Relevant Anatomic and Morphological Measurements of the Rat Spine: Considerations for Rodent Models of Human Spine Trauma

Assessment of Tie2-Rejuvenated Nucleus Pulposus Cell Transplants from Young and Old Patient Sources Demonstrates That Age Still Matters

Cell transplantation is being actively explored as a regenerative therapy for discogenic back pain. This study explored the regenerative potential of Tie2+ nucleus pulposus progenitor cells (NPPCs) from intervertebral disc (IVD) tissues derived from young (<25 years of age) and old (>60 years of age) patient donors. We employed an optimized culture method to maintain Tie2 expression in NP cells from both donor categories. Our study revealed similar Tie2 positivity rates regardless of donor types following cell culture. Nevertheless, clear differences were also found, such as the emergence of significantly higher (3.6-fold) GD2 positivity and reduced (2.7-fold) proliferation potential for older donors compared to young sources. Our results suggest that, despite obtaining a high fraction of Tie2+ NP cells, cells from older donors were already committed to a more mature phenotype. These disparities translated into functional differences, influencing colony formation, extracellular matrix production, and in vivo regenerative potential. This study underscores the importance of considering age-related factors in NPPC-based therapies for disc degeneration. Further investigation into the genetic and epigenetic alterations of Tie2+ NP cells from older donors is crucial for refining regenerative strategies. These findings shed light on Tie2+ NPPCs as a promising cell source for IVD regeneration while emphasizing the need for comprehensive understanding and scalability considerations in culture methods for broader clinical applicability.

Transspinal Focused Ultrasound Suppresses Spinal Reflexes in Healthy Rats

OBJECTIVES

Low-intensity, focused ultrasound (FUS) is an emerging noninvasive neuromodulation approach, with improved spatial and temporal resolution and penetration depth compared to other noninvasive electrical stimulation strategies. FUS has been used to modulate circuits in the brain and the peripheral nervous system, however, its potential to modulate spinal circuits is unclear. In this study, we assessed the effect of trans-spinal FUS (tsFUS) on spinal reflexes in healthy rats.

MATERIALS AND METHODS

tsFUS targeting different spinal segments was delivered for 1 minute, under anesthesia. Monosynaptic H-reflex of the sciatic nerve, polysynaptic flexor reflex of the sural nerve, and withdrawal reflex tested with a hot plate were measured before, during, and after tsFUS.

RESULTS

tsFUS reversibly suppresses the H-reflex in a spinal segment-, acoustic pressure- and pulse-repetition frequency (PRF)-dependent manner. tsFUS with high PRF augments the degree of homosynaptic depression of the H-reflex observed with paired stimuli. It suppresses the windup of components of the flexor reflex associated with slower, C-afferent, but not faster, A- afferent fibers. Finally, it increases the latency of the withdrawal reflex. tsFUS does not elicit neuronal loss in the spinal cord.

CONCLUSIONS

Our study provides evidence that tsFUS reversibly suppresses spinal reflexes and suggests that tsFUS could be a safe and effective strategy for spinal cord neuromodulation in disorders associated with hyperreflexia, including spasticity after spinal cord injury and painful syndromes.

Treatment affects load to failure and microdamage accumulation in healthy and osteolytic rat vertebrae

Bone turnover and microdamage are impacted by the presence of skeletal metastases which can contribute to increased fracture risk. Treatments for metastatic disease may further impact bone quality. This exploratory study aimed to establish an initial understanding of microdamage accumulation and load to failure in healthy and osteolytic rat vertebrae following focal and systemic cancer treatment (docetaxel (DTX), stereotactic body radiotherapy (SBRT), or zoledronic acid (ZA)). Osteolytic spine metastases were developed in 6-week-old athymic female rats via intracardiac injection of HeLa human cervical cancer cells (day 0). Additional rats served as healthy controls. Rats were either untreated, received SBRT to the T10-L6 vertebrae on day 14 (15 Gy, two fractions), DTX on day 7 or 14, or ZA on day 7. Rats were euthanized on day 21. Tumor burden was assessed with bioluminescence images acquired on day 14 and 21, histology of the excised T11 and L5 vertebrae, and ex-vivo μCT images of the T13-L4. Microstructural parameters (bone volume/total volume, trabecular number, spacing, thickness, and bone mineral density) were measured from L2 vertebrae. Load to failure was measured with axial compressive loading of the L1-L3 motion segments. Microdamage accumulation was labeled in T13 vertebrae with BaSO4 staining and was visualized with high resolution μCT imaging. Microdamage volume fraction was defined as the ratio of BaSO4 to bone volume. DTX administered on day 7 reduced tumor growth significantly (p < 0.05). Microdamage accumulation was found to be increased by the presence of metastases but was reduced by all treatments with ZA showing the largest improvement in HeLa cell injected rats. Load to failure was decreased in untreated and SBRT HeLa cell injected rats compared to healthy controls (p < 0.01). There was a moderate negative correlation between load to failure and microdamage volume fraction in vertebrae from rats injected with HeLa cells (R = -0.35, p = 0.031). Strong correlations were also found between microstructural parameters and load to failure and microdamage accumulation. Several factors, including the presence of osteolytic lesions and use of cancer therapies, influence microdamage accumulation and load to failure in rat vertebrae. Understanding the impact of these treatments on fracture risk of metastatic vertebrae is important to improve management of patients with spinal metastases.

Custom-engineered hydrogels for delivery of human iPSC-derived neurons into the injured cervical spinal cord

Cervical damage is the most prevalent type of spinal cord injury clinically, although few preclinical research studies focus on this anatomical region of injury. Here we present a combinatorial therapy composed of a custom-engineered, injectable hydrogel and human induced pluripotent stem cell (iPSC)-derived deep cortical neurons. The biomimetic hydrogel has a modular design that includes a protein-engineered component to allow customization of the cell-adhesive peptide sequence and a synthetic polymer component to allow customization of the gel mechanical properties. In vitro studies with encapsulated iPSC-neurons were used to select a bespoke hydrogel formulation that maintains cell viability and promotes neurite extension. Following injection into the injured cervical spinal cord in a rat contusion model, the hydrogel biodegraded over six weeks without causing any adverse reaction. Compared to cell delivery using saline, the hydrogel significantly improved the reproducibility of cell transplantation and integration into the host tissue. Across three metrics of animal behavior, this combinatorial therapy significantly improved sensorimotor function by six weeks post transplantation. Taken together, these findings demonstrate that design of a combinatorial therapy that includes a gel customized for a specific fate-restricted cell type can induce regeneration in the injured cervical spinal cord.

Lumbar endplate microfracture injury induces Modic-like changes, intervertebral disc degeneration and spinal cord sensitization - an in vivo rat model

BACKGROUND CONTEXT

Endplate (EP) injury plays critical roles in painful IVD degeneration since Modic changes (MCs) are highly associated with pain. Models of EP microfracture that progress to painful conditions are needed to better understand pathophysiological mechanisms and screen therapeutics.

PURPOSE

Establish in vivo rat lumbar EP microfracture model and assess crosstalk between IVD, vertebra and spinal cord.

STUDY DESIGN/SETTING

In vivo rat EP microfracture injury model with characterization of IVD degeneration, vertebral remodeling, spinal cord substance P (SubP), and pain-related behaviors.

METHODS

EP-injury was induced in 5 month-old male Sprague-Dawley rats L4-5 and L5-6 IVDs by puncturing through the cephalad vertebral body and EP into the NP of the IVDs followed by intradiscal injections of TNFα (n=7) or PBS (n=6), compared with Sham (surgery without EP-injury, n=6). The EP-injury model was assessed for IVD height, histological degeneration, pain-like behaviors (hindpaw von Frey and forepaw grip test), lumbar spine MRI and μCT, and spinal cord SubP.

RESULTS

Surgically-induced EP microfracture with PBS and TNFα injection induced IVD degeneration with decreased IVD height and MRI T2 signal, vertebral remodeling, and secondary damage to cartilage EP adjacent to the injury. Both EP injury groups showed MC-like changes around defects with hypointensity on T1-weighted and hyperintensity on T2-weighted MRI, suggestive of MC type 1. EP injuries caused significantly decreased paw withdrawal threshold, reduced axial grip, and increased spinal cord SubP, suggesting axial spinal discomfort and mechanical hypersensitivity and with spinal cord sensitization.

CONCLUSIONS

Surgically-induced EP microfracture can cause crosstalk between IVD, vertebra, and spinal cord with chronic pain-like conditions.

CLINICAL SIGNIFICANCE

This rat EP microfracture model was validated to induce broad spinal degenerative changes that may be useful to improve understanding of MC-like changes and for therapeutic screening.

Generation of direct current electrical fields as regenerative therapy for spinal cord injury: A review

Electrical stimulation (ES) shows promise as a therapy to promote recovery and regeneration after spinal cord injury. ES therapy establishes beneficial electric fields (EFs) and has been investigated in numerous studies, which date back nearly a century. In this review, we discuss the various engineering approaches available to generate regenerative EFs through direct current electrical stimulation and very low frequency electrical stimulation. We highlight the electrode-tissue interface, which is important for the appropriate choice of electrode material and stimulator circuitry. We discuss how to best estimate and control the generated field, which is an important measure for comparability of studies. Finally, we assess the methods used in these studies to measure functional recovery after the injury and treatment. This work reviews studies in the field of ES therapy with the goal of supporting decisions regarding best stimulation strategy and recovery assessment for future work.

Characterization of the L4/L5 rat facet capsular ligament macromechanical and microstructural responses to tensile failure loading

Low back pain is a prevalent condition that affects the global population. The lumbar facet capsular ligament is a source of pain since the collagenous tissue of the ligament is innervated with sensory neurons that deform with the capsule's stretch. Regional differences in the microstructural and macrostructural anatomy of the spinal facets affect its capsule's mechanical behavior. Although there are many studies of the cervical facet in human and rodent models, the lumbar capsular ligament's multiscale behavior is less well-defined. This study characterizes the macroscale and fiber-scale changes of the rat lumbar facet capsule during tensile failure loading. An integrated polarized light imaging setup captured local fiber alignment during 0.08 mm/s distraction of 7 lumbar facets. Force, displacement, strain, and circular variance were measured at several points along the failure curve: the first instance when the local collagen fiber network realigns differentially (anomalous realignment), yield, the first peak in force corresponding to the capsule's first failure, and peak force, defined as ultimate rupture. Those outcomes were compared across events. While each of force, displacement, and average maximum principal strain increased with applied tension, so did the circular variance of the collagen, suggesting that the fibers were becoming more disorganized. From the fiber alignment maps collected at each mechanical event, the number of anomalous realignment events were counted and found to increase dramatically with loading. The increased collagen disorganization and increasing regions of such disorganization in the facet capsule during loading can provide insights about how loading to the ligament afferent nerves may be activated and thereby produce pain.

Epidural and Intrathecal Drug Delivery in Rats and Mice for Experimental Research: Fundamental Concepts, Techniques, Precaution, and Application

Epidural and intrathecal routes are the most effective drug administration methods for pain management in clinical and experimental medicine to achieve quick results, reduce required drug dosages, and overcome the adverse effects associated with the oral and parenteral routes. Beyond pain management with analgesics, the intrathecal route is more widely used for stem cell therapy, gene therapy, insulin delivery, protein therapy, and drug therapy with agonist, antagonist, or antibiotic drugs in experimental medicine. However, clear information regarding intrathecal and epidural drug delivery in rats and mice is lacking, despite differences from human medicine in terms of anatomical space and proximity to the route of entry. In this study, we discussed and compared the anatomical locations of the epidural and intrathecal spaces, cerebrospinal fluid volume, dorsal root ganglion, techniques and challenges of epidural and intrathecal injections, dosage and volume of drugs, needle and catheter sizes, and the purpose and applications of these two routes in different disease models in rats and mice. We also described intrathecal injection in relation to the dorsal root ganglion. The accumulated information about the epidural and intrathecal delivery routes could contribute to better safety, quality, and reliability in experimental research.

The tracts, cytoarchitecture, and neurochemistry of the spinal cord

The human spinal cord can be described using a range of nomenclatures with each providing insight into its structure and function. Here we have comprehensively reviewed the key literature detailing the general structure, configuration of tracts, the cytoarchitecture of Rexed's laminae, and the neurochemistry at the spinal segmental level. The purpose of this review is to detail current anatomical understanding of how the spinal cord is structured and to aid researchers in identifying gaps in the literature that need to be studied to improve our knowledge of the spinal cord which in turn will improve the potential of therapeutic intervention for disorders of the spinal cord.

Spinal Cord Sensitization and Spinal Inflammation from an In Vivo Rat Endplate Injury Associated with Painful Intervertebral Disc Degeneration

Intervertebral disc (IVD) degeneration with Modic-like changes is strongly associated with pain. Lack of effective disease-modifying treatments for IVDs with endplate (EP) defects means there is a need for an animal model to improve understanding of how EP-driven IVD degeneration can lead to spinal cord sensitization. This rat in vivo study determined whether EP injury results in spinal dorsal horn sensitization (substance P, SubP), microglia (Iba1) and astrocytes (GFAP), and evaluated their relationship with pain-related behaviors, IVD degeneration, and spinal macrophages (CD68). Fifteen male Sprague Dawley rats were assigned into sham or EP injury groups. At chronic time points, 8 weeks after injury, lumbar spines and spinal cords were isolated for immunohistochemical analyses of SubP, Iba1, GFAP, and CD68. EP injury most significantly increased SubP, demonstrating spinal cord sensitization. Spinal cord SubP-, Iba1- and GFAP-immunoreactivity were positively correlated with pain-related behaviors, indicating spinal cord sensitization and neuroinflammation play roles in pain responses. EP injury increased CD68 macrophages in the EP and vertebrae, and spinal cord SubP-, Iba1- and GFAP-ir were positively correlated with IVD degeneration and CD68-ir EP and vertebrae. We conclude that EP injuries result in broad spinal inflammation with crosstalk between spinal cord, vertebrae and IVD, suggesting that therapies must address neural pathologies, IVD degeneration, and chronic spinal inflammation.

Lumbar endplate microfracture injury induces Modic-like changes, intervertebral disc degeneration and spinal cord sensitization - An In Vivo Rat Model

BACKGROUND CONTEXT : Endplate (EP) injury plays critical roles in painful IVD degeneration since Modic changes (MCs) are highly associated with pain. Models of EP microfracture that progress to painful conditions are needed to better understand pathophysiological mechanisms and screen therapeutics. PURPOSE : Establish in vivo rat lumbar EP microfracture model with painful phenotype. STUDY DESIGN/SETTING : In vivo rat study to characterize EP-injury model with characterization of IVD degeneration, vertebral bone marrow remodeling, spinal cord sensitization, and pain-related behaviors. METHODS : EP-driven degeneration was induced in 5-month-old male Sprague-Dawley rats L4-5 and L5-6 IVDs through the proximal vertebral body injury with intradiscal injections of TNFα (n=7) or PBS (n=6), compared to Sham (surgery without EP-injury, n=6). The EP-driven model was assessed for IVD height, histological degeneration, pain-like behaviors (hindpaw von Frey and forepaw grip test), lumbar spine MRI and μCT analyses, and spinal cord substance P (SubP). RESULTS : EP injuries induced IVD degeneration with decreased IVD height and MRI T2 values. EP injury with PBS and TNFα both showed MC type1-like changes on T1 and T2-weighted MRI, trabecular bone remodeling on μCT, and damage in cartilage EP adjacent to the injury. EP injuries caused significantly decreased paw withdrawal threshold and reduced grip forces, suggesting increased pain sensitivity and axial spinal discomfort. Spinal cord dorsal horn SubP was significantly increased, indicating spinal cord sensitization. CONCLUSIONS : EP microfracture can induce crosstalk between vertebral bone marrow, IVD and spinal cord with chronic pain-like conditions. CLINICAL SIGNIFICANCE : This rat EP microfracture model of IVD degeneration was validated to induce MC-like changes and pain-like behaviors that we hope will be useful to screen therapies and improve treatment for EP-drive pain.

Intra-articular MMP-1 in the spinal facet joint induces sustained pain and neuronal dysregulation in the DRG and spinal cord, and alters ligament kinematics under tensile loading

Chronic joint pain is a major healthcare challenge with a staggering socioeconomic burden. Pain from synovial joints is mediated by the innervated collagenous capsular ligament that surrounds the joint and encodes nociceptive signals. The interstitial collagenase MMP-1 is elevated in painful joint pathologies and has many roles in collagen regulation and signal transduction. Yet, the role of MMP-1 in mediating nociception in painful joints remains poorly understood. The goal of this study was to determine whether exogenous intra-articular MMP-1 induces pain in the spinal facet joint and to investigate effects of MMP-1 on mediating the capsular ligament’s collagen network, biomechanical response, and neuronal regulation. Intra-articular MMP-1 was administered into the cervical C6/C7 facet joints of rats. Mechanical hyperalgesia quantified behavioral sensitivity before, and for 28 days after, injection. On day 28, joint tissue structure was assessed using histology. Multiscale ligament kinematics were defined under tensile loading along with microstructural changes in the collagen network. The amount of degraded collagen in ligaments was quantified and substance P expression assayed in neural tissue since it is a regulatory of nociceptive signaling. Intra-articular MMP-1 induces behavioral sensitivity that is sustained for 28 days (p < 0.01), absent any significant effects on the structure of joint tissues. Yet, there are changes in the ligament’s biomechanical and microstructural behavior under load. Ligaments from joints injected with MMP-1 exhibit greater displacement at yield (p = 0.04) and a step-like increase in the number of anomalous reorganization events of the collagen fibers during loading (p ≤ 0.02). Collagen hybridizing peptide, a metric of damaged collagen, is positively correlated with the spread of collagen fibers in the unloaded state after MMP-1 (p = 0.01) and that correlation is maintained throughout the sub-failure regime (p ≤ 0.03). MMP-1 injection increases substance P expression in dorsal root ganglia (p < 0.01) and spinal cord (p < 0.01) neurons. These findings suggest that MMP-1 is a likely mediator of neuronal signaling in joint pain and that MMP-1 presence in the joint space may predispose the capsular ligament to altered responses to loading. MMP-1-mediated pathways may be relevant targets for treating degenerative joint pain in cases with subtle or no evidence of structural degeneration.

Deleterious effects of whole-body vibration on the spine: A review of in vivo, ex vivo, and in vitro models

Occupational exposure to whole-body vibration is associated with the development of musculoskeletal, neurological, and other ailments. Low back pain and other spine disorders are prevalent among those exposed to whole-body vibration in occupational and military settings. Although standards for limiting exposure to whole-body vibration have been in place for decades, there is a lack of understanding of whole-body vibration-associated risks among safety and healthcare professionals. Consequently, disorders associated with whole-body vibration exposure remain prevalent in the workforce and military. The relationship between whole-body vibration and low back pain in humans has been established largely through cohort studies, for which vibration inputs that lead to symptoms are rarely, if ever, quantified. This gap in knowledge highlights the need for the development of relevant in vivo, ex vivo, and in vitro models to study such pathologies. The parameters of vibrational stimuli (eg, frequency and direction) play critical roles in such pathologies, but the specific cause-and-effect relationships between whole-body vibration and spinal pathologies remain mostly unknown. This paper provides a summary of whole-body vibration parameters; reviews in vivo, ex vivo, and in vitro models for spinal pathologies resulting from whole-body vibration; and offers suggestions to address the gaps in translating injury biomechanics data to inform clinical practice.

Effects of systemically administered abaloparatide, an osteoanabolic PTHrP analog, as an adjuvant therapy for spinal fusion in rats

BACKGROUND

Abaloparatide is a parathyroid hormone receptor agonist that increases bone formation and reduces vertebral and nonvertebral fracture risk in women with postmenopausal osteoporosis. Animal studies indicate abaloparatide stimulates vertebral bone formation and enhances bony bridging and biomechanical stability of fracture calluses.

AIMS

The current study is evaluating the potential utility for abaloparatide as an adjunct therapy for spinal fusions.

MATERIAL AND METHODS

The effects of 14 or 28 days of daily subcutaneous injections of abaloparatide (20 μg/kg/d) or vehicle were evaluated in 32 male Sprague-Dawley rats starting 1 day after noninstrumented posterolateral fusion (PLF) with bone autograft. Fusion mass microarchitecture was analyzed by micro-computed tomography (micro-CT) and serum markers of bone formation and bone resorption were evaluated. Motion segments were scored in a blinded manner as fused or unfused by postmortem radiography and manual palpation.

RESULTS

Abaloparatide-treated rats showed higher bone formation (serum osteocalcin) at day 14 and 28 compared with vehicle controls, without increases in the bone resorption marker serum TRACP-5b. Micro-CT showed greater trabecular number in fusion masses from the abaloparatide group vs vehicle controls at day 14. Manual palpation and radiography indicated no fusions in either group at day 14, whereas 25% of vehicle-treated rats and 50% of abaloparatide-treated rats had bilateral fusion at day 28.

DISCUSSION AND CONCLUSION

In summary, this rat PLF model showed that abaloparatide treatment was associated with higher levels of the bone formation marker osteocalcin, improved fusion mass architecture, and a non- significant 2-fold higher fusion rate compared with vehicle.

Epidural electrical stimulation effectively restores locomotion function in rats with complete spinal cord injury

Epidural electrical stimulation can restore limb motor function after spinal cord injury by reactivating the surviving neural circuits. In previous epidural electrical stimulation studies, single electrode sites and continuous tetanic stimulation have often been used. With this stimulation, the body is prone to declines in tolerance and locomotion coordination. In the present study, rat models of complete spinal cord injury were established by vertically cutting the spinal cord at the T8 level to eliminate disturbance from residual nerve fibers, and were then subjected to epidural electrical stimulation. The flexible extradural electrode had good anatomical topology and matched the shape of the spinal canal of the implanted segment. Simultaneously, the electrode stimulation site was able to be accurately applied to the L2–3 and S1 segments of the spinal cord. To evaluate the biocompatibility of the implanted epidural electrical stimulation electrodes, GFAP/Iba-1 double-labeled immunofluorescence staining was performed on the spinal cord below the electrodes at 7 days after the electrode implantation. Immunofluorescence results revealed no significant differences in the numbers or morphologies of microglia and astrocytes in the spinal cord after electrode implantation, and there was no activated Iba-1⁺ cell aggregation, indicating that the implant did not cause an inflammatory response in the spinal cord. Rat gait analysis showed that, at 3 days after surgery, gait became coordinated in rats with spinal cord injury under burst stimulation. The regained locomotion could clearly distinguish the support phase and the swing phase and dynamically adjust with the frequency of stimulus distribution. To evaluate the matching degree between the flexible epidural electrode (including three stimulation contacts), vertebral morphology, and the level of the epidural site of the stimulation electrode, micro-CT was used to scan the thoracolumbar vertebrae of rats before and after electrode implantation. Based on the experimental results of gait recovery using three-site stimulation electrodes at L2–3 and S1 combined with burst stimulation in a rat model of spinal cord injury, epidural electrical stimulation is a promising protocol that needs to be further explored. This study was approved by the Animal Ethics Committee of Chinese PLA General Hospital (approval No. 2019-X15-39) on April 19, 2019.

Comparison of the anatomical morphology of cervical vertebrae between humans and macaques: related to a spinal cord injury model

Non-human primates are most suitable for generating cervical experimental models, and it is necessary to study the anatomy of the cervical spine in non-human primates when generating the models. The purpose of this study was to provide the anatomical parameters of the cervical spine and spinal cord in long-tailed macaques (Macaca fascicularis) as a basis for cervical spine-related experimental studies. Cervical spine specimens from 8 male adult subjects were scanned by micro-computed tomography, and an additional 10 live male subjects were scanned by magnetic resonance imaging. The measurements and parameters from them were compared to those of 12 male adult human subjects. Additionally, 10 live male subjects were scanned by magnetic resonance imaging, and the width and depth of the spinal cord and spinal canal and the thickness of the anterior and posterior cerebrospinal fluid were measured and compared to the relevant parameters of 10 male adult human subjects. The tendency of cervical parameters to change with segmental changes was similar between species. The vertebral body, spinal canal, and spinal cord were significantly flatter in the human subjects than in the long-tailed macaques. The cerebrospinal fluid space in the long-tailed macaques was smaller than that in the human subjects. The anatomical features of the cervical vertebrae of long-tailed macaques provide a reference for establishing a preclinical model of cervical spinal cord injury.

Mapping of the Spinal Sensorimotor Network by Transvertebral and Transcutaneous Spinal Cord Stimulation

Transcutaneous stimulation is a neuromodulation method that is efficiently used for recovery after spinal cord injury and other disorders that are accompanied by motor and sensory deficits. Multiple aspects of transcutaneous stimulation optimization still require testing in animal experiments including the use of pharmacological agents, spinal lesions, cell recording, etc. This need initially motivated us to develop a new approach of transvertebral spinal cord stimulation (SCS) and to test its feasibility in acute and chronic experiments on rats. The aims of the current work were to study the selectivity of muscle activation over the lower thoracic and lumbosacral spinal cord when the stimulating electrode was located intravertebrally and to compare its effectiveness to that of the clinically used transcutaneous stimulation. In decerebrated rats, electromyographic activity was recorded in the muscles of the back (m. longissimus dorsi), tail (m. abductor caudae dorsalis), and hindlimb (mm. iliacus, adductor magnus, vastus lateralis, semitendinosus, tibialis anterior, gastrocnemius medialis, soleus, and flexor hallucis longus) during SCS with an electrode placed alternately in one of the spinous processes of the VT12-VS1 vertebrae. The recruitment curves for motor and sensory components of the evoked potentials (separated from each other by means of double-pulse stimulation) were plotted for each muscle; their slopes characterized the effectiveness of the muscle activation. The electrophysiological mapping demonstrated that transvertebral SCS has specific effects to the rostrocaudally distributed sensorimotor network of the lower thoracic and lumbosacral cord, mainly by stimulation of the roots that carry the sensory and motor spinal pathways. These effects were compared in the same animals when mapping was performed by transcutaneous stimulation, and similar distribution of muscle activity and underlying neuroanatomical mechanisms were found. The experiments on chronic rats validated the feasibility of the proposed stimulation approach of transvertebral SCS for further studies.

Pain After Whole-Body Vibration Exposure Is Frequency Dependent and Independent of the Resonant Frequency: Lessons From an In Vivo Rat Model

Occupational whole-body vibration (WBV) increases the risk of developing low back and neck pain; yet, there has also been an increased use of therapeutic WBV in recent years. Although the resonant frequency (fr) of the spine decreases as the exposure acceleration increases, effects of varying the vibration profile, including peak-to-peak displacement (sptp), root-mean-squared acceleration (arms), and frequency (f), on pain onset are not known. An established in vivo rat model of WBV was used to characterize the resonance of the spine using sinusoidal sweeps. The relationship between arms and fr was defined and implemented to assess behavioral sensitivity-a proxy for pain. Five groups were subjected to a single 30-min exposure, each with a different vibration profile, and a sham group underwent only anesthesia exposure. The behavioral sensitivity was assessed at baseline and for 7 days following WBV-exposure. Only WBV at 8 Hz induced behavioral sensitivity, and the higher arms exposure at 8 Hz led to a more robust pain response. These results suggest that the development of pain is frequency-dependent, but further research into the mechanisms leading to pain is warranted to fully understand which WBV profiles may be detrimental or beneficial.

Error Measurement Between Anatomical Porcine Spine, CT Images, and 3D Printing

RATIONALE AND OBJECTIVES

3D printers are increasingly used in medical applications such as surgical planning, creation of implants and prostheses, and medical education. For the creation of reliable 3D printed models of the vertebral column, processing must be performed on CT images. This processing must be assessed and validated so that any error of the printed model can be recognized and minimized.

MATERIAL AND METHODS

In order to perform this validation, 10 CT scans of porcine lumbar spinal vertebra were used, which were then dissected and scanned again. CT image processing was performed to obtain a mesh and perform 3D printing.

RESULTS

There was no statistical difference among the four different levels of vertebrae measurements (first CT images, second CT images, anatomical piece of porcine bone and 3D printing of porcine bone; One Way repeated measure ANOVA, F < F_crit, p value > α = 0.05). The Intraclass Correlation also revealed a mean intraclass correlation coefficient (3,1) = 0.9553, which describes the reliability of all four levels in addition to the reliability of the data between porcine samples subjected to different levels of measurement. This shows that the average error is less than 1 mm.

CONCLUSIONS

The measurements of models created with 3D printers using the pipeline described in this paper have an average error of 0.60 mm with CT images and 0.73 mm with anatomical piece. Thus, 3D printed models accurately reflect in vivo bones and provide accurate 3D impressions to assist in surgical planning.

Activation of the sciatic nerve evoked during epidural spinal cord stimulation in rodents

Aim

To validate the use of motor activation thresholds (MoT) to titrate stimulation amplitudes for spinal cord stimulation in rodent models.

Methods

We recorded thresholds for MoT and sciatic compound action potentials in ten Sprague-Dawley rats implanted with epidural electrodes. Strength duration curves were fitted to the threshold values.

Results

Activation thresholds were in the same order for both MoT and sciatic compound action potentials.

Conclusion

Many of the large, myelinated fibers traversing the dorsal columns in the rodent spine are activated at similar current levels to MoT. Epidural stimulation in rodents needs to be applied at amplitudes close to MoT to activate these axons.

Dietary polyphenols as a safe and novel intervention for modulating pain associated with intervertebral disc degeneration in an in-vivo rat model

Developing effective therapies for back pain associated with intervertebral disc (IVD) degeneration is a research priority since it is a major socioeconomic burden and current conservative and surgical treatments have limited success. Polyphenols are naturally occurring compounds in plant-derived foods and beverages, and evidence suggests dietary supplementation with select polyphenol preparations can modulate diverse neurological and painful disorders. This study tested whether supplementation with a select standardized Bioactive-Dietary-Polyphenol-Preparation (BDPP) may alleviate pain symptoms associated with IVD degeneration. Painful IVD degeneration was surgically induced in skeletally-mature rats by intradiscal saline injection into three consecutive lumbar IVDs. Injured rats were given normal or BDPP-supplemented drinking water. In-vivo hindpaw mechanical allodynia and IVD height were assessed weekly for 6 weeks following injury. Spinal column, dorsal-root-ganglion (DRG) and serum were collected at 1 and 6 weeks post-operative (post-op) for analyses of IVD-related mechanical and biological pathogenic processes. Dietary BDPP significantly alleviated the typical behavioral sensitivity associated with surgical procedures and IVD degeneration, but did not modulate IVD degeneration nor changes of pro-inflammatory cytokine levels in IVD. Gene expression analyses suggested BDPP might have an immunomodulatory effect in attenuating the expression of pro-inflammatory cytokines in DRGs. This study supports the idea that dietary supplementation with BDPP has potential to alleviate IVD degeneration-related pain, and further investigations are warranted to identify the mechanisms of action of dietary BDPP.

Sex Differences in Rat Intervertebral Disc Structure and Function Following Annular Puncture Injury

STUDY DESIGN

A rat puncture injury intervertebral disc (IVD) degeneration model with structural, biomechanical, and histological analyses.

OBJECTIVE

To determine if males and females have distinct responses in the IVD after injury.

SUMMARY OF BACKGROUND DATA

Low back pain (LBP) and spinal impairments are more common in women than men. However, sex differences in IVD response to injury have been underexplored, particularly in animal models where sex differences can be measured without gender confounds.

METHODS

Forty-eight male and female Sprague Dawley rats underwent sham, single annular puncture with tumor necrosis factor α (TNFα) injection (1×), or triple annular puncture with TNFα injection (3×) surgery. Six weeks after surgery, lumbar IVDs were assessed by radiologic IVD height, spinal motion segment biomechanical testing, histological degeneration grading, second harmonic generation (SHG) imaging, and immunofluorescence for fibronectin and α-smooth muscle actin.

RESULTS

Annular puncture injuries significantly increased degenerative grade and IVD height loss for males and females, but females had increased degeneration grade particularly in the annulus fibrosus (AF). Despite IVD height loss, biomechanical properties were largely unaffected by injury at 6 weeks. However, biomechanical measures sensitive to outer AF differed by sex after 3× injury-male IVDs had greater torsional stiffness, torque range, and viscoelastic creep responses. SHG intensity of outer AF was reduced after injury only in female IVDs, suggesting sex differences in collagen remodeling. Both males and females exhibited decreased cellularity and increased fibronectin expression at injury sites.

CONCLUSION

IVD injury results in distinct degeneration and functional healing responses between males and females. The subtle sex differences identified in this animal model suggest differences in response to IVD injury that might explain some of the variance observed in human LBP, and demonstrate the need to better understand differences in male and female IVD degeneration patterns and pain pathogenesis.

LEVEL OF EVIDENCE

N/A.

Synchrotron tomography of intervertebral disc deformation quantified by digital volume correlation reveals microstructural influence on strain patterns

The intervertebral disc (IVD) has a complex and multiscale extracellular matrix structure which provides unique mechanical properties to withstand physiological loading. Low back pain has been linked to degeneration of the disc but reparative treatments are not currently available. Characterising the disc's 3D microstructure and its response in a physiologically relevant loading environment is required to improve understanding of degeneration and to develop new reparative treatments. In this study, techniques for imaging the native IVD, measuring internal deformation and mapping volumetric strain were applied to an in situ compressed ex vivo rat lumbar spine segment. Synchrotron X-ray micro-tomography (synchrotron CT) was used to resolve IVD structures at microscale resolution. These image data enabled 3D quantification of collagen bundle orientation and measurement of local displacement in the annulus fibrosus between sequential scans using digital volume correlation (DVC). The volumetric strain mapped from synchrotron CT provided a detailed insight into the micromechanics of native IVD tissue. The DVC findings showed that there was no slipping at lamella boundaries, and local strain patterns were of a similar distribution to the previously reported elastic network with some heterogeneous areas and maximum strain direction aligned with bundle orientation, suggesting bundle stretching and sliding. This method has the potential to bridge the gap between measures of macro-mechanical properties and the local 3D micro-mechanical environment experienced by cells. This is the first evaluation of strain at the micro scale level in the intact IVD and provides a quantitative framework for future IVD degeneration mechanics studies and testing of tissue engineered IVD replacements. STATEMENT OF SIGNIFICANCE: Synchrotron in-line phase contrast X-ray tomography provided the first visualisation of native intact intervertebral disc microstructural deformation in 3D. For two annulus fibrosus volumes of interest, collagen bundle orientation was quantified and local displacement mapped as strain. Direct evidence of microstructural influence on strain patterns could be seen such as no slipping at lamellae boundaries and maximum strain direction aligned with collagen bundle orientation. Although disc elastic structures were not directly observed, the strain patterns had a similar distribution to the previously reported elastic network. This study presents technical advances and is a basis for future X-ray microscopy, structural quantification and digital volume correlation strain analysis of soft tissue.

Development of a traumatic cervical dislocation spinal cord injury model with residual compression in the rat

BACKGROUND

Preclinical spinal cord injury models do not represent the wide range of biomechanical factors seen in human injuries, such as spinal level, injury mechanism, velocity of spinal cord impact, and residual compression. These factors may be responsible for differences observed between experimental and clinical study results, especially related to the controversial issue of timing of surgical decompression.

NEW METHOD

Somatosensory Evoked Potentials were used to: a) characterize residual compression depths in a dislocation model, and b) evaluate the physiological effect of whether or not the spinal cord was decompressed following the initial injury, prior to the application of residual compression. Modifications to vertebral clamps and the development of a novel surgical frame allowed us to conduct surgical and injury procedures in a controlled manner without the risk of additional damage to the spinal cord. Behavioural outcomes were evaluated following varying dislocation displacements, in addition to the survivability of 4 h of residual compression following a traumatic injury.

RESULTS

Residual compression immediately following the initial dislocation demonstrated significantly different electrophysiological response compared to when the residual compression was delayed.

COMPARISON WITH EXISTING METHOD

There are currently no other residual compression models that utilize a dislocation injury mechanism. Many residual compression studies have demonstrated the effectiveness of early decompression, however the compression of the spinal cord is often not representative of clinical traumatic injuries. Preclinical studies typically model residual compression using a sustained force through quasi-static application, when human injuries often occur at high velocities, followed by a sustained displacement occlusion of the spinal canal.

CONCLUSIONS

This study has validated several novel procedural approaches and injury parameters, and provided critical details to implement in the development of a traumatic cervical dislocation SCI model with residual compression.

Computer simulation of lumbar flexion shows shear of the facet capsular ligament

BACKGROUND CONTEXT

The lumbar facet capsular ligament (FCL) is a posterior spinal ligament with a complex structure and kinematic profile. The FCL has a curved geometry, multiple attachment sites, and preferentially aligned collagen fiber bundles on the posterior surface that are innervated with mechanoreceptive nerve endings. Spinal flexion induces three-dimensional (3D) deformations, requiring the FCL to maintain significant tensile and shear loads. Previous works aimed to study 3D facet joint kinematics during flexion, but to our knowledge none have reported localized FCL surface deformations likely created by this complex structure.

PURPOSE

The purpose of this study was to elucidate local deformations of both the posterior and anterior surfaces of the lumbar FCL to understand the distribution and magnitude of in-plane and through-plane deformations, including the prevalence of shear.

STUDY DESIGN/SETTING

The FCL anterior and posterior surface deformations were quantified through creation of a finite element model simulating facet joint flexion using a realistic geometry, physiological kinematics, and fitted constitutive material.

METHODS

Geometry was obtained from the micro-CT data of a healthy L3-L4 facet joint capsule (n=1); kinematics were extracted from sagittal plane fluoroscopic data of healthy volunteers (n=10) performing flexion; and average material properties were determined from planar biaxial extension tests of L4-L5 FCLs (n=6). All analyses were performed with the non-linear finite element solver, FEBio. A grid of equally spaced 3×3 nodes on the posterior surface identified regional differences within the strain fields and was used to create comparisons against previously published experimental data. This study was funded by the National Institutes of Health and the authors have no disclosures.

RESULTS

Inhomogeneous in-plane and through-plane shear deformations were prominent through the middle body of the FCL on both surfaces. Anterior surface deformations were more pronounced because of the small width of the joint space, whereas posterior surface deformations were more diffuse because the larger area increased deformability. We speculate these areas of large deformation may provide this proprioceptive system with an excellent measure of spinal motion.

CONCLUSIONS

We found that in-plane and through-plane shear deformations are widely present in finite element simulations of a lumbar FCL during flexion. Importantly, we conclude that future studies of the FCL must consider the effects of both shear and tensile deformations.