Hyoid changes following orthopedic treatment of mandibular prognathism.

In this study 30 patients exhibiting mandibular prognathism were treated by the orthopedic chin-cup appliance for a three-year period. The patients began treatment at the age of six years and completed treatment at the age of nine years. As previously reported, chin-cup treatment resulted in a "clockwise" facial rotation with the mandible moving relatively posteriorly and inferiorly. Hyoid position measured in this study also tended to move slightly posteriorly but was primarily displaced in an inferior direction. The positional alteration exceeded in amount what might have been expected by growth alone. The inferior direction of change in hyoid and associated structures and the resultant lack of encroachment on the vital pharyngeal passageways suggests that stability and patency of the pharyngeal airway is a primary factor in hyoid positioning. The duration and "growth guidance" aspects of treatment allowed for functional as well as morphologic adaption to the altered hyoid position.