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). Mean ratios between the maxillary distance J-J and the mandibular distances varied less than mean differences between maxillary and mandibular distances. The results of this study suggest that: (1) a film-porionic plane distance of 13 cm can be adopted as a standard for taking posteroanterior cephalographs, (2) in the evaluation of transverse relations between the jaws, ratios may be more accurate than absolute differences between maxillary and mandibular widths, (3) the landmarks commonly used to evaluate these widths (J and AG) seem appropriate, but their effectiveness in orthodontic diagnosis and treatment planning needs further investigation.