A model of acute compressive spinal cord injury with a minimally invasive balloon in goats.

Research into spinal cord injury depends upon animal models of trauma. While investigations using small animals have yielded critical insights into the cellular mechanisms of neurotrauma, no effective therapies have been translated to human clinical treatments. There are considerable differences in pathophysiology, scale, and anatomical organization between rodents and primates. Here, the established method of inflating balloons to compress the cord within the spinal canal was adapted for use in goats. By using surgical techniques to insert a kyphoplasty balloon, spinal cord injury was accomplished with minimal trauma to the surrounding tissues, as is common in other traumatic models. Dye volumes of 0, 1.26 ± 0.18, and 2.82 ± 0.20 mL were injected into the balloon to produce spinal occupancies of 0%, 33 ± 2%, and 89 ± 4%, as evaluated by X-ray and computerized tomography imaging. A significant dose response was observed for the different levels of trauma, with reduced conduction of somatosensory evoked potentials and impaired mobility 7 days after injury. From the strong correlations between injection volume, balloon pressure, spinal occupancy, nerve function, and animal behavior, we conclude that hydraulic compression in goats is a useful model of spinal cord injury.

Semi-interpenetrating network of polyethylene glycol and photocrosslinkable chitosan as an in-situ-forming nerve adhesive.

An ideal adhesive for anastomosis of severed peripheral nerves should tolerate strains imposed on rejoined nerves. We use blends of photocrosslinkable 4-azidobenzoic acid-modified chitosan (Az-C) and polyethylene glycol (PEG) as a new in-situ-forming bioadhesive for anastomosing and stabilizing the injured nerves. Cryo-scanning electron microscopy suggests that the polymer blends form a semi-interpenetrating network (semi-IPN), where PEG interpenetrates the Az-C network and reinforces it. Az-C/PEG semi-IPN gels have higher storage moduli than Az-C gel alone and fibrin glue. Nerves anastomosed with an Az-C/PEG gel tolerate a higher force than those with fibrin glue prior to failure. A series of ex vivo and in vitro cell experiments indicate the Az-C/PEG gels are compatible with nerve tissues and cells. In addition, Az-C/PEG gels release PEG over a prolonged period, providing sustained delivery of PEG, a potential aid for nerve cell preservation through membrane fusion. Az-C/PEG semi-IPN gels are promising bioadhesives for repairing severed peripheral nerves not only because of their improved mechanical properties but also because of their therapeutic potential and tissue compatibility.

Acrolein-mediated injury in nervous system trauma and diseases.

Acrolein, an α,ß-unsaturated aldehyde, is a ubiquitous pollutant that is also produced endogenously through lipid peroxidation. This compound is hundreds of times more reactive than other aldehydes such as 4-hydroxynonenal, is produced at much higher concentrations, and persists in solution for much longer than better known free radicals. It has been implicated in disease states known to involve chronic oxidative stress, particularly spinal cord injury and multiple sclerosis. Acrolein may overwhelm the anti-oxidative systems of any cell by depleting glutathione reserves, preventing glutathione regeneration, and inactivating protective enzymes. On the cellular level, acrolein exposure can cause membrane damage, mitochondrial dysfunction, and myelin disruption. Such pathologies can be exacerbated by increased concentrations or duration of exposure, and can occur in normal tissue incubated with injured spinal cord, showing that acrolein can act as a diffusive agent, spreading secondary injury. Several chemical species are capable of binding and inactivating acrolein. Hydralazine in particular can reduce acrolein concentrations and inhibit acrolein-mediated pathologies in vivo. Acrolein scavenging appears to be a novel effective treatment, which is primed for rapid translation to the clinic.

Functional and mechanical evaluation of nerve stretch injury.

Peripheral nerves undergo tensile loading in common physiological conditions, but stretch can also induce nerve pathology, impairing electrophysiological conduction. The level of strain nerves can tolerate and the functional deficits which result from exceeding this threshold are not thoroughly understood. To examine these phenomena, a novel system for tensile electrophysiology was created using a grease gap-recording chamber paired with a computerized micromanipulator and load cell. Guinea pig sciatic nerves were stretched beyond their maximum physiologic length to examine the effects of tension on signal conduction. Mechanical and electrophysiological data such as load, position, compound action potential amplitude, and signal latency were recorded in real-time. While 5% strain did not affect conduction, further elongation decreased amplitude approximately linearly with strain. These experiments verify the findings of prior studies into nerve stretch, and demonstrate the utility of this apparatus for investigating the mechanical and electrophysiological properties of nerves undergoing strain.

Rapidly photo-cross-linkable chitosan hydrogel for peripheral neurosurgeries.

Restoring continuity to severed peripheral nerves is crucial to regeneration and enables functional recovery. However, the two most common agents for coaptation, sutures and fibrin glues, have drawbacks such as inflammation, pathogenesis, and dehiscence. Chitosan-based adhesives are a promising alternative, reported to have good cytocompatibility and favorable immunogenicity. A photo-cross-linkable hydrogel based on chitosan is proposed as a new adhesive for peripheral nerve anastomosis. Two Az-chitosans were synthesized by conjugating 4-azidobenzoic acid with low (LMW, 15 kDa) and high (HMW, 50-190 kDa) molecular weight chitosans. These solutions formed a hydrogel in less than 1 min under UV light. The LMW Az-chitosan was more tightly cross-linked than the HMW variant, undergoing significantly less swelling and possessing a higher rheological storage modulus, and both Az-chitosan gels were stiffer than commercial fibrin glue. Severed nerves repaired by Az-chitosan adhesives tolerated longitudinal forces comparable or superior to fibrin glue. Adhesive exposure to intact nerves and neural cell culture showed both Az-chitosans to be nontoxic in the acute (minutes) and chronic (days) time frames. These results demonstrate that Az-chitosan hydrogels are cytocompatible and mechanically suitable for use as bioadhesives in peripheral neurosurgeries.

Biomimetic nerve scaffolds with aligned intraluminal microchannels: a "sweet" approach to tissue engineering.

In this paper, we outline a method for the fabrication of biomimetic hollow fiber and hollow fiber bundles with high aspect ratios. The manufacturing process utilizes a melt spinning technique with caramelized sucrose as a core template. Encapsulation of the sucrose with a thin layer of degradable polymer and selective dissolution of the sucrose core produced tubes and tube aggregates with geometries similar to biologic analogs. The manufacturing process requires no specialized equipment and minimal quantities of organic solvent/polymer. These scaffolds were shown to induce nerve and glial cell alignment in vitro and may be further customized to integrate with other tissue or cell culture systems.

Ethyl-cyanoacrylate is acutely nontoxic and provides sufficient bond strength for anastomosis of peripheral nerves.

Anastomosis is a common technique for the union of severed nerve trunks. This is commonly performed with sutures, a process that can be both time consuming and injurious to tissue. One promising alternative to suturing is the use of adhesives to join the severed segments. Cyanoacrylate-based glues have been used clinically as a surgical adhesive for soft tissues. However, the acute effects of these glues on nerve electrophysiology and the tensile strength of the rejoined tissues have not been evaluated. Using a guinea pig model, we analyzed the mechanical properties of transected sciatic nerves repaired with epineural application of ethyl-cyanoacrylate and the short term consequences of cyanoacrylate application on impulse conduction. Results showed that nerves coapted with ethyl-cyanoacrylate were capable of bearing in vivo forces. Additionally, no acute effects on conduction were observed in uninjured sciatic nerves exposed to ethyl-cyanoacrylate. In conjunction with long term in vivo reports from literature, the current results support the use of cyanoacrylates in nerve repair.