Knee pain is incredibly common. In the United States, it's responsible for about 1/3 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 swollen or torn ligaments, meniscus (cartilage) tears, and runner's knee. But the knee is a complex joint, and there's plenty more that can go wrong.
Other conditions that cause knee pain include: