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Characteristics of mandibular advancement surgery dependent on lower facial height differences in study of skeletal Class II patients with long-term stability
orthodontic waves 66 (2007) 1–8
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Research paper
Characteristics of mandibular advancement surgery dependent on lower facial height differences in study of skeletal Class II patients with long-term stability Kazunori Fukui a,*, Raymond M. Sugiyama b, Ronald M. Kaminishi c, Yoshiaki Matsuyama a, Eiko Kuroda d, Toshihiko Himuro a a
Department of Oral Growth and Development, Division of Orthodontics and Dentofacial Orthopedics, Ohu University School of Dentistry, Japan b Private Orthodontic Practice in Los Alamitos, Loma Linda University, USA c Private Oral/Maxillo-facial Surgery Practice in Bellflower, University of Southern California, USA d Ohu University Dental Hospital, Japan
article info
abstract
Article history:
We studied patients who underwent mandibular advancement surgery and showed a
Received 12 September 2006
stabilized occlusal condition at 5 years after surgery. The patients were classified into 2
Received in revised form
groups [long face type (LF group: 16 cases), short face type (SF group: 10 cases)] according to
20 December 2006
immediate presurgery face type. To clarify the characteristics of the operative method
Accepted 25 December 2006
formulated based on lower facial height, morphological changes during the 5-year post-
Published on line 26 January 2007
operative period were compared between the groups.
Keywords:
two groups, though lower facial height showed a tendency to increase in group SF. The
Mandibular advancement surgery
proximal segment was advanced to the anterosuperior position in both groups.
There was no statistically significant difference in amount of advancement between the
Lower facial height
To determine postoperative stability, in the LF group, there were no statistically significant differences for lower face height. In the SF group, the condylar axis increased in the period from
Stability
immediately after to 6 months after surgery. Correlation analysis revealed that the backward rotation of the proximal segment was greater during the first 6 months, as the amount of surgical advancement of the chin was larger. As a result, the chin was advanced during surgery and the proximal segment was advanced toward the anterosuperior position in both groups. In the SF group, the characteristics of the operative design increased the lower facial height. # 2007 Elsevier Ltd and the Japanese Orthodontic Society. All rights reserved.
1.
Introduction
There are a number of reports of relapse condition in cases in which the mandible was advanced surgically [1–11]. Factors related to such relapse are considered to be amount of man-
dibular advancement [1,2], operative procedure [1–6], fixation method utilized [1,2,5], morphological problems of the mandible concerned with tipping of the mandibular plane [4], positioning of the condylar head [6–8], effects of the suprahyoid muscles [9], and preoperative orthodontic treatment [10,11].
* Corresponding author. E-mail address: [email protected] (K. Fukui). 1344-0241/$ – see front matter # 2007 Elsevier Ltd and the Japanese Orthodontic Society. All rights reserved. doi:10.1016/j.odw.2007.01.001
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orthodontic waves 66 (2007) 1–8
To obtain stability following orthognathic surgery in patients with short and long face types [3,4], maxillary osteotomy alone and that with simultaneous two-jaw surgery have been frequently employed for the long-face type, as one-jaw surgery is generally excluded from consideration due to difficulties in attaining postoperative stability. Accordingly, we speculated that the optimum operative procedures for skeletal Class II patients with a flat inferior border of the mandible and for those with a high-pitched border are different. To clarify the characteristics of the optimum operative procedures based on lower facial height, patients who underwent one-jaw advancement surgery at a clinic in southern California and showed stable occlusion at 5 years after surgery were studied. They were classified into two groups by cluster analysis of the maxillofacial morphology using longitudinal and vertical factors of the upper and lower jaws as indices, and morphological changes were followed for 5 years after surgery.
2.
Materials and methods
2.1.
Subjects
The patients were 39 adult Caucasian females who underwent a bilateral sagittal split osteotomy during the period from 1982 to 1993 at Long Beach Memorial Medical Center, in southern California. Their mean age at the time of surgery was 29 years 3 months (range, 17 years 0 months to 45 years 2 months). In all cases, the bone segment was fixed by rigid fixation using a screw with a direct interosseous wire placed between the segments at the superior border on each side [2,7]. A suprahyoid myotomy was performed concomitantly [9]. The term of intermaxillary fixation using an orthodontic appliance with a wire was a mean 8.5 weeks. Patients who did not undergo a genioplasty and demonstrated a close occlusion at 5 years after surgery were selected, while those with facial asymmetry, cleft palate, and a large number of prosthetic appliances were excluded. For our analysis, a total of 186 lateral cephalometric radiographs taken in the central occlusal position using the usual method were examined. All tracing was done by the same examiner. After setting the landmarks, measurements were obtained (Figs. 1 and 2). The radiograph examination periods were as follows: immediately before surgery (T1, preoperative 1–2 months), immediately after surgery (T2, postoperative 1–3 months), postoperative 6 months (T3, postoperative 5–7 months), postoperative 1 year (T4, postoperative 10 months to 1 year 5 months), postoperative 3 years (T5, postoperative 2 years 6 months to 3 years 4 months), and postoperative 5 years (T6, postoperative 4 years 6 months to 5 years 3 months).
3.
Measuring procedures
3.1.
Measurements
3.1.1.
Angular measurements (in degrees) (Fig. 1)
SNP: The angle formed by the S–N and N-Pog planes. SNA: The angle formed by the S–N and N–A planes.
Fig. 1 – Cephalometric angular measurements. (1) SNP; (2) SNA; (3) SNB; (4) ANB; (5) SN–MP; (6) U1–SN; (7) L1–SN; (8) condylar axis. C: The vertical line passing through point. CN1 and CN2: positioned between the two halves of the line segment connecting points.
SNB: The angle formed by the S–N and N–B planes. ANB: Difference between SNA and SNB values. SN–MP: The angle formed by the S–N and mandibular planes. U1–SN: The angle formed by the S–N plane and the axis of the central incisor of the maxilla.
Fig. 2 – Cephalometric linear measurements. (1) U1–PP; (2) L1–MP; (3) overjet; (4) overbite; (5) POG (X); (6) POG (Y); (7) wits appraisal.
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For the measurements, the condylar axis was plotted by superimposing the contour of the distal segment taken at completion of preoperative orthodontic treatment for each subsequent tracing. When a right–left difference was present postoperatively, the side with the larger variation was measured.
3.1.2.
Fig. 3 – The three cephalometric measurements used for classification were: (1) ANB (8); (2) SN–MP (8); (3) ALFH (mm).
L1–SN: The angle formed by the S–N plane and the axis of the central incisor of the mandible. Condylar axis [12]: The vertical line passing through point C, which is positioned between the two halves of the line segment connecting points CN1 and CN2, and at a right angle with the line segment (Fig. 1). Points CN1 and CN2 were obtained from the minimum distance between the anterior and posterior margin of the neck of the mandible on the tracing of the lateral cephalometric radiograph taken at the completion of preoperative orthodontic treatment. For the angularity of the distal segment, the angle formed by the condylar axis and Frankfort horizontal plane was obtained.
Linear measurements (in mm) (Fig. 2)
ALFH: The vertical distance between ANS and pog, perpendicular to the F–H plane. U1–PP: The distance the perpendicular line dropped from U1 to the palatal plane. L1–MP: The distance the perpendicular line dropped from L1 to the mandibular plane. Overjet: The horizontal distance of U1 and L1 to the occlusal plane. Overbite: The vertical distance of U1 and L1 to the occlusal plane. Pog(X): The horizontal distance of the pogonion in the coordination system, where the nasion is the origin, the S– N plane is the X-axis, and the vertical line passing through the nasion and at a right angle with the X-axis is the Y-axis. Pog(Y): The vertical distance from the S–N plane to the pogonion. Wits appraisal: The horizontal distance of the points where the perpendicular lines, dropped from points A and B to the occlusal plane, intersected.
3.2. Classification of the face type immediately before surgery (Figs. 3 and 4) Lateral cephalometric radiographs of the 39 patients taken just before surgery were used for classifying the face types. Three variables were selected; ANB as the longitudinal
Fig. 4 – Classification of facial form before surgery (Ward method).
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Table 1 – Comparison between group LF and group SF at different intervals after surgery Group
Operated age (range)
T1
T2
T3
T4
T5
T6
Group LF Group SF
29 years 10 months (18 years 9 months to 45 years 2 months) 28 years 7 months (17 years 0 months to 38 years 10 months)
26 13
26 13
26 13
23 13
20 12
16 10
Total
29 years 3 months (17 years 0 months to 45 years 2 months)
39
39
39
36
32
26
Subject: female only. T1: immediately before surgery, T2: immediately after surgery, T3: after 6 months, T4: after 1 year, T5: after 3 years and T6: after 5 years.
were analyzed by repeated measure ANOVA. Measurement values that showed a statistically significant difference were studied using Scheffe’s multiple comparison. In addition, Spearman’s correlation coefficient by rank was used to clarify the relationship between the amount of changes caused by surgery and postoperative changes at the different radiographic examination periods.
variable, ALFH as the vertical variable, and SN–MP as the mandibular morphology [13,14]. The subjects were classified into two groups by cluster analysis, which was performed using Ward’s method, a type of combinational method. The results of face type classification by cluster analysis are shown in the dendrogram in Fig. 4. Fifteen cases on the left side of the dendrogram formed the short face (SF) group and 23 cases on the right formed the long face (LF) group. Of those 38 patients, 26 (10 and 16 in the SF and LF groups, respectively) had data available from the entire postoperative 5-year follow-up period and their results were used for our analysis (Table 1).
3.5.
Statistical analysis
Statistical analysis was done with a personal computer (Lavie, PC-LG21 FSEJM, NEC, Japan) using SPSS 13.0 J software (SPSS, Tokyo).
3.3. Comparison of immediate presurgical and postsurgical measurements within and between the groups
4. The eight angular and seven linear measurements were determined using the lateral cephalometric radiographs taken at each examination (Figs. 1 and 2), with comparisons made with an unpaired t-test. For assessing the changes caused by surgery within each group, we used an unpaired t-test to examine the changes that occurred between T1 and T2.
3.4.
Results
4.1. Immediate presurgical comparisons between the groups (Table 2) At period T1, the LF group showed a statistically significantly larger Pog(X) value (+7.5 mm), and the SF group showed significantly smaller values for Pog(Y) ( 5.1 mm) and SN–MP ( 9.48). For the denture pattern, the LF group showed a significantly larger L1–MP value of 4.18 and U1–SN value of 5.98. In addition, the SF group had a significantly increased overbite of +3.1 mm.
Postoperative stability
To examine postoperative stability in each group, the measurements obtained at each examination after surgery
Table 2 – Cephalometric measurements of group LF and group SF before surgery Measuring member
SNP (8) SNA (8) SNB (8) ANB (8) SN–MP (8) U1–SN (8) L1–SN (8) Condylar axis (8) U1–PP (mm) L1–MP (mm) Overjet (mm) Overbite (mm) Pog (X) (mm) Pog (Y) (mm) Wits appraisal (mm)
Group LF (N = 16)
Group SF (N = 10)
Mean
S.D.
Mean
S.D.
74.0 80.4 72.6 7.8 37.6 105.5 43.9 70.3 31.0 44.0 7.8 0.9 33.8 114.2 6.2
3.1 3.4 2.5 1.9 4.9 7.0 18.2 5.4 4.0 2.5 2.1 1.7 7.3 5.1 2.5
76.1 81.2 74.3 6.9 28.2 99.6 45.0 71.0 28.1 39.9 6.9 4.0 26.3 109.1 6.5
2.6 2.1 2.9 1.7 4.4 6.2 12.5 2.7 2.9 2.9 2.3 1.1 5.3 5.2 2.7
N.S.: not significant. *p < 0.05; **p < 0.001.
Group difference
2.1 0.8 1.7 0.9 9.4 5.9 1.1 0.7 2.9 4.1 0.9 3.1 7.5 5.1 0.3
Significance
N.S. N.S. N.S. N.S. ** *
N.S. N.S. N.S. **
N.S. ** * *
N.S.
Table 3 – Mean changes before and after surgery Measuring member
Group LF (N = 16) Just before surgery
SNP (8) SNA (8) SNB (8) ANB (8) SN–MP (8) U1–SN (8) L1–SN (8) Condylar axis (8) U1–PP (mm) L1–MP (mm) Overjet (mm) Overbite (mm) Pog (X) (mm) Pog (Y) (mm) Wits appraisal (mm)
74.0 80.4 72.6 7.8 37.6 105.5 43.9 70.3 31.0 44.0 7.8 0.9 33.8 114.2 6.2
S.D. 3.1 3.4 2.5 1.9 4.9 7.0 18.2 5.4 4.0 2.5 2.1 1.7 7.3 5.1 2.5
Significance
Immediately after surgery Mean 77.6 80.3 76.0 4.0 39.3 104.1 43.6 67.5 30.8 44.1 2.1 0.4 27.1 117.2 1.3
Just before surgery
S.D. 4.0 3.7 3.5 1.9 7.0 8.6 6.0 5.8 3.9 2.3 0.9 0.6 8.9 5.3 2.1
Group SF (N = 10)
Mean 3.6 0.1 3.4 3.8 1.7 1.4 0.3 2.8 0.2 0.1 5.7 0.5 6.7 3.0 4.9
*
N.S. * **
N.S. N.S. N.S. *
N.S. N.S. **
N.S. *
N.S. **
76.1 81.2 74.3 6.9 28.2 99.6 45.0 71.0 28.1 39.9 6.9 4.0 26.3 109.0 6.5
S.D. 2.6 4.5 2.9 2.8 4.4 6.2 12.5 2.7 2.8 2.8 2.3 1.1 5.3 5.2 2.7
Difference
Significance
Immediately after surgery Mean 78.4 81.3 77.4 3.9 33.3 100.7 45.1 67.9 28.1 40.0 1.8 0.7 20.3 114.1 0.1
S.D. 3.2 5.1 3.3 4.7 5.2 7.1 9.1 3.6 2.5 2.1 1.2 1.2 5.6 5.1 3.2
2.3 0.1 3.1 3.0 5.1 1.1 0.1 3.1 0.0 0.1 5.1 3.3 6.0 5.1 6.6
N.S. N.S. * * *
N.S. N.S. *
N.S. N.S. ** ** *
orthodontic waves 66 (2007) 1–8
Mean
Difference
* **
N.S.: not significant. *p < 0.05; **p < 0.001.
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4.2.
Changes caused by surgery (Table 3)
In the LF group, SNP and SNB were increased by 3.78 and 3.48, respectively. Further, the ANB, Pog(X), Wits appraisal, condylar axis, and overjet values were significantly decreased by 3.88, 6.7 mm, 4.9 mm, 2.88, and 5.7 mm, respectively. In the SF group, SNB, Pog(Y) and SN–MP were increased by 3.18, 5.1 mm and 5.18, respectively. Further, the ANB, Pog(X), Wits appraisal, condylar axis, overjet and overbite values were significantly decreased by 3.08, 6.0 mm, 6.6 mm, 3.18, 5.1 mm and 3.3 mm, respectively. Fig. 5 – Changes in the condylar axis of group SF after surgery.
4.3. Comparisons of amount of changes during surgery and intermaxillary fixation between the groups (Table 4)
classifying the maxillofacial and skeletal morphology of the patients immediately after surgery into two groups by cluster analysis. Comparisons of the groups revealed that the chin of LF group patients was positioned more inferior and posterior as compared to the SF group, while the mandibular plane and lower incisor height were increased in the LF group. In addition, overjet was greater in the SF group. On the basis of these findings, we concluded that the patients were classified properly in terms of morphology, as those in the SF group were the short-face type, and the LF group had the angle Class II dental and skeletal malocclusion of the long-face type. SN–MP was increased in the SF group, while it showed no change in LF group following surgery. It has been reported that relapse is caused by an elongation of the medial pterygoid muscle after the chin has been surgically advanced to the anterosuperior position [8]. In cases with excessive anterosuperior advancement of the chin expected, simultaneous twojaw surgery that corrects the anterosuperior advancement of the chin by the impaction of the upper jaw segment toward the superior position has been applied [4]. We considered that the operative procedure for the two groups was designed to prevent excessive counterclockwise rotation of the chin as a means to cope with possible relapse.
SNP and U1–SN were significantly larger in the LF group by 1.38 and 2.58, respectively, while U1–SN was significantly reduced by 2.58. In the SF group, Pog(Y) and SN–MP were significantly increased by 2.0 mm and 3.48, respectively, while overbite was significantly reduced by 2.8 mm.
4.4.
Postoperative stability (Table 5 and Figs. 5 and 6)
In the SF group, the condylar axis measurement increased from 67.98 to 72.78 from stage T2 to T3 ( p < 0.05), whereas no significant change was observed in the LF group during that period. As for the relationship between the amount of advancement caused by surgery and postoperative changes (Fig. 6), positive correlations (r = 0.66, p < 0.05) were observed for the Pog(X) value from stage T1 to T2 stage and the condylar axis value from stage T2 to T3.
5.
Discussion
In the present study, the three variables ANB, SN–MP, and ALFH, which represented longitudinal and vertical measurements, and mandibular morphology, respectively, were selected for
Table 4 – Comparison of mean changes of group LF to group SF during the surgery period Measuring member
Group LF (N = 16) Mean
SNP (8) SNA (8) SNB (8) ANB (8) SN–MP (8) U1–SN (8) L1–SN (8) Condylar axis (8) U1–PP (mm) L1–MP (mm) Overjet (mm) Overbite (mm) Pog (X) (mm) Pog (Y) (mm) Wits appraisal (mm)
3.6 0.1 3.4 3.8 1.7 1.4 0.3 2.8 0.2 0.1 5.7 0.5 6.7 3.0 4.9
N.S.: not significant. *p < 0.05; **p < 0.001.
S.D. 1.3 0.6 1.2 0.9 4.4 2.7 16.3 2.3 1.3 1.3 2.4 1.8 2.6 2.0 2.4
Group SF (N = 10) Mean 2.3 0.1 3.1 3.0 5.1 1.1 0.1 3.1 0.0 0.1 5.1 3.4 6.0 5.1 6.6
Group difference
Significance
S.D. 0.9 0.2 0.9 1.1 1.5 2.9 5.5 3.0 1.6 1.4 2.4 1.7 1.6 1.1 2.2
1.3 0.2 0.3 0.8 3.4 2.5 0.4 0.3 0.2 0.0 0.6 2.9 0.7 2.1 1.7
*
N.S. N.S. N.S. * *
N.S. N.S. N.S. N.S. N.S. **
N.S. *
N.S.
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orthodontic waves 66 (2007) 1–8
Table 5 – Cephalometric measurements in group LF and group SF after surgery period Measuring member
Immediately after surgery Mean
6 months
1 year
3 years
5 years
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
Group LF (N = 16) SNP (8) SNA (8) SNB (8) ANB (8) SN–MP (8) U1–SN (8) L1–SN (8) Condylar axis (8) U1–PP (8) L1–MP (8) Overjet (mm) Oberbite (mm) Pog (X) (mm) Pog (Y) (mm) wits appraisal (mm)
77.6 80.3 76.0 4.0 39.3 104.1 43.6 67.5 30.8 44.1 2.1 0.4 27.1 117.2 1.3
4.0 3.7 3.5 1.9 7.0 8.6 6.0 5.8 3.9 2.3 0.9 0.6 8.9 5.3 2.1
77.5 80.4 76.0 4.2 38.9 104.7 45.6 67.0 30.9 44.4 2.7 1.4 27.9 117.0 1.4
1.0 0.9 0.9 0.4 1.7 1.8 1.7 1.6 0.9 0.7 0.1 0.2 2.3 1.3 0.6
77.2 80.3 75.9 4.4 39.7 101.3 46.4 67.1 31.2 44.3 3.1 1.6 27.9 116.8 2.0
0.9 0.8 0.9 0.5 1.7 1.7 1.6 1.6 0.9 0.6 0.2 0.2 2.1 1.2 0.6
76.7 80.4 75.4 4.8 40.1 101.1 46.7 67.7 31.3 44.4 3.6 1.0 28.9 116.5 2.3
1.0 0.9 0.8 0.5 1.6 1.7 1.7 1.6 0.9 0.7 0.2 0.2 2.1 1.2 0.6
76.7 80.3 75.4 5.3 40.1 100.8 46.3 67.7 32.1 44.4 3.8 0.9 29.1 116.6 2.3
1.0 0.9 0.9 0.5 1.6 1.7 1.7 1.6 0.9 0.7 0.2 0.2 2.1 1.2 0.6
Group SF (N = 10) SNP (8) SNA (8) SNB (8) ANB (8) SN–MP (8) U1–SN (8) L1–SN (8) Condylar axis (8) U1–PP (8) L1–MP (8) Overjet (mm) Oberbite (mm) Pog (X) (mm) Pog (Y) (mm) Wits appraisal (mm)
78.4 81.3 77.4 3.9 33.3 100.7 45.1 67.9 28.1 40.0 1.8 0.7 20.3 114.1 0.1
3.2 5.1 3.3 4.7 5.2 7.1 9.1 3.6 2.5 2.1 1.2 1.2 5.6 5.1 3.2
78.6 81.5 77.5 4.0 32.0 103.2 45.8 72.7* 27.6 40.1 2.7 0.9 21.5 111.6 0.5
1.0 0.7 1.0 0.5 1.9 1.9 2.7 0.7 0.9 0.6 0.4 1.2 1.9 1.7 0.7
78.7 81.3 77.3 4.5 32.8 100.8 45.6 72.5 27.8 40.4 2.5 1.0 22.3 111.4 1.4
1.0 0.6 1.0 0.6 1.9 2.0 3.0 0.6 0.9 0.8 0.3 1.2 1.9 1.6 0.8
78.4 81.8 77.2 4.5 32.3 101.0 45.2 71.3 27.8 40.2 2.9 1.0 22.4 111.1 1.6
1.0 0.6 1.0 0.6 1.9 2.1 3.1 0.7 0.9 0.8 0.3 1.2 1.9 1.6 0.7
78.3 81.8 77.2 4.4 32.3 100.9 45.2 71.2 27.8 40.1 2.8 1.1 22.5 111.0 1.7
1.0 0.6 1.0 0.6 2.0 2.1 3.0 0.9 0.9 0.8 0.3 1.2 1.8 1.5 0.7
*
p < 0.05 (difference between immediately after surgery and 6 months, scheffe).
In the LF group, the amount of change in the vertical component of the chin was minimized by the operative design, as an increase in facial height might cause vertical disharmony of the profile. There was no difference in horizontal amount of advancement of the chin during surgery between the two groups. However, SNP values were increased more in the LF group. The results also suggested that the amount of anterior advancement tended to be larger in the LF group patients, in whom the chin was positioned more posterior as compared to the immediate presurgical skeletal morphology. The proximal segment was rotated counterclockwise in both groups during surgery. In the SF group, it was suggested that the proximal segment might also be rotated clockwise, due to the effect of intermaxillary fixation and postoperative orthodontic treatment performed from immediately after surgery to postoperative 6 months, which showed a tendency to be restored to the preoperative position after postoperative 6 months. Our short-term observations of the proximal segment after surgery revealed that the amount of advancement was smaller in rigid fixation cases than that in wire fixation cases. However, it has been reported that those changes continued in the head of the mandible [11]. In both groups, a bone suture in the upper edge of the anterior border of the mandibular
ramus was added to screw fixation of the bone segment. In cases of bone segment fixation using a circummandibular wire, it has been reported that counter-clockwise rotation of the proximal segment is the result. Accordingly, a superior border figure-eight wire has been used for preventing advancement of the fixation using a wire alone [2,6]. In the present LF group, advancement of the distal segment was not
Fig. 6 – Correlation between the surgical changes and each evaluation points at different intervals after surgery.
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observed postoperatively. Thus, it is considered that the application of two kinds of fixation might be effective to prevent relapse. For the denture pattern, the upper incisors demonstrated lingual tipping in the LF group. It was considered that this might have been caused by the effect of intermaxillary fixation [10,11,15], as we used cephalometric radiographs obtained after the release of intermaxillary fixation for the immediate postsurgical measurements. As for overbite, the amount of change was greater in the SF group, which corresponded to the increased direction of SN–MP and Pog(Y). The suprahyoid muscle acts to apply traction to the chin toward the inferior and posterior directions. It has been speculated that this muscle causes relapse after mandibular advancement surgery in long-face type cases with an open bite [9]. As for the relationship between deep bite and the suprahyoid muscle in short-face type patients, it is advantageous to attain postoperative stability, as the combination acts to maintain occlusal height. Accordingly, a suprahyoid myotomy is excluded from the indications for short-face type cases with an associated deep bite. It is considered that a procedure to prevent posterior relapse should be given priority, as this method was employed in all of the cases examined in the present study. With the present operative design for obtaining long-term stability, the chin was advanced in the anterior direction and the proximal segment was positioned in the anterosuperior position in both groups. Further, the operative design used to increase the lower facial height was notably effective for short-face type patients.
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[3] Van Sickels JE, Richardson DA. Stability of orthognathic surgery, a review of rigid fixation. Br J Oral Maxillofac Surg 1996;34:279–85. [4] Proffit WR, Turvey TA, Phillips C. Orthognathic surgery: a hierarchy of study. Int J Adult Orthod Orthognath Surg 1996;11:191–204. [5] Van Sickels JE. A comparative study of bicortical screws and suspention wires versus bicortical screws in large mandibular advancements. J Oral Maxillofac Surg 1991;49:1293–6. [6] Epker BN, Wessberg GA. Mechanisms of early skeletal relapse following surgical advancement of the mandible. Br J Oral Surg 1982;20:175–82. [7] Watzke IM, Turvey TA, Phillips C, Proffit WR. Stability of mandibular advancement after sagittal osteotomy with screw or wire fixation: a comparative study. J Oral Maxillofac Surg 1990;48:108–21. [8] Isacson RJ. Movement of proximal and distal segments after mandibular ramus osteotomies. J Oral Surg 1978;36:263–8. [9] Wessberg GA, Schendel SA, Epker BN. The role of suprahyoid myotomy in surgical advancement of the mandible via sagittal split ramus osteotomies. J Oral Maxillofac Surg 1982;40:273–7. [10] Nemeth DZ, Rodrigues-Garcia RCM, Sakai S, Hatch JP, Van Sickels JE, Bays RA, et al. Bilateral sagittal split osteotomy and temporomandibular disorders; Rigid fixation versus wire fixation. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2000;89:29–34. [11] Finn RA, Throckmorton GS, Bell WH, Legan HL. Biomechanical considerations in the surgical correction of mandibular deficiency. J Oral Surg 1980;38:257–64. [12] Fukui K, Tadokoro T, Himuro T, Yamaguchi T, Ohno T. Postoperative evaluation of mandibular prognathism corrected by sagittal splitting osteotomy. J Jpn Orthod Soc 1989;48:48–58. [13] Tsunori M, Mashita M, Kasai K. Relationship between facial types and tooth and bone characteristics of the mandible obtained by CT scanning. Angle Orthodon 1998;68: 557–62. [14] Ogawa T, Kawasaki H, Takahashi O, Aboshi H, Kasai K. Application od a fourier series to analysis of the relationship between mandibular form and facial morphology. J Oral Sci 2000;42:93–100. [15] Himuro T, Tadokoro T, Sasamoto F, Yamaguchi T, Takada K, Adachi F, et al. Changes during intermaxillary fixation following sagittal split osteotomy of mandibular prognathism. J Tohoku Dent Univ 1984;11:156–64.
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Research paper
Characteristics of mandibular advancement surgery dependent on lower facial height differences in study of skeletal Class II patients with long-term stability Kazunori Fukui a,*, Raymond M. Sugiyama b, Ronald M. Kaminishi c, Yoshiaki Matsuyama a, Eiko Kuroda d, Toshihiko Himuro a a
Department of Oral Growth and Development, Division of Orthodontics and Dentofacial Orthopedics, Ohu University School of Dentistry, Japan b Private Orthodontic Practice in Los Alamitos, Loma Linda University, USA c Private Oral/Maxillo-facial Surgery Practice in Bellflower, University of Southern California, USA d Ohu University Dental Hospital, Japan
article info
abstract
Article history:
We studied patients who underwent mandibular advancement surgery and showed a
Received 12 September 2006
stabilized occlusal condition at 5 years after surgery. The patients were classified into 2
Received in revised form
groups [long face type (LF group: 16 cases), short face type (SF group: 10 cases)] according to
20 December 2006
immediate presurgery face type. To clarify the characteristics of the operative method
Accepted 25 December 2006
formulated based on lower facial height, morphological changes during the 5-year post-
Published on line 26 January 2007
operative period were compared between the groups.
Keywords:
two groups, though lower facial height showed a tendency to increase in group SF. The
Mandibular advancement surgery
proximal segment was advanced to the anterosuperior position in both groups.
There was no statistically significant difference in amount of advancement between the
Lower facial height
To determine postoperative stability, in the LF group, there were no statistically significant differences for lower face height. In the SF group, the condylar axis increased in the period from
Stability
immediately after to 6 months after surgery. Correlation analysis revealed that the backward rotation of the proximal segment was greater during the first 6 months, as the amount of surgical advancement of the chin was larger. As a result, the chin was advanced during surgery and the proximal segment was advanced toward the anterosuperior position in both groups. In the SF group, the characteristics of the operative design increased the lower facial height. # 2007 Elsevier Ltd and the Japanese Orthodontic Society. All rights reserved.
1.
Introduction
There are a number of reports of relapse condition in cases in which the mandible was advanced surgically [1–11]. Factors related to such relapse are considered to be amount of man-
dibular advancement [1,2], operative procedure [1–6], fixation method utilized [1,2,5], morphological problems of the mandible concerned with tipping of the mandibular plane [4], positioning of the condylar head [6–8], effects of the suprahyoid muscles [9], and preoperative orthodontic treatment [10,11].
* Corresponding author. E-mail address: [email protected] (K. Fukui). 1344-0241/$ – see front matter # 2007 Elsevier Ltd and the Japanese Orthodontic Society. All rights reserved. doi:10.1016/j.odw.2007.01.001
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orthodontic waves 66 (2007) 1–8
To obtain stability following orthognathic surgery in patients with short and long face types [3,4], maxillary osteotomy alone and that with simultaneous two-jaw surgery have been frequently employed for the long-face type, as one-jaw surgery is generally excluded from consideration due to difficulties in attaining postoperative stability. Accordingly, we speculated that the optimum operative procedures for skeletal Class II patients with a flat inferior border of the mandible and for those with a high-pitched border are different. To clarify the characteristics of the optimum operative procedures based on lower facial height, patients who underwent one-jaw advancement surgery at a clinic in southern California and showed stable occlusion at 5 years after surgery were studied. They were classified into two groups by cluster analysis of the maxillofacial morphology using longitudinal and vertical factors of the upper and lower jaws as indices, and morphological changes were followed for 5 years after surgery.
2.
Materials and methods
2.1.
Subjects
The patients were 39 adult Caucasian females who underwent a bilateral sagittal split osteotomy during the period from 1982 to 1993 at Long Beach Memorial Medical Center, in southern California. Their mean age at the time of surgery was 29 years 3 months (range, 17 years 0 months to 45 years 2 months). In all cases, the bone segment was fixed by rigid fixation using a screw with a direct interosseous wire placed between the segments at the superior border on each side [2,7]. A suprahyoid myotomy was performed concomitantly [9]. The term of intermaxillary fixation using an orthodontic appliance with a wire was a mean 8.5 weeks. Patients who did not undergo a genioplasty and demonstrated a close occlusion at 5 years after surgery were selected, while those with facial asymmetry, cleft palate, and a large number of prosthetic appliances were excluded. For our analysis, a total of 186 lateral cephalometric radiographs taken in the central occlusal position using the usual method were examined. All tracing was done by the same examiner. After setting the landmarks, measurements were obtained (Figs. 1 and 2). The radiograph examination periods were as follows: immediately before surgery (T1, preoperative 1–2 months), immediately after surgery (T2, postoperative 1–3 months), postoperative 6 months (T3, postoperative 5–7 months), postoperative 1 year (T4, postoperative 10 months to 1 year 5 months), postoperative 3 years (T5, postoperative 2 years 6 months to 3 years 4 months), and postoperative 5 years (T6, postoperative 4 years 6 months to 5 years 3 months).
3.
Measuring procedures
3.1.
Measurements
3.1.1.
Angular measurements (in degrees) (Fig. 1)
SNP: The angle formed by the S–N and N-Pog planes. SNA: The angle formed by the S–N and N–A planes.
Fig. 1 – Cephalometric angular measurements. (1) SNP; (2) SNA; (3) SNB; (4) ANB; (5) SN–MP; (6) U1–SN; (7) L1–SN; (8) condylar axis. C: The vertical line passing through point. CN1 and CN2: positioned between the two halves of the line segment connecting points.
SNB: The angle formed by the S–N and N–B planes. ANB: Difference between SNA and SNB values. SN–MP: The angle formed by the S–N and mandibular planes. U1–SN: The angle formed by the S–N plane and the axis of the central incisor of the maxilla.
Fig. 2 – Cephalometric linear measurements. (1) U1–PP; (2) L1–MP; (3) overjet; (4) overbite; (5) POG (X); (6) POG (Y); (7) wits appraisal.
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3
For the measurements, the condylar axis was plotted by superimposing the contour of the distal segment taken at completion of preoperative orthodontic treatment for each subsequent tracing. When a right–left difference was present postoperatively, the side with the larger variation was measured.
3.1.2.
Fig. 3 – The three cephalometric measurements used for classification were: (1) ANB (8); (2) SN–MP (8); (3) ALFH (mm).
L1–SN: The angle formed by the S–N plane and the axis of the central incisor of the mandible. Condylar axis [12]: The vertical line passing through point C, which is positioned between the two halves of the line segment connecting points CN1 and CN2, and at a right angle with the line segment (Fig. 1). Points CN1 and CN2 were obtained from the minimum distance between the anterior and posterior margin of the neck of the mandible on the tracing of the lateral cephalometric radiograph taken at the completion of preoperative orthodontic treatment. For the angularity of the distal segment, the angle formed by the condylar axis and Frankfort horizontal plane was obtained.
Linear measurements (in mm) (Fig. 2)
ALFH: The vertical distance between ANS and pog, perpendicular to the F–H plane. U1–PP: The distance the perpendicular line dropped from U1 to the palatal plane. L1–MP: The distance the perpendicular line dropped from L1 to the mandibular plane. Overjet: The horizontal distance of U1 and L1 to the occlusal plane. Overbite: The vertical distance of U1 and L1 to the occlusal plane. Pog(X): The horizontal distance of the pogonion in the coordination system, where the nasion is the origin, the S– N plane is the X-axis, and the vertical line passing through the nasion and at a right angle with the X-axis is the Y-axis. Pog(Y): The vertical distance from the S–N plane to the pogonion. Wits appraisal: The horizontal distance of the points where the perpendicular lines, dropped from points A and B to the occlusal plane, intersected.
3.2. Classification of the face type immediately before surgery (Figs. 3 and 4) Lateral cephalometric radiographs of the 39 patients taken just before surgery were used for classifying the face types. Three variables were selected; ANB as the longitudinal
Fig. 4 – Classification of facial form before surgery (Ward method).
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Table 1 – Comparison between group LF and group SF at different intervals after surgery Group
Operated age (range)
T1
T2
T3
T4
T5
T6
Group LF Group SF
29 years 10 months (18 years 9 months to 45 years 2 months) 28 years 7 months (17 years 0 months to 38 years 10 months)
26 13
26 13
26 13
23 13
20 12
16 10
Total
29 years 3 months (17 years 0 months to 45 years 2 months)
39
39
39
36
32
26
Subject: female only. T1: immediately before surgery, T2: immediately after surgery, T3: after 6 months, T4: after 1 year, T5: after 3 years and T6: after 5 years.
were analyzed by repeated measure ANOVA. Measurement values that showed a statistically significant difference were studied using Scheffe’s multiple comparison. In addition, Spearman’s correlation coefficient by rank was used to clarify the relationship between the amount of changes caused by surgery and postoperative changes at the different radiographic examination periods.
variable, ALFH as the vertical variable, and SN–MP as the mandibular morphology [13,14]. The subjects were classified into two groups by cluster analysis, which was performed using Ward’s method, a type of combinational method. The results of face type classification by cluster analysis are shown in the dendrogram in Fig. 4. Fifteen cases on the left side of the dendrogram formed the short face (SF) group and 23 cases on the right formed the long face (LF) group. Of those 38 patients, 26 (10 and 16 in the SF and LF groups, respectively) had data available from the entire postoperative 5-year follow-up period and their results were used for our analysis (Table 1).
3.5.
Statistical analysis
Statistical analysis was done with a personal computer (Lavie, PC-LG21 FSEJM, NEC, Japan) using SPSS 13.0 J software (SPSS, Tokyo).
3.3. Comparison of immediate presurgical and postsurgical measurements within and between the groups
4. The eight angular and seven linear measurements were determined using the lateral cephalometric radiographs taken at each examination (Figs. 1 and 2), with comparisons made with an unpaired t-test. For assessing the changes caused by surgery within each group, we used an unpaired t-test to examine the changes that occurred between T1 and T2.
3.4.
Results
4.1. Immediate presurgical comparisons between the groups (Table 2) At period T1, the LF group showed a statistically significantly larger Pog(X) value (+7.5 mm), and the SF group showed significantly smaller values for Pog(Y) ( 5.1 mm) and SN–MP ( 9.48). For the denture pattern, the LF group showed a significantly larger L1–MP value of 4.18 and U1–SN value of 5.98. In addition, the SF group had a significantly increased overbite of +3.1 mm.
Postoperative stability
To examine postoperative stability in each group, the measurements obtained at each examination after surgery
Table 2 – Cephalometric measurements of group LF and group SF before surgery Measuring member
SNP (8) SNA (8) SNB (8) ANB (8) SN–MP (8) U1–SN (8) L1–SN (8) Condylar axis (8) U1–PP (mm) L1–MP (mm) Overjet (mm) Overbite (mm) Pog (X) (mm) Pog (Y) (mm) Wits appraisal (mm)
Group LF (N = 16)
Group SF (N = 10)
Mean
S.D.
Mean
S.D.
74.0 80.4 72.6 7.8 37.6 105.5 43.9 70.3 31.0 44.0 7.8 0.9 33.8 114.2 6.2
3.1 3.4 2.5 1.9 4.9 7.0 18.2 5.4 4.0 2.5 2.1 1.7 7.3 5.1 2.5
76.1 81.2 74.3 6.9 28.2 99.6 45.0 71.0 28.1 39.9 6.9 4.0 26.3 109.1 6.5
2.6 2.1 2.9 1.7 4.4 6.2 12.5 2.7 2.9 2.9 2.3 1.1 5.3 5.2 2.7
N.S.: not significant. *p < 0.05; **p < 0.001.
Group difference
2.1 0.8 1.7 0.9 9.4 5.9 1.1 0.7 2.9 4.1 0.9 3.1 7.5 5.1 0.3
Significance
N.S. N.S. N.S. N.S. ** *
N.S. N.S. N.S. **
N.S. ** * *
N.S.
Table 3 – Mean changes before and after surgery Measuring member
Group LF (N = 16) Just before surgery
SNP (8) SNA (8) SNB (8) ANB (8) SN–MP (8) U1–SN (8) L1–SN (8) Condylar axis (8) U1–PP (mm) L1–MP (mm) Overjet (mm) Overbite (mm) Pog (X) (mm) Pog (Y) (mm) Wits appraisal (mm)
74.0 80.4 72.6 7.8 37.6 105.5 43.9 70.3 31.0 44.0 7.8 0.9 33.8 114.2 6.2
S.D. 3.1 3.4 2.5 1.9 4.9 7.0 18.2 5.4 4.0 2.5 2.1 1.7 7.3 5.1 2.5
Significance
Immediately after surgery Mean 77.6 80.3 76.0 4.0 39.3 104.1 43.6 67.5 30.8 44.1 2.1 0.4 27.1 117.2 1.3
Just before surgery
S.D. 4.0 3.7 3.5 1.9 7.0 8.6 6.0 5.8 3.9 2.3 0.9 0.6 8.9 5.3 2.1
Group SF (N = 10)
Mean 3.6 0.1 3.4 3.8 1.7 1.4 0.3 2.8 0.2 0.1 5.7 0.5 6.7 3.0 4.9
*
N.S. * **
N.S. N.S. N.S. *
N.S. N.S. **
N.S. *
N.S. **
76.1 81.2 74.3 6.9 28.2 99.6 45.0 71.0 28.1 39.9 6.9 4.0 26.3 109.0 6.5
S.D. 2.6 4.5 2.9 2.8 4.4 6.2 12.5 2.7 2.8 2.8 2.3 1.1 5.3 5.2 2.7
Difference
Significance
Immediately after surgery Mean 78.4 81.3 77.4 3.9 33.3 100.7 45.1 67.9 28.1 40.0 1.8 0.7 20.3 114.1 0.1
S.D. 3.2 5.1 3.3 4.7 5.2 7.1 9.1 3.6 2.5 2.1 1.2 1.2 5.6 5.1 3.2
2.3 0.1 3.1 3.0 5.1 1.1 0.1 3.1 0.0 0.1 5.1 3.3 6.0 5.1 6.6
N.S. N.S. * * *
N.S. N.S. *
N.S. N.S. ** ** *
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Mean
Difference
* **
N.S.: not significant. *p < 0.05; **p < 0.001.
5
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4.2.
Changes caused by surgery (Table 3)
In the LF group, SNP and SNB were increased by 3.78 and 3.48, respectively. Further, the ANB, Pog(X), Wits appraisal, condylar axis, and overjet values were significantly decreased by 3.88, 6.7 mm, 4.9 mm, 2.88, and 5.7 mm, respectively. In the SF group, SNB, Pog(Y) and SN–MP were increased by 3.18, 5.1 mm and 5.18, respectively. Further, the ANB, Pog(X), Wits appraisal, condylar axis, overjet and overbite values were significantly decreased by 3.08, 6.0 mm, 6.6 mm, 3.18, 5.1 mm and 3.3 mm, respectively. Fig. 5 – Changes in the condylar axis of group SF after surgery.
4.3. Comparisons of amount of changes during surgery and intermaxillary fixation between the groups (Table 4)
classifying the maxillofacial and skeletal morphology of the patients immediately after surgery into two groups by cluster analysis. Comparisons of the groups revealed that the chin of LF group patients was positioned more inferior and posterior as compared to the SF group, while the mandibular plane and lower incisor height were increased in the LF group. In addition, overjet was greater in the SF group. On the basis of these findings, we concluded that the patients were classified properly in terms of morphology, as those in the SF group were the short-face type, and the LF group had the angle Class II dental and skeletal malocclusion of the long-face type. SN–MP was increased in the SF group, while it showed no change in LF group following surgery. It has been reported that relapse is caused by an elongation of the medial pterygoid muscle after the chin has been surgically advanced to the anterosuperior position [8]. In cases with excessive anterosuperior advancement of the chin expected, simultaneous twojaw surgery that corrects the anterosuperior advancement of the chin by the impaction of the upper jaw segment toward the superior position has been applied [4]. We considered that the operative procedure for the two groups was designed to prevent excessive counterclockwise rotation of the chin as a means to cope with possible relapse.
SNP and U1–SN were significantly larger in the LF group by 1.38 and 2.58, respectively, while U1–SN was significantly reduced by 2.58. In the SF group, Pog(Y) and SN–MP were significantly increased by 2.0 mm and 3.48, respectively, while overbite was significantly reduced by 2.8 mm.
4.4.
Postoperative stability (Table 5 and Figs. 5 and 6)
In the SF group, the condylar axis measurement increased from 67.98 to 72.78 from stage T2 to T3 ( p < 0.05), whereas no significant change was observed in the LF group during that period. As for the relationship between the amount of advancement caused by surgery and postoperative changes (Fig. 6), positive correlations (r = 0.66, p < 0.05) were observed for the Pog(X) value from stage T1 to T2 stage and the condylar axis value from stage T2 to T3.
5.
Discussion
In the present study, the three variables ANB, SN–MP, and ALFH, which represented longitudinal and vertical measurements, and mandibular morphology, respectively, were selected for
Table 4 – Comparison of mean changes of group LF to group SF during the surgery period Measuring member
Group LF (N = 16) Mean
SNP (8) SNA (8) SNB (8) ANB (8) SN–MP (8) U1–SN (8) L1–SN (8) Condylar axis (8) U1–PP (mm) L1–MP (mm) Overjet (mm) Overbite (mm) Pog (X) (mm) Pog (Y) (mm) Wits appraisal (mm)
3.6 0.1 3.4 3.8 1.7 1.4 0.3 2.8 0.2 0.1 5.7 0.5 6.7 3.0 4.9
N.S.: not significant. *p < 0.05; **p < 0.001.
S.D. 1.3 0.6 1.2 0.9 4.4 2.7 16.3 2.3 1.3 1.3 2.4 1.8 2.6 2.0 2.4
Group SF (N = 10) Mean 2.3 0.1 3.1 3.0 5.1 1.1 0.1 3.1 0.0 0.1 5.1 3.4 6.0 5.1 6.6
Group difference
Significance
S.D. 0.9 0.2 0.9 1.1 1.5 2.9 5.5 3.0 1.6 1.4 2.4 1.7 1.6 1.1 2.2
1.3 0.2 0.3 0.8 3.4 2.5 0.4 0.3 0.2 0.0 0.6 2.9 0.7 2.1 1.7
*
N.S. N.S. N.S. * *
N.S. N.S. N.S. N.S. N.S. **
N.S. *
N.S.
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orthodontic waves 66 (2007) 1–8
Table 5 – Cephalometric measurements in group LF and group SF after surgery period Measuring member
Immediately after surgery Mean
6 months
1 year
3 years
5 years
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
Mean
S.D.
Group LF (N = 16) SNP (8) SNA (8) SNB (8) ANB (8) SN–MP (8) U1–SN (8) L1–SN (8) Condylar axis (8) U1–PP (8) L1–MP (8) Overjet (mm) Oberbite (mm) Pog (X) (mm) Pog (Y) (mm) wits appraisal (mm)
77.6 80.3 76.0 4.0 39.3 104.1 43.6 67.5 30.8 44.1 2.1 0.4 27.1 117.2 1.3
4.0 3.7 3.5 1.9 7.0 8.6 6.0 5.8 3.9 2.3 0.9 0.6 8.9 5.3 2.1
77.5 80.4 76.0 4.2 38.9 104.7 45.6 67.0 30.9 44.4 2.7 1.4 27.9 117.0 1.4
1.0 0.9 0.9 0.4 1.7 1.8 1.7 1.6 0.9 0.7 0.1 0.2 2.3 1.3 0.6
77.2 80.3 75.9 4.4 39.7 101.3 46.4 67.1 31.2 44.3 3.1 1.6 27.9 116.8 2.0
0.9 0.8 0.9 0.5 1.7 1.7 1.6 1.6 0.9 0.6 0.2 0.2 2.1 1.2 0.6
76.7 80.4 75.4 4.8 40.1 101.1 46.7 67.7 31.3 44.4 3.6 1.0 28.9 116.5 2.3
1.0 0.9 0.8 0.5 1.6 1.7 1.7 1.6 0.9 0.7 0.2 0.2 2.1 1.2 0.6
76.7 80.3 75.4 5.3 40.1 100.8 46.3 67.7 32.1 44.4 3.8 0.9 29.1 116.6 2.3
1.0 0.9 0.9 0.5 1.6 1.7 1.7 1.6 0.9 0.7 0.2 0.2 2.1 1.2 0.6
Group SF (N = 10) SNP (8) SNA (8) SNB (8) ANB (8) SN–MP (8) U1–SN (8) L1–SN (8) Condylar axis (8) U1–PP (8) L1–MP (8) Overjet (mm) Oberbite (mm) Pog (X) (mm) Pog (Y) (mm) Wits appraisal (mm)
78.4 81.3 77.4 3.9 33.3 100.7 45.1 67.9 28.1 40.0 1.8 0.7 20.3 114.1 0.1
3.2 5.1 3.3 4.7 5.2 7.1 9.1 3.6 2.5 2.1 1.2 1.2 5.6 5.1 3.2
78.6 81.5 77.5 4.0 32.0 103.2 45.8 72.7* 27.6 40.1 2.7 0.9 21.5 111.6 0.5
1.0 0.7 1.0 0.5 1.9 1.9 2.7 0.7 0.9 0.6 0.4 1.2 1.9 1.7 0.7
78.7 81.3 77.3 4.5 32.8 100.8 45.6 72.5 27.8 40.4 2.5 1.0 22.3 111.4 1.4
1.0 0.6 1.0 0.6 1.9 2.0 3.0 0.6 0.9 0.8 0.3 1.2 1.9 1.6 0.8
78.4 81.8 77.2 4.5 32.3 101.0 45.2 71.3 27.8 40.2 2.9 1.0 22.4 111.1 1.6
1.0 0.6 1.0 0.6 1.9 2.1 3.1 0.7 0.9 0.8 0.3 1.2 1.9 1.6 0.7
78.3 81.8 77.2 4.4 32.3 100.9 45.2 71.2 27.8 40.1 2.8 1.1 22.5 111.0 1.7
1.0 0.6 1.0 0.6 2.0 2.1 3.0 0.9 0.9 0.8 0.3 1.2 1.8 1.5 0.7
*
p < 0.05 (difference between immediately after surgery and 6 months, scheffe).
In the LF group, the amount of change in the vertical component of the chin was minimized by the operative design, as an increase in facial height might cause vertical disharmony of the profile. There was no difference in horizontal amount of advancement of the chin during surgery between the two groups. However, SNP values were increased more in the LF group. The results also suggested that the amount of anterior advancement tended to be larger in the LF group patients, in whom the chin was positioned more posterior as compared to the immediate presurgical skeletal morphology. The proximal segment was rotated counterclockwise in both groups during surgery. In the SF group, it was suggested that the proximal segment might also be rotated clockwise, due to the effect of intermaxillary fixation and postoperative orthodontic treatment performed from immediately after surgery to postoperative 6 months, which showed a tendency to be restored to the preoperative position after postoperative 6 months. Our short-term observations of the proximal segment after surgery revealed that the amount of advancement was smaller in rigid fixation cases than that in wire fixation cases. However, it has been reported that those changes continued in the head of the mandible [11]. In both groups, a bone suture in the upper edge of the anterior border of the mandibular
ramus was added to screw fixation of the bone segment. In cases of bone segment fixation using a circummandibular wire, it has been reported that counter-clockwise rotation of the proximal segment is the result. Accordingly, a superior border figure-eight wire has been used for preventing advancement of the fixation using a wire alone [2,6]. In the present LF group, advancement of the distal segment was not
Fig. 6 – Correlation between the surgical changes and each evaluation points at different intervals after surgery.
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observed postoperatively. Thus, it is considered that the application of two kinds of fixation might be effective to prevent relapse. For the denture pattern, the upper incisors demonstrated lingual tipping in the LF group. It was considered that this might have been caused by the effect of intermaxillary fixation [10,11,15], as we used cephalometric radiographs obtained after the release of intermaxillary fixation for the immediate postsurgical measurements. As for overbite, the amount of change was greater in the SF group, which corresponded to the increased direction of SN–MP and Pog(Y). The suprahyoid muscle acts to apply traction to the chin toward the inferior and posterior directions. It has been speculated that this muscle causes relapse after mandibular advancement surgery in long-face type cases with an open bite [9]. As for the relationship between deep bite and the suprahyoid muscle in short-face type patients, it is advantageous to attain postoperative stability, as the combination acts to maintain occlusal height. Accordingly, a suprahyoid myotomy is excluded from the indications for short-face type cases with an associated deep bite. It is considered that a procedure to prevent posterior relapse should be given priority, as this method was employed in all of the cases examined in the present study. With the present operative design for obtaining long-term stability, the chin was advanced in the anterior direction and the proximal segment was positioned in the anterosuperior position in both groups. Further, the operative design used to increase the lower facial height was notably effective for short-face type patients.
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