The effect of alterations in horizontal X-ray beam angulation and bucco-lingual cavity width on the radiographic depth of approximal cavities.

The effects of changes in horizontal X-ray beam angulation and different cavity shape on images of approximal cavities on bite-wing radiographs were examined using a laboratory model. Cylindrical-, rectangular- and cone-shaped bur cut cavities were prepared in the approximal surfaces of 108 teeth and their depth determined using silicone putty impressions. Spatial relationships were controlled by fixing specimens into perspex blocks. Standardized radiographs were taken at baseline (0 degree) and then using pre-angled wedges to imitate horizontal X-ray beam movements of 5, 10 and 15 degrees in both mesial and distal directions. Radiographic cavity depth to the nearest 0.1 mm was measured. Over the range of X-ray beam movements, more than 72% of cavities deepened compared to baseline. As bucco-lingual cavity width increased the magnitude of the depth change increased. For some cylindrical cavities increases equating to 200% over baseline were seen; in some rectangular cavities increases of up to 400% over baseline occurred. The present study suggests that changes in irradiation geometry from baseline are likely to result in a cavity appearing to deepen. In a clinical context over-treatment is therefore the most likely outcome.