On the growth of the mandible.

In a cross-sectional cephalometric study of over 800 normal white American children we show that the average ANB angle is roughly twice as large as the currently employed norms for that variable. The angle is relatively the same in females from 6 to 20 years of age, but there is a definite tendency for older males to exhibit a smaller ANB angle. The dynamics of this decrease is essentially the fact that, in the male, the mandible continues to grow steadily (relative to the maxilla and nasion) throughout much of the postpuberal stage, whereas this tendency is not exhibited in the female. The mandible and maxilla, and the relationship between these structures, have long been recognized as important components of craniofacial morphology, both in the study of racial variations and in the diagnosis and treatment of dentofacial anomalies. It is, therefore, of considerable importance to derive variables which reflect this relationship and to produce normative values for these variables in various population groups. Many studies of facial angles and proportions designed to measure this relationship have been made but, with few exceptions, the sample sizes were small with pooled ages and sexes. A case in point is the angle defined by the points subspinale (Downs’ point A), nasion and supramentale (Downs’ point B), commonly referred to as the ANB angle in the orthodontic literature. This angle is of particular importance in cephalometric analyses since the lines NA and NB are readily located in a lateral head film and provide convenient reference lines from which to measure incisal positions and inclinations. Steiner (‘59) chose a set of craniofacial norms, “. . . which express our concept of a normal average American child of average age,” the norm for the ANB angle being set at 2” . He implored the reader to, “Please bear in mind that these are rough estimates, to be used as a starting point from which to vary and must be modified by other factors, not only pogonion to the line NB, but also age, sex, race, growth potential and individual variations within these and other groupings,” but provided little insight into how these modifications should be accomplished. In this paper we investigate, using cephalometric methods, the distribution of the ANB angle in a large sample of “normal” individuals for several age groups and both sexes. The intent is to test the hypothesis that an ANB angle of 2” is “normal” and to investigate the dependence of this angle on age and sexual dimorphism. METHODS AND MATERIALS The cephalograms of children presenting “normal” dental occlusion were obtained as part of a study of normal growth conducted at the Philadelphia Center for Research in Child Growth between 1948 and 1968, and were selected from a group of 2500 white elementary and secondary school children. The project director, Dr. W. M. Krogman, “. . . took in substance children who were in “good medical health’ and who had no more than the usual so-called mild ‘childhood illnesses’ . . . “good dental health” . . . a low DMF index . . . and all four first permanent molars in place.” The socioeconomic distribution of these children was concentrated in the mid-level stratum. Their ethnic distribution was Northern European (German, Scandinavian), Southern European (Italian) and Scotch, Irish and English. Middle and Eastern Europe were represented by children of Galician, Ukranian, Polish (largely Ashkenazic Jews) and Russian ancestry. In essence, those selected for the survey were felt to be, 111 AM. J . PHYS. ANTHROP., 36. 111-118. 112 GEOFFREY F. WALKER AND CHARLES J. KOWALSKI Fig. 1 The morphological structures comprising the mathematical model. “. . . reasonably representative of the [white] children of the city of Philadelphia,” during the period studied. The cephalograms of this group of “normals” were selected by one of the authors (GFW), who also devised the computerized method used in this study (Walker, ’67) and supervised the transformation of the radiographic information into digitized form for ready access by statistical computing programs. The data were processed using a consoleoriented statistical computing program, caIled CONSTAT, developed by the Statistical Research Laboratory at the University of Michigan. The roentgenographic cephalograms of the children included in the study were reduced to this mathematical model by the following procedure: (1) The outer contours of the individual skull bones were traced by experienced technicians to produce the sagittal profile of the craniofacial complex. Figure 1 illustrates the actual structures