Effect of mandibular setback surgery on the posterior airway size

Maija Liukkonen, DDS Assistant Professor Department of Oral Development and Orthodontics Institute of Dentistry University of Turku Kimmo Vähätalo, DDS Senior Oral and Maxillofacial Surgeon Timo Peltomäki, DDS, PhD Associate Professor Department of Oral Development and Orthodontics Institute of Dentistry University of Turku Consultant Orthodontist Jaakko Tiekso, DDS Research Associate Risto-Pekka Happonen, DDS, PhD Professor and Head Department of Oral and Maxillofacial Surgery Institute of Dentistry University of Turku Reprint requests: Dr Timo Peltomäki Institute of Dentistry, University of Turku Lemminkaisenkatu 2 FIN-20520 Turku Finland Fax: +358-3-333-8356 E-mail: timo.peltomaki@utu.fi Int J Adult Orthod Orthognath Surg Vol. 17, No. 1, 2002 Effect of mandibular setback surgery on the posterior airway size Treatment of dentofacial deformities with jaw osteotomies has an effect on airway anatomy, and therefore, mandibular setback surgery has the potential to diminish airway size. The purpose of this study was to evaluate the long-term effect of mandibular setback surgery on the airway size. The material consisted of pre- and postoperative (minimum 1 year) lateral radiograms of 22 individuals (18 females and 4 males) with a mean age of 30 years, who had undergone mandibular setback surgery to correct skeletal Class III discrepancies. Hard and soft tissue points were digitized with a Numonics Accugrid digitizer and analyzed with Xmetrix software. A paired t test was used to evaluate the difference between pre- and postoperative measurements. In addition, Pearson s coefficient correlation was calculated to reveal the possible association between the skeletal change in relation to the change in airway size. The mean value for the initial SNA was 81.3 degrees, 85.4 degrees for SNB, 36.9 degrees for S-Na/MP, and for the posterior airway, 10.5 mm and 12.0 mm retropalatinally and retrolingually, respectively. At the postoperative evaluation, SNB was 80.7 degrees, S-Na/MP 41.0 degrees, and the posterior airway retropalatinally 8.3 mm and retrolingually 9.8 mm. Statistical analysis revealed a highly significant correlation between the change in the ANB angle and in the S-Na/MP angle versus the change in the upper airway size, both retropalatinally and retrolingually. Mandibular setback surgery with posterior rotation may gradually result in increased upper airway resistance in cases where neuromuscular adaptation is insufficient to compensate for the reduction in the airway size. Therefore, large anteroposterior discrepancies should be corrected by combined maxillary and mandibular osteotomies. (Int J Adult Orthod Orthognath Surg 2002;41 46) Treatment of dentofacial deformities with jaw osteotomies has an effect on oropharyngeal morphology. Consequently, mandibular advancement has been successfully used to increase airways in patients with obstructive sleep apnea 1,2 ;however, less encouraging results have also been published. 3 On the other hand, mandibular setback surgery is known to reduce airway size 4 7 even, in some rare cases, to the extent that a sleep-related breathing disorder may develop. 8,9 This was demonstrated in a case which led to the creation of the present evaluation. 41 A 50-year-old man was referred to the at Turku University Central Hospital,, because of snoring and excessive daytime sleepiness. Clinical, radiologic, and polysomnographic studies showed that the patient had partial upper airway obstruction with an oxygen saturation of 79% at its lowest. The posterior airway dimension was, retrolingually, 4 mm wide. He had Class I occlusion with 2 mm overjet and 4 mm overbite. Anamnestic records revealed that he had had a skeletal Class III relationship with anterior

42 Liukkonen et al Table 1 Pre- and postoperative skeletal values (mean ± SD) and changes following mandibular setback surgery in 22 patients Preoperative T1 Postoperative T1 Difference T1 T2 Paired Patient Mean SD Mean SD Mean SD t test T1 T2 SNB 85.4 4.6 80.7 4.2 4.8 3.5 0.0000 82.4 72.4 ANB 4.2 3.0 0.5 2.8 4.7 3.4 0.0000 8.0 1.8 Cd-Gn 122.0 7.5 115.2 7.4 6.8 4.6 0.0000 136.8 120.8 S-Na/MP 36.9 6.7 41.0 7.8 4.2 5.8 0.0029 43.9 57.6 NL/ML 29.1 6.8 33.2 8.4 4.0 6.1 0.0051 33.9 47.5 Gonial angle 136.4 8.2 139.8 9.5 3.4 6.2 0.0166 146.1 157.0 ANS-Me 67.5 8.0 67.6 7.4 0.1 3.6 0.8846 79.9 84.4 Na-Me 117.5 10.7 117.1 9.7 0.4 3.5 0.5744 137.3 139.7 S-Go 73.4 6.5 71.6 6.5 1.8 3.7 0.0358 83.6 78.0 Na-S-Ah 87.9 4.5 89.3 5.4 1.7 3.9 0.0855 88.1 98.9 S-Ah 100.0 9.4 100.3 10.9 0.3 4.3 0.7635 124.8 119.8 NSL/OPT 97.4 8.1 98.6 10.2 1.2 8.4 0.5104 103.6 122.3 Values of patient described are indicated in bold. SD = standard deviation. crossbite 15 years earlier and no reported breathing problems. The skeletal and dental discrepancy had been treated with 12 mm mandibular setback surgery using vertical ramus osteotomy. At 2-year postoperative follow-up, the SNB angle was found to be 10 degrees less than at initial examination, while the mandibular plane angle had increased by 13.7 degrees. Retropalatinally, airway space had been reduced from 8.7 mm to 5.0 mm, and retrolingually, from 9.1 mm to 6.5 mm. Body mass index was 24 before the orthognathic surgery and is presently, ie 15 years later, 22. Reduction in airway space after setback surgery had probably been a causative factor in the patient s developing partial upper airway obstruction. Hence, interest was evoked in studying other patients treated with the use of the same approach. The aim of the investigation was to evaluate the long-term effect of mandibular setback surgery on airway size, and particularly, to examine whether the presented patient had any specific features, which may have contributed to the development of partial upper airway obstruction. Materials and methods The material consisted initially of 33 patients who had undergone mandibular setback surgery to correct skeletal Class III discrepancy at the Institute of Dentistry, University of Turku,, during the years 1980 to 1992. Ultimately, 22 individuals (18 females and 4 males) with natural dentition and good-quality preoperative (Table 1) and postoperative (Table 2; min. 1 year, mean 19.9 months, range 12 to 55 months) available lateral cephalograms were included for the investigation. The mean age was 30 years (range 17 to 45 years) and mean body mass index 22.4 (n = 17, range 17 to 32) at the time of surgery. Surgical techniques included bilateral vertical ramus osteotomy (VRO) with maxillomandibular fixation (MMF) in 17 cases, bilateral sagittal split osteotomy (BSSO) with rigid fixation in 4 cases, and 1 patient with BSSO and MMF. The majority of the patients were operated on during the early 1980s, which explains the high number of VRO cases. Furthermore, by that time Le Fort I osteotomy had not yet become a routine procedure at the authors clinic.

Lateral cephalograms were performed with the subject standing with his or her head in the cephalometer in a natural head position and teeth in the intercuspal position. A tracing was made on each cephalogram, and conventional hard and soft tissue cephalometric points (Figs 1 and 2) were digitized with a Numonics Accugrid digitizer (Numonics Co, Montgomeryville, PA) and analyzed with X- Int J Adult Orthod Orthognath Surg Vol. 17, No. 1, 2002 43 Table 2 Pre- and postoperative airway size values (mean ± SD) and changes following mandibular setback surgery in 22 patients Preoperative T1 Postoperative T1 Difference T1 T2 Paired Patient Mean SD Mean SD Mean SD t test T1 T2 TB-phw1 12.0 4.8 9.8 4.0 2.3 3.6 0.0067 9.1 6.5 UP-phw2 10.5 3.6 8.3 3.7 2.2 2.3 0.0002 8.7 5.0 ad1 24.1 3.2 24.2 3.5 0.1 2.4 0.8958 21.3 24.5 ad2 22.8 3.2 22.9 3.5 0.1 2.4 0.8385 19.3 23.9 PNS-u 31.6 4.1 32.9 3.9 1.3 3.0 0.0514 34.3 37.7 Values of patient described are indicated in bold. SD = standard deviation. Fig 1 Skeletal reference points on the lateral skull radiographs: A = A- point; Aa = most anterior point on the first cervical vertebra; Ah = most anterosuperior point on the hyoid bone; Ar: articular; ANS = anterior nasal spine; B = B-point; Ba = basion; Co = condylion; cv2ip = most posteroinferior point on the corpus of the second cervical vertebra; cv2tg = tangent point on the dorsal contour of the odontoid process of the second cervical vertebra; Gn = gnathion; Go = gonion; Md1A = apex of the mandibular incisor; Md1T = incisal edge of the mandibular incisor; Mx1A = apex of the maxillary incisor; Mx1T = incisal edge of the maxillary incisor; Me = menton; Na = nasion; PNS = posterior nasal spine; S = sella. Fig 2 Pharyngeal reference points on the lateral skull radiographs: Ad1 = point of intersection of adenoid tissue/dorsal pharyngeal wall with a line PNS-Ba; Ad2 = point of intersection of adenoid tissue/dorsal pharyngeal wall with a line from PNS perpendicular to S-Ba line; U = tip of the soft palate; TB = point on the posterior aspect of the tongue closest to the dorsal pharyngeal wall; UP = point on the posterior aspect of the soft palate closest to the dorsal pharyngeal wall; phw1 = point on the dorsal pharyngeal wall closest to TB; phw2 = point on the dorsal pharyngeal wall closest to UP. metrix software (Smart Systems, ). The cephalometric analysis consisted of 12 linear and 12 angular measurements. Two different X-ray units were used with different magnifications. This was taken into consideration by reducing the linear dimensions to actual size. A paired t test was used to evaluate the difference between pre- and postoperative measurements. In addition, Pearson s

44 Liukkonen et al Table 3 Pearson s correlation coefficients between skeletal and airway size changes TB-phw1 UP-phw2 SNB 0.595** 0.593** ANB 0.619** 0.557** Cd-Gn 0.608** 0.610** S-Na/MP 0.585** 0.609** NL/MP 0.592** 0.543* Gonial angle 0.553** 0.681** Na-Me 0.375 0.478 S-Go 0.210 0.242 ANS-Me 0.360 0.429 See Fig 2 for lateral skull radiograph pharyngeal reference points. **P <.01. *P <.05. correlation coefficient was calculated to reveal possible associations between changes in the mandibular parameters in relation to the changes in airway size. Two orthodontists (M.L. and T.P.) examined the tracings and the location of the landmarks. Discrepancies between the examiners were resolved by arbitration. The reproducibility of the measurements was tested by repeated digitation in all patients (except the one presented in the introduction) at least 1 month later by the same investigator. Errors in digitation for each variable were calculated from the formula 10 : d 2 /2n where d is the difference between the first and the second measurement. Results Double measurements revealed errors ranging between 0.4 and 1.2 degrees in the angular measurements and between 0.2 and 0.7 mm in the linear measurements.the errors were considered to have an insignificant effect on the results as far as the reliability of the measurements is concerned. At operation and during the follow-up period, the SNB angle was found to have decreased on average by 4.8 degrees, while the ANB angle increased on average by 4.7 degrees. Total mandibular length (Cd-Gn) had decreased on average by 6.8 mm. All these changes were statistically highly significant (P <.001). While the mandibular plane and gonial angles were observed to have increased in a statistically significant manner (4.2 degrees and 3.4 degrees; P <.01 and P <.05, respectively), no statistically significant increase in the anterior face height (Na-Me) or in the lower facial height (ANS-Me) was found. On the other hand, statistically significant shortening of the posterior face height (S-Go) had occurred at operation and/or during the follow-up period. Measurements of hyoid bone and head posture, which were taken both pre- and postoperatively, showed no changes (Table 1). Preoperative airway size was found to be, on average, 10.5 mm retropalatinally and 12.0 mm retrolingually, respectively. At the follow-up examination, these measurements were 9.8 mm and 8.3 mm, respectively a decrease which was statistically significant. No statistically significant changes were found in the other airway measurements (Table 2). Correlation analysis revealed a statistically significant correlation between decrease in airway size and the mandibular measurements (Table 3). The greater the reduction in the mandibular linear measurements and the greater the increase in the mandibular plane angle, the greater the reduction in the airway measurements retropalatinally and retrolingually. Discussion In the present study, airway size has been evaluated by using a 2-dimensional view of a 3-dimensional structure. A good correlation between the airway dimension measured on lateral cephalograms and on 3-dimensional computer tomography has, however been reported, 11,12 validating the use of lateral cephalograms in airway size analysis. According to the suggestion by Solow et al, 13 airway size was measured as the narrowest dimension from the base of

the tongue or the soft palate to the posterior pharyngeal wall, not related to any skeletal structure, for example, the gonial region (B-point), which has probably undergone changes at osteotomy. A comparable method was used in a recent study by Mehra et al. 1 Adaptive changes are known to occur in the soft and hard tissues after osteotomies, 14 19 so all the present patients were examined at least 1 year postoperatively. The present correlation analysis indicates that airway size reduction is related to the amount of mandibular setback. Interestingly, posterocaudal (clockwise) rotation was also found to be related to the amount of airway size decrease. Accordingly, counterclockwise surgical rotation of not only the mandible but also the maxilla has been successfully used to gain maximal airway size increase in obstructive sleep apnea patients. 1 Knowledge of minimal critical airway size would be of great importance when osteotomies are planned to avoid the possible development of sleep-related breathing disorders. When evaluated from lateral cephalograms, the normal value for retrolingual airway size is 10 to 12 mm. 2,20,21 Hochban et al 5 have reported preoperative airway space to be larger than average in prognathic patients, and, following mandibular setback, airway size decreases only to values within normal range. The findings of the present study, however, do not support this postulate: Preoperative airway size was found to be, on average, 12 mm at the tongue base, thereby corresponding well with normal values. It seems evident that the patient we described was an exceptional case (among this group of patients) with specific features, which probably contributed to the development of partial upper airway obstruction. The patient was extremely dolichofacial, initially with an S-Na/ mandibular plane angle of 44 degrees (normal value 32 degrees), and posterior airway size preoperatively about the same as the whole group postoperatively (Tables 1 and 2). Furthermore, he had the greatest amount of mandibular setback (12 mm) with posterior rotation (13.7 degrees). While the group, on average, did not display changes in the hyoid bone or head Int J Adult Orthod Orthognath Surg Vol. 17, No. 1, 2002 45 posture, this patient showed posterior and upward relocation of the hyoid bone and increase in the craniocervical angle. The latter change can be interpreted as an attempt to maintain airway patency. 22 All the changes can well explain the symptoms the patient had gradually developed after surgery, even without being overweight. The features are comparable to those of other patients presented earlier who had developed upper airway obstruction following mandibular setback. 5,8,9 No other patient from the group has paid a return visit to our clinic with breathing problems. In conclusion, it is evident that mandibular setback surgery with posterior rotation may cause narrowing of the posterior airway, and may be a causative factor in the gradual development of a breathing disorder (upper airway resistance). This may particularly occur if predisposing factors, such as specific craniofacial type and/or obesity, are present or individual neuromuscular adaptation is insufficient to compensate for the reduction in airway size. Therefore, careful airway analysis should be performed, particularly in connection with large anteroposterior discrepancies. Once indicated by skeletal and airway analysis, such cases should be corrected by combined maxillary and mandibular osteotomies. References 1. Mehra P, Downie M, Pita MC, Wolford LM. Pharyngeal airway space changes after counterclockwise rotation of the maxillomandibular complex. Am J Orthod Dentofacial Orthop 2001;120: 154 159. 2. Riley RM, Powell NB, Guilleminault C. Obstructive sleep apnea syndrome: A surgical protocol for dynamic upper airway reconstruction J Oral Maxillofac Surg 1993;51:742 747. 3. Yu LF, Pogrel MA, Ajayi M. Pharyngeal airway changes associated with mandibular advancement. J Oral Maxillofac Surg 1994;52:40 43;discussion44. 4. Greco JM, Frohberg U, Van Sickels JE. Long-term airway space changes after mandibular setback using bilateral sagittal split osteotomy. Int J Oral Maxillofac Surg 1990;9:103 105. 5. Hochban W, Schurmann R, Brandenburg U, Conradt R. Mandibular setback for surgical correction of mandibular hyperplasia Does it provoke sleep-related breathing disorders? Int J Oral Maxillofac Surg 1996;25:333 338. 6. Tselnik M, Pogrel MA. Assessment of the pharyngeal airway space after mandibular setback surgery. J Oral Maxillofac Surg 2000;58:282-285;discussion285 287.

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