The J-sign is when the kneecap subluxes laterally each time the knee moves into terminal extension.

Page updated January 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 there is some instability of the patella as the knee goes into extension. This may be because of a shallow groove at the upper end of the trochlea or the patella may be positioned a bit high, or some other structural problem may be present.

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Is a J-sign always accompanied by problems?

A positive J-sign must be interpreted in the light of other symptoms, such as subluxation.

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Peer-reviewed papers

  • Quote:

    "....chronic recurrent patellar instability is difficult to treat and may involve a variety of underlying pathology...."

    Citation: Beckert MW, Albright JC, Zavala J, Chang J, Albright JP. Clinical Accuracy of J-Sign Measurement Compared to Magnetic Resonance Imaging. Iowa Orthop J. 2016;36:94-7. PMID: 27528843; PMCID: PMC4910780.

  • Quote:

    [no-lexicon]"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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Dr Sheila Strover (Editor)
BSc (Hons), MB BCh, MBA

See biography...