Rib fractures are usually caused by direct trauma to the chest wall. In these cases, the rib is pushed inwards, possibly producing a hemothorax or pneumothorax. Rib fractures are notoriously subtle to detect on standard chest radiographs because they are frequently nondisplaced. In these cases, dedicated rib series with oblique views may reveal the fracture site. This case illustrates the point that a large amount of force is required to fracture the upper ribs because they are shielded by overlying structures. This patient was in an automobile accident causing severe injury to the chest.
AP chest radiograph. This demonstrates multiple left lateral rib fractures. The patient sustained a pneumothorax as well and a chest tube was placed. 
Axial CT. This image is acquired through the shoulder and shows a comminuted fracture of the glenoid as well as one of the rib fractures identified in the chest radiograph.