Genetic disorders of heterotopic ossification: Fibrodysplasia ossificans progressiva and progressive osseous heteroplasia

Fibrodysplasia ossificans progressiva (FOP) is a rare heritable disorder of connective tissue characterized by congenital malformations of the great toes, and progressive heterotopic endochondral ossification (HEO) in characteristic anatomical patterns. HEO may also occur sporadically following joint replacement, central nervous system trauma, athletic injury, war wounds, atherosclerosis, and valvular heart disease. Bony masses from FOP immobilize joints and cause contractures and deformity. Ossification around the hips, typically present by the third decade of life, often prevents ambulation. Skeletal anomalies and soft-tissue ossification are radiological hallmarks of FOP. Bone scintigraphy detects abnormalities in soft tissue before HEO can be demonstrated radiographically. Removal of FOP lesions is often followed by significant recurrence of HEO. As with FOP, progressive osseous heteroplasia (POH) is an autosomal dominant genetic disorder of heterotopic ossification. POH commonly begins in the dermis and progresses to deeper tissues by an intramembranous, rather than an endochondral, pathway.