The J-sign is when the kneecap 'pops out' (subluxes) to the outer side each time the knee moves into the fully straight position (terminal extension).

Page updated March 2024 by Dr Sheila Strover (Clinical Editor)

The J-sign

The dotted line shows the trajectory of the patella as the knee is straightened. The 'J' is actually an 'inverted-J'.

 

What does a positive J-sign indicate?

The presence of a J-sign indicates that the trochlear groove of the femur, in which it runs, is not managing to constrain it as the knee goes into extension.

This may be because of a shallow groove at the upper end of the trochlea (trochlear dysplasia) or the patella may be positioned a bit high (patella alta), or some other structural problem may be present.

 

  • Quote from peer-reviewed paper:

    "The J-sign on clinical examination is suggestive of patellar maltracking and potential instability." - [paper links to a video]

    Citation: Hadidi O, Ellanti P, Lincoln M, Hogan N. The J-sign in patellar maltracking. BMJ Case Rep. 2018 Feb 14;2018:bcr2017222887. doi: 10.1136/bcr-2017-222887. PMID: 29444791; PMCID: PMC5847943.

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Is a J-sign always accompanied by problems?

Some people exhibit a positive J-sign but do not complain of any symptoms.

 

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-

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

See biography...


See also -


From the Experts -

  • Expert view - The J-Sign - by Dr Lars Blønd (Knee Surgeon)


Journal interpretation - patellar instability

Journal interpretation - 2023 - Framing Patellar Instability: From Diagnosis to the Treatment of the First Episode

- Authors: Maggioni DM et al. and summarised for readers by Dr Sheila Strover (Clinical Editor)