**11. Heterotopic ossification**

Heterotopic ossification (HO) is defined by the presence of bone in soft tissue where bone tissue normally does not exist, and it usually takes place around large joints (Medina et al., 2008). Its etiology is unknown, but is frequently precipitated by trauma, spinal cord injury or central nervous system injury (Shebab et al., 2002). The incidence of HO varies widely between populations. The incidence after hip replacement ranges from 16% to 53%; among patients with spinal cord injury HO develops in 20-25% and in brain injury patients the incidence of HO ranges from 10 to 20% (Medina et al., 2008; as cited in Vanden & Vanderstraeten, 2005). Around 20% of the patients who have an HO will develop functional limitation and it will be severe in 8% to 10% (Medina et al., 2008; as cited in Buschbacher, 1992; Subbarao, 1999). HO may closely mimic the presentation of cellulitis, osteomyelitis, or thrombophlebitis. HO can even be confused with some bone tumors such as osteosarcoma or osteochondroma. To resolve such diagnostic uncertainty and to prevent functional limitations, clinicians often request bone scanning and other imaging studies for patients at risk (Shebab et al., 2002). Radiography, MRI and CT have low sensibility in early stages of HO and three-phase bone scintigraphy (**Fig. 7**) is the most sensitive imaging modality during this period. It is also useful for its monitoring (Vanden & Vanderstraeten, 2005).

Fig. 7. Bone Scanning: increased uptake around the left hip consistent with Heterotopic Ossification.

First and second phases of three-phase bone scintigraphy are especially sensitive to detect incipient HO, which may be diagnosed 2.5 weeks after injury. Findings on the third phase may become positive approximately 1 week later. Radiographic studies do not show any change for at least 5- 8 weeks after initial injury (Shebab et al., 2002; as cited in Freed et al., 1982; Orzel & Rudd, 1985). Activity on the delayed bone scans usually peaks in a few months and after that the intensity of HO activity progressively lessens and thus the uptake of the radiotracer on the scans, which return toward normal within 12 months. However, in some cases activity remains slightly elevated even though the underlying HO has become mature (Shebab et al., 2002; as cited in Tibone et al., 1978). During the course of HO, bone scans made on follow-up may show radiotracer uptake on third phase even after flow and blood-pool images have returned to normal. Serial bone scans have been used successfully to monitor the metabolic activity of HO and determine the appropriate time for surgical resection, if needed, and to predict postoperative recurrence (Shebab et al., 2002; as cited in Freed et al., 1982; Muheim et al., 1973; Rossier et al., 1973; Tanaka et al., 1977). For the differential diagnosis of osteomyelitis complementary imaging with gallium-67 citrate (for spinal infection) or indium-111-labeled autologous leukocytes (for the appendicular skeleton) may be necessary, as commented in that section. Gallium-67 citrate uptake in HO is proportional to the uptake of 99m Tc-diphosphonates, in contrast to the relatively greater gallium-67 citrate uptake characteristic of osteomyelitis (Shebab et al., 2002).

A diagnostic-treatment algorithm of heterotopic ossification has been proposed, and threephase bone scintigraphy has been recommended, after clinical signs and laboratory test, for its diagnosis in patients without HO but with high risk factor. If clinical signs and symptoms are present but initial radiographic studies are normal, bone scan should be repeated after 4- 6 weeks, and when scintigraphic studies have displayed the HO, it should be made every three months during the first year. Bone scan has also been proposed during the follow-up after HO removal to monitor possible recurrences (Medina et al., 2008).
