Distribution of the mandibular incisor-mandibular plane angle in "normal" individuals.

W. B. Downs (Amer J Orthodont 34:812, 1948) presented findings from a study to determine the "normal range of variation" for certain variables that represented components of the dentofacial complex. He studied the distribution of these variables in 20 individuals with "clinically excellent occlusions," who ranged in age from 12 to 17 years and who were about equally divided with respect to sex. Among the components of craniofacial morphology studied by Downs was the mandibular incisor-mandibular plane angle. Earlier investigators include Margolis (Amer J Orthodont 29:571, 1943), Noyes, Rushing, and Sims (Angle Orthodant 13:60, 1943), and Speidel and Stoner (Amer J Orthodont 35:536, 1944), who agreed closely on ranges and on a mean value of 900. Downs' group yielded a mean of 91.40; the measurements ranged from 81.5 to 97°. These figures have been accepted generally as representative of "normal variation" despite the fact that they are based on small samples from a special population. We are reporting the findings of a larger study of the distribution of this angle and we note that "normal variation" is considerably different from that reported by Downs. This investigation is based on cephalometric data from radiographs of children with normal dental occlusion. They were part of a survey of normal growth conducted at the Philadelphia Center for Research in Child Growth between 1948 and 1968 and were selected from a group of 2,500 white elementary and secondary school children. The project director,