The back of the kneecap rests in a groove in the end of the thigh bone called the femoral groove. The kneecap is covered with cartilage, which helps it glide around smoothly in the femoral groove as you move your knee in different directions. A number of conditions and injuries can cause kneecap pain. In many cases, it occurs when the knee is not correctly aligned. This abnormal alignment leads to pressure and friction between the kneecap and the thigh bone. The tendons and ligaments that attach the kneecap to the bones of the knee joint become irritated and inflamed, causing pain. Patellofemoral syndrome is the most common cause of knee pain in people under 45 years of age, and women are more likely to develop the condition than men.