Effect of film-object distance on posteroanterior cephalometric measurements: suggestions for standardized cephalometric methods.

Posteroanterior (PA) cephalographs are used for diagnosis of transverse skeletal and dentoalveolar relationships. Unlike lateral head films, the variability of the PA radiograph has not been assessed. The purpose of this study was to evaluate the effect of film-object distance (film-ear rod, or film-porionic axis distance: FPD) and head angulation on transverse measurements from PA cephalographs. Seventeen skulls were selected from a total of 45 on the basis of strict criteria, including a stable reproducible centric occlusion. Radiographs were taken of each skull at the FPD of 11, 12, 13, and 14 cm. At the FPD of 11 and 14 cm, additional radiographs were taken at the angulations of +5 degrees and -5 degrees from the Frankfort horizontal (FH). Distances between the following landmarks were measured on the radiographs with digital calipers accurate to 0.01 mm: J (on the lateral contour of the maxilla), Ag (at the antegonial notch), Go (Gonion), Mb (buccal surface of mandibular first molar), IR (inner ramal point at the intersection of mandibular ramus and body). No clinically significant difference existed between measurements at the different FPDs evaluated. For this reason, and because the majority of PA films taken of 59 human subjects were within a FPD range below and including 13 cm, regression analyses were computed only at FPD = 13 cm. Correlation coefficients r between the distance AG-AG and distances J-J, IR-IR, and Mb-Mb were 0.71, 0.75, and 0.68, respectively. Transverse measurements were not significantly affected by a head angulation within 10 degrees (FH -5 degrees to +5 degrees).(ABSTRACT TRUNCATED AT 250 WORDS)