It is unusual to find rheumatoid nodules in synovium1. We report a case of an invasion of medullary bone by pannus tissue associated with rheumatoid nodules.
A 46-year-old woman presented with a 4-year history of persistent pain in her left third metacarpophalangeal (MCP) joint. She was diagnosed with rheumatoid arthritis 13 years ago and was taking methotrexate and leflunomide treatments for 9 years. On admission, physical examination was unremarkable except for mild tenderness in the third MCP joint. Plain radiographs of the left hand showed a cystic lesion occupying the entire third proximal phalanx (Figure 1A). Enchondroma was suspected. However, magnetic resonance imaging of the left hand revealed an intramedullary cystic lesion and pannus tissue in the third proximal phalanx. The pannus tissue eroded the cortical bone and invaded the medulla. Pericortical and intramedullary pannus tissues were excised. Pathologic examination of the biopsy specimen showed 3 rheumatoid nodules in the pannus tissue. Figure 1B shows 1 of the nodules with fibrinoid necrosis and palisading histiocytes.
Rheumatoid nodules in synovium are rare1. To our knowledge, invasion of medullary bone by pannus tissue associated with rheumatoid nodules has not been reported previously. In our case, it led to misinterpretation of the lesion as enchondroma, a benign tumor of the cartilage mostly involving tubular bones of the hands.