Use of as internal standard in subtraction radiography to assess initial periodontal bone changes

Objectives. To assess the use of an internal reference when performing histogram analyses in digital subtraction images and to determine the ability of the method to detect initial bone lesions. Methods. Fifty-one Royal Air Force recruits had standardized vertical bitewing radiographs and clinical assessment of attachment level recorded annually over three years. Subtraction analyses of crestal bone changes at the mesial surface of the upper right first molar were compared with changes at the mesial surface of the same tooth. Changes over the periods from age 17 to 18 years and age 18 to 20 years were monitored using two subtraction procedures. Results. Reproducibility studies revealed that Pearson correlations between duplicate measurements of the test site alone (16 crest; r = 0.74) were lower than those between duplicate measurements of changes where misalignment was controlled for, either as a difference (16 crest- 16 tooth; r = 0.93) or ratio (16 crest/16 tooth; r = 0.93). We used the differences between the mean subtraction density for 16 crest and 16 tooth as our measure of change in bone density. For the duplicate measurements, the standard deviation of these differences was 3.9: a difference of +/- 7.8 was therefore taken as a threshold value for evidence of real change. Bone gain was noted between the ages of 17 and 18 years (16/21 subjects), but some early bone loss was seen between 18 and 20 years (12/21 subjects), with four subjects showing changes significantly greater than the method error. There were no associations between the clinical and radiographic observations. Conclusion. Use of a control site in subtraction radiography improves the reproducibility; such systems can detect small changes in alveolar bone which may assist in early diagnosis of the initial periodontal lesion which may precede observable clinical changes.