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.
Peer-reviewed papersQuote 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.
Is a J-sign always accompanied by problems?
Some people exhibit a positive J-sign but do not complain of any symptoms.
Peer-reviewed papersQuote from peer-reviewed paper:
"....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.