Patellar instability is when the kneecap does not ride smoothly in the groove of the femur, popping over to one side, or de-railing.


Patellar subluxation or dislocation?

When it pops to one side, that is known as patellar subluxation. When it completely derails, that is known as patellar dislocation. These disorders sit on an instability continuum - from pain in the front of the knee to permanent dislocation of the kneecap.

patellar instability continuum

Patellofemoral instability

Factors contributing to an unstable patella

An ebook by Dr Lars Blond about factors contributing to an unstable patella

How TT-TG Distance is measured, and what it means to the patient with patellar instability.

Interpretation by the Editor of a 2017 published medical paper studying long term problems in patients whose original patellar dislocations were not surgically managed.

Video demonstrating the J sign

This is the Editor's interpretation of a paper published in the orthopaedic literature in 2016.

An overview by Dr Lars Blond.




Isolated Medial Patellofemoral Ligament Reconstruction with Semitendinosus Tendon Allograft. Ridley TJ, Macalena JA and Arendt EA. JBJS Essent Surg Tech. 2018 Mar 28; 8(1): e5.

See also:

Factors contributing to an Unstable Patella - ebook