A nerve is a cable of specialized fibers called axons, which transmit electrical and chemical signals between the brain, spinal cord, and the rest of the body. When a nerve is cut, this line of communication is instantly severed, leading to a loss of motor function, sensation, or both, in the affected area. A cut nerve can be repaired, but the process is complex, location-dependent, and rarely results in a perfect restoration of function. Recovery relies on the body’s intrinsic biological mechanisms and timely surgical intervention.
Peripheral vs. Central Nerve Repair
The body’s capacity for nerve repair differs significantly between the peripheral and central nervous systems. Nerves in the periphery, which extend outside of the brain and spinal cord, have a significant ability to regenerate following injury. This potential is due to the cellular environment and the presence of Schwann cells, which actively support and guide the regrowth of damaged axons.
In contrast, nerves within the brain and spinal cord (the central nervous system) generally lack this regenerative capacity. The central nervous system environment is inhibitory to growth, primarily because of molecules released by specialized cells called oligodendrocytes and the formation of dense scar tissue. Therefore, the majority of successful nerve repair efforts, both natural and surgical, focus on the peripheral nervous system.
The Body’s Natural Repair Process
When a peripheral nerve is severed, the segment of the axon detached from the cell body immediately begins Wallerian degeneration. Within 24 to 48 hours, the distal axon and its surrounding myelin sheath break down into fragments. Specialized immune cells called macrophages, along with resident Schwann cells, migrate to the injury site to clear this cellular debris, creating a clean pathway for regrowth.
As the debris is cleared, Schwann cells in the remaining nerve sheath proliferate and align themselves into continuous columns, forming the bands of Büngner. These bands serve as a guiding scaffold, releasing neurotrophic factors that encourage the proximal axon to sprout. The part of the axon still connected to the cell body then produces multiple fine projections, known as axonal sprouts, which attempt to enter the Schwann cell tubes.
Once a sprout successfully enters a band of Büngner, it begins the slow process of elongation toward the target organ or muscle. The growth rate of a regenerating axon averages about 1 millimeter per day, or roughly one inch per month. This slow pace is a major factor in determining the time needed for functional recovery, especially when the injury is far from the target. The Schwann cells eventually wrap around the new axon to form a new myelin sheath, which is necessary for fast and efficient signal transmission.
Surgical Approaches to Repairing Severed Nerves
When a nerve is completely severed and a gap exists, the body’s natural repair process is often insufficient to bridge the distance. Surgical intervention is required to realign the nerve ends and provide a structural pathway for regenerating axons. For clean cuts with minimal tissue loss, a direct repair, or neurorrhaphy, is the preferred method. The surgeon sutures the outer sheath of the nerve together, ensuring the internal bundles of axons are tension-free and accurately aligned for successful regrowth.
If the injury results in a gap too large for a tension-free direct repair, the surgeon must bridge the defect using a grafting technique. The standard for bridging larger gaps is the nerve autograft, which involves harvesting a segment of a less-critical sensory nerve (such as the sural nerve) and using it to connect the severed ends. The harvested segment provides a natural scaffold—the preserved Schwann cell tubes—for the regenerating axons to follow across the defect.
For smaller gaps, typically less than three centimeters, an alternative is the use of a nerve conduit. These are hollow, bioabsorbable tubes made from materials like collagen or polyglycolic acid, used to enclose the gap and guide the axonal sprouts. Conduits prevent scar tissue from infiltrating the repair site and concentrate the neurotrophic factors released by the nerve ends. While autografts remain the benchmark for longer defects, conduits offer the advantage of avoiding the donor site morbidity associated with harvesting a patient’s own nerve.
Prognosis and Variables Affecting Outcome
Even after successful anatomical repair, the final outcome of functional recovery is highly variable and depends on several factors. The age of the patient is a primary predictor, with children and younger patients generally experiencing better recovery than adults. Younger individuals exhibit a stronger regenerative capacity and benefit from shorter distances for the axon to travel to the target tissue.
The location of the injury significantly affects the prognosis, as the regenerating axon must reach its target organ, such as a muscle, before that tissue atrophies beyond repair. Injuries closer to the target muscle have a more urgent time constraint. Conversely, injuries high up on a limb, requiring a long regeneration distance, can take many months or years to show functional return.
Other variables include the length of the nerve gap requiring repair, with longer defects correlating with poorer outcomes, and the specific nerve injured. Some nerves, like the radial nerve, have a better intrinsic regenerative potential compared to others, such as the ulnar or peroneal nerves. Recovery is a long-term process, and functional results may continue to improve for up to two years after the initial repair.