The outer rim is wider and better supplied with fibres and blood vessels.
The lateral (outer) meniscus is more 'O-shaped' than the medial meniscus, which is more 'C-shaped'. At the far ends are the two 'horns'.
Areas of the outer rim, especially on the medial meniscus, are intimately related to the capsule.
What does the knee meniscus do?
The menisci are the shock absorbers of the knee. Their crescentic shape, with a wider outer rim and a flattened inner rim, allow them to absorb the vertical forces going through the joint, channelling the forces towards the outer edge where the fibre bundles are more tightly packed. Thus the outer edge absorbs more stress.
Components of the knee meniscus
The two menisci - medial and lateral - are shaped differently.
The lateral meniscus is 'O'-shaped and is quite mobile. The capsular attachments at the outer rim are not strong, and there is a defect in the capsular attachment where the popliteus tendon passes. So it can glide when the knee is twisted, and tears are less common.
The medial meniscus, in comparison, is 'C'-shaped with strong capsular attachments and is relatively immobile, unable to glide much when the knee is twisted, and thus suffering more tears.
Blood supply of the meniscus
The blood supply of the meniscus comes from the outer capsule. This means that the outer rim of each meniscus is well supplied with blood, and tears in this area have a good opportunity to heal. On the other hand, the blood supply progressively reduces as it approaches the inner rim, so tears of the inner rim do not heal well.
When a surgeon describes the position of a tear he may refer to it being in the 'red' zone (implying the outer rim) or in the 'white' zone (implying the inner rim.
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