We attempted to quantify periarticular bone atrophy from radiographs of the proximal phalanx in patients with rheumatoid arthritis (RA) by means of the National Institutes of Health (NIH) Image computer program. The degree of brightness or darkness in four squares, each 20 × 20 pixels, in the right third proximal phalanx was measured using NIH Image, and the mean value of the 400 pixels was defined as the brightness/darkness index (BDI). The BDI was used to express bone volume. The BDI value was set at zero for an area of complete darkness and at 255 for an area of maximum brightness. The mean coefficient of variation in our hospital was 2.28%. The BDI was measured in 54 RA patients and 146 normal volunteers. The mean BDI at the midpoint of the diaphysis was 100 in RA patients and 176 in normal volunteers, while at the medial side of the proximal end it was 75 and 145, respectively. The difference between normal volunteers and RA patients was greatest in younger people. In some young RA patients, the BDI was significantly low at the medial side of the proximal end, clearly demonstrating periarticular bone atrophy. Periarticular bone atrophy can be quantified using the NIH Image computer program.
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