When the patella jumps completely out of the walls of the groove in the femur bone, and the joint looks horribly deformed.

-

An overview by Dr Lars Blond.

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.

-