InService Insights: Targeted muscle reinnervation to avoid neuroma or nerve injury
Chronic neuropathic pain after surgical procedures is a challenging and debilitating problem. While some operations such as amputations and inguinal hernia repair have a greater likelihood of nerve injury or symptomatic neuroma formation, this phenomenon can be seen after a variety of operations – making this condition relevant to all surgical specialties. Additionally, neuropathic pain can develop after trauma. This topic recently has become clinically popular and carries a growing probability of appearing on future exams, which makes it a logical focus of InService Insights.
Targeted muscle reinnervation
Targeted muscle reinnervation (TMR) is a technique by which proximal sensory nerve endings are coapted to distal motor nerve targets to allow axonal regeneration to have an appropriate distal target, thereby preventing neuroma formation and its symptoms. First described by Todd Kuiken, MD, PhD, in 2004 as a technique for improved prosthetic control in upper-limb amputees, TMR has been primarily used to treat or prevent chronic neuropathic pain in lower-extremity amputation stumps.
Transfer of a sensory nerve to a denervated muscle segment allows the regenerating axon of the sensory nerve to find distal motor-end plates of the denervated muscle. This prevents the axons of the regenerating sensory nerve from forming a neuroma. The muscle segment that's denervated must be carefully chosen to not affect function. This is fairly straightforward in lower-extremity amputations but needs to be more thoughtful when TMR is being done in intact limbs.
There are three common indications for TMR: unreconstructable neuromas; management of nerve endings in acute amputations; and treatment of chronic amputation stump or phantom limb pain. The definition of unreconstructable neuroma may vary, but some consider nerve gaps of 2-3cm better suited for TMR as opposed to reconstruction.
The nerve of interest is first identified and freed from any surrounding scar tissue. If a neuroma is present, the nerve is sharply transected proximal to the neuroma in a healthy segment of the nerve. A branch of a motor nerve to an adjacent muscle is identified and transected. In an intact extremity, the branch should be a minor branch so as to not compromised muscle function. In an amputation stump, any distal motor nerve can be used. Using a nerve stimulator, the motor component of the distal target nerve and the sensory component of the proximal nerve are identified. The two ends are then coapted under loupe magnification using permanent monofilament 6-0 or 7-0 suture to approximate the epineurium. The coaptation site should be close to the muscle to avoid tension.
Regenerative peripheral nerve interface
Regenerative peripheral nerve interface (RPNI) is an alternative to TMR. Also originally developed for myoelectric control of prostheses, RPNI offers a simpler way to treat and/or prevent neuromas, as it doesn't require nerve-to-nerve coaptation.
The principle behind RPNI is to cover the nerve ending with a free muscle graft, thereby providing a denervated and devascularized piece of muscle into which sprouting axons can find motor-end plates. The muscle graft must be thin (no more than 5mm thick), since it survives in the same way as a skin graft. In addition, the nerve provides trophic factors that aid in muscle-graft survival and eventual revascularization. The larger the nerve, the more muscle grafts are needed to ensure that all of the regenerating axons will have a distal target.
Indications for RPNI are similar to those for TMR. The difference between the two is in the technique, with RPNI being easier to perform – since it doesn't require nerve coaptation and thus loupe magnification is not required.
The transected nerve is freed of adhesions proximally. The neuroma bulb is resected sharply with a blade. If the nerve ending is in a weight-bearing area, it's repositioned to a more favorable location. For small sensory nerves, a single RPNI will often suffice. However, for larger mixed nerves, intraneural dissection to separate the fascicles along the epineurium is required to accommodate multiple RPNIs, to assure that the regenerating axons will have ample denervated muscle fibers to reinnervate. A donor muscle is identified and a small, thin piece of muscle is sharply harvested in the direction of the muscle fibers. In the lower extremity, vastus lateralis serves as a good donor site, as muscle can be harvested without compromising donor muscle function. The muscle graft should be no more than 5mm thick and approximately 15mm in width. It's cleared of all fat. The epineurium is then sewn to the muscle with 6-0 Prolene to secure the nerve in place. The muscle edges are then reapproximated over the nerve.
The benefits of TMR and RPNI have been clearly demonstrated in reducing chronic pain or phantom limb pain after lower-extremity amputation.6 The principles can be applied anywhere nerves are injured or damaged, both to treat and prevent neuropathic pain.
Dr. Parikh is PGY-6 at at Houston Methodist Hospital.