Myositis ossificans in psoas muscle after lumbar spine fracture.

STUDY
DESIGN: A case report.
OBJECTIVE: To report a case of myositis ossificans in the psoas muscle after lumbar spine fracture. SUMMARY OF BACKGROUND DATA: Myositis ossificans is a benign condition of heterotopic bone formation in skeletal muscle. It predominantly occurs in large muscles of extremities following injury. Occurrence in the psoas muscle is rare with no previous reports of this condition associated with vertebral fracture in patients without hematologic abnormality.
METHODS: A 56-year-old man was referred to our hospital for a mass located in the left psoas muscle and an L3 compression fracture. He complained of low back and left leg pain after a fall 2 weeks previously. Magnetic resonance (MR) imaging obtained 12 days after trauma showed an L3 compression fracture with a left psoas mass with low T1 and high T2 signal intensity, associated edema, and enhancement. Computed tomography (CT) scans showed a suspicious slightly high-density mass in the left psoas area. The radiologic features of the mass were consistent with a hematoma.
RESULTS: After 1 week, CT scans showed calcification at the hematoma site. Histologic examination following CT-guided biopsy demonstrated new bone formation, atrophy of skeletal muscle fibers, and fibrosis. Lymphocytes were focally infiltrating. Irregularly anastomosing bony trabeculae and fibroblastic cell proliferation were noted. On follow-up MRI and CT scans 2 months later, the previous calcified mass was no longer evident.
CONCLUSION: Myositis ossificans usually occurs in extremities and is very uncommon in the psoas muscle. We report psoas myositis ossificans that developed from hemorrhage and edema associated with a lumbar fracture and required only 2 months to regress completely. When one finds a soft tissue mass with surrounding edema in the psoas muscle combined with a vertebral compression fracture, myositis ossificans should be included in the differential diagnosis.