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Patellar Instability – a Continuum

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

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

Drop down the arrows in the following sections to read more about each of the forms that patellar instability can take...

Anterior Knee Pain

Subluxation with 'giving way'

A First Dislocation Event

Episodic or Recurrent Dislocation

Habitual ('Obligatory') Dislocation

Permanent Dislocation

Lars BløndKnee & shoulder surgeon

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 preseence of trochlear dysplasia.


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.

20120115 Lukas Patellar J Sign (on the YouTube channel of Jondy Cohen)

An explanation of the mechanism of patellar tracking in the trochlear groove of the femur, and an overview abnormality of 'trochlear dysplasia' as a cause of patellar instability.

Habitual dislocation of the patella - on the YouTube channel of Arindam Bandyopadhaya.