Myositis ossificans of the psoas muscle: MRI findings

Clinical History

A 31-year-old female patient presented with a four-week history of right-sided lower back pain.

Imaging Findings

The patient presented with a four-week history of right-sided lower back pain. Her past medical history was unremarkable except for a three months old labour. She denied of having any specific trauma. Laboratory tests were performed, the values of which were normal except for an elevated erythrocyte sedimentation rate. Lumbar X-ray examination did not show any pathology. With a presumptive diagnosis of lumbar disc pathology, the patient was sugggested to undergo an MRI examination. T1-weighted sequences showed a barely seen iso-hyperintense lesion with minimal displacement and mass effect, located in the right psoas muscle, close to L3–L4 intervertebral disc (Fig. 1a). On T2-weighted and especially STIR sequences, the lesion was more readily apparent with well-defined borders and hyperintense central region surrounded by a hypointense rim (Fig. 1b, c). There was also an excessive, diffuse hyperintensity consistent with edema in psoas muscle surrounding this lesion. Both the lesion (except from its hypointense rim) and the surrounding edema showed a marked enhancement after gadolinium injection (Fig. 1d). There were no pathologic signal changes or contrast enhancement of the nearby intervertebral disc and the vertebral bodies. The lesion was thought to be compatible with myositis ossificans and to confirm the presence of an osteoid substance at the periphery of the lesion, a CT scan was performed. The results of the CT scan revealed the presence of an ellipsoid lesion measuring 8–20 mm in size with a well-demarcated ossified peripheral rim surrounding the central non-ossified region of low attenuation (Fig. 2). The findings were considered to be consistent with myositis ossificans and a conservative management was adopted. Her symptoms resolved two-months later and a follow-up MRI examination was performed, which showed complete disappearance of the surrounding perilesional edema (Fig. 3).

Discussion

Myositis ossificans is a benign, localized, self-limiting an uncommon disease with unknown pathogenesis. There is a non-neoplastic heterotopic bone and cartilage in the soft tissues in or adjacent to the muscle and in close proximity to the bone. This lesion commonly affects large muscles of the extremities, such as elbow, thigh, buttocks, and less often shoulder and calf. Ä°t tends to occur in adolescents and young adults (1, 2). Radiologic findings are parallel to the histologic pattern of maturation. Ä°n the first 2 weeks after trauma, soft tissue swelling without mineralization is found. After 4–6 weeks the lesion shows a rim of curvilinear peripheral ossification. Ä°n the final healing stage, the mature lesions ultimately demonstrate a dense, and diffuse ossification and reveal regression in size. Maturation is completed in about 5–6 months. The most characteristic radiological findings are radiological evolution and a peripheral rim-like calcification, reflecting a zone phenomenon (1, 2). CT is the preferred cross-sectional imaging method for detecting heterotopic mineralization (1). MRI appearance of myositis ossificans also correlates well with the histologic findings, zone phenomenon (progression of maturation from the periphery of the lesion) and evolution (maturation in five to six months). On MRI, myositis ossificans is most often seen as a relatively well-defined, inhomogeneous soft tissue mass. Diffuse surrounding edema is a prominent finding in the early lesions, imaged within eight weeks of the onset of symptoms (2, 3). Some reactive edema in the adjacent bone marrow could also be found. Ä°n early and intermediate lesions, myositis ossificans appears on T2-weighted SE MRI images as an inhomogeneous soft-tissue mass with increased signal intensity. Curvilinear and irregular areas of decreased signal intensity surrounding and coursing through the lesions could be seen especially in the intermediate type. Ä°n the early lesions on the corresponding T1-weighted images, an isointense signal with muscle and some mass effect with displacement is seen. Ä°n the intermediate lesions, fluid-fluid levels could be seen due to the occurrence of haemorrhage. After the administration of gadopentetate dimeglumine, a marked enhancement was found especially in the early lesions. Mature lesions are well-defined and inhomogeneous masses with a signal intensity approximating that of fat on both T1- and T2-weighted images without the associated edema and enhancement. On all pulse sequences, decreased signal changes related to bone trabeculae are seen surrounding the lesion and are also found within the lesion (3). Early-intermediate myositis ossificans lesions may be confused with soft tissue sarcomas.Ä°n mature lesions, the main differential diagnosis is with parosteal osteosarcoma.

Differential Diagnosis List

Final Diagnosis

Myositis ossificans of the psoas muscle.

URL:	https://www.eurorad.org/case/2365
DOI:	10.1594/EURORAD/CASE.2365
ISSN:	1563-4086