The frontal sinus and n andibutar growth prediction P.E. Rossouw,* BSc, BChD, Hons BChD, MChD, C.J. Lombard,** BSc, MSc, PhD, and A.M.P. Harris,* BChD, Hons BSc, DTE Tygerberg, South Africa The skeletal growth patterns of 103 subjects with Class I and III malocclusions were cepha[ometrically analyzed as advocated by Ricketts el al? to assess abnormal mandibular growth. The surface area (mm 2) of the frontal sinus was assessed by a Summagraphics decoder linked to a microcomputer. The results indicate that there is a significant correlation between maxillary length, mandibular length, symphysis width, condylar length, and frontal sinus size on a lateral cephalogram. The frontal sinus can possibly be used as an additional indicator when one is predicting mandibular growth. (AMJ ORTHOD DENTOFAO ORTHOP 1991 ;100:542-6.)

G r o w t h prediction has been a controversial topic ever since it was advocated by Ricketts.' Prediction would involve forecasting a change in direction or different growth rates for two patients who were the same age, sex, and race on the basis of some prior knowledge, such as a cephalometric measurement. 2 A main problem associated with patients who have Class III malocclusion has to do with the differential diagnosis in these cases. Would treatment involve only orthodontic therapy, or would the patients require surgery in the corrective therapeutics? This decision usually has to be made during the patient's childhood; therefore growth and development are important considerations in the prediction of abnormal or excessive growth. The typical characteristics of the growing person with Class [II malocclusion have previously been described. 3-4 An error in the diagnosis of these cases of excessive mandibular prognathism may indicate an error in attainment of a favorable prognosis. 5 Even normal growth during adolescence will favor relapse in the patient with Class III malocclusion, and accurate prediction would therefore be helpful. ~ There is a small but significant linear correlation between the shape of the mandible and a change in gonial angle. 7 Bjrrk, 8 with the use of implants, reported his observations of anatomic characteristics of individual mandibles that grew abnormally both vertically and horizontally. Rieketts 9 proposed a method based on the

This project was supported by the Medical Research Council, Tygerberg, *Department of Orthodontics, University of Stellenbosch. **Institute for Bin. statistics, Medical Research Council, Tygerberg. 8/1/25571

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theory that the mandible grows along an arc, which is basically in agreement with Moss's theory that the mandible grows in a logarithmic spiral. 10Mitchell, Jordan, and Ricketts ~ evaluated Rickett's method for predicting the size and form of the mandible and found the arcial method valid for the prediction of mandibular growth. Hand-wrist radiographs, however, will improve the accuracy of the amount of growth expected during shortrange predictions. It would seem therefore that it is possible to predict the form of the mandible with the use of an individualized m e t h o d ) A visual treatment objective t2-~4 should be developed with long-term results in mind. Certainly, the maxillomandibular relationship seen 2 years after the start of treatment in a growing child may not be the same at maturity. Therefore treatment of a case (whether Angle Class I, II, or III) to proper facial balance at the age of 12 years may prove unsuccessful at the age of 25 years. This consideration is especially true in cases in which an abnormally large amount of mandibular growth is predicted for the teen years. 3 Schulhof, Nakamura, and Williamson 2 found that a high cranial deflection (28 ° -_+ 3°), short porion location ( - 3 7 mm +__ 2.5 mm), forward ramus position (75.5 ° +_ 2.8°), and Class III molar relation ( - 3 . 0 m m _ 2.6 mm) were telltale signs that excessive mandibular growth was likely to occur. It has been demonstrated that cases exhibiting a wide symphysis and a long condyle axis also tend to indicate excessive mandibular growth. Ricketts 3 claims that with the use of these six measurements accuracy in predicting abnormal mandibular growth has increased to approximately 9O%. Acromegaly is associated with prominent frontal sinuses and overgrowth of the jawbone, and one usually

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Fro~ltal sitars and mandibular growth prediction

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Fig. 1. Six specific cephalometric measurements to predict excessive mandibular growth,za

finds a Class III-type prognathic mandible in these cases.~5 The frontal sinus bud is present at birth in the ethmoid region but is not evident radiographically until the fifth year, when it projects above the orbital rim.J6 Rapid growth of the sinuses continues until the age of 12 years, when they reach nearly adult size. ~7 Joffe ~ found frontal sinus enlargement to be associated with prognathic subjects, but no indication was given as to the correlation with growth-prediction indicators. Tanner 19 found that the annual height (stature) growth increments in children reached a plateau at 16 years in boys and 14 years in girls, and it was thought that these, too, are the ages at which frontal sinus enlargement ceased. This was supported by Brown, Molleson, and Chinn, 2° who found during a study on lateral cephalograms that the main enlargement of the fi'ontal sinuses ceased at 151/2 years in boys and 133/4 years in girls. This suggests that the increase in the sinuses follows the trend in growth in bone lengths very closely. The aim of this study was to assess whether a large frontal sinus size could be correlated with excessive mandibular growth. This, together with the other known indicators mentioned, would enable the orthodontist to make an even more accurate prediction of expected excessive mandibular growth. MATERIALS AND METHODS

A random selection of 103 cephalograms, consisting of 53 adult skeletal Class I growth patterns and 50 adult

Fig. 2. Photograph of measurements of the frontal sinus on the computer.

skeletal Class III growth patterns of both male and female white subjects were analyzed. The six measurements described by Ricketts 7 (Fig. 1) were used to determine the presence of abnormal mandibular growth. The size of the frontal sinus was expressed in square millimeters as measured on a digitizer connected to a microcomputer (Summagraphics decoder to Olivetti M24, Ing C. Olivetti e C.S.p.A., Ivrea(To), Italy) (Fig. 2). The ANB angle al was used to ensure that the subjects conformed to skeletal growth differences. Although the ANB angle has its shortcomings, it is still widely accepted as an indicator of maxillomandibular harmony. "-2 The facial axis angle 23 and the FMA angle 24 were measured to give an indication of growth direction. Maxillary length and mandibular length 25 were measured to give an indication of the size of the bones (Table I). All these measurements were checked for accuracy by two investigators familiar with the analyses. The results of this cross-sectional study were analyzed with the SAS. Spearman correlation coefficients were used to assess associations between the frontal

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Rossouw, Lombard, (md Ht~rris

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