Human motor endplate remodeling after traumatic nerve injury.

OBJECTIVE: Current management of traumatic peripheral nerve injuries is variable with operative decisions based on assumptions that irreversible degeneration of the human motor endplate (MEP) follows prolonged denervation and precludes reinnervation. However, the mechanism and time course of MEP changes after human peripheral nerve injury have not been investigated. Consequently, there are no objective measures by which to determine the probability of spontaneous recovery and the optimal timing of surgical intervention. To improve guidance for such decisions, the aim of this study was to characterize morphological changes at the human MEP following traumatic nerve injury. METHODS: A prospective cohort (here analyzed retrospectively) of 18 patients with traumatic brachial plexus and axillary nerve injuries underwent biopsy of denervated muscles from the upper extremity from 3 days to 6 years after injury. Muscle specimens were processed for H & E staining and immunohistochemistry, with visualization via confocal and two-photon excitation microscopy. RESULTS: Immunohistochemical analysis demonstrated varying degrees of fragmentation and acetylcholine receptor dispersion in denervated muscles. Comparison of denervated muscles at different times postinjury revealed progressively increasing degeneration. Linear regression analysis of 3D reconstructions revealed significant linear decreases in MEP volume (R = -0.92, R2 = 0.85, p = 0.001) and surface area (R = -0.75, R2 = 0.56, p = 0.032) as deltoid muscle denervation time increased. Surprisingly, innervated and structurally intact MEPs persisted in denervated muscle specimens from multiple patients 6 or more months after nerve injury, including 2 patients who had presented > 3 years after nerve injury. CONCLUSIONS: This study details novel and critically important data about the morphology and temporal sequence of events involved in human MEP degradation after traumatic nerve injuries. Surprisingly, human MEPs not only persisted, but also retained their structures beyond the assumed 6-month window for therapeutic surgical intervention based on previous clinical studies. Preoperative muscle biopsy in patients being considered for nerve transfer may be a useful prognostic tool to determine MEP viability in denervated muscle, with surviving MEPs also being targets for adjuvant therapy.

Establishing validity of the fundamentals of spinal surgery (FOSS) simulator as a teaching tool for orthopedic and neurosurgical trainees.

BACKGROUND CONTEXT: Pedicle screw placement is a demanding surgical skill as a spine surgeon can face challenges including variations in pedicle morphology and spinal deformities. Available CT simulators for spine pedicle placement can be very costly and hands-on cadaver courses are limited by specimen availability and are not readily accessible. PURPOSE: To conduct validation of a simulated training device for essential spine surgery skills. DESIGN: Cross-sectional, empirical study of physician performance on a surgical simulator model. SAMPLE: Spine attending physicians and residents from four different academic institutions across the United States. OUTCOME MEASURES: Performance metrics on two surgical simulator tasks. METHODS: After IRB approval, an inexpensive ($30) simulator was developed to test two main psychomotor tasks (1) creation of the pedicle screw path with a standard gearshift probe without cortical breaks and (2) the ability to palpate for the presence or absence of cortical breaches as well as determine the location of wall defects. Orthopedic and neurosurgery residents (N=72) as well as spine attending surgeons (N=26) participated from four different institutions. To test construct validity, performance metrics were compared between participants of different training status through one-way analysis of variance and linear regression analysis, with significance set at p<.05. RESULTS: Spine attending surgeons consistently scored higher than the residents, in the screw trajectory task with triangular base (p=.0027) and defect probing task (p=.0035). In defect probing, performance improved with linear trend by number of residency training years with approaching significance (p=.0721). In that task, independent of institutional affiliation, PGY-2 residents correctly identified an average of 1.25±0.43 fewer locations compared with attending physicians (p=.0049). More than 80% of the spine attendings reported they would use the simulator for training purposes. CONCLUSIONS: This low-cost fundamentals of spine surgery simulator detected differences in performances between spine attending surgeons and surgical residents. Programs should consider implementing a simulator such as fundamentals of spine surgery to assess and develop pedicle screw placement ability outside of the operating room.

Attenuation of Robust Glial Scar Formation Facilitates Functional Recovery in Animal Models of Chronic Nerve Compression Injury.

BACKGROUND: Late surgery for chronic nerve compression injuries usually improves sensation but rarely reverses motor atrophy. We hypothesized that a persistent glial scar after chronic nerve compression injury might account for poor motor recovery and that degradation of the glial scar as an adjunct to surgical decompression would improve functional recovery. METHODS: A previously described model of chronic nerve compression injury was created in C57BL/6 mice and Sprague-Dawley rats, and the nerves were harvested early or late after electrophysiological confirmation of the injury. Western blot, polymerase chain reaction, and quantitative immunohistochemical analyses were performed to determine levels of chondroitin sulfate proteoglycans and extracellular matrix molecules. Subsets of mice were treated either with surgical decompression alone or with decompression coupled with intraepineurial injection of a low dose (0.1 µgµL) or a high dose (0.2 µg/µL) of chondroitinase ABC at 6 weeks after injury. RESULTS: Aggrecan showed the greatest change in mRNA and protein levels at the early and late time points following creation of the chronic nerve compression injury. Quantitative immunohistochemical analysis revealed early aggrecan upregulation localized primarily to the endoneurium and late upregulation localized to the perineurium and epineurium (p < 0.0105). Quantitative immunohistochemical analysis for collagen IV, laminin-α2, and fibronectin also showed early upregulation with perineurial scarring. Quantitative immunohistochemical analysis and Western blot analysis for aggrecan demonstrated a marked increase in the endoneurium at the early time points and upregulation of expression in the epineurium and perineurium at the late time points. Decompression along with intraepineurial injection of high-dose chondroitinase ABC at 6 weeks after creation of the compression injury resulted in marked attenuation of decorin and aggrecan expression with functional improvement in nerve conduction velocity. CONCLUSIONS: Significant upregulation of chondroitin sulfate proteoglycans and other extracellular matrix components contributes to the pathogenesis of compression neuropathies in murine models. The administration of chondroitinase ABC degrades these chondroitin sulfate proteoglycans and improves functional recovery after chronic nerve compression injury; thus, it can be considered as a possible therapeutic adjunct.

Surgical repair in humans after traumatic nerve injury provides limited functional neural regeneration in adults.

Traumatic nerve injuries result in devastating loss of neurologic function with unpredictable functional recovery despite optimal medical management. After traumatic nerve injury and denervation, regenerating axons must traverse a complex environment in which they encounter numerous barriers on the way to reinnervation of their target muscle. Outcomes of surgical intervention alone have unfortunately reached a plateau, resulting in often unsatisfactory functional recovery. Over the past few decades, many improvements were developed to supplement and boost the results of surgical repair. Biological optimization of Schwann cells, macrophages, and degradation enzymes have been studied due to the key roles of these components in axonal development, maintenance and response to injury. Moreover, surgical techniques such as nerve grafting, conduits, and growth factor supplementation are also employed to enhance the microenvironment and nerve regeneration. Yet, most of the roadblocks to recovery after nerve injury remain unsolved. These roadblocks include, but are not limited to: slow regeneration rates and specificity of target innervation, the presence of a segmental nerve defect, and degeneration of the target end-organ after prolonged periods of denervation. A recognition of these limitations is necessary so as to develop new strategies to improve functional regeneration for these life changing injuries.

Biophysical stimulation induces demyelination via an integrin-dependent mechanism.

OBJECTIVE: Chronic nerve compression (CNC) injuries occur when peripheral nerves are subjected to sustained mechanical forces, with increasing evidence implicating Schwann cells as key mediators. Integrins, a family of transmembrane adhesion molecules that are capable of intracellular signaling, have been implicated in a variety of biological processes such as myelination and nerve regeneration. In this study, we seek to define the physical stimuli mediating demyelination and to determine whether integrin plays a role in the demyelinating response. METHODS: We used a previously described in vitro model of CNC injury where myelinating neuron-Schwann cell cocultures were subjected to independent manipulations of hydrostatic pressure, hypoxia, and glucose deprivation in a custom bioreactor. We assessed whether demyelination increased in response to applied manipulation and determined whether integrin-associated signaling cascades are upregulated. RESULTS: Biophysical stimulation of neural tissue induced demyelination and Schwann cell proliferation without neuronal or glial cytotoxicity or apoptosis. Although glucose deprivation and hypoxia independently had minor effects on myelin stability, together they potentiated the demyelinating effects of hydrostatic compression, and in combination, significantly destabilized myelin. Biophysical stimuli transiently increased phosphorylation of the integrin-associated tyrosine kinase Src within Schwann cells. Silencing this integrin signaling cascade blocked Src activation and prevented pressure-induced demyelination. Colocalization analysis indicated that Src is localized within Schwann cells. INTERPRETATION: These results indicate that myelin is sensitive to CNC injury and support the novel concept that myelinating cocultures respond directly to mechanical loading via activating an integrin signaling cascade.

Nerve compression activates selective nociceptive pathways and upregulates peripheral sodium channel expression in Schwann cells.

Chronic nerve compression (CNC) injuries, such as carpal tunnel syndrome, are common musculoskeletal conditions that affect patients with debilitating loss of sensory function and pain. Although early detection and treatment are important, our understanding of pain-related molecular mechanisms remains largely unclear. Here we investigate these mechanisms using an animal model for CNC injury. To confirm that CNC injury induces pain, we assessed expression of c-fos, a gene that is rapidly expressed in spinal sensory afferents in response to painful peripheral stimuli, and TNF-alpha and IL-6, two proinflammatory cytokines that are crucial to development of inflammatory-mediated pain. Results show c-fos upregulation 1-2 weeks postinjury in the absence of TNF-alpha or IL-6 expression, indicating increased neural sensitivity without an inflammatory response. This is consistent with previous studies that showed no morphologic evidence of inflammation in the CNC model. Surprisingly, we also found de novo expression of Na(V)1.8, a sodium channel linked to the development of neuropathic pain, in endoneurial Schwann cells following injury. Until now, Na(V)1.8 expression was thought to be restricted to sensory neurons. CNC injury appears to be a unique model of noninflammatory neuropathic pain. Further investigation of the underlying molecular basis could yield promising targets for early diagnosis and treatment.

Functional assessment after sciatic nerve injury in a rat model.

The aim of this study is to evaluate the effectiveness of Sciatic Function Index (SFI) and Basso, Beattie, and Bresnahan (BBB) Locomotor Rating in assessing peripheral nerve injuries. SFI is a standard method for evaluating crush and transected peripheral nerve injuries, likewise BBB for spinal cord injury. Models of chronic nerve compression (CNC), crush, and transection injury were created on Sprague-Dawley rats and functional outcomes were measured using BBB and SFI at 1-week interval for 6 weeks. All injury models showed high correlation between SFI and BBB scores. With crush injury, the SFI showed near complete recovery while BBB showed residual deficits 6 weeks after injury. Both the BBB and SFI were unable to detect motor deficits in 6-week CNC animals. The BBB score should be considered as an adjunct in evaluating peripheral nerve recovery and may be more sensitive in detecting residual deficits than SFI after crush-type injuries.

Macrophage depletion alters the blood-nerve barrier without affecting Schwann cell function after neural injury.

Previous work has shown that, during the early phases of chronic nerve compression (CNC) injury, axonal pathology is absent while Schwann cells undergo a dramatic process of cellular turnover with marked proliferation. It is known that macrophages may release Schwann cell mitogens, so we sought to explore the role of macrophages in CNC injury by selectively depleting the population of hematogenously derived macrophages in nerves undergoing CNC injury by injecting clodronate liposomes at days 1, 3, and 6 postinjury and evaluating both the integrity of the blood-nerve barrier (BNB) and Schwann cell function. Integrity of the BNB was evaluated by intravenously injecting Evans blue albumin (EBA), and Schwann cell number was determined via stereologic techniques. The BNB was clearly altered by 2 weeks postinjury and continued to disintegrate at later time points. Macrophage depletion attenuated this response at all observed time points. Quantification of Schwann cell nuclei in CNC nerves showed no differences between compressed sections of macrophage-depleted and nondepleted animals. Although macrophages are largely responsible for the increased vascular permeability associated with CNC injury, it is likely that the Schwann cell response to CNC injury is not influenced by macrophage-derived mitogenic signals but rather must be mediated via alternative mechanisms.

Local down-regulation of myelin-associated glycoprotein permits axonal sprouting with chronic nerve compression injury.

Chronic nerve compression (CNC) injuries induce a robust Schwann cell proliferation in a distinct spatial and temporal pattern, which is accompanied by an increase in the number of small un-myelinated axons in the area of the injury. These findings suggest that this local proliferation of Schwann cells may induce local axonal sprouting. Here, we use quantitative electron microscopic techniques to define the nature of this sprouting response, and explore whether the local sprouting is in response to down-regulation of expression of myelin-associated glycoprotein (MAG) by proliferating Schwann cells. Axonal sprouting was observed without evidence of Wallerian degeneration in the outer region of CNC-injured nerves with a noticeable increase in Remak bundles within this region of injury. Immunolabeling of teased nerve fibers and Western blot analysis of nerves from CNC-injured animals revealed a local down-regulation of MAG protein within the zone of injury. Moreover, local delivery of purified MAG protein intraneurally at the time of CNC model creation abrogates the axonal sprouting response. These data demonstrate that CNC injury triggers axonal sprouting and suggests that a local down-regulation of MAG within the peripheral nerve secondary to CNC injury is the critical signal for the sprouting response.