The knee joint - made up of bone, cartilage, fluid and ligaments - is a common site for painful injuries. One in three Americans older than age 45 have knee pain. Treatment depends on the cause.

Chronic knee pain is pain lasting for at least two or three months. At some point, damage has occurred in the knee joint, causing the pain. People of all ages can suffer from chronic knee pain, especially if they have had an injury in the past or have arthritis.

Cartilage is a robust cushion covering all of our joints. In the knee joint, this cushion is about three-sixteenths of an inch thick. It covers the entire knee joint surface....