The knee is the largest joint in your body. It is a complex joint made up of bones, cartilage, and ligaments. The cartilage of the knee acts as a cushion and sliding surface. When healthy, cartilage prevents the bones in the joint from rubbing against each other. However, when the joint is affected by arthritis, the bones come into contact and cause pain. Injuries, aging, and degenerative diseases such as arthritis can cause cartilage to break down.