniscus occurs in place of the lateral meniscus 97% of the time. The normal lateral meniscus is crescent shaped when looked at from above, with the opening of the crescent facing the center of the knee. The normal lateral meniscus is also shaped like a wedge from the front, so that the crescent tapers toward the center of the knee joint. A discoid meniscus resembles a complete circular disc instead of a crescent from above and is block shaped (i.e., it does not taper toward the center of the knee). In addition, the normal lateral meniscus is properly anchored to the tibia and femur by ligaments so that it is stable inside the knee. A discoid lateral meniscus may lack these attachments and may be very loose within the knee.

Causes

It is unclear what causes a discoid meniscus. A discoid meniscus is currently believed to be a congenital malformation and not a developmental variant, because a normal meniscus never resembles a discoid meniscus during its development. One theory is that when a developing meniscus does not form proper attachments to the tibia and femur, it is very loose, and to compensate for this instability, it develops discoid features, such as being circular and block shaped.

Symptoms

Many people with a discoid meniscus never have any symptoms. It is likely that in these cases, the discoid meniscus is properly attached and not loose. Young children who have a discoid meniscus that is not properly attached to the knee present with “snapping knee” syndrome. In these cases, the child experiences episodes of dramatic snapping and popping in the knee. These snaps occur spontaneously as the knee straightens and usually produce temporary pain and apprehension. In very young children (between the ages of 3 and 4), the snapping may be audible but is usually painless. The snap represents the loose discoid meniscus moving more than it should inside the knee. Older children with a discoid meniscus that is properly attached often do not have any symptoms unless they injure their knee. These patients have a higher risk of tearing their discoid meniscus after a trauma. They present with pain and swelling on the outer aspect of the knee after a knee injury.

Diagnosis

A history of snapping inside the knee in a young child should alert the practitioner to evaluate for a discoid meniscus. On physical exam, the clinician should feel for a dramatic clunk on the outer aspect of the knee as the knee is flexed and rotated (McMurray test). Sometimes a bulge may be visible on the outer aspect of the knee with knee flexion. The clunk and bulge likely represent an unstable discoid lateral meniscus. Because discoid menisci occur in both knees 25% of the time, the other knee should be carefully examined as well.

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