Methods: Baseline and six-month bitewing radiographs were obtained using an extra-oral alignment apparatus on nine periodontally healthy subjects. The radiographs were digitised and analysed using Image Pro-Plus® Software. Measurements made were: mesio-distal tooth width (L1), cusp tip to cemento-enamel junction (CEJ) (L2), inter-proximal area (PG), CEJ to alveolar crest (T1) and the angle between alveolar crestal bone and the long axis of the teeth (AN). The measurements were compared at baseline and six-months in order to investigate any alterations in film alignment, as changes in alveolar bone levels would not be expected in these subjects. Subtraction radiography was also performed. All variables were re-measured on a separate occasion. Intra-Class Correlation Coefficients (ICC) were used to investigate the repeatability of measurements on each image. Changes between baseline and six-month measurements were analysed using paired sample t-tests to investigate the reliability of radiographic image acquisition. For subtraction radiography, match error values were used to test the geometric alignment of images.
Results: The repeatability of measurements was high (ICC>0.80). There were no statistically significant differences between baseline and six-month measurements of L1, L2, PG, T1 and AN (p>0.05). Subtraction radiography confirmed good geometric alignment between images.
Conclusion: The extra-oral alignment apparatus investigated in this study enabled reliable images to be acquired. The linear, area and angular measurements had a high degree of repeatability. This technique would allow accurate measurement of any changes in bone levels in patients with periodontal disease.