My friend's daughter has what they called discoid meniscus in her left knee. I've also heard the term in sports. What exactly is the meniscus?

Your knee is a complicated joint in that it has the knee cap protecting where the femur (thigh bone) and shin bone (tibia) meet. The knee needs tendons to help move the muscles, and it also needs cartilage - the menisci - to protect the knee. You have two menisci. One is the lateral meniscus (on the outside of your knee) and the other is the medial meniscus (on the inside of the knee).

The menisci attach to the tibia, staying in place with ligaments, sinewy tissue. The menisci are important because they act as shock absorbers, they stabilize the knee, allowing the body weight be distributed evenly across it, rather than on one or two points directly on the knee, and they lubricate the cartilage.

So, the menisci are important parts of your knee and should be treated if there are any injuries.