There is a common public misconception that nerves once injured will not recover. In general, this is not true. The nerve cells that supply the muscles and skin of the body live either within the spinal cord for motor fibers, or in a local swelling at the spine called the dorsal root ganglion for sensory fibers. Those cells send out a long, thin process called the axon to the muscles and skin. When a nerve is cut in the arm, it is the axon that is cut. The nerve cells are usually still alive and will try to grow back to their normal territory. Depending on the amount of scar in the area, age of the patient and a number of other factors, the nerve may or may not be successful in regenerating. The nerve fibers grow back at a rate of approximately one inch per month. If a muscle loses its nerve supply for more than one to two years, it dies. So even if the nerve eventually makes it back to its muscle, motor recovery will not occur in those cases where the injury is more than twenty inches or so from the muscles the nerve supplies. Significant scar along or within the nerve may prohibit the axon regrowth. In some cases, the area of scar can be surgically removed and a skin nerve from somewhere else in the body can be used to bridge the gap created between the ends of the nerve. This is called a graft. The nerve cells can then grow through the graft and eventually get back to the muscles and skin. Recovery in that situation depends on the length of the graft, time from injury to repair, and distance from injury to the muscles and skin supplied by the nerve.
When a nerve is injured, it is not always severed completely. In some cases, the nerve may not function after injury but recovers over hours to days. This type of injury can be thought of as a concussion of the nerve. Since the nerve is intact, recovery in that situation is usually very good and surgery is rarely needed. There are varying degrees of injury between these two extremes of rapid complete recovery and the need for grafting. Immediately after the injury, it is typically not possible to determine whether a nerve will recover spontaneously or will need surgery. Periodic observation, examination, and electrodiagnostic studies (EMG) help in the decision making process to determine which cases need to go to surgery. When surgery is performed, the nerve is explored and the surrounding scar is removed. The nerve is then stimulated to see whether the impulse can get across the area of injury and be recorded further down on the nerve. If the impulse gets across the injury, the chance for recovery is good. If not, nerve grafting may be considered.