The patella (kneecap) attaches to the femur (thigh bone) and tibia (shin bone) by tendons. The patella fits into a groove at the end of the femur (trochlear groove) and slides up and down as the knee bends and straightens. Patellofemoral instability occurs when the kneecap moves outside of this groove.

There are two types of patellofemoral instability: 1) Traumatic patellofemoral dislocation typically results from an injury to the knee where the kneecap gets completely pushed out of the groove, and 2) Chronic patellofemoral instability results in the kneecap only slides partly out of the groove. This is also known as a subluxation.

A dislocating or subluxing kneecap can be an extremely debilitating condition, particularly in the young active population. It can see children, adolescents and adults limit their activity because of fear of recurrent dislocations. Some people may even find that walking is difficult and their kneecaps pop out very easily.


  • Pain, swelling, stiffness
  • Difficulty walking on the affected limb
  • A buckling, catching, or locking sensation in the knee
  • A noticeable deformity in the affected knee
  • Most patients also experience a sensation that the kneecap has shifted or moved out of place. Usually, the kneecap will move back in on its own but sometimes it will need to be put back in place by a physician.
  • With chronic patellofemoral subluxations, the pain may be less severe than in a traumatic injury.


It’s important to undergo a thorough assessment of all the factors that may predispose your kneecap to dislocate. X-rays and MRI scans are required to understand your anatomy and to generate a surgical procedure that is tailored to you.


If your kneecap doesn’t go back into place on its own, Dr. Maine and QLRC will most likely recommend rest and often a knee brace and crutches. After a period of rest, she will typically recommend physical therapy to strengthen the muscles in the knee that help keep the kneecap from sliding out of the groove.

There may be some circumstances where Dr. Maine may recommend surgery – for instance, in instances of acute dislocations here cartilage may have been knocked loose. Another instance in which surgery may be needed is if there are ruptured ligaments in the knee. In this case, reconstruction of the ligament may be recommended.

In children, certain procedures such as guided growth may be necessary to improve alignment of the legs to reduce the risk of recurrent dislocations in the future. It is critical not to miss the opportunity to perform this as once the growth plates have closed, corrective surgery is much more complex and riskier.