Imaging description

Despite the advances in imaging techniques diagnosing myositis ossificans remains a challenging problem, especially in early stages. It is important to remember that no one set of imaging findings obtained at one time can characterize all the stages of myositis ossificans. In fact the imaging findings rapidly change depending on the degree of maturation of the lesion. Initially radiographs can be entirely normal. Within 2 weeks after the injury radiography may show a soft tissue mass along with thin uninterrupted periosteal reaction in the adjacent bone. The periosteal reaction is more often seen in children where it can sometimes be thick and lamellated. At this stage the best indication that the lesion is not a neoplasm is the fact that the periosteal reaction is uninterrupted. Biopsies of the soft tissue mass, at this early stage, reveal rapidly proliferating mesenchyme that may be mistaken for a sarcoma. By 3–4 weeks the lesion starts to form osteoid which in the following weeks begins to mineralize. At this stage radiographs show clumps of calcifications within the soft tissue mass. By 5–8 weeks a zoning pattern appears which represents the most important diagnostic feature of myositis ossificans (Figures 87.1 and 87.2). This presents radiographically as peripheral calcifications followed by ossifications which surround the lesion. At 2 months the ossifications become thicker and denser, and later on the lesion starts to appear essentially similar to mature trabecular bone. By 3–5 months the lesion becomes diffusely ossified, smaller, and may eventually adhere to the adjacent bone (Figure 87.1, G and H).