Excerpt from Myositis Ossificans

Synonyms, Key Words, and Related Terms: myositis ossificans, fibrodysplasia ossificans progressiva (hereditary type), myositis ossificans circumscripta, myositis ossificans progressiva, soft tissue ossification, progressive ossifying myositis, myositis ossificans traumatica, heterotopic osteogenesis, progressive osseous heteroplasia

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Background

Myositis ossificans is a misnomer and the term fibrodysplasia ossificans progressiva (FOP), as used by McKusick, is preferred. It is a rare autosomal dominant disorder characterized by skeletal malformation and progressive, disabling heterotopic osteogenesis. The condition was first described by Guy Patin in 1692.

Pathophysiology

Myositis ossificans manifests in 2 forms.

Myositis ossificans circumscripta can develop either in response to soft tissue injury (eg, blunt trauma, stab wound, fracture/dislocation, surgical incision) or can occur without known injury. Proposed mechanisms for atraumatic myositis ossificans include nondocumented trauma, repeated small mechanical injuries, and nonmechanical injuries caused by ischemia or inflammation.

Recently Shore et al reported mapping FOP to chromosome 2q23-24 by linkage analysis. They have further identified an identical heterozygous mutation (617G®R206H) in the glycine-serine (GS) domain of the activin A receptor type I (ACVR1) gene, a bone morphogenic protein (BMP) type I receptor in all affected individuals thus far examined.

Frequency

International

A recent report suggested the incidence of FOP to be less than 1 in 10^-7.

Race

Myositis ossificans progressiva occurs in all races.

Sex

Myositis ossificans progressiva demonstrates no definitive sexual predilection because the slight male predominance overall probably relates to differences in physical activity levels between the genders.

Age

Nonhereditary myositis ossificans is uncommon in children; fewer than 6.7% of cases occur in the first decade of life.
In FOP, average age of onset is 5 years, with a reported onset range from fetus to 25 years.

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