Myositis ossificans is an inflammatory pseudotumor characterised by the formation of heterotopic ossification. The exact pathogenesis remains unclear but mostly it occurs as a metaplastic response to trauma. A rapidly growing, painful inflammatory mass is the most common presentation. It has been confused with deep vein thrombosis or cellulitis because of the rapidity of growth and associated inflammatory component. Typically myositis ossificans passes through three stages- acute, (day 0-7) subacute (day 7-15) and late phase (>day 15). If biopsied early in the acute or subacute phase it can give a false diagnosis of sarcoma. It is only in the late phase, which is also known as the maturation phase that mature bone production is seen at the periphery.
The typical zone phenomenon in MO corresponds to mature well organised lamellar bone at the periphery with an intermediate osteoid zone and a central immature fibroblastic non-ossified zone. Plain radiographs appear normal in the early stages or may demonstrate a soft tissue opacity. Calcification is usually apparent by 2-6 weeks and radiographs repeated later in the course of this pathological process demonstrate peripheral ossification surrounding a central clear zone by around 8 weeks. In some cases, the string sign; which is a radiolucent cleft separating this ossification from the cortex of the underlying bone, is seen which helps in the differential diagnosis.
Ultrasound typically depicts MO as a central hypoechoic area surrounded by echogenic rim corresponding to ossification and a third peripheral hypoechoic area. The most sensitive modality to depict the zone phenomenon of MO is ultrasound, which can demonstrate this finding before ossification is seen on any other modality
CT is best to depict the peripheral ossification. A clear cleft or plane is present between the ossified mass and the underlying bone. MRI appearance can vary depending upon the stage of the disease. There is often inflammatory oedema extending beyond the mass. Peripheral hypo intensity corresponding to ossification can be seen as low signal areas on all sequences.
The diagnosis is based on clinical history of antecedent trauma, rapidly growing inflammatory painful mass with characteristic imaging findings of peripheral ossification and zone phenomenon. Histology can be misleading in early cases because of the metaplastic response and significant mitosis.
Most cases of MO are secondary to trauma, and present as a rapidly growing, painful, tender mass; though in few cases no antecedent history of trauma is found. It is also well documented in patients with spinal cord injury. During the recent vaccination drive for COVID-19 virus, there were reports of myositis after COVID-19 vaccination along with other vaccination adverse side reactions like pain, oedema, and redness. Because of the direct intra-muscular inoculation of the vaccine the muscle is exposed to the modified contaminant agent which elicits an immune response to the antigen. This inflammatory response can be because of direct toxicity of the injected vaccine and its components or patients’ immune response to the injected antigen. There was a temporal association between the vaccination and the symptoms with complete resolution of symptoms with conservative management; thereby suggesting the diagnosis of COVID-19 vaccine-related myositis ossificans as the cause.
