Mandibular prognathism: a cineradiographic analysis of mandibular and hyoid bone movements before and after surgical correction.

A study was undertaken to investigate the effect of surgical treatment of mandibular prognathism on mandibular and hyoid movements during speech and swallowing in fourteen individuals. The material consisted of cineradiographic records of adult prognathic patients before and after the surgical correction. The records were obtained with a 16 mm, cinecamera at a speed of thirty-two frames per second. Image intensification was used, and the head of the patient was positioned in a cephalostat during the recording. Postoperative recordings were taken at about 1 month after the removal of intermaxillary fixation. To improve the accuracy of the analysis, templates of the skeletal structures were prepared. The following cephalometric landmarks were located and marked on the template: the tip of the upper central incisor, the occlusal plane, pogonion, and hyoid point. The coordinates of the cephalometric points were recorded by means of a Lysta dental digitizer linked to a Hewlett-Packard 9815A desk-top computer. The significance of the difference between the pre- and postoperative values was determined by the Student's test. The results were examined for variations in the vertical and horizontal ranges of movement and in the surface areas covered by pogonion and hyoid during movement. Correlation between several pairs of variants was tested. Large individual variations were found in the pattern and range of movement of the mandible and hyoid both pre- and postoperatively. The coefficients of correlation between several parameters showed some significant correlation, which disappeared after surgical correction. A general trend for a reduction in most parameters studied was shown. Some correlation between the amount of setback and the mandibular movement was also found. The hyoid and mandibular movements in tongue-reduction cases do not differ significantly. Lines of further investigation are suggested, with larger and possible more homogeneous samples.