Nerves that give sensation to the skin and power muscles in the arm travel from the neck through the front of the shoulder and down to the hand. These nerves run close to the humerus -- the upper arm bone -- making them vulnerable to injury when this bone fractures. Nerve damage can cause numbness in the fingers.
The shoulder is a ball-and-socket joint formed by the round head of the humerus and the glenoid fossa, part of the shoulder blade. The area of bone just below the ball-like head of the humerus is known as the humeral neck. Fractures in this part of the bone are usually caused by a fall on an outstretched hand. They sometimes occur with direct trauma, such as from a motor vehicle accident.
The axillary nerve wraps around the neck of the humerus. Injuries to this nerve often occur with a humeral neck fracture. The axillary nerve powers several shoulder muscles, causing weakness and numbness at the outside of the upper arm. Damage to this nerve does not cause hand numbness.
The radial nerve travels along the humerus bone. This nerve is most frequently injured with humeral fractures, but more typically with fractures in the middle to bottom third of the bone instead of the humeral neck. This nerve powers the muscles that straighten the fingers and bend the wrist backward.
The radial nerve gives sensation to the back side of the upper arm, forearm and hand. In the hand, it provides sensation to the back of the thumb, index, middle and thumb-side of the ring fingers, up to the middle joint. Radial nerve damage can occur when the bone is broken or as the bone pieces are put back into place during treatment.
Radial nerve damage after humeral fracture causes numbness and tingling in the back of the hand and fingers. Pain can also occur in this area. Weakness in the muscles that straighten the fingers and wrist makes movement difficult. With severe injury, "wrist drop" can occur, making a person unable to straighten the fingers or lift the hand. Symptoms typically occur soon after injury. However, a study published in 2010 by "Musculoskeletal Journal of Hospital for Special Surgery" describes a case in which symptoms developed 9 weeks after the fracture.
According to the authors of a study published in 2010 by "The Open Orthopaedics Journal," 90 percent of radial nerve injuries from humeral fracture spontaneously recover. The other 10 percent may need surgical intervention to repair the nerve. Spontaneous recovery can occur over several weeks or months after injury.
Splinting is often used to maintain hand function while the radial nerve is recovering. A thermoplastic splint is made by a specialized occupational or physical therapist. The base of the splint is molded along the back of the forearm, covering approximately two-thirds of its length. A metal outrigger is attached to the splint and positioned out over the large knuckles of the hand. Slings are placed under each finger and attached to the outrigger with rubber bands. The splint holds the wrist in a slightly backward-bent position and allows the fingers to bend. The rubber bands bring the fingers back up into a straight position so the fingers can be used.