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Changes in natural head position after orthognathic surgery in skeletal Class III patients
ORIGINAL ARTICLE
Changes in natural head position after orthognathic surgery in skeletal Class III patients Dohyun Cho,a Dong-Soon Choi,b Insan Jang,b and Bong-Kuen Chac Gangneung, South Korea
Introduction: The purpose of this study was to evaluate the change in natural head position (NHP) after orthognathic surgery in skeletal Class III patients. Methods: We used pretreatment (T1) and posttreatment (T2) cephalometric radiographs and T1 and T2 lateral facial photographs of 20 skeletal Class III patients (mean age, 21.6 years), with 20 skeletal Class I patients (mean age, 22.2 years) as the controls. The Class III patients had undergone mandibular setback surgery, and the patients in the control group had received conventional orthodontic treatment. All lateral facial photographs were recorded in NHP. The true vertical line (TVL) was transferred from the photograph to the cephalometric radiograph, and then the angle between the TVL and the Frankfort horizontal plane (TVL/FH) was measured. A t test and a paired t test were used to verify the differences between the 2 groups, and between the T1 and T2 measurements in each group. Results: The mean TVL/FH at T1 was significantly greater in the Class III group than in the Class I group; this indicated that the Class III group showed head flexion. However, the mean TVL/FH of the Class III group decreased by 3.1 at T2; this indicated head extension, and it did not significantly differ from that of the Class I group. Nineteen of the 20 Class I patients showed minimal or no change in their TVL/FH (1.5 to 1.5 ) at T2. On the other hand, 6 of the 20 Class III patients showed more than a 4.5 decrease in their TVL/FH at T2. Conclusions: Most of the Class I patients showed minimal or no change in their NHP at T2, but some Class III patients had changes in their NHP that tended toward head extension after mandibular setback surgery. Thus, soft tissue analysis using the TVL in NHP may not be reliable for some skeletal Class III patients who undergo mandibular setback surgery. (Am J Orthod Dentofacial Orthop 2015;147:747-54)
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ince the introduction of cephalometric radiography, orthodontists have used cephalometric analysis to plan orthodontic treatment and to evaluate the treatment results. Intracranial structures have been considered stable reference landmarks. Consequently, the sella-nasion plane (SN plane) and the Frankfort horizontal plane (FH plane) have been most commonly used as horizontal reference planes. However, some authors have reported individual variations of intracranial landmarks.1 Natural head position (NHP) is a standardized and reproducible position of the head in an upright posture From Gangneung-Wonju National University, Gangneung, South Korea. a PhD student, Department of Orthodontics, College of Dentistry. b Associate professor, Department of Orthodontics, College of Dentistry. c Professor, Department of Orthodontics, College of Dentistry. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Address correspondence to: Dong-Soon Choi, Department of Orthodontics, Gangneung-Wonju National University, Dental Hospital, 120 Gangneung Daehangno, Gangneung City, Gangwon Province, South Korea 210-702; e-mail, [email protected]. Submitted, October 2013; revised and accepted, January 2015. 0889-5406/$36.00 Copyright Ó 2015 by the American Association of Orthodontists. http://dx.doi.org/10.1016/j.ajodo.2015.01.026
when the visual axis is horizontal.2 Many reports have confirmed the reproducibility of the NHP. Cooke3 reported in a 5-year longitudinal study on the reproducibility of the NHP method errors of 1.9 after 1 to 2 hours, 2.3 after 3 to 6 months, and 3.0 after 5 years. Peng and Cooke4 also reported that NHP has been shown to be remarkably reproducible (to 2.2 ) even after 15 years. In 1981, Spradley et al5 suggested that the subnasale true vertical line (TVL) may provide a useful tool for research on the soft tissue changes associated with both orthodontic and orthognathic surgical treatments. Viazis6 and Lundstr€ om and Lundstr€ om7 reported a cephalometric analysis using the true horizontal line based on NHP. Arnett and Bergman8,9 presented an organized and comprehensive approach to facial analysis using the NHP for orthodontic and surgical treatments. In 1999, Arnett et al10 presented a new soft tissue cephalometric analysis tool using the subnasale TVL for diagnosis and treatment planning for orthognathic surgery. Sugawara and Kawamura11 also used a TVL 5 mm ahead of the subnasale for soft tissue analysis for planning the surgical treatment objective. However, Vig et al12 suggested that orthognathic surgery most likely shifts the center of mass of the 747
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head, thereby affecting head position. Arnett and McLaughlin13 recommended adjusting the head position during NHP registration because patients with Class II and Class III facial types tend to compensate for their head position. However, evidence of this is lacking, and there is no information on how much adjustment is needed. There have been a few studies on the changes in NHP after orthognathic surgery. In the studies of Wenzel et al14 and Achilleos et al,15 the surgical setback of the mandible was shown to have decreased the nasopharyngeal airway space and increased the craniocervical angulation. However, there was no information on the TVL change. In 1991, Phillips et al16 reported no significant change in NHP a year after 1-jaw surgery (mandibular setback) or 2-jaw surgery (maxillary intrusion and mandibular setback). However, their study had no control group, and the ear rods and the forehead positioner in the cephalostat might have affected the real NHP of the patients. In 2005, Savjani et al17 found no significant change in NHP after orthognathic surgery. However, their study focused only on the vertical changes in the face after orthognathic surgery. Kim18 reported no change in NHP immediately after orthognathic surgery, but there was no information on the classifications of the malocclusions, types of surgery, or a control group. The literature review did not clarify whether there had been any changes in NHP after orthognathic surgery in Class III patients. If NHP changes after mandibular setback surgery, soft tissue analysis using TVL for the surgical treatment objective may yield an unsatisfactory facial profile. The purpose of this study was to evaluate the change in NHP after orthognathic surgery in skeletal Class III patients. MATERIAL AND METHODS
In this retrospective study, we used pretreatment (T1) and posttreatment (T2) lateral cephalometric radiographs and T1 and T2 lateral facial photographs. The T1 lateral cephalometric radiographs and facial photographs were taken at the beginning of the treatment, and the T2 data were recorded immediately after the removal of the orthodontic appliances. Twenty subjects in the Class III group (11 male, 9 female; age range, 15.8-41.5 years; mean age, 21.6 6 5.7 years) were consecutively selected from the skeletal Class III patients who had undergone mandibular setback surgery from 2005 to 2012 at Gangneung-Wonju National University Dental Hospital in Korea, and had their T1 and T2 lateral facial photographs taken in NHP. Twenty subjects for the control group were selected from skeletal Class I patients (9 male, 11 females; age range, 16.7-37.3 years; mean
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Table I. Sample description Skeletal Class III (n 5 20) Age (y) Treatment duration (y) ANB ( ) Frankfort-mandibular plane angle ( )
Mean 21.6 2.6 2.2 29.8
SD 5.7 1.1 3.1 5.9
Control (n 5 20) Mean 22.3 2.2 2.3 28.3
SD 4.6 0.5 0.9 4.8
age, 22.2 6 4.5 years; ANB angle range, 0.4 -4.0 ) who had received conventional orthodontic treatment, and had their T1 and T2 lateral facial photographs taken in NHP. The exclusion criteria for the control group were (1) temporomandibular disorder, (2) tonsillar hypertrophy, (3) mouth breathing, and (4) anterior open bite or crossbite. The sample is described in Table I. Eight patients in the control group were treated without extractions, and 12 were treated with 4 premolar extractions. Seven patients in the Class III group were treated without extractions; 8 were treated with 2 maxillary premolar extractions; 2 were treated with 1 unilateral maxillary premolar extraction; 2 were treated with 4 premolar extractions; and 1 patient was congenitally missing 2 maxillary lateral incisors. The mean duration of their postsurgical orthodontic treatment after their mandibular setback surgery was 10.4 months in the Class III group. The protocols of this study were approved by the ethics committee (number 2010-1-3) of Gangneung-Wonju National University Dental Hospital. The patients were instructed to stand in their natural position and to stare at their own eyes reflected in a 60 3 90 cm mirror at a distance of 1 m so that their lateral facial photographs in NHP could be taken. They were instructed to maintain their habitual occlusion with maximum intercuspation and to relax their lips while their lateral facial photographs were taken. A metal chain with a pendulum was attached to the background to show the true vertical plane in the facial photograph. The angle formed by the metal chain and the E-line (a line through the nasal tip and soft tissue pogonion) was measured in the facial photograph (Fig 1). This angle was transferred to the cephalometric tracing to draw the TVL.19 Lateral cephalometric radiographs (CX-90SP; Asahi Roentgen Ind, Kyoto, Japan) of all the patients were taken in the patients' habitual occlusion and with relaxed lips, with the same instructions as those when their lateral facial photographs were shot. Nineteen landmarks and 6 reference planes were traced on the cephalometric radiographs using acetate paper and 0.5-mm pencils. The SN and FH planes of the T1 cephalograms were transferred to the T2 cephalograms with
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Fig 2. Cephalometric reference planes for the linear and angular measurements: TVL, a true vertical line through 5 mm ahead of subnasale (Sn); SN, sella-nasion line; FH, Frankfort horizontal plane; SVL, a perpendicular line to the FH plane through sella. Fig 1. The metal chain represented the true vertical plane. The angle formed by the metal chain and the Eline was measured on the lateral facial photograph and transferred to the cephalometric tracing to draw the TVL.
cranial-base superimpositions. Then in each T1 and T2 tracing, 7 linear and 4 angular measurements were made using Quick Ceph Studio software (Quick Ceph Systems, San Diego, Calif), and 1 linear measurement and 3 angular measurements were manually made with a protractor and a digital caliper. The linear measurements were calibrated by considering the magnification ratio of the radiographs (110%). All measurements and analyses were performed by 1 examiner (D.C.). The measurement points and reference planes used in this study are shown in Figure 2. The TVL was drawn to pass 5 mm ahead of subnasale. The angles between the TVL and SN plane (TVL/SN) and between the TVL and FH plane (TVL/FH) were evaluated to investigate the changes in NHP. In this study, the flexion and extension of the head in NHP were defined as an increase and a decrease in the TVL/SN and the TVL/FH, respectively. The distance from pogonion to a line perpendicular to the FH plane through sella (SVL) was measured to evaluate the amount of setback in the surgery group. One examiner (D.C.) measured 10 randomly selected cephalometric radiographs and facial photographs, and repeated the TVL transfer and the cephalometric
measurements 2 weeks later to determine the method errors of the cephalometric measurements. The method errors wereqcalculated using Dahlberg's formula20 (method ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P 2 error 5 d = 2n , where d is the difference between the 2 measurements of a pair, and n is the number of samples). The maximum method errors of the measurements were 0.4 mm in mandibular length and 0.4 in TVL/FH. The intraexaminer correlation coefficients of the repeated measurements were greater than 0.963 (TVL-A0 ). Statistical analysis
The normal distribution of the measurements was confirmed with the Shapiro-Wilk test (P .0.05). The equality of the variances was checked using the Levene test. An independent t test was used to verify the difference between the mean values of the 2 groups. A paired t test was used to determine the differences between the T1 and T2 measurements in each group. Statistical significance was determined at the 5% level of confidence. All statistical analyses were performed with software (version 18.0; SPSS, Chicago, Ill). RESULTS
The T1 measurements between the groups are compared in Table II. The skeletal Class III group showed
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Table II. Comparison of the measurements at pretreatment (T1) between the groups Skeletal Class III Variable TVL/SN ( ) TVL/FH ( ) SNA ( ) SNB ( ) ANB ( ) Maxillary length (mm) Mandibular length (mm) Pogonion to SVL (mm) Frankfort-mandibular plane angle ( ) Lower anterior face height (mm) Overjet (mm) Overbite (mm) Upper lip to E-line (mm) Lower lip to E-line (mm) Facial contour angle (G0 -Sn-Pog0 ) ( )
Mean 83.1 91.8 80.1 82.4 2.2 82.4 120.9 66.0 29.8 72.1 1.3 0.2 3.3 0.9 1.1
SD 3.6 3.6 2.2 4.2 3.1 4.6 7.8 9.2 5.9 5.1 3.0 2.7 2.8 2.3 6.8
Control Mean 79.5 88.3 80.5 78.2 2.3 83.9 113.4 57.2 28.3 70.7 3.4 1.0 0.9 3.0 9.9
Class III vs Control SD 3.5 3.8 3.2 3.2 0.9 4.2 6.2 6.0 4.8 5.3 1.7 1.5 1.9 2.8 3.6
P value 0.002 0.005 0.646 0.001 0.000 0.296 0.002 0.001 0.373 0.419 0.000 0.261 0.000 0.014 0.000
Significance y y
NS y z
NS y z
NS NS z
NS z
* z
t test: *P \0.05; yP \0.01; zP \0.001. NS, Not significant.
Table III. Comparison of the changes after treatment (T2-T1) between the groups Skeletal Class III Variable TVL/SN ( ) TVL/FH ( ) SNA ( ) SNB ( ) ANB ( ) Maxillary length (mm) Mandibular length (mm) Pogonion to SVL (mm) Frankfort-mandibular plane angle ( ) Lower anterior face height (mm) Overjet (mm) Overbite (mm) Upper lip to E-line (mm) Lower lip to E-line (mm) Facial contour angle (G0 -Sn-Pog0 ) ( )
Mean 3.1 3.1 0.9 3.2 4.1 0.9 4.8 5.7 3.2 0.8 4.8 1.5 2.4 0.4 8.1
SD 4.1 4.2 0.9 2.2 2.0 1.5 3.9 4.9 3.2 3.0 3.0 2.8 1.8 1.8 4.6
P value* 0.003 0.004 0.000 0.000 0.000 0.013 0.000 0.000 0.000 0.268 0.000 0.032 0.000 0.315 0.000
Control Mean 0.3 0.3 0.2 0.5 0.3 0.2 0.2 0.4 0.1 0.5 0.0 1.3 0.8 1.6 0.5
SD 0.8 0.9 1.4 0.8 1.2 1.1 1.2 1.4 1.1 1.0 2.0 1.7 1.5 2.0 1.5
Class III vs Control P value* 0.104 0.150 0.515 0.012 0.292 0.404 0.571 0.238 0.711 0.039 0.992 0.003 0.027 0.002 0.195
P valuey 0.007z 0.008z 0.005 0.000 0.000 0.102 0.000 0.000 0.000 0.083z 0.000 0.793 0.000 0.053 0.000
Significance § § § k k
NS k k k
NS k
NS k
NS k
*Paired t test; yt test; zt test assuming unequal variances was used; §P \0.01; kP \0.001. NS, Not significant.
mandibular prognathism, concave profiles, and reverse overjets, unlike the control group. The mean TVL/SN and TVL/FH measurements were 83.1 and 91.8 , respectively, in the skeletal Class III group, and both were significantly greater than those in the control group (TVL/SN, 79.5 ; TVL/FH, 88.3 ). Table III compares the changes after treatment between the groups. As a result of the orthognathic surgery, the SNB angle and pogonion to SVL decreased to 3.2 and 5.7 mm, respectively, and the ANB angle and overjet increased to 4.1 and 4.8 mm, respectively, in
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the skeletal Class III group. The TVL/SN and TVL/FH angles significantly decreased by 3.1 in the Class III group, but there were no changes in the control group. The changes in the TVL/SN and TVL/FH also significantly differed between the groups. Figure 3 shows the distribution of the patients according to the changes in their TVL/FH measurements after treatment. Most Class I patients (19 of 20 subjects) had minimal or no change in their TVL/FH. On the other hand, 6 of the 20 Class III patients (30%) showed more than a 4.5 head extension.
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Fig 3. Distribution of the Class III and Class I patients according to the changes in their TVL/FH angles after treatment. The Class III subjects tended to show head extension, and the Class I subjects showed minimal or no change in their TVL/FH.
Table IV. Comparison of the measurements at posttreatment (T2) between the groups Skeletal Class III Variable TVL/SN ( ) TVL/FH ( ) SNA ( ) SNB ( ) ANB ( ) Maxillary length (mm) Mandibular length (mm) Pogonion to SVL (mm) Frankfort-mandibular plane angle ( ) Lower anterior face height (mm) Overjet (mm) Overbite (mm) Upper lip to E-line (mm) Lower lip to E-line (mm) Facial contour angle (G0 -Sn-Pog0 ) ( )
Mean 80.0 88.7 81.0 79.1 1.9 83.3 116.1 60.3 33.0 71.3 3.5 1.7 0.9 0.5 9.2
Control
SD 3.8 3.6 2.3 3.1 2.1 4.3 5.6 5.8 4.9 4.5 0.7 1.1 2.3 2.6 4.6
Mean 79.2 88.0 80.3 77.7 2.6 84.1 113.2 57.5 28.3 71.2 3.4 2.3 0.1 1.4 10.4
Class III vs Control SD 3.4 3.8 3.6 3.2 1.4 4.0 6.2 5.7 4.6 5.1 1.0 1.2 1.9 3.0 3.7
P value 0.456 0.550 0.475 0.156 0.206 0.553 0.136 0.140 0.004 0.964 0.680 0.112 0.133 0.325 0.365
Significance NS NS NS NS NS NS NS NS * NS NS NS NS NS NS
t test: *P \0.01. NS, Not significant.
Table IV compares the T2 results of the groups. After the orthodontic treatment and the orthognathic surgery, the skeletal Class III group showed no significant differences in its dental and skeletal measurements compared with the control group, except for the Frankfort-mandibular plane angle. The TVL/SN angles at posttreatment were 80.0 in the skeletal Class III
group and 79.2 in the control group, and the TVL/FH angles were 88.7 in the Class III group and 88.0 in the control group. The TVL/SN and TVL/FH angles did not differ significantly between the groups. Because the TVL changed after the orthognathic surgery in the Class III group, the soft tissue analysis using the TVL at T1 was compared with the soft tissue analysis
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Table V. Differences of the soft tissue analysis with the same T2 tracings between using TVL at T1 and using TVL at T2 Using TVL at T1 Distance (mm) TVL to A0 TVL to UL TVL to LL TVL to B0 TVL to Pog0
Mean 4.9 0.8 3.3 10.6 11.0
SD 0.3 2.0 2.5 3.5 4.9
Using TVL at T2 Mean 4.8 0.3 1.7 8.1 7.8
SD 0.2 2.3 3.1 4.5 6.0
Differences Mean 0.1 1.1 1.7 2.5 3.3
SD 0.2 1.3 2.1 2.9 3.8
P value 0.020 0.002 0.002 0.001 0.001
Significance * y y y y
Paired t test: *P \0.05; yP \0.01. A0 , Soft tissue A-point; UL, upper lip; LL, lower lip; B0 , soft tissue B-point; Pog0 , soft tissue pogonion.
using the TVL at T2 (Table V). The results showed significant differences. The greatest difference was in the distance from the TVL to soft tissue pogonion (3.3 mm), followed by the distance from the TVL to soft tissue B-point (2.5 mm), the TVL to the lower lip (1.7 mm), the TVL to the upper lip (1.1 mm), and the TVL to soft tissue A-point (0.1 mm). DISCUSSION
As shown in Table III, the patients who had undergone orthognathic surgery showed significant changes in their NHP. Our findings differed from those of Phillips et al.16 This divergence might have been due to the differences in the NHP registration method in the cephalometric analysis. In the study of Phillips et al, some cephalograms were taken with the subjects in NHP while they were seated, whereas others were taken with a fluid level (spirit level) attached to the temporal crest area of the subject before the cephalograms were taken.21 In our study, however, all NHP measurements were recorded in lateral facial photographs with the subjects standing, with no interference of ear rods and forehead support in the cephalostats. Then the TVL measurements on the lateral facial photographs were transferred to the cephalometric radiographs. This indirect recording method7,19,22 can provide a more precise NHP registration than direct recording of NHP on the cephalogram, and it is much simpler than the method that uses the fluid level. However, this indirect NHP registration could have errors if the strain of the mentalis muscle differs while the photographs and the cephalogram images are being taken. In this study, the skeletal Class III patients showed a mean head extension of 3.1 at 10.4 months after their mandibular setback surgery. It is well known that surgical setback of the mandible decreases the pharyngeal airway space,14,15,23 and mouth breathers tend to extend their heads to expand their airway.24,25 Therefore, the head extension observed in the setback surgery group can be explained by the changes in the airway space, although the mean amount of the
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mandibular setback was small (5.7 mm from pogonion to SVL). The study of Vig et al12 is interesting because the total nasal obstruction was found to have resulted in progressive extension of the head, whereas removal of the nasal obstruction resulted in return of the head position to its baseline values. However, Solow et al26 reported no significant correlation between the obstructed airway and the head position (TVL/FH). Achilleos et al15 also reported no changes in the TVL/FH at 6 months and at 3 years after mandibular setback surgery, whereas Solow et al26 found a strong relationship between an obstructed airway and an increase in the craniocervical angulation (ie, an increase in the angle formed by the nasion-sella line and the cervical vertebrae tangent) and the forward inclination of the cervical column. Achilleos et al15 also observed an increase in the craniocervical angulation in patients who had undergone mandibular setback osteotomy, and they explained it as a compensatory mechanism for maintaining the airway space. We did not investigate the craniocervical angulation because our main focus of interest was not the airway dimensions but the TVL for the soft tissue analysis. An investigation of airway adequacy, such as pharyngeal space, craniocervical angulation, and hyoid bone position, might show more interesting results. In future studies, the inclinometer method,27 which uses an eyeglass with inclinometers to measure and transfer NHP, may be more useful in clarifying the association between airway dimensions and changes in the head position or the inclinations of patients who had undergone orthognathic surgery. Another explanation may be a change in the neuromuscular balance around the head and neck. The posterior cervical extensor muscles balance with the anterior muscle group, which includes the suprahyoid, infrahyoid, and platysma muscles.28 Valk et al29 reported that the distance between menton and the hyoid bone decreased in the mandibular setback group; consequently, the length and the tension of the suprahyoid and the infrahyoid muscles decreased as well. Therefore,
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Fig 4. Superimpositions of the pretreatment (solid lines) and posttreatment (dotted lines) cephalometric tracings: A, superimposition at nasion using the intracranial reference line (SN line); B, superimposition at nasion using the extracranial reference line (TVL), which shows an average 3.1 of head extension.
the extension of the head position may be explained by the mechanism for creating a new balance between the anterior and posterior muscles. Some authors have suggested a correlation among head position, gravity, and psychosocial factors. Vig et al12 suggested that mandibular setback orthognathic surgery can cause backward relocation of the center of mass of the head that can influence head position, although their experiment did not provide satisfactory evidence. According to the literature review of Hunt et al,30 orthognathic patients experience psychosocial benefits as a result of orthognathic surgery, including improved self-confidence, body and facial images, and social adjustment. Wenzel et al14 mentioned the psychosocial factors that stimulate patients to lift their heads; this reinforces the changes in head position from the biologic changes. In this study, airway spaces, muscle activities, and psychosocial factors were not assessed. In the future, 3-dimensional assessments of airway spaces, electromyography of the posterior and anterior muscles around the head and neck, and assessments of the psychosocial factors will be needed to identify the factors that affect head position. It is interesting that the skeletal Class III patients with mandibular prognathism had a greater TVL/FH angle (91.8 ) than did the control group with a normal skeletal pattern (88.3 ) (Table II). The prognathic facial profile tends to be masked by the flexion of the head, which reduces the facial concavity by decreasing the prominence
of the chin. However, as shown in Table IV, the TVL/FH of the patients in the Class III group, whose prognathic chin and dental malocclusion were normalized by orthognathic surgery, became similar to that of the control group. This indicates that orthognathic surgery for patients with mandibular prognathism can simultaneously correct the biologic and psychologic compensations for the skeletal anomaly, even while improving the skeletal and dental relationship and the facial esthetics. However, changes in head position after orthognathic surgery cause changes in the reference plane based on NHP; these yield different outcomes, unlike the presurgical treatment objective planned by the TVL analysis. The greatest difference was found at soft tissue pogonion (TVL-Pog0 , 3.3 mm). Therefore, as shown in Figure 4, the prognathic facial profile can remain in some patients, even if the orthognathic surgery is performed as planned. According to Halazonetis,31 natural head orientation, a subjective position oriented by the orthodontist, was also influenced by facial morphology; therefore, the use of natural head orientation would result in underestimation of mandibular prognathism. Thus, it seems reasonable for the NHP of the patient to be adjusted so that it would be extended during the pretreatment NHP registration, as Arnett and McLaughlin13 suggested. Simply put, a 3.1 head extension at the pretreatment NHP recording or an adjustment of the TVL may be recommended according to our results (Table III), but there is a limitation in this mean value for clinical use because of the large interindividual
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variations (Fig 3). Further studies on the factors related to NHP are needed to predict the change in NHP after orthognathic surgery. CONCLUSIONS
1. Most of the Class I patients who had received conventional orthodontic treatment showed a minimal or no change in their NHP, but some Class III patients who had undergone mandibular setback surgery showed a change in their NHP, which tended toward head extension. 2. The soft tissue analysis with the TVL in NHP might not be reliable for some skeletal Class III patients who undergo mandibular setback surgery. REFERENCES 1. Jacobson A. The “Wits” appraisal of jaw disharmony. Am J Orthod 1975;67:125-38. 2. Moorrees CFA. Natural head position—a revival. Am J Orthod Dentofacial Orthop 1994;105:512-3. 3. Cooke MS. Five-year reproducibility of natural head posture: a longitudinal study. Am J Orthod Dentofacial Orthop 1990;97: 489-94. 4. Peng L, Cooke MS. Fifteen-year reproducibility of natural head posture: a longitudinal study. Am J Orthod Dentofacial Orthop 1999;116:82-5. 5. Spradley FL, Jacobs JD, Crowe DP. Assessment of the anteroposterior soft-tissue contour of the lower facial third in the ideal young adult. Am J Orthod 1981;79:316-25. 6. Viazis AD. A cephalometric analysis based on natural head position. J Clin Orthod 1991;25:172-81. 7. Lundstr€ om F, Lundstr€om A. Natural head position as a basis for cephalometric analysis. Am J Orthod Dentofacial Orthop 1992; 101:244-7. 8. Arnett GW, Bergman RT. Facial keys to orthodontic diagnosis and treatment planning. Part I. Am J Orthod Dentofacial Orthop 1993; 103:299-312. 9. Arnett GW, Bergman RT. Facial keys to orthodontic diagnosis and treatment planning—part II. Am J Orthod Dentofacial Orthop 1993;103:395-411. 10. Arnett GW, Jelic JS, Kim J, Cummings DR, Beress A, Worley CM Jr, et al. Soft tissue cephalometric analysis: diagnosis and treatment planning of dentofacial deformity. Am J Orthod Dentofacial Orthop 1999;116:239-53. 11. Sugawara J, Kawamura H. Principle and practice of contemporary surgical orthodontics. Osaka, Japan: Tokyo Rinsho Shuppan; 2000. 12. Vig PS, Rink JF, Showfety KJ. Adaptation of head posture in response to relocating the center of mass: a pilot study. Am J Orthod 1983;83:138-42.
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13. Arnett GW, McLaughlin RP. Facial and dental planning for orthodontists and oral surgeons. St Louis: Mosby; 2004. p. 95-6. 14. Wenzel A, Williams S, Ritzau M. Changes in head posture and nasopharyngeal airway following surgical correction of mandibular prognathism. Eur J Orthod 1989;11:37-42. 15. Achilleos S, Krogstad O, Lyberg T. Surgical mandibular setback and changes in uvuloglossopharyngeal morphology and head posture: a short- and long-term cephalometric study in males. Eur J Orthod 2000;22:383-94. 16. Phillips C, Snow MD, Turvey TA, Proffit WR. The effect of orthognathic surgery on head posture. Eur J Orthod 1991;13:397-403. 17. Savjani D, Wertheim D, Edler R. Change in cranio-cervical angulation following orthognathic surgery. Eur J Orthod 2005;27:268-73. 18. Kim DY. A study on the reproducibility on natural head position before and after orthognathic surgery [thesis]. Chuncheon, South Korea: Hallym University; 2011. 19. Leitao P, Nanda RS. Relationship of natural head position to craniofacial morphology. Am J Orthod Dentofacial Orthop 2000; 117:406-17. 20. Dahlberg G. Statistical methods for medical and biological students. New York: Interscience Publications; 1940. 21. Showfety KJ, Vig PS, Matteson S. A simple method for taking natural-head-position cephalograms. Am J Orthod 1983;83: 495-500. 22. Lundstr€om F, Lundstr€ om A. Clinical evaluation of maxillary and mandibular prognathism. Eur J Orthod 1989;11:408-13. 23. Guilleminault C, Riley R, Powell N. Sleep apnea in normal subjects following mandibular osteotomy with retrusion. Chest 1985;88: 776-8. 24. Linder-Aronson S. Respiratory function in relation to facial morphology and the dentition. Br J Orthod 1979;6:59-71. 25. Ricketts RM. Forum on the tonsil and adenoid problem in orthodontics. Respiratory obstruction syndrome. Am J Orthod 1968; 54:495-507. 26. Solow B, Slersbaek-Nlelsen S, Greve E. Airway adequacy, head posture, and craniofacial morphology. Am J Orthod 1984;86:214-23. 27. Malkoc S, Usumez S, Nur M, Donaghy CE. Reproducibility of airway dimensions and tongue and hyoid positions on lateral cephalograms. Am J Orthod Dentofacial Orthop 2005;128:513-6. 28. Preston B. The upper airway and cranial morphology. In: Graber LW, Vanarsdall RL Jr., Vig KW, editors. Orthodontics: current principles and techniques. 5th ed. St Louis: Mosby; 2011. p. 162-5. 29. Valk JW, Zonnenberg AJJ, van Maanen CJ, van Wonderen OGT. The biomechanical effects of a sagittal split ramus osteotomy on the relationship of the mandible, the hyoid bone, and the cervical spine. Am J Orthod Dentofacial Orthop 1992;102:99-108. 30. Hunt OT, Johnston CD, Hepper PG, Burden DJ. The psychosocial impact of orthognathic surgery: a systematic review. Am J Orthod Dentofacial Orthop 2001;120:490-7. 31. Halazonetis DJ. Estimated natural head position and facial morphology. Am J Orthod Dentofacial Orthop 2002;121:364-8.
American Journal of Orthodontics and Dentofacial Orthopedics
Changes in natural head position after orthognathic surgery in skeletal Class III patients Dohyun Cho,a Dong-Soon Choi,b Insan Jang,b and Bong-Kuen Chac Gangneung, South Korea
Introduction: The purpose of this study was to evaluate the change in natural head position (NHP) after orthognathic surgery in skeletal Class III patients. Methods: We used pretreatment (T1) and posttreatment (T2) cephalometric radiographs and T1 and T2 lateral facial photographs of 20 skeletal Class III patients (mean age, 21.6 years), with 20 skeletal Class I patients (mean age, 22.2 years) as the controls. The Class III patients had undergone mandibular setback surgery, and the patients in the control group had received conventional orthodontic treatment. All lateral facial photographs were recorded in NHP. The true vertical line (TVL) was transferred from the photograph to the cephalometric radiograph, and then the angle between the TVL and the Frankfort horizontal plane (TVL/FH) was measured. A t test and a paired t test were used to verify the differences between the 2 groups, and between the T1 and T2 measurements in each group. Results: The mean TVL/FH at T1 was significantly greater in the Class III group than in the Class I group; this indicated that the Class III group showed head flexion. However, the mean TVL/FH of the Class III group decreased by 3.1 at T2; this indicated head extension, and it did not significantly differ from that of the Class I group. Nineteen of the 20 Class I patients showed minimal or no change in their TVL/FH (1.5 to 1.5 ) at T2. On the other hand, 6 of the 20 Class III patients showed more than a 4.5 decrease in their TVL/FH at T2. Conclusions: Most of the Class I patients showed minimal or no change in their NHP at T2, but some Class III patients had changes in their NHP that tended toward head extension after mandibular setback surgery. Thus, soft tissue analysis using the TVL in NHP may not be reliable for some skeletal Class III patients who undergo mandibular setback surgery. (Am J Orthod Dentofacial Orthop 2015;147:747-54)
S
ince the introduction of cephalometric radiography, orthodontists have used cephalometric analysis to plan orthodontic treatment and to evaluate the treatment results. Intracranial structures have been considered stable reference landmarks. Consequently, the sella-nasion plane (SN plane) and the Frankfort horizontal plane (FH plane) have been most commonly used as horizontal reference planes. However, some authors have reported individual variations of intracranial landmarks.1 Natural head position (NHP) is a standardized and reproducible position of the head in an upright posture From Gangneung-Wonju National University, Gangneung, South Korea. a PhD student, Department of Orthodontics, College of Dentistry. b Associate professor, Department of Orthodontics, College of Dentistry. c Professor, Department of Orthodontics, College of Dentistry. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Address correspondence to: Dong-Soon Choi, Department of Orthodontics, Gangneung-Wonju National University, Dental Hospital, 120 Gangneung Daehangno, Gangneung City, Gangwon Province, South Korea 210-702; e-mail, [email protected]. Submitted, October 2013; revised and accepted, January 2015. 0889-5406/$36.00 Copyright Ó 2015 by the American Association of Orthodontists. http://dx.doi.org/10.1016/j.ajodo.2015.01.026
when the visual axis is horizontal.2 Many reports have confirmed the reproducibility of the NHP. Cooke3 reported in a 5-year longitudinal study on the reproducibility of the NHP method errors of 1.9 after 1 to 2 hours, 2.3 after 3 to 6 months, and 3.0 after 5 years. Peng and Cooke4 also reported that NHP has been shown to be remarkably reproducible (to 2.2 ) even after 15 years. In 1981, Spradley et al5 suggested that the subnasale true vertical line (TVL) may provide a useful tool for research on the soft tissue changes associated with both orthodontic and orthognathic surgical treatments. Viazis6 and Lundstr€ om and Lundstr€ om7 reported a cephalometric analysis using the true horizontal line based on NHP. Arnett and Bergman8,9 presented an organized and comprehensive approach to facial analysis using the NHP for orthodontic and surgical treatments. In 1999, Arnett et al10 presented a new soft tissue cephalometric analysis tool using the subnasale TVL for diagnosis and treatment planning for orthognathic surgery. Sugawara and Kawamura11 also used a TVL 5 mm ahead of the subnasale for soft tissue analysis for planning the surgical treatment objective. However, Vig et al12 suggested that orthognathic surgery most likely shifts the center of mass of the 747
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head, thereby affecting head position. Arnett and McLaughlin13 recommended adjusting the head position during NHP registration because patients with Class II and Class III facial types tend to compensate for their head position. However, evidence of this is lacking, and there is no information on how much adjustment is needed. There have been a few studies on the changes in NHP after orthognathic surgery. In the studies of Wenzel et al14 and Achilleos et al,15 the surgical setback of the mandible was shown to have decreased the nasopharyngeal airway space and increased the craniocervical angulation. However, there was no information on the TVL change. In 1991, Phillips et al16 reported no significant change in NHP a year after 1-jaw surgery (mandibular setback) or 2-jaw surgery (maxillary intrusion and mandibular setback). However, their study had no control group, and the ear rods and the forehead positioner in the cephalostat might have affected the real NHP of the patients. In 2005, Savjani et al17 found no significant change in NHP after orthognathic surgery. However, their study focused only on the vertical changes in the face after orthognathic surgery. Kim18 reported no change in NHP immediately after orthognathic surgery, but there was no information on the classifications of the malocclusions, types of surgery, or a control group. The literature review did not clarify whether there had been any changes in NHP after orthognathic surgery in Class III patients. If NHP changes after mandibular setback surgery, soft tissue analysis using TVL for the surgical treatment objective may yield an unsatisfactory facial profile. The purpose of this study was to evaluate the change in NHP after orthognathic surgery in skeletal Class III patients. MATERIAL AND METHODS
In this retrospective study, we used pretreatment (T1) and posttreatment (T2) lateral cephalometric radiographs and T1 and T2 lateral facial photographs. The T1 lateral cephalometric radiographs and facial photographs were taken at the beginning of the treatment, and the T2 data were recorded immediately after the removal of the orthodontic appliances. Twenty subjects in the Class III group (11 male, 9 female; age range, 15.8-41.5 years; mean age, 21.6 6 5.7 years) were consecutively selected from the skeletal Class III patients who had undergone mandibular setback surgery from 2005 to 2012 at Gangneung-Wonju National University Dental Hospital in Korea, and had their T1 and T2 lateral facial photographs taken in NHP. Twenty subjects for the control group were selected from skeletal Class I patients (9 male, 11 females; age range, 16.7-37.3 years; mean
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Table I. Sample description Skeletal Class III (n 5 20) Age (y) Treatment duration (y) ANB ( ) Frankfort-mandibular plane angle ( )
Mean 21.6 2.6 2.2 29.8
SD 5.7 1.1 3.1 5.9
Control (n 5 20) Mean 22.3 2.2 2.3 28.3
SD 4.6 0.5 0.9 4.8
age, 22.2 6 4.5 years; ANB angle range, 0.4 -4.0 ) who had received conventional orthodontic treatment, and had their T1 and T2 lateral facial photographs taken in NHP. The exclusion criteria for the control group were (1) temporomandibular disorder, (2) tonsillar hypertrophy, (3) mouth breathing, and (4) anterior open bite or crossbite. The sample is described in Table I. Eight patients in the control group were treated without extractions, and 12 were treated with 4 premolar extractions. Seven patients in the Class III group were treated without extractions; 8 were treated with 2 maxillary premolar extractions; 2 were treated with 1 unilateral maxillary premolar extraction; 2 were treated with 4 premolar extractions; and 1 patient was congenitally missing 2 maxillary lateral incisors. The mean duration of their postsurgical orthodontic treatment after their mandibular setback surgery was 10.4 months in the Class III group. The protocols of this study were approved by the ethics committee (number 2010-1-3) of Gangneung-Wonju National University Dental Hospital. The patients were instructed to stand in their natural position and to stare at their own eyes reflected in a 60 3 90 cm mirror at a distance of 1 m so that their lateral facial photographs in NHP could be taken. They were instructed to maintain their habitual occlusion with maximum intercuspation and to relax their lips while their lateral facial photographs were taken. A metal chain with a pendulum was attached to the background to show the true vertical plane in the facial photograph. The angle formed by the metal chain and the E-line (a line through the nasal tip and soft tissue pogonion) was measured in the facial photograph (Fig 1). This angle was transferred to the cephalometric tracing to draw the TVL.19 Lateral cephalometric radiographs (CX-90SP; Asahi Roentgen Ind, Kyoto, Japan) of all the patients were taken in the patients' habitual occlusion and with relaxed lips, with the same instructions as those when their lateral facial photographs were shot. Nineteen landmarks and 6 reference planes were traced on the cephalometric radiographs using acetate paper and 0.5-mm pencils. The SN and FH planes of the T1 cephalograms were transferred to the T2 cephalograms with
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Fig 2. Cephalometric reference planes for the linear and angular measurements: TVL, a true vertical line through 5 mm ahead of subnasale (Sn); SN, sella-nasion line; FH, Frankfort horizontal plane; SVL, a perpendicular line to the FH plane through sella. Fig 1. The metal chain represented the true vertical plane. The angle formed by the metal chain and the Eline was measured on the lateral facial photograph and transferred to the cephalometric tracing to draw the TVL.
cranial-base superimpositions. Then in each T1 and T2 tracing, 7 linear and 4 angular measurements were made using Quick Ceph Studio software (Quick Ceph Systems, San Diego, Calif), and 1 linear measurement and 3 angular measurements were manually made with a protractor and a digital caliper. The linear measurements were calibrated by considering the magnification ratio of the radiographs (110%). All measurements and analyses were performed by 1 examiner (D.C.). The measurement points and reference planes used in this study are shown in Figure 2. The TVL was drawn to pass 5 mm ahead of subnasale. The angles between the TVL and SN plane (TVL/SN) and between the TVL and FH plane (TVL/FH) were evaluated to investigate the changes in NHP. In this study, the flexion and extension of the head in NHP were defined as an increase and a decrease in the TVL/SN and the TVL/FH, respectively. The distance from pogonion to a line perpendicular to the FH plane through sella (SVL) was measured to evaluate the amount of setback in the surgery group. One examiner (D.C.) measured 10 randomly selected cephalometric radiographs and facial photographs, and repeated the TVL transfer and the cephalometric
measurements 2 weeks later to determine the method errors of the cephalometric measurements. The method errors wereqcalculated using Dahlberg's formula20 (method ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P 2 error 5 d = 2n , where d is the difference between the 2 measurements of a pair, and n is the number of samples). The maximum method errors of the measurements were 0.4 mm in mandibular length and 0.4 in TVL/FH. The intraexaminer correlation coefficients of the repeated measurements were greater than 0.963 (TVL-A0 ). Statistical analysis
The normal distribution of the measurements was confirmed with the Shapiro-Wilk test (P .0.05). The equality of the variances was checked using the Levene test. An independent t test was used to verify the difference between the mean values of the 2 groups. A paired t test was used to determine the differences between the T1 and T2 measurements in each group. Statistical significance was determined at the 5% level of confidence. All statistical analyses were performed with software (version 18.0; SPSS, Chicago, Ill). RESULTS
The T1 measurements between the groups are compared in Table II. The skeletal Class III group showed
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Table II. Comparison of the measurements at pretreatment (T1) between the groups Skeletal Class III Variable TVL/SN ( ) TVL/FH ( ) SNA ( ) SNB ( ) ANB ( ) Maxillary length (mm) Mandibular length (mm) Pogonion to SVL (mm) Frankfort-mandibular plane angle ( ) Lower anterior face height (mm) Overjet (mm) Overbite (mm) Upper lip to E-line (mm) Lower lip to E-line (mm) Facial contour angle (G0 -Sn-Pog0 ) ( )
Mean 83.1 91.8 80.1 82.4 2.2 82.4 120.9 66.0 29.8 72.1 1.3 0.2 3.3 0.9 1.1
SD 3.6 3.6 2.2 4.2 3.1 4.6 7.8 9.2 5.9 5.1 3.0 2.7 2.8 2.3 6.8
Control Mean 79.5 88.3 80.5 78.2 2.3 83.9 113.4 57.2 28.3 70.7 3.4 1.0 0.9 3.0 9.9
Class III vs Control SD 3.5 3.8 3.2 3.2 0.9 4.2 6.2 6.0 4.8 5.3 1.7 1.5 1.9 2.8 3.6
P value 0.002 0.005 0.646 0.001 0.000 0.296 0.002 0.001 0.373 0.419 0.000 0.261 0.000 0.014 0.000
Significance y y
NS y z
NS y z
NS NS z
NS z
* z
t test: *P \0.05; yP \0.01; zP \0.001. NS, Not significant.
Table III. Comparison of the changes after treatment (T2-T1) between the groups Skeletal Class III Variable TVL/SN ( ) TVL/FH ( ) SNA ( ) SNB ( ) ANB ( ) Maxillary length (mm) Mandibular length (mm) Pogonion to SVL (mm) Frankfort-mandibular plane angle ( ) Lower anterior face height (mm) Overjet (mm) Overbite (mm) Upper lip to E-line (mm) Lower lip to E-line (mm) Facial contour angle (G0 -Sn-Pog0 ) ( )
Mean 3.1 3.1 0.9 3.2 4.1 0.9 4.8 5.7 3.2 0.8 4.8 1.5 2.4 0.4 8.1
SD 4.1 4.2 0.9 2.2 2.0 1.5 3.9 4.9 3.2 3.0 3.0 2.8 1.8 1.8 4.6
P value* 0.003 0.004 0.000 0.000 0.000 0.013 0.000 0.000 0.000 0.268 0.000 0.032 0.000 0.315 0.000
Control Mean 0.3 0.3 0.2 0.5 0.3 0.2 0.2 0.4 0.1 0.5 0.0 1.3 0.8 1.6 0.5
SD 0.8 0.9 1.4 0.8 1.2 1.1 1.2 1.4 1.1 1.0 2.0 1.7 1.5 2.0 1.5
Class III vs Control P value* 0.104 0.150 0.515 0.012 0.292 0.404 0.571 0.238 0.711 0.039 0.992 0.003 0.027 0.002 0.195
P valuey 0.007z 0.008z 0.005 0.000 0.000 0.102 0.000 0.000 0.000 0.083z 0.000 0.793 0.000 0.053 0.000
Significance § § § k k
NS k k k
NS k
NS k
NS k
*Paired t test; yt test; zt test assuming unequal variances was used; §P \0.01; kP \0.001. NS, Not significant.
mandibular prognathism, concave profiles, and reverse overjets, unlike the control group. The mean TVL/SN and TVL/FH measurements were 83.1 and 91.8 , respectively, in the skeletal Class III group, and both were significantly greater than those in the control group (TVL/SN, 79.5 ; TVL/FH, 88.3 ). Table III compares the changes after treatment between the groups. As a result of the orthognathic surgery, the SNB angle and pogonion to SVL decreased to 3.2 and 5.7 mm, respectively, and the ANB angle and overjet increased to 4.1 and 4.8 mm, respectively, in
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the skeletal Class III group. The TVL/SN and TVL/FH angles significantly decreased by 3.1 in the Class III group, but there were no changes in the control group. The changes in the TVL/SN and TVL/FH also significantly differed between the groups. Figure 3 shows the distribution of the patients according to the changes in their TVL/FH measurements after treatment. Most Class I patients (19 of 20 subjects) had minimal or no change in their TVL/FH. On the other hand, 6 of the 20 Class III patients (30%) showed more than a 4.5 head extension.
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Fig 3. Distribution of the Class III and Class I patients according to the changes in their TVL/FH angles after treatment. The Class III subjects tended to show head extension, and the Class I subjects showed minimal or no change in their TVL/FH.
Table IV. Comparison of the measurements at posttreatment (T2) between the groups Skeletal Class III Variable TVL/SN ( ) TVL/FH ( ) SNA ( ) SNB ( ) ANB ( ) Maxillary length (mm) Mandibular length (mm) Pogonion to SVL (mm) Frankfort-mandibular plane angle ( ) Lower anterior face height (mm) Overjet (mm) Overbite (mm) Upper lip to E-line (mm) Lower lip to E-line (mm) Facial contour angle (G0 -Sn-Pog0 ) ( )
Mean 80.0 88.7 81.0 79.1 1.9 83.3 116.1 60.3 33.0 71.3 3.5 1.7 0.9 0.5 9.2
Control
SD 3.8 3.6 2.3 3.1 2.1 4.3 5.6 5.8 4.9 4.5 0.7 1.1 2.3 2.6 4.6
Mean 79.2 88.0 80.3 77.7 2.6 84.1 113.2 57.5 28.3 71.2 3.4 2.3 0.1 1.4 10.4
Class III vs Control SD 3.4 3.8 3.6 3.2 1.4 4.0 6.2 5.7 4.6 5.1 1.0 1.2 1.9 3.0 3.7
P value 0.456 0.550 0.475 0.156 0.206 0.553 0.136 0.140 0.004 0.964 0.680 0.112 0.133 0.325 0.365
Significance NS NS NS NS NS NS NS NS * NS NS NS NS NS NS
t test: *P \0.01. NS, Not significant.
Table IV compares the T2 results of the groups. After the orthodontic treatment and the orthognathic surgery, the skeletal Class III group showed no significant differences in its dental and skeletal measurements compared with the control group, except for the Frankfort-mandibular plane angle. The TVL/SN angles at posttreatment were 80.0 in the skeletal Class III
group and 79.2 in the control group, and the TVL/FH angles were 88.7 in the Class III group and 88.0 in the control group. The TVL/SN and TVL/FH angles did not differ significantly between the groups. Because the TVL changed after the orthognathic surgery in the Class III group, the soft tissue analysis using the TVL at T1 was compared with the soft tissue analysis
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Table V. Differences of the soft tissue analysis with the same T2 tracings between using TVL at T1 and using TVL at T2 Using TVL at T1 Distance (mm) TVL to A0 TVL to UL TVL to LL TVL to B0 TVL to Pog0
Mean 4.9 0.8 3.3 10.6 11.0
SD 0.3 2.0 2.5 3.5 4.9
Using TVL at T2 Mean 4.8 0.3 1.7 8.1 7.8
SD 0.2 2.3 3.1 4.5 6.0
Differences Mean 0.1 1.1 1.7 2.5 3.3
SD 0.2 1.3 2.1 2.9 3.8
P value 0.020 0.002 0.002 0.001 0.001
Significance * y y y y
Paired t test: *P \0.05; yP \0.01. A0 , Soft tissue A-point; UL, upper lip; LL, lower lip; B0 , soft tissue B-point; Pog0 , soft tissue pogonion.
using the TVL at T2 (Table V). The results showed significant differences. The greatest difference was in the distance from the TVL to soft tissue pogonion (3.3 mm), followed by the distance from the TVL to soft tissue B-point (2.5 mm), the TVL to the lower lip (1.7 mm), the TVL to the upper lip (1.1 mm), and the TVL to soft tissue A-point (0.1 mm). DISCUSSION
As shown in Table III, the patients who had undergone orthognathic surgery showed significant changes in their NHP. Our findings differed from those of Phillips et al.16 This divergence might have been due to the differences in the NHP registration method in the cephalometric analysis. In the study of Phillips et al, some cephalograms were taken with the subjects in NHP while they were seated, whereas others were taken with a fluid level (spirit level) attached to the temporal crest area of the subject before the cephalograms were taken.21 In our study, however, all NHP measurements were recorded in lateral facial photographs with the subjects standing, with no interference of ear rods and forehead support in the cephalostats. Then the TVL measurements on the lateral facial photographs were transferred to the cephalometric radiographs. This indirect recording method7,19,22 can provide a more precise NHP registration than direct recording of NHP on the cephalogram, and it is much simpler than the method that uses the fluid level. However, this indirect NHP registration could have errors if the strain of the mentalis muscle differs while the photographs and the cephalogram images are being taken. In this study, the skeletal Class III patients showed a mean head extension of 3.1 at 10.4 months after their mandibular setback surgery. It is well known that surgical setback of the mandible decreases the pharyngeal airway space,14,15,23 and mouth breathers tend to extend their heads to expand their airway.24,25 Therefore, the head extension observed in the setback surgery group can be explained by the changes in the airway space, although the mean amount of the
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mandibular setback was small (5.7 mm from pogonion to SVL). The study of Vig et al12 is interesting because the total nasal obstruction was found to have resulted in progressive extension of the head, whereas removal of the nasal obstruction resulted in return of the head position to its baseline values. However, Solow et al26 reported no significant correlation between the obstructed airway and the head position (TVL/FH). Achilleos et al15 also reported no changes in the TVL/FH at 6 months and at 3 years after mandibular setback surgery, whereas Solow et al26 found a strong relationship between an obstructed airway and an increase in the craniocervical angulation (ie, an increase in the angle formed by the nasion-sella line and the cervical vertebrae tangent) and the forward inclination of the cervical column. Achilleos et al15 also observed an increase in the craniocervical angulation in patients who had undergone mandibular setback osteotomy, and they explained it as a compensatory mechanism for maintaining the airway space. We did not investigate the craniocervical angulation because our main focus of interest was not the airway dimensions but the TVL for the soft tissue analysis. An investigation of airway adequacy, such as pharyngeal space, craniocervical angulation, and hyoid bone position, might show more interesting results. In future studies, the inclinometer method,27 which uses an eyeglass with inclinometers to measure and transfer NHP, may be more useful in clarifying the association between airway dimensions and changes in the head position or the inclinations of patients who had undergone orthognathic surgery. Another explanation may be a change in the neuromuscular balance around the head and neck. The posterior cervical extensor muscles balance with the anterior muscle group, which includes the suprahyoid, infrahyoid, and platysma muscles.28 Valk et al29 reported that the distance between menton and the hyoid bone decreased in the mandibular setback group; consequently, the length and the tension of the suprahyoid and the infrahyoid muscles decreased as well. Therefore,
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Fig 4. Superimpositions of the pretreatment (solid lines) and posttreatment (dotted lines) cephalometric tracings: A, superimposition at nasion using the intracranial reference line (SN line); B, superimposition at nasion using the extracranial reference line (TVL), which shows an average 3.1 of head extension.
the extension of the head position may be explained by the mechanism for creating a new balance between the anterior and posterior muscles. Some authors have suggested a correlation among head position, gravity, and psychosocial factors. Vig et al12 suggested that mandibular setback orthognathic surgery can cause backward relocation of the center of mass of the head that can influence head position, although their experiment did not provide satisfactory evidence. According to the literature review of Hunt et al,30 orthognathic patients experience psychosocial benefits as a result of orthognathic surgery, including improved self-confidence, body and facial images, and social adjustment. Wenzel et al14 mentioned the psychosocial factors that stimulate patients to lift their heads; this reinforces the changes in head position from the biologic changes. In this study, airway spaces, muscle activities, and psychosocial factors were not assessed. In the future, 3-dimensional assessments of airway spaces, electromyography of the posterior and anterior muscles around the head and neck, and assessments of the psychosocial factors will be needed to identify the factors that affect head position. It is interesting that the skeletal Class III patients with mandibular prognathism had a greater TVL/FH angle (91.8 ) than did the control group with a normal skeletal pattern (88.3 ) (Table II). The prognathic facial profile tends to be masked by the flexion of the head, which reduces the facial concavity by decreasing the prominence
of the chin. However, as shown in Table IV, the TVL/FH of the patients in the Class III group, whose prognathic chin and dental malocclusion were normalized by orthognathic surgery, became similar to that of the control group. This indicates that orthognathic surgery for patients with mandibular prognathism can simultaneously correct the biologic and psychologic compensations for the skeletal anomaly, even while improving the skeletal and dental relationship and the facial esthetics. However, changes in head position after orthognathic surgery cause changes in the reference plane based on NHP; these yield different outcomes, unlike the presurgical treatment objective planned by the TVL analysis. The greatest difference was found at soft tissue pogonion (TVL-Pog0 , 3.3 mm). Therefore, as shown in Figure 4, the prognathic facial profile can remain in some patients, even if the orthognathic surgery is performed as planned. According to Halazonetis,31 natural head orientation, a subjective position oriented by the orthodontist, was also influenced by facial morphology; therefore, the use of natural head orientation would result in underestimation of mandibular prognathism. Thus, it seems reasonable for the NHP of the patient to be adjusted so that it would be extended during the pretreatment NHP registration, as Arnett and McLaughlin13 suggested. Simply put, a 3.1 head extension at the pretreatment NHP recording or an adjustment of the TVL may be recommended according to our results (Table III), but there is a limitation in this mean value for clinical use because of the large interindividual
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variations (Fig 3). Further studies on the factors related to NHP are needed to predict the change in NHP after orthognathic surgery. CONCLUSIONS
1. Most of the Class I patients who had received conventional orthodontic treatment showed a minimal or no change in their NHP, but some Class III patients who had undergone mandibular setback surgery showed a change in their NHP, which tended toward head extension. 2. The soft tissue analysis with the TVL in NHP might not be reliable for some skeletal Class III patients who undergo mandibular setback surgery. REFERENCES 1. Jacobson A. The “Wits” appraisal of jaw disharmony. Am J Orthod 1975;67:125-38. 2. Moorrees CFA. Natural head position—a revival. Am J Orthod Dentofacial Orthop 1994;105:512-3. 3. Cooke MS. Five-year reproducibility of natural head posture: a longitudinal study. Am J Orthod Dentofacial Orthop 1990;97: 489-94. 4. Peng L, Cooke MS. Fifteen-year reproducibility of natural head posture: a longitudinal study. Am J Orthod Dentofacial Orthop 1999;116:82-5. 5. Spradley FL, Jacobs JD, Crowe DP. Assessment of the anteroposterior soft-tissue contour of the lower facial third in the ideal young adult. Am J Orthod 1981;79:316-25. 6. Viazis AD. A cephalometric analysis based on natural head position. J Clin Orthod 1991;25:172-81. 7. Lundstr€ om F, Lundstr€om A. Natural head position as a basis for cephalometric analysis. Am J Orthod Dentofacial Orthop 1992; 101:244-7. 8. Arnett GW, Bergman RT. Facial keys to orthodontic diagnosis and treatment planning. Part I. Am J Orthod Dentofacial Orthop 1993; 103:299-312. 9. Arnett GW, Bergman RT. Facial keys to orthodontic diagnosis and treatment planning—part II. Am J Orthod Dentofacial Orthop 1993;103:395-411. 10. Arnett GW, Jelic JS, Kim J, Cummings DR, Beress A, Worley CM Jr, et al. Soft tissue cephalometric analysis: diagnosis and treatment planning of dentofacial deformity. Am J Orthod Dentofacial Orthop 1999;116:239-53. 11. Sugawara J, Kawamura H. Principle and practice of contemporary surgical orthodontics. Osaka, Japan: Tokyo Rinsho Shuppan; 2000. 12. Vig PS, Rink JF, Showfety KJ. Adaptation of head posture in response to relocating the center of mass: a pilot study. Am J Orthod 1983;83:138-42.
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