Patellar subluxation

In patellar subluxation, when the patient straightens the knee the patella suddenly shifts over the the lateral side. When a surgeon sees this during knee examination he will record it in the notes as a 'positive 'J-sign'' - a sudden swing of the patella to the outer aspect with each knee extension, after which the patella returns back into the groove.

What causes patellar subluxation?

Patients experiencing patellar subluxation may have any of several underlying structural problems. These include -

trochlear dysplasia
patella alta
increased tibial external torsion
valgus deformity
medialised trochlear groove
lateralised tibial tubercle
combination of the above

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How to check for a Positive J-sign

Dr Lars Blønd explains how the surgeon can confirm patellar subluxation in the examination room by eliciting the J-sign.

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Relevant links on KNEEguru

A-Z Keywords

Patellar instability, J-sign

eBooks

Factors contributing to an Unstable Patella