EARLY TREATMENT SYMPOSIUM

Biomechanics of maxillaryexpansion and protraction inClass III patientsPeter Ngan, DMDMorgantown, WVa

During a 1981 pre-sentation at anAmerican Asso-

ciation of Orthodontistsmeeting, David Turpin1

recommended that earlytreatment should be con-sidered for young pa-tients who present withpositive factors such asconvergent facial type,anteroposterior function-al shift, symmetrical con-

dyle growth, mild skeletal disharmony, some remaininggrowth, good cooperation, no familial prognathism, andgood facial esthetics. For patients who present withnegative factors, he suggested delaying treatment untilgrowth was completed. In the 20 years that havepassed, what have we learned to help us better treat ourpatients?

We now know, for example, that Class III patientswith maxillary deficiency can be treated quite success-fully with facemask therapy in conjunction with max-illary expansion. In a prospective clinical trial, 20patients with skeletal Class III malocclusion weretreated consecutively with maxillary expansion and aprotraction facemask.2 A positive overjet was obtainedin all of them after 6 to 9 months of treatment. Thesechanges were caused by forward movement of themaxilla, backward and downward rotation of the man-dible, proclination of the maxillary incisors, and retro-clination of the mandibular incisors. The molar rela-tionship was overcorrected to a Class I or II dental arch

relationship, and the overbite was reduced with asignificant increase in lower facial height.

We have also learned that overcorrection is a key tolong-term stability. At the end of the 4-year observationperiod, and after half of the patients completed theirpubertal growth spurt, 15 of the 20 (75%) maintained apositive overjet or an end-to-end incisal relationship.Patients who reverted to a negative overjet were foundto have excess horizontal mandibular growth. Clini-cally, the success rate of treating Class III patients withmaxillary expansion and a protraction facemask is atbest 50% to 60% at the completion of the pubertalgrowth spurt.

PREDICTING MANDIBULAR GROWTH

Can we predict mandibular growth to improve theprognosis of early treatment? Bjork3 used a singlecephalogram to identify 7 structural signs of extrememandibular growth rotation occurring during growth.The 7 signs are related to the inclination of the condylarhead, the curvature of the mandibular canal, the shapeof the lower border of the mandible, the inclination ofthe symphysis, the interincisal angle, the intermolarangle, and the anterior lower face height.

Aki et al4 proposed the use of symphyseal morphol-ogy to predict the direction of mandibular growth.Mandibles that grew in an anterior direction wereassociated with reduced height, increased depth, a smallratio, and a large angle of the symphysis.

To predict abnormal growth, Schulhof et al5 calcu-lated the sum of the deviations of molar relationship,cranial deflection, porion location, and ramus positionfrom the norm with the Rocky Mountain Data System.A sum greater than 4 indicates probable increasedmandibular growth. However, the accuracy of predic-tion is about 70%.

The magnitude and direction of maxillary andmandibular growth can be determined by using serialcephalograms. Musich6 proposed a growth treatmentresponse vector (GTRV) analysis to warn of excessivemandibular growth after early orthopedic treatment.

Professor and Chair, Department of Orthodontics, School of Dentistry, WestVirginia University.Presented at the International Symposium on Early Orthodontic Treatment,February 8-10, 2002; Phoenix, Ariz.Am J Orthod Dentofacial Orthop 2002;121:582-3Copyright © 2002 by the American Association of Orthodontists.0889-5406/2002/$35.00 � 0 8/1/124168doi:10.1067/mod.2002.124168

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GTRV equals the horizontal growth of A-point dividedby the horizontal growth of B-point. The norm forpatients age 6 to 16 years is 0.77. If the ratio falls below0.60, the patient might need surgical treatment. Withthe GTRV, clinicians can use early treatment as a toolrather than a shortcoming when deciding whether toinitiate early treatment.

Discriminant analysis of long-term results of earlytreatment identified several variables that had predic-tive values. One study found the inclination of thecondylar head to be predictive; the maxillomandibularvertical relationship together with the width of themandibular arch can predict success or failure of earlyClass III treatment in 95% of cases.7 Another study8

found that the position of the mandible, the ramallength, the corpus length, and the gonial angle canpredict successful outcomes with 95% accuracy andunsuccessful outcomes with 70% accuracy.

Variability in response to maxillary protraction wasnoted in our longitudinal study. Horizontal protractionof the maxilla ranged from –0.8 to 5.5 mm, and verticalmovement of the maxilla ranged from –3.5 to 5.0 mm.The ability to maintain a positive overjet during theobservation period depends on the differential horizon-tal growth of the maxilla and the mandible aftertreatment. Without treatment, there is tremendous indi-vidual growth variation. Creekmore and Radney9 statedthat “individual growth responses were not predictable,but looking at individual changes, we see tremendousvariation. Is it no wonder, then, that the same ortho-dontic treatment does not elicit the same response forall individuals since individuals do not grow the samewithout treatment.”

TREATMENT INDICATORS

What are the treatment indications for Class IIIpatients? The facemask is a most effective tool for

treating skeletal Class III malocclusion with a retrusivemaxilla and a hypodivergent growth pattern. Patientspresenting initially with some degree of anterior man-dibular shift and a moderate overbite have an improvedtreatment prognosis. Correcting the anterior crossbiteusually results in a downward and backward rotation ofthe mandible that diminishes its prognathism. Thepresence of an overbite helps to maintain the immediatedental correction after treatment. For patients present-ing with a hyperdivergent growth pattern and a minimaloverbite, a bonded palatal expansion appliance to con-trol vertical eruption of the molars is recommended.During retention, a mandibular retractor or a Class IIIactivator with a posterior bite block can be used forvertical control.

REFERENCES

1. Turpin DL. Early Class III treatment. Presentation at 81st annualsession, American Association of Orthodontists, San Francisco;1981.

2. Ngan PW, Hagg U, Yiu C, Wei SHY. Treatment response andlong-term dentofacial adaptations to maxillary expansion andprotraction. Semin Orthod 1997;3:255-64.

3. Bjork A. Prediction of mandibular growth rotation. Am J Orthod1969;55:585-99.

4. Aki T, Nanda RS, Currier GF, Nanda SK. Assessment of sym-physis morphology as a predictor of the direction of mandibulargrowth. Am J Orthod Dentofacial Orthop 1994;106:60-9.

5. Schulhof RJ, Nakamura S, Williamson WV. Prediction of abnor-mal growth in Class III malocclusions. Am J Orthod 1977;71:421-30.

6. Musich D. Growth treatment response vector analysis. Personalcommunication, November 1, 2001.

7. Franchi L, Baccetti T, Tollaro I. Predictive variables for theoutcome of early functional treatment of Class III malocclusion.Am J Orthod Dentofacial Orthop 1997;112:80-6.

8. Ghiz M, Ngan P, Gunel E. Cephalometric variables to predictfuture success of Class III orthopedic treatment [abstract #1158].J Dent Res 2001;80:180.

9. Creekmore T, Radney L. Frankel appliance therapy: orthopedic ororthodontic? Am J Orthod 1983;83:89-108.

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