Myositis ossificans (MO) and fibro-osseous pseudotumour of digits are localised self-limiting lesions composed of reactive hypercellular fibrous tissue and bone (1). They often grow rapidly and biopsy may reveal mitotic activity and hypercellularity, and they are therefore classic pseudosarcomas.
These conditions can occur throughout life, but are most common in young adulthood (mean age 32 years). Males are more frequently affected than females (3:2), although FP more commonly occurs in females.
Soft tissue injury is the most common initiating event. MO can develop anywhere in the body but is most common in areas which are readily injured, such as the extremities, thigh, buttock and shoulder. It usually develops in skeletal muscle. Similar lesions occur in the subcutaneous tissue (panniculitis ossificans), tendons or fascia (fasciitis ossificans).
Early in the development of the lesion, there is classically swelling and pain. Calcification appears after 2-6 weeks, often in an eggshell like-distribution (hence the synonym myositis ossificans circumscripta). Over time, lesions become hard and well demarcated.
Lesions range in size from 2 to 12cm with a mean of around 5cm. Macroscopically they have a gritty white periphery and a soft tan haemorrhagic centre. On microscopy, early lesions appear highly cellular, comprising proliferating fibroblasts. There can be well-formed bony trabeculae peripherally.
Treatment is usually expectant with anti-inflammatory medication. Lesions can be removed after they have matured. Recurrence is unusual. The use of bisphosphonates has been described (2).
1. WHO Classification of tumours of soft tissue and bone. 4th Edition. 2013
2. Mani-Babu S et al. Quadriceps traumatic myositis ossificans in a football player: management with intravenous pamidronate. Clin J Sport Med. 2014 Sep;24(5):e56-8