Gonial angle distortion in lateral head films: a methodologic study.

The distortion of the gonial angle in lateral roentgenographic head films was studied in skull material. The left and the right gonial angles were determined craniometrically and roentgenocephalometrically, and the observations were compared. Mean difference of 8.48 degrees and 5.15 degrees between the roentgenocephalometric and the craniometric registrations were found for the side closer to the film and the side farther away from it, respectively. For the intermediate gonial angle, the difference was 6.65 degrees. The differences were all statistically significant. It was demonstrated that they were mainly attributable to a systematic method error in the roentgenocephalometric method, involving a magnification of the gonial angles. The magnification was significantly larger on the side closer to the film than on the side farther away from it. A pronounced individual variation in gonial angle distortion was observed. For intermediate gonial angle it ranged between magnifications of 1.17 degrees and 17.54 degrees. The distortion is closely associated with the form of the mandible. It is largest in mandibles with a pronounced divergence of the two halves of the corpus and a pronounced upward convergence of the rami. It is smallest in mandibles with a modest divergence of the two halves of the corpus and a pronounced upward divergence of the rami. By some very special combinations of corpus divergence and ramus inclination, a correct reproduction may be obtained. No such cases were found in the sample studied, but there can hardly be any doubt that they exist. There is an association between corpus divergence and ramus inclination. Maximum magnifying effect of the forward-extending leg tends to coincide with the maximum magnifying effect of the upward-extending leg. Conversely, minimum magnifying effect of the forward-extending leg tends to coincide with a dimishing effect of the upward-extending leg. This is the clue to an understanding of the great individual variation in gonial angle distortion. Generally speaking, lateral head films do not permit reliable registrations of the gonial angle. At present no method is available for an exact assessment of the distortion in head films of the single living person.