Patellar subluxation is when the patella rides suddenly up the side of its groove, but does not fully dislocate.

Page updated September 2023 by Dr Sheila Strover (Clinical Editor)

subluxaton of the patella seen from above

As the patient straightens the knee, the patella suddenly shifts over the the lateral side. The event during knee examination is recorded as a positive 'J-sign' - a sudden swing of the patella to the outer aspect with each knee extension, after which it returns to the groove.


What causes patellar subluxation?

Patients experiencing patellar subluxation may have any of several underlying structural problems. These include -

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Positive J-sign

Dr Lars Blønd explains how the surgeon can confirm patellar subluxation in the examination room by eliciting the J-sign.

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kneecap subluxation

Dr Sheila Strover (Editor)
BSc (Hons), MB BCh, MBA

See biography...


Relevant material -

Surgeon commentary -

Dr Lars Blønd2017 - Patellar Instability – a Continuum - by Dr Lars Blønd (Knee Surgeon)


eBook on patellar instability

eBook - Factors contributing to an Unstable Patella

Written by Dr Lars Blønd (Knee Surgeon). Free to download.