Introduction

Peripheral nerve injuries are common and can be functionally devastating. Improved understanding of nerve topography, injury, and regeneration has led to advances in peripheral nerve surgery and increased treatment options for our patients.

Traditionally, when a tensionless repair cannot be performed, a nerve autograft is employed to bridge the defect. However, autografts can be associated with additional surgical sites, increased operative time for harvesting, and potential donor site morbidity, including sensory loss, scarring, and debilitating neuroma formation. Further, in the cases of large, segmental or multiple limb injuries, the amount of expendable autograft becomes a limitation. For these reasons, alternatives including nerve allotransplantation and processed nerve allografts were developed.

More recently, vascularized composite tissue allotransplantation has become a reality for the treatment of devastating injuries involving traumatic amputations or significant facial disfigurement. Since 1998, over 100 hand and 30 face transplants have been performed, with published reports of 72 hand and 19 face transplantations. The function of these transplanted parts is dependent on specificity and efficacy of the nerve regeneration. The purpose of this chapter is to explore and review nerve autograft alternatives. Specifically, we focus on the biology of nerve allografts, review the clinical experience of nerve allotransplantation, and discuss nerve regeneration in the setting of vascularized composite allotransplantation (VCA). We will also briefly describe the clinical evidence for the use of nerve conduits and processed nerve allografts.

Peripheral Nerve Regeneration

A peripheral nerve injury initiates a highly regulated and sophisticated sequence of events. This process is dependent on the degree of nerve injury. Establishing the severity of the injury will help predict if recovery is possible and if surgical intervention is needed. Seddon first classified nerve injury into neurapraxia, axonotmesis, and neurotmesis. Sunderland and Mackinnon later revised the classification system ( Table 68.1 ). A neurapraxia involves localized myelin damage. The axon is preserved and thus the nerve does not undergo degeneration. Axonal disruption, in contrast, leads to Wallerian degeneration distal to the site of injury with coordinated activation of Schwann cells (SCs) and macrophages to remove myelin and axonal debris.

Proximal to the injury, mitogenic cytokines released by the injured axons induce SCs to change into a proliferating phenotype. These proliferating SCs organize into columns along the basement membrane of endoneurial tubes and are referred to as “bands of Büngner” and they support axonal regeneration. Axons are initially pruned and then begin to sprout and regenerate along these bands to reestablish nerve continuity and reach the end-organ target. In humans, this regenerative rate is reported to occur at 1–3 mm/day.

Segmental nerve defects or tension at the site of injury will disrupt the normal cascade of events preventing regeneration. Traditionally, interposed autografts have served as a bridge or structural framework to allow regeneration to proceed and are considered the gold standard. They provide the necessary medium for regeneration, including viable SCs and extracellular matrix, despite undergoing Wallerian degeneration themselves. Autografts are typically chosen based on their caliber, the length of the nerve gap, donor site morbidity, and ease of harvest. Due to the ease of harvest, the long length, and minimal donor site morbidity, one of the most commonly used autografts is the sural nerve. Other suitable nerves for interpositional grafts are reviewed in Table 68.2 .

Nerve Allotransplantation

In large, segmental, complex, or multi-limb nerve injuries, the amount of available autograft sources may be limited. Nerve allografts, from either a cadaver or living donor provide an abundant source of nerve graft. They also avoid the donor site morbidity of nerve autografts, but require systemic immunosuppression for their use. In the presence of adequate immunosuppression, nerve allografts provide equal regeneration and function of an autograft. Studies even demonstrate enhanced regeneration in nerve allografts due to the regeneration-enhancing effects of the immunosuppressant, tacrolimus.

Systemic immunosuppression has known risks. These include, but are not limited to, increased risk for opportunistic infections, nephrotoxicity, and decreased cancer immunosurveillance, resulting in malignancy. Due to the toxicity and risks of immunosuppression, the indication for nerve allografts has been narrow, and careful selection of the patients is important. Currently, nerve allotransplantation is reserved for unique clinical situations of devastating nerve injuries that are not amenable to nerve transfers or nerve autografting, and conservative management would result in a nonfunctional limb or nonmanageable pain.

Nerve allotransplantation is unique when compared with solid organ and composite tissue transplantation. The nerve allografts act as a biological and temporary conduit through which host motor and sensory axons traverse to reach the host end-organ target. Thus, nerve allografts require only temporary systemic immunosuppression, which can be withdrawn once the axons have crossed the allograft.

Schwann Cells and Immunology of Nerve Allotransplantation

As previously discussed, SCs are an integral part of injured axonal recovery as they provided both neurotrophic factors and structural integrity. In both nerve autografts and allografts, SCs are essential for supporting axonal regeneration and subsequent remyelination. In nerve allografts, both donor and host SCs proliferate and support axonal regeneration as long as adequate immunosuppression is present ( Fig. 68.1A ).

Schwann cell migration in a nerve allograft.

(A) Nerve autograft contains native, non-immunogenic, Schwann cells that are capable of supporting axonal regeneration. (B) Nerve allografts contain immunogenic non-host Schwann cells capable of supporting axonal regeneration under immunosuppression (yellow box). (C) Migration of host Schwann cells, to slowly replace donor Schwann cells, occurs from both proximal and distal nerve stumps into the donor graft to support axonal regeneration. (D) Over time, the entire donor Schwann cell population of the nerve allograft is replaced with host Schwann cells. (E) After complete replacement of the donor Schwann cell population, immunosuppression can be withdrawn without risk of the host initiating an immunogenic response to the allograft tissue.

Host SC migration is essential in the process of nerve regeneration across a nerve allograft. It occurs from both the proximal and distal nerves. Eventually, donor SCs are lost, despite adequate immunosuppression, but this is from an unclear mechanism and may involve chronic rejection or episodes of subtherapeutic immunosuppression. Our laboratory explored SC migration and found, under immunosuppression, host SC migration into the graft is delayed. However, after regeneration was completed and immunosuppression was stopped, host SC migration occurred immediately to fill in the gaps left by the loss of the rejected donor SCs along the graft ( Fig. 68.1E ).

SCs are also known to play a large immunogenic role in nerve allograft rejection. They express major histocompatibility complex I (MHC-I), a molecule on cellular membranes that is responsible for displaying fragments of intracellular proteins to T cells to either trigger or prevent an immune response. Transplanted SCs also increase their expression of MHC II molecules, a family of molecules found on antigen-presenting cells and B-cell lymphocytes, increasing their host rejection response. For this reason, SCs are recognized as a prime target of the host alloimmune response. Because SCs play a large role in both axonal regeneration and allograft rejection, immunosuppression cannot be safely withdrawn until regeneration has reached the end-target organ. Functionally, removal of immunosuppression prior to repopulation of the donor graft by host SCs will result in donor SC death and a subsequent, devastating conduction block ( Fig. 68.2 ).

Effects of premature immunosuppression withdrawal in nerve allograft.

(A) Nerve autograft contains native, nonimmunogenic, Schwann cells that are capable of supporting axonal regeneration. (B,C) Under immunosuppression, host Schwann cells migrate from both the proximal and distal nerve stumps into the donor graft to slowly replace donor Schwann cells capable of supporting axonal regeneration. (D) Premature withdrawal of immunosuppression results in rejection of donor Schwann cells and segmental loss of myelination. (E) Demyelination results in conduction block.

Clinical Experience with Nerve Allotransplantation

As discussed previously, the indications for nerve allotransplantation are limited due to the risks of immunosuppression. Currently, nerve allotransplantation is reserved for unique clinical situations of devastating nerve injuries that are not amenable to nerve transfers or nerve autografting. Patient selection and a multidisciplinary team approach are critical to the success of this procedure.

In contrast to solid organ or VCA transplantation, nerve allografts require only temporary systemic immunosuppression, which can be withdrawn once the axons have crossed the allograft. However, detection of rejection during regeneration is difficult with nerve allotransplantation. In organ transplantation, measurements of organ function, such as serum creatinine and liver transaminases, provide clinicians with objective data. In nerve allografts, the grafts are buried and functional assays do not exist. In nerve allotransplantation, once rejection is detected, either by local inflammation or erythema, it is usually too late to rescue the graft with modifications of the immunosuppression. Thus, having a sound immunosuppressive protocol, adequate patient selection and clinical suspicion, and ensuring patient compliance is essential.

The senior author (S.E.M.) has significant clinical expe­rience with nerve allotransplantation. The results of the initial seven cases have previously been reported. Since that publication, an additional five patients have undergone reconstruction with nerve allografts. All of these cases are summarized in Table 68.3 . In the first 11 cases, nerve allografts were used to reconstruct large segmental defects in the extremities. Between 72 cm and 248 cm of nerve allograft pretreated with cold preservation were used in each case and in conjunction with the available nerve autograft in 9 of the 11 patients.

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  Most cases were in younger patients (age range: 3–54 years).

  
  
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  Patient 6 lost the graft due to acute rejection from non-compliance with immunosuppression.

  
  
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  Most patients recovered protective sensation with varying degrees of motor reinnervation.

  
  
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  Motor reinnervation was most robust in proximal muscle groups.

  
  
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  The average length of time patients remained on FK-506 was 20.1 months. This was dictated by clinical evidence of nerve growth beyond the distal end of the allograft.

In contrast to the first 11 cases of allotransplantation, the most recent patient underwent nerve allotransplantation for significant pain after a thoracotomy. This 54-year-old man had recalcitrant right intercostal neuralgia following two thoracic operations 8 years prior to presentation to our clinic. Following his second thoracic procedure (a pleural leak repair after a right upper lobe resection), he developed severe pain localized to the dermatomes of T5-T8. His dysesthetic pain was unresponsive to conservative therapy ( Fig. 68.3 ). In a novel use of allograft transplantation, two 48 cm nerve allografts were looped between the T5-T6 and T7-T8 intercostal nerves following exposure and transection of the proximal nerve roots. Each was looped to connect two consecutive levels and allowed the regenerating intercostal nerves to taper off, preventing neuroma formation and pain. His neuralgia pain subsided by 7 months after his reconstruction. Autografts were avoided in this patient to avoid a second site of neuropathic pain. At 30 months postoperatively, the patient reported his pain was entirely resolved in the thoracic distribution and only limited to the spinal incision site. On a visual analog scale, he rated his pain a 3 out of 10. He controls his pain with topical anesthetic patches and activity modifications. He reported the procedure was “life-changing.”

Case study: Patient 12 (see Table 68.3 ).

This 54-year-old man developed severe right intercostal neuralgia following a thoracotomy. X-ray images: (A) demonstrate the location of the thoracotomy and site of the intercostal nerve injury. Clinical examination demonstrated injury to T5–T8 intercostal nerves. (B) Exposure, decompression, and transection of the injured right T5–8 intercostal nerves (N) in preparation for grafting. (C) Two 48 cm cadaveric nerve allografts were harvested from the donor to be used as looped grafts between the intercostal nerves. (D) Looped cadaveric nerve graft in situ. The two proximal ends of T5–6 were looped together, as were the proximal ends of T7–8. (E) Cadaveric nerve (N) graft loops were mobilized into the exposure site. (F) The pre- (left) and postoperative (right) pain evaluation demonstrates significant reduction in right intercostal neuropathic pain 7 months following the procedure.

Our current clinical protocol involves exhausting all available autografts followed by the use of ABO-blood typed nerve allograft matched donors harvested within 24 h of death. More recently, living, related donors have also been used, as this can eliminate the waiting period for a matched donor and can decrease immunogenicity. To further reduce allogenicity, our protocol utilizes the University of Wisconsin Cold Storage Solution at 4°C for 7 days prior to implantation. We also use multiple small-diameter allograft cables as these revascularized more consistently than larger-diameter grafts. Our immunosuppression regimen incorporates the initiation of FK-506, 3 days preoperatively to maximize the regenerative potential. Our current protocol is detailed in Table 68.4 .

Nerve Regeneration in Vascularized Composite Allotransplantation

The long-term functional outcome in vascularized composite allotransplantation (VCA) is dependent on the quality of nerve regeneration and muscle reinnervation of transplanted parts. With successful results from the multiple hand and face transplants performed to date, VCA is a viable option to treat devastating upper extremity and facial injuries. Understanding the nuances of nerve regeneration in these transplanted parts is necessary and critical to maintain the functional benefit of these procedures.

A number of differences between nerve regeneration in a nerve allograft versus that in VCA exist. Unlike isolated nerve allografting, which only requires temporary immunosuppression, VCA requires lifelong immunosuppression to prevent the rejection of the more immunogenic tissues such as skin, muscle, and bone. As mentioned earlier, nerve allografts depend on the migration of SCs from both the distal and proximal host nerve stumps into the graft to ensure axonal regeneration and ultimately function of the nerve. In VCA, there are no distal host SCs, as the entire distal neuromuscular unit is composed of donor tissue. Therefore, repopulation of the transplanted nerve with host SCs is dependent on the proximal stump only ( Fig. 68.4 ). Currently, it is unknown if there is a limit to how far host SCs are able to migrate to functionally repopulate a nerve in a transplanted part. Further, in nerve allografts, SC migration in the graft was stimulated by episodes of subclinical rejection in an effort to replace depleted donor SCs. In the presence of continuous immunosuppressive regimens, as required in VCA, it is unknown to what extent host SC migration will be stimulated and how this will affect long-term function.

Rejection of nerves in composite tissue allografts.

(A) Composite tissue allografts include nerve and associated Schwann cells from the donor (muscle and gray portion of nerve) transplanted into the host (yellow). (B) If early acute rejection can be abated, nerve regeneration toward the target will continue. (C) Maintenance immunosuppression prevents rejection, allowing axonal regeneration (aided by host Schwann cells migrating from the proximal nerve stump and donor Schwann cells), leading to innervation of the target end organ. (D) In the event of a significant rejection episode prior to repopulation of the donor nerve with host Schwann cells, a loss of donor Schwann cells can occur. (E) The loss of Schwann cells results in a conduction block and permanent functional deficit.

Long-term characterization of host and donor SCs in VCA is not yet fully understood. It is believed that donor SCs are no different from host SCs in their ability to support nerve regeneration. However, lapses in therapeutic immunosuppression causing acute rejection and early loss of donor SCs may have a more pronounced clinical effect in VCA, leading to demyelination and resulting in conduction blocks or a devastating permanent functional failure. Until immunosuppressive strategies are developed to induce tolerance to all donor tissue, the late loss of donor SCs from unexpected episodes of rejection is a significant concern.

Currently, VCA has a very specific application involving severely injured or disfigured patients who have adequate motivation to undergo a major surgical procedure, lifelong immunosuppression, and a lengthy rehabilitation process. Despite this, patients who undergo the procedure remain very satisfied and are recovering neurologic function. Length-dependent functional motor and sensory axonal regeneration has been observed in both facial and upper extremity transplants. Compared with more proximal transplants, distal hand transplants have demonstrated both a superior sensory and motor recovery, with reported return of intrinsic motor function, and discriminative sensation. Forearm and upper arm transplant patients have demonstrated return of sensation over longer time periods, but inconsistent and commonly protective sensation at best. The motor recovery of these more proximal transplants appears to continually improve over the first 5 years, but is commonly limited to elbow and forearm animation.

Future strategies to enhance nerve regeneration in VCA should consider the delicate balance between donor and host SCs. Strategies to optimize donor SC migration into regenerating transplanted nerves within a therapeutic immunosuppression environment should be developed. Keeping donor SCs alive to support axonal regeneration during this time period is important, and may help prevent rejection or functional failure over the lifetime of the patient.