Translational bioengineering strategies for peripheral nerve regeneration: opportunities, challenges, and novel concepts

Peripheral nerve injuries remain a challenging problem in need of better treatment strategies. Despite best efforts at surgical reconstruction and postoperative rehabilitation, patients are often left with persistent, debilitating motor and sensory deficits. There are currently no therapeutic strategies proven to enhance the regenerative process in humans. A clinical need exists for the development of technologies to promote nerve regeneration and improve functional outcomes. Recent advances in the fields of tissue engineering and nanotechnology have enabled biomaterial scaffolds to modulate the host response to tissue repair through tailored mechanical, chemical, and conductive cues. New bioengineered approaches have enabled targeted, sustained delivery of protein therapeutics with the capacity to unlock the clinical potential of a myriad of neurotrophic growth factors that have demonstrated promise in enhancing regenerative outcomes. As such, further exploration of combinatory strategies leveraging these technological advances may offer a pathway towards clinically translatable solutions to advance the care of patients with peripheral nerve injuries. This review first presents the various emerging bioengineering strategies that can be applied for the management of nerve gap injuries. We cover the rationale and limitations for their use as an alternative to autografts, focusing on the approaches to increase the number of regenerating axons crossing the repair site, and facilitating their growth towards the distal stump. We also discuss the emerging growth factor-based therapeutic strategies designed to improve functional outcomes in a multimodal fashion, by accelerating axonal growth, improving the distal regenerative environment, and preventing end-organs atrophy.

Sustained IGF-1 delivery ameliorates effects of chronic denervation and improves functional recovery after peripheral nerve injury and repair

Functional recovery following peripheral nerve injury is limited by progressive atrophy of denervated muscle and Schwann cells (SCs) that occurs during the long regenerative period prior to end-organ reinnervation. Insulin-like growth factor 1 (IGF-1) is a potent mitogen with well-described trophic and anti-apoptotic effects on neurons, myocytes, and SCs. Achieving sustained, targeted delivery of small protein therapeutics remains a challenge. We hypothesized that a novel nanoparticle (NP) delivery system can provide controlled release of bioactive IGF-1 targeted to denervated muscle and nerve tissue to achieve improved motor recovery through amelioration of denervation-induced muscle atrophy and SC senescence and enhanced axonal regeneration. Biodegradable NPs with encapsulated IGF-1/dextran sulfate polyelectrolyte complexes were formulated using a flash nanoprecipitation method to preserve IGF-1 bioactivity and maximize encapsulation efficiencies. Under optimized conditions, uniform PEG-b-PCL NPs were generated with an encapsulation efficiency of 88.4%, loading level of 14.2%, and a near-zero-order release of bioactive IGF-1 for more than 20 days in vitro. The effects of locally delivered IGF-1 NPs on denervated muscle and SCs were assessed in a rat median nerve transection-without- repair model. The effects of IGF-1 NPs on axonal regeneration, muscle atrophy, reinnervation, and recovery of motor function were assessed in a model in which chronic denervation is induced prior to nerve repair. IGF-1 NP treatment resulted in significantly greater recovery of forepaw grip strength, decreased denervation-induced muscle atrophy, decreased SC senescence, and improved neuromuscular reinnervation.

Insulin-Like Growth Factor-1: A Promising Therapeutic Target for Peripheral Nerve Injury

Patients who sustain peripheral nerve injuries (PNIs) are often left with debilitating sensory and motor loss. Presently, there is a lack of clinically available therapeutics that can be given as an adjunct to surgical repair to enhance the regenerative process. Insulin-like growth factor-1 (IGF-1) represents a promising therapeutic target to meet this need, given its well-described trophic and anti-apoptotic effects on neurons, Schwann cells (SCs), and myocytes. Here, we review the literature regarding the therapeutic potential of IGF-1 in PNI. We appraised the literature for the various approaches of IGF-1 administration with the aim of identifying which are the most promising in offering a pathway toward clinical application. We also sought to determine the optimal reported dosage ranges for the various delivery approaches that have been investigated.

Mechanisms and outcomes of the supercharged end-to-side nerve transfer: a review of preclinical and clinical studies

Proximal peripheral nerve injuries often result in poor functional outcomes, chiefly because of the long time period between injury and the reinnervation of distal targets, which leads to muscle and Schwann cell atrophy. The supercharged end-to-side (SETS) nerve transfer is a recent technical innovation that introduces donor axons distally into the side of an injured nerve to rapidly innervate and support end organs while allowing for additional reinnervation after a proximal repair at the injury site. However, the mechanisms by which donor axons grow within the recipient nerve, contribute to muscle function, and impact the regeneration of native recipient axons are poorly understood. This uncertainty has slowed the transfer's clinical adoption. The primary objective of this article is to comprehensively review the mechanisms underpinning axonal regeneration and functional recovery after a SETS nerve transfer. A secondary objective is to report current clinical applications in the upper limb and their functional outcomes. The authors also propose directions for future research with the aim of maximizing the clinical utility of the SETS transfer for peripheral nerve surgeons and their patients.

Glial Cell Line-Derived Neurotrophic Factor and Chondroitinase Promote Axonal Regeneration in a Chronic Denervation Animal Model

Functional recovery following nerve injury declines when target re-innervation is delayed. Currently, no intervention exists to improve outcomes after prolonged denervation. We explored the neuroregenerative effects of glial cell line-derived neurotrophic factor (GDNF) and chondroitinase (CDN) in a chronic denervation animal model. A fibrin-based sustained delivery method for growth factors was optimized in vitro and in vivo, and then tested in our animal model. GDNF, CDN, and GDNF+CDN were injected into the denervated stump at the time of nerve repair. Histomorphometry and retrograde labeling were used to assess axonal regeneration. The mechanisms promoting such regeneration were explored with immunofluorescence. Five weeks after repair, the GDNF+CDN group had the highest number and maturity of axons. GDNF was noted to preferentially promote axonal maturity, whereas CDN predominantly increased the number of axons. GDNF favored motor neuron regeneration, and upregulated Ki67 in Schwann cells. CDN did not favor motor versus sensory regeneration and was noted to cleave inhibitory endoneurial proteoglycans. Early measures of nerve regeneration after delayed repair are improved by activating Schwann cells and breaking down the inhibitory proteoglycans in the distal nerve segment, suggesting a role for GDNF+CDN to be translated for human nerve repairs.

Macroporous nanofiber wraps promote axonal regeneration and functional recovery in nerve repair by limiting fibrosis

Functional outcomes following nerve repair remain suboptimal. Scarring at the repair site is a major impediment to regeneration. A biomaterial scaffold applied around the coaptation site that decreases inflammation holds great potential in reducing scarring, enhancing axonal growth, and improving functional recovery. In this study, we evaluated the effect of a macroporous nanofiber wrap, comprised of nonwoven electrospun poly-ε-caprolactone (PCL), in improving axonal regeneration in a rat sciatic nerve cut and direct repair model. Controls consisted of conventional epineurial repair. We also evaluated our wrap against the commercially available AxoGuard wrap. At five weeks following repair, the nanofiber wrap group showed a significantly decreased intraneural macrophage invasion and collagen deposition at the repair site. This was associated with increased expression of the anti-inflammatory cytokine (IL-10), decreased expression of the pro-inflammatory cytokine (TNF-α), and a decrease in the M1:M2 macrophage phenotype ratio. These findings suggest that this nanofiber wrap, with its unique macroporosity, is modulating the inflammatory response at the repair site by polarizing macrophages towards a pro-regenerative M2 phenotype. Concomitantly, a higher number of regenerated axons was noted. At sixteen weeks, the nanofiber wrap resulted in enhanced functional recovery as demonstrated by electrophysiology, neuromuscular re-innervation, and muscle histology. When compared to the AxoGuard wrap, the nanofiber wrap showed similar inflammation at the repair site and similar nerve morphometric findings, but there was a trend towards a lower overall number of macrophages invading the wrap wall. These results demonstrate favorable outcomes of the macroporous nanofiber wrap in promoting neuroregeneration and functional recovery following nerve repair. STATEMENT OF SIGNIFICANCE: Electrospun nanofiber scaffolds, with specific fiber and pore sizes, were shown to modulate the immune response and create a regenerative environment. In this paper, we present a macroporous nanofiber wrap, made of poly-ε-caprolactone, to be applied at the coaptation site in primary nerve repair. We show that it regulates the inflammatory response at the repair site and decreases scarring/fibrosis. This results in enhanced axonal regeneration, allowing a higher number of axons to cross the suture line and reach the target muscle in a timely fashion. Functional outcomes are thus improved.

A Novel Rodent Orthotopic Forelimb Transplantation Model That Allows for Reliable Assessment of Functional Recovery Resulting From Nerve Regeneration

Improved nerve regeneration and functional outcomes would greatly enhance the utility of vascularized composite allotransplantation (VCA) such as hand and upper extremity transplantation. However, research aimed at achieving this goal has been limited by the lack of a functional VCA animal model. We have developed a novel rat midhumeral forelimb transplant model that allows for the characterization of upper extremity functional recovery following transplantation. At the final end point of 12 weeks, we found that animals with forelimb transplantation including median, ulnar and radial nerve coaptation demonstrated significantly improved grip strength and forelimb function as compared to forelimb transplantation without nerve approximation (grip strength: 1.71N ± 0.57 vs. no appreciable recovery; IBB scale: 2.6 ± 0.7? vs. 0.8 ± 0.40; p = 0.0005), and similar recovery to nerve transection-and-repair only (grip strength: 1.71N ± 0.57 vs. 2.03 ± 0.42.6; IBB scale: 2.6 ± 0.7 vs. 2.8 ± 0.8; p = ns). Moreover, all forelimb transplant animals with nerve coaptation displayed robust axonal regeneration with myelination and reduced flexor muscle atrophy when compared to forelimb transplant animals without nerve coaptation. In conclusion, this is the first VCA small-animal model that allows for reliable and reproducible measurement of behavioral functional recovery in addition to histologic evaluation of nerve regeneration and graft reinnervation.

Microsurgery in the burn population - a review of the literature

The management of patients suffering from burn injury poses unique challenges for the reconstructive surgeon, both in the acute and delayed settings. Once resuscitative measures are optimized and hemodynamic stability is achieved, early burn debridement and coverage is performed. Traditionally, this consists of excision of devitalized tissue and subsequent coverage using split thickness skin grafts. However, in certain instances, and depending on the extent and nature of the burn injury, skin grafting (or even local tissue rearrangement) may not be a reasonable option. in these cases, free tissue transfer may provide a viable reconstructive alternative. While free flap reconstruction is rare in burn surgery, particularly in the acute setting, burn injuries that expose vital structures, such as tendon, nerve, bone, or deep vessels, require robust flap coverage. in the delayed setting, unsightly scar formation and contracture often occurs secondary to skin graft coverage. These significant patient morbidities are often amenable to free tissue transfer as well. This review article discusses the indications, applications, and problems with free flap surgery for burn injuries in both the acute and delayed setting, and summarizes the available literature on microsurgical free tissue transfer for burn management.

Neoadjuvant chemotherapy and short-term morbidity in patients undergoing mastectomy with and without breast reconstruction

IMPORTANCE

Neoadjuvant chemotherapy (NC) is increasingly being used in patients with breast cancer, and evidence-based reports related to its independent effects on morbidity after mastectomy with immediate breast reconstruction are limited.

OBJECTIVE

To determine the effect of NC on 30-day postoperative morbidity in women undergoing mastectomy with or without immediate breast reconstruction.

DESIGN, SETTING, AND PARTICIPANTS

All women undergoing mastectomy with or without immediate breast reconstruction from January 1, 2005, through December 31, 2011, at university and private hospitals internationally were analyzed using the American College of Surgeons National Surgical Quality Improvement Program 2005-2011 databases. Patients who received NC were compared with those without a history of NC to estimate the relative odds of 30-day postoperative overall, systemic, and surgical site morbidity using model-wise multivariable logistic regression.

EXPOSURE

Neoadjuvant chemotherapy.

MAIN OUTCOMES AND MEASURES

Thirty-day postoperative morbidity (overall, systemic, and surgical site).

RESULTS

Of 85,851 women, 66,593 (77.6%) underwent mastectomy without breast reconstruction, with 2876 (4.3%) receiving NC; 7893 patients were excluded because of missing exposure data. The immediate breast reconstruction population included 19,258 patients (22.4%), with 820 (4.3%) receiving NC. After univariable analysis, NC was associated with a 20% lower odds of overall morbidity in the group undergoing mastectomy without breast reconstruction (odds ratio [OR], 0.80; 95% CI, 0.71-0.91) but had no significant effect in the immediate breast reconstruction group (OR, 0.98; 95% CI, 0.79-1.23). After adjustment for confounding, NC was independently associated with lower overall morbidity in the group undergoing mastectomy without breast reconstruction (OR, 0.61; 95% CI, 0.51-0.73) and the immediate tissue expander reconstruction subgroup (OR, 0.49; 95% CI, 0.30-0.84). Neoadjuvant chemotherapy was associated with decreased odds of systemic morbidity in 4 different populations: complete sample (OR, 0.59; 95% CI, 0.49-0.71), mastectomy without breast reconstruction (OR, 0.59; 95% CI, 0.48-0.72), any immediate breast reconstruction (OR, 0.57; 95% CI, 0.37-0.88), and the tissue expander subgroup (OR, 0.41; 95% CI, 0.23-0.72).

CONCLUSIONS AND RELEVANCE

Our study supports the safety of NC in women undergoing mastectomy with or without immediate breast reconstruction. Neoadjuvant chemotherapy is associated with lower overall morbidity in the patients undergoing mastectomy without breast reconstruction and in those undergoing tissue expander breast reconstruction. In addition, the odds of systemic morbidity were decreased in patients undergoing mastectomy with and without immediate breast reconstruction. The mechanisms behind the protective association of NC remain unknown and warrant further investigation.

Treatment of a paediatric patient with a distal lower extremity traumatic wound using a dermal regeneration template and NPWT

Degloving injuries are common in trauma and represent a spectrum of complex wounds, the management of which may be highly challenging especially in the paediatric population. In severe injuries leading to wounds reaching tendon and bone, vascularity is compromised precluding traditional wound management, and sometimes necessitating amputation. This report highlights the use of a dermal regeneration template combined with vacuum-assisted closure (VAC) in the treatment of complex traumatic degloving wounds. Here, we present a case of a five-year-old boy who sustained a high-energy shear injury to his lower extremity that resulted in an extensive degloving wound involving the distal third of his leg and the dorsum of his foot. After debridement, the patient underwent VAC combined with a dermal skin substitute placement, followed by split-thickness skin grafting. The extremity healed with no complications and without the need for amputation or flap reconstruction, achieving satisfactory recovery of range of motion and favourable cosmetic results.

Functional abdominal wall reconstruction using an innervated abdominal wall vascularized composite tissue allograft: a cadaveric study and review of the literature

BACKGROUND

Large, composite abdominal wall defects represent complex problems requiring a multidisciplinary approach for reconstruction. Abdominal wall vascularized composite allotransplantation (AW-VCA) has been successfully performed in 21 patients, already receiving solid organ transplants, to provide immediate abdominal closure. The current study aims to establish a novel anatomic model for AW-VCA that retains motor and sensory function in an effort to preserve form and function while preventing complications.

METHODS

Three fresh cadaver torsos were obtained. Dissection was started in the midaxillary line bilaterally through the skin and subcutaneous fascia until the external oblique was encountered. The thoracolumbar nerves were identified and measurements were obtained. A peritoneal dissection from the costal margin to pubic symphysis was performed and the vascular pedicle was identified for subsequent microsurgical anastomosis.

RESULTS

The mean size of the abdominal wall graft harvested was 615 ± 120 cm(2). The mean time of abdominal wall procurement was ∼150 ± 12 minutes. The mean number of thoracolumbar nerves identified was 5 ± 1.4 on each side. The mean length of the skeletonized thoracolumbar nerves was 7.8 ± 1.7 cm. The cross-sectional diameter of all nerves as they entered the rectus abdominis was greater than 2 mm.

CONCLUSIONS

Motor function and sensory recovery is expected in other forms of vascularized composite allotransplantation, such as the hand or face; however, this has never been tested in AW-VCA. This study demonstrates feasibility for the transplantation of large, composite abdominal wall constructs that potentially retains movement, strength, and sensation through neurotization of both sensory and motor nerves.

Robotic harvest of the rectus abdominis muscle: principles and clinical applications

Harvest of the rectus abdominis muscle requires an abdominal incision as well as violation of the anterior rectus sheath, creating the potential for significant surgical-site morbidity (bulges, hernias, infections, seromas). Laparoscopic or endoscopic techniques, although feasible, have not become popular among plastic surgeons due to multiple technical shortcomings. Robotic surgery on the other hand has an easier learning curve, enhanced precision, tremor elimination, motion scaling, high resolution, three-dimensional optics and an intuitive interface. As a result of these advantages, robotic surgery has permeated into the plastic surgery specialty, assuming a role in the harvest of the latissimus dorsi muscle flap and other reconstructive procedures. In this review, the authors discuss its applicability in the harvest of the rectus abdominis muscle.

Transoral robotic reconstructive surgery

Transoral robotic surgery (TORS) has emerged as a technique that allows head and neck surgeons to safely resect large and complex oropharyngeal tumors without dividing the mandible or performing a lip-split incision. These resections provide a reconstructive challenge because the cylinder of the oropharynx remains closed and both physical access and visualization of oropharyngeal anatomy is severely restricted. Transoral robotic reconstruction (TORRS) of such defects allows the reconstructive surgeon to inset free flaps or perform adjacent tissue transfer while seeing what the resecting surgeon sees. Early experience with this technique has proved feasible and effective. Robotic reconstruction has many distinct advantages over conventional surgery, and offers patients a less morbid surgical course. In this review, we discuss the clinical applicability of transoral robotic surgery in head and neck reconstruction, highlighting the benefits and limitations of such an approach, and outlining the guidelines for its utilization.

The role of a dedicated staff psychiatrist in modern burn centers

Psychiatric disorders may be more common in burn-injured subjects than in the general population, and oftentimes contribute to the injury itself. Even in the absence of underlying psychiatric illnesses, burn patients may still benefit from a psychiatric evaluation during and after their hospitalization. In this regard, we included a dedicated psychiatry service in our multidisciplinary burn team. We review herein the course of burn patients that were offered psychiatric evaluation and highlight the benefits of such a program. We conducted an IRB-approved retrospective chart review of burn subjects admitted to our institution between June 15, 2009 and April 30, 2010 and identified 83 patients that were examined by our psychiatrist. Indications for consultation, history of psychiatric illness and substance abuse, as well as administered drugs, were recorded. Among the 83 evaluated patients, 48 (57.8%) had a preexisting psychiatric disorder and 36 (43.4%) suffered from substance abuse. The most common indications for consultation were pain (28.1%), alcohol dependence (25.8%), anxiety (24.7%), illicit drug abuse (16.8%), depression (15.7%), post-traumatic stress disorder (8.9%), and sleep disturbances (8.9%). Pharmacotherapy was initiated in 75 patients (90.3%). 31 (37.3%) had neither a psychiatric disorder nor a history of substance abuse, although 26 of them (83.9%) still received drugs for psychiatric conditions. The inclusion of a dedicated psychiatrist as part of our burn team has improved our comprehensive burn care. In the overwhelming majority of cases, even in the absence of preexisting psychiatric illnesses, consultation resulted in pharmacologic intervention and enhanced patient care.

A critical analysis of rejection in vascularized composite allotransplantation: clinical, cellular and molecular aspects, current challenges, and novel concepts

Advances in microsurgical techniques and immunomodulatory protocols have contributed to the expansion of vascularized composite allotransplantation (VCA) with very encouraging immunological, functional, and cosmetic results. Rejection remains however a major hurdle that portends serious threats to recipients. Rejection features in VCA have been described in a number of studies, and an international consensus on the classification of rejection was established. Unfortunately, current available diagnostic methods carry many shortcomings that, in certain cases, pose a great diagnostic challenge to physicians especially in borderline rejection cases. In this review, we revisit the features of acute skin rejection in hand and face transplantation at the clinical, cellular, and molecular levels. The multiple challenges in diagnosing rejection and in defining chronic and antibody-mediated rejection in VCA are then presented, and we finish by analyzing current research directions and novel concepts aiming at improving available diagnostic measures.

Cutaneous collateral axonal sprouting re-innervates the skin component and restores sensation of denervated Swine osteomyocutaneous alloflaps

Reconstructive transplantation such as extremity and face transplantation is a viable treatment option for select patients with devastating tissue loss. Sensorimotor recovery is a critical determinant of overall success of such transplants. Although motor function recovery has been extensively studied, mechanisms of sensory re-innervation are not well established. Recent clinical reports of face transplants confirm progressive sensory improvement even in cases where optimal repair of sensory nerves was not achieved. Two forms of sensory nerve regeneration are known. In regenerative sprouting, axonal outgrowth occurs from the transected nerve stump while in collateral sprouting, reinnervation of denervated tissue occurs through growth of uninjured axons into the denervated tissue. The latter mechanism may be more important in settings where transected sensory nerves cannot be re-apposed. In this study, denervated osteomyocutaneous alloflaps (hind- limb transplants) from Major Histocompatibility Complex (MHC)-defined MGH miniature swine were performed to specifically evaluate collateral axonal sprouting for cutaneous sensory re-innervation. The skin component of the flap was externalized and serial skin sections extending from native skin to the grafted flap were biopsied. In order to visualize regenerating axonal structures in the dermis and epidermis, 50um frozen sections were immunostained against axonal and Schwann cell markers. In all alloflaps, collateral axonal sprouts from adjacent recipient skin extended into the denervated skin component along the dermal-epidermal junction from the periphery towards the center. On day 100 post-transplant, regenerating sprouts reached 0.5 cm into the flap centripetally. Eight months following transplant, epidermal fibers were visualized 1.5 cm from the margin (rate of regeneration 0.06 mm per day). All animals had pinprick sensation in the periphery of the transplanted skin within 3 months post-transplant. Restoration of sensory input through collateral axonal sprouting can revive interaction with the environment; restore defense mechanisms and aid in cortical re-integration of vascularized composite allografts.

Preoperative Anemia and Postoperative Outcomes in Immediate Breast Reconstructive Surgery: A Critical Analysis of 10,958 Patients from the ACS-NSQIP Database

BACKGROUND

Preoperative anemia is independently associated with adverse outcomes after general and cardiac surgery. Outcomes after breast reconstruction are not established. We assessed the effect of preoperative anemia on 30-day postoperative morbidity and length of hospital stay (LOS) in patients undergoing immediate breast reconstruction.

METHODS

We identified patients undergoing immediate breast reconstruction from 2008 to 2010 from the American College of Surgeons' National Surgical Quality Improvement Program database (a prospective outcomes-based registry from hospitals worldwide). De-identified data were obtained for demographics, preoperative risk factors, 30-day morbidity, and LOS. Morbidity variables included flap/graft/prosthesis, cardiac, respiratory, neurological, urinary, wound, and venous thromboembolism outcomes. Logistic regression assessed the crude and adjusted effect of anemia (hematocrit <36%) on postoperative 30-day morbidity. Measures of central tendency of LOS were compared across increasing severities of anemia in patients developing adverse events versus controls.

RESULTS

The study population included 10,958 patients; 1556 (16.74%) had preoperative anemia. Crude odds ratio for 30-day morbidity was significantly higher in anemic patients, unadjusted odds ratio = 1.33 (P < 0.008). This prevailed after extensive adjustment for confounding, yielding an adjusted odds ratio = 1.38 (P < 0.03). Patients who experienced adverse effects had protracted LOS, and the presence of anemia significantly amplified this effect.

CONCLUSIONS

These data provide new insight into the effect of anemia in immediate breast reconstruction, demonstrating an independent association between preoperative anemia and 30-day morbidity. These findings suggest treating anemia when possible; however, prospective studies should explore the efficacy, safety, and cost-effectiveness of such treatments.

Cardiovascular dysfunction in burns: review of the literature

Major burn injury produces substantial hemodynamic and cardiodynamic derangements, which contribute to the development of sepsis, multiple organ failure, and death. Cardiac stress is the hallmark of the acute phase response and its severity determines postburn outcomes, with poorer outcomes associated with cardiac dysfunction. With available evidence from the literature, the present is a comprehensive review of cardiac dysfunction in burns as well as the different monitoring modalities.