EFFECT OF FILM-OBJECT DISTANCE ON POSTEROANTERIOR CEPHALOMETRIC MEASUREMENTS - SUGGESTIONS FOR STANDARDIZED CEPHALOMETRIC METHODS

Posteroanterior (PA) cephalographs are used for diagnosis of transvers e skeletal and dentoalveolar relationships. Unlike lateral head films, the variability of the PA radiograph has not been assessed. The purpo se of this study was to evaluate the effect of film-object distance (f ilm-ear rod, or film-porionic axis distance: FPD) and head angulation on transverse measurements from PA cephalographs. Seventeen skulls wer e selected from a total of 45 on the basis of strict criteria, includi ng 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 1 4 cm, additional radiographs were taken at the angulations of + 5 degr ees and - 5 degrees from the Frankfort horizontal (FH). Distances betw een the following landmarks were measured on the radiographs with digi tal calipers accurate to 0.01 mm: J (on the lateral contour of the max illa), Ag (at the antegonial notch), Go (Gonion), Mb (buccal surface o f mandibular first molar), IR (inner ramal point at the intersection o f mandibular ramus and body), No clinically significant difference exi sted between measurements at the different FPDs evaluated. For this re ason, 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, 0.7 5, and 0.68, respectively, Transverse measurements were not significan tly affected by a head angulation within 10 degrees (FH -5 degrees to +5 degrees). Mean ratios between the maxillary distance J-J and the ma ndibular 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 tra nsverse relations between the jaws, ratios may be more accurate than a bsolute differences between maxillary and mandibular widths, (3) the l andmarks commonly used to evaluate these widths (J and AG) seem approp riate, but their effectiveness in orthodontic diagnosis and treatment planning needs further investigation.