POA leads to complete ankylosis of the affected joints limiting their range of motion and the independence of the patient. The clinical manifestations resemble those of inflammation which include pain, warmth, swelling, and limited range of motion. These symptoms and signs simulate thrombophlebitis, cellulitis, or osteomyelitis.
Three studies, serum alkaline phosphatase levels, bone scintigrams, and X-ray, are essential for diagnosing POA. Alkaline phosphatase is the earliest to become abnormal. It begins rising within one week of the injury (1). Typically it is attributed to healing fractures, occult biliary disease or normal bone growth in young patients. The three-phase bone scan becomes abnormal with increased blood flow and blood pool as early as 19 days following the injury (12,2), the third-phase turns abnormal one to two weeks later. The radiographs turn positive later in the process at 4-5 weeks following the accident.
The importance of bone scanning lies at the two ends of the pathologic process. In the early stages (before any calcifications are seen on X-ray) and, when only the flow study is abnormal, starting therapy with Didronel can effectively prevent further bone formation. Didronel is a diphosphonate compound (Etidronate Disodium) that has a potent in vivo and in vitro inhibitory effect on calcification.
In the later stages of the process treatment consists of surgical resection of the heterotopic bone. However, the resection should be performed only when the ectopic bone is mature and when the active bone formation has ceased (4). This can be demonstrated by falling levels of alkaline phosphatase or by quantitative bone scans. Regions of interest are drawn on the affected joint and on a normal bone (usually L5), and ratio of activity of calculated (3). When the process is stable the ratios remain constant over time and resection of the ectopic bone scan can be performed with little possibility of recurrence.
2) Resnick and Niwayama. Diagnosis of bone and joint disorders. Vol. 3, pp. 2412-2415.
3) Toshiyuki T, et al. Quantitative assessment of POA and its maturation on serial bone scans. Radiology 1977; 123:217-221.
4) Freed JH, et al. The use of the three-phase bone scan in the early diagnosis of heterotopic ossification and in the evaluation of Didronel therapy. Paraplegia 1982; 20:208-216.
5) Yutaka S, et al. Demonstration of myositis ossifications by Tc-99m pyrophosphate bone scanning. Radiology 1974; 111:663-669.
J. Anthony Parker, MD PhD, email@example.com