Meniscal extrusion

Illustration to show the meniscus sliding over the rim of the underlying tibial plateau.

Illustration showing a meniscus avulsing over the edge of the tibial plateau.

MRI definition of meniscal extrusion

MRI scans help to define the degree of extrusion.

Normally the meniscus is held in place by the meniscal roots and ligaments at the meniscal horns, and meniscotibial and meniscofemoral ligaments around much of the edge.

If its moorings become incompetent, normal vertical pressure pushes the meniscus body over the edge of the tibial plateau, leaving the bones in contact with one another.

"....Meniscal extrusion on MRI scan is defined as ≥3 mm of external displacement of the meniscus with respect to the central outer rim or edge of the tibial plateau quantified in the midcoronal plane for an unloaded knee...Due to convexity of the lateral femoral and tibial condyles, the lateral compartment is more adversely affected than the medial...."

Shreyash M Gajjar et al

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The extrusion is usually detected radiologically.

The surgeon may need to probe carefully to identify the tear during arthroscopy.

What is the cause of meniscal extrusion?

Several factors may lead to extrusion:

meniscal root avulsion
disruption of the meniscus fibres from injury
degeneration of the meniscus fibres from age

A tear of the posterior root of the medial meniscus is a common cause of extrusion.

After such an injury the meniscus may become incompetent at absorbing the vertical forces that go through the joint. The extrusion is usually detected radiologically.