Age / Sex : 37 / M

Age / Sex :
14/M

Chief complaint : Painful hard mass in left deltoid region for 3-4 weeks

1) What is your impression?

Two weeks later, you can see the final diagnosis with a brief discussion of this case.

* Send Application Answers to In Sook Lee, MD (lis@pusan.ac.kr)

Courtesy :

myositis ossificans

Discussion

Findings:

Simple radiography showed ovoid ossified mass in the lateral aspect of left shoulder region. There was incidental non-ossifying fibroma in the left humerus.

MRI showed a ovoid mass lesion in the left deltoid muscle. It showed multilayered or target appearing signal intensity zones with a central dark singal intensity nodule and peripheral dark signal intensity rim on all image sequences. The remained zone was high signal intensity on fat-suppressed T2WI and isosigal to that of adjacent muscle on T1WI. Diffuse intense enhancement was noted within the mass. It was also accompanied by extensive enhancement of surrounding tissues and increased vascularities adjacent to the mass (Not shown here).

 Differential Diagnosis: Early MO lesions should be differentiated from inflammatory disease or soft tissue tumor with intralesional calcifications and perilesional edema, such as osteosarcoma (with its parosteal and extraosseous variants), synovial sarcoma, rhabdomyosarcoma, MFH, etc.

Diagnosis:  Myositis ossificans

Discussion: Myositis ossificans is a benign, solitary, self-limiting, heterotopic bone formation typically occurring within the skeletal muscle. There are numerous synonyms including pseudomalignant osseous tumor of soft tissue, extraosseous localized nonneoplastic bone and cartilage formation, pseudomalignant myositis ossificans, heterotopic ossification. MO is not an inflammatory process but a proliferative mesenchymal response to a sufficient initiating injury to the soft tissue, not necessarily a muscle, eventually leading to localized ossification. In the first few days vascularized proliferative fibroblastic cells are appeared in the injured tissue. With maturation of the lesion, a typical zonal pattern develops with three distinct zones: the center consists of rapidly proliferating fibroblasts with area of hemorrhage and necrotic muscles; the intermediate zone consists of osteoblasts with immature osteoid formation. The peripheral zone is composed of mature bone. Peripheral bone formation begins usually at 6-8 weeks. At 5-6 months, the lesion can completely ossify with formation of a cortex and marrow spaces. With maturation, the lesion classically regresses in size, and in about 30% of cases may eventually resolve sponataneously. Imaging findings are dependent on the stage of MO.

References:

1.     Gindele, D. Schwamborn, K. Tsironis and G. Benz-Bohm. Myositis ossificans traumatica in young children: report of three cases and review of the literature. Skeletal radiology 2000;30(7): 451-459

2.     KransdorfMJ, Meis JM, Jelinek JS. Myositis ossificans: MR appearance with radiologic pathologic correlation. AJR Am J Roentgenol 1991;157: 1243–1248