Fibrodysplasia ossificans progressiva (FOP), previously known as myositis ossificans progressiva (MOP), is a rare, inherited disorder characterised by progressive fibrosis and ossification of muscles, tendons, fasciae, aponeuroses, and ligaments of multiple sites. It is disabling and ultimately fatal.
FOP should not be confused with myositis ossificans circumscripta, which is usually post traumatic.
FOP is a very rare disease, with an estimated prevalence of ~1 per 2 million. There is no racial/ethnic, or gender predilection 3. The process and symptoms of heterotopic ossification starts between 2 and 5 years old.
Most cases arise from sporadic mutations, although some are inherited in an autosomal dominant fashion 3.
FOP can be evaluated using plain radiographs, CT and/or MRI. MRI is mostly useful for more subtle oedema which would not be seen on the other modalities. Characteristic features include:
- hallux valgus
- monophalangic first toe
- shortened metacarpals
- pseudoexostoses (ossification of ligamentous insertions)
- microdactyly of the first metacarpal/metatarsal
- neck muscle oedema
- C2-C7 facet joint fusion
Treatment and prognosis
This is a progressive, fatal disease with the median survival being 45 years.
Since the process of heterotopic ossification is quite unique, differential diagnosis on a long term basis is quite limited. However, for punctual abnormalities, the following differential diagnosis should be considered:
- 1. Hagiwara H, Aida N, Machida J et-al. Contrast-enhanced MRI of an early preosseous lesion of fibrodysplasia ossificans progressiva in a 21-month-old boy. AJR Am J Roentgenol. 2003;181 (4): 1145-7. AJR Am J Roentgenol (full text) - Pubmed citation
- 2. Fisher DR, Hinke ML. Musculoskeletal case of the day. AJR Am J Roentgenol. 1986;146 (5): 1087-93. AJR Am J Roentgenol (citation) - Pubmed citation
- 3. Kaplan FS, Le Merrer M, Glaser DL et-al. Fibrodysplasia ossificans progressiva. Best Pract Res Clin Rheumatol. 2008;22 (1): 191-205. doi:10.1016/j.berh.2007.11.007 - Free text at pubmed - Pubmed citation