Correlation between change of tongue area and skeletal stability after correction of mandibular prognathism

Abstract

The purpose of this study was to investigate the correlation between postoperative stability and a change in tongue area after treatment of mandibular prognathism. Twenty‐six patients, who were treated for mandibular prognathism using intraoral vertical ramus osteotomy, were evaluated cephalometrically. A set of three standardized lateral cephalograms were obtained from each participant preoperatively (T1), immediately postoperatively (T2), and after 2 years postoperatively (T3). Student t test and Pearson correlation coefficient were used for statistical analysis. Immediately after the surgery (T12), the setback of the menton (Me) was 12.9 mm (p < 0.001) and the tongue area had significantly increased to 105.8 mm² (p = 0.047). At a 2‐year follow‐up to examine postsurgical stability (T23), the Me exhibited a forward movement of 0.6 mm (p = 0.363) and the tongue area had significantly decreased to 124.3 mm² (p = 0.004). Pearson correlation coefficient test revealed no statistical significance between postoperative stability and change in tongue area. The tongue area significantly increased during the T12 period and decreased during the T23 period. There is no significant correlation between postoperative skeletal relapse and a change in tongue area.

Introduction

The tongue is a muscular structure that occupies the mouth floor and is probably the most active, functional part of the oropharyngeal system. The tongue is divided into anterior and posterior parts and is directly influenced by modifications in the dentoskeletal environment, especially in the mandible [1]. The anterior portion of the tongue is approximately two‐thirds of the total length of the tongue and is attached to the lingual surface of the mandible. The root of the posterior portion of the tongue is attached to the hyoid bone by the hyoglossi and genioglossi muscles. It also forms a part of the anterior wall of the oropharynx and connects with the soft palate, epiglottis, and pharynx by the glossopalatine arches, glossoepiglottic mucous membrane, and superior pharyngeal constrictor muscle [[2], [3], [4]]. A study by Yamaoka et al [5] observed that the tongue roots of patients with distocclusion (Angle class II occlusion) were positioned further back than those of patients with Angle class III malocclusion.

In mandibular prognathism, the tongue must compensate for altered physiological functions caused by severe dental and skeletal deformities [6]. Studies [[7], [8], [9], [10]] have indicated that the position of the tongue and hyoid bone shift backward after mandibular setback surgery for the treatment of mandibular prognathism. Therefore, the positional changes that occur in the tongue after mandibular setback surgery might cause postural adaptation of the tongue to preserve the airway, which can influence surgical stability [11].

No studies have addressed whether changes in the tongue are associated with postoperative mandibular stability. In the present study, we investigated whether changes in the tongue area affect the stability of the mandible position following intraoral vertical ramus osteotomy (IVRO).

Methods

Twenty‐six patients (18 women, 8 men; aged from 17 years to 34 years) were diagnosed with mandibular prognathism. The inclusion criteria of the present study included the following: (1) an Angle class III malocclusion with mandibular protrusion; (2) no history of trauma or other congenital craniofacial abnormality; (3) no growth of the mandible; and (4) underwent bilateral IVRO only. Patients who had tongue thrust habits were not excluded from the study and were treated with physical therapy during the orthodontic procedure. All patients received consultations from orthodontists and oral maxillofacial surgeons prior to the surgery. After careful data analysis and assessment, detailed treatment plans and steps were formulated. We used the cervical vertebral maturation method discussed in a study by Baccetti et al [12] to determine the surgical timing for the treatment of mandibular prognathism. All patients were determined to be in the sixth maturational stage (final stage), which was at least 2 years after the peak of growth. Patients were examined by cephalograms preoperatively (T1), immediately postoperatively (T2), and 2 years postoperatively (T3) to evaluate the postoperative changes in tongue areas and related mandibular positions. Reference points and tongue areas are shown in Figure 1. The reference points and definitions used in this study were as follows: S, sella; N, nasion; Me (menton), the most inferior point on the mandibular symphysis; H, the most superior and anterior point of hyoid bone; G, the most prominent point of the mandibular symphyseal posterior border; V, vallecula epiglottica; TT, tongue tip.

Figure 1.

Reference points and tongue area (pink color). G  =  the most prominent point of the mandibular symphyseal posterior border; H  =  the most superior and anterior point of hyoid bone; Me  =  menton; N  =  nasion; S  =  sella; TT  =  tongue tip. V  =  vallecula epiglottica.

The two reference lines were as follows: (1) X axis, constructed by drawing a line through the nasion 7° above the sella ‐ nasion (SN) line; (2) Y axis, constructed by drawing a line through the sella (S) perpendicular to the X axis. The magnitude of setback and tongue area was measured. The surgical changes were defined as follows: postsurgical immediate change (T12), 2‐year postsurgical stability (T23), and final 2‐year postsurgical change (T13). Postoperative changes at the reference points during each period (T12, T23, and T13) were quantified to estimate statistical parameters, including the mean value and standard deviation. In the present study, there were only eight men, which was too small of a sample size to investigate the relationship between men and women and to provide entirely corrective postoperative results for both sexes. Therefore, we evaluated the 26 patients as a group for preoperative and postoperative comparisons.

A statistical analysis was performed using a paired t test at a confidence level of 95%. Additionally, a Pearson's correlation coefficient analysis was performed to examine the statistical significance of correlations among the Me, H, and tongue area. The significance level was set at p < 0.05. This study was reviewed and approved by the Human Investigation Review Committee at the Kaohsiung Medical University Hospital (KMUH‐IRB‐20140173).

Results

A superimposed cephalogram of tongue area (T1, T2, T3) is shown in Figure 2. 1, 2 show that the immediate surgical changes (T12) of the Me and H demonstrated significant setbacks of 12.9 mm (p < 0.001) and 5.1 mm (p = 0.005), respectively. The Me and H were also positioned significantly downward at 2.5 mm (p = 0.024) and 11.4 mm (p < 0.0001), respectively. The tongue area significantly increased to 105.8 mm² (p = 0.047). Table 3 shows the Pearson analysis that was used to study the T12 correlation of tongue area changes between Me and H. In the horizontal direction, the correlation coefficients of Me and H were 0.017 (p = 0.935) and 0.049 (p = 0.812), respectively. In the vertical direction, the correlation coefficients of Me and H were 0.245 (p = 0.227) and 0.165 (p = 0.421), respectively.

Figure 2.

Superimposed cephalogram of the tongue area (T1, T2, T3): Black line (T1), blue line (T2), and red line (T3). G  =  the most prominent point of the mandibular symphyseal posterior border; H  =  most superior and anterior point of hyoid bone; T1  =  preoperatively; T2  =  immediately postoperatively; T3  =  2 years postoperatively; TT  =  tongue tip; V  =  vallecula epiglottica.

Table 1.

Student's t test for significance for various cephalometric parameters of menton (Me) in T12, T23, and T13.

Me (mm)	Mean	SD	p	Significant
Horizontal change
T12	−12.9	5.13	<0.001	*
T23	0.6	3.49	0.363	NS
T13	−12.3	3.46	<0.001	*
Vertical change
T12	2.5	5.25	0.024	*
T23	−2.8	7.34	0.070	NS
T13	−0.3	6.48	0.845	NS

* Significant, p  <  0.05.

NS  =  not significant; SD  =  standard deviation; T12  =  immediate surgical changes; T13  =  2‐year surgical change; T23  =  postoperative stability.

Table 2.

Student's t test for significance for various cephalometric parameters of hyoid (H) and area of tongue in T12, T23, and T13.

Variable	Mean	SD	p	Significant
Horizontal change (mm)
T12	−5.1	8.24	0.005	*
T23	0.4	7.22	0.760	NS
T13	−4.7	5.28	<0.001	*
Vertical change (mm)
T12	11.4	5.95	<0.001	*
T23	−9.1	9.59	<0.001	*
T13	2.3	8.76	0.196	NS
Area of tongue (mm2)
T12	105.8	253.48	0.047	*
T23	−124.3	196.31	0.004	*
T13	−18.4	250.05	0.715	NS

* Significant p  <  0.05.

NS  =  not significant; SD  =  standard deviation; T12  =  Immediate surgical changes; T13  =  2‐year surgical change; T23  =  postoperative stability.

Table 3.

Pearson correlation testing (significant, p  <  0.05) area changes of tongue between menton (Me) and hyoid (H) in T12.

Variable	Correlation coefficient	p	Significant
Horizontal
T12 (Me)	0.017	0.935	NS
T12 (H)	0.049	0.812	NS
Vertical
T12 (Me)	0.245	0.227	NS
T12 (H)	0.165	0.421	NS

NS  =  not significant (p  ≥  0.05); T12  =  immediate surgical changes.

At a 2‐year follow‐up evaluating postsurgical stability (T23), Me and H exhibited forward movements of 0.6 mm (p = 0.363) and 0.4 mm (p = 0.760), respectively. In the vertical direction, the Me moved upward by 2.8 mm (p = 0.070) and the H moved significantly upward by 9.1 mm (p < 0.001). The tongue area tongue significantly decreased to 124.3 mm² (p = 0.004). Table 4 shows the Pearson analysis that was used to study the T23 correlation of tongue area changes between Me and H. In the horizontal direction, the correlation coefficients of Me and H were −0.020 (p = 0.923) and −0.140 (p = 0.946), respectively. In the vertical direction, the correlation coefficients of Me and H were −0.168 (p = 0.411) and −0.197 (p = 0.334), respectively.

Table 4.

Pearson correlation testing (significant, p  <  0.05) area changes of tongue between menton (Me) and hyoid (H) in T23.

Variable	Correlation coefficient	p	Significant
Horizontal
T23 (Me)	−0.020	0.923	NS
T23 (H)	−0.140	0.946	NS
Vertical
T23 (Me)	−0.168	0.411	NS
T23 (H)	−0.197	0.334	NS

NS  =  not significant (p  ≥  0.05); T23  =  postoperative stability.

After a 2‐year follow‐up (T13), the final movements of Me and H demonstrated significant setbacks of 12.3 mm (p < 0.001) and 4.7 mm (p < 0.001), respectively. Although Me and H had moved upward by 0.3 mm (p = 0.845) and downward 2.3 mm (p = 0.196), respectively, the results were not significant. The tongue area decreased to 18.4 mm² (p = 0.715), which was also not significant. Table 5 shows the Pearson analysis that was used to study the T13 correlation of tongue area changes between Me and H. In the horizontal direction, the correlation coefficients of Me and H were 0.013 (p = 0.951) and −0.038 (p = 0.853), respectively. In the vertical direction, the correlation coefficients of Me and H were 0.324 (p = 0.106) and 0.265 (p = 0.191), respectively. The tongue area changes revealed no significance between Me and H in the T12, T23, and T13 periods.

Table 5.

Pearson correlation testing (significant, p  <  0.05) area changes of tongue between menton (Me) and hyoid (H) in T13.

Variable	Correlation coefficient	p	Significant
Horizontal
T13 (Me)	0.013	0.951	NS
T13 (H)	−0.038	0.853	NS
Vertical
T13 (Me)	0.324	0.106	NS
T13 (H)	0.265	0.191	NS

NS  =  not significant (p  ≥  0.05); T13  =  2‐year surgical change.

Discussion

The hyoid bone supports the tongue between the mandible and the throat and acts as an attachment area for several tongue muscles. The hyoid bone primarily comprises a central part, known as the body, and a pair of projecting bones, known as the lesser cornu and the greater cornu. The muscles and ligaments are attached to the greater and lesser cornu to connect them to the floor of the mouth, the tongue, the epiglottis, the pharynx, the larynx, the mandible, and the styloid process of the temporal bone [[2], [3], [4]]. Therefore, the primary function of the hyoid bone is to anchor the tongue, aiding in tongue movement and swallowing. A study by Adamidis and Spyropoulos [13] noted that the hyoid bone of an Angle class III malocclusion was positioned more forward than those of patients with Angle class I malocclusion.

A study by Bench [14] investigated cervical and tongue development and observed that the hyoid bone position was relatively consistent with the cervical vertebrae. Once a person reaches adulthood, the hyoid bone approaches the height of the fourth cervical vertebra. Studies [[7], [9], [10], [15]] have reported that the hyoid bone was shifted forward following a mandibular advancement surgery and backward by a mandibular setback surgery. Therefore, mandibular setback following surgery for mandibular prognathism indicated that not only do substantial changes occur in mandible position and pattern, but also that the tongue and hyoid bone, which are attached to the muscle group and the mandible, also change positions and patterns.

Hwang et al [16] performed IVRO and found a point B setback of 5.43 mm and an H setback of 4.85 mm immediately after surgery, indicating an H/B ratio of 89.3%. A study by Eggensperger et al [17] reported a Me bone setback of 6.3 mm and a hyoid bone setback of 2.5 mm, indicating an H/Me ratio of 39.7% and a hyoid bone downward movement of 7.5 mm. In the present study, the Me exhibited a setback of 12.9 mm and the H exhibited a setback of 5.1 mm, indicating an H/Me ratio of 39.5% at the immediate postsurgical change (T12). Our patients demonstrated a greater amount of setback and a smaller H/Me ratio than those reported by Hwang et al [16]. This suggests that a greater mandibular setback does not result in a similar amount of backward movement in the hyoid bone. To avoid a narrowing of the pharyngeal airway space, a significant inferior movement of the hyoid bone replaced the backward movement itself. Therefore, our finding was similar to that of the study by Eggensperger et al [17] at the T12 period.

Hasebe et al [18] observed a setback of 6.7 mm at the point pogonion (Pog) and a setback of 4.4 mm at the hyoid bone (H), indicating an H/Pog ratio of 65.7%, and a downward movement of 3 mm in the H after a 6‐month follow‐up. Hwang et al [16] found that the H/B ratio decreased to 39.8% after a 1.5‐year postoperative follow‐up. Wickwire and Proffit [19] stated that the hyoid bone shifts backward after surgery and attempted to return it to its original position but were unable to do so. In agreement with Wickwire and Proffit's report [19], the H positions of our patients were still significantly backward and downward even 2 years after the surgeries. In our study (T13), the Me setback was 12.3 mm and the H setback was 4.7 mm, indicating an H/Me ratio of 38.2%. Our results were similar to those of Hwang et al [16] at the final follow‐up (T13). Therefore, we found that H returned to a new balanced position, approximately to 40% of the Me setback.

A study by Do et al [20] also found that there is a statistical trend for patients with sleep‐disordered breathing (SDB) to have larger tongue sizes compared to patients without SDB, but cannot account for disease severity of SDB. While performing mandibular setback surgery, the tongue was moved backward and the morphology was changed. Therefore, the pharyngeal airway space was reduced, which may lead to the onset of obstructive sleep apnea. It has often been hypothesized that a large tongue leads to an enlargement of the mandible and therefore contributes to the development of mandibular prognathism. Yoo et al [21] found that the tongue volume is accounted for by the combined horizontal and vertical location of the chin and symphysis, but does not support the conventional clinical surmise that a large tongue volume is inherent in patients with mandibular prognathism.

Jakobsone et al [22] evaluated area changes in the upper airway after bimaxillary correction of a class III malocclusion. They found that the tongue significantly increased in length to 4.8 mm and decreased in area to 62 mm². In our study, the tongue area significantly increased to 150.8 mm² immediately following the surgery period (T12). Even the amount of setback and increase in tongue area in our study were greater than those reported in other studies, but our patients did not develop obstructive sleep apnea. Therefore, immediate mandibular setback not only moves the H backward, but also affects the neck muscles, which respond immediately by producing various head positions. The synergy between the suprahyoid and infrahyoid muscles interferes with the sense of balance of the head position. The adjustment of head position could adapt to the position changes of the tongue and narrowing of the pharyngeal airway space. Therefore, the tongue area of our patients only decreased to 18.4 mm² at the postoperative 2‐year follow‐up (T13), which was insignificant. However, a lack of three‐dimensional volumetric data, which would reflect entire morphological changes in the tongue, remains a limitation. In future studies, three‐dimensional imaging may help bridge the gap between tongue area and volume analysis.

Ko et al [23] investigated the alteration of masticatory electromyographic activity and stability of orthognathic surgery in patients with skeletal class III malocclusions. They found that the mandible showed a significant setback of 10.19 mm and a relapse of 1.12 mm (10.99%). They concluded that a larger relapse of mandibular setback occurred in patients with greater masticatory muscle activity. In the present study, we investigated the correlation between changes in tongue area and amount of setback in Me and H. In the T12 period, we found that a greater amount of setback (Me and H) did not lead to significant increases in tongue area, and the patency of the pharyngeal airway space was maintained. Considering the postoperative relapse of the mandible (T23), a Pearson correlation also revealed no significant correlation (p = 0.923) with changes in tongue area. In the final follow‐up (T13), Me and H also showed no association with changes in tongue area, suggesting that the activity of tongue‐related muscles did not cause the significant postoperative relapse.

Kim et al [24] compared the stability after mandibular setback surgery in patients with skeletal class III malocclusions with and without presurgical orthodontics. They concluded that a surgery‐first approach without presurgical orthodontic treatment was less stable than a conventional orthognathic surgery for mandibular prognathism. The unstable occlusion may cause imbalanced muscular activity, which may lead to asymmetrical activity of the muscles, especially in the masseter muscles and tongue. Based on our findings, the immediate postoperative (T12) tongue area was significantly increased, which may lead to more skeletal relapse if there is an unstable postoperative occlusion. In the present study, all patients required a preoperative orthodontic treatment to achieve a better, stable postoperative occlusion and to minimize mandibular relapse. Therefore, occlusal stability and related muscle activities must be considered prior to performing a surgery‐first approach.

Conclusion

After surgical treatment for mandibular prognathism, the tongue area significantly increased in the T12 period and decreased in the T23 period. In the final follow‐up (T13), the tongue area decreased, but not at a significant level. There is no significant correlation between postoperative skeletal relapse and a change in tongue area.

Acknowledgments

This work was partially supported by a grant (KMUH105‐5M42) from Kaohsiung Medical University Hospital, Taiwan.