Investigation of changes in hyoid bone position with chin cap and face mask therapy in individuals with skeletal class III anomaly: A cephalometric analysis

The aim of this study was to examine retrospectively the changes in the hyoid bone position following the use of chin cup and face mask appliances in the treatment of skeletal Class III malocclusions and to compare the outcomes of these two different treatments. To understand the differences in hyoid bone position changes resulting from chin cup and face mask therapy compared to natural growth, changes in untreated skeletal Class I and Class III individuals were also investigated. Accordingly, the study was designed to comprise four distinct groups. A total of 66 individuals were included in the study, with 18 individuals in each treatment group and 15 individuals in each control group. The average age of individuals in the chin cup and face mask groups was 10.25±2.08 years and 10.99±1.98 years, respectively. The average age of individuals in the Class I and Class III control groups was 10.71±1.17 years and 10.46±1.27 years, respectively. For the chin cup group, a total force of 700 g was applied along a direction passing through the condyle and chin tip. In the face mask group, a protraction force of 400 g per side was applied at an angle of 30° to the occlusal plane. The study material consisted of 132 cephalometric radiographs and 132 hand-wrist radiographs collected before (T0) and after (T1) treatment in the treatment groups, and before (K0) and after (K1) observation periods in the control groups. The mean treatment durations for the chin cup and face mask groups were 15.89±4.39 months and 14.39±4.65 months, respectively. The mean observation periods for the Class I and Class III control groups were 15.8±3.76 months and 13.93±3.17 months, respectively. The data obtained from cephalometric analyses were analyzed using the Wilcoxon signed-rank test to evaluate within-group changes (T1-T0). The Kruskal-Wallis analysis method and Mann-Whitney U analysis method were applied for inter-group comparisons. The relationship between the hyoid bone and other parameters was determined using Spearman's rank correlation coefficient. In the cephalometric evaluation, it was found that the hyoid bone was positioned more anteriorly in the face mask and chin cup groups compared to the control groups. When the changes in distance measurements were examined, it was concluded that the sagittal relationship between the hyoid and cervical vertebrae tended to remain stable with growth and treatment. It was observed that individuals with skeletal Class I and Class III growth patterns had different directions and amounts of mandibular growth, and this differential growth was accompanied by hyoid movement. In the chin cup group, it was concluded that the more the anteroposterior growth of the mandible was restricted, the more the hyoid bone moved inferiorly. Although the inferior movement of the hyoid bone resulting from treatment in the chin cup and face mask groups was found to be significant within the group, it was not found to be different from the changes resulting from growth and development.