A 16-year-old patient presented with right thigh pain that began three weeks earlier after he was injured in a lacrosse game. He was involved in a collision, with direct trauma to his right thigh, but was able to complete the game. The pain gradually increased over the next few weeks, and he noticed a new palpable mass within the anterolateral thigh.

On examination, a firm, tender mass was palpated over the lateral quadriceps. The patient had pain on knee flexion, and range of motion was limited to 100 degrees. No overlying skin changes or gross deformities were present. Point-of-care ultrasonography was performed, followed by radiography (Figure 1 and Figure 2).

Question

Based on the patient's history, physical examination, and imaging findings, which one of the following is the most likely diagnosis?

• A. Ewing sarcoma.

• B. Femoral stress fracture.

• C. Myositis ossificans.

• D. Osteosarcoma.

Discussion

The answer is C: myositis ossificans. Myositis ossificans, also known as posttraumatic heterotopic ossification, is a nonmalignant ossification of soft tissue structures, usually the quadriceps, brachialis, and adductor muscles.¹ The condition commonly occurs in athletes following an injury but may also occur after surgery. Myositis ossificans is believed to result from inflammation-induced differentiation of fibroblasts into osteoblasts.¹