Upper limb loss affects more than half a million individuals in the United States, with estimates that those numbers may double by the year 2050.[1] Trauma remains the leading cause of amputation, followed by ischemia and infection. Regardless of the diagnosis, upper limb amputations are devastating for patients and their families, given the essential role the upper limb plays in interacting with our environment.

The advent of myoelectric prosthetics, initially introduced nearly 50 years ago, has had several major advancements since inception, which have afforded significant reconstruction benefits to upper extremity amputees. Unlike cosmetic and body-powered prosthetics, myoelectric prosthetics use detectable surface electromyographic (EMG) signals from muscles in the residual limb to control the prosthesis. This difference provides improved grip strength and decreased energy demands, and potentially even reduces overuse injuries to the residual limb. Conventional direct-control myoelectric prostheses translate EMG signals from one muscle to provide one prosthetic function. However, the advent of pattern recognition algorithms allows for a combination of EMG patterns across the residual limb to predict intended movements.[1] [2] And yet, it is important to consider ongoing limitations of myoelectric prostheses, including prosthetic weight, battery life, cost, durability, socket fit, and moisture that may interfere with the quality of EMG signals interpreted by surface electrodes.[3]

Targeted muscle reinnervation (TMR) has been a critical procedure to improve further the degrees of freedom and intuitive control of myoelectric prostheses. TMR uses free nerve ends that have lost their distal target to reinnervate or switch innervate residual muscles that can then serve as a biological amplifier to control a myoelectric prosthetic.[4] For example, in an above-elbow amputee, the median nerve can be transferred into the branch to one head of the biceps muscle by cutting the motor branch of the musculocutaneous nerve as it enters that biceps muscle head and suturing the median nerve into it just before the motor entry point. This allows the median nerve to reinnervate this head of the biceps to prevent a median nerve neuroma and provide an intuitive signal for grasp once reinnervation occurs. Thus, TMR nerve transfers have improved the intuitive control for amputees and increased the number of signals available for myoelectric prosthetics and have been shown to play a role in preventing and treating neuroma and phantom limb pain.[2] [5] [6] [7] [8] [9] We know that painful neuromas affect up to half of the major-limb amputees, a barrier to prosthetic use.[7] [8] [9] TMR provides a purpose and new function for transected peripheral nerves by reinnervating a residual muscle to avoid neuroma formation. In one study of TMRs performed for amputees, 92% (11/12) of the patients who had acute TMR at the time of amputation had no neuroma pain at follow-up.[10] In the same study, 23 patients had a secondary TMR for painful neuromas and 87% experienced resolution of their neuroma pain. Similar results have also been demonstrated using regenerative peripheral nerve interval (RPNI), in which a denervated free muscle graft is used as the recipient for the amputated terminal nerve.

While both TMR and RPNI have demonstrated successful outcomes in treating and preventing neuromas, at our institution, TMR is preferred in the upper extremity when myoelectric signals are needed for the donor's nerve and in large mixed motor sensory nerves. RPNI is typically utilized when no myoelectric signal is required for prosthetic control, such as in sensory-only nerves, and in combination with TMR in the lower extremity when multiple nerves are being managed. Additionally, RPNI can preserve native muscle bulk by avoiding further denervation of the residual limb and the need for a secondary incision or raising large flaps. Technical pearls for performing RPNI include using an appropriately sized graft, specifically a 3 × 1 × 1 cm graft, dividing larger nerves into grouped fascicles as needed to reduce axonal input to the graft, securing the nerve ends within the muscle graft through an epimysial-to-epineural suture and then wrapping the muscle graft closed to contain the nerve further and avoid axonal escape. The RPNI unit is then tucked and buried deep in the wound to prevent direct contact or mechanical irrigation at this site.

In summary, TMR and RPNI can reduce amputees' phantom limb pain and residual limb pain. They can even prevent and treat neuromas in amputees and nonamputees when nerve reconstruction is impossible. Both TMR and RPNI have been used for nearly all peripheral nerves, demonstrating benefits for neuroma pain management and even selective treatment in the management of neuropathic pain. TMR is an effective strategy for improving intuitive myoelectric prosthetic control in combination with pattern recognition. Well-designed studies are still needed to determine the superiority and appropriate utilization algorithm for TMR and RPNI versus alternative techniques such as end-to-side neurorrhaphy, nerve cap or connector, or relocation nerve grafting with an allograft.

Article published online:
25 August 2023

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