Heterotopic ossification and pseudoarthrosis in the shoulder following encephalitis. A case report and review of the literature.

Heterotopic bone formation, or myositis ossificans, is common, particularly following trauma, total hip arthroplasty, spinal cord injury, severe head injury, and long-term coma. Although the mechanism is unknown, the pathogenesis is assumed to depend on transformation of mesenchymal cells to bone forming cells in response to a variety of stimuli. The clinical findings, laboratory data, roentgenograms, and radionuclide studies are standard aids in the diagnosis of heterotopic ossification. The treatment usually consists of range-of-motion exercise, nonsteroidal antiinflammatory drugs, X-ray therapy, disodium etidronate (EHDP), and excisional surgery. Reported here is a rare case of periarticular heterotopic ossification in the shoulder of a 38-year-old woman following head injury and 13 months in a coma. The unusual feature was the development of a pseudoarthrosis within the heterotopic bone. The patient's shoulder became markedly stiff with the development of a heterotopic pseudoarthrosis. Excision of the heterotopic bone and pseudoarthrosis was performed to improve the range of motion. Clinical roentgenographic, radionuclide, and pathologic observations are presented on the formation of a synovial joint within the heterotopic bone.