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A 53-year-old woman had extraskeletal osteosarcoma that developed from a soft tissue bony mass present on the volar aspect of the left wrist for 4 years. Initially, the bony mass was soft and movable, but during the first year it became hard and fixed. The patient had no history of trauma. Because the lesion did not grow or cause any symptoms, the patient did not come to the hospital until 4 years after she first noticed the lesion. Radiologically, the bony mass had features characteristic of mature myositis ossificans, showing "eggshell" ossification. A nonmineralized soft tissue mass occurred between the surface of the radius and the bony shell. Histologically, a high-grade osteosarcoma was present between the surface of the radius and the well-differentiated bone tissue, which included fatty and hematopoietic marrow. All the findings indicated that our patient had an extremely rare case of malignant transformation of myositis ossificans.

To our knowledge, no previous direct associations have been made between generalized myositis ossificans and pharmacological therapy. We report a case of generalized periarticular myositis ossificans associated with the use of curare and diazepam. The previously reported associations of myositis ossificans with tetanus and burns may be misleading. It is possible that it is not the disease process itself (e.g., tetanus, severe burn) that precipitates heterotopic ossification, but the treatment of these ailments. These observations suggest the importance of early mobilization and restrained use of immobilizing drugs. Further investigation is warranted with regard to the predisposing factors of generalized myositis ossificans and to its prevention.

We have described the pre- and postgadolinium MR appearance in one case of acute myositis ossificans. The unenhanced sequences can be helpful in excluding malignancy, particularly when viewed serially. While use of gadolinium allowed primary sarcoma to be ruled out in this case, it was not useful in the exclusion of early abscess formation or necrotic metastasis.

Noninfectious, nonneoplastic reactive processes of the hand, such as myositis ossificans circumscripta, pseudomalignant osseous tumor of soft tissue, and florid reactive periostitis, appear similar radiologically and histologically and are often difficult to differentiate. Magnetic resonance (MR) findings in two such lesions are reported. The extensive reactive change in the extraosseous soft tissue and the bone marrow and the relatively small extent of ossification may be characteristic. Although low-grade infection and small osseous neoplasms with reactive changes, such as osteoid osteoma, may still remain possible causes, MR imaging provides essential evidence for including noninfective, nonneoplastic reactive processes of uncertain cause in the list of differential diagnoses.

Myositis ossificans typically presents as soft tissue swelling with progressive ossification on radiographs. Since magnetic resonance imaging (MRI) is commonly used to evaluate soft tissue masses, we analyzed eight MR examinations in seven patients with myositis ossificans to determine if typical patterns were present. One acute lesion had homogeneous intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images. Two subacute lesions had low signal intensity margins with slightly increased signal intensity centers on T1-weighted images and very high signal intensity on T2-weighted images. Five chronic lesions had two different patterns. All five were well-defined with low signal intensity borders. Three had signal intensity patterns characteristic of fat on T1-weighted and T2-weighted images. The other two lesions had intermediate signal intensity on T1-weighted images and slightly increased signal intensity on T2-weighted images. We conclude that typical MR appearances of myositis ossificans do exist. A low signal intensity rim is a common finding. However, these patterns are not unique to myositis ossificans and resemble those that have been reported in other lesions. It is important to be aware of the spectrum of MR findings of myositis ossificans when considering the differential diagnosis of a soft tissue mass.

Ultrasonic evaluation of a soft tissue mass of the thigh was performed and suggested the diagnosis of myositis ossificans. Correlation with routine radiographic studies is made, as well as with the follow-up radiographic examination.