Treatment Effects of Occipitomental Anchorage ... - Science Direct

ORIGINAL ARTICLE

Treatment Effects of Occipitomental Anchorage Appliance of Maxillary Protraction Combined with Chincup Traction in Children with Class III Malocclusion Hsiang-Chien Lin,1 Hong-Po Chang,2* Hsin-Fu Chang3 Background/Purpose: Little information related to the treatment effects of the occipitomental anchorage (OMA) appliance of maxillary (Mx) protraction combined with chincup traction is available. The aim of this study was to investigate the treatment effects of the OMA orthopedic appliance on patients with Class III malocclusion. Methods: Pretreatment and post-treatment cephalometric records of 20 consecutively treated patients with Class III malocclusions were evaluated and compared with a matched sample of untreated Class III control subjects. Results: The OMA appliance is effective for correcting skeletal Class III malocclusion in growing children. The treatment effects of this orthopedic appliance were considered to be from both skeletal and dentoalveolar changes. The skeletal effects were mainly obtained by stimulating forward growth of the Mx complex with negligible rotation of the Mx plane and restraining forward advancement of the mandible (Mn) with backward and downward rotation of the Mn plane. The observed dentoalveolar effects were mostly due to the labial tipping movement of the Mx incisors. Conclusion: Our results suggest that the OMA orthopedic appliance can correct the mesial jaw relationship and negative incisal overjet. This appliance is effective for correcting skeletal Class III malocclusion with both midface deficiency and Mn prognathism in growing children. [J Formos Med Assoc 2007;106(5): 380–391] Key Words: chincup traction, growing children, maxillary protraction, occipitomental (OMA) appliance, skeletal Class III malocclusion

type.4,10 Such skeletal Class III cases result from growth disharmony between the mandible and maxilla, thus producing a concave facial profile. Treatment of skeletal Class III malocclusion in growing children remains one of the most challenging problems confronting the practicing orthodontist.

Most studies have shown the incidence of Caucasian population afflicted with Angle Class III malocclusion to be below the 5% level.1–4 However, Class III malocclusion is a common clinical problem in orthodontic patients of Asian or Mongoloid descent.5–9 Studies indicate that 63–73% of Class III malocclusions are of a skeletal

©2007 Elsevier & Formosan Medical Association .

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1

Department of Orthodontics, Taipei Medical University Hospital and School of Dentistry, Taipei Medical University, Taipei, 2Department of Orthodontics, Kaohsiung Medical University Hospital, and Faculty of Dentistry and Graduate Institute of Dental Sciences, College of Dental Medicine, Kaohsiung Medical University, Kaohsiung, and 3Department of Orthodontics, National Taiwan University Hospital and School of Dentistry, College of Medicine, National Taiwan University, Taipei, Taiwan. Received: August 2, 2006 Revised: September 26, 2006 Accepted: February 6, 2007

380

*Correspondence to: Dr Hong-Po Chang, Department of Orthodontics, Kaohsiung Medical University, 100 Shih-Chuan 1st Road, Kaohsiung 807, Taiwan. E-mail: [email protected]

J Formos Med Assoc | 2007 • Vol 106 • No 5

Effects of OMA appliance

Skeletal Class III malocclusion can be present with maxillary (Mx) retrusion, mandibular (Mn) protrusion, or some combination of the two.11 A number of treatment protocols have been used to address skeletal Class III cases, including the chincup, the face mask and the Mx protraction combined with chincup traction.12,13 The chincup is recommended in growing patients who have a moderately protrusive mandible and a relatively normal anteroposterior (AP) position and maxilla size. The face mask is recommended for developing skeletal Class III patients with Mx deficiency and relatively normal AP position of mandible or slightly Mn protrusion. The occipitomental anchorage (OMA) appliance of Mx protraction combined with chincup traction is recommended for growing skeletal Class III patients showing both midface deficiency and Mn prognathism. One of the first papers to demonstrate the clinical effects of the OMA appliance appeared in 1944.13 Except for a few case reports of the OMA orthopedic appliance,14–17 little information concerning its treatment effects is available. The purpose of the present study was to cephalometrically evaluate the treatment effects of the OMA orthopedic appliance on the skeletal and dental changes of dentofacial morphology in developing skeletal Class III malocclusions.

Materials and Methods Twenty children (10 boys and 10 girls) with Class III malocclusion consecutively treated with OMA appliance of Mx protraction combined with chincup traction were selected as the treated group. The selection criteria were sella-nasion-A (SNA) = 78°–81°, sella-nasion-B (SNB) = 81°–84° and Anasion-B (ANB) = −6°–0° for Class III patients with both midface deficiency and Mn prognathism. This group of skeletal Class III patients usually have negative incisal overjet and Class III molar relationship. The ANB angle should be not smaller than −7°. At the beginning of treatment, they were a mean age of 9 years 11 months, and the mean treatment duration was 1 year 4 months. At the J Formos Med Assoc | 2007 • Vol 106 • No 5

end of OMA treatment, the Mx and Mn incisors achieved a minimal or positive overjet. The molars were near or true Class I relationship. Often, a vertical-pull chincup may be necessary to prevent the relapse of treated skeletal Class III malocclusion.18 Treatment (or at least supervision) is essential in mixed dentition cases until the full eruption of the permanent teeth. Fixed orthodontic treatment may be necessary as a final phase of treatment to align malposed teeth after establishment of a proper skeletal sagittal relationship. The untreated Class III group consisted of 20 children (10 boys and 10 girls) with a similar mean age (9 years 6 months old at the initial stage) and a similar mean observation period (1 year 5 months). Records of the untreated Class III subjects were obtained from the growth study material which had been collected as a control group in this study. The use of archival radiographs conformed with institutional standards, since the human subjects had participated after providing informed consent to a protocol that had been reviewed and approved by the Institutional Review Board of Kaohsiung Medical University.

OMA appliance The OMA appliance used in the present study is shown in Figure 1. The force for Mx protraction of 200–250 g was applied at each side. Elastics ran at an angle of 5°–10° downward to the occlusal plane from the buccal hooks on the upper first molars to the horns extending from the chincup. The retraction force on the mandible by chincup was 200–250 g each side. The upper first molars were reinforced for anchorage by a modified Nance appliance connecting the lingual bar with the vertical tubes and posts (ST Lock, Sankin Trading Co., Tokyo, Japan) semifixed on the lingual aspect of the bands on the Mx molars. Patients were instructed to wear the appliance for 12 hours/day.

Cephalometric analysis Lateral cephalometric radiographs taken at two different times, at the pretreatment or initial stage (T1) and at the post-treatment or final stage (T2), of two groups were used. The magnification of 381

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A 2

1 29

5 28

3 4 6

27

8 13

7

26

25 23

14 17

9 10 11 15

12 18 19

24 16

20

22 21

B

Figure 1. Occipitomental anchorage (OMA) appliance of maxillary protraction combined with chincup traction: (A) facial photograph with OMA appliance; (B) modified Nance appliance semifixed with bands on the first molars. The entire maxillary base is pulled forward with elastics from buccal hooks of the first molars to the horns extending from the chincup.

each cephalograph used in this study was 10%, which was not corrected. Twenty-nine craniofacial landmarks were identified and digitized (Figure 2). A horizontal reference plane (x-axis) and a vertical reference plane (y-axis) of the x–y coordinate system were created.19,20 The x-axis was registered on sella and defined by the Frankfort horizontal parallel plane. The y-axis was perpendicular to the x-axis passing sella (Figure 3). Horizontal and vertical linear measurements were made on the following eight landmarks 382

Figure 2. Cephalometric landmarks used in this study: 1. S = sella; 2. N = nasion; 3. Rh = rhinion; 4. Or = orbitale; 5. PtmS = pterygomaxillare superior; 6. Ptm = pterygomaxillare; 7. PNS = posterior nasal spine; 8. MPP = midpalatal point; 9. ANS = anterior nasal spine; 10. A = A point; 11. Pr = prosthion; 12. UIE = upper incisor incisal edge; 13. UIA = upper incisor root apex; 14. UMC = upper first molar mesial contact; 15. LIE = lower incisor incisal edge; 16. LIA = lower incisor root apex; 17. LMC = lower first molar mesial contact; 18. Id = infradentale; 19. B = B point; 20. Pog = pogonion; 21. Gn = gnathion; 22. Me = menton; 23. Go = gonion; 24. GoI = gonion interception; 25. AB = anterior border of ramus; 26. PB = posterior border of ramus; 27. Ar = articulare; 28. Cd = condylion; 29. Po = porion.

relative to the x–y coordinate system: anterior nasal spine (ANS), posterior nasal spine (PNS), point A, point B, incisal edge of the Mx incisor (UIE) or upper incisor incisal edge (Mx1), mesial contact point of the Mx first molar (UMC) or upper first molar mesial contact (Mx6), incisal edge of the Mn incisor (LIE) or lower incisor incisal edge (Mn1), and mesial contact point of the Mn first molar (LMC) or lower first molar mesial contact (Mn6). Mx length was measured by the projected distance between the pterygomaxillary fissure (Ptm) and point A to the maxillary plane (A-Ptm/MxP). Mn length was measured by the distance between condylion (Cd) and gnathion (Gn) (Cd–Gn). Measurements of the SNA, SNB, ANB, SN–MxP, SN–RP, SN–MnP, gonial, U1 to FH and L1 to MnP angles, incisal overbite and overjet were taken. The software used in this study was written in the “MATALAB 5.3” (Version 2.3, CAESAR Lab, NCKU, Taiwan). J Formos Med Assoc | 2007 • Vol 106 • No 5


Y

performed on the cephalometric changes (T2–T1) between the treated and control groups (intergroup comparison).

N

S

X

Results PNS

ANS A Mx6 Mn1 Mn6 Mx1 B

Figure 3. X–Y axis (reference planes) and cephalometric landmarks.

In order to assess errors involved in cephalometric tracing and digitizing, 30 randomly selected lateral cephalographs were traced and digitized. The same cephalographs were retraced and redigitized after an interval of 1 week. Correlations between the double measurements were then analyzed for both angular and linear measurements.21 Method error (ME) was calculated by the Dahlberg formula: ME = √Σd2/2n22 where d is the difference between two measurements of a pair and n is the number of double measurements. The MEs were < 0.30 mm for linear measurements and not exceeding 0.60° for angular measurements. The reliability coefficients were from 0.973 to 1.00, indicating a high level of reliability. The combined ME in locating, superimposing and measuring the sagittal and vertical changes of the different cephalometric landmarks in the x–y coordinate system did not exceed 0.85 mm for any of the variables measured. The reliability coefficients were from 0.956 to 1.00, indicating a high level of reliability.

Statistical analysis The two-sample t test was performed on T1 between the treated and control groups. The paired t test was performed between T1 and T2 (intragroup comparison) and the two-sample t test was J Formos Med Assoc | 2007 • Vol 106 • No 5

In the comparison of the treated and control groups at T1, there was no statistically significant difference except for the vertical height of the Mx and Mn molars (Mx6-y = 3.0 mm and Mn6-y = 3.2 mm). Thus, the control group was reasonably selected and satisfied the same criteria as the treated group. The two-sample t test was performed on the cephalometric changes (T2–T1) between the treated and control groups (Table 3). Therefore, these two groups can still be compared with these differences.

Skeletal changes The skeletal effects on the Mx complex showed a statistically significant forward and downward movement in point A when compared in the treated group (Table 1; 2.7 mm, p < 0.001; 1.3 mm, p < 0.01). Significant forward and downward movement of point A was also observed in the control group (Table 2; 0.9 mm, p < 0.01; 2.1 mm, p < 0.001). In comparison between the two groups, the treatment change was statistically significant forward (p < 0.001) but not significantly downward in point A (Table 3). In addition, there was a statistically significant change in the Mx length (A-Ptm/MxP) between the two groups (Table 3; treated group, 1.9 mm and control group, 0.8 mm; p = 0.001). The significance in the mean change of SNA angle between the two groups (Table 3; treated group, 1.8° and control group, 0.5°; p = 0.001) also suggests the treatment effect of the Mx complex. The palate showed a statistically significant forward and downward displacement in point ANS when compared with the treated group (Table 2; 2.6 mm, p < 0.001 and 1.3 mm, p < 0.001). Significant forward and downward displacement of point ANS was also observed in the untreated group (Table 1; 1.1 mm, p < 0.01 and 1.8 mm, p < 0.001). 383

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Table 1. Cephalometric changes in treated group T1

T2

T2–T1

t test

Mean

SD

Mean

SD

Mean

p

Horizontal (mm) ANS-x PNS-x A-x B-x Mx1-x Mx6-x Mn1-x Mn6-x

67.8 19.9 64.0 66.4 68.6 42.1 71.5 44.7

4.3 2.2 4.1 5.8 6.2 4.2 6.0 4.7

70.4 20.5 66.7 66.0 74.6 45.5 71.5 46.5

4.6 2.1 4.5 5.5 5.4 5.5 5.4 4.7

2.6 0.7 2.7 −0.4 6.0 3.4 0.0 1.8

< 0.001 0.027 < 0.001 0.615 < 0.001 < 0.001 0.961 0.005

Vertical (mm) ANS-y PNS-y A-y B-y Mx1-y Mx6-y Mn1-y Mn6-y

43.3 43.1 48.4 89.2 70.6 61.6 67.5 68.8

3.3 2.8 3.5 5.5 3.6 3.4 5.5 3.8

44.6 44.8 49.7 92.6 71.6 65.4 70.0 72.0

3.5 3.4 3.7 5.5 4.6 4.0 5.2 4.3

1.3 1.7 1.3 3.4 1.0 3.8 2.5 3.2

< 0.001 < 0.001 0.003 < 0.001 0.038 < 0.001 < 0.001 < 0.001

Angular (°) SNA SNB ANB SN/MnP SN/RP SN/MxP Gonial U1 to FH L1 to MnP

79.7 81.9 −2.1 33.4 90.0 8.7 123.5 116.1 88.2

3.3 3.3 1.8 5.1 3.9 2.8 5.3 8.1 8.1

81.6 81.3 0.3 34.2 91.7 8.1 122.5 124.3 86.9

3.5 3.4 2.0 48 4.1 3.3 5.6 4.7 8.1

1.8 −0.6 2.4 0.8 1.7 −0.6 −1.0 8.2 −1.3

< 0.001 0.124 < 0.001 0.040 0.001 0.058 0.011 < 0.001 0.324

Linear (mm) A-Ptm/MxP Cd-Gn Overbite Overjet

45.2 114.3 3.5 −2.4

2.5 6.0 3.2 1.8

47.1 117.8 1.4 3.2

3.0 7.8 1.3 1.7

1.9 3.6 −2.1 5.6

< 0.001 0.001 0.001 < 0.001

SD = standard deviation; ANS = anterior nasal spine; PNS = posterior nasal spine; Mx = maxillary; Mn = mandibular; SNA = sellanasion-A; SNB = sella-nasion-B; ANB = A-nasion-B; SN/RP = sella-nasion to ramus plane; MnP = mandibular plane; MxP = maxillary plane; Ptm = pterygomaxillary fissure; Cd = condylion; Gn = gnathion.

There was a statistically significant difference between the two groups in terms of the horizontal change of ANS (p = 0.001) but no significant difference in the vertical change (Table 3). The palate also showed a weak forward tendency and statistically significant downward displacement in point PNS when compared in the treated group (Table 2; 0.7 mm, p < 0.05 and 1.7 mm, p < 0.001). 384

No significantly forward but significantly downward (1.3 mm, p = 0.001) displacement for point PNS was also observed in the control group (Table 1). There was no significant difference between the two groups for the horizontal and vertical changes in point PNS (Table 3). When comparing the two groups, it was observed that there was a weak tendency for posterior downward J Formos Med Assoc | 2007 • Vol 106 • No 5


Table 2. Cephalometric changes in the control group T1

T2

T2–T1

t test

Mean

SD

Mean

SD

Mean

p


67.4 19.9 63.6 64.7 66.9 40.3 70.4 42.9

2.7 1.5 3.0 3.9 4.5 3.7 4.1 4.1

68.4 20.1 64.5 67.1 68.9 41.7 72.6 44.9

3.1 1.9 3.5 4.1 5.4 4.0 5.4 4.4

1.1 0.2 0.9 2.4 2.0 1.5 2.2 2.0

0.002 0.446 0.002 < 0.001 < 0.001 0.092 < 0.001 0.021


41.9 41.5 46.5 86.8 68.6 58.6 65.2 65.6

2.8 3.2 2.5 3.9 3.0 3.2 3.3 3.9

43.7 42.8 48.6 89.9 71.5 61.3 67.5 68.4

2.8 3.8 3.0 4.8 3.7 4.0 4.2 4.6

1.8 1.3 2.1 3.0 2.9 2.7 2.3 2.9

< 0.001 0.001 < 0.001 < 0.001 < 0.001 0.001 < 0.001 0.001


78.4 80.4 −1.9 35.4 90.0 8.9 125.4 113.9 88.5

2.2 1.7 1.7 3.9 3.3 3.1 6.1 6.5 5.5

78.9 81.7 −2.7 34.6 89.0 9.2 125.6 115.1 86.9

2.4 1.7 1.9 4.1 3.4 3.1 6.2 6.9 5.8

0.5 1.3 −0.8 −0.8 −1.0 0.3 0.2 1.2 1.6

0.026 < 0.001 < 0.001 0.015 0.112 0.267 0.364 0.129 0.013

Linear (mm) A-Ptm/MxP Cd-Gn Overbite Overjet

44.1 111.0 3.9 −2.9

2.3 4.6 2.5 1.1

44.9 115.4 4.5 −2.9

2.3 5.3 2.3 1.1

0.8 4.4 0.6 −0.1

0.001 < 0.001 0.019 0.635

SD = standard deviation; ANS = anterior nasal spine; PNS = posterior nasal spine; Mx = maxillary; Mn = mandibular; SNA = sellanasion-A; SNB = sella-nasion-B; ANB = A-nasion-B; SN/RP = sella-nasion to ramus plane; MnP = mandibular plane; MxP = maxillary plane; Ptm = pterygomaxillary fissure; Cd = condylion; Gn = gnathion.

rotation of the SN–MxP angle (Table 3; treated group, −0.6° and control group, 0.3°; p < 0.05). As for the skeletal effects on the mandible, no statistically significant backward but a statistically significant downward (3.4 mm; p< 0.001) movement of point B was observed in the treated group (Table 2). In the control group, the mandible showed a significant forward and downward J Formos Med Assoc | 2007 • Vol 106 • No 5

movement of point B (Table 1; 2.4 mm, p < 0.001 and 3.0 mm, p < 0.001, respectively). There was a statistically significant difference between the two groups in terms of the horizontal change (p < 0.001) but no significant difference in the vertical change of point B (Table 3). In addition, there was no significant change in the Mn length (Cd–Gn) between the two groups. The treatment changes between 385

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Table 3. Comparison of changes between OMA appliance treated and control groups Treated group

Control group

Difference

t-test

T1

T2

T2–T1

T1

T2

T2–T1

Mean

p


67.8 19.9 64.0 66.4 68.6 42.1 71.5 44.7

70.4 20.5 66.7 66.0 74.6 45.5 71.5 46.5

2.6 0.7 2.6 −0.4 6.0 3.4 0.0 1.8

67.4 19.9 63.6 64.7 66.9 40.3 70.4 42.9

68.4 20.1 64.5 67.1 68.9 41.7 72.6 44.9

1.1 0.2 0.9 2.4 2.0 1.5 2.2 2.0

1.6 0.5 1.7 −2.8 4.0 1.9 2.3 −0.2

0.001 0.245 < 0.001 < 0.001 < 0.001 0.050 0.001 0.769


43.3 43.1 48.4 89.2 70.6 61.6 67.5 68.8

44.6 44.8 49.7 92.6 71.6 65.4 70.0 72.0

1.3 1.7 1.3 3.4 1.0 3.8 2.5 3.2

41.9 41.5 46.5 86.8 68.6 58.6 65.2 65.6

43.7 42.8 48.6 89.9 71.5 61.3 67.5 68.4

1.8 1.3 2.1 3.0 2.9 2.7 2.3 2.9

−0.5 0.5 −0.8 0.3 −1.9 1.1 0.2 0.8

0.316 0.300 0.179 0.656 0.001 0.107 0.829 0.663


79.7 81.9 −2.1 33.4 90.0 8.7 123.5 116.1 88.2

81.6 81.3 0.3 34.2 91.7 8.1 122.5 124.3 86.9

1.8 −0.6 2.4 0.8 1.7 −0.6 −1.0 8.2 −1.3

78.4 80.4 −1.9 35.4 90.0 8.9 125.4 113.9 88.5

78.9 81.7 −2.7 34.6 89.0 9.2 125.6 115.1 86.9

0.5 1.3 −0.8 −0.8 −1.0 0.3 0.2 1.2 −1.6

1.3 −1.9 3.2 1.5 2.7 −0.9 −1.2 7.0 0.3

0.001 < 0.001 < 0.001 < 0.002 < 0.001 0.028 0.007 0.001 0.818

Linear (mm) A-Ptm/MxP Cd–Gn Overbite Overjet

45.2 114.3 3.5 –2.4

47.1 117.8 1.4 3.2

1.9 3.6 −2.1 5.6

44.1 111.0 3.9 −2.9

44.9 115.4 4.5 −2.9

0.8 4.4 0.6 −0.1

1.1 −0.8 −2.7 5.7

0.001 0.390 < 0.001 < 0.001

ANS = anterior nasal spine; PNS = posterior nasal spine; Mx = maxillary; Mn = mandibular; SNA = sella-nasion-A; SNB = sella-nasion-B; ANB = A-nasion-B; SN/RP = sella-nasion to ramus plane; MnP = mandibular plane; MxP = maxillary plane; Ptm = pterygomaxillary fissure; Cd = condylion; Gn = gnathion.

the two groups, the increase in the MnP (treated group, 0.8° and control group, −0.8°; p < 0.01) and ramus plane angles (treated group, 1.7° and control group, −1.0°; p < 0.001) and the decrease in the SNB (treated group, −0.6° and control group, 1.3°, p < 0.001) and gonial angles (treated group, −1.0° and control group, 0.2°, p < 0.01) also indicate the growth modification of the mandible (Table 3). 386

A subsequent change in the ANB angle obtained after treatment was also statistically significant compared with the control (Table 3; treated group, 2.4° and control group, −0.8°; p < 0.001).

Dental changes The mean change in the U1 to FH angle and the horizontal position of the Mx incisor tip J Formos Med Assoc | 2007 • Vol 106 • No 5


(Mx1-x) showed a significant labial tipping movement of the Mx incisors when compared with the treated group (Table 2; 8.2°, p < 0.001 and 6.0 mm, p < 0.0001); the changes were also significant in comparison with the control group (Table 3; p = 0.001 and p < 0.001, respectively). On the contrary, the mean change in the L1 to MnP angle and the horizontal position of the mandibular incisor tip (Mn1-x) were not statistically significant when compared with the treated group (Table 2). Even though the change in the L1 to MnP angle was not significant in comparison with the control group, the horizontal change of the Mn incisor tip was significant in comparison with the control (Table 3; treated group, 0.0 mm and control group, 2.3 mm; p = 0.001). Thus, in the treated group, there was a significant change in the incisal overjet from negative to positive values, with a mean increase of 5.7 mm (Table 3, p < 0.001). A significant forward and downward eruption of the Mx molars (Mx6-x and Mx6-y) was observed in the treated group (Table 2, p < 0.0001). But there was no significant difference in terms of horizontal and vertical change in comparison with the control group (Table 3). A significant forward and downward displacement of the Mn molars (Mn6-x and Mn6-y) was observed in both the treated and control groups (Tables 1 and 2, p < 0.01 and p = 0.001; p < 0.01 and p < 0.001), but the range of difference between the groups did not reach statistically significant levels (Table 3).

Treated example case A female case in early permanent dentition at pretreatment and post-treatment with the OMA appliance therapy in the treated group is illustrated by cephalometric radiographs with cephalometric analyses and lateral profile photographs (Figures 4 and 5).

Discussion For a long time, skeletal Class III malocclusions were thought to arise primarily from overgrowth of the mandible. Traditional management of J Formos Med Assoc | 2007 • Vol 106 • No 5

developing Class III malocclusions usually involved chincup therapy or orthopedic appliance treatment to restrain Mn growth, camouflage techniques to advance Mx1 and retract Mn1, or waiting until growth ceased to pursue orthognathic surgery. However, studies show that the skeletal Class III children with both Mx retrusion and Mn protrusion made up 15.0–28.0% of Class III patients.23,24 The orthopedic appliances used for Mx protraction may involve either a facemask/protraction headgear or be used in combination with a chincup. The Mx teeth regions become the point of force application, and the face (forehead, chin), top of head or occipital area become the sources of anchorage. Facemask or protraction headgear uses the chin and forehead for anchorage.25–27 Mx protraction headgear combined with a chincup uses the chin and the top of the head for anchorage.28–31 The OMA appliance of Mx protraction combined with a chincup uses the chin and occipital area for anchorage.13–17 The chincup portion of the orthopedic appliance applies retraction force to the mandible. Mx protraction is recommended for developing skeletal Class III patients with Mx deficiency.25–27 Appliances, which combine Mx protraction and chincup traction, are appropriate for skeletal Class III patients showing both midface deficiency and Mn prognathism.32 Combining Mx protraction therapy for the midface deficiency with the necessary Mn retraction strategy often produces satisfactory results. Skeletal Class III patients with midface deficiency and Mn prognathism are often less difficult to treat than patients with Mn prognathism alone, since some improvement may be obtained in the midface by Mx protraction and some in the mandible by chincup therapy.32 They may not respond as well, if the strategies are focused on one region only. Mx protraction from the Mx first molar area has been shown to produce posterior downward rotation of the MxP.33,34 To reduce such rotation, protraction was carried out from the Mx canine area reported in several studies30,35 or the anchorage of Mx first molars was reinforced with a 387

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Name: YHC Sex: Female Age: 11Y0M

A

SNA SNB ANB SN/MxP SN/MnP SN–RP Gonial a. U1 to FH L1 to MnP Mx1-x Mx6-x Mn1-x Mn6-x Overbite Overjet

B

SNA SNB ANB SN/MxP SN/MnP SN–RP Gonial a. U1 to FH L1 to MnP Mx1-x Mx6-x Mn1-x Mn6-x Overbite Overjet

78.7º 83.9º –5.2º 8.8º 27.8º 89.5º 118.3º 117.1º 85.1º 68.4 mm 44.2 mm 73.2 mm 50.9 mm 9.0 mm –4.1 mm

C

Name: YHC Sex: Female Age: 12Y2M

80.0º 81.5º –1.5º 9.0º 31.3º 90.8º 118.7º 131.9º 84.2º 75.1 mm 48.4 mm 71.6 mm 48.9 mm 2.2 mm 3.6 mm

D

Figure 4. Cephalometric analysis of treated example case: (A) pretreatment tracing; (B) post-treatment tracing, 1 year 2 months after occipitomental anchorage (OMA) orthopedic appliance therapy; (C) composite cephalometric tracings, superimpositions on SN at sella; (D) composite cephalometric tracings, superimpositions on maxilla, and mandible. Solid line, pretreatment; dotted line, post-treatment.

Nance appliance such as in the current study. Thus, the observed rotational change of the MxP was clinically negligible. Most Class III patients with a retruded midface display AP and vertical Mx deficiency.36 The 388

forward and downward movement of the maxilla and associated downward and backward rotation of the mandible is ideally suited for patients with vertical Mx deficiency and excessive overbite. Patients with a deep overbite and short lower J Formos Med Assoc | 2007 • Vol 106 • No 5


A

B

face height can afford greater clockwise rotation of the mandible than those with minimal overbite and a long lower face.9 Cases of Class III malocclusion with a vertical growth pattern/open bite tendency should be treated with caution when considering Mx protraction therapy. When comparing the treated and control groups in this study, the ratio between the forward movement of point A (2.7 mm − 0.9 mm = + 1.8 mm) and backward movement of point B (−0.4 mm − 2.4 mm = −2.8 mm) was approximately 1:1.6. In accordance with our results, Irie and Nakamura28 also reported that the backward repositioning of the mandible was greater than the forward movement of the maxilla in the treatment using the Mx protracting headgear with a chincup. In a study of combined Mx protraction and chincup appliance by Ishii et al,29 however, changes in the maxilla and the mandible (+2.1 mm and −2.0 mm, respectively) almost equally contributed to the correction of the AP jaw relationship. This disparity in response might have been due to variations in treatment protocol including the design of appliances, the number of hours worn per day and the overall treatment time and differences in the skeletal patterns and ages of the subjects. The current study indicates that the treated Class III patients show a backward rotation of the mandible with backward rotation of the Mn and ramus planes and decreased gonial angle but J Formos Med Assoc | 2007 • Vol 106 • No 5

Figure 5. Lateral profile photographs of treated example case: (A) pretreatment; (B) post-treatment.

without significant downward displacement of the B point as compared to the untreated controls. Backward rotation of the Mn complex is a typical skeletal effect of orthopedic chincup therapy.4,37 However, a closure of the gonial angle applied by chincup force was not revealed in the treated example case. The effect of skeletal age on response of the gonial angle with chincup therapy is yet to be determined. As with the mandible, Mx growth can be redirected by the orthopedic force applied by the Mx protraction only during the growth period.30,38 Our previous study utilized morphometric procedures, including Procrustes and thin-plate spline (TPS) analysis.39 TPS analysis revealed that major deformation consisted of forward advancement of the Mx complex with negligible rotation of the palatal plane and a forward direction of growth of the Mn condyle associated with a restriction in sagittal advancement of the chin. Considerable dentoalveolar components contributed to the correction of anterior crossbite. In this current study, the cephalometric analysis utilizing linear and angular measurements were undertaken, which showed consistent findings with our previous TPS analysis. However, description of craniofacial shape change from cephalographs is a difficult notion as conventional cephalometry is fundamentally deficient in its application to systematic shape change. TPS is a geometric 389

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morphometric technique that expresses the changes between two configurations as a continuous deformation, derived from landmarks of size-scaled configurations. TPS graphic analysis seems to provide a valuable supplement for conventional cephalometric analysis because the complex patterns of craniofacial shape changes are more easily visualized suggestive by means of grid deformations. The ultimate treatment goal of skeletal Class III patients should not only be the correction of the jaw relationship and negative incisal overjet related to mesial occlusion at that stage, but also the stabilization of the intermaxillary skeletal and dental relationships resulting from orthopedic appliance therapy. Thus, close observation and follow-up of excessive Mn development and deficient Mx growth during adolescence, particularly during phase II orthodontic treatment, is essential. In conclusion, the current study demonstrated that the OMA appliance of Mx protraction combined with chincup traction is effective for correcting skeletal Class III malocclusion with midface deficiency and Mn prognathism in growing children. The study also suggested that the treatment effects of this orthopedic appliance may be considered to be from skeletal and dentoalveolar changes. The skeletal effects were mainly obtained by stimulating forward growth of the Mx complex with negligible counterclockwise rotation of the MxP and restraining forward advancement of the mandible with backward and downward rotation of the MnP. The dentoalveolar effects were mostly due to the labial tipping movement of the Mx incisors. Further longitudinal long-term studies are required to fully ascertain the long-term stability of OMA orthopedic therapy.

Acknowledgments This study was supported by grants from the National Science Council of Taiwan (NSC91-2320B-037-022; NSC92-2320-B-037-015; NSC93-2320B-037-001). 390

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