Angular distortion in the orthopantomogram.

The orthopantomogram is in popular use in orthodontic diagnosis and monitoring treatment progress. Being a laminographic-type radiograph, its image suffers from a variable degree of linear distortion and this has limited its use as a research tool. A wire-mesh frame shaped to represent the curvature of the dentition in the jaws, with lead shots representing the long axes of the teeth as well as certain skeletal landmarks, was used to investigate the accuracy of projection of its structures on to the X-ray film of the orthopantomograph. Angular parameters were used to identify and assess both the magnitude and distribution of angular distortion of the radiographic image. The canine--premolar region of both arches expressed the greatest amount of angular distortion and variability. The third molar regions, together with the landmarks projected at a lower level on the radiograph, were the least variable. The applications and implications of the findings are discussed.