A Functional appliance by definition is one that changes the posture of the mandible, holding it open or open and forward. Pressures created by the stretch of the muscles and soft tissues are transmitted to the dental and skeletal structures ,moving teeth and modifying growth
Removable functional appliances are normally very large in size, have unstable fixation, cause discomfort, lack tactile sensibility, exert pressure on the mucous (encouraging gingivitis), reduce space for the tongue, cause difficulties in deglutition and speech and very often affect aesthetic appearance. The alteration in the mandibular posture creates added difficulties. These adverse effects make the adaptation and acceptance of these appliances more difficult.