What is the function of the meniscus in the knee? What is the consequence of tearing a meniscus? Why are most symptomatic meniscal tears removed and not repaired?
The medial and lateral meniscus together provide shock absorption, establish a broad base of contact surface and help provide stability to the knee.
It should be clear, even sans picture, that a wedge of cartilage helps absorb shock:
The meniscus also helps establish a broad base of contact surface. As shown below on the left, without a meniscus there is focal loading and higher pressure at the point of contact
Last, the meniscus is stabilizing – a so called "chock block" effect
Here's how that works:
A lost meniscus (might) leads to
- Pain with loading, such as jumping (because of lost shock absorption)
- Less anterior/posterior stability of the knee (because of loss of chock block effect)
- DJD (because lost broad base of contact surface leads to focal load bearing and increased pressure, P = F/A. High pressure stimulates bone formation and in turn a stiff, injury prone subchondral plate.
We remove menisci, in light of the above, only when we have to. And we usually have to remove symptomatic tears, because most tears don't heal. In general, only tears at the periphery have healing potential, as the blood supply enters from the capsule. Also, if the meniscus is macerated, ie torn up, there is no point in repairing it.
The key clinical question (in light of the histological truth that menisci have no pain fibers) is whether the tear is causing the pain, or whether it is just part and parcel of an arthritic picture, or an incidental finding.