Resorbable plating system stabilizes tissue-engineered intervertebral discs implanted ex vivo in canine cervical spines

Finite element modeling to predict the influence of anatomic variation and implant placement on performance of biological intervertebral disc implants

Background

Tissue-engineered intervertebral disc (TE-IVD) constructs are an attractive therapy for treating degenerative disc disease and have previously been investigated in vivo in both large and small animal models. The mechanical environment of the spine is notably challenging, in part due to its complex anatomy, and implants may require additional mechanical support to avoid failure in the early stages of implantation. As such, the design of suitable support implants requires rigorous validation.

Methods

We created a FE model to simulate the behavior of the IVD cages under compression specific to the anatomy of the porcine cervical spine, validated the FE model using an animal model, and predicted the effects of implant location and vertebral angle of the motion segment on implant behavior. Specifically, we tested anatomical positioning of the superior vertebra and placement of the implant. We analyzed corresponding stress and strain distributions.

Results

Results demonstrated that the anatomical geometry of the porcine cervical spine led to concentrated stress and strain on the posterior side of the cage. This stress concentration was associated with the location of failure of the cages reported in vivo, despite superior mechanical properties of the implant. Furthermore, placement of the cage was found to have profound effects on migration, while the angle of the superior vertebra affected stress concentration of the cage.

Conclusions

This model can be utilized both to inform surgical procedures and provide insight on future cage designs and can be adopted to models without the use of in vivo animal models.

Microphysiological system for studying contractile differences in young, active, and old, sedentary adult derived skeletal muscle cells

Microphysiological systems (MPS), also referred to as tissue chips, incorporating 3D skeletal myobundles are a novel approach for physiological and pharmacological studies to uncover new medical treatments for sarcopenia. We characterize a MPS in which engineered skeletal muscle myobundles derived from donor-specific satellite cells that model aged phenotypes are encapsulated in a perfused tissue chip platform containing platinum electrodes. Our myobundles were derived from CD56+ myogenic cells obtained via percutaneous biopsy of the vastus lateralis from adults phenotyped by age and physical activity. Following 17 days differentiation including 5 days of a 3 V, 2 Hz electrical stimulation regime, the myobundles exhibited fused myotube alignment and upregulation of myogenic, myofiber assembly, signaling and contractile genes as demonstrated by gene array profiling and localization of key components of the sarcomere. Our results demonstrate that myobundles derived from the young, active (YA) group showed high intensity immunofluorescent staining of α-actinin proteins and responded to electrical stimuli with a ~1 μm displacement magnitude compared with non-stimulated myobundles. Myobundles derived from older sedentary group (OS) did not display a synchronous contraction response. Hypertrophic potential is increased in YA-derived myobundles in response to stimulation as shown by upregulation of insulin growth factor (IGF-1), α-actinin (ACTN3, ACTA1) and fast twitch troponin protein (TNNI2) compared with OS-derived myobundles. Our MPS mimics disease states of muscle decline and thus provides an aged system and experimental platform to investigate electrical stimulation mimicking exercise regimes and may be adapted to long duration studies of compound efficacy and toxicity for therapeutic evaluation against sarcopenia.

The degenerative impact of hyperglycemia on the structure and mechanics of developing murine intervertebral discs

Introduction

Diabetes has long been implicated as a major risk factor for intervertebral disc (IVD) degeneration, interfering with molecular signaling and matrix biochemistry, which ultimately aggravates the progression of the disease. Glucose content has been previously shown to influence structural and compositional changes in engineered discs in vitro, impeding fiber formation and mechanical stability.

Methods

In this study, we investigated the impact of diabetic hyperglycemia on young IVDs by assessing biochemical composition, collagen fiber architecture, and mechanical behavior of discs harvested from 3‐ to 4‐month‐old db/db mouse caudal spines.

Results

We found that discs taken from diabetic mice with elevated blood glucose levels demonstrated an increase in total glycosaminoglycan and collagen content, but comparable advanced glycation end products (AGE) levels to wild‐type discs. Diabetic discs also contained ill‐defined boundaries between the nucleus pulposus and annulus fibrosus, with the latter showing a disorganized and unaligned collagen fiber network at this same boundary.

Conclusions

These compositional and structural changes had a detrimental effect on function, as the diabetic discs were twice as stiff as their wild‐type counterparts and demonstrated a significant resistance to deformation. These results indicate that diabetes may predispose the young disc to DDD later in life by altering patterns of extracellular matrix deposition, fiber formation, and motion segment mechanics independently of AGE accumulation.

Tissue-Engineering of Human Intervertebral Disc: A Concise Review

Intervertebral disc (IVD) represents a structure of crucial structural and functional importance for human spine. Pathology of IVD institutes a frequently encountered condition in current clinical practice. Degenerative Disc Disease (DDD), the principal clinical representative of IVD pathology, constitutes an increasingly diagnosed spinal disorder associated with substantial morbidity and mortality in recent years. Despite the considerable incidence and socioeconomic burden of DDD, existing treatment modalities including conservative and surgical methods have been demonstrated to provide a limited therapeutic effect, being not capable of interrupting or reversing natural progress of underlying disease. These limitations underline the requirement for development of novel, innovative and more effective therapeutic strategies for DDD management. Within this literature framework, compromised IVD replacement with a viable IVD construct manufactured with Tissue-Engineering (TE) methods has been recommended as a promising therapeutic strategy for DDD. Existing preliminary preclinical data demonstrate that proper combination of cells from various sources, different scaffold materials and appropriate signaling molecules renders manufacturing of whole-IVD tissue-engineered constructs a technically feasible process. Aim of this narrative review is to critically summarize current published evidence regarding particular aspects of IVD-TE, primarily emphasizing in providing researchers in this field with practicable knowledge in order to enhance clinical translatability of their research and informing clinical practitioners about the features and capabilities of innovative TE science in the field of IVD-TE.

Architecture-Promoted Biomechanical Performance-Tuning of Tissue-Engineered Constructs for Biological Intervertebral Disc Replacement

Background: Biological approaches to intervertebral disc (IVD) restoration and/or regeneration have become of increasing interest. However, the IVD comprises a viscoelastic system whose biological replacement remains challenging. The present study sought to design load-sharing two-component model systems of circular, nested, concentric elements reflecting the nucleus pulposus and annulus fibrosus. Specifically, we wanted to investigate the effect of architectural design variations on (1) model system failure loads when testing the individual materials either separately or homogeneously mixed, and (2) also evaluate the potential of modulating other mechanical properties of the model systems. Methods: Two sets of softer and harder biomaterials, 0.5% and 5% agarose vs. 0.5% agarose and gelatin, were used for fabrication. Architectural design variations were realized by varying ring geometries and amounts while keeping the material composition across designs comparable. Results: Variations in the architectural design, such as lamellar width, number, and order, combined with choosing specific biomaterial properties, strongly influenced the biomechanical performance of IVD constructs. Biomechanical characterization revealed that the single most important parameter, in which the model systems vastly exceeded those of the individual materials, was failure load. The model system failure loads were 32.21- and 84.11-fold higher than those of the agarose materials and 55.03- and 2.14-fold higher than those of the agarose and gelatin materials used for system fabrication. The compressive strength, dynamic stiffness, and viscoelasticity of the model systems were always in the range of the individual materials. Conclusions: Relevant architecture-promoted biomechanical performance-tuning of tissue-engineered constructs for biological IVD replacement can be realized by slight modifications in the design of constructs while preserving the materials’ compositions. Minimal variations in the architectural design can be used to precisely control structure–function relations for IVD constructs rather than choosing different materials. These fundamental findings have important implications for efficient tissue-engineering of IVDs and other load-bearing tissues, as potential implants need to withstand high in situ loads.

Canine models of spine disorders

Neck and low back pain are common among the adult human population and impose large social and economic burdens on health care and quality of life. Spine-related disorders are also significant health concerns for canine companions with etiopathogeneses, clinical presentations, and diagnostic and therapeutic options that are very similar to their human counterparts. Historically, induced and spontaneous pathology in laboratory rodents, dogs, sheep, goats, pigs, and nonhuman primates have been used for study of human spine disorders. While each of these can serve as useful preclinical models, they all have inherent limitations. Spontaneously occurring spine disorders in dogs provide highly translatable data that overcome many of the limitations of other models and have the added benefit of contributing to veterinary healthcare as well. For this scoping review, peer-reviewed manuscripts were selected from PubMed and Google Scholar searches using keywords: "intervertebral disc," "intervertebral disc degeneration," "biomarkers," "histopathology," "canine," and "mechanism." Additional keywords such as "injury," "induced model," and "nucleus degeneration" were used to further narrow inclusion. The objectives of this review were to (a) outline similarities in key features of spine disorders between dogs and humans; (b) describe relevant canine models; and (c) highlight the applicability of these models for advancing translational research and clinical application for mechanisms of disease, diagnosis, prognosis, prevention, and treatment, with a focus on intervertebral disc degeneration. Best current evidence suggests that dogs share important anatomical, physiological, histological, and molecular components of spinal disorders in humans, such that induced and spontaneous canine models can be very effective for translational research. Taken together, the peer-reviewed literature supports numerous advantages for use of canine models for study of disorders of the spine when the potential limitations and challenges are addressed.

Preclinical ex-vivo Testing of Anti-inflammatory Drugs in a Bovine Intervertebral Degenerative Disc Model

Discogenic low back pain (LBP) is a main cause of disability and inflammation is presumed to be a major driver of symptomatic intervertebral disc degeneration (IDD). Anti-inflammatory agents are currently under investigation as they demonstrated to alleviate symptoms in patients having IDD. However, their underlying anti-inflammatory and regenerative activity is poorly explored. The present study sought to investigate the potential of Etanercept and Tofacitinib for maintaining disc homeostasis in a preclinical intervertebral disc (IVD) organ culture model within IVD bioreactors allowing for dynamic loading and nutrient exchange. Bovine caudal IVDs were cultured in a bioreactor system for 4 days to simulate physiological or degenerative conditions: (1) Phy—physiological loading (0.02–0.2 MPa; 0.2 Hz; 2 h/day) and high glucose DMEM medium (4.5 g/L); (2) Deg+Tumor necrosis factor α (TNF-α)—degenerative loading (0.32–0.5 MPa; 5 Hz; 2 h/day) and low glucose DMEM medium (2 g/L), with TNF-α injection. Etanercept was injected intradiscally while Tofacitinib was supplemented into the culture medium. Gene expression in the IVD tissue was measured by RT-qPCR. Release of nitric oxide (NO), interleukin 8 (IL-8) and glycosaminoglycan (GAG) into the IVD conditioned medium were analyzed. Cell viability in the IVD was assessed using lactate dehydrogenase and ethidium homodimer-1 staining. Immunohistochemistry was performed to assess protein expression of IL-1β, IL-6, IL-8, and collagen type II in the IVD tissue. Etanercept and Tofacitinib downregulated the expression of IL-1β, IL-6, IL-8, Matrix metalloproteinase 1 (MMP1), and MMP3 in the nucleus pulposus (NP) tissue and IL-1β, MMP3, Cyclooxygenase-2 (COX2), and Nerve growth factor (NGF) in the annulus fibrosus (AF) tissue. Furthermore, Etanercept significantly reduced the IL-1β positively stained cells in the outer AF and NP regions. Tofacitinib significantly reduced IL-1β and IL-8 positively stained cells in the inner AF region. Both, Etanercept and Tofacitinib reduced the GAG loss to the level under physiological culture condition. Etanercept and Tofacitinib are able to neutralize the proinflammatory and catabolic environment in the IDD organ culture model. However, combined anti-inflammatory and anabolic treatment may be required to constrain accelerated IDD and relieving inflammation-induced back pain.

Restoration of physiologic loading modulates engineered intervertebral disc structure and function in an in vivo model

Tissue-engineered whole disc replacements are an emerging treatment strategy for advanced intervertebral disc degeneration. A challenge facing the translation of tissue-engineered disc replacement to clinical use are the opposing needs of initial immobilization to advantage integration contrasted with physiologic loading and its anabolic effects. Here, we utilize our established rat tail model of tissue engineered disc replacement with external fixation to study the effects of remobilization at two time points postimplantation on engineered disc structure, composition, and function. Our results suggest that the restoration of mechanical loading following immobilization enhanced collagen and proteoglycan content within the nucleus pulposus and annulus fibrosus of the engineered discs, in addition to improving the integration of the endplate region of the construct with native bone. Despite these benefits, angulation of the vertebral bodies at the implanted level occurred following remobilization at both early and late time points, reducing tensile failure properties in the remobilized groups compared to the fixed group. These results demonstrate the necessity of restoring physiologic mechanical loading to engineered disc implants in vivo, and the need to transition toward their evaluation in larger animal models with more human-like anatomy and motion compared to the rat tail.

Long-term mechanical function and integration of an implanted tissue-engineered intervertebral disc

Tissue engineering holds great promise for the treatment of advanced intervertebral disc degeneration. However, assessment of in vivo integration and mechanical function of tissue-engineered disc replacements over the long term, in large animal models, will be necessary to advance clinical translation. To that end, we developed tissue-engineered, endplate-modified disc-like angle ply structures (eDAPS) sized for the rat caudal and goat cervical spines that recapitulate the hierarchical structure of the native disc. Here, we demonstrate functional maturation and integration of these eDAPS in a rat caudal disc replacement model, with compressive mechanical properties reaching native values after 20 weeks in vivo and evidence of functional integration under physiological loads. To further this therapy toward clinical translation, we implanted eDAPS sized for the human cervical disc space in a goat cervical disc replacement model. Our results demonstrate maintenance of eDAPS composition and structure up to 8 weeks in vivo in the goat cervical disc space and maturation of compressive mechanical properties to match native levels. These results demonstrate the translational feasibility of disc replacement with a tissue-engineered construct for the treatment of advanced disc degeneration.