Heterotopic ossification (HO) following spinal cord injury (SCI) was described first by Dejerine and Ceillier in 1918 as paraosteoarthropathy. The process is one of formation of mature lamellar bone, which is indistinguishable from normal bone, in soft tissues surrounding paralyzed joints. The bone is not connected to periosteum and becomes encapsulated as it matures. The pathology is similar to that of fracture callus, except that bone forms in the connective tissue between the muscle planes. HO also is seen after other neurologic insults such as traumatic brain injury (TBI) and stroke, as well as after thermal injuries and orthopedic procedures (eg, total hip replacement). In experimental models of HO formation, ischemia and tissue expression of bone morphogenic proteins have been shown to play important roles. Bone morphogenic proteins likely act on mesenchymal stem cells present in tissue and, thus, activate them to differentiate into osteoblasts (Banovac, 2004).