When a bone breaks or cracks, the injury is called a fracture. "Fractured" and "broken" mean the same thing.
In the arm, a fracture most often occurs in the long and slender shaft of one of the three arm bones. The three arm bones are the humerus, radius and ulna.
Fractures of the humerus (upper arm bone)
The humerus is the bone that extends from the shoulder to the elbow.
In otherwise healthy people, most fractures of the humerus are caused by a direct blow to the upper arm. This often is caused by a motor vehicle accident or high-impact fall. Less often, the humerus can fracture because of a severe twist of the upper arm, a fall on an outstretched arm, or an extreme contraction of upper arm muscles.
If the bone fractures because of an extreme muscle contraction, the break curves around the bone. This is sometimes called a "spiral fracture" or a "ball-thrower's fracture." These injuries are fairly rare.
If the humerus breaks because of a low-impact bump or fall, this may mean that the bone has been weakened by an illness, such as osteoporosis or cancer. These are called pathologic fractures. Cancer-related fractures of the upper arm bone tend to occur in older people. Trauma-related fractures of the humerus tend to affect younger people.
Fractures of the radius and ulna (forearm fractures)
The forearm contains two bones, the radius and the ulna. Both extend from the elbow to the wrist. The radius is on the same side of the arm as the thumb. The ulna is on the side of the little finger.
When the forearm is fractured, either the radius or ulna may be fractured alone, or both bones may be fractured. In either case, the injury is almost always caused by a direct blow to the forearm, or by falling on an outstretched arm.
Among young Americans, forearm fractures are common in teen-agers who fall while in-line skating or skateboarding. Osteoporosis is a common risk factor for older persons with a forearm fracture.