Functional appliances are devices used to correct significant disharmony in the relationship between the upper and lower teeth. They work by influencing growth and development of a growing patient.
The effectiveness of a functional appliance depends on:
How much time the appliance is worn;
How quickly the patient is growing.
The Twin Block appliance, as the name implies, uses a removable upper and lower plate to bring the lower jaw and teeth forward and reduce the relative protrusion of the upper anterior teeth. It is effective in reducing the bite depth and bringing the teeth into a better back to front position though it has minimal long term effects on the length of the jaws.
Eating with the Twin Block in place increases the functional forces and speeds up and improves the response to treatment.
The Herbst appliance is a fixed equivalent to the Twin Block and works in a similar manner. Rods fitted from the upper molars to the lower teeth hold the lower jaw forward during function.
The Forsus appliance is a functional appliance used in a patient already fitted with braces. It is composed of a spring coil and rod mounted on the side of the appliance and moves the lower arch forward upon closure of the jaws while moving the upper molars back. It runs from the upper first molar band down to the lower archwire.