In 1918, Dejerine and Ceillier first described heterotopic ossification (HO) in paraplegic patients injured in World War I, referring to the process as paraosteoarthropathy. HO has been defined as the formation of mature lamellar bone in soft tissues. The process involves true osteoblastic activity and bone formation. HO has been reported in cases of brain injury, spinal cord injury, stroke, poliomyelitis, myelodysplasia, tabes dorsalis, carbon monoxide poisoning, spinal cord tumors, syringomyelia, tetanus, and multiple sclerosis. This condition also has been reported after burns and total hip replacement/joint arthroplasty.
Several terms have been used to describe the condition, including heterotopic ossification, ectopic ossification, and myositis ossificans. HO usually involves the large joints of the body (eg, hips, elbows, shoulders, knees). Excessive bone formation may result in significant disability by severely limiting the range of motion (ROM) of these joints (see image below).
The following 3 categories of HO have been described:
Myositis ossificans progressiva - This is a rare metabolic bone disease in children with progressive metamorphosis of skeletal muscle to bone; it is characterized by an autosomal dominant pattern of genetic transmission.
Myositis ossificans circumscripta without trauma - Also referred to as neurogenic HO, this is a localized soft-tissue ossification occurring after neurologic injury or burns.
Traumatic myositis ossificans - This condition occurs from direct injury to the muscles. Fibrous, cartilaginous, and osseous tissues near bone are affected; the muscle may not be involved.
Related Medscape Reference topics:
Heterotopic Ossification [Physical Medicine and Rehabilitation]
Heterotopic Ossification Imaging [Radiology]
Related Medscape resource:
Resource Center Joint Disorders
The specific cause and pathophysiology of heterotopic ossification (HO) remain uncertain, but the condition appears to involve the inappropriate differentiation of mesenchymal cells into osteoblastic stem cells in response to still-unidentified inducing agents.
HO may be due to an interaction between local factors (eg, the pool of available calcium in adjacent skeleton, soft-tissue edema, vascular stasis tissue hypoxia, mesenchymal cells with osteoblastic activity) and an unknown systemic factor or factors. The basic defect in HO is the inappropriate differentiation of fibroblasts into bone-forming cells. Early edema of connective tissue proceeds to tissue with foci of calcification and then to maturation of calcification and ossification.
The reported incidence of heterotopic ossification (HO) varies. In cases of severe trauma or insult to the central nervous system (CNS), 10-20% of patients develop HO, and the condition has been observed in 20% of patients with severe brain injury. The incidence is higher in patients who undergo open reduction and internal fixation of a fracture. With an elbow fracture, dislocation, or fracture-dislocation, the incidence of traumatic HO at the elbow approaches 90%. Traumatic HO of the elbow occurs in 20% of forearm fractures. Fifty-five percent of patients with hip fractures develop HO. The incidence increases to 83% if open reduction and internal fixation are performed. The incidence is similar in the upper and lower extremities.
An association has been cited between spasticity and HO. The incidence is higher in a spastic extremity; 84% of patients with HO had spasticity, and 54% of patients with HO had no spasticity. HO is seen in the elbow in 4% of patients with traumatic brain injury (TBI); however, if fracture or dislocation is associated with brain injury, the incidence of HO rises to 89%.
Related Medscape Reference topics:
Studies from Europe and Japan have shown the incidence of HO to range between 11% and 76%, depending on the population studied and on the method of detection.
Only 10-20% of all heterotopic ossification (HO) patients have functionally significant deficits.
No race predilection exists for heterotopic ossification.
The development of heterotopic ossification is independent of the patient's sex.
An increased incidence of heterotopic ossification (HO) has been found in persons over age 30 years. The incidence of HO in children appears to be lower than that in adults (8-22.5%).
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