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

patellar subluxation

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

patellar dislocation

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.

Synonyms: 
Patellofemoral instability
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Patellar instability and dislocation

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

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