Evaluation of pharyngeal

Evaluation of pharyngeal airway changes following en-masse distalization of mandibular dentition

Assistant Professor, Ragas Dental College and Hospital, Chennai, India
Associate Professor, Ragas Dental College and Hospital, Chennai, India
Professor and Head of the Department, Ragas Dental College and Hospital, Chennai, India

Introduction

After growth completion, treatment options for Class III mandibular prognathism is either by Orthognathic surgery or by camouflaging the skeletal discrepancy. Camouflaging mild to moderate Class III Malocclusion has become an attractive option as Orthognathic surgery demands strong motivation from the patients because of its biological risks and the high cost.

In the past camouflaging mandibular prognathism by distalization of mandibular dentition was done by devices like jones jig, class III elastics, and Multi-loop edgewise therapy,1,2 however, these appliances require patient compliance and complicated biomechanics. The advent of temporary anchorage devices has broadened the envelope of nonsurgical and non-extraction treatment options and simplified the biomechanics by application of force vector that was previously difficult or impossible to achieve.

The role of patency of airway in influencing the breathing pattern and its impact on malocclusion has always been an area of interest for the Orthodontist. It is well recognized that the pharyngeal dimension is influenced by the position of the tongue3 and the position of the hyoid bone is determined by the combined activity of the suprahyoid and infrahyoid muscle groups, with the upper airway dilator muscles being of great importance.4 Alteration of the sagittal position of the mandible is reported to influence the position of the tongue and hyoid bone thereby affecting the patency of airway, alteration of the tongue following mandibular setback surgery andits influence on patency of airway has been well documented in literature.5-9 However, the impact of en-masse distalization of mandibular dentition and its influence on tongue and hyoid bone has not been well documented in literature.

Anterior positioning of the tongue is considered to be one of the causative factors for Class III malocclusion, further it has been reported that Class III malocclusion has an anteriorly and superiorly placed hyoid bone.10 Mandibular setback surgery to correct mandibular prognathism results in the tongue being positioned more posteriorly and hyoid bone position altered more posteriorly and inferiorly leading to narrowing of hypopharyngeal and oropharyngeal airway.5-9 Given its implication in causing Class III malocclusion, the extent of mandibular setback must be done with diligence to avoid relapse. Although Orthodontic camouflage by en-masse distalization of mandibular dentition does not alter the position of the mandible, it could still influence the tongue and hyoid bone position. Research-based literature about the effect of en-masse distalization of mandibular dentition on tongue and hyoid bone position is limited. Thus, this study was conceived and its objectives were:

To evaluate hyoid bone position and tongue position using cephalometrics before and after en-masse distalization of mandibular dentition.
To evaluate pharyngeal airway space using cephalometrics before and after en-masse distalization of mandibular dentition.
To compare the hyoid bone position and airway space changes before and after the en-masse distalization of the mandibular dentition.

Materials and Methods

The sample comprised of 16 patients with class III skeletal malocclusion who underwent treatment by distalization of mandibular dentition within a period of 4-years, only 10 patients met the following inclusion criteria: 1) Patients having mild to moderate skeletal class III Malocclusion (ANB 00 to -20); Good quality pre- and post-treatment lateral cephalometric radiograph; 3) Completed Craniofacial growth; 4) Skeletal Class III Camouflaged with en-masse mandibular dentition distalization. The remaining 6 patients were excluded based on the following exclusion criteria: 1) Poor or missing pre-and post-treatment lateral cephalometric radiograph; 2) Discontinued patients; 3) Class III Malocclusion Camouflaged by maxillary dentition protraction or combination of mandibular dentition distalization and maxillary dentition protraction.

The patients were treated by extracting the third molars followed by bonding with 0.022 x 0.028-inch pre-adjusted edgewise brackets (Roth prescription) on the maxillary and mandibular dentitions with an initial wire of 0.014 nickel-titanium. Following aligning and leveling a 0.019 x 0.025 stainless steel wire was placed with hooks between lateral incisor and canine then 2×12 mm SS mini-implants

Figure: 1, a: Pre-Treatment Extra-oral and Intra-oral photographs of a patient treated with Buccal-shelf Mini-implant.

Figure: 1, c: Post-Treatment Extra-oral and Intra-oral photograph of a patient treated with Buccal-shelf Mini-implant.

Lateral Cephalogram of all the patients with the head in Natural head position and teeth in centric occlusion were taken using standard cephalometric procedures. Conventional lateral Cephalogram was digitalized using MICROTEX SCAN MAKER 9000XL plus at a resolution of 75 dpi and saved in JPEG format. The magnification of the radiographs was calibrated. The Standardized Digital Pre- and Post-Treatment Lateral Cehalogram of 10 samples were imported into the Dolphin software (Dolphin imaging 11.8 Premium) A customized Airway Cephalometric analysis was created according to the requirement of this study. The following are the landmarks (Table 1), Reference plane (Table 2) and measurements (Table 3) that were used in this study.

After the landmarks were identified, the reference line was imaged and measurements were obtained from the dolphin software. (figure 3)

Measurements obtained from Pre- and Post-Treatment Digital Lateral Cephalograms of the 10 samples were compared to evaluate the pharyngeal airway space and hyoid bone positional changes.

Statistical analysis:

Data was entered in Microsoft Excel spreadsheet and analysed using SPSS software (version 21).
Descriptive statistics were conducted to determine the mean and standard deviation of the variables.
For the test, a p-value of <0.05 is to be considered statistically significant.
Paired samples t-test: To assess the difference between the pre-treatment and post-treatment cephalometric measurements.

Statistical analysis:

Cephalometric landmarks were identified on the Dolphin software (Dolphin imaging 11.8 Premium) by two examiners to determine inter-examiner reliabilities.
The intra-class correlation coefficient was used to determine the inter-examiner reliabilities. The reliabilities for the 10 measurements (pre and post-treatment) were consistently high. The Cronbach’s alpha values for the inter-examiner reliability ranged from α=0.97 to α =1.000 (Excellent reliability).

Result:

Table 4 and Graph 1 shows the mean and standard deviation of the cephalometric measurements at baseline pre- and post-treatment. Table 5 shows the comparison of the measurements pre-treatment and post-treatment. The difference between all the variables pre-treatment and post-treatment was found to be statistically significant (p<0.05). The majority of the measurements show a decrease in value post-treatment except Hyoidale – Rgn and Mand plane to the hyoid bone.

The mean difference between the pre-treatment and post-treatment superior pharyngeal airway and Hyoidale – Rgn values were found to be statistically highly significant (p<0.001) (Table 5).

Paired samples t test

*p value <0.05 – significant; **<0.001 – highly significant
All the cephalometric measurements reveal a decrease in post-treatment value except in the case of Hyoidale – Rgn and Mand plane to the hyoid bone.
The above values suggest that there is a significant change between pre- and post-Treatment Cephalometric values.

Discussion

The role of the tongue has been implicated as a causative factor in the development of skeletal Class III malocclusion. Several studies have reported on the size, posture, and functioning of the tongue in Class III malocclusion.11,12 In recent times, en-masse distalization of the mandibular dentition with the help of mini implants has been suggested as a method of camouflaging skeletal Class III malocclusion. The efficiency of Mini-implant assisted en-masse mandibular dentition distalization has been documented in recent literatures.13-21 However, to one’s knowledge, none of the studies have focused on the impact of en-masse distalization of mandibular dentition on the pharyngeal airway. Thus, this study was conceived and an attempt has been made to evaluate pharyngeal airway dimension and the position of the hyoid bone after en-masse distalization of the mandibular dentition by comparing the pre- and post-treatment cephalometric measurements of pharyngeal airway space and hyoid bone position.

Pharyngeal airway space can be evaluated using various methods like computer tomography (CT), Cone-beam Computer tomography (CBCT), lateral Cephalogram, magnetic resonance imaging, and polysomnography. However, Cephalometry is the most commonly used mode of evaluation due to fact that it offers considerable advantages over other techniques like convenience, more economical and reduced radiation exposure, as well as being able to simultaneously analyse craniofacial morphology and pharyngeal airway.22 Although lateral Cephalometric measurements of PAS are two-dimensional, studies have reported it to be very reliable in diagnosing pharyngeal volumes20,21.

The results obtained from our samples show that there is a reduction of superior, middle and inferior pharyngeal airway space inferring that there is a generalized constriction of pharyngeal airway space after treatment. It has been earlier reported that changing the sagittal position of the mandible with orthognathic surgery influences the position of the tongue and hyoid bone thereby affecting the patency of airway.5.9 Similarly, there is also evidence to suggest that fixed functional appliance which facilitates mesial migration of the mandibular dentition also has an influence on pharyngeal airway space.25 These findings suggest that an alteration in the mesiodistal position of the dentition or the mandible can influence the position of the tongue and may lead to alteration in the pharyngeal airway dimension. It is logical to assume that as the mandibular dentition is distalized there is a reduction of the tongue space which in turn rolls the tongue backward, reducing the borders of pharyngeal space.

The results also indicate that distance from the 3rd cervical vertebra to hyoidale is reduced and distance from hyoidale to Retrognathion is increased showing that hyoid bone has moved to a more posterior position after treatment. Further, there is a statistically significant increase in the distance between the mandibular plane and the hyoid bone suggesting that the hyoid bone has moved to a more inferior position after the distalization of mandibular dentition. These movements of hyoid bone inferiorly and posteriorly could be because as the mandibular dentition is distalzed the tongue is carried posteriorly. However, with this posterior movement, there is a possibility that the tongue and hyoid apparatus could encroach upon the vital pharyngeal airway space. So, the hyoid apparatus is guided to an inferior position as a compensatory response to avoid compromising the vital passage. The changes in the hyoid bone position reported in this study were consistent with the immediate post-surgical findings of other studies and which suggest that this movement is a physiological adaptation to prevent hyoid related encroachment on the pharyngeal airway. However, the tendency for the hyoid bone to revert back to its original position following a few months after the surgery has been documented.5,6,26,27 If there is a similar trend for the hyoid bone and tongue to retrace its original position following en-masse distalization of the dentition, this could have a serious consequence on the stability of the correction.

This being a retrospective study it has several limitations. The 3-dimensional airway was measured using a 2-dimensional conventional cephalogram and in the future 3-dimensional volumetric data in addition to motion MRI or polysomnography procedures should be carried out especially in vulnerable patients with sleep disordered breathing. Although the size, shape, and position of the tongue are known to have an influence on the position of the teeth it is hard to evaluate the position and posture of the tongue with conventional diagnostic modality. However, the position of the tongue can be determined by ascertaining the position of the hyoid bone hence this method was employed in determining the tongue position and its impact on the pharyngeal airway. It also suffers from the limitation of a small sample size.

Conclusion

Following en-masse distalization of the dentition

The hyoid bone position was altered to a more posterior and inferior position that could be due to the altered tongue position posteriorly.
The alteration in hyoid position resulted in superior, middle and inferior pharyngeal airway constriction as evident from our results.

En-Masse distalization of mandibular dentition is effective in camouflaging skeletal Class III malocclusion in patients with mandibular prognathism. However, it is evident from the outcome of the study that the distalization of the mandibular dentition will influence the posture of the tongue as depicted by the position of the hyoid bone. This alteration of the position of the tongue can reduce the pharyngeal space and is similar to the change in tongue position compromising the airway space following mandibular set back procedure.5-9 There is a possibility that, overtime the tongue would attempt to regain its original position which may compromise the stability of the distalizized dentition similar to the relapse following mandibular setback surgery.28 However, long-term follow-up of the cases with larger sample size is essential to access the stability of the result achieved.

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