Knee pain is incredibly common. In the United States, it's responsible for about one third of all doctor's visits for muscle and bone pain. Knee pain is a special problem for athletes -- over half of all athletes endure it every year.
Some of the most common reasons for knee pain are sprained ligaments, meniscus (cartilage) tears, tendonitis, and runner's knee. But the knee is a complex joint, and there's plenty more that can go wrong.
Bursitis. A bursa is a sac of fluid that resides below the skin overlying your joints. Overuse, a fall, or repeated bending and kneeling can irritate the bursa overlying the knee cap (patella), causing pain and swelling. This is known as prepatellar bursitis, but is commonly referred to as ''housemaid's knee'' or ''preacher's knee'' since they are often caused by repetitive kneeling.
Iliotibial band syndrome. The iliotibial band is a piece of tough tissue that runs from your hip down to the outer part of your knee. If it's irritated by overuse or other problems, it can become inflamed and cause pain on the outer side of the knee.
Osgood-Schlatter Disease. This condition causes a painful bump below the knee, where a tendon from the kneecap connects to the shin. It's usually caused by overuse and irritation at the tendon insertion known as the tibial tubercle. The pain may come and go over time. It's especially common in teenage boys and girls (ages 13 and 14).
Dislocated kneecap (or patellar dislocation). In this condition, the kneecap slides out of position, causing knee pain and swelling.
Patellofemoral pain syndrome. This condition often results from issues associated with muscle imbalance, tightness, and alignment of the legs, rather than an injury. It's particularly common in teenage girls.
Patellar tendonitis. This condition results from inflammation of the tendons surrounding your knee. Tendons are tough bands of tissue that connect muscles to your bones. Overuse can make the tendons inflamed and sore. The most common type of patellar tendonitis is called ''jumper's knee.''
Loose bodies. Sometimes, a knee injury can break loose cartilage. These pieces can get stuck in the joint, causing it to freeze up and producing pain and swelling.
If an old knee injury was not properly treated, it may keep causing occasional -- or constant -- knee pain.