Christina K. Magill

Processed allografts and type I collagen conduits for repair of peripheral nerve gaps

Autografting is the gold standard in the repair of peripheral nerve injuries that are not amenable to end‐to‐end coaptation. However, because autografts result in donor‐site defects and are a limited resource, an effective substitute would be valuable. In a rat model, we compared isografts with Integra NeuraGen® (NG) nerve guides, which are a commercially available type I collagen conduit, with processed rat allografts comparable to AxoGens Avance® human decellularized allograft product. In a 14‐mm sciatic nerve gap model, isograft was superior to processed allograft, which was in turn superior to NG conduit at 6 weeks postoperatively (P < 0.05 for number of myelinated fibers both at midgraft and distal to the graft). At 12 weeks, these differences were no longer apparent. In a 28‐mm graft model, isografts again performed better than processed allografts at both 6 and 22 weeks; regeneration through the NG conduit was often insufficient for analysis in this long graft model. Functional tests confirmed the superiority of isografts, although processed allografts permitted successful reinnervation of distal targets not seen in the NG conduit groups. Processed allografts were inherently non‐immunogenic and maintained some internal laminin structure. We conclude that, particularly in a long gap model, nerve graft alternatives fail to confer the regenerative advantages of an isograft. However, AxoGen processed allografts are superior to a currently available conduit‐style nerve guide, the Integra NeuraGen®. They provide an alternative for reconstruction of short nerve gaps where a conduit might otherwise be used. Muscle Nerve, 2008

Reinnervation of the tibialis anterior following sciatic nerve crush injury: A confocal microscopic study in transgenic mice

Transgenic mice whose axons and Schwann cells express fluorescent chromophores enable new imaging techniques and augment concepts in developmental neurobiology. The utility of these tools in the study of traumatic nerve injury depends on employing nerve models that are amenable to microsurgical manipulation and gauging functional recovery. Motor recovery from sciatic nerve crush injury is studied here by evaluating motor endplates of the tibialis anterior muscle, which is innervated by the deep peroneal branch of the sciatic nerve. Following sciatic nerve crush, the deep surface of the tibialis anterior muscle is examined using whole mount confocal microscopy, and reinnervation is characterized by imaging fluorescent axons or Schwann cells (SCs). One week following sciatic crush injury, 100% of motor endplates are denervated with partial reinnervation at 2 weeks, hyperinnervation at 3 and 4 weeks, and restoration of a 1:1 axon to motor endplate relationship 6 weeks after injury. Walking track analysis reveals progressive recovery of sciatic nerve function by 6 weeks. SCs reveal reduced S100 expression within 2 weeks of denervation, correlating with regression to a more immature phenotype. Reinnervation of SCs restores S100 expression and a fully differentiated phenotype. Following denervation, there is altered morphology of circumscribed terminal Schwann cells demonstrating extensive process formation between adjacent motor endplates. The thin, uniformly innervated tibialis anterior muscle is well suited for studying motor reinnervation following sciatic nerve injury. Confocal microscopy may be performed coincident with other techniques of assessing nerve regeneration and functional recovery.

The differential effects of pathway- versus target-derived glial cell line–derived neurotrophic factor on peripheral nerve regeneration

OBJECT Glial cell line-derived neurotrophic factor (GDNF) has potent survival effects on central and peripheral nerve populations. The authors examined the differential effects of GDNF following either a sciatic nerve crush injury in mice that overexpressed GDNF in the central or peripheral nervous systems (glial fibrillary acidic protein [GFAP]-GDNF) or in the muscle target (Myo-GDNF). METHODS Adult mice (GFAP-GDNF, Myo-GDNF, or wild-type [WT] animals) underwent sciatic nerve crush and were evaluated using histomorphometry and muscle force and power testing. Uninjured WT animals served as controls. RESULTS In the sciatic nerve crush, the Myo-GDNF mice demonstrated a higher number of nerve fibers, fiber density, and nerve percentage (p < 0.05) at 2 weeks. The early regenerative response did not result in superlative functional recovery. At 3 weeks, GFAP-GDNF animals exhibit fewer nerve fibers, decreased fiber width, and decreased nerve percentage compared with WT and Myo-GDNF mice (p < 0.05). By 6 weeks, there were no significant differences between groups. CONCLUSIONS Peripheral delivery of GDNF resulted in earlier regeneration following sciatic nerve crush injuries than that with central GDNF delivery. Treatment with neurotrophic factors such as GDNF may offer new possibilities for the treatment of peripheral nerve injury.

Matching of motor-sensory modality in the rodent femoral nerve model shows no enhanced effect on peripheral nerve regeneration

The treatment of peripheral nerve injuries with nerve gaps largely consists of autologous nerve grafting utilizing sensory nerve donors. Underlying this clinical practice is the assumption that sensory autografts provide a suitable substrate for motoneuron regeneration, thereby facilitating motor endplate reinnervation and functional recovery. This study examined the role of nerve graft modality on axonal regeneration, comparing motor nerve regeneration through motor, sensory, and mixed nerve isografts in the Lewis rat. A total of 100 rats underwent grafting of the motor or sensory branch of the femoral nerve with histomorphometric analysis performed after 5, 6, or 7 weeks. Analysis demonstrated similar nerve regeneration in motor, sensory, and mixed nerve grafts at all three time points. These data indicate that matching of motor-sensory modality in the rat femoral nerve does not confer improved axonal regeneration through nerve isografts.

A New Model for Facial Nerve Research: The Novel Transgenic Thy1-GFP Rat

OBJECTIVE To introduce a Thy1-GFP transgenic rat model, whose axons constitutively express green fluorescent protein (GFP), in order to study facial nerve regeneration. Facial nerve injury can cause devastating physical and social sequelae. The functional recovery of the facial nerve can result in synkinesis and permanent axonal misrouting. Facial nerve research has been hindered by the lack of available animal models and reliable outcome measures. METHODS Transgenic Thy1-GFP rats underwent a proximal facial nerve crush injury and were imaged at 0, 1, 2, 4, and 8 weeks after injury. Nerve regeneration was assessed via confocal imaging and fluorescence microscopy. RESULTS Uninjured animals reliably demonstrated facial nerve fluorescence and had predictable anatomical landmarks. Fluorescence microscopy demonstrated the loss and reappearance of fluorescence with regeneration of axons following injury. This was confirmed with the visualization of denervation and reinnervation of zygomaticus muscle motor end plates using confocal microscopy. CONCLUSIONS The Thy1-GFP rat is a novel transgenic tool that enables direct visualization of facial nerve regeneration after injury. The utility of this model extends to a variety of clinical facial nerve injury paradigms.

Dynamic quantification of host Schwann cell migration into peripheral nerve allografts

Host Schwann cell (SC) migration into nerve allografts is the limiting factor in the duration of immunosuppression following peripheral nerve allotransplantation, and may be affected by different immunosuppressive regimens. Our objective was to compare SC migration patterns between clinical and experimental immunosuppression regimens both over time and at the harvest endpoint. Eighty mice that express GFP under the control of the Schwann cell specific S100 promoter were engrafted with allogeneic, nonfluorescent sciatic nerve grafts. Mice received immunosuppression with either tacrolimus (FK506), or experimental T-cell triple costimulation blockade (CSB), consisting of CTLA4-immunoglobulin fusion protein, anti-CD40 monoclonal antibody, and anti-inducible costimulator monoclonal antibody. Migration of GFP-expressing host SCs into wild-type allografts was assessed in vivo every 3 weeks until 15 weeks postoperatively, and explanted allografts were evaluated for immunohistochemical staining patterns to differentiate graft from host SCs. Immunosuppression with tacrolimus exhibited a plateau of SC migration, characterized by significant early migration (< 3 weeks) followed by a constant level of host SCs in the graft (15 weeks). At the endpoint, graft fluorescence was decreased relative to surrounding host nerve, and donor SCs persisted within the graft. CSB-treated mice displayed gradually increasing migration of host SCs into the graft, without the plateau noted in tacrolimus-treated mice, and also maintained a population of donor SCs at the 15-week endpoint. SC migration patterns are affected by immunosuppressant choice, particularly in the immediate postoperative period, and the use of a single treatment of CSB may allow for gradual population of nerve allografts with host SCs.

The Short- and Long-Term Effects of Seprafilm® on Peripheral Nerves: A Histological and Functional Study

Extraneural scar reduction is an important goal in peripheral nerve microsurgery. The use of biosynthetic materials, such as Seprafilm , reduces postoperative adhesions in abdominopelvic gynecologic and orthopedic surgery. The study evaluates the safety of Seprafilm in proximity to nerve tissue in a noninjury (phase 1) and injury (phase 2) model. Phase 1 groups were: (1) sciatic nerve exposure and neurolysis (n = 15), (2) Seprafilm placement superficial to the nerve (n = 15), and (3) circumferentially wrapping Seprafilm around the nerve (n = 15). Outcome measures at 45 and 90 days included wound inspection, histomorphometry, and stereological analysis of vascularity. Phase II groups were: (1) sciatic nerve cut and repair alone (n = 15) or (2) nerve wrapped with Seprafilm (n = 15). Nerves were evaluated at 18, 32, and 42 days postoperatively, and animals underwent biweekly functional walking tracks. In phase I, no significant differences were detected between groups. In phase II, fewer perineural scar bands were seen with Seprafilm . Histomorphometric differences favoring Seprafilm at 18 days and favoring control at 42 days were noted ( P < 0.05), though no differences in functional outcomes were detected. Qualitatively less perineural scar tissue was seen when using Seprafilm . No functional or histological deleterious effects were noted from placing Seprafilm on intact nerves or cut and repaired nerves.

Transgenic models of nerve repair and nerve regeneration

Abstract Objective: The mainstays of peripheral nerve research have historically involved quantifying nerve regeneration by the staining of fixed specimens at multiple time points and by assessing the function of innervated targets. We review advances in transgenic techniques that significantly improve upon standard nerve imaging. Methods: The emergence of transgenic mice whose axons or Schwann cells constitutively express chromophores and techniques enabling direct visualization of nerve regeneration over time after a nerve injury are evaluated. Results: These techniques have enabled investigators to monitor the behaviors of single axons after injury over time. Discussion: Transgenic tools that overexpress proteins or desired factors at certain targets are available, thus circumventing methodological difficulties in drug delivery, maintenance of constant neurotrophic factor concentrations and the comorbidities associated with achieving these aims. In this chapter, we will outline the advancements made in peripheral nerve research using transgenic mouse models. We focus on transgenic tools that have fluorescing nervous system components, overexpress factors at desired targets, or knockout mice with hereditable or modifiable deficits.

Same modality nerve reconstruction for accessory nerve injuries

The standard repair of a nerve gap under tension is to use a sensory autograft, such as the medial antebrachial cutaneous or the sural nerve. The practice of using sensory grafts to repair motor nerve defects is challenged by the discovery of preferential motor reinnervation and modality specific nerve regeneration. In this article, two clinical cases are presented where accessory nerve injuries are repaired with either a motor nerve transfer (a branch of C7) or a motor autograft (obturator nerve), and excellent functional results are reported. These cases provide a stimulus to consider the use of motor nerve grafts or transfers in the repair of motor nerve deficits.

Principles and New Approaches in Surgical Reconstruction

The reconstruction of oncologic defects remains a critical element in the surgical treatment of head and neck cancer. Goals of reconstruction are wound healing, vital structure protection, function, and cosmesis. In this chapter, we discuss the reconstructive ladder as it applies to defects of the oral cavity, oropharynx, nose, orbit, midface, hypopharynx, larynx, and cervical esophagus. Patient cases are shown to illustrate outlined principles. New approaches in surgical reconstruction are discussed, including salvage surgery after failed chemoradiotherapy, the use of perforator flaps, and the frontier of transoral laser microsurgery defects that require flap reconstruction.