A peripheral nerve is a cable composed of bundled fibers called axons, which transmit electrical and chemical signals between the brain, spinal cord, and the rest of the body. When a nerve is severely damaged, the lost segment must be replaced to allow these fibers to cross the gap and restore function. While the term “nerve transplant” is sometimes used, a surgeon performs a nerve graft, which is the surgical placement of tissue or a synthetic tube to bridge the two severed nerve ends. This complex procedure provides a structural pathway for the patient’s own nerve fibers to regrow, a slow biological process.
Understanding Severe Nerve Injuries
Minor nerve damage, such as a mild compression, can often heal spontaneously if the protective sheath around the axon remains intact. However, a severe injury, like a laceration or crush injury, can completely sever the nerve, leaving a gap between the two stumps. If the two ends can be sutured together without tension, a direct repair is the preferred method for recovery.
When a nerve segment is damaged, the surgeon must trim back the injured tissue, creating a deficit. If this resulting gap is greater than a few millimeters, pulling the ends together will place tension on the repair site, which hinders regeneration. For gaps exceeding five millimeters, and certainly for those larger than three centimeters, a bridging material is necessary to serve as a scaffold for the regenerating axons. This intervention also prevents the proximal nerve end from forming a painful, disorganized mass of nerve tissue called a neuroma.
Bridging the Gap: Nerve Grafting Materials
The autograft is the standard material used to bridge a significant nerve gap, involving harvesting a segment of the patient’s own sensory nerve. The sural nerve in the leg is a common donor site because it is dispensable and provides a long segment of tissue. However, its removal results in permanent numbness in a small area of the foot. Using the patient’s own tissue provides the best environment for regrowth, including supporting Schwann cells and the natural architecture of the nerve sheath.
An alternative is the allograft, which uses processed nerve tissue taken from a human cadaver donor. This donor tissue is chemically treated to remove cellular components that would trigger immune rejection, while preserving the extracellular matrix that forms the internal scaffolding. Allografts demonstrate recovery rates comparable to autografts for many gap lengths. They offer the advantage of avoiding the functional deficit and pain associated with a second surgical donor site.
For small defects, typically less than three centimeters, a conduit or nerve tube may be used instead of a graft. These are hollow, synthetic, or naturally derived tubes that encase the gap between the severed nerve ends. The tube prevents surrounding scar tissue from interfering with the repair and concentrates the body’s natural healing factors to guide the regenerating axons. However, for longer gaps, the organized internal structure provided by an autograft or allograft is required for successful regeneration.
The Biology of Nerve Regrowth
Once the graft is in place, the nerve segment distal to the injury undergoes Wallerian degeneration, typically beginning within 24 to 48 hours. During this time, the axon fibers and their insulating myelin sheaths break down and are cleared away by specialized immune cells. The remaining structures, primarily the basal lamina tubes, are lined by Schwann cells. These cells proliferate and organize into “bands of Büngner,” creating a scaffold for new growth.
Axons from the proximal, healthy nerve stump begin a slow process of sprouting and migration across the grafted material. These sprouts are guided by the internal architecture of the graft, which directs them toward the distal nerve segment. This regrowth is slow, proceeding at a rate of approximately one to three millimeters per day.
This slow rate means functional recovery is delayed and dependent on the distance between the injury site and the target muscle or skin area. For an injury high in the arm, it can take many months or years for the regenerating axons to reach the hand muscles. An early repair is essential because Schwann cells are time-sensitive in their ability to support regeneration, and target muscles can atrophy beyond the point of reinnervation.
Surgical Alternatives to Grafts
When a nerve injury is far from the target muscle, or if the resulting gap is exceptionally long, the slow rate of regeneration may cause the muscle to atrophy before the nerve fibers can reach it. In these cases, a nerve transfer is a surgical alternative that bypasses the need for a long graft. This procedure involves surgically disconnecting a less important branch of a healthy, functioning nerve.
The surgeon connects this healthy nerve to the sheath of the damaged nerve closer to the target muscle. This rerouting shortens the distance the new axons must travel to reinnervate the muscle, accelerating the return of motor function. This technique sacrifices a minor function for the restoration of a more critical one, such as using a branch of a sensory nerve to power a paralyzed hand muscle.
Nerve transfers are useful for injuries high on the limb, such as those involving the brachial plexus, where the distance to the hand is extensive. By providing a new, closer power source, the transfer maximizes the chance that the target muscle will be reinnervated before its ability to accept nerve signals is permanently lost. This approach offers a more timely and successful restoration of critical function compared to a lengthy nerve graft.