Heterotopic ossification (HO) following spinal cord injury (SCI) was first described by Dejerine and Ceillier in 1918 as paraosteoarthropathy. The ossification process involves the formation of mature lamellar bone, which is indistinguishable from normal bone, in soft tissues surrounding paralyzed joints (see the following image). The bone is not connected to periosteum and becomes encapsulated as it matures.

Extensive heterotopic ossification at the medial aspect of the left knee. View Media Gallery

The pathology is similar to that of fracture callus, except that bone forms in the connective tissue between the muscle planes (histologic findings in neurogenic HO are similar to those in healing fracture callus). HO is also seen after other neurologic insults, such as traumatic brain injury (TBI), stroke, Guillain-Barré syndrome (GBS), and after burn 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, activating the cells to differentiate into osteoblasts. [1]

The incidence of HO in SCI is between 16% and 53%, depending on the incidence reports from various institutions. Clinically significant HO develops in about 20% of patients with a SCI. [2, 3] Fortunately, only 3-5% of cases involve joint ankylosis. In SCI, HO always occurs below the level of the lesion, most commonly at the hip (70-97%). [3, 4] Other body segments, including the knee, elbow, shoulder, hand, and spine (in decreasing incidence), may also be involved.

There is no known race or sex predilection for neurogenic HO; however, the incidence of neurogenic HO after SCI is lower in pediatric patients than in adults, ranging from 3% to 10%. In addition, spontaneous resorption of the neurogenic HO is frequently seen in pediatric patients. [5]

The risk factors associated with the development of HO in patients with traumatic SCI has been studied in a case control study of 264 patients by Citak et al. According to this study, patients with spasticity, thoracic trauma, complete lesion, pneumonia, presence of tracheostomy, and urinary tract infection had a higher risk of developing HO. [6]

The following image depicts 3 common locations of HO in the hip.

Three common locations of heterotopic ossification around the hip joint. A: Anterolateral/anteromedial location; B: Inferior and medial location; and C: Location around the femoral neck and posterior. View Media Gallery

See also Spinal Cord Injuries, Autonomic Dysreflexia in Spinal Cord Injury, Functional Outcomes per level of Spinal Cord Injury, Hypercalcemia in Spinal Cord Injury, Osteoporosis and Spinal Cord Injury, Prevention of Thromboembolism in Spinal Cord Injury, Rehabilitation of Persons with Spinal Cord Injuries, and Spinal Cord Injury and Aging.