Functional appliances are devices used to correct a significant disharmony in the relationship between the upper and lower jaws. They work by influencing the growth and development of a growing patient.
The most common use of a functional appliance is to encourage the forward growth of a retrusive or "under-developed" lower jaw. The functional appliance holds the lower jaw forward over a period until the teeth, jaws and joints have "adapted" and the desired jaw position has been obtained.