The hyrax is an appliance for expanding the upper jaw. As a rule, it is used in growing children whose upper jaw is not wide enough in relation to the lower. This can be seen in the fact that the lower posterior teeth embrace the upper ones, instead of the other way around, and the space for all the teeth to align is lacking (resulting in crowding).
The appliance consists of a screw placed in the middle of the palate, which rests on both sides with two arms against the upper posterior teeth. These arms are either attached to metal bands on the teeth, or to acrylic (plastic) pads that also cover the occlusal surfaces of the posterior upper teeth.
By activating (turning) the screw, the arms push the teeth and two maxillary segments outwards, thus achieving a skeletal and dental effect. The skeletal effect refers to the expansion of the upper jaw seen in the opening of the space between the two front teeth (central diastema). After the active period, the screw is “blocked”, and a period of stabilisation follows during which new bone is created in the midline area. The dental effect refers to tooth and alveolar ridge movement. At the end of the hyrax phase the upper teeth cover the lower teeth and space is opened to align all teeth.
The hyrax is often combined with the reverse pull face mask, expanding the upper jaw and stimulating it to grow forward.