Characteristics

• Also known as heterotopic ossification.

• Benign condition involving ossification of muscle and other soft tissues.

• Majority of lesions described are secondary to trauma, including operative trauma such as total hip replacement.

• Calcification occurs within the traumatised tissue.

• Non-traumatic and hereditary forms are also described.

• Common sites involved are the large muscles of the extremities (80%), chest and back.

Clinical features

• Painful tender soft tissue mass – important to have a clear history of trauma, otherwise the suspicion of malignancy must be considered.

• Decreased range of movement of the involved musculo-tendinous unit, which reduces the range of the joints supplied by that unit.

• Pain decreases with time unlike most sinister pathology.

• May be aymptomatic and diagnosed incidentally.

• Can occur in periarticular situations, e.g. around the elbow following a paediatric supracondylar fracture.

Radiographic features

• Faint soft-tissue calcification develops in 2–6 weeks, becoming smaller and organised by 5 to 6 months.

• Separate from bone, but periosteal reaction may occur and can be mistaken for osteosarcoma.

• May occur within the muscle for example ‘riders bone’ (adductor longus); ‘fencers bone’ (brachialis); ‘dancers bone’ (soleus).

• May be periosteal at tendon insertion; Pellegrini–Stieda lesion (medial collateral ligament of knee).

• CT – depends on age of lesion. Well-defined mineralisation at periphery of lesion after 4–6 weeks with diffuse ossification in mature lesion.

• MRI – early lesions may reveal an ill-defined mass with heterogenous signal. Later, soft-tissue and bone-marrow oedema develop with decreased signal intensity surrounding the lesion due to mineralization/ossification.

Management

• Directed towards diagnosis and exclusion of sinister pathology.

• Symptomatic control with NSAIDs.

• Indomethacin is usually first line for myositis and should be started early for best results.

• Avoid painful aggressive rehabilitation/new trauma particularly after paediatric elbow trauma.

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