Calcinosis preferentially affects the thumb compared to other fingers in patients with systemic sclerosis

Although Raynaud's phenomenon often spares the thumb, clinical experience suggests conversely that, in patients with systemic sclerosis (SSc), the thumb is frequently affected by calcinosis (as is demonstrated on plain radiographs). Our aim was to investigate the hypothesis that, in patients with SSc, thumbs are more commonly affected by calcinosis than other digits.

Sixty-eight hand radiographs with at least one area of calcinosis were identified. Each digit on both hands of each patient was assigned a severity score on a scale from 0 to 3 (0 = no calcinosis, 3 = most severe). The scoring was completed twice, including and excluding the metacarpals.

Right hands were found to be associated with slightly higher scores than left hands [estimate 0.14, 95% confidence interval (CI) 0.03-0.26]. Scores were lower for other fingers compared to thumbs. There was strong evidence (p < 0.0001) of a trend of decreasing severity moving from the thumb to the little finger. There was no evidence that the pattern of severity across the fingers was different on left and right hands (p = 0.77). The results were similar whether or not metacarpals were included.

The thumb is affected by calcinosis more than other digits, followed by the index finger. This observation provides insight into the pathogenesis of SSc-related calcinosis, which may relate more to repetitive trauma than to ischaemia.