Traumatic injury and oncologic resection account for 19 percent of major lower extremity amputations each year, with the majority secondary to dysvascular disease.1 Up to 25 percent of major limb amputees will develop chronic localized pain caused by symptomatic neuromas within their residual limb.2–6 In addition to neuroma pain, amputees also often experience phantom limb pain and phantom limb sensations.
Targeted muscle reinnervation is a surgical technique originally reported in 2002 for improved control of advanced myoelectric prosthetics, and has since shown promise in treating symptomatic end-neuromas.7 Targeted muscle reinnervation transfers transected peripheral nerves in the amputated extremity to recipient motor nerves of residual muscle, reestablishing muscle innervation, preventing muscle atrophy and symptomatic neuroma and phantom limb pain, and amplifying electromyographic signals.8–11 To reinnervate muscle with the transected peripheral nerve associated with amputation, one must first acutely denervate the target muscle. The motor nerve branch entering the targeted muscle is cut and receives an immediate nerve transfer. This nerve transfer permits rapid reinnervation of the targeted muscle and provides the peripheral nerve with “somewhere to go and something to do.” Detailed descriptions of the surgical technique of targeted muscle reinnervation for transhumeral, transradial, shoulder disarticulation, and transfemoral level amputees have been published, but no detailed description for below-knee amputation exists.12–15 This article describes targeted muscle reinnervation for below-knee amputation and introduces primary targeted muscle reinnervation—the method of targeted muscle reinnervation performed concurrent with amputation.
The main objectives of primary targeted muscle reinnervation are to prevent the development of symptomatic neuromas and phantom limb pain and amplify myoelectric signals for advanced bioprosthetic limbs. Primary targeted muscle reinnervation, also termed “acute” or “concurrent” targeted muscle reinnervation, is performed at or near the time of elective amputation for multiple causes, including failed limb salvage, oncologic disease, or traumatic injury. Secondary targeted muscle reinnervation, also sometimes termed “delayed” or “chronic” targeted muscle reinnervation, refers to targeted muscle reinnervation performed months to years after index surgery.
Surgery is coordinated in conjunction with orthopedic or trauma surgery, and every effort is made to perform targeted muscle reinnervation at the time of index amputation. The below-knee amputation is performed under tourniquet control, which is then released before nerve transfers to gain hemostasis and allow for the target motor nerves to recover and aid in ease of stimulation. Regional nerve blocks are used routinely, but performed until all nerve transfers are completed. Below-knee amputation initiates with anterior incision 15 cm distal to the knee joint. Care is taken to avoid electrodissection of the major peripheral nerves. When identified, the major peripheral nerves are each sharply transected with preservation of length (Table 1). The saphenous nerve is first identified anteromedially. The tibia is then cleared and divided with an anterior bevel proximally. The lateral compartment muscles are divided and the superficial and deep peroneal nerves are identified anterior to the fibula. The fibula is exposed and transected 1 to 2 cm proximal to the residual tibia. The posterior compartment incision is carried through the posterior intermuscular septum where the tibial nerve is found. The major vessels of the leg are ligated and divided when encountered.
Attention is turned to locating motor nerve recipients for nerve transfer. The tourniquet is removed, as the motor nerves stimulate better in its absence. A nerve stimulator identifies motor nerve branches entering the target muscle with minimal additional dissection. The motor nerve is transected with straight microscissors near its entry into the muscle. An end-to-end neurorrhaphy from the peripheral nerve stump to the desired motor nerve branch is performed using interrupted 8-0 nylon epineurial stitches interspaced so that the fascicles remain within the nerve and not flaring outward. Size mismatch is addressed by dissecting a portion of the surrounding muscle and then cerclage a muscle cuff around the nerve coaptation using 4-0 or 5-0 absorbable suture; this optimizes the nerve transfer ingrowth to target motor nerve/muscle unit construct. Any combination of nerve transfer provides the amputated nerve with a target muscle, maintaining the central principle of targeted muscle reinnervation: reestablish the higher level functions of the major amputated peripheral nerve by giving this nerve somewhere to go and something to do (Table 1). (See Video, Supplemental Digital Content 1, which demonstrates primary targeted muscle reinnervation performed in a below-knee amputee with nerve transfer of saphenous, tibial, superficial and deep peroneal, and sural nerves to motor nerve targets as illustrated in Table 1, available in the “Related Videos” section of the full-text article on PRSJournal.com or, for Ovid users, available at http://links.lww.com/PRS/D208.)
Closure is next completed. Adequate soleus muscle bulk is resected to allow myodesis of gastrocnemius muscle anteriorly over the distal tibia. Myodesis is performed by drilling a transverse bicortical pilot hole through the anterior tibia to allow for one to secure the advanced gastrocnemius fascia and muscle over the distal tibia with permanent or absorbable suture. The fascia is approximated and closed in an interrupted manner, and the skin can be closed with sutures or skin staples.
Since 2015, we have performed targeted muscle reinnervation on 22 below-knee amputees, 18 primary and four secondary. None have developed symptomatic neuromas postoperatively, with a mean time since surgery of 18 months. Seventy-two percent of the primary targeted muscle reinnervation cohort experience phantom limb pain in the first month, with an abrupt decline to 19 percent at 3 months, and 13 percent at 6 months. These rates of symptomatic neuromas/phantom limb pain are of significant improvement compared with our institutional control groups. Moreover, this topic is specifically analyzed in an ongoing multi-institution study. Average time to prosthetic wear was under 3 months.
Multiple studies demonstrate the prevalence of symptomatic neuroma, reported from 2 to 25 percent, and phantom limb pain, from 9 to 67 percent.6 , 16–20 Our entire cohort of below-knee amputation targeted muscle reinnervation patients are without symptomatic neuromas. Our primary targeted muscle reinnervation below-knee amputation patients had a high incidence of early phantom limb pain, with significant improvement by 3 months and resolution by 6. This corresponds with the timeline of reinnervation, with return of voluntary muscle twitches noted at 3 months.21 , 22
Targeted muscle reinnervation may be a reliable technique for the treatment and prevention of below-knee amputation/phantom limb pain at all amputation levels, without additional morbidity to the traditional below-knee amputation procedure. The technique for below-knee amputation with primary targeted muscle reinnervation is described. We hypothesize that primary targeted muscle reinnervation has the advantage of preservation of longer peripheral nerve length than traditional traction neurectomy, permits rapid nerve ingrowth and reinnervation of target muscle, and potentially permits greater neuroplasticity to possibly alter the pain circuits and central pain up-regulation.
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