Hyoid position, pharyngeal airway and head posture in relation to relapse after the mandibular setback in skeletal Class III

This study evaluates the process of relapse after mandibular setback surgery by an analysis of the role of craniofacial morphology, hyoid position, pharyngeal airway and head posture. Subjects examined were 62 patients who received the sagittal split ramus osteotomies (SSRO). Changes of the craniofacial and related structures were evaluated from the serial cephalograms up to 3 years after the surgery. Results indicated that mandibular relapse represented by Pg occurred mostly within 6 months after the surgery. A net setback of the mandible was 9.1 mm and the superior move was 1.7 mm, with a reduction of 7.2 mm in mandibular length, 4.2 mm in ramus height, 3.7 mm in posterior face height, 2.6° in gonial angle, an increase of 2.9° in mandibular plane angle (MPA) by the last examination. Hyoid bone moved backward and downward and head posture was raised. The forward relapse of Pg was correlated with the changes of ANB, MPA, ramus height and hyoid position. Only hyoid position was predictably correlated with mandibular morphology and head posture. These findings suggest that mandibular setback alters the relationship among the hyoid position, pharyngeal airway and the head posture. It might be critical, therefore, relapse is closely monitored and controlled before the full healing of fragments and new muscular balance is established.