(20) The Change of the Hyoid Bone Position After Orthognathic Surgery Using 3-Dimensional Computed Tomography in Mandibular Prognathism

Abstract: Statements of the problem
Many studies have been reported on the hyoid bone, which are thought to affect the stability of the dentition and the mandible after orthognathic surgery. Most of these studies are mainly conducted using cephalometric radiographs, but the overlap and difference in magnification makes it difficult to evaluate accurately and there was a possibility of error due to flat analysis.
The 3-dimensional computed tomography have recently been widely used in orthodontic patients for diagnosis, treatment plan, surgical indicators, evaluation of surgical results, and follow-up studies. Changes in the upper airway and hyoid bone have also been reported several times.
The study was conducted on patients who underwent VRO surgery in department of Oral and Maxillofacial Surgery, YONSEI University Dental Hospital, hyoid bone positional changes of the pre‐surgery, post‐surgery and post retention period were assessed by cone‐beam computed tomography images in patients with mandibular prognathism.

Materials and methods
Subjects and CT data analysis
This study was conducted with the tomographic records of 11 patients with skeletal class III underwent orthognathic surgery at Department of Oral and Maxillofacial Surgery, Gangnam Severance Hospital from 2020 to 2021. Inclusion criteria is Skeletal class III malocclusion. Exclusion criteria is Lack of follow-up CT and patients with history of previous orthognathic surgery. CT scans were acquired Pre OP (T0); and Post OP (T1) and 6 months (T2) postoperatively. Mimics (version 21.0; Materialise Dental, Leuven, Belgium) was used to conduct 3D image reconstruction.
Statistical analysis
A Pearson correlation test was done to identify the correlation between the amount of setback and the changes of the hyoid bone with the SPSS version 12.0 software (SPSS Inc., Chicago, IL, USA).

Results of investigation & Outcomes data
The mean age of the patients was 21.3 years and the mean amount of mandibular setback was 5.94±2.43 mm measured from the B-point.
The amount of movement of hyoid bone as follows. (Table 1)
Correlations between the amount of mandibular setback and the changes of the hyoid bone as follows. (Table 2)

Conclusions
In this study, we were able to examine the changes in the hyoid bone with CBCT images after mandibular setback surgery. From this study, we observed the following:
1. The hyoid bone moved backward and downward and returned to the original position at the 6 months.
2. There were some significant, positive correlations between the amount of mandibular setback and the amount of posterior and inferior movement of the hyoid bone.

List of two references
1. Yamashita, A. L., Iwaki Filho, L., Leite, P. C. C., de Lima Navarro, R., Ramos, A. L., Previdelli, I. T. S., ... & Iwaki, L. C. V. (2017). Three-dimensional analysis of the pharyngeal airway space and hyoid bone position after orthognathic surgery. Journal of Cranio-Maxillofacial Surgery, 45(9), 1408-1414.
2. Efendiyeva, R., Aydemir, H., Karasu, H., & Toygar-Memikoğlu, U. (2014). Pharyngeal airway space, hyoid bone position, and head posture after bimaxillary orthognathic surgery in Class III patients: long-term evaluation. The Angle Orthodontist, 84(5), 773-781.

Displacement of hyoid bone in X-, Y-, Z- axes before and after surgery, 6 months

Correlations between the amount of mandibular setback and the changes of the hyoid bone