Patellar Instability can dramatically reveal itself with a sudden episode of dislocation of the patella (kneecap).

First published in 2017, and reviewed August 2023 by Dr Sheila Strover (Clinical Editor)

Although most people are familiar with the distress experienced by someone who has had a dislocated patella, fewer realise that instability can range from nagging pain in the front of the knee, without any obvious instability, through episodic 'giving way' when the patella almost derails but not quite, then there may be dislocation events where the patella jumps right out of the groove.

The continuum continues on to more severe instability where the patella habitually dislocates with every knee bend, and right up to rare cases where the patella is permanently dislocated and never goes back into the groove.


Patellar Instability

Patellar instability can take the following forms ...

  • Anterior Knee Pain - Anterior knee pain is a term that is bandied around a lot in the knee world. It is not a diagnosis, but is simply a descriptive term to describe distressing pain that seems to be localised to the front of the knee.
    There may be a number of causes, but one of these is a patella that does not 'track' comfortably within the groove of the femur for the whole motion of bending and straightening while bearing weight over all the different surfaces that one may traverse. What is important is to make sure not to be dismissed with this as a 'diagnosis' or told that your pain in the front of the knee is due to 'chondromalacia related to anterior knee pain'. Don't worry about that term right now - it just means softening of the joint cartilage - but take away the message that if you have anterior knee pain then find a good specialist and make sure that patellar instability or patellar maltracking is excluded as a cause.
  • Subluxation with 'giving way' - The term 'giving way' is something that is clearly understood by any patient who has experienced this. They may be happily walking along, perhaps chatting to friends, when suddenly the knee buckles, and the person stumbles or falls. The knee is extremely painful, but they can hobble a bit and then continue to walk and it all soon settles down. There are several conditions in the knee that can cause giving way, but one of them is subluxation of the patella.
    subluxaton of the patella seen from above In this condition the patella suddenly lurches to the edge of the groove, and nearly pops right over the rim, but just in time it returns back into its rightful place. The giving way may be due to a reflex inhibition of the quads muscles with the sudden acute pain. As the quads are the muscles that work to straighten the knee, the reflex inhibition suddenly stops the muscle firing, and the person stumbles.

  • A First Dislocation Event - A first dislocation of the patella is a shock to the patient, with the knee grossly deformed and very painful. Even the inexperienced emergency room doctor will be alarmed at the apparent deformity caused by the derailed patella.

    Reduction is usually quickly performed once the routine tests are done, and the patient able to return home on crutches.

    Damage may include a chip of bone from the groove and/or the patella itself, and perhaps tears to the soft tissue structures to the side of the patella, but these may only be fully revealed on a follow-up MRI scan. Some patients who experience a first dislocation may have had previous symptoms of subluxation, but most patients have not.
  • Episodic or Recurrent Dislocation - The first time the patella dislocates it is always a big event, and then some of these patients go on to develop recurrent patellar instability. Sometimes the patella goes out so easily that they in the end hardly feel any pain - it is just uncomfortable. And there is a whole spectrum of this. Some will have recurrent patellar instability once a year - or every second year - and some will have it nearly every month - so it is very different from patient to patient.

    The first few occasions will be very stressful and probably require hospital intervention to reduce the patella back into the groove, but with time the patient often learns to do this on their own. This is a reason why the true incidence of episodic dislocation is not really known for sure.
  • Habitual ('Obligatory') Dislocation - In habitual dislocation, the patella dislocates completely with every knee flexion and spontaneously relocates during extension without pain and swelling. If the patella is firmly retained in the midline the person is unable to bend the knee more than 30–70° unless the patella is released and allowed to dislocate, when a full range of movement again becomes possible.

    ​Generally it is diagnosed in childhood, and may not be obvious if the child has a chubby joint. The parents may be concerned only because the child has a 'funny-looking knee', but sometimes it is brought to attention because the child has problems with running. The underlying problem is typically a 'dysplastic trochlea' - where the groove only begins much lower than normal and the lateral side of the groove is underformed (hypoplastic).

    When diagnosed in adults, generally the patient is more symptomatic, complaining of pain in the front of the knee, swelling and noises in the knee, and problems with running and with stairs.​
  • Permanent Dislocation - This situation is rare and generally causes excitement amongst the medical professionals when they see it because the knee looks so bizarre! These patients have had a dislocation or a number of dislocations, but now their anatomy has accommodated to the patella being permanently outside of the groove and they just live with it.
Dr Lars Blond


"My special interest with regards to patellar instability is trochlear dysplasia. For this condition I think X-rays and ultrasound can be used as screening tools - but they can never outclass the MRI scan. Also you need to look at the uppermost part of the trochlea - this is where the dysplasia is localised. I try to encourage the young orthopaedic surgeons to always to look at the MRI themselves. They need to get used to checking for the presence of trochlear dysplasia.


Dislocated kneecap versus dislocated knee

Please be aware that a 'dislocated kneecap' or patella is a very different condition from a 'dislocated knee'. A dislocated knee is a catastrophic injury, ripping several of the major knee ligaments connecting the two long bones. In a dislocated kneecap, in contrast, the two long bones are still well connected together via their ligaments. It is the kneecap that moved from its alignment, and some of its local ligaments may be torn.