The maxillary (upper jaw) bone features a suture down the middle of the palate. Beginning in the early teens this suture begins to fuse (earlier in girls, later in boys) and at some point by early adulthood it is no longer possible to expand the bone simply with an orthodontic appliance. In adults, surgery is required to aid the expansion. During the window of opportunity between the time the 1st molars erupt around age 6 and until college age the bone may be expanded laterally by either a removable appliance like a retainer with a screw in the middle or by an expander cemented to the teeth. This is termed orthopedic expansion as we are actually modifying the bone. There are three main justifications for maxillary expansion: 1) The maxilla is so narrow that it does not fit the width of the lower arch leading to a posterior crossbite. This may be masked if the lower posterior teeth are tipped inwards towards the tongue to compensate for the narrowness of the upper jaw. 2) Crowding is due to the narrowness of the jaw. Expansion creates a space between the central incisors which is then used to align the teeth. A related issue is protruding incisors. Much like a balloon squeezed in on the sides a narrow arch will cause the maxillary incisors to protrude. 3) To alleviate crowding in a normal arch form. Care must be taken in this area because overexpansion may be unstable; expansion in the maxilla may not be matched in the mandible because the mandible cannot be expanded in the same manner; overexpansion may not be esthetically pleasing in a smaller facial structure leading to a stuffed chipmunk appearance.
The main two variants of fixed expanders are either spring activated or screw activated. The spring devices are a W-arch or a Quad helix. The orthodontist expands the spring and then cements it on to the teeth to provide slow steady pressure outwards and it is then activated approximately once a month. The screw activated types are collectively referred to as Rapid Palatal Expanders or RPEs and are cemented to the teeth. Haas expanders cover part of the palatal tissue; bonded expanders cover all the posterior tooth surfaces and are usually used for children in a mixed dentition stage especially as an anchor point for reverse pull headgear and a conventional RPE is cemented to 2 or 4 teeth. As the name implies, RPEs work much more rapidly to expand the suture at a rate of a fraction of a mm a day over 1 to 3 weeks. It is then left in the mouth for approximately 2 months while the expanded bone recalcifies. There are proponents of a Schwarz expansion appliance for the mandibular arch but unlike the maxillary arch the bone in the mandible does not have a suture that can be expanded. An expansion appliance in the mandibular arch will simply tip the teeth outwards in an unstable manner.