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Introduction to the knee meniscus

Note the spelling -

  • ONE MENISCUS (singular) (pronounced menis-kiss)
  • TWO MENISCI    (plural)    (pronounced menis-sky)

Front of the knee showing the meniscus

 

The menisci are the two C-shaped white shock-absorbers of the knee - wedged horizontally between the femur (thighbone) and the tibia (shinbone), filling in the incongruency between the rounded ends (condyles) of the former and the flattened top of the latter - cushioning any impact when walking, running or jumping.

This illustration on the left is of a knee looked at from the front, with the skin slit open and the kneecap (patella) pulled over to the outer side. It is not a fully accurate drawing, but gives a reasonable idea of the position of the menisci in the joint. The meniscus on the inner side is called the medial meniscus and that on the outer side is called the lateral meniscus.

The two menisci differ in shape and mobility, and because of its anatomy it is the medial one that is more commonly injured.

meniscus transplant

Look at this photo of two real menisci just before being used for meniscal transplant.

The lateral meniscus (left of photo) is more O-shaped and quite highly mobile, able to slide forwards and backwards with knee movement. In addition there is a tendon (popliteus tendon) that passes along one edge, which breaks the attachment of the meniscus rim to the capsule of the joint at that point, and this adds to the mobility of the lateral meniscus.

The medial meniscus (right of photo) is quite different. It is larger and more C-shaped rather than O-shaped, and tightly bound to the capsular structures and to the medial collateral ligament along the outer rim. It moves very little with the movement of the knee. It is this inflexibility which leads to the medial meniscus being torn more frequently than the lateral meniscus. The lateral one can move and absorb impact, while the medial one simply rips.

 

Anterior and posterior horns of the meniscus

The pointed parts of the meniscus at the back and front of the knee are called the 'horns' of the meniscus. The ones at the back are the 'posterior' horns and the ones at the front are the 'anterior' horns. The thicker middle bit is the 'body' of the meniscus, with the thicker 'lateral rim' on the outer side and the flattened 'medial rim' on the inner side.

The posterior horn is hard to reach because the bones of femur and tibia are in the way. The surgeon reaches this part by asking his assistant to force the knee into a knock-knee or bow-leg position, and then approaches it around the bulky cruciate ligaments through the notch of the femur or under the rounded end of the femur.

The anterior horn is also difficult to operate on, because it is so close to the entry hole (portal) where the surgeon introduces his instruments and camera. This part of the meniscus is sometimes torn right off (avulsed) from its attachment.

Meniscus terminology

When a sportsperson says "I have torn my cartilage", what he should be saying is "I have torn my meniscus". You see, anatomists used to call the knee menisci 'semi-lunar cartilages' (which means 'half-moon-shaped' cartilages), and the 'semi-lunar' bit was often dropped.

However, the gristle at the ends of the long bones is also covered in a material which is called 'hyaline cartilage', and when it became clear that patients were getting muddled about the two different structures both tagged 'cartilage', doctors began calling the one 'meniscus' (previously the semi-lunar cartilage) and the other 'cartilage' (meaning hyaline cartilage).

Unfortunately, the confusion over the terms still persists.

So, to avoid confusion -

  • Drop the plural term 'cartilages' completely
  • Refer to 'menisci' if you mean the shock absorbers between the femur (thighbone) and tibia (shinbone)
  • Refer to 'cartilage' when you mean the hyaline cartilage at the end of the long bones (the 'gristle')
  • Don't let the surgeon patronise you by using the old terminology and referring to your 'cartilages' when he really means your 'menisci'. Ask him to clarify the terminology or to draw a picture.
  • Try not to be confused by the many internet articles still muddling up the old and new terminology.
Updated: 16 Apr, 2013
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