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 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.

Patellofemoral instability

Patellar instability and dislocation

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




Chondral Injury in Patellofemoral Instability. Lording T, Lustig S, Servien E and Neyret P. Cartilage. 2014 Jul; 5(3): 136–144.

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