Discoid Meniscus-Overview and Pathology

Normal meniscus of the knee is semilunar in shape. In discoid meniscus the meniscus of the knee is discoid rater than semilunar in shape.  This is a common cause of popping or snapping knee.

The lateral meniscus is most frequently affected, though on occasion, the condition may occur in the medial meniscus.

Involvement is often bilateral.

There is no difference in occurence in either sex.

The condition is diagnosed comparatively rarely, and it may often go unrecognized. Familial incidence of discoid lateral meniscus is known.

Pathology

The affected meniscus is made of fibrocartilage, oval or roughly circular in shape. The corresponding surface of the tibial plateau will be almost entirely covered by the discoid meniscus. Variations may occur where anterior horn and body will be a solid mass, whereas the posterior horn will be normal.

Cystic degeneration with centrally located cavities  can occur in Discoid meniscus.

Histology shows mucoid degeneration due to increased wear. Meniscal tears are common.

There are three types of discoid lateral meniscus

• Primitive
• Intermediate
• Infantile.

The primitive type

This type of meniscus is a complete disc.  This gets damaged due to opposing movement between the superior and inferior surfaces  which lead to tears, ridges and separation from surfaces.

Cause of snapping

The movement of the femoral condyle over the ridges that produces the snapping sensation and sound. Because of hypermobility of the posterior part of the lateral meniscus, it frequently gets displaced medially, gets caught between femoral condyle and tibial plateau leading to loud click.

The intermediate type

The intermediate type i also complete but thickness is lesser than primitive and size is smaller. Fate is also similar to primitive type.

The infantile type

The infantile type is discoid in shapebut the size is normal. tears occur here as in normal meniscus.

Another classification of  discoid meniscus is based on arthroscopic visualization. It describes three types

Wrisberg-ligament type

Wrisberg-ligament type in which the lateral meniscus has not attached to the tibial plateau posteriorly except for meniscofemoral ligament (ligament of Wrisberg). This ligament is taut and does not accommodate the normal flexion and extension of the knee.

The hypermobility of the posterior part of the lateral meniscus results in its secondary hypertrophic thickening.

Usually this variety is symptomatic.

Complete type

The menisci are disc-shaped, covering the entire tibial plateau.  The attachments of the lateral meniscus are intact and there is no hyperlaxity or abnormal motion.

Complete type of discoid lateral meniscus is usually an incidental finding at arthroscopy

Incomplete type

This from the complete type only in size.