Cephalometric superimposition on the cranial base: a review and a comparison of four methods.

Spatial change in the jaws of growing persons is often evaluated by superimposing cephalometric tracings made at different points in time. Methods of superimposition vary according to structures used as references within the skull. This study compares four different superimposition methods. The sample consisted of 26 patients (13 boys, 13 girls) treated for Class II, Division 1 malocclusions with extraction of the four first premolars. Tracings of pretreatment (average age for boys, 12.5 years; for girls, 12.2 years) and posttreatment (average age for boys, 15.4 years; for girls, 14.9 years) cephalograms were superimposed according to the following methods: (1) best fit of anterior cranial base anatomy, (2) superimposition on SN line, registered at S, (3) superimposition on registration point R with Bolton-nasion planes parallel, and (4) superimposition on basion-nasion (Ricketts), registered at point CC (4) and point N (4a). Differences in amount of change among the superimposition methods were assessed independently for each of the following landmarks: PNS, ANS, A, B, Pog, Gon. On each patient and for each landmark, ten distances--the paired differences of five posttreatment positions obtained by methods 1, 2, 3, 4, and 4a--were evaluated. Two methods were compared at a time. A t test examined the average difference for each comparison. Because all differences between all paired methods were significant (P less than 0.01), t tests were then viewed under the hypothesis that a difference less than or equal to 1 mm was insignificant clinically. Clinically-statistically significant differences were found only for boys and for the total sample between methods 4a and each of methods 1, 2, and 3. As method 4a is advocated to assess changes of point A (Ricketts), this method gives, for the same person, an interpretation of anterior maxillary change in position different from the other methods. Conclusions about facial changes may be made only in reference to the superimposition method.