Three bones meet within the knee joint: the femur, or thighbone, the tibia, or shinbone, and the patella, the kneecap. The joint is stabilized by connective tissues known as ligaments, which connect bones to bones within the body.
Because of trauma or laxity (looseness) of the ligaments in the knee, the kneecap may slide out of the groove it rests in. When it does so, but doesn't leave the joint completely, a patient experiences subluxation. If the patella becomes completely dislodged from the joint, it is known as dislocation.