Patellar subluxation is when the patella rides suddenly up the side of its groove, but does not fully dislocate.
Page updated September 2023 by Dr Sheila Strover (Clinical Editor)
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.
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
Positive J-sign
Dr Lars Blønd explains how the surgeon can confirm patellar subluxation in the examination room by eliciting the J-sign.
Synonyms:
kneecap subluxation
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Relevant material -
2017 - 
eBook - Factors contributing to an Unstable Patella
Written by Dr Lars Blønd (Knee Surgeon). Free to download.