A 20 year old professional footballer presented with pain and swelling of the left thigh after a kick three weeks previously. He had sustained a number of injuries in the past including a previous calf haematoma because of the nature of his profession but none had been as severe. Clinical examination revealed a normal gait. There was definite but diffuse swelling of the extensor compartment of the left thigh that measured 1 cm more in circumference compared with the normal right side at the level of the mid-thigh. It was tender but no discrete mass could be felt. There was also slight limitation of knee flexion but extension was full and no hip abnormality was detected. The patient was otherwise well with no constitutional symptoms. The rest of the physical examination was normal. Plain radiographs of the left thigh were obtained. What abnormality does figure 1 show? What is the differential diagnosis? Figure 1 Plain radiograph of the left thigh. The clinical history points strongly to a post-traumatic sequel. This may have involved either the bone or soft tissues. Bony involvement may be attributable to a fracture or stress fracture. The first develops acutely after the injury whereas the second has a more insiduous course and often develops at a later stage because of repeated weight bearing, for example, if the patient returns to training after the initial insult. A detailed history as to the time course of the symptoms in relation to the injury is particularly useful in this regard. In this patient, the normal gait suggests that a fracture or stress fracture is unlikely. Post-traumatic soft tissue lesions include haematoma, myositis ossificans, or a muscle tear with or without associated muscle herniation. The diagnosis of muscle herniation can usually be confirmed clinically by demonstrating the presence of a soft tissue …