Farhana Ghaffar*, Mubassar Fida**, Attiya Shaikh***
How to CITE:
J Pak Dent Assoc 2010;19(1): 19 - 23
The aim of this study was to determine change in the position of hyoid bone with functional appliance treatment.
METHODOLOGY:
The present study was carried out using pre-functional and post-functional cephalographs of patients attending the Orthodontic Clinics at the Aga Khan University Hospital, Karachi, from 2003 to 2007. This study was conducted on 30 Pakistani subjects (12 males and 18 females) aged between 9 to 14 years, treated with removable or fixed functional appliances. Cephalometric points, linear and angular measurements assessing position of hyoid bone on pre-and-post functional cephalographs were recorded. Paired sample t-test was used to depict the change in the position of hyoid bone following functional appliance treatment.
RESULTS:
Linear and angular cephalometric measurements depicting the position of hyoid bone after functional appliance treatment were statistically insignificant.
CONCLUSION:
There was no statistically significant change observed in the position of hyoid bone with functional appliance treatment.
KEY WORDS:
Hyoid bone, functional appliance, cephalographs.
Introduction
The hyoid bone is involved in three basic functions: deglutition, phonation and respiration. The importance of hyoid bone is based upon its unique relation with other structures. It provides insertion for muscles, ligaments and fascia from the mandible, cranium and cervical spine. It is unique and different since it has no bone, joint nor articular relationship with the anterior cervical spine, however, it has a definite relationship with the anterior cervical spine through the cervical fascia. Two groups of muscles function concomitant with those of the hyoid bone, the suprahyoid and the infrahyoid. These muscles depend upon the hyoid bone and its relationship with anterior cervical spine and the shoulder girdle for normal function. The hyoid bone has a predominant role in determining the cervical spine physiologic curvature. The suprahyoids have direct effect upon mandibular dynamics by controlling activities of the levator and mandibular muscles1.
The relationship between the hyoid bone and mandible stays constant from the age of three years. Hyoid bone position tends to remain at the inferior half of C-3 vertebral body and the superior half of C-41. When all permanent teeth except the third molars are erupted the hyoid bone position is near and above a line connecting the third vertebra and the most inferior portion of the chin2. During puberty, the hyoid usually moves slightly anterior but not in cranial or caudal directions1. This maturational descent of the hyoid bone is thought to be closely related to speech3,4 and deglutition5. Early descent of the hyoid bone is hypothesized to account for the evolutionary adaptation of speech development, used to resonate the sound spectrum with the frequency of a harmonic6. In contrast, late descent, which is readily observed in middle-aged or older men with obstructive sleep apnea7,8
patients after mandibular setback surgery9 might be associated with airway patency because airway resistance appears to increase with age or after the surgery. The normal position of the hyoid bone is thought to have a wide range of variability. It is not clear whether late descent occurs in normal adults. Several studies10-17 have tried to determine the actual position of the hyoid bone relative to cervical spine and hyoidal functional relationships with the craniomandibular system. These studies have shown changes in the position of the hyoid bone and in pharyngeal size with the mandibular advancement. In the field of orthodontics, mandibular advancement therapy with removable or fixed functional appliances is a well established and versatile procedure used to treat mandibular retrognathism with patients in their pubertal phase. Although the position of hyoid bone has been assessed with orthodontic treatment in a local study18, however, the current study was performed with special focus to determine the change in the position of hyoid bone after functional appliance treatment
Methodology
The present study was a cross-sectional comparative study carried out at the Orthodontic Clinic, the Aga Khan University and Hospital, Karachi, from 2003 to 2007. Pakistani patients treated with removable or fixed functional appliances (twin block, bionator and herbst) having good quality lateral cephalographs taken at two time periods (pre-treatment, T0, and end of active functional appliance treatment, T1) of patients were recorded. The exclusion criteria was patients with systemic diseases having an effect on growth and development and previous orthodontic and/or orthopedic treatment.
The study sample consisted of 30 patients, having 12 males and 18 females, aged between 9 to 14 years. The lateral cephalographs were taken in natural head position with lips relaxed and teeth in occlusion. To ensure consistency between the pre-and post-functional treatment cephalographs, the length of cranial base structures were compared.
Magnification errors between T0 and T1, if any, were corrected using a constant-conversion factor . Each pair of pre-and post-functional treatment lateral cephalographs was hand-traced for hyoid position on standard acetate paper at the same sitting to minimize tracing error. Cephalometric points, linear and angular measurements depicting change in the position of hyoid bone on pre-and post-functional cephalographs used in the study are as follows10
Hy-S: linear distance from sella (S) the estimated center of the sella turcica to hyoidale (Hy) the most anterior and superior point of the hyoid body.
Hy-Mp: linear distance measured from Hy perpendicular to the mandibular plane, formed by a line connecting gonion (Go) the most posterior, inferior, and outer point of the mandible angle and menton (Me) the most inferior point on the mandibular symphysis.
Hy-Rgn: linear distance from Hy to retrognathion (Rgn), the most posterior point of the mandible symphysis.
C3-Rgn: linear distance from the most inferior, anterior point on the third cervical vertebra (C3) to Rgn.
C3-Hy: linear distance between C3 and Hy.
Go-Hy-Me: angle measured from Go to Hy to Me.
Data analysis was undertaken using the Statistical Package for Social Sciences for Windows version 16.0 (SPSS Inc., Chicago, Illinois, USA). Pre-and post-functional treatment differences in the position of hyoid bone were calculated by employing the paired sample t-test and p value of <0.05 was considered to be significant. To rule out measurement error cephalographs of 15 patients were randomly selected by the principal examiner and were measured again after a period of one month..
Results
Mean age and gender distribution of the patients included in the study are shown in Table I
Table II shows the type of functional appliance used with the mean treatment time period. This table depicts that majoriy of the patients in our study were treated with bionator removable functional appliance
Table III depicts the change in the position of hyoid bone at To (pre-functional treatment time) and T1 (post-Functional treatment time). This table shows statistically insignificant difference in the position of hyoid after functional appliance treatment
Discussion
Cervical spine is of great importance in the fields of medicine as well as dentistry. At age three, the cranium, cervical spine and mandible have a functional relation through soft tissue tensile forces that are generated by functional position of the hyoid bone and the hyoid
System. Hyoid system relates the cranium to the mandible through the digastrics sling, the cervical spine through the cervical fascia and the shoulder girdle through the infrahyoid and suprahyoids1. Hyoid bone also plays an important role in keeping the airway patent. Upper airway structures tend to collapse in the supine position. The compensatory reflexes acting on pharyngeal dilators holding the hyoid bone forward help in preventing pharyngeal narrowing during the life span19,20. As craniomandibular, cervical , hyoid and airway regions are considered as single functional unit, patients having disorders in the anterior neck region such as tension in the throat, difficulty in deglutition, change in voice and respiratory disorders should also be carefully checked for dysfunction in the airway and hyoid system1.
Studies11-17 have shown that there are changes in the position of the hyoid bone and in pharyngeal size in connection with mandibular advancement surgeries. Recent studies suggest that lower position of hyoid bone occurs over time in males21 or after mandibular setback surgery9,22 in relation to upper airway patency. These findings suggest that the progressive inferior migration of the hyoid bone might be a physiologic phenomenon, not pathologic, related to an increase of airway resistance or airway length. Since relationship between the craniomandibular, cervical and hyoid regions can be modified by removable jaw orthopaedic appliances, the current study was conducted with an idea to assess the change in the position of hyoid bone with functional appliance treatment. The results depicted that the mean treatment time of functional appliance in our study was 11.8 months with bionator and twin block being used as removable functional appliances and herbst as fixed functional appliance. Further it was observed that majority of the patients were treated with removable functional appliances and amongst them bionator was the most common. The measurements Hy-S, Hy-Rgn, C3-Rgn, C3-Hy and Go-Hy-Me showed a difference in their measurement with downward movement of hyoid bone with functional appliance treatment, however, none of the measurements were statistically significant. Our study results are in agreement with Jan’s18 study who assessed post-treatment changes in hyoid bone position in dental Class I, II and III patients. His study results concluded that there were no statistically significant changes in the position of the hyoid bone irrespective of age, gender and the type of malocclusion. Similarly, Grant23 concluded that the hyoid bone position is constant in the three types of malocclusion and believed that the musculature rather than the occlusion of the teeth determines the position of the hyoid bone. Likewise Cuozzo and Bowman24 studied the position of hyoid bone in Class I, II and III malocclusions during deglutition and found no difference in the position and the functional pattern of the hyoid bone. However, Ricketts25 was of the opinion that the hyoid bone is not stable and always changes with the change in the mandibular position. Graber26 studied the change in the hyoid bone with orthopaedic treatment in children with mandibular prognathism and depicted that hyoid bone moved slightly posteriorly and inferiorly but the change was statistically insignificant.
Pae et al10 in their study concluded that the significant change in the position of hyoid bone was independent of age or obesity but was related to facial type, as classified by the steepness of the lower margin of the mandible. Their study depicted that a brachyfacial person has a hyoid bone position closer to the mandibular plane and more posteriorly located towards the cervical vertebrae. In contrast, dolichofacial and normal subjects have a hyoid bone positioned more inferiorly and anteriorly than their brachyfacial counterparts. They also concluded that the lower hyoid bone position is the main characteristic of patients with obstructive sleep apnea.
Previous studies11-17 have suggested that the change in the oropharyngeal complex caused by mandibular surgery may be only a temporary phenomenon observed at a relatively short-term post-operative stage. Schendel and Epker14 reported that the hyoid bone tends to return almost to its pre-operative position after a certain post-operative period following mandibular advancement with maxillomandibular fixation. LaBanc and Epker15 reported immediate post-operative movement of the hyoid bone in an anterior direction, but at the same time they emphasized the highly variable nature of the post-operative position of the hyoid. Eggensperger et al11 reported total relapse to the original position after the first post-operative year and observed that the hyoid bone was located more superior than it had been post-operatively. Hayes et al13 also showed the hyoid bone to be located superiorly to its pre-operative position by about 3.7mm. Similarly, Chung et al17 did a study to compare positional changes of the hyoid bone and postsurgical compensation in mandibular position in patients who received either wire or rigid fixation after surgery and found no significant relationship. Few studies11,14,15 concluded that this post-operative change was attributed to tendons and muscles and changes in their location of attachment to the bones as well as the changes in the tendon-bone interface while some studies27,28 emphasize that stretching of the suprahyoid muscles with mandibular advancement procedures posteriorly directed forces is a primary factor leading to skeletal relapse.
Hyoid bone has always been a topic of interest in the field of orthodontics. As functional appliance bring a gross amount of change in the patients profile, occlusion, speech and masticatory function, this study was conducted to depict the effect of functional appliance treatment on the position of hyoid bone. According to Jan18 if the hyoid bone is in the same position after functional appliance treatment then the soft tissues must still be in the same balanced rest position, thus reducing the chances of relapse due to soft tissue forces. However, difference observed in the position of hyoid bone beyond its normal range increases the chances of skeletal relapse. As indicators of skeletal relapse are limited, therefore the position of hyoid bone can be used as a good indicator to predict relapse tendencies. Depicting change in the position of hyoid bone after functional appliance treatment will not only alarm about the risk of post-functional skeletal relapse but will be of great help to the orthodontist in guiding certain active measures to be taken with fixed mechanotherapy such as wear of a removable anterior inclined plane to prevent post-functional relapse21
Conclusion
There was no significant change observed in the position of hyoid bone with functional appliance treatment.
ACKNOWLEDGEMENTS
I would like to thank Dr. Rashna Aga, for her valuable help in the statistical analysis, special appreciation for Dr. Munizeh Khan, Dr. Nabila Anwar, and Dr.Sarwat Memon for their
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