Chondromyxoid fibroma involving the metacarpophalangeal joint

Chondromyxoid fibroma is a rare benign bony tumor classically occurring in the metaphyseal region of the long bones surrounding the knee, but also found with relative frequency in other long bones, the pelvis, ribs, and small foot bones. The small bones of the hand, however, are rarely involved. The tumor is considered a physeal plate remnant and may involve the epiphysis, diaphysis, or both along with its metaphyseal origin. It may cause cortical expansion and destruction, but consistently respects the periosteal boundary. We report a case of histopathologically confirmed chondromyxoid fibroma involving a metacarpal, proximal phalanx, and synovial tissues of the metacarpophalangeal joint. A 29-year-old woman presented with pain, weakness, and a mass of the right ring finger metacarpophalangeal joint that had persisted since prior surgery to remove an extraosseous chondroma. Prominent soft tissue swelling and limited range of motion were noted, corresponding to magnetic resonance imaging findings of lytic changes in the right ring finger metacarpal and proximal phalanx, surrounded by an irregularly enhancing mass with soft tissue extension. Open biopsy revealed calcified cartilaginous synovial deposits suspicious for synovial chondromatosis, with erosion into surrounding bones. Histologic examination revealed an unusual chondromyxoid fibroma with joint involvement. The patient is >15 months out from synovectomy, intralesional curettage, and placement of demineralized bone matrix and allograft cancellous chips, and is without signs of recurrence. This benign, curable lesion should be included in the differential diagnosis of a bony lesion, even when a joint and adjacent bones are involved.