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.

2005年7月13日 · 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.

Orthop J Sports Med. 2021 Apr 2;9(4):2325967121994548. doi: 10.1177/2325967121994548. PMID: 33869646; PMCID: PMC8020235. Back to top Trochlear dysplasia and kneecap instability Trochlear dysplasia is commonly associated with an unstable patella (kneecap), and patellar subluxation or dislocation. Quote from peer-reviewed paper:

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.

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

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.

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. J Pers Med. 2023 Aug 2;13(8):1225. doi: 10.3390/jpm13081225. PMID: 37623475; PMCID: PMC10456090.

If, as the knee nears extension, the patella lurches to the side, the patient is said to exhibit a J sign. This is an indication of considerable bony and/or soft tissue abnormality at the top of the trochlear groove .