So the J-sign is the lateral subluxation of the patella each time the patella goes into terminal extension. This patient was dislocating frequently. Investigation by MRI showed that he had underlying trochlear dysplasia - an abnormality of the upper end of the groove - which was giving rise to these high forces on the patella as his knee moved ...

"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.

Jul 13, 2005 · What does this mean "she has quite large J sign with active extension. The knee is clinically stable. Marked patellar subluxation laterally but joint space is fairly well maintained on standing 45', positive sage, chondramalacia patella with ticompartmental degenerative changes, most severe in patellofemoral joint.

2012 - Video of Dr Lars Blønd (Knee Surgeon) explaining the relationship of the J-sign to patellar instability Peer-reviewed paper - Journal interpretation - 2016 - First time patellar dislocation in children – risk factors.

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.

Classically with severe trochlear dysplasia the patient experiences there may be a visible subluxation near full extension, where the patella suddenly slips laterally. This is called the 'J-sign'. Back to top

Framing Patellar Instability: From Diagnosis to the Treatment of the First Episode Maggioni DM, Giorgino R, Messina C, Albano D, Peretti GM, Mangiavini L. Framing Patellar Instability: From Diagnosis to the Treatment of the First Episode.

There may also be a 'J-sign' where the abnormal excursion of the patella is visible when the patient holds the limb over the edge of the examination couch and flexes and contacts the knee.

J-sign search for term 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) Joint search for term A joint is a junction between two bones. Joint cartilage search for term

The clinician may also be able to elicit a 'J-sign' as the knee goes into extension, when the patient lies on the coach with the leg over the side and actively bends and straightens the knee. Measurements from X-rays may also have indicated a high-riding patella, or patella alta possibly with some trochlear dysplasia.