Image Interpretation: Focused ultrasound of the right thigh demonstrates echogenic mineralization within the vastus intermedius muscle measuring 10.22 cm in diameter with small surrounding hypoechoic fluid collection. These findings are consistent with myositis ossificans within the vastus intermedius.

Teaching Pearl: Myositis ossificans is a self-limited condition of benign, heterotopic bone growth often following trauma in athletes. The most common sites of occurrence include the brachialis, quadriceps, and adductor muscle groups and there is often an inciting injury such as a contusion, hematoma, or microinjury from overuse. Patients may present with localized pain, a palpable mass, and adjacent joint stiffness with associated reduced range of motion. A lesion in the thigh can also result in reduced weight-bearing ability and decreased knee flexion. Primary treatment for myositis ossificans includes gradual rehabilitation of flexibility and strength over a two month to one year period, with minimization of pain. Persistent symptoms may warrant surgical excision. Diagnostic ultrasound can lead to early identification of a lesion, prevent inaccurate diagnosis of a muscle strain, and is useful for monitoring progression. Ultrasound findings appear 2 weeks prior to radiographs, which usually develop 3-4 weeks post injury. Early phases demonstrate decreased echogenicity of the muscle with sheets of peripheral echogenic material. With increasing ossification, coarse calcifications accompanied by acoustic shadowing appear often parallel to the diaphysis. Findings must be differentiated from a malignant lesion such as osteosarcoma by identifying the "zonal phenomenon" of myositis ossificans on ultrasound, most evident at least two weeks after injury. The zonal phenomenon is marked by the lesion progressively calcifying from the periphery to the center, as opposed to osteosarcomas which calcify from the center to the periphery. Additionally, myositis ossificans will not have connection to the adjacent bone.