Judy DeMeyere, age 16, of St. Paul, Minn., for her question:

HOW DOES THE KNEE WORK?

The knee moves like a hinge. It is more likely to be damaged than most other joints because it cannot twist without injury.

The patella, as the kneecap is called, is in front of the joint and protects it from bruises. The small, flat, triangular bone is not directly connected with any other bone. Muscle attachments hold it in place.

The femur, or thigh bone, and the tibia, or large bone of the lower leg, are connected in three ways: by strong, cordlike tissues called ligaments, by muscles and by a synovial capsule. The synovial capsule, which surrounds the joint, is made of strong connective tissue.

One group of muscles bends the knee and another group straightens it. Cords called tendons attach the muscles to the bones.