OPTION 3

Functional appliances work on two broad principles:
Ⅰ. Force transmission 
Ⅱ. Force elimination
Ⅰ. Force transmission
Compressive stress and strain are transmitted directly or indirectly on the structures involved and as a result there is a primary alteration in function with a secondary adaptation in form. Most of the fixed and removable functional appliances work on this principle. Directly acting appliances act directly on the skeletal and dental structures and include lip bumper, oral screen etc. Indirectly acting appliances include almost all the functional appliances like Activator, Bionator, Herbst, etc. These indirectly acting appliances transmit forces by altering the position of the mandible through muscle forces. Some of the appliances like activator act directly as well as indirectly.
Ⅱ. Force elimination
This principle involves the elimination of abnormal muscular or other environmental factors which try to modify the normal function and hence the proper form of the structures. The elimination of these aberrant forces restores the normal function and aids in proper and normal development of the orofacial structures. The typical example is Functional regulator or Frankel appliance.
⸙ Key Concept:         

Principle of functional appliance. (A) Force elimination. (B) Force application.
The most important principle underlying the functional appliance therapy is the adaptation between form and function.
֍ Functional appliances induce change either in form or function.
֍ Neuromuscular adaptation allows the form and function to get adjusted