In the United States, there are approximately 185,000 amputations each year (not 40,000) and there are nearly 2 million amputees in total. Nearly half of all amputations occur following trauma or as part of oncologic treatment. The remaining cases are secondary to vascular disease, diabetes, infection, sepsis, and other vasculopathies.1 Between a quarter and a half of all amputee patients experience chronic localized pain secondary to symptomatic neuromas and/or phantom limb pain (PLP).2–6 Neuromas are disorganized bundles of nerve fibers that form at the end of a divided or damaged nerve secondary to aberrant nerve fiber regeneration. Their development can have a significant negative impact on the functional rehabilitation and overall outcome of patients with amputations because of chronic pain and subsequent problems with fitting of prosthetics. Management of troublesome neuromas and the pain they cause can be challenging with traditional medical and surgical approaches.
TMR is a surgical procedure that was originally designed to provide intuitive control of upper limb prostheses following amputation, through transfer of residual nerves to otherwise redundant denervated target muscle. It was noticed early on that many TMR patients noted a reduction in pain, sometimes with complete resolution of neuroma-related pain in an affected limb following this procedure.1–6 A rabbit amputation model was subsequently developed to study the effects of TMR. This showed that after 10 weeks, transferred nerves were found to more closely resemble that of their uninjured counterparts than that of a neuroma.7 Since then, multicenter patient trials have shown that TMR provides a consistent long-term benefit of pain relief and symptoms associated with both symptomatic neuroma and phantom limb pain.8
Coaptation of the amputated nerve stumps to recipient motor nerve branches in TMR encourages organized nerve regeneration into the denervated target muscles, thus preventing the chaotic and misdirected nerve growth that typically leads to neuroma formation. This critical element of nerve-to-nerve coaptation differentiates TMR from other neuroma operations where nerve ends may be buried in muscle or bone or left to retract in other soft tissue. The central principle underlying the nerve transfers in TMR surgery is to reestablish a function for the divided nerve. This surgery uses a normal open surgical approach and is not minimally invasive. It does not restore normal localized function, nor promote growth factors for healing. Wallerian degeneration occurs when a nerve is divided and there is degeneration of the proximal nerve back to the next node of Ranvier. This is a process that occurs predictably in peripheral nerve divisions irrespective of subsequent surgery. It is after this point that the TMR process and principles differ.1–4
4. Ducic I, Mesbahi AN, Attinger CE, Graw K. The role of peripheral nerve surgery in the treatment of chronic pain associated with amputation stumps. Plast Reconstr Surg 2008;121(3):908–914, discussion 915–917 There are a number of things that are useful in the preoperative workup for TMR in patients with a painful neuroma. These start with an office consultation to discuss the procedure with the patient and set their expectations as well as perform a physical examination. A useful and reliable way to assess whether neuroma surgery is likely to be beneficial is to inject local anesthetic into the area where pain originates, i.e., around the presumed neuroma to assess the effects of doing so. Physical examination and palpation can guide the injection to the site of a neuroma. Tinel’s sign or ultrasound guidance can help with this. In essence, if the block proves effective, i.e., it alleviates the patient’s usual pain, then they may be a good candidate for TMR. This is probably the most useful single test to be performed preoperatively. An MRI may also be ordered if further visualization and characterization of the neuroma is required. It is useful for the patient to record a pain and medication diary, documenting daily symptoms, duration, intensity, and type of pain. This can help assess the effectiveness of surgery postoperatively by repeating the diary exercise. Neither injection of steroid or botulinum toxin would be helpful in this setting.1,2
When performing TMR during or following amputation, there is normally a size discrepancy between the donor and target nerves because the motor nerve branches tend to be small, whereas the peripheral nerves tend to be far larger as they would originally have divided distally to form different individual nerves. This size discrepancy is generally well tolerated and the key point is to reestablish the higher-level functions of the major amputated peripheral nerve by giving this nerve somewhere to go. The peripheral nerves which are to be transferred are identified visually by careful dissection and without a nerve stimulator, but it is the target motor nerves which are identified with the aid of a nerve stimulator following their visualization. The motor nerve target is transected near its entry point to muscle, preserving only enough length to perform suture neurorrhaphy (~2 mm), as this gives the shortest distance for nerve regeneration to occur. The neurorrhaphy is performed end to end with an epineural suture technique. Individual fascicular or group fascicular coaptation is not normally required.1
6. Harris AM, Althausen PL, Kellam J, Bosse MJ, Castillo R; Lower Extremity Assessment Project (LEAP) Study Group. Com- plications following limb-threatening lower extremity trauma. J Orthop Trauma 2009;23(1):1–6
There are a number of recommended nerve transfer combinations in TMR in order to reduce the likelihood of symptomatic end neuroma. These relate to each individual amputation level. The recommended muscle sites for the tibial nerve in BKA are the soleus, tibialis posterior, and gastrocnemius. The semitendinosus is recommended for the common peroneal component of the sciatic nerve during above knee amputation, the gracilis is recommended for the saphenous nerve in the above knee amputation, and the peroneus brevis is indicated for the deep peroneal nerve in a BKA. However, many combinations of nerve transfer can provide the divided nerve with a target muscle and maintain the central principle of TMR: to reestablish the higher level functions of the major amputated peripheral nerve by giving this nerve a direction and function. Other popular combinations include those shown in Tables 87.1 through 87.5 Table 87.1 Nerve Transfers for Below-Knee Amputation TMR Donor Nerve Target Motor Nerve Branch Tibial Medial or lateral gastrocnemius Tibialis posterior Medial or lateral soleus Deep peroneal Tibialis anterior Peroneus longus Peroneus brevis Medial soleus Superficial peroneal Peroneus longus Peroneus brevis Sural Tibialis posterior Soleus Table 87.2 Nerve Transfers for Above-Knee Amputation TMR Donor Nerve Target Motor Nerve Branch Tibial component of sciatic nerve Biceps femoris Common peroneal component of sciatic nerve Semitendinosus Saphenous Gracilis Table 87.3 Nerve Transfers for Forequarter Amputation TMR Donor Nerve Target Motor Nerve Branch Median nerve Pectoralis major Musculocutaneous nerve Clavicular head nerve branch of pectoralis major Ulnar nerve Pectoralis minor Radial nerve Long thoracic Thoracodorsal Table 87.4 Nerve Transfers for Transhumeral Amputation TMR Donor Nerve Target Motor Nerve Branch Medial antebrachial cutaneous nerve End-to-side to motor nerve branch of distal brachialis Lateral antebrachial cutaneous nerve Motor nerve branch of radial nerve innervating brachialis (where present) or brachioradialis Median nerve Musculocutaneous nerve motor branch to short head of biceps brachii Ulnar nerve Musculocutaneous nerve motor branch to brachialis Musculocutaneous nerve End-to-side to musculocutaneous nerve motor nerve branch of long head of biceps brachii Radial nerve Motor nerve branch of lateral head of triceps Table 87.5 Nerve Transfers for Transradial Amputation TMR Donor Nerve Target Motor Nerve Branch Median nerve Flexor digitorum superficialis Ulnar Flexor carpi ulnaris Superficial radial nerve Anterior interosseous nerve Flexor digitorum profundus