Roentgenographic assessment of overhanging margins of dental restorations. Part I. Significance of variations in horizontal projection. HILDING BJORN, HAKAN HAKANSSON and OWE JOHANSSON. Odont. Revy 24: 127-140 (1973). The influence of various factors on the roentgenographic reproduction and assessment of overhanging margins of dental restorations was studied partly theoretically in a trigonometrical model and partly by measurements on roentgenographs obtained in experiments with a dry mandible. The trigonometrical model made possible the derivation of a formula for the influence of various factors on the image size. These factors were the size of the overhang, the focus-object and the object-film distances, the horizontal angle between the 110

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central ray and the plane of the film, and the position of the object along an axis parallel with the film-plane. The magnification and distortion in the roentgenographic image of small overhanging margins are of minor importance within clinically acceptable variations in horizontal angulation of the roentgen beam. Maximum errors are 12 % and 16 % for focusobject distances corresponding to "long and short cone" techniques respectively. In the experimental part of the study the measurements on roentgenographs were compared with direct measurements of the overhangs with a surface detector. These comparisons failed to show any overestimations of the size of the overhangs in the roentgenographic measurements. In clinical work the size of overhangs thus will not be seriously overrated due to projection factors, while there is some risk of underestimation. 7 references. Hakan Hakansson, Malmo