The present investigation studied the relation ship of hyoid bone position to skeletal relapse in mandibular prognathism patients who underwent mandibular sagittal split osteotomy. The study material consisted of presurgical and 6-months postsurgical profile cephalograms of 43 patients.
At point Gn, 1.4% horizontal and 7.7% vertical skeletal relapse was found following mandibular osteotomy in mandibular prognathism without anterior open-bite. In mandibular prognathism with anterior open-bite, the horizontal relapse was 4.7% and the vertical relapse 24.5%.
Postoperatively, the hyoid bone moved posterosuperiorly or posteroinferiorly. This change in hyoid bone position indicates that some physiologic mechanisms operate to maintain pharyngeal airway patency. The group of patients with posteroinferior displacement of the hyoid bone after surgery was found to have a large relapse. This relapse was suggested to be related to increased geniohyoid contracting force of mandibular opening components by vector analysis, because the interplay of forces in the suporting structure of the hyoid bone has important effects on mandibular form and function. It therefore appeared that the stability of the surgical outcone could be associated in some way with the change in hyoid position.
Distance H-G continued to show a large reduction effected by posterior repositioning of the mandible by surgery. However, distance Sp-H remained only slightly reduced. This may indicate that the stylohyoid muscle surporting the hyoid bone and the mandible plays an important role in vertical skeletal relapse.
