Back pain is one of the most common physical conditions in the United States, for which approximately 15% of the population will visit a doctor every year. The most common type of back pain is low back pain (LBP) and millions of dollars are spent every year healthcare are a due to LBP. Although poorly understood, low back pain has been associated to interveterbral disc (IVD) degeneration. The IVD is an important structure that helps maintaining normal skeletal support. It is composed of three different tissues called the annulus fibrosus (AF), and the nucleus pulposus (NP), attached to a cartilage endplate (CEP) at its top and bottom surfaces. The AF tissue is composed of chondrocyte-like cells, while the NP tissue is composed of notochordal cells at a young age, which are replaced by mature NP cells later in life. Common signs of degeneration are the inability to maintain extracellular matrix integrity and calcification of the cartilage endplate. Extracellular matrix synthesis and cartilage endplate calcification are closely related to production of adenosine triphosphate (ATP) or energy metabolism of the cells. AF and NP tissues are known to be structurally and compositionally different; therefore it is believed that their metabolic pathways are also distinct. The objective of this study was to determine the differences between AF and NP cells, specifically in their energy metabolism with and without dynamic loading.

This study investigates the changes due to crosslinking treatment in stiffness, permeability, and glycosaminoglycan (GAG) content of bovine intervertebral discs. The objective of this study was to determine the mechanical and biochemical effects of crosslinking treatment on lumbar bovine tissue. Previous studies have found that crosslinking can increase stiffness and permeability in the intervertebral disc. These changes have not yet been investigated by confined compression, stress-relaxation tests of young bovine tissue. Eleven lumbar motion segments were harvested from calf spines and soaked in a saline solution or one of four crosslinking treatments (genipin, methylglyoxal, proanthrocyanidin, and EDC). Five mm diameter samples were removed from the midannulus region at anterior / anterior-lateral locations, confined in a saline bath, swelled to equilibrium, and tested in confined compression stress-relaxation to 15% strain in 5% increments. Radial samples were also harvested, treated with saline solution and EDC, and tested in the same manner. The aggregate modulus and hydraulic permeability were calculated using the nonlinear biphasic theory. Swelling pressure was calculated as the load at swelling equilibrium. GAG content was measured using the dimethylmethylene blue assay. Differences with P value < 0.05 were considered significant. In the axial orientation, all crosslinking treatments except methyglyoxal at least doubled the aggregate modulus relative to soaked controls (P less than 0.05). Genipin treatment resulted in 78% lower axial permeability, proanthrocyanidin (PA) 50% lower, and EDC treatment 84% lower relative to soaked controls (P &lt; 0.05). GAG content measured in the methyglyoxal treatment group was 25% lower than in soaked control group. Genipin (G), proanthrocyanidin (PA), and EDC treatment increased the swelling pressure by at least 65% (P less than 0.05). In the radial orientation, EDC treatment increased the stiffness by 75%, and did not significantly affect the permeability or swelling pressure. Some crosslinking treatments proved effective in increasing the stiffness and swelling pressure of the disc. The increased swelling pressure in G, PA, and EDC treatment groups relative to soaked controls suggests reduced GAG leaching during soaking treatment, further confirmed by the reduction in permeability in these groups.

Tissue engineering of the annulus fibrosus (AF) is challenging due to its complex lamellar structure. Polyurethane scaffolds have shown promise in AF tissue engineering. The current study examines whether matrix protein coatings (collagen type I, fibronectin, or vitronectin) would enhance cell attachment and promote cell and collagen orientation that more closely mimics native AF. The results demonstrate that the greatest cell attachment occurred with fibronectin (Fn)-coated scaffolds. Cells on Fn-coated scaffolds were also aligned parallel to scaffold fibers, a process that involved α5β1 integrin, determined by integrin-specific blocking antibodies. The inhibition of this integrin reduced AF cell spreading and alignment and the changes in cell shape were regulated by the actin cytoskeleton, demonstrated using cytochalasin D inhibitor. Cells on Fn-coated scaffolds formed fibrillar Fn, synthesized significantly more collagen, and showed alignment of type I collagen that more closely mimics native AF therefore facilitating the development of the tissue in vitro.

Tissue engineering of the annulus fibrosus (AF) is challenging due to its complex lamellar structure. Polyurethane scaffolds have shown promise in AF tissue engineering. The current study examines whether matrix protein coatings (collagen type I, fibronectin, or vitronectin) would enhance cell attachment and promote cell and collagen orientation that more closely mimics native AF. The results demonstrate that the greatest cell attachment occurred with fibronectin (Fn)-coated scaffolds. Cells on Fn-coated scaffolds were also aligned parallel to scaffold fibers, a process that involved α5β1 integrin, determined by integrin-specific blocking antibodies. The inhibition of this integrin reduced AF cell spreading and alignment and the changes in cell shape were regulated by the actin cytoskeleton, demonstrated using cytochalasin D inhibitor. Cells on Fn-coated scaffolds formed fibrillar Fn, synthesized significantly more collagen, and showed alignment of type I collagen that more closely mimics native AF therefore facilitating the development of the tissue in vitro.

Ce travail de thèse a pour ambition d'apporter une contribution à l'étude de la croissance du disque intervertébral. Il s'inscrit dans un projet de recherche qui vise à développer de nouveaux outils prédictifs afin d'améliorer le traitement de pathologies liées au phénomène de croissance des cartilages et des fibrocartilages. Un second objectif est d'apporter des éléments de compréhension qui pourraient être utiles pour le développement de techniques novatrices d'ingénierie tissulaire.La première partie expérimentale de ce travail consiste en l'identification de propriétés mécanique quasi-statique de l'annulus fibrosus du disque intervertébral, au travers d'une loi de comportement hyperélastique anisotrope, ainsi que d'un champ de déformations résiduelles présent au sein de l'annulus fibrosus, par le biais d'un outil de corrélation d'images numériques. Cette double identification permet à la fois d'estimer le champ de contrainte in vivo de l'annulus fibrosus, mais aussi d'obtenir une trace historique du processus de croissance qui nous servira de référence dans la suite de l'étude.La prise en compte des déformations résiduelles dans un modèle numérique d'annulus fibrosus a permis de constater l’homogénéisation spatiale des déformations lors de chargements physiologiques. Ces résultats soulignent l’importance de la considération des déformations résiduelles dans l’estimation des déformations et des contraintes subies in vivo par l’annulus fibrosus.De plus, un scénario de croissance de l'annulus fibrosus associé à deux critères mécaniques générateurs de croissance ont aussi été implémentés en utilisant la méthode des éléments finis. Les résultats de cette étude numérique n'ont pas permis de reproduire le champ de déformations résiduelles estimé expérimentalement. Seul le modèle de croissance utilisant un critère mécanique anisotrope prenant en compte la direction des fibres présentes dans l’annulus fibrosus et dans le cas d’un chargement omettant volontairement un chargement vertical sur l’annulus fibrosus a permis de reproduire qualitativement les déformations résiduelles tangentielles mesurées expérimentalement.Afin de compléter la compréhension du phénomène de croissance du cartilage, une étude biomécanique d'un modèle in vitro de cartilage de synthèse a mis en évidence l'impact du facteur de croissance TGF-beta3 sur la rigidité de la matrice extra-cellulaire. Une corrélation forte entre les expressions géniques des cellules de cartilage et les propriétés mécaniques de la matrice extra-cellulaire a été trouvée. Cette corrélation forte entre l’activité cellulaire et la rigidité de la matrice extra-cellulaire, couplée à la difficulté des modèles de croissance actuels uniquement basés sur des critères mécaniques, ouvre d’intéressantes perspectives d’études sur la compréhension du phénomène de croissance sous contrainte mécanique. L'étude approfondie de cet échantillon permettra à terme d'enrichir les modèles de croissance afin de prendre en compte les différents phénomènes physiques présent au sein des tissus biologiques. / This thesis aims to contribute to the study of the intervertebral disc growth. It is a part of a global reasearch program which focuses on the development of new predictive tools to improve treatments related to cartilage and fibrocartilage growth phenomenon pathologies. A second objective is to provide elements of understanding that could be useful for the development of innovative tissue engineering technics.The first experimental part of this work consists in the identification of quasi-static mechanical properties of the annulus fibrosus of the intervertebral disk, through an anisotropic hyperelastic constitutive law, as well as a residual strains field present within the annulus fibrosus, through a digital image correlation tool. This double identification makes it possible both to estimate the stress field in vivo of the annulus fibrosus, but also to obtain a historical trace of the growth process which will serve us as a reference in the rest of the study.We note the spatial homogenization of the deformations during physiological loadings when residual strains field is take into account in a numerical model of annulus fibrosus. These results underline the importance of the consideration of residual strains in the estimation of the deformations and stresses undergone by the annulus fibrosus.In addition, a growth scenario of the annulus fibrosus associated with two mechanical growth criterias has also been implemented using the finite element method. The results of this numerical study did not make it possible to reproduce the residual strains field estimated experimentally. Only the growth model using an anisotropic mechanical criterion taking into account the fibers direction present in the annulus fibrosus and in the case of a deliberately omitted load in the vertical direction on the annulus fibrosus has made it possible to qualitatively reproduce the tangential residual strains measured experimentally.In order to complete the understanding of cartilage growth, a biomechanical study of a synthetic in vitro cartilage model revealed the impact of the TGF-beta3 growth factor on the extracellular matrix cartilage stiffness. A strong correlation between the cartilage cell gene expressions and the mechanical properties of the extracellular matrix was found. This strong correlation between cellular activity and the rigidity of the extracellular matrix, coupled with the difficulty of the current growth models based solely on mechanical criteria, opens up interesting prospects for studies on the understanding of the phenomenon of growth under mechanical stimulus. In-depth study of this sample will eventually enrich the growth models to take into account the different physical phenomena present in biological tissues.

Low back pain represents a significant concern in the United States, with 70% of individuals experiencing symptoms at some point in their lifetime. Although the specific cause of low back pain remains unclear, symptoms have been strongly associated with degeneration of the intervertebral disc. Insufficient nutritional supply to the disc is believed to be a major mechanism for tissue degeneration. Understanding nutrients' transport in intervertebral disc is crucial to elucidate the mechanisms of disc degeneration, and to develop strategies for tissue repair (in vivo), and tissue engineering (in vitro). Transport in intervertebral disc is complex and involves a series of electromechanical, chemical and biological coupled events. Despite of the large amount of studies performed in the past, transport phenomena in the disc are still poorly understood. This is partly due to the limited number of available experimental techniques for investigating transport properties, and the paucity of theoretical or numerical methods for systematically predicting the mechanisms of solute transport in intervertebral disc. In this dissertation, a theoretical and experimental approach was taken in order to investigate the mechanisms of solute transport and binding interactions in intervertebral disc. New imaging techniques were developed for the experimental determination of diffusive and binding parameters in biological tissues. The techniques are based on the principle of fluorescence recovery after photobleaching, and allow the determination of the anisotropic diffusion tensor, and the rates of binding and unbinding of a solute to the extracellular matrix of a biological tissue. When applied to the characterization of transport properties of intervertebral disc, these methods allowed the establishment of a relationship between solute anisotropic and inhomogeneous diffusivity and the unique morphology of human lumbar annulus fibrosus. A mixture theory for charged hydrated soft tissues was presented as a framework for theoretical investigations on solute transport and binding interactions in cartilaginous tissues. Based on this theoretical framework and on experimental observations, a finite element model was developed to predict solute diffusive-convective-reactive transport in cartilaginous tissues. The numerical model was applied to simulate the effect of mechanical loading on solute transport and binding interactions in cartilage explants and intervertebral disc.

Replacing a diseased disc with a tissue engineered disc has the potential to restore normal spinal biomechanics. However, recreating the interface between annulus fibrosus (AF) and vertebral bone (VB) will be necessary to facilitate proper function of the implant in vivo. This study characterizes the native bovine AF-VB interface and assesses adult human discs. The AF insertion site in humans and cows is uniquely differentiated from other soft tissue-bone interfaces, as AF collagen fibers anchor into the calcified region of vertebral endplate through a zone of hyaline cartilage and have a different organization in inner and outer AF. Mineralization-associated proteins are present in this region and the chondroid tissue undergoes calcification over time. Based on these observations an in vitro AF culture system was developed and demonstrated that AF cells can induce mineralization. Understanding mechanism(s) regulating AF mineralization will help develop conditions to ensure proper integration of bioengineered AF.

Replacing a diseased disc with a tissue engineered disc has the potential to restore normal spinal biomechanics. However, recreating the interface between annulus fibrosus (AF) and vertebral bone (VB) will be necessary to facilitate proper function of the implant in vivo. This study characterizes the native bovine AF-VB interface and assesses adult human discs. The AF insertion site in humans and cows is uniquely differentiated from other soft tissue-bone interfaces, as AF collagen fibers anchor into the calcified region of vertebral endplate through a zone of hyaline cartilage and have a different organization in inner and outer AF. Mineralization-associated proteins are present in this region and the chondroid tissue undergoes calcification over time. Based on these observations an in vitro AF culture system was developed and demonstrated that AF cells can induce mineralization. Understanding mechanism(s) regulating AF mineralization will help develop conditions to ensure proper integration of bioengineered AF.

Tissue engineering the annulus fibrosus (AF) for use in a functional intervertebral disc replacement is a promising alternative to current treatments of degenerative disc disease. Polycarbonate urethane (PU) scaffolds have demonstrated the ability to support AF cell attachment and matrix synthesis and are suitable for tissue engineering the AF. The present study investigates the effects of the physical and biochemical environment on AF cells grown on aligned nanofibrous PU scaffolds. First, the effect of dynamic spinner flask culture and fibronectin pre-coating on tissue formation was analyzed and then the role of scaffold fibre tension on annulus fibrosus cells was examined using a tailored culture system. The results of these studies demonstrated that AF cells are sensitive to differences in biochemical cues at the scaffold surface and their physical environment and respond by altering their cellular responses and, potentially by manipulating their microenvironments, including the physical characteristics of the PU-ADO scaffolds.

Tissue engineering the annulus fibrosus (AF) for use in a functional intervertebral disc replacement is a promising alternative to current treatments of degenerative disc disease. Polycarbonate urethane (PU) scaffolds have demonstrated the ability to support AF cell attachment and matrix synthesis and are suitable for tissue engineering the AF. The present study investigates the effects of the physical and biochemical environment on AF cells grown on aligned nanofibrous PU scaffolds. First, the effect of dynamic spinner flask culture and fibronectin pre-coating on tissue formation was analyzed and then the role of scaffold fibre tension on annulus fibrosus cells was examined using a tailored culture system. The results of these studies demonstrated that AF cells are sensitive to differences in biochemical cues at the scaffold surface and their physical environment and respond by altering their cellular responses and, potentially by manipulating their microenvironments, including the physical characteristics of the PU-ADO scaffolds.