On the use of the SNA and SNB angles in cephalometric analyses

Abstract

T he SNA and SNB angles have long been recognized as informative indicators of upper and lower facial prognathism and as useful guides in the diagnosis and treatment of malocclusion. As such, these angles play fundamental roles in a number of cephalometric analyses, such as those proposed by Down& 2 and Steiner. 3, 4 It is therefore of considerable interest to determine normative values for these variables. In an attempt to do this, Down+ studied the distribution of these variables in twenty persons with “clinically excellent occlusions, ” ranging from 12 to 17 years of age and about equally divided with respect to sex. He found a mean value of 81 degrees for the XNA angle (ranging from 78 to 84 degrees) and a mean value of 78 degrees (ranging from 75 to 81 degrees) for the SNB angle, and these figures have been generally accepted as representative of “normal variation” despite the fact that they are based on a small sample from a rather special population. Steiner3 chose a set of craniofacial norms, “. . . which express our concept of a normal average American child of average age,” the norms for the SNA and SNB angles being set at 83 and 80 degrees respectively. 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 . . . age, sex, race, growth potential and mdividual variations within these and other groupings,” lbut he provided little insight into how these modifications should be implemented in practice. In the present article we will investigate, using cephalometric methods, the distribution of the SNA and SNB angles in a large sample of “normal” persons for several age groups and both sexes. The intent is to estimate the norms, and the variation about these norms, for the SNA and SNB angles and to compare these with the values obtained by Downs and Steiner. We shall also investigate sexual dimorphism and the dependence of these angles on age.