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On-line version ISSN 2176-9451
Dental Press J. Orthod. vol.16 no.5 Maringá Sept./Oct. 2011
Camila GornicI; Paula Paiva do NascimentoII; Camilo Aquino MelgaçoIII; Antonio Carlos de O. RuellasIV; Paulo José D'Albuquerque MedeirosV; Eduardo Franzotti Sant'AnnaVI
IGraduate, School of Dentistry - Federal
University of Rio de Janeiro (UFRJ)
IIMSc student in Orthodontics, Department of Pediatric Dentistry and Orthodontics, School of Dentistry - UFRJ
IIIMSc in Orthodontics and PhD student, Department of Pediatric Dentistry and Orthodontics, School of Dentistry - UFRJ
IVPhD in Orthodontics and Adjunct Professor, Department of Pediatric Dentistry and Orthodontics, School of Dentistry - UFRJ
VPhD in Dentistry, UFRJ. Head Professor of Surgery, School of Dentistry - UERJ
VIPhD in Orthodontics, School of Dentistry - UFRJ. Adjunct Professor, Department of Pediatric Dentistry and Orthodontics, School of Dentistry - UFRJ
OBJECTIVE: The aim of this study was to
evaluate the effects of orthognathic surgery for mandibular setback - with and
without combined maxillary surgery - on the upper airways (UA).
METHODS: Immediate lateral preoperative and postoperative cephalometric radiographs of 17 Class III patients were evaluated. Measurements of airway space (AS) diameter were taken in the sagittal plane in the hypopharyngeal and oropharyngeal regions, and changes in hyoid bone position were also recorded. Paired t-test and Pearson's coefficient were applied seeking for potential associations between skeletal and AS changes.
RESULTS: Significant AS reduction was noted in the hypopharyngeal region (mean= 3.10 mm, p= 0.024). The hyoid bone was displaced inferiorly and posteriorly, thereby reducing its distance to the anterior mandibular region. No quantitative correlation could be established between anteroposterior AS reduction and mandibular setback. However, there was a strong correlation between initial AS diameter and the amount of reduction observed in the hypopharynx, but only moderate correlation with the oropharynx.
CONCLUSIONS: Mandibular setback can cause significant UA narrowing, especially in the inferior-most portion (hypopharynx). Therefore, special attention should be given to UA evaluation when formulating an orthosurgical treatment plan since the potential deleterious effects of these changes on functions of the patients should not be overlooked.
Keywords: Orthognathic surgery. Mandibular setback. Airways. Oropharynx. Hypopharynx.
The upper airway (UA) - whose major component is the pharynx - consists of a tube that extends from the nostrils to the larynx (glottis). This structure can be didactically divided into: Nasopharynx (superior-most region, related to the choanae), velopharynx (posterior region of the uvula), oropharynx (base of tongue) and hypopharynx (inferior-most region, posterior to the hyoid bone). The UA walls consist only of soft tissue and, therefore, is not supported by any rigid structure such as bone or cartilage. Thus, the mechanical support that ensures the continued opening of this structure in opposition to negative pressure during inhalation movements results from tension and contraction of the muscles that surround it.11 This mechanism is due in large part to muscular insertions in the genial tubercles, which enable the mandible to be closely involved in the function and support of the tongue and related soft tissues. The genioglossus, geniohyoid and infrahyoid muscles are linked to the oropharynx and hypopharynx. The tongue muscles play an important role in maintaining airway opening, since they form the anterior wall of the pharynx in that segment.
Several procedures have been developed in the hope of increasing the pharyngeal airway space in patients with obstructive sleep apnea syndrome (OSAS). The goal would be to pull the tongue muscles anteriorly through mandibular advancement, thereby decreasing airway resistance to enhance air flow efficiency.14
On the other hand, one might question whether the opposite procedure, i.e., mandibular setback, could cause any reduction in pharyngeal airway space. This is a pertinent question since mandibular setback surgical technique is widely used in the treatment of Class III dentofacial deformities.
The effects of orthognathic surgery on the airways of healthy patients have not been fully explained in the literature, especially with regard to the risk of these patients developing OSAS in the postoperative period.
The purpose of this study was to assess and quantify, by means of cephalometric analysis, changes in the airways caused by orthognathic surgery involving mandibular setback, more specifically in regions directly related to the mandible, corresponding to the oropharynx and hypopharynx.
MATERIAL AND METHODS
Seventeen patients who had been subjected to orthodontic treatment in preparation for surgical correction of Class III dentoskeletal deformities were selected for this study. The surgeries were performed in the Department of Oromaxillofacial Surgery of the Pedro Ernesto State Hospital in 2006.
Six adult men and 11 adult women whose maxillofacial growth had already ceased were evaluated.
Among the patients, 14 underwent combined orthognathic surgery for maxillary advancement and mandibular setback, whilst the remaining patients (3) were subjected to mandibular setback only. The surgical technique consisted of Le Fort I type maxillary osteotomy with rigid internal fixation. In performing mandibular setback, the technique of choice was bilateral vertical osteotomy with maxillomandibular fixation for a period of 2 to 3 weeks depending on the degree of occlusal stability.
Preoperative lateral cephalometric X-rays obtained up to 7 months prior to surgery as well as immediate postoperative radiographs taken up to one week after surgery were evaluated. Pre- and postoperative radiographs were obtained using the same device and in accordance with the standards of the lateral cephalometric technique.
All cephalograms were traced on acetate paper and all measurements were performed by the same examiner. Upper airway references were obtained according to the methodology advanced by Frohberg et al,3 which can be seen in Figure 1.
Soft tissue measurements
1) Diameter of oropharyngeal airway (dark blue): Distance between the points determined by the intersection between the plane that extends from point B (the deepest point on the outer contour of the mandibular alveolar process) to the gonion (Go - point determined by the intersection of the bisector of the angle formed by the lines tangent to the posterior and inferior borders of the mandible and the gonial angle), and the anterior and posterior walls of the pharynx. This measure intends to quantify the changes undergone by the oropharynx after surgery. Point B was selected as a reference as it constitutes the principal region of muscle insertion in the mandible, i.e., the genial tubercles.
2) Airway diameter in the hypopharynx region (green): Distance between the points defined by the intersection between the line extending from the most superoanterior point of the hyoid bone to the most anteroinferior point of the third cervical vertebra, and the anterior and posterior walls of the pharynx. This measure aims at quantifying the changes undergone by the hypopharynx after surgery.
3) Mandibular position (yellow): Measures the shortest distance between point B and a line perpendicular to SN (sella-nasion) passing through S (here called x). This measure discloses the amount of mandibular setback achieved through surgery.
4) SNA - Angle formed by the SN and NA lines (nasion-point A), where A is the deepest point on the contour of the premaxilla, which determines the degree of maxillary retrusion relative to the cranial base.
5) SNB - Angle formed between the SN and NB lines (nasion-point B), which expresses the degree of mandibular protrusion relative to the cranial base.
6) GoGn-SN - Angle determined by the intersection between the line joining the gonial (Go) and gnathion (Gn) points, and the SN line.
7) FMA - Angle determined by the intersection of the mandibular plane (line passing through the menton (Me) and tangent to the lower border of the mandible in the region of the gonial angle), with the Frankfort horizontal plane (junction of the Porium, Po, and orbitale, Or, points).
These last two angular measures (GoGn-SN and FMA) help to diagnose potential mandibular rotations in the vertical direction that can result from surgery, consequently inducing changes in the oropharyngeal reference plane.
8) Hy-SN (purple), the smallest distance between the most anterosuperior point on the body of the hyoid bone (Hy) and the SN line, revealing the vertical position of the hyoid bone relative to the cranial base.
9) Hy-x (orange), the smallest distance between Hy and line x, enabling the evaluation of the anteroposterior position of the hyoid bone.
10) Hy-B (light blue), distance between the Hy and point B, reflecting the relationship between the hyoid bone and the anterior region of the mandibular body.
Data were collected from the patients and all variables were used in the cephalometric analysis. Descriptive analysis including mean, standard deviation and minimum and maximum values measured on the preoperative and postoperative radiographs as well as the paired t-test are shown in Table 1. Some variables were compared with the purpose of establishing correlations between them, using Pearson's correlation coefficient (Table 2).
In the cases analyzed in this study, the mandible experienced a mean setback of 7.32 mm after surgery. The impact on the airway space was evidenced by a mean reduction of 0.97 mm in the oropharynx and 3.41 mm in the hypopharynx. The initial mean diameters of these spaces were 16.88 mm and 13.05 mm, respectively.
This reduction was statistically significant only in the region of the hypopharynx (P=0.025), as shown by the paired t-test (Table 1). In a study8 where the smallest diameter in the posterior region of the tongue (which in most cases corresponds to the hypopharynx) was measured, the same result was found: Airway space reduction, in agreement with other authors.12
Despite this reduction in the hypopharynx, this study was unable to establish a significant quantitative correlation between this reduction and the mandibular setback, i.e., mandibular setback causes airway narrowing, but not proportionally. However, there was a strong correlation between the initial diameter and the amount of AS reduction in the hypopharynx (r= -0.728, p<0.01). In the oropharyngeal region there was moderate correlation (r=-0.526, p<0.05) between the initial diameter and the reduction observed in the airway after surgery. Thus, patients presenting extensive airway space in the preoperative period tend to experience greater AS reduction after mandibular setback surgery.
Because most patients underwent combined orthognathic surgery, one should take into account the influence of maxillary surgery on the final position of the mandible. This influence can be observed by a change in mandibular plane angulation, as assessed by measuring GoGn-SN. The correlation found between this measure and changes in the oropharynx (r=0.511, p<0.05) indicates a direct link between this space and mandibular rotation, i.e., when GoGn-SN increases (clockwise mandibular rotation) the diameter of the oropharynx is likely to increase as well. This finding may explain the variability found in the behavior of oropharyngeal soft tissues, where it was found that 57% of patients who underwent mandibular setback associated with maxillary intervention showed an increase in oropharyngeal space. Otherwise, three patients who underwent mandibular setback alone experienced only a reduction in this space. Another explanation for this variation may be ascribed to a difficulty in standardizing tongue position during radiograph acquisition, since such position is very unstable and constantly changes in response to physiological movements.²
A study evaluating patients who underwent mandibular setback surgery alone found a reduction in airway space (in the mandibular plane) in all cases, establishing a correlation between these two variables (r=0.52).5
Another significant finding was that the hyoid bone was displaced inferiorly (mean=4.26 mm) in response to mandibular setback. Horizontally, bone movement varied with a slight prevalence of posterior displacement (2.08 mm), following the mandibular movement. Other authors4,7,12 also observed inferior and posterior displacement of the hyoid bone. In contrast, anterior displacement of this bone has also been reported.15
Reduction in the distance between the hyoid bone and the anterior mandible region (mean=4.88 mm) was also a significant finding, which raised the question of how the suprahyoid musculature might behave. Changes in this muscle's tone are likely to take place over time to compensate for the decrease in this distance. The largest reduction found for this distance was 18 mm, corresponding to 27% of the original distance exhibited by this particular individual.
In this study, cephalometric radiographs were used to evaluate the upper airways. Although these X-rays provide a two-dimensional view of a three-dimensional structure, studies in the literature have shown adequate correlation between measurements obtained from lateral radiographs and CT scans, validating the former's use for this purpose.6,9
Magnetic resonance evaluation of airway behavior after orthognathic surgery has shown no signs of edema in patients' airways in the immediate postoperative period.10 It is therefore safe to assert that the airway changes observed in this study were not masked by tissue edema but result in fact from movements induced by bone surgery.
This study focused on assessing changes in the immediate postoperative period. Other authors, however, followed their patients for longer periods: 3 months, 6 months, 1 year and up to three years. After one year, no significant tendency was found showing that airway soft tissues tend to return to their initial dimensions.5 Another study,7 which used computed tomography, also demonstrated this same result. A two-year or longer follow-up actually suggests that changes occur in the airways over time after mandibular setback surgery.13 Moreover, some authors argue that the hyoid bone tends to return to its original position, probably because mandible position tends to relapse forward and upward even though the hyoid bone does not quite extend as far as the mandible.4
Although there are many studies on this subject in the literature, any comparison between them is complicated by the use of a wide range of different methodologies, which are still rather limited due to the nature of airway tissues and their dynamic structure. Further studies are warranted to clinically evaluate the impact of changes observed in cephalometric radiographs. Are such changes sufficient to cause dysfunction or are they within the scope of physiological limits? The answer to this question requires that patients be interviewed for possible symptoms of discomfort, breathing or swallowing difficulties, snoring, apnea or other symptoms that might exert a genuine impact on the health of these individuals.
Mandibular setback orthognathic surgery for treatment of cases with Class III facial deformity can lead to significant upper airway narrowing, especially in the lower portion, which comprises the hypopharynx (3.41 mm). Careful attention should therefore be given to the evaluation of this area in lateral view cephalometric radiographs during the stage of orthosurgical treatment planning, as potential deleterious effects, not only as a result of this reduction but also due to other alterations in crucial patient functions, should not be overlooked.
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Contact address Submitted: August 20, 2008
Eduardo Franzotti Sant'Anna
Av. Brigadeiro Trompowsky, s/n
CEP: 21.949-900 - Ilha do Fundão - Rio de Janeiro/RJ, Brazil
Revised and accepted: March 2, 2009
Submitted: August 20, 2008