Development of Improved Confined Compression Testing Setups for Use in Stress Relaxation Testing of Viscoelastic Biomaterials.

The development of cell-based biomaterial alternatives holds significant promise in tissue engineering applications, but it requires accurate mechanical assessment. Herein, we present the development of a novel 3D-printed confined compression apparatus, fabricated using clear resin, designed to cater to the unique demands of biomaterial developers. Our objective was to enhance the precision of force measurements and improve sample visibility during compression testing. We compared the performance of our innovative 3D-printed confined compression setup to a conventional setup by performing stress relaxation testing on hydrogels with variable degrees of crosslinking. We assessed equilibrium force, aggregate modulus, and peak force. This study demonstrates that our revised setup can capture a larger range of force values while simultaneously improving accuracy. We were able to detect significant differences in force and aggregate modulus measurements of hydrogels with variable degrees of crosslinking using our revised setup, whereas these were indistinguishable with the convectional apparatus. Further, by incorporating a clear resin in the fabrication of the compression chamber, we improved sample visibility, thus enabling real-time monitoring and informed assessment of biomaterial behavior under compressive testing.

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.

Tensile energy dissipation and mechanical properties of the knee meniscus: relationship with fiber orientation, tissue layer, and water content.

Introduction: The knee meniscus distributes and dampens mechanical loads. It is composed of water (∼70%) and a porous fibrous matrix (∼30%) with a central core that is reinforced by circumferential collagen fibers enclosed by mesh-like superficial tibial and femoral layers. Daily loading activities produce mechanical tensile loads which are transferred through and dissipated by the meniscus. Therefore, the objective of this study was to measure how tensile mechanical properties and extent of energy dissipation vary by tension direction, meniscal layer, and water content. Methods: The central regions of porcine meniscal pairs (n = 8) were cut into tensile samples (4.7 mm length, 2.1 mm width, and 0.356 mm thickness) from core, femoral and tibial components. Core samples were prepared parallel (circumferential) and perpendicular (radial) to the fibers. Tensile testing consisted of frequency sweeps (0.01-1Hz) followed by quasi-static loading to failure. Dynamic testing yielded energy dissipation (ED), complex modulus (E*), and phase shift (δ) while quasi-static tests yielded Young's Modulus (E), ultimate tensile strength (UTS), and strain at UTS (εUTS). To investigate how ED is influenced by the specific mechanical parameters, linear regressions were performed. Correlations between sample water content (φw) and mechanical properties were investigated. A total of 64 samples were evaluated. Results: Dynamic tests showed that increasing loading frequency significantly reduced ED (p < 0.05). Circumferential samples had higher ED, E*, E, and UTS than radial ones (p < 0.001). Stiffness was highly correlated with ED (R2 > 0.75, p < 0.01). No differences were found between superficial and circumferential core layers. ED, E*, E, and UTS trended negatively with φw (p < 0.05). Discussion: Energy dissipation, stiffness, and strength are highly dependent on loading direction. A significant amount of energy dissipation may be associated with time-dependent reorganization of matrix fibers. This is the first study to analyze the tensile dynamic properties and energy dissipation of the meniscus surface layers. Results provide new insights on the mechanics and function of meniscal tissue.

Effect of molecular weight and tissue layer on solute partitioning in the knee meniscus.

Objective: Knee meniscus tissue is partly vascularized, meaning that nutrients must be transported through the extracellular matrix of the avascular portion to reach resident cells. Similarly, drugs used as therapeutic agents to treat meniscal pathologies rely on transport through the tissue. The driving force of diffusive transport is the gradient of concentration, which depends on molecular solubility. The meniscus is organized into a core region sandwiched between the tibial and femoral superficial layers. Structural differences exist across meniscal regions; therefore, regional differences in solubility are also hypothesized. Methods: Samples from the core, tibial and femoral layers were obtained from 5 medial and 5 lateral porcine menisci. The partition coefficient (K) of fluorescein, 3 â€‹kDa and 40 â€‹kDa dextrans in the layers of the meniscus was measured using an equilibration experiment. The effect of meniscal compartment, layer, and solute molecular weight on K was analyzed using a three-way ANOVA. Results: K ranged from a high of ∼2.9 in fluorescein to a low of ∼0.1 in 40 â€‹kDa dextran and was inversely related to the solute molecular weight across all tissue regions. Tissue layer only had a significant effect on partitioning of 40k Dex solute, which was lower in the tibial surface layer relative to the core (p â€‹= â€‹0.032). Conclusion: This study provides insight into depth-dependent partitioning in the meniscus, indicating the limiting effect of the meniscus superficial layer on solubility increases with solute molecular size. This illustrates how the surface layers could potentially reduce the effectiveness of drug delivery therapies incorporating large molecules (>40 â€‹kDa).

Biomechanical properties of porcine meniscus as determined via AFM: Effect of region, compartment and anisotropy.

The meniscus is a fibrocartilaginous tissue that plays an essential role in load transmission, lubrication, and stabilization of the knee. Loss of meniscus function, through degeneration or trauma, can lead to osteoarthritis in the underlying articular cartilage. To perform its crucial function, the meniscus extracellular matrix has a particular organization, including collagen fiber bundles running circumferentially, allowing the tissue to withstand tensile hoop stresses developed during axial loading. Given its critical role in preserving the health of the knee, better understanding structure-function relations of the biomechanical properties of the meniscus is critical. The main objective of this study was to measure the compressive modulus of porcine meniscus using Atomic Force Microscopy (AFM); the effects of three key factors were investigated: direction (axial, circumferential), compartment (medial, lateral) and region (inner, outer). Porcine menisci were prepared in 8 groups (= 2 directions x 2 compartments x 2 regions) with n = 9 per group. A custom AFM was used to obtain force-indentation curves, which were then curve-fit with the Hertz model to determine the tissue's compressive modulus. The compressive modulus ranged from 0.75 to 4.00 MPa across the 8 groups, with an averaged value of 2.04±0.86MPa. Only direction had a significant effect on meniscus compressive modulus (circumferential > axial, p = 0.024), in agreement with earlier studies demonstrating that mechanical properties in the tissue are anisotropic. This behavior is likely the result of the particular collagen fiber arrangement in the tissue and plays a key role in load transmission capability. This study provides important information on the micromechanical properties of the meniscus, which is crucial for understanding tissue pathophysiology, as well as for developing novel treatments for tissue repair.

Heterogeneity of dynamic shear properties of the meniscus: A comparison between tissue core and surface layers.

Damage to the meniscus has been associated with excessive shear loads. Aimed at elucidating meniscus pathophysiology, previous studies have investigated the shear properties of the meniscus fibrocartilaginous core. However, the meniscus is structurally inhomogeneous, with an external cartilaginous envelope (tibial and femoral surface layers) wrapping the tissue core. To date, little is known about the shear behavior of the surface layers. The objective of this study was to measure the dynamic shear properties of the surface layers and derive empirical relations with their composition. Specimens were harvested from tibial and femoral surface layers and core of porcine menisci (medial and lateral, n = 10 each). Frequency sweep tests yielded complex shear modulus (G*) and phase shifts (δ). Mechanical behavior of regions was described by a generalized Maxwell model. Correlations between shear moduli with water and glycosaminoglycans content of the tissue regions were investigated. The femoral surface had the lowest shear modulus, when compared to core and tibial regions. A 3-relaxation times Maxwell model satisfactorily interpreted the shear behavior of all tissue regions. Inhomogeneous tissue composition was also observed, with water content in the surface layers being higher when compared with tissue core. Water content negatively correlated with shear properties in all regions. The lower measured shear properties in the femoral layer may explain the higher prevalence of meniscal tears on the superior surface of the tissue. The heterogenous behavior of the tissue in shear provides insight into meniscus pathology and has important implications for efforts to tissue engineer replacement tissues.

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.

Mechanical properties of meniscal circumferential fibers using an inverse finite element analysis approach.

The extracellular matrix (ECM) of the meniscus is a gel-like water solution of proteoglycans embedding bundles of collagen fibers mainly oriented circumferentially. Collagen fibers significantly contribute to meniscal mechanics, however little is known about their mechanical properties. The objective of this study was to propose a constitutive model for collagen fibers embedded in the ECM of the meniscus and to characterize the tissue's pertinent mechanical properties. It was hypothesized that a linear fiber reinforced viscoelastic constitutive model is suitable to describe meniscal mechanical behavior in shear. It was further hypothesized that the mechanical properties governing the model depend on the tissue's composition. Frequency sweep tests were conducted on eight porcine meniscal specimens. A first cohort of experimental data resulted from tissue specimens where collagen fibers oriented parallel with respect to the shear plane were used. This was done to eliminate the contribution of collagen fibers from the mechanical response and characterize the mechanical properties of the ECM. A second cohort with fibers orthogonally oriented with respect to the shear plane that were used to determine the elastic properties of the collagen fibers via inverse finite element analysis. Our testing protocol revealed that tissue ECM mechanical behavior could be described by a generalized Maxwell model with 3 relaxation times. The inverse finite element analysis suggested that collagen fibers can be modeled as linear elastic elements having an average elastic modulus of 287.5 ± 62.6 MPa. Magnitudes of the mechanical parameters governing the ECM and fibers were negatively related to tissue water content.

Viscoelastic and equilibrium shear properties of human meniscus: Relationships with tissue structure and composition.

The meniscus is crucial in maintaining the knee function and protecting the joint from secondary pathologies, including osteoarthritis. Although most of the mechanical properties of human menisci have been characterized, to our knowledge, its dynamic shear properties have never been reported. Moreover, little is known about meniscal shear properties in relation to tissue structure and composition. This is crucial to understand mechanisms of meniscal injury, as well as, in regenerative medicine, for the design and development of tissue engineered scaffolds mimicking the native tissue. Hence, the objective of this study was to characterize the dynamic and equilibrium shear properties of human meniscus in relation to its anisotropy and composition. Specimens were prepared from the axial and the circumferential anatomical planes of medial and lateral menisci. Frequency sweeps and stress relaxation tests yielded storage (G') and loss moduli (G″), and equilibrium shear modulus (G). Correlations of moduli with water, glycosaminoglycans (GAGs), and collagen content were investigated. The meniscus exhibited viscoelastic behavior. Dynamic shear properties were related to tissue composition: negative correlations were found between G', G″ and G, and meniscal water content; positive correlations were found for G' and G″ with GAG and collagen (only in circumferential samples). Circumferential samples, with collagen fibers orthogonal to the shear plane, exhibited superior dynamic mechanical properties, with G' ~70 kPa and G″ ~10 kPa, compared to those of the axial plane ~15 kPa and ~1 kPa, respectively. Fiber orientation did not affect the values of G, which ranged from ~50 to ~100 kPa.

Computational Modeling Intervertebral Disc Pathophysiology: A Review.

Lower back pain is a medical condition of epidemic proportion, and the degeneration of the intervertebral disc has been identified as a major contributor. The etiology of intervertebral disc (IVD) degeneration is multifactorial, depending on age, cell-mediated molecular degradation processes and genetics, which is accelerated by traumatic or gradual mechanical factors. The complexity of such intertwined biochemical and mechanical processes leading to degeneration makes it difficult to quantitatively identify cause-effect relationships through experiments. Computational modeling of the IVD is a powerful investigative tool since it offers the opportunity to vary, observe and isolate the effects of a wide range of phenomena involved in the degenerative process of discs. This review aims at discussing the main findings of finite element models of IVD pathophysiology with a special focus on the different factors contributing to physical changes typical of degenerative phenomena. Models presented are subdivided into those addressing role of nutritional supply, progressive biochemical alterations stemming from an imbalance between anabolic and catabolic processes, aging and those considering mechanical factors as the primary source that induces morphological change within the disc. Limitations of the current models, as well as opportunities for future computational modeling work are also discussed.

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.

Compressive Properties and Hydraulic Permeability of Human Meniscus: Relationships With Tissue Structure and Composition.

The meniscus is crucial in maintaining knee function and protecting the joint from secondary pathologies, including osteoarthritis. The meniscus has been shown to absorb up to 75% of the total load on the knee joint. Mechanical behavior of meniscal tissue in compression can be predicted by quantifying the mechanical parameters including; aggregate modulus (H) and Poisson modulus (ν), and the fluid transport parameter: hydraulic permeability (K). These parameters are crucial to develop a computational model of the tissue and for the design and development of tissue engineered scaffolds mimicking the native tissue. Hence, the objective of this study was to characterize the mechanical and fluid transport properties of human meniscus and relate them to the tissue composition. Specimens were prepared from the axial and the circumferential anatomical planes of the tissue. Stress relaxation tests yielded the H, while finite element modeling was used to curve fit for ν and K. Correlations of moduli with water and glycosaminoglycans (GAGs) content were investigated. On average H was found to be 0.11 ± 0.078 MPa, ν was 0.32 ± 0.057, and K was 2.9 ± 2.27 × 10-15 m4N-1s-1. The parameters H, ν, and K were not found to be statistically different across compression orientation or compression level. Water content of the tissue was 77 ± 3.3% while GAG content was 8.79 ± 1.1%. Interestingly, a weak negative correlation was found between H and water content (R2 ~ 34%) and a positive correlation between K and GAG content (R2 ~ 53%). In conclusion, while no significant differences in transport and compressive properties can be found across sample orientation and compression levels, data trends suggest potential relationships between magnitudes of H and K, and GAG content.

Effects of Oxygen Concentration and Culture Time on Porcine Nucleus Pulposus Cell Metabolism: An in vitro Study.

Low back pain is a common ailment that affects millions of individuals each year and is linked to degeneration of the intervertebral discs in the spine. Intervertebral disc degeneration is known to result from an imbalance in anabolic and catabolic activity by disc cells. Due to the avascular nature of the intervertebral disc, oxygen deficiency may occur in the central nucleus pulposus (NP). The resulting hypoxia affects matrix regulation and energy metabolism of disc cells, although the mechanisms are not fully understood. This study investigates in vitro glucose consumption and gene expression by NP cells over time under varying oxygen tensions. Notochordal porcine NP cells were cultured in agarose discs at 21, 5, or 1% oxygen tension for 1, 5, or 10 days. The expression of 10 key matrix genes, as well as Brachyury (T), by NP cells was analyzed using RT-PCR. Glucose consumption was measured using a two-point method. Results show that culture time and oxygen tension significantly affect glucose consumption rates by porcine NP cells. There were also significant changes in T expression based on oxygen level and culture time. The 1% oxygen tension had a significantly higher T expression on day 10 than the other two groups, which may indicate a better maintenance of the notochordal phenotype. MMP 1 and 13 expression increased over time for all groups, while only the 5% group showed an increase over time for MMP 3. TIMP expression followed the direction of MMPs but to a lesser magnitude. Five percent and twenty-one percent oxygen tensions led to decreases in anabolic gene expression while 1% led to increases. Oxygen concentration and culture time significantly impacted glucose consumption rate and the gene expression of matrix regulatory genes with hypoxic conditions most accurately maintaining the proper NP phenotype. This information is valuable not only for understanding disc pathophysiology, but also for harnessing the potential of notochordal NP cells in therapeutic applications.

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 ±â€¯2.00 × 10-12 m2/s (n = 10), while that of oxacillin was 2.26 ±â€¯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.

Hydraulic permeability of meniscus fibrocartilage measured via direct permeation: Effects of tissue anisotropy, water volume content, and compressive strain.

Hydraulic permeability is an important material property of cartilaginous tissues, governing the rate of fluid flow, which is crucial to tissue biomechanics and cellular nutrition. The effects of strain, anisotropy, and region on the hydraulic permeability in meniscus tissue have not been fully elucidated. Using a one-dimensional direct permeation test, we measured the hydraulic permeability within statically compressed porcine meniscus specimens, prepared such that the explants were in either the axial or circumferential direction of either the central or horn (axial direction only) region of the medial and lateral menisci. A constant flow was applied and the pressure difference was measured using pressure transducers. Specimens were tested under 10-20% compressive strain. Permeability values were in the range of 1.53-1.87 × 10-15 m4/Ns, which is comparable to values found in the literature. Permeability was significantly anisotropic, being higher in the circumferential direction than in the axial direction. Additionally, there was a significant negative correlation between strain level and permeability for all groups. Lastly, no statistically significant difference was found between permeability coefficients from different regional locations. This study provides important information regarding structure-function relationships in meniscal tissues that helps to elucidate biomechanics and transport in the tissue, and can aid in the understanding of the tissue's role in the function of the knee joint and onset of osteoarthritis.

The nutrition of the human meniscus: A computational analysis investigating the effect of vascular recession on tissue homeostasis.

The meniscus is essential to the functioning of the knee, offering load support, congruency, lubrication, and protection to the underlying cartilage. Meniscus degeneration affects ∼35% of the population, and potentially leads to knee osteoarthritis. The etiology of meniscal degeneration remains to be elucidated, although many factors have been considered. However, the role of nutritional supply to meniscus cells in the pathogenesis of meniscus degeneration has been so far overlooked. Nutrients are delivered to meniscal cells through the surrounding synovial fluid and the blood vessels present in the outer region of the meniscus. During maturation, vascularization progressively recedes up to the outer 10% of the tissue, leaving the majority avascular. It has been hypothesized that vascular recession might significantly reduce the nutrient supply to cells, thus contributing to meniscus degeneration. The objective of this study was to evaluate the effect of vascular recession on nutrient levels available to meniscus cells. This was done by developing a novel computational model for meniscus homeostasis based on mixture theory. It was found that transvascular transport of nutrients in the vascularized region of the meniscus contributes to more than 40% of the glucose content in the core of the tissue. However, vascular recession does not significantly alter nutrient levels in the meniscus, reducing at most 5% of the nutrient content in the central portion of the tissue. Therefore, our analysis suggests that reduced vascularity is not likely a primary initiating source in tissue degeneration. However, it does feasibly play a key role in inability for self-repair, as seen clinically.

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.

Electrical conductivity and ion diffusion in porcine meniscus: effects of strain, anisotropy, and tissue region.

The purpose of the present study was to investigate the effects of mechanical strain, anisotropy, and tissue region on electrical conductivity and ion diffusivity in meniscus fibrocartilage. A one-dimensional, 4-wire conductivity experiment was employed to measure the electrical conductivity in porcine meniscus tissues from two tissue regions (horn and central), for two tissue orientations (axial and circumferential), and for three levels of compressive strain (0%, 10%, and 20%). Conductivity values were then used to estimate the relative ion diffusivity in meniscus. The water volume fraction of tissue specimens was determined using a buoyancy method. A total of 135 meniscus samples were measured; electrical conductivity values ranged from 2.47mS/cm to 4.84mS/cm, while relative ion diffusivity was in the range of 0.235 to 0.409. Results show that electrical conductivity and ion diffusion are significantly anisotropic (p<0.001), both being higher in the circumferential direction than in the axial direction. Additionally, the findings show that compression significantly affects the electrical conductivity with decreasing conductivity levels corresponding to increased compressive strain (p<0.001). Furthermore, there was no statistically significant effect of tissue region when comparing axial conductivity in the central and horn regions of the tissue (p=0.999). There was a positive correlation between tissue water volume fraction and both electrical conductivity and relative ion diffusivity for all groups investigated. This study provides important insight into the electromechanical and transport properties in meniscus fibrocartilage, which is essential in developing new strategies to treat and/or prevent tissue degeneration.

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.