Age / Sex : 39 / F

Chief complaint: Right inguinal pain for 9 years

What is your diagnosis?

Two weeks later, you can see the final diagonosis with a brief discussion of this case (Please submit only one answer).

Courtesy : Yun Sun Choi, Eulji Hospital, Eulji University

Discussion

Answer: Heterotopic ossification (myositis ossificans)

Findings:

Anteroposterior radiograph of the hip shows an ossified mass adjacent to the right acetabulum. MRI shows an ovoid mass in the right iliopsoas muscle. The lesion is well defined mass to have mainly high signal intensity due to formation of fatty marrow on both T1-weighted and PD-weighted FSE MR images. Coronal fat suppression T2-weighted FSE MR image shows high signal intensity within the inferior aspect of the lesion. The lesion has peripheral low signal intensity rim on all image sequences. After contrast injection, heterogeneous enhancement is noted within the mass. There is no perilesional edema or extensive enhancement of surrounding tissues.

Differential Diagnosis:

1. Parosteal osteosarcoma


2. Osteochondroma


3. Soft tissue tumor with intralesional calcifications


4. Chronic avulsion injury

Discussion:

Heterotopic ossification (HO) or myositis ossificans is a benign, solitary, heterotopic bone formation typically occurring within the striated muscle, tendons, ligaments, fasciae, and aponeuroses. HO is not an inflammatory process but a proliferative mesenchymal response to a sufficient initiating injury to the soft tissue, 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 spontaneously.

Imaging findings are dependent on the stage of HO. Radiographs may reveal a soft tissue mass with faint peripheral calcification visible by 7 to 10 days. Over the next few weeks, floccular calcifications develop, and the most peripheral calcification becomes coarser and denser. The central zone of the lesion remains relatively lucent, and by 2 months, a well-defined cortex is seen peripherally. At MR imaging, early lesions may be difficult to detect or appear as a swelling or nodule isointense to the surrounding musculature on T1-weighted images, with diffuse or peripheral enhancement. The surrounding muscles are usually markedly edematous in the early stages, a feature not frequently seen in sarcomas and an important diagnostic finding. In the intermediate stage, lesions demonstrate a variable appearance. The center is isointense or hyperintense to normal muscle on T1-weighted images. On T2-weighted images, lesions tend to be inhomogeneous, with a variable, but predominantly high signal center, and irregular focal areas of intralesional decreased signal intensity. Perilesional edema decreases and a nonspecific pattern of enhancement may be seen. A variably thick rim of low signal on all sequences corresponds to the calcified peripheral zone. Mature lesions generally return low signal on all sequences, due to intralesional ossification, fibrosis, and hemosiderin deposition, although areas of cystic change may also be evident. Areas of signal isointense to normal bone marrow correspond to intralesional fatty marrow formation. Perilesional edema is not seen around mature lesions, an important marker for inactivity when surgical excision is considered.

References:

1. Resnick D. Diseases of soft tissue and muscle. In: Resnick D, Kransdorf MJ, eds. Bone and joint imaging. 3rd ed. Philadelphia, Pa: Saunders, 2005;682-684


2. Tyler P, Saifuddin A. The imaging of myositis ossificans. Semin Musculoskelet Radiol 2010;14:201-216


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