Patellar subluxation

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?
• Positive J-sign

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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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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