Osgood-Schlatter disease. Usually affecting preteen and young teenage boys, this condition is caused by repetitive stress on the upper area of the tibia, where the bone is growing. In children with this condition, the patellar tendon (which connects the knee cap and tibia) becomes inflamed and may even tear away from the tibia.

Dislocated kneecap. This occurs when an injury causes the patella, or kneecap, to move out of position. The movement of the kneecap is always visible and, often, intensely painful.

Iliotibial band syndrome. This syndrome occurs when a band of tissue rubs against the outer portion of your femur (thigh bone), causing sharp, burning pain on the outer side of the knee. Although  this can result from a direct injury to the knee, often the cause is the stress of long-term use, such as long-distance running.

Plica syndrome. This condition occurs when bands of synovial tissue, called plica, are irritated by overuse or injury. Symptoms may include knee pain, swelling, locking and weakness.