Bizarre parosteal osteochondromatous proliferation of bone (Nora's lesion)

Bizarre parosteal osteochondromatous proliferation was first described in 1983, when Nora et al. reported 35 examples of a proliferative lesion involving the small bones of the hands and, less often, the feet. No examples involving the large bones were reported. We present 65 cases of this condition, five from the Mayo Clinic files and 60 from our consultation files, including 34 females and 31 males; patients' ages ranged from 8 to 73 years (average, 33.9 years). Thirty-six lesions involved the hands, 10 the feet, 17 the long bones, one the skull, and one an unknown site. The long bones involved were the radius (three lesions), ulna (six), fibula (two), femur (three), tibia (two), and humerus (one lesion). Roentgenograms typically showed a heavily calcific mass attached to the underlying cortex and having a broad base. Histologically, a large amount of hypercellular cartilage showed maturation to trabecular bone, which frequently contained spindle cells in the intertrabecular spaces, an appearance that could lead to a mistaken diagnosis of parosteal osteosarcoma. A helpful feature was the distinct blue tinctorial characteristic of the bone in the lesion. Follow-up information was available for 40 patients; approximately 55% had a recurrence. No metastasis had been reported in any of the 65 cases. Bizarre parosteal osteochondromatous proliferations are a form of heterotopic ossification and should not be mistaken for chondrosarcoma or osteosarcoma.