A subluxation is a partial dislocation. The kneecap, or patella, can sublux out of its normal position more easily when the thigh muscles are weak, when the patella is not firmly held by the surrounding tendons and ligaments, or when there is a problem with the alignment or structure of the knee bones.
A patellar subluxation feels like the kneecap is shifting or jamming out of place. Usually, a subluxated patella returns to its normal position by itself. But repeated incidents can damage the cartilage on the back of the patella or stretch the connective ligaments.