Functional appliances are worn to help in the development of the jaw. They may also modify the development of the lips, cheeks & tongue. There are many types of functional appliances. Most of them are removable, although some of them are fixed onto the teeth, such as the Herbst appliance.
Removable appliances are more hygienic and less likely to break or cause tooth decay. They connect the upper and lower dentition and jaws together by means of plastic and wires. They may be made as two separate pieces for the top & bottom jaws, or they may be fused together as a single piece that fits both the top & bottoms jaws in a certain position. The plastic or wires guide the tongue, lips, cheeks & jaws by stretching, shielding, or re-positioning, producing a more conducive environment for the tissues to grow & develop.