Fracture is synonymous with break. Several types of bony fractures exists. The most common and well-known type is equivalent to breaking a pencil. This usually occurs with a traumatic event and is easily diagnosed by X-ray. Compression fractures of the vertebral body are also common in orthopedics as a sudden cause of low back pain, usually present in an elderly patient who has fallen. In the presence of osteoporosis, or other bony pathology, rendering the bone weaker than usual compression fractures of the vertebral body become quite common (see compression fracture). Hairline fractures are fractures that are less easily seen on X-ray and do not cause any displacement of the two bony fragments. Sometimes the trauma of the fracture also breaks the skin and, in many cases, more than one fragment of bone is involved. A stress fracture is often seen in sports medicine where there is no one single event that suddenly breaks the bone. Rather, a stress fracture occurs when multiple repetitive microfractures to the bone are caused repeatedly over time when an athlete overdoes it. This is also sometimes seen in soldiers in boot camp and other training protocol, or workers starting a new job that requires a lot of walking or other repetitive activities.