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Nerves controlling muscles are best repaired with similar nerves

May 14, 2007

When repairing severed or damaged motor nerves with a donor nerve graft, surgeons have traditionally used a sensory nerve from another area of the patient's body. However, these patients often do not fully regain function in the injured area.

But now a team of surgeons at Washington University School of Medicine in St. Louis and Barnes-Jewish Hospital has found that repairing a motor nerve in rats with an intact motor nerve yields better results than using a sensory nerve. The research appeared in the March issue of the journal Microsurgery.

Motor nerves control movement in the muscles, while sensory nerves receive sensory stimuli, such as pain. A significant difference between the two types of nerves is that motor nerves have much larger axons, the thread-like extensions of the nerve cell that carry nerve impulses throughout the body.

The researchers, led by Gregory H. Borschel, M.D., a plastic and reconstructive surgeon at the School of Medicine and senior author of the paper, defines the question of this work as seeking to determine why motor nerves were regenerating more successfully than sensory nerves. Was it because of the nerve's own structure, or architecture, or because supporting cells such as Schwann cells were boosting the regeneration"

To find an answer, the researchers broke down the nerve architecture by chopping up motor, sensory and mixed nerves. They divided the minced nerves into groups by type, inserted the mush into tiny silicone tubes and encouraged severed motor nerves to regenerate through the mixtures in the tubes.

The researchers found that disrupting the nerve's architecture by mincing it abolished the benefit of repairing a motor nerve with an intact motor nerve. "It turned out there was no difference in regeneration using motor versus sensory nerves through the chopped-nerve tissue," Borschel said.

Several factors contributed to the results, he said. "We know that the axons, or nerve fibers, in the motor nerves are bigger, while the sensory nerve fibers are smaller," he said. "When the nerves are trying to regenerate using a motor nerve as a graft, it's easier for them to use the larger axons of another motor nerve, although the reason why is not clear."

The results could eventually translate into improved treatment for humans who have nerve damage from industrial, recreational or auto accidents.

"The research data is very compelling," Borschel said. "The evidence presented through this study could represent a paradigm shift from what we currently do in the operating room. The current standard of treatment for fixing a gap in a motor nerve is to use a sensory nerve, but we believe that if you use a motor nerve instead of a sensory nerve, then the outcome would be better."

The surgeons in the Division of Plastic and Reconstructive Surgery have begun using motor nerves grafts in limited patient cases with good results, Borschel said, but to clearly demonstrate the difference between motor nerve grafting and sensory nerve grafting in humans, much more study is needed.

One obstacle to the use of more motor nerve grafts is that the human body has a limited number of expendable motor nerves. Currently, surgeons are able to use the nerve from the gracilis muscle along the inner thigh or the latissimus dorsi along the side of the torso.

"This study, in conjunction with other related work from our laboratory, will likely result in a shift away from the use of traditional sensory nerve grafts to the much more permissive motor nerve grafts for reconstruction of injury," said Susan E. Mackinnon, M.D., the Sydney M., Jr. and Robert H. Shoenberg Professor and Head of the Division of Plastic and Reconstructive Surgery at the School of Medicine.

Washington University School of Medicine