Knee problems fall into two main categories: inflammation and mechanical difficulties.

Irritation, infection or injury can cause a variety of painful inflammatory "itises" in the knee. The most common are tendinitis (inflammation of the tendon connecting the kneecap to the leg bone), synovitis (inflammation of the membrane behind the kneecap), bursitis (inflammation of any of the 14 sacs or bursae around the knee, especially the one in front of the kneecap) and arthritis (inflammation or wearing out of the connective tissues in the joint).

The most common mechanical problems in the knee involve:

*The patella, or kneecap. If the patella is out of alignment, it fails to track properly in its groove, eventually causing chondromalacia, or erosion of the cartilage. The kneecap is lined with a synovial membrane, which secretes fluid that helps lubricate the knee and reduce friction. When the synovial membrane produces too much fluid, it causes "water on the knee," which may have to be drained with a needle periodically.

*Cartilage. The knee contains two types of cartilage, the glossy articular cartilage on the ends of the leg bones and the two fibrous, crescent-shaped shock absorbers between the bones. Cartilage injuries are the most common cause of knee surgery. Loose, frayed or torn cartilage can cause a clicking or popping sound when the knee is flexed. In severe cases, torn cartilage will block the normal motion of the knee or lock it in one position. Damaged cartilage usually can't heal itself, except on the edges near adjoining tissue, where the blood supply is better.

*Ligaments. They may be stretched (strained), partially torn (sprained) or completely torn. Like rubber bands, ligaments can stretch only so far before they snap. The collateral ligaments, one on each side of the knee, are the most commonly injured. If they are torn, the knee loses its side-to-side stability and the lower leg wobbles like a bad elevator. A more serious injury involves the pair of cruciate ligaments that cross in the center of the knee. The cruciate ligaments usually will not heal on their own, and are much more complicated to repair.

In many cases, of course, an injury involves a combination of inflammatory and mechanical problems.

The goal of initial treatment of a knee injury should be to reduce the swelling and the risk of further injury. It is sometimes abbreviated as RICE (rest, ice, compression and elevation). A general rule: If the pain persists, or the knee is loose, weak and wobbly, see a doctor as soon as possible.

As with so many medical problems, the best approach is preventive. Staying in shape, keeping the leg muscles fit, keeping body weight down, wearing shock-absorbent athletic shoes, warming up and stretching properly before exercise -- all reduce the chance of a knee injury.