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