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The Lowdown on Patellar Instability (Knee Cap Dislocations)

Patellar or kneecap dislocations are part of the overall condition called patellofemoral instability. It is more common in children but can become recurrent and extend into adulthood.

It is a complex problem and causes knee pain and significant disability and restriction of your activities, sports and day to day life! Here a full run down of everything you need to know about what it is and how we can diagnose it and ultimately prevent it happening again and give you your life back.

Yours or your child's kneecap (patella) is usually right where it should be—resting in a groove at the end of the thighbone (femur - trochlear groove). When the knee bends and straightens, the patella moves straight up and down within this groove. Sometimes, the patella slides too far to one side or the other. When this occurs — such as after a hard blow or fall — the patella can completely or partially dislocate. Sometimes this can occur with a simple twist and is due to the laxity of the ligaments or under-development of the groove (trochlea). This is something you are born with.

When the patella slips out of place — whether a partial or complete dislocation — it typically causes pain and loss of function and can lock your knee. Even if the patella slips back into place by itself, it will still require treatment to relieve the knee pain and swelling that will follow. Be sure to take your child to the doctor for a full examination to identify any damage to the knee joint and surrounding soft tissues as the act of the knee cap dislocating and going back into joint can knock of the highly specialised articular cartilage that covers the surface of the joint. This is an emergency and may require urgent surgery. Contact us if this happens or attend ED.


There are a several ways in which the kneecap can become unstable or dislocate. In many cases, the patella dislocates with very little force because of an abnormality in the structure of yours or your child's knee.

A shallow or uneven groove in the femur can make dislocation more likely.

Some children's ligaments are looser, making their joints extremely flexible and more prone to patellar dislocation. This occurs more often in girls, and the problem may affect both knees.

Children with cerebral palsy and Down syndrome may have kneecaps that dislocate frequently due to imbalance and muscle weakness.

Rarely, children are born with unstable kneecaps causing dislocations at a very early age, often without pain.

In children or patients with normal knee structure, patellar dislocations are often the result of a direct blow or a fall onto the knee. This incidence is more common in high-impact sports, such as football or rugby.


We would want to examine your or your child’s knee and check to see how the leg is aligned and whether there is any rotational abnormalities that can increase the chances of patellar instability and recurrent dislocations. We also want to look for the laxity of the medial patellofemoral ligament (MPFL). tis is lax with hypermobility and in those where they have had damage to the ligament holding the knee cap in position. Patella Alta or the knee cap riding high can also increase the risk of dislocation.

MRI scans would be ordered to look at the smooth articular cartilage to check for damage and also to look at how well developed the groove the knee cap runs in is developed. In the acute injury this MRI is vital at excluding a significant cartilage injury that might need urgent surgery.

Surgical Treatment

If yours or your child's patella dislocates multiple times, or continues to be unstable despite physiotherapy and bracing, surgery may be recommended to correct the problem. The type of surgery will depend on the cause of the unstable kneecap.

Surgical treatments often involve reconstructing the ligaments that hold the patella in place. This surgery is sometimes performed arthroscopically—using a tiny camera and miniature surgical instruments inserted through small incisions. Other surgeries include realignment of the angle of the leg or rotation of the leg with an osteotomy, a tibial tubercle or tuberosity ostetomy to move the attachment of the patella on to the shin bone (tibia) to allow it to glide in a straight line in the groove (trochlea) and finally if the groove is under-developed we may need to recreate the groove with a trochleoplasty.

At Wessex Knee Mr Frame uses his 10 year experience in 3D printing to create models from CT scan of your knee to help plan the complex knee reconstruction surgery required to get you back on your feet.

If you have any questions or want to make an appointment contact us here.

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