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Meniscal Tears

Meniscal Tears

OVERVIEW

There are two menisci in each knee (medial (inside) and lateral(outside)).  They are a very special type of cartilage, and act like shock absorbers within the knee to reduce pressure and stress across the joint.

The medial meniscus is on the inside of the knee, it is attached to the ligaments on the inside of the knee (MCL) and the joint capsule.  As it is more firmly attached to the deep structures of the knee, it is more commonly injured than the lateral meniscus. It takes approximately 50% of the weight across the medial (inside) compartment of the knee. The lateral meniscus is on the outside of the knee, it is more mobile and ‘C’ shaped than the medial meniscus.  It takes up to 80% of the weight across the outside compartment fo the knee.

In cross section the meniscus looks like a triangle – with the apex (tip) on the inside and the base on the outside.  The two parts of the meniscus like a ‘bow tie’ on an MRI scan.

Only the outer 1/3 of the meniscus gets a significant blood supply, but the inner 2/3 gets the majority of its oxygen and nutrients from the joint fluid and has minimal blood supply.  As a consequence, tears of the inner part of the meniscus often do not heal (these are called white on white tears), whereas tears of the outer meniscus can heal (red on red tears).

There can be variations of the natural anatomy which can cause the meniscus to tear – “Discoid” Meniscus.  This is where the meniscus develops into a complete semicircular shape.  It lacks the stability of a normal meniscus, and consequently can tear more easily.

 

Normal and torn menisci

Meniscal blood vessels above indicated in red in the outer part of the meniscus
Above: Types of Discoid Meniscus
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