Patellar instability braces are designed to push the patella towards the midline (medially), aligning the extensor mechanism optimally in the groove of the femur, but may also affect the joint by improving warmth, sensation and circulation.
They are generally made of a sleeve of soft material, such as Neoprene, and have a cutout for the patella. Most include some kind of buttress, usually on the lateral side, or a raised ring ('do-nut') protecting the whole patella.
They may be 'static' (the majority) where there is no actual force applied to the patella, or 'dynamic' (e.g. Palumbo), where elasticised arms provide both force and counterforce to the patella. Patellar tendon supports (e.g. Cho-Pat strap) may also be thought of as patellar braces.