Heterotopic Ossification

The term heterotopic ossification (HO) describes bone formation at an abnormal anatomical site, usually in soft tissue. HO can be classified into the following 3 types:

• Myositis ossificans progressiva (fibrodysplasia ossificans progressiva) - This disorder is among the rarest genetic conditions, with an incidence of 1 case per 2 million persons. Transmission is autosomal dominant with variable expression. The condition is characterized by (a) recurrent, painful soft-tissue swelling that leads to HO and (b) congenital malformation of the great toe. There is no treatment for this form of HO. Limited benefits have been reported using corticosteroids and etidronate. Most patients die early from restricted lung disease and pneumonia; however some patients live productive lives.^[1]
• Traumatic myositis ossificans - In this condition, a painful area develops in muscle or soft tissue following a single blow to the area, a muscle tear, or repeated minor  The painful area gradually develops masses with a cartilaginous consistency; within 4-7 weeks, a solid mass of bone can be felt. Common sites include the pectoralis major, the biceps, and thigh muscles. A nontraumatic type of myositis ossificans also may exist.
• Neurogenic heterotopic ossification - This condition is the one that comes to mind when the generic phrase heterotopic ossification is used. This type of HO is the subject of this article. The various terms mentioned at the outset all refer to this type of HO.

In 1918, Dejerine and Ceilier first described HO in patients with spinal cord injury (SCI) from the First World War. Now HO is recognized as a fairly common sequela of SCI, especially [2