Overview
Patellar tracking disorder occurs when the kneecap (patella) shifts out of place as the leg bends or straightens. In most cases, the kneecap shifts too far toward the outside of the leg, although in a few people it shifts toward the inside. Your knee joint is a complex hinge that joins the lower leg bones (tibia and fibula) with the thighbone (femur). The kneecap is held in place in the front of the knee joint by tendons on the top and bottom and by ligaments on the sides. A layer of cartilage lines the underside of the kneecap, helping it glide along the groove at the end of your thighbone. The kneecap can shift or rotate off track if the groove is too shallow or if the cartilage is damaged. Ligaments, tendons, or muscles that are too loose or too tight may also lead to a misaligned kneecap.
Causes
Shape of the patella
Too tight/too loose muscles and tendons in the leg, foot, or hip areas
Damage to cartilage
Overuse
Severe blow the inside of the knee
Genetics
Symptoms
Discomfort or pain especially when going down stairs, sitting for prolonged periods of time, standing up from sitting or squatting
Popping, grinding, slipping, or catching sensation of the kneecap when bending/straightening the leg
Sensation that the knee is giving way
Diagnosis
Physical examination and medical history consultation
X-rays
CT scan
MRI
Treatment
Rest from aggravating activities
Stretching and strengthening exercises
Physical therapy
Surgery
Source: http://www.emedicinehealth.com/patellar_tracking_disorder-health/article_em.htm