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Effects of functional therapy on hyoid bone, pharyngeal airway and tongue position in class II patients

Aim: The aim of this study is to evaluate the change in the position of hyoid bone, pharyngeal airway, and tongue position in subjects with skeletal Class II malocclusion treated with Twin block therapy. Materials and Methods: This study was performed on pre and post treatment lateral cephalograms of 21 patients with class II malocclusion treated with Twin block appliance, (mean age of (12.24± 1.22). Paired-t test was used to compare the amount of differences of mean value before and after treatment. A value of P<0.05 was considered as significant. Result: Hard tissue result showed significant difference between pre-and post-treatment values of ANB, Hy-B, Hy-Me & HyCL. But no significant difference was seen between the pre-and post-treatment values of Hy-SN' & Hy-FHP. Soft tissue examination also found significant difference between pre-and post-treatment values of SPAS, but no significant difference was observed between the pre-and post-treatment values of MAS, IAS, PT-CL & PT-SNˈ. Conclusion: ANB angle was found to be significantly improved after functional treatment in skeletal class II group. The mandible has showed forward movement with respect to hyoid bone and the hyoid bone has been found to be shifted anteriorly and downward. Superior pharyngeal space was found to be significantly improved after functional therapy whereas there was no significant change with respect to tongue.


Introduction
The hyoid bone is located in the anterior portion of the neck and has no bony articulation. 1 It is connected to the pharynx, mandible, and cranium through muscles and ligaments and act as an anchoring structure for the tongue. Through their muscle attachment, the hyoid bone plays an important role in maintaining an airway, regulating deglutition and mandibular movement. 2 The effect of the change in anteroposterior position of the mandible on hyoid bone position and the pharyngeal airway space is well documented. [3][4][5] Studies have shown that changes in hyoid bone position tend to be related to changes in mandibular position. 6 It has also been suggested that mandibular advancement with protrusion splint or orthognatic surgery is associated with a proportionate increase in oropharyngeal dimensions. 4 The aim of functional therapy in young adults is to achieve maximum skeletal correction. As the hyoid bone is attached to the mandible through the muscle, Orthodontic myofunctional therapy can effect the position of hyoid bone by altering the mandibular position. These changes can be assessed with the use of radiographs such as lateral cephalograms, which are routinely used in orthodontic diagnosis and treatment planning Skeletal Class II discrepancy with mandibular deficiency is considered as a risk factor for upper airway disorders and oropharyngeal airway deficiencies. [7][8][9][10] In order to prevent the potential respiratory problems early orthodontic treatment of mandibular deficiencies in skeletal class II patients would be beneficial. 11 Numerous types of functional appliances for correction of Class II relationship results in sagittal mandibular growth during adolescence. These appliances are also used to prevent apneas or airway problems by forcing the mandible, hyoid bone, tongue and soft palate forward, thus causing an increase in oropharyngeal dimensions. 12,13 This study was done to evaluate the changes in hyoid bone position, pharyngeal airway and tongue position after treatment with twin block functional appliance in patients with class II div I malocclusion.

Materials and Methods
Pre-and post-treatment lateral cephalograms of 21 class II div 1 subjects treated with twin block were obtained from the orthodontic record files from the Department of Orthodontics and Dentofacial Orthopaedics and from the outpatient Department of Institute of Dental Sciences, Bareilly.
All cephalograms used in the study were obtained in natural head position as recommended by (Broadbent et al. 1975), on a digital x-ray machine (Allengers Smart PAN 2K15030009-D9). The subjects were positioned in the cephalostat with the sagittal plane at a right angle to the path of the X-rays, Frankfort plane parallel to the horizontal plane, teeth in centric occlusion, and the lips relaxed. During exposure of the X-ray, subjects were guided to stand still with the mandible in relaxed position to ensure that no strain or change in head posture occurred while the head was fixed in the cephalostat. Kodak X-ray films (8″ × 10″) were exposed at 80 kVp, 10 mA for 0.8 s from a fixed distance of 60 inches by following the standard technique employed. The lateral head cephalograms were traced on acetate tracing sheets 0.5 μm in thickness using a sharp pencil on a view box using transilluminated light in a dark room.
Measurement analysed were related to horizontal and vertical change in hyoid bone position, tongue positon and aslo change in the air way dimensions.

Result
Statistical analysis of the data was done using the statistical package for the social science (SPSS 22.0). Paired-t test was used to compare the amount of differences of mean value before and after treatment. A value of P<0.05 was considered as significant. Pearson correlation was used to find correlation between hard tissue and soft tissue, P< 0.05 consider statistically significant. The data was compiled using Microsoft excel sheet (windows 2007).
The pre-treatment & post-treatment hard and soft tissue measurements of subjects and their comparison have been displayed in Table 1 and 2. A highly significant difference was observed between pre-and post-treatment values of ANB (p<.001) with mean difference of 2.32º.A highly significant difference was observed between pre-and post-treatment values of Hy-B and Hy-Me (p<.001). It means that there was significant increase in distance between hyoid to point B (2.30mm) and hyoid to menton (2.73mm).A significant difference was observed between pre-and post-treatment values of Hy-CL (p<.015). It means that there was significant increase in distance between hyoid to CL (1.47 mm). However, there was no significant difference seen between the pre-and post-treatment values of Hy-SN' & Hy-FHP. It shows that there was no significant difference in the position of hyoid bone in vertical plane.
A significant difference was observed between preand post-treatment values of SPAS (p<.007). It means that there was significant increase in SPAS (1.23 mm), but no significant difference was observed between the pre-and post-treatment values of MAS & IAS. Also, there was no significant difference between pre-and post-treatment values of PT-CL & PT-SNˈ Showing that there was no significant difference in tongue position in both sagittal and vertical plane following functional appliance therapy.  .373 ns-p>0.05, *-p<0.05, * *-p<0.01, ***-p<0.001

Discussion
Pre and post-treatment values after functional appliance treatment in class II subjects shows significant difference. In the present study the ANB angle was found to be significantly reduced (p<.001) after functional appliance therapy. Study by Toth and McNamara 14 also found reduction in ANB angle by 1.8˚ in patients treated with the twin block appliance. Likewise, Illing et al 15 also found statistically significant reduction in ANB angle when compared to controls after twin block appliance therapy.
Anteroposterior movement of the hyoid bone with respect to the mandible is represented by Hy-Me and Hy-B and with respect to cervical vertebrae by Hy-CL. In the present study, Hy-Me and Hy-B were found to be significantly increased (p<.001) which means that either the mandible moved forward or the hyoid bone moved posteriorly. However HY-CL distance was also found to be significantly increased (p<.015) showing that hyoid bone moved forward with functional appliance treatment. Study by Geeta Verma et al 6 also showed increased in Hy-B, Hy-Me and Hy-CL following functional treatment.
Vertical position of hyoid is represented by HY-SN (p> .509) and HY-FHP (p> .764). Our study showed, hyoid bone moved non-significantly in an inferior direction after twin block therapy. Study by Zhou's et al 16 also showed a downward displacement of the hyoid bone after functional treatment. In contrast to the present study Robertson CJ 17 reported that hyoid bone moved upwards following functional mandibular advancement in the short-term; however, it moved downwards towards its original position in the longterm as a compensatory action.
The mean value of the various soft tissue parameters have been summarized in Table 2. The results of this study indicate that following treatment with functional appliance, the tongue move forward (PT-CL) but, change was not significant (p>.079). However in a study done by S Yassaei et al 18 following treatment with functional appliance tongue showed significantly forward movement. Also in our study tongue shows non-significant (p>.373) downward displacement in vertical plane (PT-SN') after functional therapy. Similar result were obtained by S Yassaei et al 18 in which they reported non-significant downward displacement of tongue following functional appliance.
Increased in upper airway width SPAS was found to be significantly increased (p<.007) after treatment with functional appliance in the present study, whereas the middle pharyngeal airway space (MAS) (p> .277) and lower pharyngeal airway space (IAS) (p> .643) increased but the change was not significant. Batool Ali et al 19 in their study reported, that the upper airway width, upper airway thickness and nasopharyngeal depth considerably improved after twin block treatment whereas middle and lower pharyngeal dimensions were not affected with functional appliances.
In this study upper airway width was increased by 1.24 mm. The increased in upper airway width found in the study was comparable with the study conducted by Han et al 20 in which an increase of 2 mm in upper airway width was found in skeletal class-II subjects who had undergone treatment with Bionator. In contrast to our findings, study by Ozbek et al 12 reported an increase in upper, middle and lower airway dimensions whereas few other studies found an increase only in the superior and inferior pharyngeal dimensions. 21,22 The present study was conducted to find out any variation in the position of hyoid bone, position of tongue and pharyngeal airway in skeletal class II group and to find out if the upper mentioned structures shows any change after the functional appliance therapy. The result revealed that the position of hyoid bone shifted significantly anteriorly and non significantly downward after the functional appliance therapy in skeletal class II malocclusion. Pharyngeal airway space and tongue position also changed.

Conclusion
After functional appliance treatment in skeletal class II subjects. The hyoid bone has been found to be shifted anteriorly and downward. Superior pharyngeal space was found to be significantly improved after functional therapy whereas there was no significant change with respect to tongue.