Age / Sex : 23 / F

Chief complaint: Rt. hip pain for 1 month

1) What is your impression?

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

(Quiz는 quiz일 뿐이오니 답안은 한개만 보내주시기 바라오며, 복수의 답안을 보내주시는 분은 정답이 포함되어 있더라도 부득이 semi-correct answer로 처리토록 하겠습니다.)

Courtesy: 홍성환 (Sung Hwan Hong), 서울대학교병원 (Seoul National University Hospital)

Myositis ossificans ( Heterotopic ossification)

Discussion

Findings:

Radiography

-  an ossified mass with peripheral rim of calcification overlapped on the right ischial tuberosity

MRI

- slightly T1 hyperintense and T2 hypointense soft tissue mass in the right adductor magnus

- prominent enhancement after contrast administration

- extensive edema and enhancement in surrounding soft tissue

CT

- soft tissue mass with peripheral rim-like calcification (zonal phenomenon)

Differential Diagnosis:

1. Myositis ossificans (Heterotopic ossification)

2. Soft tissue sarcoma

Diagnosis: Myositis ossificans (Heterotopic ossification)

Discussion: Myositis ossificans (MO) or heterotopic ossification is characterized by abnormal heterotopic bone formation involving striated muscle, tendons, ligaments, fasciae, and aponeuroses.

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.

CT is the imaging modality of choice because it optimally demonstrates the cross sectional zonal pattern, with excellent definition of both soft tissues and calcification.

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 scan, 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 sequences, 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 is seen. A variably thick rim of low signal on all pulse sequences corresponds to the calcified peripheral zone. Mature lesions generally return low signal on all sequences, due to intralesional ossification, fibrosis, and hemosiderin deposition. Areas of signal isointense to normal bone marrow correspond to intralesional fatty marrow formation.

The differential diagnoses for MO in the acute or subacute setting include muscle abscess, soft tissue sarcoma, focal myositis, and rhabdomyolysis. As for the more mature lesions in which there is marked bone formation, parosteal osteosarcoma, osteochondroma, and chronic avulsion injuries should be considered when the lesion arises adjacent to the cortex. MO has a characteristic peripheral calcification pattern, not typically seen in other calcifying soft tissue lesions.

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