Panoramic zonography of the midface: a clinician's guide to vertical magnification

Surgical procedures involving the maxilla require details regarding the topography of the nasal floor and that of the maxillary sinus. Panoramic zonography can provide this information with a minimum of radiation exposure and may be used in certain instances when other imaging modalities cannot. Spatially dependent variations in vertical magnification inherent to panoramic zonography using the midface program were determined. Radiographic lengths of marker wires attached at seven locations along the maxillary arch of dry skulls were measured. Radiographs were obtained at 0, -10 and -20 mm shifts. Analysis of variance and ranking did not reveal any significant differences in magnification between skulls. However, a dichotomy between the anterior and posterior portions of the maxillary arch in terms of vertical magnification was found at all three shift positions. In addition, at all marker locations, the magnification diminished as a function of decreasing shift position. These variations are due to the fact that teeth within the arch are not all located at the central plane of the focal trough generated by the unit. The results provide the clinician with vertical magnification factors from which actual measurements can be derived from zonographs.