A facemask — also known as a reverse pull headgear or protraction facemask — is an orthodontic appliance worn on the outside of the face that works in combination with an expander inside the mouth to guide the growth of the upper jaw forward. It is used to correct a specific type of bite problem called an underbite caused by an underdeveloped upper jaw — and it is most effective when used during periods of active growth in children and young adolescents. Here is your complete guide.