Spatial relation between a rigid (digital) intraoral X-ray receptor and longitudinal axes of maxillary teeth

The purpose of this study was to quantify the existing (inevitable) angle which in intraoral radiology appears between tooth length axis and receptor caused by the anatomical situation. Especially in the upper jaw, due to its arched anatomy, a true "paralleling technique" is not achievable. The angulation necessarily causes distortion and a foreshortening of the image; hence, the foreshortened image leads to misinterpretations in diagnostics. We investigated the effects of the realistic angulation on these image deteriorating factors. Two hundred ninety-four plaster models of the upper jaw were collected, and the angles between a dummy receptor and the axes of the central incisor or the first molar were measured. For evaluation, a rigid dummy of an intraoral charge-coupled device (CCD) receptor (30 mm × 40 mm) was used. The mean angulation evaluated for central incisors was 36.7° (range 19-56°) and for first molars 42.5° (range 26-56°). This leads to a foreshortening of the tooth ranging from 5.4% to 44.1% in the image, when magnification is neglected. Large angles of up to 56°, in both incisor and molar region, result in a relevant underestimation of true tooth length up to 44%. It is important to note that this error cannot be simply corrected by means of local magnification correction. Techniques should be developed that allow for automated assessment of the effective angle to provide information for distortion correction.