Early abnormalities of pisiform and triquetrum in rheumatoid arthritis.

Erosions on the triquetrum and pisiform are frequent in early rheumatoid arthritis and occur characteristically at 3 sites. (a) A shallow erosion on the proximal medial triquetral "bare" area is related to synovial proliferation at the margin of the radiocarpal cavity, although compressive forces from an adjacent long ulnar styloid may be contributory; (b) an erosive abnormality on the distal medial triquetrum is related to synovial proliferation along the margin of the midcarpal joint; and (c) ring-like erosions on apposing surfaces of the triquetrum and pisiform are produced by pannus within the pisiform-triquetral compartment.