The kneecap (patella) moves through a groove in the middle of the femur when your knee bends.

When it slips out of its groove, this is called patella instability or subluxation. When the kneecap comes all the way out of its groove, it is called a dislocation.

A dislocated patella is often called a dislocated knee, although a true dislocated knee joint involves the femur bone and tibia bone coming apart. Patella subluxation and dislocation usually affects adolescents and sportspeople with a contact injury.

You may have heard a pop at the time of injury and/or seen the kneecap sitting over to the outside of the knee. It may have popped back on its own or needed help from a medical professional. The inside of the knee is usually painful and the whole knee can swell. The inside of the knee is likely to stay tender to touch.

The knee will usually feel weak and unstable, and may collapse if you try to walk.

If your kneecap partially or fully comes out many times, the subluxation may not be painful although the progressive cartilage damage can lead to pain. You usually either dislike the sensation or just can’t trust the knee for sports or daily activities.

If you dislocate your kneecap during sport, it is usually diagnosed by the team’s medical staff or physiotherapists. You will usually visit the emergency department for scans and to relocate the kneecap if necessary. They will take x-rays to check for fractures or chunks of cartilage that have bone attached. You will then be put in a non-bending knee brace and given crutches and painkillers.

Seeing an orthopaedic knee specialist or a sports physiotherapist as soon as possible is vital They can ensure there are no other injuries and start the recovery process.

You also need an MRI to check for loose cartilage chunks as they don’t show up in x-rays.