A 17-year-old male basketball player presents with right lateral thigh pain for the past 3 weeks. He had a collision with another player 5 weeks ago that resulted in a bruise that has since resolved. He is mildly tender over the lateral mid-thigh in the soft tissues and has a decreased knee flexion. You obtain X-rays (Figure 1). What is your suspected diagnosis? What is your initial workup in the ED? What imaging confirms the diagnosis? What is your management and disposition?
Figure 1: AP and lateral radiographs of the right femur
Myositis ossificans, which in this case is acquired from a traumatic injury. This is identified by the radiopaque mass within the muscle body on the AP and lateral views of the femur representing the calcifications (Figure 2).
Figure 2: AP and lateral radiographs of the right femur. Notice the radiopaque mass in the muscle consistent with a calcification (green arrows).
Pearl: Traumatic myositis ossificans occurs when damaged muscle body improperly heals leading to partial ossification of the muscle fibers. Occasionally you can feel a hard mass corresponding to the area of ossification in the soft tissues of the muscle on exam.
Pearl: Most commonly occurs in the brachialis, quadriceps femoris, and adductor muscles. 
The prognosis is generally favorable with most patients recovering with conservative therapy. A small minority will need surgery. This patient will likely return to light activity within 3 months and pre-injury level at around 6 months.