from link below:

The knee is a complex joint, capable of large movements and stresses many times our body weight. It has a loose, cup-and-saucer configuration allowing shifting leverage. This requires more ligaments (strings between bones) and lubricated pouches (bursae) than other body joints.

This sloppy skeletal instability of the knee makes it depend on the support of strong thigh muscles. It is also the reason for the unique menisci, two in each knee.

There is one meniscus on each half of the knee-joint rim, the inner and outer side of the knee. Menisci are important to:

1. Space and stabilize the two main knee bones
2. Increase surface contact between the bones
3. Absorb shock
4. Help joint nutrition from the synovial fluid
5. Lock the knees straight when standing
6. Control knee movements better.