orthodontic treatment changes of chin position

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ORIGINAL ARTICLE

Orthodontic treatment changes of chin positionin Class II Division 1 patientsMark B. LaHaye,a Peter H. Buschang,b R. G. “Wick” Alexander,b,c and Jim C. Boley b,d

Thibodoux, La, and Dallas, Tex

Introduction: Because most patients with skeletal Class II malocclusions also have mandibular deficiencies,

treatment plans should include improvement in chin projection. On that basis, the purposes of this study

were to (1) determine how Class II treatment affects anteroposterior (AP) chin position in growing subjects

and (2) ascertain the most important determinants of AP chin position. Methods: Pretreatment and

posttreatment lateral cephalograms of 67 treated patients (25 extraction headgear and Class II elastics, 23

nonextraction headgear, and 19 Herbst) were collected, traced, and digitized. The average pretreatment age

was 12.2 years (range, 9-14 years), and the average treatment duration was 30.2 months (range, 17-65

months). Cephalometric changes were compared with 29 matched untreated Class II controls. Mandibular

superimpositions were used to evaluate condylar growth and true mandibular rotation. Results: All 3

treatment methods produced normal dental relationships and restricted or inhibited AP maxillary growth, withno significant improvement of AP chin position. Differences between changes in vertical position of the

maxilla, maxillary and mandibular molars, and condylar growth could not reliably predict changes in chin

position. Analyses demonstrated that true mandibular rotation was the primary determinant of AP chin

position. Stepwise multiple regression showed that, combined with true mandibular rotation, condylar growth

and movements of the glenoid fossa accounted for 81% of the variation in AP changes of pogonion.

Conclusions: Contemporary treatments do not adequately address mandibular deficiencies. Future treat-

ments must incorporate true mandibular rotation into Class II skeletal correction. (Am J Orthod Dentofacial

Orthop 2006;130:732-41)

T

hroughout the history of orthodontics, clinicianshave been faced with the challenge of correct-

ing skeletal Class II malocclusions. It has beenreported that approximately 15% to 30% of Americanchildren have Class II malocclusions, comprising about20% to 30% of all orthodontic patients.1 They typicallyhave retrusive chin positions,2-7 and the skeletal ClassII pattern is generally not self-correcting.3,8-10

Three commonly used methods of Class II correc-tion have been (1) extraction therapy with headgear andClass II elastics (Ext HG), (2) nonextraction headgeartreatment (NE HG), and (3) functional appliances, suchas the Herbst. Headgear treatment, both extraction andnonextraction, causes orthopedic changes in the maxilla

in addition to orthodontic tooth movement.11-18

Head-gear treatment predictably achieves normal dental ClassI molar and canine relationships, proper overbite and

overjet, and significant anteroposterior (AP) improve-ments. Treatment does not, however, generally cause

significant improvements in the AP relationship of thechin.14,16,19-22 Some studies even reported unfavorablebackward rotation of the mandible after headgear ther-apy.14,15,17,18,23-25

Herbst treatment, which can also produce accept-able dental results, has been shown to cause an overallincrease in mandibular length as well as headgeareffects on the maxilla.26-29 Initial improvements inmandibular growth might diminish during fixed appli-ance treatment.2,28 Studies have also shown either nochange or a slight increase in the mandibular planeangle with Herbst treatment.26,29-31 This suggests that

Herbst treatment produces results similar to headgeartreatment, although no well-controlled comparisons of treatment effects on chin position are available.

The inability of contemporary treatment approachesto adequately address the chin is important for profileconsiderations. People frequently seek orthodontictreatment because of facial disharmony32; straighterprofiles and more prominent chins are preferred esthet-ically over retruded chin positions.33,34 Correction of Class II Division 1 malocclusions requires maintenanceof normal AP maxillary growth and greater than normalAP mandibular growth. Headgear and functional appli-

aPrivate practice, Thibodoux, La.bFaculty, Baylor College of Dentistry, Dallas, Tex.cPrivate practice, Arlington, Tex.dPrivate practice, Richardson, Tex.Reprint requests to: Peter H. Buschang, Department of Orthodontics, BaylorCollege of Dentistry, 3302 Gaston Ave, Dallas, TX 75246; e-mail,[email protected], November 2004; revised and accepted, February 2005.0889-5406/$32.00Copyright © 2006 by the American Association of Orthodontists.doi:10.1016/j.ajodo.2005.02.028

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ance approaches that restrain maxillary growth whileallowing mandibular growth to catch up producestraighter profiles, but less than desired facial angles.

Possible mechanisms for improving AP chin posi-

tion include (1) increase in mandibular size, (2) repo-sitioning of the glenoid fossa, and (3) counterclockwiseor forward rotation of the mandible. Implant andhistological studies show that the anterior aspect of thechin is extremely stable.35-37 Schudy38 proposed thatthe growth of the condyles must exceed the sum of thevertical growth of the corpus of the maxilla and verticalgrowth of the maxillary and mandibular processes if animprovement in chin position is to occur. Apparentlysupporting this hypothesis, Sinclair and Little39 attrib-uted anterior repositioning of the chin to greater con-dylar growth than maxillary growth.

Alternatively, true mandibular rotation might play amore fundamental or primary role in determining APchin position. Buschang and Santos-Pinto40 developedmathematical models showing that the AP position of pogonion in growing children and adolescents, bothtreated and untreated, is most closely associated withtrue rotation of the mandible, as defined by Solow andHouston,41 followed by AP condylar growth and theAP position of the glenoid fossa, respectively.40 Thesefindings were later validated by Thompson et al.42

Because of the lack of studies comparing matchedsamples, the purposes of this study were to evaluatemaxillary and, particularly, mandibular changes asso-

ciated with 3 commonly accepted forms of Class IItreatment. Our aims were to (1) determine how varioustreatments affect chin position, (2) test the validity of Schudy’s pogonion formula38 concerning the determi-nants of chin position, and (3) validate the mathemat-ical model regarding determinants of chin position.

MATERIAL AND METHODS

The treated sample (n 67) consisted of 3 groupsof consecutively treated patients from the records of 3private-practice orthodontists, each using a differentmethod of Class II correction. To be considered for this

study, the treated patients had to meet the followingselection criteria: 1. Class II Division 1 malocclusion: half-step Class II molar and canine relationship; 2. Class IIskeletal relationships; 3. approximately equal numbers of boys and girls; 4. growing white children (9-14 years);5. complete records including acceptable pretreatmentand posttreatment cephalograms, pretreatment dentalmodels, and intraoral photographs; 6. mandibular defi-ciency, defined as smaller than average pretreatmentSNB angle according to age- and sex-specific norms43;7. vertical growth tendencies, defined as greater thanaverage pretreatment mandibular plane angle (SN-

GoGn) based on age- and sex-specific norms43; and 8.successfully treated by dental criteria: Class I molar andcanine relationship, adequate overbite (2-4 mm) andoverjet (1-3 mm).

The Ext HG group included 25 patients (12 boys,13 girls) treated with 4 premolar extractions in a typicalTweed edgewise manner with extensive use of tip-backbends, anchorage preparation, and Class II elastics.Various types of headgear (high-pull J hook, combi-pull, high-pull bow Hickam) were used. The meanduration of treatment was 34.2 10.5 months. Theirmean pretreatment age was 12.1 2 years.

The NE HG sample consisted of 23 patients (11boys, 12 girls) who were treated with the Alexanderstraightwire appliance in conjunction with cervical–pull headgear and nonextraction therapy. These patientswere instructed to wear the headgear a minimum of 14hours per day. The average duration of treatment was25.2 10 months. Their mean pretreatment age was12.7 2 years.

The Herbst group consisted of 19 patients (9 boys,10 girls) treated with stainless steel crown Herbstappliances for an average of 12.7 7 months, followedby fixed edgewise appliances. The total mean treatmenttime was 31.3 10 months. Their mean pretreatmentage was 11.7 3 years.

The untreated control group included children whowere followed longitudinally at the Human Growth andResearch Center at the University of Montreal. They

were from 3 school districts in Montreal representingvarious socioeconomic strata of the larger population.44

The sample consisted of 29 untreated Class II Division1 white subjects (14 boys, 15 girls) who were matchedto the treated sample for age, sex, ANB angle, andSN-GoGn angle. The initial observation was at 12.4 1.5 years of age, and they were followed for 2.2 0.6years.

Cephalometric methods

The analyses describe growth and modeling of 12skeletal landmarks (Fig 1), identified using standard

definitions.43 All cephalograms were hand traced anddigitized by 1 investigator using Dentofacial Planner(Dentofacial Software, Toronto, Ontario, Canada). Thelinear measurements were adjusted to eliminate mag-nification.

Traditional measurements were used to determineAP changes in the maxilla and the mandible (SNA,SNB, ANB, and SN-Pg angles) and changes in themandibular plane angle (SN-GoGn). To evaluate con-dylar growth, true rotation (ie, rotation of the mandibleindependent of the modeling changes, also called totalrotation36) and mandibular molar movement in the

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mandible, mandibular superimpositions were per-formed by using natural reference structures.36 Theradiographic tracings were oriented based on the

following structures: (1) anterior contour of the chin,(2) inner contour of the cortical plate at the lowerborder of the symphysis, (3) distinct trabecularstructures in the symphysis, (4) contour of themandibular canal, and (5) third-molar tooth germbefore root formation. Anterior and posterior stablereference landmarks were marked on the pretreat-ment (T1) tracing (Fig 1). The posttreatment (T2)tracing was superimposed on the mandible as de-scribed above, and the reference structures weretransferred to the second, superimposed, tracing.

The horizontal and vertical movements of selected

landmarks were described based on rectangular coor-dinates (X,Y). A horizontal reference line (RL), wasoriented based on the T1 sella-nasion plane minus 7°,registering on T1 sella (Fig 2). For example, the APchange in pogonion was measured parallel to RL, andthe vertical change was measured perpendicular to RL.Horizontally, an anterior change was recorded as pos-itive, and a posterior change was recorded as negative.Vertically, a superior change was recorded as positive,and an inferior change was recorded as negative (Fig2). True rotation was determined as the angular changebetween the T1 and T2 mandibular reference lines,

drawn through the posterior reference point (PRP) andthe anterior reference point (ARP), relative to RL.Replicate analyses showed no significant systematicerrors. Random method errors ranged from 0.07 to 1.2,with menton horizontal showing the greatest error.

Fig 1. Cephalometric landmarks, anterior and posterior reference points, and reference line.

Fig 2. AP and vertical cephalometric landmark posi-

tions measured parallel and perpendicular to SN-7°.


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Statistical methods

To account for group differences in duration be-tween T1 and T2, the changes were annualized. In otherwords, all changes given in the text and in Tables II andIII were standardized to represent changes per yearrather than changes over the entire treatment. Thedistributions of all variables were normal based on theskewness and kurtosis statistics. Analyses of variance(ANOVA) were used to evaluate group differences.Scheffé tests were performed for post-hoc analysis of group differences.

Stepwise linear regression was performed to deter-mine the independent variables that were most closelyassociated with the AP movements of pogonion (de-pendent variable). The linear regression equation takesthe form of:

Y1X12X2 · · · kXk

where, , 1, 2, . . . , and k are constants and X1,X2, . . . , and Xk are independent variables combinedlinearly to explain variation in the dependent vari-able (Y). To evaluate Schudy’s pogonion formula,38

the first regression included vertical maxillary

growth, vertical growth of the maxillary and man-dibular first molars, and condylar growth as theindependent variables. The second regression evalu-ated the relative contributions of condylar growth,fossa displacement, and true mandibular rotation onthe AP position of pogonion.

RESULTS

Mean pretreatment values showed that all 4 groupsincluded skeletal Class II subjects with ANB anglesranging from 5.5° to 6.4° (Table I). The SNA and SNB

angles indicate normal positions of the maxilla andmandibular retrusion, respectively. Based on the SN-GoGn angle, the mandibular plane of all groups wasslightly greater than normal, indicating vertical growthtendencies. There were no significant pretreatmentdifferences among the 4 groups.

The SNA and ANB angles were the only measure-ments showing significant treatment effects (Table II).They decreased in all treated groups and remainedunchanged in the untreated control sample. In contrastto the treated groups, which showed no changes, theuntreated controls showed a small (0.3°) but statisti-

Table I. Pretreatment skeletal relationships of Herbst, nonextraction headgear, extraction headgear, and untreatedcontrol groups

Measurement

Herbst (n 19) NE HG (n 23) Ext HG (n 25)Controls(n 28)

Groupdifferences

Mean SD Mean SD Mean SD Mean SD P value

SNA (°) 82.0 3.5 80.8 2.4 82.6 2.9 81.6 2.0 .129SNB (°) 76.2 3.2 74.9 2.3 76.2 2.7 76.1 2.1 .231ANB (°) 5.8 1.3 5.9 1.1 6.4 1.3 5.5 1.2 .062SN-Pg 77.4 3.2 76.0 1.9 76.8 2.6 76.5 2.2 .305SN-GoGn (°) 34.2 3.9 34.6 2.5 35.9 2.8 36.2 3.0 .082Age (y) 11.7 1.9 12.7 1.1 12.1 1.0 12.4 0.7 .057

Table II. Annualized angular (° per year) treatment changes in Herbst, NE HG, and Ext HG, and changes for untreatedcontrols

Measurement

Herbst NE HG Ext HG Controls Group differences

Mean SD Mean SD Mean SD Mean SD P value Post hoc†

ANB (°) 0.7* 0.5 1.2* 0.6 1.4* 0.6 0.0 1.1 .000 1,2,3,4SNA (°) 0.7* 1.1 1.0* 0.7 1.3* 0.8 0.3 1.0 .003 2SNB (°) 0.0 1.0 0.2 0.7 0.2 0.6 0.3* 0.7 .055 NSSN-Pg 0.0 0.9 0.3* 0.8 0.4* 0.6 0.0 0.8 .144 NSSN-GoGn (°) 0.3 0.9 0.3 1.2 0.2 0.8 0.1 1.2 .355 NSTrue rotation 0.1 0.9 0.7* 1.3 0.5* 0.9 0.2 1.8 .321 NS

*Significant changes between T1 and T2 (P .05).†Significant group differences based on Scheffé test P .05: 1, Controls and Herbst; 2, Controls and Ext HG; 3, Controls and NE HG; 4, Ext HGand Herbst. NS, No significant group differences at .05 level.




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cally significant decrease in SNB angle. AlthoughSN-Pg increased during treatment in the NE HG andExt HG groups, the changes were too small to producestatistically significant group differences. Similarly, the

mandible rotated forward in the NE HG and Ext HGgroups, but the effect was not sufficient to producegroup differences. There were no significant changes inSN-GoGn angle.

Treatment restricted maxillary growth but had nosignificant effect on the mandible in AP measurements.Point A remained stationary in the Herbst and NE HGgroups, moved posteriorly in the Ext HG group, andmoved anteriorly in the control group (Table III).Significant retraction of Point A was found in the ExtHG group when compared with all other groups.Pogonion and menton showed significant anterior

movements in all groups. The glenoid fossa showedslight posterior movements in the Ext HG and controlgroups. The maxillary molar showed significant for-ward movement in the Ext HG and control groups.Mandibular superimpositions showed posterior growthof condylion for all groups except the NE HG group,which had no significant AP changes. The mandibularmolar moved forward in all 4 groups, with greaterforward movements in the Ext HG group than in thecontrols.

With the exception of the glenoid fossa, there weresignificant vertical changes for all measurements in all

groups; the changes for the NE HG group were greaterthan the changes of the other 3 groups (Table III). Withthe exception of condylion and the mandibular molar, theother measurements showed inferior movements ranging

between 0.1 and 4.7 mm per year. Condylion grewsuperiorly 2.2 to 4.4 mm per year, and the mandibularmolar erupted 0.7 to 1.5 mm per year.

The first regression analysis, evaluating Schudy’spogonion formula,38 showed that vertical condylargrowth was most closely associated with the AP move-ments of pogonion. The regression indicated 0.385 mmof anterior movement of pogonion for every 1 mm of superior condylar growth. No other variables enteredthe regression. Superior condylar growth alone ex-plained 25% (R .50) of the variation in the APmovements of pogonion; it produced estimates of AP

change of pogonion that were within 1.1 mmapproximately 68% of the time.

The second regression analysis (Table IV) showedthat true rotation was most closely associated (R .66)with AP movement of pogonion. As shown in Figure 3,the first step of the regression predicted 0.694 mm of anterior movements of pogonion for every 1° of trueforward rotation. For example, assuming 2° for trueforward rotation, 2.076 mm of anterior movement of pogonion would be predicted (.688 [2 * .694]).Horizontal condylar growth, horizontal fossa remodel-ing, vertical fossa remodeling, and vertical condylar

Table III. Annualized horizontal and vertical treatment changes (mm per year) in Herbst, NE HG, and Ext HGheadgear, and changes for untreated controls

Herbst NE HG Ext HG Controls Group differences

Measurement Mean SD Mean SD Mean SD Mean SD P value Post hoc†

HorizontalPg 0.7* 1.0 1.4* 1.7 1.2* 1.2 0.5* 0.7 .027 NSMe 0.7* 1.2 1.5* 1.6 1.2* 1.3 0.5* 0.9 .029 NSFossa 0.0 1.3 0.1 0.9 0.6* 1.1 0.4* 0.9 .227 NSPoint A 0.1 0.6 0.1 0.9 0.6 1.0 0.7* 0.5 .000 1,4,5U6 0.3 0.9 0.2 1.3 0.6* 1.0 1.0* 0.6 .069 NSCo sup 0.7* 0.7 0.3 1.4 0.9* 0.7 0.5* 1.2 .292 NSL6 sup 1.2* 0.9 1.1* 0.8 1.7* 1.1 0.6* 0.4 .003 1

VerticalPg 2.3* 1.0 4.4* 1.5 2.7* 1.4 2.2* 1.0 . 000 2,3,4Me 2.3* 1.0 4.7* 1.5 2.9* 1.3 2.3* 1.0 . 000 2,3,4Fossa 0.2 1.5 0.9* 1.1 0.8* 2.0 0.1* 0.6 .018 NSA point 1.2* 0.9 3.0* 1.0 0.6* 1.0 1.0* 0.6 .000 2,3,4

U6 0.8* 0.9 2.5* 1.2 1.5* 0.9 1.5* 0.7 .000 2,3,4Co sup 2.6* 1.3 4.4* 1.8 2.7* 1.3 2.2* 1.3 .000 2,3,4L6 sup 0.9* 0.7 1.5* 0.9 0.7* 0.6 0.8* 0.5 .001 2,3,4

*Significant changes at. 05 level.†Significant group differences based on Scheffé test P .05: 1, Controls and Herbst; 2, Controls and Ext HG; 3, Controls and NE HG; 4, Ext HGand Herbst; 5, Ext HG and Herbst. NS, No significant group differences at .05 level.See Fig 1 for definitions.


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growth were the second, third, fourth, and fifth vari-ables to enter the regression, respectively. They pro-duced a multiple regression of 0.90, which accountedfor 81% of the variation in AP chin movement. Thestandard error of the estimate indicated that these 5variables predicted the AP movements of pogonionwithin 0.55 mm approximately 68% of the time.

DISCUSSION

The 3 treatment approaches produced Class I dentalrelationships and reduced the maxillomandibular dis-crepancies by restricting or inhibiting maxillary

growth. Although the mandible came forward duringtreatment, it did not come forward any more thanexpected in the untreated Class II subjects, who main-tained their original ANB angles. In other words, theanterior growth of the maxilla was compromised tocoordinate with a mandible that was moving forwardbut remained retruded.

In comparison with previous studies, the Ext HGgroup showed greater correction of the maxilloman-dibular relationship because of a larger reduction of SNA angle and perhaps less detrimental effects on SNBangle. SNA and ANB angle decreases were approxi-

mately twice as large as changes previously reportedfor extraction treatment over comparable treatmentperiods.16,19,21,22,45,46 Most studies also reported de-creases in SNB angles,16,19,21,22 but the results show thatExt HG treatment can maintain or slightly improve theSNB angle. This suggests that the Ext HG maintainedbetter control of the mandible by preventing worsening of the mandibular position. Despite the belief that mesialmolar movement during space closure allows for a greaterpositive mandibular response,16,46-49 the results showedno significant response compared with the controls and,more importantly, no improvement in mandibular posi-

tion. Although the AP corrections were greater thanpreviously reported, the underlying problem of chin defi-ciency was clearly not addressed. At best, Ext HGtreatment maintains the mandibular plane angle and APmandibular position.

Nonextraction headgear treatment also produced greaterreductions in SNAandANBangles than previously reportedfor comparable treatments.12,14-16,19,21,45,46,50,51 TheSNB and mandibular plane angles were relativelymaintained; this is consistent with previously re-ported changes ranging from 2.3° to 2.2° for themandibular plane angle and 0.37° to 0.83° for SNB

Table IV . Stepwise linear regression analyses of effects of true rotation, horizontal condylar growth, horizontal fossadisplacement, and vertical condylar growth on AP movements of pogonion

StepConstant

() Variable 1 1

Variable2 2

Variable3 3

Variable4 4

Variable5 5 R P SEE

1 0.688 True rotation 0.694 — — — — — — — — 0.657 .000 0.932 0.364 True rotation 0.851 Co H 0.459 — — — — — — 0.741 .000 0.833 0.224 True rotation 1.072 Co H 0.778 Fo H 0.482 — — — — 0.804 .000 0.744 0.164 True rotation 0.972 Co H 0.783 Fo H 0.671 Fo V 0.348 — — 0.882 .000 0.595 0.233 True rotation 0.834 Co H 0.738 Fo H 0.644 Fo V 0.365 Co V 0.157 0.900 .000 0.55

SEE, Standard error of estimate; Co H, Horizontal condylar growth; Fo H, horizontal fossa displacement; Co V, vertical condylar growth.

-3

-2

-1

0

12

3

4

5

6

-4 -3 -2 -1 0 1 2 3 4

True Rotation (deg)

P g H ( m

m )

Y=.688-(X*-.694)

R=.657; R 2=.432

Fig 3. Linear regression relating horizontal movement of pogonion (Y) with true mandibular rotation

(X).




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angle.12-16,19-21,46,50-52 These results confirm that NEHG treatment, when used with proper mechanics,does not cause undesirable mandibular rota-tion.12,14,53 The mandibular plane angle was held

relatively constant throughout treatment. As with ExtHG treatment, the NE HG results show that APcorrection was produced primarily by restricting orinhibiting maxillary growth and preventing undesir-able changes in mandibular position.

The AP correction in the Herbst group was less thanpreviously reported due to the lack of mandibularchanges. The amount of SNA angle reduction comparedfavorably with amounts previously reported.2,26,29,53 ANBangle changes, however, were less29,53 because no changeoccurred in the SNBangle. Previous studies reported SNBangle increases of 0.2° to 1.4°.2,26,29,53 This suggests thatinitial improvements from the Herbst appliance might notroutinely be maintained over the course of fixed treat-ment,2,28 particularly for patients with vertical tendencies.Herbst treatment achieved Class I dental correction pri-marily by maxillary growth restriction, with no significantincrease in condylar growth or mandibular length, asreported by others.27,54,55

Schudy’s pogonion formula38 was not supported bythe results. Of the variables that comprised the formula,only vertical condylar growth entered into the regres-sion equation, explaining 25% of the variability in theAP movement of pogonion. This suggests that verticalgrowth of the maxilla and vertical development of the

maxillary and mandibular dentoalveolar processes donot add additional information for predicting changes inAP chin position. For example, the difference betweencondylar and maxillary vertical changes was almosttwice as great in the NE HG group as in the Ext HGgroup, but there were little or no differences in anteriormovement of pogonion or mandibular rotation. Al-though significant amounts of vertical maxillary andcondylar growth occur during facial development, itappears they might play a more secondary role in APpositional changes of the mandible. Because this studydoes not support the notion that vertical growth of the

maxilla as well as vertical maxillary and mandibulardentoalveolar growth are primary determinants of APchin position, investigators should be cautious whenmaking inferences about the determinants of forwardmandibular movement.

The most important determinant of anterior chinmovements during growth and treatment was truerotation. Buschang and Santos-Pinto40 originally pro-duced mathematical models showing that mandibularrotation was the most important determinant of hori-zontal movements of the chin in untreated children andadolescents. Thompson et al42 reported a correlation of

0.69 between the horizontal movements of pogonionand true rotation; this closely approximates our results(R 0.64). As shown in Figure 3, the greater thecounterclockwise or forward true rotation of the man-

dible, the greater the anterior movement of pogonion.Conversely, if the mandible rotates backwards, indi-cated by a positive value of true rotation, pogonionshows unfavorable posterior movement. The casesdescribed by Björk and Skieller56 showed that anteriormovements of the chin were strongly related with truerotation of the mandible. This suggests that compensa-tory changes in condylar and dentoalveolar develop-ment can occur secondarily to mandibular rotation, anotion that is supported by morphologic studies evalu-ating patients with airway restrictions57-59 and muscu-lar deficiencies.60-64

The multivariate models showed that accurate pre-dictions of chin movements require additional informa-tion beyond true rotation. As previously shown,40,42 themultiple regressions showed that true rotation, whencombined with horizontal and vertical changes in theglenoid fossa and condyle, also accounted for approx-imately 90% of the variation of AP chin position.Together, these studies suggest that an optimal skeletalClass II correction will result from a treatment ap-proach that provides the greatest amount of forwardmandibular rotation, which effectively allows for ante-rior movement of the chin point while controllingundesirable inferior movement of the chin.

If the rotation of the mandible plays the primaryrole, with or without treatment, then it becomes clearthat true rotation must be addressed when attempting toproduce greater anterior chin projection. By focusingClass II correction on methods that control eruption andintrude teeth, greater amounts of true mandibular rota-tion and greater improvements in chin projection mightbe expected. Studies showed that chewing exercises,performed from approximately 4 weeks to 2 years, canproduce 2° to 2.5° of mandibular plane closure.65,66

Posterior bite blocks have also been shown to intrudeposterior teeth and decrease mandibular plane an-

gles.67-71 Vertical chincups, with or without headgear,can redirect condylar growth, increase posterior facialheight, and reduce the mandibular plane angle.72-74

Removable molar intrusion appliances have also beenproposed to intrude maxillary molars and allow favor-able forward rotation.75 These methods produce favor-able results, but most depend on excellent patientcooperation.

In the future, implants could provide orthodontists apredictable, compliance–free method of inhibiting orreducing dentoalveolar development and thus closingthe mandibular plane, rotating the mandible forward,


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and improving the profile. Although no clinical studieshave been performed, animal studies and case reportsshow that mini-implants, miniscrews, and titaniumminiplates can intrude molars from 0.5 mm per month

to 5 mm over the course of treatment, often closing themandibular plane.75-78 Unlike removable appliances,implants do not require compliance, and they provideabsolute anchorage, which eliminates the side effectsproduced by reciprocal forces and minimizes theamount of force needed. The future offers great prom-ise for a compliance-free, predictable method of pro-ducing true forward mandibular rotation and improve-ments in chin projection.

CONCLUSIONS

In this study evaluating the effect of extraction

headgear and Class II elastics, nonextraction headgear,and Herbst treatment, we made the following conclu-sions.

1. Methods commonly used to correct Class II skeletalmalocclusions produce no significant improve-ments in AP chin position. Skeletal Class II correc-tion in growing adolescents results primarily frommaxillary growth restriction or inhibition.

2. AP changes in chin position cannot be accuratelypredicted by Schudy’s pogonion formula38 (ie,based on condylar growth, vertical growth of themaxilla, and vertical maxillary and mandibulardentoalveolar growth).

3. Validating previously established mathematicalmodels, approximately 80% of the variability in APmovement of the chin can be explained by truerotation, AP and vertical condylar growth, and APmovement or drift of the glenoid fossa. True man-dibular rotation is the most important determinantof AP changes of chin position.

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Editors of the International Journal of Orthodontia (1915-1918),

International Journal of Orthodontia & Oral Surgery (1919-1921),

International Journal of Orthodontia, Oral Surgery and Radiography (1922-1932),

International Journal of Orthodontia and Dentistry of Children (1933-1935), International Journal of Orthodontics and Oral Surgery (1936-1937), American

Journal of Orthodontics and Oral Surgery (1938-1947), American Journal of

Orthodontics (1948-1986), and American Journal of Orthodontics and Dentofacial

Orthopedics (1986-present)

1915 to 1932 Martin Dewey1931 to 1968 H. C. Pollock1968 to 1978 B. F. Dewel1978 to 1985 Wayne G. Watson1985 to 2000 Thomas M. Graber2000 to present David L. Turpin



orthodontic treatment changes of chin position

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