The kneecap or patella is part of the extensor mechanism, which is in charge of bringing your knee up straight when bent. The kneecap runs inside of a groove located in the femur or thigh bone. As long as your kneecap stays in its groove in the knee, you can walk, run, sit, stand, and move easily. When the kneecap slips out of the groove, problems and pain often result.
When the quadriceps (part of the extensor mechanism) is weak or when the groove is uneven or too shallow, the kneecap can slide off, resulting in a partial or complete dislocation. A direct blow to the kneecap when falling or practising contact sports, can also pop the kneecap out of place. This occurs more commonly in young women due to their anatomy and soft tissue laxity.