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 patellar dislocation  
Patellar subluxation - when the patella rides to one side up the wall of the trochlear groove, but does not pop out. Patellar dislocation - when the patella jumps right out of the groove.  


Patellar subluxation or dislocation?

patellar instability continuum

Patellar instability is a continuum. Patients may progress from one level of instability to another. 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.

Patellofemoral instability

The unstable patella - congenital or acquired?

Instability in the patella means that it jumps to the edge or out of the normal track in the femur in which it lies.

Patellar instability and dislocation

Discussing how the anatomy can affect the tendency to dislocate the patella.




Medial Patella Subluxation: Diagnosis and Treatment. McCarthy MA and Bollier MJ. Iowa Orthop J. 2015; 35: 26–33.

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.

How to Deal With Chronic Patellar Instability. Vicente Sanchis-Alfonso. Sports Health. 2016 Jan; 8(1): 86–90.

See also:

Factors contributing to an Unstable Patella - ebook