Patellar dislocation is a painful and distressing event, when the patella jumps clean out of the restraining walls of the groove in which it normally rides.

dislocation of the patella with torn MPFL


Illustration showing how a torn medial patello-femoral ligament (MPFL) can allow the kneecap to dislocate to the lateral (outer) side.


What happens during a patellar dislocation?

In the first dislocation event the patella usually dislocates to the lateral side. This is due to the anatomy of the region. At the same time other structures may be injured, such as the medial patellofemoral ligament which may be torn and there may also be a small fracture on the medial side of the patellar and some associated bone bruising.


Why can't I bend my knee after dislocating it?

During a patellar dislocation, the kneecap jumps out of the underlying groove in which it usually glides. If the restraining MPFL is torn, it may not return to the groove after the event, but may remain caught on the other side with the knee bent, and attempts to straighten the knee will be very painful.

An anaesthetic with muscle relaxation may be needed to allow the clinician to return the kneecap to the groove, which will allow the leg to be straight again.


Can a dislocated knee be permanent?

With proper management at the time, it should always be possible to return the kneecap to the groove. A delay in treatment with poor underlying anatomy may lead to a permanent disfigurement.


Management of patellar dislocation

A first event may be managed conservatively, with a period of immobilisation, after which sports may be limited for some months. If there is a subsequent event, the patient is usually subjected to a full investigation of any underlying causes and surgery may be planned. This may include a reconstruction of the MPFL. In patients where there is a marked problem with the anatomy, such as trochlear dysplasia, corrective surgery may be undertaken.


kneecap dislocation

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