Effects of protraction mechanics on the midface

ORIGINAL ARTICLE Effects of protraction mechanics on the midface Valmy Pangrazio-Kulbersh, DDS, MS,a Jeff Berger, BDS, DipOrtho,b and Gerald Kersten, DMD, MS Detroit, Mich., and Victoria, B.C., Canada Forty patients with Class III maxillary deficiencies were each treated with a bonded maxillary palatal expansion appliance followed by protraction. Nineteen of the 40 patients were retained with a Frankel III appliance. This group was compared with 24 Class I patients treated solely with bonded expansion appliance mechanotherapy. To determine at which level protraction mechanics affects the maxilla, the Walker’s analysis and other cephalometric measurements were used. The protraction group showed significant increases (p < .05) in the following measurements: ANB angle, Wits, A perpendicular to nasion and in sella to A point. Anterior molar movement, without changes in posterior nasal spine or upper incisor to SN, was evident (p < .05). Favorable change in the facial profile was noted. There were no changes in the angles between sella-nasion and its relationship with the Frankfurt, occlusal, palatal, and mandibular planes. Walker’s analysis showed no change in the position of orbitale. The control group did not demonstrate any significant changes in the position of the maxillary complex as a result of expansion mechanics. The retention group maintained the position of the maxilla postprotraction. Facial contour was maintained and other profile related variables improved. (Am J Orthod Dentofacial Orthop 1998;114: 484-91)

S

keletal Class III malocclusions are considered one of the most complex and difficult orthodontic problems to diagnose and treat. Much controversy and uncertainty surrounds the effects and stability of early treatment with this type of skeletal pattern. In many Class III skeletal cases, the discrepancy is usually the result of a combination of maxillary and mandibular dysplasia. Ellis and McNamara1 found that almost one third of 302 adult patients with Class III malocclusions had a combination of maxillary retrusion and mandibular prognathism. The sample exhibited 19.5% maxillary skeletal retrusion with a normally positioned mandible whereas 19.1% demonstrated a normal positioned maxilla combined with mandibular prognathism. Sue et al.2 found that 62% of the cases examined had a component of maxillary skeletal retrusion as part of the diagnosis. In a subsequent study, Guyer et al.3 found maxillary retrusion with a normally positioned mandible to be present in 23% of their 13 to 15 year old sample. From these and other studies, it appears that early treatment of Class III malocclusions commonly involves correction of the maxilla with, or without, manipulation of mandibular growth.

From the University of Detroit Mercy, School of Dentistry. aAssociate Professor, Department of Orthodontics. bClinical Associate Professor, Department of Orthodontics. cIn private practice. Reprint requests to: Dr. Jeff Berger, 600 Tecumseh Road East, Suite 241, Windsor, Ontario, Canada N8X 4X9. Copyright © 1998 by the American Association of Orthodontists. 0889-5406/98/$5.00 + 0 8/1/87457

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A review of the literature reveals that early treatment with a maxillary protraction appliance is effective in correcting the Class III malocclusion due to maxillary retrusion.4-8 Class III skeletal patterns often exhibit a high incidence of deficient transverse maxillary growth7 and maxillary expansion is often the first treatment procedure. Furthermore, McNamara6 and Turley5 have recommended the use of bonded rapid palatal expansion appliance for several days before beginning protraction in order to facilitate maxillary movement. After active protraction of the maxilla, retention is vitally important in maintaining the treatment effects of the facemask. Petit8 suggested the Fränkel III regulator be used for 6 months postprotraction. Whereas other Class III studies 9-11 have shown that anterior displacement of the maxilla and redirection of mandibular position result in a favorable change in skeletal relationships, few studies have attempted to demonstrate the effect of protraction at various levels of the maxillofacial complex. Leonard and Walker12 developed a cephalometric analysis for the diagnosis of midfacial retrusion at the Le Fort II level. This analysis, Table I, aids the clinician in determining the level at which the maxilla is deficient in the Class III patient. The purpose of this study is to evaluate the effects of protraction mechanotherapy on the maxilla and midfacial region. Retention of the protraction treatment with Fränkel III functional appliances was also evaluated.

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MATERIAL AND METHODS

Table 1.

Lateral cephalograms of patients who received maxillary expansion and protraction were evaluated and compared with cephalograms of a Class I alternate treatment group who were treated by maxillary expansion only. The protraction group consisted of 40 caucasian patients, 24 females and 16 males, with mean age of 8.9 (range, 6.5 to 12 years) and all pre-sesamoid maturation, as assessed radiographically. For the purpose of this study, the patients were carefully selected as having Class III midfacial deficiency as determined by specific inclusion criteria measurements that included: SNA of less than 80°, ANB of less than 1°,3 Wits of less than –1 mm13 and A point to nasion perpendicular.14 In order to prevent excluding Class III cases in which a high SNA value was present because of an abnormally short cranial base, the SN (sella-nasion) length was corrected to standard values,14 as tabulated in the Atlas of Craniofacial Growth.15 The SNGOGN value was not a measurement used to select the cases for this study. The protraction group was treated by a technique that uses a maxillary bonded rapid palatal expansion appliance,6 in which bilateral protraction hooks were embedded in the cuspid region of the appliance. The mean expansion time was 5.7 weeks (range, 1 to 12 weeks). A protraction force of 400 to 600 g per side was used with an anteroinferior force vector of 30° to the occlusal plane. The elastics were attached to a small or medium sized Reverse-pull face crib (Great Lakes Orthodontics Ltd., Tonawanda, NY) according to the facial dimensions of the patient. Mean protraction time was 31 weeks (range, 12 to 56 weeks). Patients were instructed to wear elastics 14 to 16 hours a day until an overjet of 5 mm was obtained with the appliance in place. At the completion of active protraction, patients were placed either in retention with a Fränkel III appliance, or continued comprehensive treatment with fixed appliances if all secondary teeth had erupted. Of the total cases studied, 19 patients (7 male and 12 female) with a mean age of 9.2 years were treated with a Fränkel III appliance immediately after removal of the protraction devices. The mean treatment time with the Fränkel III appliance was 14 months (range, 7 to 24 months). The experimental group was compared with an alternate treatment group consisting of 24 caucasian patients (13 male and 11 female) with a mean age of 9.4 years (range, 7.0 to 12.5) who had exhibited Class I skeletal and dental characteristics. This group of patients displayed a maxillary transverse deficiency and also was treated with a bonded rapid palatal expansion appliance. The mean expansion time was 9.6 weeks (range, 2 to 12 weeks).

Angle

485

Walker’s analysis

SNOr NOrA SOr:SN SOr:SA Or-NA (mm)

Mean

Standard deviation

57.83 128.90 0.85 0.67 13.52

±4.18 ±8.13 ±0.35 ±0.27 ±2.45

Cephalograms were taken before treatment, immediately after protraction, and at completion of Fränkel III treatment. All cephalograms were taken in centric relation position and were standardized for magnification and traced manually by the same operator. Ten cephalograms were retraced and analyzed for reproducibility. Using paired Student’s t tests, no statistically significant difference was observed. The measurements used in this study are illustrated in Figs. 1, 2, and 3. An analysis of variance (ANOVA) was used to derive the significance levels of each variable between the two groups. A P value of .05 or less was considered to be significant. Descriptive statistics were used to calculate the means for each variable at any given time during the study. RESULTS Immediate post-protraction findings

The protraction group, Table II, showed a significant change in the ANB angle (P = .00) with a mean increase of 2.3° immediately post-protraction. However, there were no statistically significant changes in the SNA (P = .160) or SNB (P = .45) angles. The Wits analysis displayed a significant change of 3.9 mm (P = .00) in the relationship between denture bases. The expansion group, Table III, did not show a significant change in SNA, SNB, ANB, or Wits. The linear measurement of sella to A point (SA) increased significantly (P = .027), indicating a downward and forward movement of the maxilla with a mean change in the sample of 2.4 mm post-protraction. The measurement of N perpendicular to A point (Aperp) was also significantly changed (P = .016) by A point moving anteriorly 2.0 mm. The expansion group did not show a change in sella to A point (P = .48) or N perpendicular to A point (P = .860). The maxillary first molars (SNpMOLAR) moved mesially 2.4 mm (P = .003) resulting in a significant change in molar position. Posterior nasal spine (PNS) did not show any significant change in position (P = .398). There was no change in upper incisal angulation (SNI) (P = .958). The expansion group did not show a change in molar position (P = .149), PNS (P = .747), or upper incisal angulation (P = .123).

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Fig 1. Linear cephalometric measurements.

The change in ANB angle was reflected in a significant favorable change in the facial contour angle (P = .001) with a mean increase of 5.1° immediately after protraction. The position of upper lip at subnasale (SnGLPg) also improved significantly (P = .002) with a mean increase of 2.5 mm. The expansion group did not show any statistically significant change in the facial contour angle (P = .848) or upper lip protrusion (ULP) (P = .660). There was no significant angular changes in the following horizontal reference planes in either group: sella nasion to Frankfort horizontal (SN-FH), sella nasion to palatal plane (SN-PP), sella nasion to

occlusal plane (SN-OP), or sella nasion to mandibular plane (SN-MP). The vertical position of both the occlusal plane (OP) and the palatal plane (PP) remained relatively stable when measured along a perpendicular traced midpoint from the SN reference plane. At a more superior level, the variables of Walker’s analysis for diagnosis of midfacial deficiency did not show a statistically significant difference in the protraction or expansion groups. The analysis indicates that there was no change in the position of the maxilla at the Le Fort II level as represented by orbitale as a reference.

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Fig 2. Angular cephalometric measurements.

Fränkel III: Retention findings

In the Fränkel III group, Table IV, there was a significant change in ANB (P = .042) of 1.7° from the initial to immediate post-protraction. This change continued to increase an additional 0.8° (P = .27) during retention. During the FR III retention period, the Wits continued to improve 1.2 mm, however, this change was not statistically significant (P = .230). Sella to A point (SA) increased significantly (P = .020) 3.0 mm immediate post-protraction and an additional 2.6 mm during retention (P = .020). N perpendicular to A point (A-perp) was also significantly changed (P = .036) with a 1.8 mm mean forward movement of A point immediate post-protraction. Continued forward movement of 1.3 mm was observed during the retention period, but this was not statistically significant (P = .14).

The maxillary molars (SNpMOLAR) moved forward significantly (P = .018) with a mean change of 2.0 mm immediate post-protraction and 2.2 mm during retention (P = .021). There was no change in posterior nasal spine position noted in the Fränkel III retention group immediate post-protraction or during retention. The facial contour angle showed a significant improvement by increasing 3.7° (P = .004) immediate post-protraction, however, no significant change was noted during the retention period. An increase in upper lip protrusion (SnGLPg) of 1.9 mm (P = .049) was noted post-protraction with no significant change during retention. Other profile-related changes that were significant at the end of retention were a mean increase in upper lip length (ULL) of 2.1 mm (P = .032) and in lower lip length (LLL) of 3.1 mm (P = .040).

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Fig 3. A, Walker’s analysis, and B, profile measurements.

The vertical position of A point (SNpA), palatal plane (SNpPP), occlusal plane (SNpOP) and menton (SNpMe), to SN perpendicular did not change significantly post-protraction. However, during retention with the FR III, there was a significant change in the vertical position of A point to SN perpendicular (P = .023) increasing 2.3 mm. The palatal plane also moved inferiorly (P = .002) 3.8 mm as did the occlusal plane (P = .000) 4.7 mm. Menton moved inferior 6.6 mm in relation to SN perpendicular (P = .009) during retention with the FR III appliance. There was no significant change in the mandibular plane angle (P = .799) or in the other horizontal reference planes as represented by the angles sella nasion to Frankfort horizontal (P = .870), SN to palatal plane (P = .306), and SN to occlusal plane (P = .453) post protraction and postretention. The Walker’s analysis did not show a significant

change in the ratio of SA:SO (Sella A point:Sella Orbitale) immediate post-protraction. However, a mean decrease in the ratio of .018 was significant (P = .027) postretention. The other variables of Walker’s analysis, NOrA (P = .929), SNor (P = .849) were not indicative of a change in the position or orbitale postprotraction or during retention. DISCUSSION Post-protraction results

The post-protraction results indicated that there was significant movement of A point in a downward and forward direction as seen by an increase in the measurement from sella to A point. As a result of a statistically significant change in the horizontal vector of A point to nasion perpendicular and no change in the vertical measurement of A point perpendicular from sella nasion, one could conclude that the majority of the

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American Journal of Orthodontics and Dentofacial Orthopedics Volume 114, Number 5

Protraction group (n = 40), preprotraction and postprotraction values Table II.

Variable

Initial

Linear cephalometric measurements (mm) SN 69.4 SnpMOLAR 21.2 SNpPNS 16.1 SNpOP 63.3 SNpPP 45.3 SA 81.0 SNpA 53.9 SNpMe 107.4 Wits –5.3 Aperp –4.2 Angular cephalometric measurements SN-FH 9.6 SN-PP 7.1 SN-OP 17.7 SN-MP 35.7 SNA 80.0 SNB 79.5 ANB 0.6 SNPg 79.7 SNI 107.0 Walker’s analysis NOrA (degrees) 128.2 SOr (degrees) 55.9 SA/SO ratio 0.72 Profile measurements F contour (degree) 8.17 NLA (degree) 105.0 ULL (mm) 20.3 LLL (mm) 43.5 SnGLPg (mm) 3.8 ULP (mm) 4.2 LLP (mm) 5.3

Postprotraction

Mean change

Standard deviation

70.1 23.6 16.3 65.9 46.6 83.4 55.7 111.1 –1.4 –2.2

0.7 2.4* 0.2 2.6 1.3 2.4* 1.8 3.7 3.9* 2.0*

2.49 2.54 3.52 2.65 2.54 2.98 2.62 3.39 1.06 1.26

9.6 7.8 19.2 36.7 81.4 78.5 2.9 78.9 107.5

0.0 0.7 1.5 1.0 1.4 –1.0 2.3* –0.8 0.5

1.92 2.24 2.89 2.92 1.93 1.63 0.63 1.86 3.83

128.0 55.7 0.71

–0.2 –0.2 –0.01

9.72 2.74 0.02

13.2 104.0 21.1 44.3 6.3 5.5 5.2

5.1* –1.0 0.8 0.8 2.5* 1.3 –0.1

2.15 4.78 3.52 3.63 1.20 1.28 1.36

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Alternate treatment group (n = 24), preexpansion and postexpansion values Table III.

Variable

Initial

Linear cephalometric measurements (mm) SN 71.5 SnpMOLAR 21.3 SNpPNS 16.9 SNpOP 65.5 SNpPP 44.9 SA 82.4 SNpA 54.9 SNpMe 107.4 Wits –0.6 Aperp 0.0 Angular cephalometric measurements SN-FH 10.4 SN-PP 7.7 SN-OP 19.9 SN-MP 35.2 SNA 79.9 SNB 77.1 ANB 3.1 SNPg 77.4 SNI 103.2 Walker’s analysis NOrA (degrees) 126.1 SOr (degrees) 53.4 SA/SO ratio 0.71 Profile measurements F contour (degree) 13.7 NLA (degree) 110.7 ULL (mm) 21.0 LLL (mm) 42.8 SnGLPg (mm) 6.5 ULP (mm) 4.7 LLP (mm) 3.9

Postprotraction

Mean change

Standard deviation

72.0 22.5 16.6 66.8 45.8 83.1 55.8 109.6 –0.8 –0.1

0.5 1.2 –0.3 1.3 0.9 0.7 1.1 2.2 –0.2 –0.1

2.39 2.78 3.29 2.56 2.29 3.04 2.42 3.59 0.77 1.15

10.6 7.8 20.3 35.8 80.1 76.8 3.2 77.4 101.0

0.2 0.1 0.4 0.6 0.2 –0.4 0.1 0.0 –2.2

1.74 1.96 2.89 2.63 1.93 1.67 0.81 1.98 3.54

125.6 53.3 0.70

0.5 –0.1 –0.01

9.56 2.85 0.03

13.9 109.8 20.8 43.6 6.7 4.8 3.9

0.2 –1.1 –0.2 0.8 0.2 0.1 1.0

2.46 4.81 2.75 3.96 1.41 1.10 1.87

*Denotes statistical significance P < .05.

*Denotes statistical significance P < .05.

movement in A point was in a horizontal direction. Reinforcing the horizontal change in the position of A point, there was a statistically significant change in the Wits analysis without a change in the angulation of the functional occlusal plane. It has been established that sella nasion generally grows 1 mm per year in length.16 The results coincided with this trend and demonstrated 0.7 mm of growth in the treatment period of 31 weeks. The distance from sella to A point normally increases 0.8 mm per year in the group of 9 year olds. The results demonstrated that A point repositioned 2.4 mm, therefore, one could conclude that growth may have played a role in the change in the position of the maxilla but that 1.7 mm of this change was due to protraction mechanics. The amount of forward maxillary movement due to the effects of the

expansion appliance was minimal possibly due to the different type of expansion appliance in this study, as compared with the Haas type of expansion device.17,18 The SNA and SNB angles did not show a statistically significant change. The mean increase in SNA of 1.4° and the mean decrease in the SNB of 1° account for the significant increase of 2.4° in ANB. The position of posterior nasal spine remained stable, possibly because of the concomitant appositional growth that takes place at the posterior border of the maxilla.19 One could speculate that the forward movement of the maxillary first molars was the result of forward alveolar repositioning. The molars may have also migrated into either the existing primate spaces or into additional arch length created by the expansion process. Because there was no change in the maxillary

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American Journal of Orthodontics and Dentofacial Orthopedics November 1998

Fränkel III group (n = 19) initial, postprotraction and post-Fränkel values Table IV.

Variable

Initial

Linear cephalometric measurements (mm) SN 69.5 SnpMOLAR 19.6 SNpPNS 16.5 SNpOP 60.0 SNpPP 41.2 SA 79.8 SNpA 51.9 SNpMe 103.2 Wits –5.0 Aperp –4.4 Angular cephalometric measurements SN-FH 9.1 SN-PP 7.5 SN-OP 18.6 SN-MP 34.6 SNA 79.5 SNB 79.7 ANB –0.2 SNPg 79.8 SNI 108.9 Walker’s analysis NOrA (degrees) 125.1 SOr (degrees) 55.8 SA/SO ratio 0.718 Profile measurements F contour (degree) 7.1 NLA (degree) 103.3 ULL (mm) 19.1 LLL (mm) 41.5 SnGLPg (mm) 3.5 ULP (mm) 4.1 LLP (mm) 5.5

Postprotraction

PostFränkel

Standard deviation

70.1 21.6* 16.7 61.4 42.1 82.8* 53.6 104.8 –1.8* –2.6*

72.0 23.8* 17.0 66.1* 45.9* 85.4* 55.9* 111.4* –0.6 –1.3

2.38 2.70 3.72 2.48 2.00 2.61 2.57 3.26 0.89 0.70

9.3 8.1 19.0 34.8 81.0 79.5 1.5* 79.5 110.2

9.2 6.9 18.1 34.4 81.4 79.1 2.3 79.1 107.8

1.82 1.59 3.23 2.85 1.73 1.75 0.81 2.00 3.02

125.6 55.5 0.723

125.4 55.2 0.705*

8.93 2.66 0.03

10.8* 101.9 19.6 42.4 5.4* 4.7 5.9

11.8 107.1 21.7* 45.5* 6.1 5.0 5.3

2.75 5.42 2.36 4.32 1.09 1.26 1.54

*Denotes statistical significance P < .05.

incisor angulation, one would not expect movement to have occurred at the expense of forward incisor molar repositioning. Further studies are necessary to confirm these findings. The stability of the horizontal reference planes would suggest that there was good control of the vertical force vectors during treatment. The changes seen in the anterior repositioning of the maxilla resulted in favorable alteration in the profile. One could assume that the increase in the facial contour angle was the result of anterior movement of A point because SNB remained relatively stable in the sample. The significant change in position of subnasale, as measured from the glabella pogonion line, would also indicate that the improvement in profile was the result of anterior movement of A point.

Walker’s analysis demonstrated that no significant angular changes occurred in the position of orbitale as a result of protraction of the maxilla. However, the ratio of SA:SOr was near significance at the P = .085 level, which shows that this may be the result of an increase in the SA distance and a stable SOr value. The ineffectiveness of the protraction mechanics at the level of orbitale could be due to the amount of force used in this sample, which was lower than the 800 to 1500 g as described by Petit.8 Further studies using higher force levels could support this point. The alternate treatment group did not show a significant change in any of the variables measured. This would lead one to believe that changes seen in the protraction group were the result of the protraction mechanics and not from the expansion appliance. Fränkel III Retention Results

In the Fränkel III retention group, there was a significant change in ANB (P = .042) of 1.7° from the initial to immediate post-protraction. This change continued to increase an additional 0.8° (P = .27) during retention. SNA (P = .266) and SNB (P = .720) values did not demonstrate any significant changes in this Fränkel retention group. The Wits value increased 3.2 mm from initial to post-protraction with a statistical significance of P = .011, indicating a positive change in the interrelationship between the dental bases. During the FR III retention period the Wits continued to improve 1.2 mm, however, this change was not statistically significant (P = .230). The vertical measurement from sella to A point increased significantly immediately post-protraction and continued to increase during retention. This increase could be explained by the downward repositioning of the maxilla as depicted by the vertical measurement from sella nasion perpendicular to A point as well as the vertical increases in SN to the occlusal, palatal planes and to menton. Therefore, it could be postulated that the Fränkel III appliance had a significant effect on the vertical development of the face. The values for Wits and A perpendicular increased significantly during protraction. During FR III retention, these measurements decreased slightly due to a decrease in the angulation of the occlusal and palatal planes. Therefore one could speculate that the anteroposterior position of A point remained stable during retention. Furthermore, within this group, the growth factor plays a larger role as the total mean treatment time from post-protraction through retention was 1.2 years. Because A point grows 0.8 mm per year (measured along SA vector),16 one would expect approxi-

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American Journal of Orthodontics and Dentofacial Orthopedics Volume 114, Number 5

mately .9 mm of movement due to growth. Because the results indicated a total of 2.6 mm of movement in A point during the retention period, one could suggest that the majority of the effects of the protraction were maintained and that continued forward and downward growth of the maxilla resulted from FR III wear. The findings substantiate the results of McNamara and Huge15 who reported forward repositioning of the maxilla as well as an increase in lower anterior face height with the use of the Fränkel III appliance. The Walker’s Analysis indicated that the Fränkel III appliance had no effect on the position of the midface at the level of orbitale. With the use of an implant study, Iseri and Solow20 have demonstrated that appositional growth on the orbitale floor will compensate for vertical and horizontal sutural displacement of the maxillary complex. This finding, although observed over a substantially longer period, may account for the lack of effect on orbitale observed in this study. The significant anterior repositioning of the maxillary molars immediately post-protraction as well as during the retention period could be the result of either migration into the primate or leeway spaces or additional arch length created with expansion, but not due to incisor repositioning because no significant change in this measurement was noted. The facial contour and upper lip protrusion (SnGLPg and ULP) were improved significantly postprotraction and continued to improve during retention with the FR III. The upper and lower lip length increased significantly. This increase would be in accordance with the vertical change induced by Fränkel III appliance wear and to natural growth expected during the treatment period.21 CONCLUSION

The present investigation has demonstrated that protraction forces on the maxilla will result in movement of the maxilla in a downward and forward direc-

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tion. However, no significant changes were observed in the position of other midfacial structures with the use of Walker’s analysis. The Fränkel III appliance was determined to be an effective method for maintaining the results obtained by protraction of the maxilla. REFERENCES 1. Ellis E, McNamara JA. Components of adult Class III malocclusion. J Oral Maxillofac Surg 1984;42:295-305. 2. Sue G, Chanoca SJ, Turley PK, Itoh J. Indicators of skeletal Class III growth. J Dent Res 1987;66:343. 3. Guyer EC, Ellis E, McNamara JA, Behrents RG. Components of Class III malocclusion in juveniles and adolescents. Angle Orthod 1986;56:7-30. 4. Delaire J. L’articulation du fronto-maxillaire: basis theoretiques et principles generaux d’application de forces extra-orales postero-anterieures sur masque ortopedique. Rev Stomat Paris 1976;77:921-30. 5. Turley P. Orthopedic correction of Class III malocclusion with palatal expansion and a custom protraction headgear. J Clin Orthod 1988;22:314-25. 6. McNamara JA. An orthopedic approach to the treatment of Class III malocclusion in young patients. J Clin Orthod 1987;21:598-608. 7. Hata S, Itoh T, Nakagawa M, Kamogashira K, Ichiakawa K, Matsumoto M, et al. Biomechanical effects of maxillary protraction on the craniofacial complex. Am J Orthod Dentofacial Orthop 1987;91:305-11. 8. Petit H. Adaptations following accelerated facial mask therapy. In: McNamara JA, Ribbens KA, Howe RP, eds. Clinical alteration to the growing face. Monograph 14, Craniofacial Growth Series. Center for human growth and development, University of Michigan, Ann Arbor, Mich. 1983. 9. Tanne K, Hiraga J, Kakiuchi K, Yamagata Y, Sakuda M. Biomechanical effect of anterior directed extraoral forces on the craniofacial complex: a study using finite element method. Am J Orthod Dentofac Orthop 1989;95:200-7. 10. Nanda R. Biomechanical and clinical considerations of a modified protraction headgear. Am J Orthod 1980;78:125-39. 11. Ishii H, Morita S, Takeuchi Y, Nakamura S. Treatment effects of combined maxillary protraction and chin cup appliance in severe skeletal Class III cases. Am J Orthod Dentofacial Orthop 1987;92:304-12. 12. Leonard M, Walker GF. A cephalometric guide to the diagnosis of midface hypoplasia at the Le fort II level. J Oral Surg 1977;35:21-4. 13. Jacobson A. The “Wits” appraisal of jaw disharmony. Am J Orthod 1975;67:125-38. 14. McNamara JA. A method of cephalometric evaluation. Am J Orthod 1984;86:449-69. 15. McNamara JA, Huge SA. The functional regulator (FR3) of Frankel. Am J Orthod 1985;88:409-24. 16. Riolo ML, Moyers RE, McNamara JA, Hunter S. An atlas of craniofacial growth. Center for Human Growth and Development. Monograph 2, Craniofacial growth series, University of Michigan, Ann Arbor, Mich. 1974. 17. Haas AJ. Rapid expansion of the maxillary dental arch and nasal cavity by opening the mid-palatal suture. Angle Orthod 1961;31:73-90. 18. Haas AJ. The treatment of maxillary deficiency by opening the mid-palatal suture. Angle Orthod 1965;35:200-17. 19. Enlow DH. Handbook of facial growth. Philadelphia: W.B. Saunders Co. 1975. 20. Iseri H, Solow B. Average surface remodeling of the maxillary base and the orbital floor in female subjects from 8 to 25 years: an implant study. Am J Orthod 1995;107:49-57. 21. Vig PS, Cohen M. Vertical growth of the lips: a serial cephalometric study. Am J Orthod 1979;75:405-15.