Relationship between solute transport properties and tissue morphology in human annulus fibrosus

Effect of compressive and tensile forces on glucose concentration and cell viability within the intervertebral disc: A finite element study

Understanding the role of mechanical force on tissue nutrient transport is essential, as sustained force may affect nutrient levels within the disc and initiate disc degeneration. This study aims to evaluate the time-dependent effects of different compressive force amplitudes as well as tensile force on glucose concentration and cell viability within the disc. Based on the mechano-electrochemical mixture theory, a multiphasic finite element model of the lumbar intervertebral disc was developed. The minimum glucose concentration and minimum cell density in both normal and degenerated discs were predicted for different compressive force amplitudes, tensile force, and corresponding creep time. Under high compressive force, the minimum glucose concentration exhibited an increasing and then decreasing trend with creep time in the normal disc, whereas that of the degenerated disc increased, then decreased, and finally increased again. At steady state, a higher compressive force was accompanied by a lower glucose concentration distribution. In the degenerated disc, the minimum cell density was negatively correlated with creep time, with a greater range of affected tissue under a higher compressive force. For tensile force, the minimum glucose concentration of the degenerated disc raised over time. This study highlighted the importance of creep time, force magnitude, and force type in affecting nutrient concentration and cell viability. Sustained weight-bearing activities could deteriorate the nutrient environment of the degenerated disc, while tensile force might have a nonnegligible role in effectively improving nutrient levels within the degenerated disc.

Unveiling interactions between intervertebral disc morphologies and mechanical behavior through personalized finite element modeling

Introduction: Intervertebral Disc (IVD) Degeneration (IDD) is a significant health concern, potentially influenced by mechanotransduction. However, the relationship between the IVD phenotypes and mechanical behavior has not been thoroughly explored in local morphologies where IDD originates. This work unveils the interplays among morphological and mechanical features potentially relevant to IDD through Abaqus UMAT simulations. Methods: A groundbreaking automated method is introduced to transform a calibrated, structured IVD finite element (FE) model into 169 patient-personalized (PP) models through a mesh morphing process. Our approach accurately replicates the real shapes of the patient's Annulus Fibrosus (AF) and Nucleus Pulposus (NP) while maintaining the same topology for all models. Using segmented magnetic resonance images from the former project MySpine, 169 models with structured hexahedral meshes were created employing the Bayesian Coherent Point Drift++ technique, generating a unique cohort of PP FE models under the Disc4All initiative. Machine learning methods, including Linear Regression, Support Vector Regression, and eXtreme Gradient Boosting Regression, were used to explore correlations between IVD morphology and mechanics. Results: We achieved PP models with AF and NP similarity scores of 92.06\% and 92.10\% compared to the segmented images. The models maintained good quality and integrity of the mesh. The cartilage endplate (CEP) shape was represented at the IVD-vertebra interfaces, ensuring personalized meshes. Validation of the constitutive model against literature data showed a minor relative error of 5.20%. Discussion: Analysis revealed the influential impact of local morphologies on indirect mechanotransduction responses, highlighting the roles of heights, sagittal areas, and volumes. While the maximum principal stress was influenced by morphologies such as heights, the disc's ellipticity influenced the minimum principal stress. Results suggest the CEPs are not influenced by their local morphologies but by those of the AF and NP. The generated free-access repository of individual disc characteristics is anticipated to be a valuable resource for the scientific community with a broad application spectrum.

Assessing the role of surface layer and molecular probe size in diffusion within meniscus tissue

Diffusion within extracellular matrix is essential to deliver nutrients and larger metabolites to the avascular region of the meniscus. It is well known that both structure and composition of the meniscus vary across its regions; therefore, it is crucial to fully understand how the heterogenous meniscal architecture affects its diffusive properties. The objective of this study was to investigate the effect of meniscal region (core tissue, femoral, and tibial surface layers) and molecular weight on the diffusivity of several molecules in porcine meniscus. Tissue samples were harvested from the central area of porcine lateral menisci. Diffusivity of fluorescein (MW 332 Da) and three fluorescence-labeled dextrans (MW 3k, 40k, and 150k Da) was measured via fluorescence recovery after photobleaching. Diffusivity was affected by molecular size, decreasing as the Stokes' radius of the solute increased. There was no significant effect of meniscal region on diffusivity for fluorescein, 3k and 40k dextrans (p>0.05). However, region did significantly affect the diffusivity of 150k Dextran, with that in the tibial surface layer being larger than in the core region (p = 0.001). Our findings contribute novel knowledge concerning the transport properties of the meniscus fibrocartilage. This data can be used to advance the understanding of tissue pathophysiology and explore effective approaches for tissue restoration.

Diffusivity of Human Cartilage Endplates in Healthy and Degenerated Intervertebral Disks

The cartilage endplates (CEPs) on the superior and inferior surfaces of the intervertebral disk (IVD), are the primary nutrient transport pathways between the disk and the vertebral body. Passive diffusion is responsible for transporting small nutrient and metabolite molecules through the avascular CEPs. The baseline solute diffusivities in healthy CEPs have been previously studied, however alterations in CEP diffusion associated with IVD degeneration remain unclear. This study aimed to quantitatively compare the solute diffusion in healthy and degenerated human CEPs using a fluorescence recovery after photobleaching (FRAP) approach. Seven healthy CEPs and 22 degenerated CEPs were collected from five fresh-frozen human cadaveric spines and 17 patients undergoing spine fusion surgery, respectively. The sodium fluorescein diffusivities in CEP radial and vertical directions were measured using the FRAP method. The CEP calcification level was evaluated by measuring the average X-ray attenuation. No difference was found in solute diffusivities between radial and axial directions in healthy and degenerated CEPs. Compared to healthy CEPs, the average solute diffusivity was 44% lower in degenerated CEPs (Healthy: 29.07 μm2/s (CI: 23.96-33.62 μm2/s); degenerated: 16.32 μm2/s (CI: 13.84-18.84 μm2/s), p < 0.001). The average solute diffusivity had an inverse relationship with the degree of CEP calcification as determined by the normalized X-ray attenuation values (ß = -22.19, R2 = 0.633; p < 0.001). This study suggests that solute diffusion through the disk and vertebral body interface is significantly hindered by CEP calcification, providing clues to help further understand the mechanism of IVD degeneration.

Improving Mechanical Properties of Tendon Allograft through Rehydration Strategies: An In Vitro Study

Allogenic tendons grafts sourced from intrasynovial tendons are often used for tendon reconstruction. Processing is achieved through repetitive freeze-thaw cycles followed by lyophilization. Soaking the lyophilized tendon in saline (0.9%) for 24 h is the standard practice for rehydration. However, data supporting saline rehydration over the use of other hydrating solutions are scant. The purpose of the current study was to compare the effects of different rehydration solutions on biomechanical properties of lyophilized tendon allograft. A total of 36 canine flexor digitorum profundus tendons were collected, five freeze-thaw cycles followed by lyophilization were performed for processing, and then divided into three groups rehydrated with either saline solution (0.9%), phosphate-buffered saline (PBS), or minimum essential medium (MEM). Flexural stiffness, tensile stiffness, and gliding friction were evaluated before and after allograft processing. The flexural moduli in both fibrous and fibrocartilaginous regions of the tendons were measured. After lyophilization and reconstitution, the flexural moduli of both the fibrocartilaginous and non-fibrocartilaginous regions of the tendons increase significantly in the saline and MEM groups (p < 0.05). Compared to the saline and MEM groups, the flexural moduli of the fibrocartilaginous and non-fibrocartilaginous regions of tendons rehydrated with PBS are significantly lower (p < 0.05). Tensile moduli of rehydrated tendons are significantly lower than those of fresh tendons for all groups (p < 0.05). The gliding friction of rehydrated tendons is significantly higher than that of fresh tendons in all groups (p < 0.05). There is no significant difference in either tensile moduli or gliding friction between tendons treated with different rehydration solutions. These results demonstrate that allograft reconstitution can be optimized through careful selection of hydrating solution and that PBS could be a better choice as the impact on flexural properties is lower.

Controversies in spine research: Organ culture versus in vivo models for studies of the intervertebral disc

Intervertebral disc degeneration is a common cause of low back pain, the leading cause of disability worldwide. Appropriate preclinical models for intervertebral disc research are essential to achieving a better understanding of underlying pathophysiology and for the development, evaluation, and translation of more effective treatments. To this end, in vivo animal and ex vivo organ culture models are both widely used by spine researchers; however, the relative strengths and weaknesses of these two approaches are a source of ongoing controversy. In this article, members from the Spine and Preclinical Models Sections of the Orthopedic Research Society, including experts in both basic and translational spine research, present contrasting arguments in support of in vivo animal models versus ex vivo organ culture models for studies of the disc, supported by a comprehensive review of the relevant literature. The objective is to provide a deeper understanding of the respective advantages and limitations of these approaches, and advance the field toward a consensus with respect to appropriate model selection and implementation. We conclude that complementary use of several model types and leveraging the unique advantages of each is likely to result in the highest impact research in most instances.

Strain-Dependent Diffusivity of Small and Large Molecules in Meniscus

Due to lack of full vascularization, the meniscus relies on diffusion through the extracellular matrix to deliver small (e.g., nutrients) and large (e.g., proteins) to resident cells. Under normal physiological conditions, the meniscus undergoes up to 20% compressive strains. While previous studies characterized solute diffusivity in the uncompressed meniscus, to date, little is known about the diffusive transport under physiological strain levels. This information is crucial to fully understand the pathophysiology of the meniscus. The objective of this study was to investigate strain-dependent diffusive properties of the meniscus fibrocartilage. Tissue samples were harvested from the central portion of porcine medial menisci and tested via fluorescence recovery after photobleaching to measure diffusivity of fluorescein (332 Da) and 40 K Da dextran (D40K) under 0%, 10%, and 20% compressive strain. Specifically, average diffusion coefficient and anisotropic ratio, defined as the ratio of the diffusion coefficient in the direction of the tissue collagen fibers to that orthogonal, were determined. For all the experimental conditions investigated, fluorescein diffusivity was statistically faster than that of D40K. Also, for both molecules, diffusion coefficients significantly decreased, up to ∼45%, as the strain increased. In contrast, the anisotropic ratios of both molecules were similar and not affected by the strain applied to the tissue. This suggests that compressive strains used in this study did not alter the diffusive pathways in the meniscus. Our findings provide new knowledge on the transport properties of the meniscus fibrocartilage that can be leveraged to further understand tissue pathophysiology and approaches to tissue restoration.

EFFECTS OF SOLUTE SIZE AND TISSUE COMPOSITION ON MOLECULAR AND MACROMOLECULAR DIFFUSIVITY IN HUMAN KNEE CARTILAGE

Objective

Articular cartilage is an avascular tissue. Accordingly, diffusivity represents a fundamental transport mechanism for nutrients and other molecular signals regulating its cell metabolism and maintenance of the extracellular matrix. Understanding how solutes spread into articular cartilage is crucial to elucidating its pathologies, and to designing treatments for repair and restoration of its extracellular matrix. As in other connective tissues, diffusivity in articular cartilage may vary depending both its composition and the specific diffusing solute. Hence, this study investigated the roles of solute size and tissue composition on molecular diffusion in knee articular cartilage.

Design

FRAP tests were conducted to measure diffusivity of five molecular probes, with size ranging from ~332Da to 70,000Da, in human knee articular cartilage. The measured diffusion coefficients were related to molecular size, as well as water and glycosaminoglycan (GAG) content of femoral and tibial condyle cartilage.

Results

Diffusivity was affected by molecular size, with the magnitude of the diffusion coefficients decreasing as the Stokes radius of the probe increased. The values of diffusion coefficients in tibial and femoral samples were not significantly different from one another, despite the fact that tibial samples exhibited significantly higher water content and lower GAG content of the femoral specimens. Water content did not affect diffusivity. In contrast, diffusivities of large molecules were sensitive to GAG content.

Conclusions

This study provides new knowledge on the mechanisms of diffusion in articular cartilage. Our findings can be leveraged to further investigate osteoarthritis and to design treatments for cartilage restoration or replacement.

Tendon Multiscale Structure, Mechanics, and Damage Are Affected by Osmolarity of Bath Solution

One of the most common bath solutions used in musculoskeletal mechanical testing is phosphate buffered saline (PBS). In tendon, swelling induced by physiological PBS results in decreased tendon modulus and induces microstructural changes. It is critical to evaluate the multiscale mechanical behavior of tendon under swelling to interpret prior work and provide information to design future studies. We compared the effects of physiological PBS and 8% polyethylene glycol and saline bathing solutions on tendon multiscale tendon mechanics and damage as well as microstructure with TEM in order to understand the effect of swelling on tendon. At the tissue level, tendons in PBS had a lower modulus than SPEG samples. PBS samples also showed an increased amount of non-recoverable sliding, which is an analog for microscale damage. SPEG had a higher microscale to tissue-scale strain ratio, showing the fibrils experienced less strain attenuation. From the TEM data, we showed the fibril spacing of SPEG samples was more similar to fresh control than PBS. We concluded that swelling alters multiscale mechanics and damage in addition to tendon microstructure. Future mechanical testing should consider using SPEG as a bath solution with an osmotic pressure which preserves fresh tissue water content.

Molecular and macromolecular diffusion in human meniscus: relationships with tissue structure and composition

OBJECTIVE

To date, the pathophysiology of the meniscus has not been fully elucidated. Due to the tissue's limited vascularization, nutrients and other molecular signals spread through the extracellular matrix via diffusion or convection (interstitial fluid flow). Understanding transport mechanisms is crucial to elucidating meniscal pathophysiology, and to designing treatments for repair and restoration of the tissue. Similar to other fibrocartilaginous structures, meniscal morphology and composition may affect its diffusive properties. The objective of this study was to investigate the role of solute size, and tissue structure and composition on molecular diffusion in meniscus tissue.

DESIGN

Using a custom FRAP technique developed in our lab, we measured the direction-dependent diffusivity in human meniscus of six different molecular probes of size ranging from ∼300Da to 150,000Da. Diffusivity measurements were related to sample water content. SEM images were used to investigate collagen structure in relation to transport mechanisms.

RESULTS

Diffusivity was anisotropic, being significantly faster in the direction parallel to collagen fibers when compared the orthogonal direction. This was likely due to the unique structural organization of the tissue presenting pores aligned with the fibers, as observed in SEM images. Diffusion coefficients decreased as the molecular size increased, following the Ogston model. No significant correlations were found among diffusion coefficients and water content of the tissue.

CONCLUSIONS

This study provides new knowledge on the mechanisms of molecular transport in meniscal tissue. The reported results can be leveraged to further investigate tissue pathophysiology and to design treatments for tissue restoration or replacement.

Nutrient supply and nucleus pulposus cell function: effects of the transport properties of the cartilage endplate and potential implications for intradiscal biologic therapy

OBJECTIVE

Intradiscal biologic therapy is a promising strategy for managing intervertebral disc degeneration. However, these therapies require a rich nutrient supply, which may be limited by the transport properties of the cartilage endplate (CEP). This study investigated how fluctuations in CEP transport properties impact nutrient diffusion and disc cell survival and function.

DESIGN

Human CEP tissues harvested from six fresh cadaveric lumbar spines (38-66 years old) were placed at the open sides of diffusion chambers. Bovine nucleus pulposus (NP) cells cultured inside the chambers were nourished exclusively by nutrients diffusing through the CEP tissues. After 72 h in culture, depth-dependent NP cell viability and gene expression were measured, and related to CEP transport properties and biochemical composition determined using fluorescence recovery after photobleaching and Fourier transform infrared (FTIR) spectroscopy.

RESULTS

Solute diffusivity varied nearly 4-fold amongst the CEPs studied, and chambers with the least permeable CEPs appeared to have lower aggrecan, collagen-2, and matrix metalloproteinase-2 gene expression, as well as a significantly shorter viable distance from the CEP/nutrient interface. Increasing chamber cell density shortened the viable distance; however, this effect was lost for low-diffusivity CEPs, which suggests that these CEPs may not provide enough nutrient diffusion to satisfy cell demands. Solute diffusivity in the CEP was associated with biochemical composition: low-diffusivity CEPs had greater amounts of collagen and aggrecan, more mineral, and lower cross-link maturity.

CONCLUSIONS

CEP transport properties dramatically affect NP cell survival/function. Degeneration-related CEP matrix changes could hinder the success of biologic therapies that require increased nutrient supply.

Mechanotransduction and cell biomechanics of the intervertebral disc

Mechanical loading of the intervertebral disc (IVD) initiates cell-mediated remodeling events that contribute to disc degeneration. Cells of the IVD, nucleus pulposus (NP) and anulus fibrosus (AF), will exhibit various responses to different mechanical stimuli which appear to be highly dependent on loading type, magnitude, duration, and anatomic zone of cell origin. Cells of the NP, the innermost region of the disc, exhibit an anabolic response to low-moderate magnitudes of static compression, osmotic pressure, or hydrostatic pressure, while higher magnitudes promote a catabolic response marked by increased protease expression and activity. Cells of the outer AF are responsive to physical forces in a manner that depends on frequency and magnitude, as are cells of the NP, though they experience different forces, deformations, pressure, and osmotic pressure in vivo. Much remains to be understood of the mechanotransduction pathways that regulate IVD cell responses to loading, including responses to specific stimuli and also differences among cell types. There is evidence that cytoskeletal remodeling and receptor-mediated signaling are important mechanotransduction events that can regulate downstream effects like gene expression and posttranslational biosynthesis, all of which may influence phenotype and bioactivity. These and other mechanotransduction events will be regulated by known and to-be-discovered cell-matrix and cell-cell interactions, and depend on composition of extracellular matrix ligands for cell interaction, matrix stiffness, and the phenotype of the cells themselves. Here, we present a review of the current knowledge of the role of mechanical stimuli and the impact upon the cellular response to loading and changes that occur with aging and degeneration of the IVD.

Diffusion of antibiotics in intervertebral disc

Delivering charged antibiotics to the intervertebral disc is challenging because of the avascular, negatively charged extracellular matrix (ECM) of the tissue. The purpose of this study was to measure the apparent diffusion coefficient of two clinically relevant, charged antibiotics, vancomycin (positively charged) and oxacillin (negatively charged) in IVD. A one-dimensional steady state diffusion experiment was employed to measure the apparent diffusion coefficient of the two antibiotics in bovine coccygeal annulus fibrosus (AF) tissue. The averaged apparent diffusion coefficient for vancomycin under 20% compressive strain was 7.94 ± 2.00 × 10^-12 m²/s (n = 10), while that of oxacillin was 2.26 ± 0.68 × 10^-10 m²/s (n = 10). A student's t-test showed that the diffusivity of vancomycin was significantly lower than that of oxacillin. This finding may be attributed to two factors: solute size and possible binding effects. Vancomycin is approximately 3 times larger in molecular weight than oxacillin, meaning that steric hindrance likely plays a role in the slower transport. Reversible binding between positive vancomycin and the negative ECM could also slow down the rate of diffusion. Therefore, more investigation is necessary to determine the specific relationship between net charge on antibiotic and diffusion coefficients in IVD. This study provides essential quantitative information regarding the transport rates of antibiotics in the IVD, which is critical in using computational modeling to design effective strategies to treat disc infection.

Intervertebral Disk Nutrients and Transport Mechanisms in Relation to Disk Degeneration: A Narrative Literature Review

Objective

The purpose of this paper was to review the literature regarding the mechanisms leading to degeneration in intervertebral disks and to discuss contributing mechanical and biological factors.

Methods

The inclusion criteria for the literature review were research studies conducted in the last 3 decades with free full-text available in English. Review articles and articles pertaining to temporomandibular joints and joints of the body other than the intervertebral disk were excluded. The following databases were searched: PubMed, EBSCOhost, and Google Scholar through September 9, 2016.

Results

A total of 57 articles were used in this review. Intervertebral disk cells require glucose for sustainability and oxygen to synthesize matrix components. Nutrients enter the disk via 2 vascular supply routes: capillary beds of end plates and the peripheral annulus fibrosus. Solute size, shape and charge, compression, and metabolic demand all influence the efficiency of nutrient transport, and alterations of any of these factors may have effects on nutrient transport and, potentially, disk degeneration.

Conclusions

Progressive nutrient transport disruptions may actively contribute in advancing the phases of degenerative disk disease. Such disruptions include dysfunctional loading and spinal position, lack of motion, high frequency loading, disk injury, aging, smoking, an acidic environment, and a lack of nutrient bioavailability.

Exposure to buffer solution alters tendon hydration and mechanics

A buffer solution is often used to maintain tissue hydration during mechanical testing. The most commonly used buffer solution is a physiological concentration of phosphate buffered saline (PBS); however, PBS increases the tissue's water content and decreases its tensile stiffness. In addition, solutes from the buffer can diffuse into the tissue and interact with its structure and mechanics. These bathing solution effects can confound the outcome and interpretation of mechanical tests. Potential bathing solution artifacts, including solute diffusion, and their effect on mechanical properties, are not well understood. The objective of this study was to measure the effects of long-term exposure of rat tail tendon fascicles to several concentrations (0.9-25%) of NaCl, sucrose, polyethylene glycol (PEG), and SPEG (NaCl+PEG) solutions on water content, solute diffusion, and mechanical properties. We found that with an increase in solute concentration the apparent water content decreased for all solution types. Solutes diffused into the tissue for NaCl and sucrose, however, no solute diffusion was observed for PEG or SPEG. The mechanical properties changed for both NaCl solutions, in particular after long-term (8h) incubation the modulus and equilibrium stress decreased compared to short-term (15min) for 25% NaCl, and the cross sectional area increased for 0.9% NaCl. However, the mechanical properties were unchanged for both PEG and SPEG except for minor alterations in stress relaxation parameters. This study shows that NaCl and sucrose buffer solutions are not suitable for long-term mechanical tests. We therefore propose using PEG or SPEG as alternative buffer solutions that after long-term incubation can maintain tissue hydration without solute diffusion and produce a consistent mechanical response.

Effect of Strain, Region, and Tissue Composition on Glucose Partitioning in Meniscus Fibrocartilage

A nearly avascular tissue, the knee meniscus relies on diffusive transport for nutritional supply to cells. Nutrient transport depends on solute partitioning in the tissue, which governs the amount of nutrients that can enter a tissue. The purpose of the present study was to investigate the effects of mechanical strain, tissue region, and tissue composition on the partition coefficient of glucose in meniscus fibrocartilage. A simple partitioning experiment was employed to measure glucose partitioning in porcine meniscus tissues from two regions (horn and central), from both meniscal components (medial and lateral), and at three levels of compression (0%, 10%, and 20%). Partition coefficient values were correlated to strain level, water volume fraction, and glycosaminoglycan (GAG) content of tissue specimens. Partition coefficient values ranged from 0.47 to 0.91 (n = 48). Results show that glucose partition coefficient is significantly (p < 0.001) affected by compression, decreasing with increasing strain. Furthermore, we did not find a statistically significant effect of tissue when comparing medial versus lateral (p = 0.181) or when comparing central and horn regions (p = 0.837). There were significant positive correlations between tissue water volume fraction and glucose partitioning for all groups. However, the correlation between GAG content and partitioning was only significant in the lateral horn group. Determining how glucose partitioning is affected by tissue composition and loading is necessary for understanding nutrient availability and related tissue health and/or degeneration. Therefore, this study is important for better understanding the transport and nutrition-related mechanisms of meniscal degeneration.

Region-media coupling in characterization and modelling of the disc annulus single lamella swelling

The annulus fibrosus (AF) swelling property, which is correlated with its rheological and viscoelastic properties, plays a significant role in disc nutrition and mechanical loading justification during daily activities as well as designing scaffolds for tissue engineering applications. The objective of this study was twofold: firstly to characterize the AF single lamella swelling kinetics in different regions and solutions at the temperature range of 35-40 °C and secondly to use the swelling results as a baseline to model (independent to swelling media and anatomic region) the AF single lamella swelling. It was found that the AF single lamella swelling kinetics (equilibrium swelling ratio and swelling rate) depends on anatomic region and swelling media; however, its trend for different swelling media (ionic and molecular solution) is similar and the majority of hydration occurs during first 20% of equilibrium swelling time (about 20 min). Change in swelling rate constant in circumferential direction depends on the solution type. It decreases from anterior to lateral regions for water, PBS and glucose solution and remains constant-or its change is negligible-from lateral to posterolateral regions. The effect of temperature (in the range of 35-40 °C) on swelling kinetics was seen to be negligible. It was also understood that it is possible to present a model (independent to swelling media type) to predict the swelling kinetics of posterior and posterolateral AF single lamella, as these locations are less sensitive to the swelling media.

A computational model for investigating the effects of changes in bioavailability of insulin-like growth factor-1 on the homeostasis of the intervertebral disc

Insulin-like growth factor-1 (IGF-1) is well-known for upregulating cell proliferation and biosynthesis of the extracellular matrix in the intervertebral disc (IVD). Pathological conditions, such as obesity or chronic kidney disease cause IGF-1 deficiency in plasma. How this deficiency impacts disc homeostasis remains unknown. Pro-anabolic approaches for the treatment of disc degeneration based on enhancing IGF-1 bioavailability to tissue-cells are considered, but knowledge of their effectiveness in enhancing cellular anabolism of a degenerated disc is limited. In this study, we developed a computational model for disc homeostasis specifically addressing the role of IGF-1 in modulating both extracellular matrix biosynthesis and cellularity in the IVD. This model was applied to investigate how changes in IGF-1 bioavailability, namely deficiency or enhancement of growth factor, affect disc health. In this study, it was found that IGF-1 deficiency mainly affects the biosynthesis of ECM components, especially in the most external regions of the IVD such as the cartilage endplates and the outer portion of annulus fibrosus. Also, a total of three approaches for increasing IGF-1 bioavailability as a therapy for degenerated IVDs were investigated. It was found that all these strategies are only beneficial to those disc regions receiving sufficient nutritional supply (i.e., the outmost IVD regions), while they exacerbate tissue degradation in malnourished regions (i.e., inner portion of the disc). This suggests that pro-anabolic growth factor-based therapies are limited in that their success strongly depends on an adequate nutritional supply to the IVD tissue, which is not guaranteed in degenerated discs.

Effects of Tobacco Smoking on the Degeneration of the Intervertebral Disc: A Finite Element Study

Tobacco smoking is associated with numerous pathological conditions. Compelling experimental evidence associates smoking to the degeneration of the intervertebral disc (IVD). In particular, it has been shown that nicotine down-regulates both the proliferation rate and glycosaminoglycan (GAG) biosynthesis of disc cells. Moreover, tobacco smoking causes the constriction of the vascular network surrounding the IVD, thus reducing the exchange of nutrients and anabolic agents from the blood vessels to the disc. It has been hypothesized that both nicotine presence in the IVD and the reduced solute exchange are responsible for the degeneration of the disc due to tobacco smoking, but their effects on tissue homeostasis have never been quantified. In this study, a previously presented computational model describing the homeostasis of the IVD was deployed to investigate the effects of impaired solute supply and nicotine-mediated down-regulation of cell proliferation and biosynthetic activity on the health of the disc. We found that the nicotine-mediated down-regulation of cell anabolism mostly affected the GAG concentration at the cartilage endplate, reducing it up to 65% of the value attained in normal physiological conditions. In contrast, the reduction of solutes exchange between blood vessels and disc tissue mostly affected the nucleus pulposus, whose cell density and GAG levels were reduced up to 50% of their normal physiological levels. The effectiveness of quitting smoking on the regeneration of a degenerated IVD was also investigated, and showed to have limited benefit on the health of the disc. A cell-based therapy in conjunction with smoke cessation provided significant improvements in disc health, suggesting that, besides quitting smoking, additional treatments should be implemented in the attempt to recover the health of an IVD degenerated by tobacco smoking.

Effect of Static Compressive Strain, Anisotropy, and Tissue Region on the Diffusion of Glucose in Meniscus Fibrocartilage

Osteoarthritis (OA) is a significant socio-economic concern, affecting millions of individuals each year. Degeneration of the meniscus of the knee is often associated with OA, yet the relationship between the two is not well understood. As a nearly avascular tissue, the meniscus must rely on diffusive transport for nutritional supply to cells. Therefore, quantifying structure–function relations for transport properties in meniscus fibrocartilage is an important task. The purpose of the present study was to determine how mechanical loading, tissue anisotropy, and tissue region affect glucose diffusion in meniscus fibrocartilage. A one-dimensional (1D) diffusion experiment was used to measure the diffusion coefficient of glucose in porcine meniscus tissues. Results show that glucose diffusion is strain-dependent, decreasing significantly with increased levels of compression. It was also determined that glucose diffusion in meniscus tissues is anisotropic, with the diffusion coefficient in the circumferential direction being significantly higher than that in the axial direction. Finally, the effect of tissue region was not statistically significant, comparing axial diffusion in the central and horn regions of the tissue. This study is important for better understanding the transport and nutrition-related mechanisms of meniscal degeneration and related OA in the knee.

Modeling the role of IGF-1 on extracellular matrix biosynthesis and cellularity in intervertebral disc

The insulin-like growth factor-1 (IGF-1) is a well-known anabolic agent for intervertebral disc (IVD), promoting both proteoglycan (PG) biosynthesis and cell proliferation. Accordingly, it is believed that IGF-1 may play a central role in IVD homeostasis. Furthermore, the exogenous administration of IGF-1 has been proposed as a possible therapeutic strategy for disc degeneration. The objectives of this study were to develop a new computational framework for describing the mechanisms regulating IGF-mediated homeostasis in IVD, and to apply this numerical tool for investigating the effectiveness of exogenous administration of IGF-1 for curing disc degeneration. A diffusive-reactive model was developed for describing competitive binding of IGF-1 to its binding proteins and cell surface receptors, with the latter reaction initiating the intracellular signaling mechanism leading to PG production and cell proliferation. Because PG production increases cell metabolic rate, and cell proliferation increases nutritional demand, nutrients transport and metabolism were also included into the model, and co-regulated, together with IGF-1, IVD cellularity. The sustainability and the effectiveness of IGF-mediated anabolism were investigated for conditions of pathologically insufficient nutrient supply, and for the case of exogenous administration of IGF-1 to degenerated IVD. Results showed that pathological nutrients deprivation, by decreasing cellularity, caused a reduction of PG biosynthesis. Also, exogenous administration of IGF-1 was only beneficial in well-nourished regions of IVD, and exacerbated cell mortality in malnourished regions. These findings remark the central role of nutrition in IVD health, and suggest that adequate nutritional supply is paramount for achieving a successful IGF-based therapy for disc degeneration.

Spinal traction promotes molecular transportation in a simulated degenerative intervertebral disc model

STUDY DESIGN

Biomechanical experiment using an in situ porcine model.

OBJECTIVE

To find the effect of traction treatment on annulus microstructure, molecular convection, and cell viability of degraded discs.

SUMMARY OF BACKGROUND DATA

Spinal traction is a conservative treatment for disc disorders. The recognized biomechanical benefits include disc height recovery, foramen enlargement, and intradiscal pressure reduction. However, the influence of traction treatment on annulus microstructure, molecular transportation, and cell viability of degraded discs has not been fully investigated.

METHODS

A total of 48 thoracic discs were dissected from 8 porcine spines (140 kg, 6-month old) within 4 hours after killing them and then divided into 3 groups: intact, degraded without traction, and degraded with traction. Each disc was incubated in a whole-organ culture system and subjected to diurnal loadings for 7 days. Except for the intact group, discs were degraded with 0.5 mL of trypsin on day 1 and a 5-hour fatigue loading on day 2. From day 4 to day 6, half of the degraded discs received a 30-minute traction treatment per day (traction force: 20 kg; loading: unloading = 30 s: 10 s). By the end of the incubation, the discs were inspected for disc height loss, annulus microstructure, molecular (fluorescein sodium) intensity, and cell viability.

RESULTS

Collagen fibers were crimped and delaminated, whereas the pores were occluded in the annulus fibrosus of the degraded discs. Molecular transportation and cell viability of the discs decreased after matrix degradation. With traction treatment, straightened collagen fibers increased within the degraded annulus fibrosus, and the annulus pores were less occluded. Both molecular transportation and cell viability increased, but not to the intact level.

CONCLUSION

Traction treatment is effective in enhancing nutrition supply and promoting disc cell proliferation of the degraded discs.

LEVEL OF EVIDENCE

N/A.

Measurement of three-dimensional anisotropic diffusion by multiphoton fluorescence recovery after photobleaching

The multiphoton fluorescence recovery after photobleaching (MP-FRAP) technique has been developed to measure the three-dimensional (3D) solute diffusion within biological systems. However, current 3D MP-FRAP models are based on isotropic diffusion and spatial domain analysis. The 3D anisotropic diffusion and frequency domain analysis for MP-FRAP measurements are rarely studied. In this study, a new technique is demonstrated for the quantitative and non-destructive determination of 3D anisotropic solute diffusion tensors within biological fibrosis tissues by multiphoton photobleaching and spatial Fourier analysis (SFA). Compared to the spatial domain analysis based MP-FRAP techniques, this SFA-based method has the capability for determining the 3D anisotropic diffusion tensors as well as the flexibility for satisfying initial and boundary conditions. First, a close-form solution of the 3D anisotropic diffusion equation is derived by solely using SFA. Next, this new method is validated by computer-simulated MP-FRAP experiments with pre-defined 3D anisotropic diffusion tensors as well as experimental diffusion measurements of FITC-Dextran (FD) molecules in aqueous glycerol solutions. Finally, this MP-FRAP technique is applied to the measurement of 3D anisotropic diffusion tensors of FD molecules within porcine tendon tissues. This study provides a new tool for complete determination of 3D anisotropic solute diffusion tensor in biological tissues.

Oligomers modulate interfibril branching and mass transport properties of collagen matrices

Mass transport within collagen-based matrices is critical to tissue development, repair, and pathogenesis, as well as the design of next-generation tissue engineering strategies. This work shows how collagen precursors, specified by intermolecular cross-link composition, provide independent control of collagen matrix mechanical and transport properties. Collagen matrices were prepared from tissue-extracted monomers or oligomers. Viscoelastic behavior was measured in oscillatory shear and unconfined compression. Matrix permeability and diffusivity were measured using gravity-driven permeametry and integrated optical imaging, respectively. Both collagen types showed an increase in stiffness and permeability hindrance with increasing collagen concentration (fibril density); however, different physical property–concentration relationships were noted. Diffusivity was not affected by concentration for either collagen type over the range tested. In general, oligomer matrices exhibited a substantial increase in stiffness and only a modest decrease in transport properties when compared with monomer matrices prepared at the same concentration. The observed differences in viscoelastic and transport properties were largely attributed to increased levels of interfibril branching within oligomer matrices. The ability to relate physical properties to relevant microstructure parameters, including fibril density and interfibril branching, is expected to advance the understanding of cell–matrix signaling, as well as facilitate model-based prediction and design of matrix-based therapeutic strategies.

Relationship between anisotropic diffusion properties and tissue morphology in porcine TMJ disc

OBJECTIVE

To investigate the relationship between anisotropic solute diffusion properties and tissue morphology in porcine temporomandibular joint (TMJ) discs.

DESIGN

TMJ discs from eleven pigs aged 6-8 months were divided into five regions: anterior, intermediate, posterior, lateral, and medial. The transport properties and tissue morphology were investigated in three orthogonal orientations: anteroposterior (AP), mediolateral (ML), and superoinferior (SI). The anisotropic diffusivity of fluorescein (332 Da) in the right discs was determined by the fluorescence recovery after photobleaching (FRAP) protocols. The tissue morphology in the left discs was quantified by scanning electron microscopy.

RESULTS

The diffusivities of fluorescein in the TMJ disc were significantly anisotropic, except for the anterior region. In the medial, intermediate, and lateral regions, the diffusion along the fiber orientation (i.e., AP direction) was significantly faster than the diffusion in ML and SI directions. In the posterior region, the diffusion along the fiber orientation (i.e., ML direction) was significantly faster than the diffusion in AP and SI directions. The diffusion in the anterior region was mostly isotropic with the lowest degree of diffusion anisotropy, as well as collagen fiber alignment, likely due to the multi-directional fiber arrangement. The anterior region had the highest mean diffusivity [65.6 (49.3-81.8) μm(2)/s] in the disc, likely due to its high water content. The overall average diffusivity of fluorescein across the TMJ disc was 57.0 (43.0-71.0) μm(2)/s.

CONCLUSIONS

The solute diffusion in porcine TMJ discs was strongly anisotropic and inhomogeneous, which associated with tissue structure (i.e., collagen fiber alignment) and composition (e.g., water content).

Nutrient transport in human annulus fibrosus is affected by compressive strain and anisotropy

The avascular intervertebral disc (IVD) receives nutrition via transport from surrounding vasculature; poor nutrition is believed to be a main cause of disc degeneration. In this study, we investigated the effects of mechanical deformation and anisotropy on the transport of two important nutrients--oxygen and glucose--in human annulus fibrosus (AF). The diffusivities of oxygen and glucose were measured under three levels of uniaxial confined compression--0, 10, and 20%--and in three directions--axial, circumferential, and radial. The glucose partition coefficient was also measured at three compression levels. Results for glucose and oxygen diffusivity in AF ranged from 4.46 × 10(-7) to 9.77 × 10(-6) cm(2)/s and were comparable to previous studies; the glucose partition coefficient ranged from 0.71 to 0.82 and was also similar to previous results. Transport properties were found to decrease with increasing deformation, likely caused by fluid exudation during tissue compression and reduction in pore size. Furthermore, diffusivity in the radial direction was lower than in the axial or circumferential directions, indicating that nutrient transport in human AF is anisotropic. This behavior is likely a consequence of the layered structure and unique collagen architecture of AF tissue. These findings are important for better understanding nutritional supply in IVD and related disc degeneration.

Strain tunes proteolytic degradation and diffusive transport in fibrin networks

Proteolytic degradation of fibrin, the major structural component in blood clots, is critical both during normal wound healing and in the treatment of ischemic stroke and myocardial infarction. Fibrin-containing clots experience substantial strain due to platelet contraction, fluid shear, and mechanical stress at the wound site. However, little is understood about how mechanical forces may influence fibrin dissolution. We used video microscopy to image strained fibrin clots as they were degraded by plasmin, a major fibrinolytic enzyme. Applied strain causes up to 10-fold reduction in the rate of fibrin degradation. Analysis of our data supports a quantitative model in which the decrease in fibrin proteolysis rates with strain stems from slower transport of plasmin into the clot. We performed fluorescence recovery after photobleaching (FRAP) measurements to further probe the effect of strain on diffusive transport. We find that diffusivity perpendicular to the strain axis decreases with increasing strain, while diffusivity along the strain axis remains unchanged. Our results suggest that the properties of the fibrin network have evolved to protect mechanically loaded fibrin from degradation, consistent with its function in wound healing. The pronounced effect of strain upon diffusivity and proteolytic susceptibility within fibrin networks offers a potentially useful means of guiding cell growth and morphology in fibrin-based biomaterials.

Intervertebral disk nutrition: a review of factors influencing concentrations of nutrients and metabolites

The biomechanical behavior of the intervertebral disk ultimately depends on the viability and activity of a small population of resident cells that make and maintain the disk's extracellular matrix. Nutrients that support these cells are supplied by the blood vessels at the disks' margins and diffuse through the matrix of the avascular disk to the cells. This article reviews pathways of nutrient supply to these cells; examines factors that may interrupt these pathways, and discusses consequences for disk cell survival, disk degeneration, and disk repair.

Complex loading affects intervertebral disc mechanics and biology

BACKGROUND

Complex loading develops in multiple spinal motions and in the case of hyperflexion is known to cause intervertebral disc (IVD) injury. Few studies have examined the interacting biologic and structural alterations associated with potentially injurious complex loading, which may be an important contributor to chronic progressive degeneration.

OBJECTIVE

This study tested the hypothesis that low magnitudes of axial compression loading applied asymmetrically can induce IVD injury affecting cellular and structural responses in a large animal IVD ex-vivo model.

METHODS

Bovine caudal IVDs were assigned to either a control or wedge group (15°) and placed in organ culture for 7 days under static 0.2MPa load. IVD tissue and cellular responses were assessed through confined compression, qRT-PCR, histology and structural and compositional measurements, including Western blot for aggrecan degradation products.

RESULTS

Complex loading via asymmetric compression induced cell death, an increase in caspase-3 staining (apoptosis), a loss of aggrecan and an increase in aggregate modulus in the concave annulus fibrosis. While an up-regulation of MMP-1, ADAMTS4, IL-1β, and IL-6 mRNA, and a reduced aggregate modulus were induced in the convex annulus.

CONCLUSION

Asymmetric compression had direct deleterious effects on both tissue and cells, suggesting an injurious loading regime that could lead to a degenerative cascade, including cell death, the production of inflammatory mediators, and a shift towards catabolism. This explant model is useful to assess how injurious mechanical loading affects the cellular response which may contribute to the progression of degenerative changes in large animal IVDs, and results suggest that interventions should address inflammation, apoptosis, and lamellar integrity.

Measurement of diffusion in articular cartilage using fluorescence correlation spectroscopy

BACKGROUND

Fluorescence correlation spectroscopy (FCS) provides information about translational diffusion of fluorescent molecules in tiny detection volumes at the single-molecule level. In normal states, cartilage tissue lacks vascularity, so chondrocyte metabolism depends on diffusion for molecular exchanges. The abundant extracellular matrix (ECM) of cartilage is maintained by a limited number of chondrocytes. ECM plays an important role in the regulation of chondrocyte functions. In this study, FCS was used to measure diffusion behaviors of albumin, the major protein of the intra-articular space, using normal and degenerated cartilage. Preliminary investigation of fluorescence dyes including Alexa 488, Rhodamine 6G and Rhodamine 123 was conducted to evaluate their properties in cartilage.

RESULTS

The results indicate that the diffusion behaviors of fluorescently labeled albumin can be observed using FCS in both normal and chemically degenerated cartilage.

CONCLUSIONS

This work demonstrates the capability of FCS for direct measurement of diffusion in cartilaginous ECM. When the diffusion characteristics of fluorescent probes in ECM are clarified using FCS evaluation, FCS will be applicable as a method for early diagnosis of osteoarthritis, which is accompanied by increased abnormalities of ECM and also as tool for evaluating bio-engineered artificial cartilage for autologous chondrocyte implantation.

Simultaneous measurement of anisotropic solute diffusivity and binding reaction rates in biological tissues by FRAP

Several solutes (e.g., growth factors, cationic solutes, etc.) can reversibly bind to the extracellular matrix (ECM) of biological tissues. Binding interactions have significant implications on transport of such solutes through the ECM. In order to fully delineate transport phenomena in biological tissues, knowledge of binding kinetics is crucial. In this study, a new method for the simultaneous determination of solute anisotropic diffusivity and binding reaction rates was presented. The new technique was solely based on Fourier analysis of fluorescence recovery after photobleaching (FRAP) images. Computer-simulated FRAP tests were used to assess the sensitivity and the robustness of the method to experimental parameters, such as anisotropic solute diffusivity and rates of binding reaction. The new method was applied to the determination of diffusivity and binding rates of 5-dodecanoylaminofluorescein (DAF) in bovine coccygeal annulus fibrosus (AF). Our findings indicate that DAF reversibly binds to the ECM of AF. In addition, it was found that DAF diffusion in AF is anisotropic. The results were in agreement with those reported in previous studies. This study provides a new tool for the simultaneous determination of solute anisotropic diffusion tensor and rates of binding reaction that can be used to investigate diffusive-reactive transport in biological tissues and tissue engineered constructs.

Anisotropic solute diffusion tensor in porcine TMJ discs measured by FRAP with spatial Fourier analysis

A new method solely based on spatial Fourier analysis (SFA) was developed to completely determine a two-dimensional (2D) anisotropic diffusion tensor in fibrous tissues using fluorescence recovery after photobleaching (FRAP). The accuracy and robustness of this method was validated using computer-simulated FRAP experiments. This method was applied to determine the region-dependent anisotropic diffusion tensor in porcine temporomandibular joint (TMJ) discs. The average characteristic diffusivity of 4 kDa FITC-Dextran across the disc was 26.05 ± 4.32 μm²/s which is about 16% of its diffusivity in water. In the anteroposterior direction, the anterior region (30.99 ± 5.93 μm²/s) had significantly higher characteristic diffusivity than the intermediate region (20.49 ± 5.38 μm²/s) and posterior region (20.97 ± 2.46 μm²/s). The ratio of the two principal diffusivities represents the anisotropy of the diffusion and ranged between 0.45 and 0.51 (1.0 = isotropic). Our results indicated that the solute diffusion in TMJ discs is inhomogeneous and anisotropic. These findings suggested that diffusive transport in the TMJ disc is dependent on tissue composition (e.g., water content) and structure (e.g., collagen orientation). This study provides a new method to quantitatively investigate the relationship between solute transport properties and tissue composition and structure.