Tuesday, June 20, 2017

Peroneal nerve injuries - one tough cookie!

While the majority of my practice is in the upper extremity, one of the things I enjoy the most about specializing in nerve surgery is getting to team up with colleagues from all of the different orthopedic/plastic subspecialties. Several times a month, I see patients with nerve injuries in their legs - sometimes with compressive neuropathies like tarsal tunnel syndrome, or those who have had nerve injuries from motor vehicle accidents or falls. 

One of the more common lower extremity nerve injuries that I see involves the peroneal nerve. I wanted to spend some time writing about these injuries to help out any of my patients (or anybody else out there) seeking more information about them.
Image result for aaos peroneal nerve
What is the peroneal nerve? The common peroneal nerve is a branch of the sciatic nerve that "comes into its own" at the lower part of the thigh, just above the knee; it then continues on the outside (lateral) part of the knee and wraps around the neck of the fibula bone (the skinny bone on the outside of the shin). The common peroneal nerve is "common" because it has 2 of its own branches - the deep branch and superficial branch. Both of these nerve branches have motor and sensory jobs. The deep peroneal nerve supplies the very important muscles that allow you to raise your ankle (tibialis anterior) and toes (extensor hallucis longus and extensor digitorum longus), but also carries the sensation from the space between your big toe and second toe. The superficial peroneal nerve supplies the muscles that swing your ankle outside (eversion by the peroneus longus and brevis muscles) and carries sensation from the top part of your foot (closer to the second through little toes).

How does a peroneal nerve injury occur? The peroneal nerve gets stretched during injuries that place stress on the outside of the knee - such as dislocations of the knee, ligamentous damage to the knee (such as the ACL or LCL), or fractures of the femur, tibia or fibula. These injuries can happen after falls or motor vehicle accidents, or from many other causes. As the peroneal nerve is stretched during the injury, the inner portions of the nerve may stretch and snap/rupture. In severe injuries, the entire nerve may rupture. Scar tissue forms around the injured parts of the nerve and the nerve can no longer conduct the motor and sensory functions. 

What is the prognosis? Recovery after any nerve injury (upper or lower extremity) is tough - it takes a long time for recovery and the function/strength is never "quite the same" as before the iinjury. Unfortunately, peroneal nerve injuries are among the toughest to treat - there tends to be a lot of scarring aroud the nerve ends as the very strong calf muscles pull the nerve ends apart. Additionally, the internal composition of the peroneal nerve isn't as favorable for a fast recovery. Despite advances in microsurgery, the results from nerve repair, nerve grafting, and nerve transfers have not been as successful for the peroneal nerve as nerves in the shoulder/arm (brachial plexus). 

What are the treatment options? If the peroneal nerve injury is diagnosed early enough, your surgeon may choose to explore the peroneal nerve and remove scar tissue around/within the nerve (neurolysis). If there is a gap between the nerve ends, the surgeon may choose to place a nerve graft to provide a scaffold for nerve regeneration. If the gap is too big for a chance of recovery with a graft, one option is to perfrom a nerve transfer, in which an expendable working nerve (to either curl the toes or press the ankle down) is connected to the muscles supplied by the peroneal nerve. There are two big issues with nerve transfers for peroneal nerve palsies: it's really hard to re-train the brain to make the nerve transfer work and the results from these transfers have not been reliably good. For this reason, your surgeon may discuss tendon transfers with you - in this procedure, working tendons are re-routed to help bring the ankle and toes up. This is the most reliable "tried and true" procedure, and is often what we use for patients with this tough problem. It doesn't help with sensation to the top of the foot. One of the best parts about working at Washington University Orthopedics is that I get to team up with some of the best foot and ankle surgeons in the country - we work together to get patients the best outcome possible.  

Overall, this is a very challenging injury. I am very honest with my patients about the tough road ahead of them. I will try my best to restore the innervation to the muscles and sensation of the foot, but tendon transfers may be necessary in the future. Seeing eye-to-eye on expectations and goals for functional recovery is important for all nerve injuries, but especially for peroneal nerve injuries. 

Christopher J. Dy, MD MPH
My Bio at Washington University Orthopedics

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