A dislocated kneecap is an uncommon injury usually caused by a sudden change in direction when the leg is planted on the ground.
It often happens during sports or dancing, but can also occur when getting out of a chair. It's mostly seen in teenagers.
Normally, the kneecap (patella) sits over the front of the knee joint. It glides over a groove in the joint when you bend or straighten your knee.
When the kneecap dislocates, it comes out of this groove and the supporting tissues can overstretch or tear.
A dislocated kneecap should be treated promptly and investigated properly. It takes about six weeks to fully heal.
What are the signs?
When a kneecap dislocates, it shouldn't be hard to spot as the knee usually looks deformed. Other signs and symptoms are:
you may hear a "crack"
the kneecap may feel out of joint
the surrounding tissue swells up very quickly
it's extremely painful
you'll be unable to walk
Often the kneecap will spontaneously correct itself soon afterwards, which can mean the diagnosis is not made when it's later seen by a healthcare professional.
What should I do?
If you think you've dislocated your kneecap for the first time and it hasn't corrected itself, call an ambulance. Sit with your leg outstretched and keep it still.
Either straighten your knee or ask someone to gently lift up your foot. The kneecap will usually correct itself and the pain should rapidly fade.
If it's not the first time you've dislocated your kneecap and you've managed to gently manipulate the kneecap back into place, with no major injury, there may be no need to go to hospital.
You can manage any swelling by holding an ice pack to your knee for 10 to 15 minutes every hour for the first day of your injury, and every few hours for the few days that follow.
How is it diagnosed and treated?
If it's the first time you've dislocated your kneecap, the leg will be immediately splinted by paramedics and you'll be taken to your nearest hospital A&E department.
If the kneecap hasn't corrected itself by the time you get to hospital, a doctor will need to manipulate the kneecap back into place in a procedure called a reduction.
You may be given medication to sedate you to ensure you are relaxed and free from pain.
Once the kneecap is back in place, you may have further tests to check the bones are in the correct position and there is no other injury.
You'll be sent home with painkillers and your leg will be immobilised in a detachable splint (plaster casts are rarely used nowadays).
You'll need about six weeks of physiotherapy to recover.
Surgery would only be necessary if there was a fracture with a piece of bone floating in the knee.
Recovering at home
The knee may be very painful during the first few days and you'll probably need to take painkillers, such as paracetamol or ibuprofen. Always follow the dosage instructions on the packet.
If this doesn't control the pain, see your GP – there may be a fracture, so you'll need an X-ray if you haven't had one already. Your GP can prescribe a stronger painkiller, such as codeine, if necessary.
The splint should be kept on for comfort only and should be removed when you have control of the leg and for supervised exercises with a physiotherapist. This allows you to strengthen the muscles that stabilise the kneecap.
A physiotherapist should give you some exercises, such as straight leg raises, to do at home to help strengthen the leg muscles and improve the movement of your knee. You probably won't need supervised treatment if you're managing recurrent dislocations yourself.
It takes about six weeks to fully recover from a dislocated kneecap.
Dislocating the kneecap again
More than half of those who first dislocated their kneecap when they were young will dislocate it again.
The chance of another dislocation depends partly on how well the muscles surrounding the joint heal.
The risk is higher if you're obese, because of the extra strength needed by the muscles to control the movements of the kneecap, and if you have hypermobile joints.
There is a better chance of preventing the kneecap dislocating again if you regularly do the exercises your physiotherapist recommends.
Surgery may occasionally be needed if the kneecap keeps dislocating.