Bone Scintigraphy in Para-osteo-arthropathy

Zaim Badra, M.D.

Paul Doherty, M.D.

February 2, 1988

Case Presentation:

A 22 year-old man was involved in a motor vehicle accident in which he sustained a complete fracture dislocation of C5 on C6 with C6 completely anterior to C5. He was treated with subsequent decrease in the dislocation. Initially he was classified as complete motor loss and incomplete sensory loss. Two weeks after his admission he started complaining of hip pain, swelling in both legs and limited range of motion.

Findings:

A Tc-99m MDP bone scintigram of the pelvis and hips and an X-ray showed the presence of para-osteo-arthropathy (arrows). Regions-of-interest have been placed over the lumbar spine and right proximal femur.

Discussion:

POA is characterized by deposition of calcium in soft tissues primarily around the joints. All joints are susceptible to the disease, but most commonly it involves the hip joint, the knees, and the shoulders. Heterotopic bone formation (HBF) is a common complication of spinal cord injury and has been reported in 16% to 54% of paraplegic cases (1). Less commonly it can follow acute anoxia, head injury, CVA, encephalomyelitis, poliomyelitis, multiple sclerosis, and burns (2). The common factor among these conditions is immobilization. No definite cause for HBF has been found. Local factors such as continuous pressure, decubitus ulceration, and trauma from physical therapy, have been suggested; however, the occurrence of POA in areas remote from pressure joints or decubitus ulcers and in the absence of physical manipulation has been documented. Systemic factors such as hypoproteinemia and urinary tract infection have been implicated, but studies failed to prove the relation (2). Finally, it is believed that the ossification result from transformation of primitive mesenchymal derived cells present in soft tissues into bone forming cells in response to a variety of stimuli.

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.

References:

1) Orzel JO, Rudd TF. Heterotopic bone formation: clinical, laboratory, and imaging correction. JNM 1985; 26:125-132.

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.

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J. Anthony Parker, MD PhD, jap@nucmed.bih.harvard.edu