Computational modelling of articular joints with biphasic cartilage: recent advances, challenges and opportunities.

Biphasic models have been widely used to simulate the time-dependent biomechanical response of soft tissues. Modelling techniques of joints with biphasic weight-bearing soft tissues have been markedly improved over the last decade, enhancing our understanding of the function, degenerative mechanism and outcomes of interventions of joints. This paper reviews the recent advances, challenges and opportunities in computational models of joints with biphasic weight-bearing soft tissues. The review begins with an introduction of the function and degeneration of joints from a biomechanical aspect. Different constitutive models of articular cartilage, in particular biphasic materials, are illustrated in the context of the study of contact mechanics in joints. Approaches, advances and major findings of biphasic models of the hip and knee are presented, followed by a discussion of the challenges awaiting to be addressed, including the convergence issue, high computational cost and inadequate validation. Finally, opportunities and clinical insights in the areas of subject-specific modeling and tissue engineering are provided and discussed.

Quantification of Cartilage Poroelastic Material Properties Via Analysis of Loading-Induced Cell Death.

Articular cartilage (AC) is a load-bearing tissue that covers long bones in synovial joints. The biphasic/poroelastic mechanical properties of AC help it to protect joints by distributing loads, absorbing impact forces, and reducing friction. Unfortunately, alterations in these mechanical properties adversely impact cartilage function and precede joint degeneration in the form of osteoarthritis (OA). Thus, understanding what factors regulate the poroelastic mechanical properties of cartilage is of great scientific and clinical interest. Transgenic mouse models provide a valuable platform to delineate how specific genes contribute to cartilage mechanical properties. However, the poroelastic mechanical properties of murine articular cartilage are challenging to measure due to its small size (thickness ∼ 50 microns). In the current study, our objective was to test whether the poroelastic mechanical properties of murine articular cartilage can be determined based solely on time-dependent cell death measurements under constant loading conditions. We hypothesized that in murine articular cartilage subjected to constant, sub-impact loading from an incongruent surface, cell death area and tissue strain are closely correlated. We further hypothesized that the relationship between cell death area and tissue strain can be used-in combination with inverse finite element modeling-to compute poroelastic mechanical properties. To test these hypotheses, murine cartilage-on-bone explants from different anatomical locations were subjected to constant loading conditions by an incongruent surface in a custom device. Cell death area increased over time and scaled linearly with strain, which rose in magnitude over time due to poroelastic creep. Thus, we were able to infer tissue strain from cell death area measurements. Moreover, using tissue strain values inferred from cell death area measurements, we applied an inverse finite element modeling procedure to compute poroelastic material properties and acquired data consistent with previous studies. Collectively, our findings demonstrate in the key role poroelastic creep plays in mediating cell survival in mechanically loaded cartilage and verify that cell death area can be used as a surrogate measure of tissue strain that enables determination of murine cartilage mechanical properties.

Pericellular Matrix Formation and Atomic Force Microscopy of Single Primary Human Chondrocytes Cultured in Alginate Microgels.

One of the main components of articular cartilage is the chondrocyte's pericellular matrix (PCM), which is critical for regulating mechanotransduction, biochemical cues, and healthy cartilage development. Here, individual primary human chondrocytes (PHC) are encapsulated and cultured in 50 µm diameter alginate microgels using drop-based microfluidics. This unique culturing method enables PCM formation and manipulation of individual cells. Over ten days, matrix formation is observed using autofluorescence imaging, and the elastic moduli of isolated cells are measured using AFM. Matrix production and elastic modulus increase are observed for the chondrons cultured in microgels. Furthermore, the elastic modulus of cells grown in microgels increases ≈ten-fold over ten days, nearly reaching the elastic modulus of in vivo PCM. The AFM data is further analyzed using a Gaussian mixture model and shows that the population of PHCs grown in microgels exhibit two distinct populations with elastic moduli averaging 9.0 and 38.0 kPa. Overall, this work shows that microgels provide an excellent culture platform for the growth and isolation of PHCs, enabling PCM formation that is mechanically similar to native PCM. The microgel culture platform presented here has the potential to revolutionize cartilage regeneration procedures through the inclusion of in vitro developed PCM.

Positive outcomes following Autologous Matrix-Induced Chondrogenesis (AMIC) in the treatment of retropatellar chondral lesions: a retrospective analysis of a patient registry.

BACKGROUND: The patellofemoral joint is a challenging environment for treating chondral defects. Among the surgical options for the treatment of chondral defects, the single-stage Autologous Matrix-Induced Chondrogenesis (AMIC) procedure uses a porcine collagen I/III membrane to enhance bone-marrow stimulation. However, longer term outcomes data are rare for this specific indication. In order to provide real-world information, an ongoing registry has been established to record patient data and outcomes when AMIC is used to treat chondral and osteochondral lesions. METHODS: Patient data were retrieved from an ongoing, prospective, multisite registry of patients who had undergone AMIC treatment of chondral defects. We identified 64 patients who had undergone AMIC for patellofemoral chondral defects and for whom pre-operative and at least 1 post-operative score were available were included in this retrospective data analysis. Outcomes were assessed via the KOOS, VAS pain, and the Lysholm scores. Outcomes at the post-operative time-points were analysed using a factorial ANOVA with post-hoc testing while linear regression was used to assess associations between the change in the Lysholm score and lesion size. RESULTS: There was a significant improvement in Lysholm, VAS pain, and KOOS scores from pre-operative to the 1st year post-operative (p < 0.001), and this was maintained during the follow-up. CONCLUSIONS: The forces exerted on the patellofemoral joint make this a challenging scenario for chondral repair. Our data demonstrates that the AMIC procedure with a collagen I/III membrane is an effective treatment for retropatellar cartilage lesions, and provides reliable results, with decreased pain and improved function. Importantly, these improvements were maintained through the follow-up period.

Engineered biomechanical microenvironment of articular chondrocytes based on heterogeneous GelMA hydrogel composites and dynamic mechanical compression.

Tissue-engineered articular cartilage constructs are currently not able to equal native tissues in terms of mechanical and biological properties. A major cause lies in the deficiency in engineering the biomechanical microenvironment (BMME) of articular chondrocytes. In this work, to engineer the BMME of articular chondrocytes, heterogeneous hydrogel structures of gelatin methacrylated (GelMA) containing differential-stiffness domains were first fabricated, and then periodic dynamic mechanical stimulations were applied to the hydrogel structures. The chondrocyte phenotype of ATDC5 cells was enhanced as the spatial differentiation in stiffness was increased in the hydrogel structures and was further strengthened by dynamic mechanical stimulation. It was speculated that the mechanical signals generated by the engineered BMME were sensed by the cells through the integrin ß1-FAK signaling pathway. This study revealed the key role of the combined effects of differential and dynamic BMME on the chondrocyte phenotype, which could provide theoretical guidance for highly active tissue-engineered articular cartilage.

Arthritic Microenvironment-Dictated Fate Decisions for Stem Cells in Cartilage Repair.

The microenvironment and stem cell fate guidance of post-traumatic articular cartilage regeneration is primarily the focus of cartilage tissue engineering. In articular cartilage, stem cells are characterized by overlapping lineages and uneven effectiveness. Within the first 12 weeks after trauma, the articular inflammatory microenvironment (AIME) plays a decisive role in determining the fate of stem cells and cartilage. The development of fibrocartilage and osteophyte hyperplasia is an adverse outcome of chronic inflammation, which results from an imbalance in the AIME during the cartilage tissue repair process. In this review, the sources for the different types of stem cells and their fate are summarized. The main pathophysiological events that occur within the AIME as well as their protagonists are also discussed. Additionally, regulatory strategies that may guide the fate of stem cells within the AIME are proposed. Finally, strategies that provide insight into AIME pathophysiology are discussed and the design of new materials that match the post-traumatic progress of AIME pathophysiology in a spatial and temporal manner is guided. Thus, by regulating an appropriately modified inflammatory microenvironment, efficient stem cell-mediated tissue repair may be achieved.

Harnessing knee joint resident mesenchymal stem cells in cartilage tissue engineering.

Osteoarthritis (OA) is a widespread clinical disease characterized by cartilage degeneration in middle-aged and elderly people. Currently, there is no effective treatment for OA apart from total joint replacement in advanced stages. Mesenchymal stem cells (MSCs) are a type of adult stem cell with diverse differentiation capabilities and immunomodulatory potentials. MSCs are known to effectively regulate the cartilage microenvironment, promote cartilage regeneration, and alleviate OA symptoms. As a result, they are promising sources of cells for OA therapy. Recent studies have revealed the presence of resident MSCs in synovial fluid, synovial membrane, and articular cartilage, which can be collected as knee joint-derived MSCs (KJD-MSC). Several preclinical and clinical studies have demonstrated that KJD-MSCs have great potential for OA treatment, whether applied alone, in combination with biomaterials, or as exocrine MSCs. In this article, we will review the characteristics of MSCs in the joints, including their cytological characteristics, such as proliferation, cartilage differentiation, and immunomodulatory abilities, as well as the biological function of MSC exosomes. We will also discuss the use of tissue engineering in OA treatment and introduce the concept of a new generation of stem cell-based tissue engineering therapy, including the use of engineering, gene therapy, and gene editing techniques to create KJD-MSCs or KJD-MSC derivative exosomes with improved functionality and targeted delivery. These advances aim to maximize the efficiency of cartilage tissue engineering and provide new strategies to overcome the bottleneck of OA therapy. STATEMENT OF SIGNIFICANCE: This research will provide new insights into the medicinal benefit of Joint resident Mesenchymal Stem Cells (MSCs), specifically on its cartilage tissue engineering ability. Through this review, the community will further realize promoting joint resident mesenchymal stem cells, especially cartilage progenitor/MSC-like progenitor cells (CPSC), as a preventive measure against osteoarthritis and cartilage injury. People and medical institutions may also consider cartilage derived MSC as an alternative approach against cartilage degeneration. Moreover, the discussion presented in this study will convey valuable information for future research that will explore the medicinal benefits of cartilage derived MSC.

Prochondrogenic effect of decellularized extracellular matrix secreted from human induced pluripotent stem cell-derived chondrocytes.

Cartilage is mainly composed of chondrocytes and the extracellular matrix (ECM), which transmits important biochemical and biomechanical signals necessary for differentiation and homeostasis. Human articular cartilage has a low ability for regeneration because it lacks blood vessels, nerves, and lymphatic vessels. Currently, cell therapeutics, including stem cells, provide a promising strategy for cartilage regeneration and treatment; however, there are various hurdles to overcome, such as immune rejection and teratoma formation. In this study, we assessed the applicability of stem cell-derived chondrocyte ECM for cartilage regeneration. Human induced pluripotent stem cell (hiPSC)-derived chondrocytes (iChondrocytes) were differentiated, and decellularized ECM (dECM) was successfully isolated from cultured chondrocytes. Isolated dECM enhanced the in vitro chondrogenesis of iPSCs when recellularized. Implanted dECM also restored osteochondral defects in a rat osteoarthritis model. A possible association with the glycogen synthase kinase-3 beta (GSK3ß) pathway demonstrated the fate-determining importance of dECM in regulating cell differentiation. Collectively, we suggest the prochondrogenic effect of hiPSC-derived cartilage-like dECM and offer a promising approach of a noncellular therapeutic for articular cartilage reconstruction without cell transplantation. STATEMENT OF SIGNIFICANCE: Human articular cartilage has low ability for regeneration and cell culture-based therapeutics could aid cartilage regeneration. Yet, the applicability of human induced pluripotent stem cell-derived chondrocyte (iChondrocyte) extracellular matrix (ECM) has not been elucidated. Therefore, we first differentiated iChondrocytes and isolated the secreted ECM by decellularization. Recellularization was performed to confirm the pro-chondrogenic effect of the decellularized ECM (dECM). In addition, we confirmed the possibility of cartilage repair by transplanting the dECM into the cartilage defect in osteochondral defect rat knee joint. We believe that our proof-of-concept study will serve as a basis for investigating the potential of dECM obtained from iPSC-derived differentiated cells as a non-cellular resource for tissue regeneration and other future applications.

Development and experimental validation of a dynamic numerical model for human articular cartilage.

The purpose of this study was to create a preliminary set of experimentally validated Finite Element Analysis (FEA) models, in order to predict the dynamic mechanical behaviour of human articular cartilage (AC). Current models consider static loading with limited independent experimental validation, while the models for this study assess dynamic loading of AC, with direct comparison and validation to physical testing. Three different FEA models of AC were constructed, which considered both linear elastic and hyperelastic models; Neo-Hookean and Ogden. Models were validated using the data collected from compression testing of human femoral heads across 0-1.7 MPa (quasi-static tests and dynamic mechanical analysis). The linear elastic model was inadequate, with a 10-fold over prediction of the displacement dynamic amplitude. The Neo-Hookean model accurately predicted the dynamic amplitude but failed to predict the initial compression of the cartilage, with a 10 times overprediction. The Ogden model provided the best results, with both the initial compression lying within one standard deviation of that observed in the validation data set, and the dynamic amplitude of the same order of magnitude. In conclusion, this study has found that the fast dynamic response of human AC is best represented by a third order Ogden model.

Computational assessment on the impact of collagen fiber orientation in cartilages on healthy and arthritic knee kinetics/kinematics.

BACKGROUND: The inhomogeneous distribution of collagen fiber in cartilage can substantially influence the knee kinematics. This becomes vital for understanding the mechanical response of soft tissues, and cartilage deterioration including osteoarthritis (OA). Though the conventional computational models consider geometrical heterogeneity along with fiber reinforcements in the cartilage model as material heterogeneity, the influence of fiber orientation on knee kinetics and kinematics is not fully explored. This work examines how the collagen fiber orientation in the cartilage affects the healthy (intact knee) and arthritic knee response over multiple gait activities like running and walking. METHODS: A 3D finite element knee joint model is used to compute the articular cartilage response during the gait cycle. A fiber-reinforced porous hyper elastic (FRPHE) material is used to model the soft tissue. A split-line pattern is used to implement the fiber orientation in femoral and tibial cartilage. Four distinct intact cartilage models and three OA models are simulated to assess the impact of the orientation of collagen fibers in a depth wise direction. The cartilage models with fibers oriented in parallel, perpendicular, and inclined to the articular surface are investigated for multiple knee kinematics and kinetics. FINDINGS: The comparison of models with fiber orientation parallel to articulating surface for walking and running gait has the highest elastic stress and fluid pressure compared with inclined and perpendicular fiber-oriented models. Also, the maximum contact pressure is observed to be higher in the case of intact models during the walking cycle than for OA models. In contrast, the maximum contact pressure is higher during running in OA models than in intact models. Additionally, parallel-oriented models produce higher maximum stresses and fluid pressure for walking and running gait than proximal-distal-oriented models. Interestingly, during the walking cycle, the maximum contact pressure with intact models is approximately three times higher than on OA models. In contrast, the OA models exhibit higher contact pressure during the running cycle. INTERPRETATION: Overall, the study indicates that collagen orientation is crucial for tissue responsiveness. This investigation provides insights into the development of tailored implants.

Incorporation of Magnesium Ions into an Aptamer-Functionalized ECM Bioactive Scaffold for Articular Cartilage Regeneration.

The regeneration and reconstruction of articular cartilage (AC) after a defect are often difficult. The key to the treatment of AC defects lies in regeneration of the defect site and regulation of the inflammatory response. In this investigation, a bioactive multifunctional scaffold was formulated using the aptamer Apt19S as a mediator for mesenchymal stem cell (MSC)-specific recruitment and the enhancement of cellular chondrogenic and inflammatory regulation through the incorporation of Mg2+. Apt19S, which can recruit MSCs in vitro and in vivo, was chemically conjugated to a decellularized cartilage extracellular matrix (ECM)-lysed scaffold. The results from in vitro experiments using the resulting scaffold demonstrated that the inclusion of Mg2+ could stimulate not only the chondrogenic differentiation of synovial MSCs but also the increased polarization of macrophages toward the M2 phenotype. Additionally, Mg2+ inhibited NLRP3 inflammasome activation, thereby decreasing chondrocyte pyroptosis. Subsequently, Mg2+ was incorporated into the bioactive multifunctional scaffold, and the resulting scaffold promoted cartilage regeneration in vivo. In conclusion, this study confirms that the combination of Mg2+ and aptamer-functionalized ECM scaffolds is a promising strategy for AC regeneration based on in situ tissue engineering and early inflammatory regulation.

Characterizing site-specific mechanical properties of knee cartilage with indentation-relaxation maps and machine learning.

Knee cartilage experiences site-specific focal lesion and degeneration, which is likely associated with tissue inhomogeneity and nonuniform mechanical stimuli in the joint, for which a complete picture remains to be depicted. The present study aimed to develop a methodology to quantify knee cartilage inhomogeneity using porcine knee specimens. Automated indentation-relaxation and needle probing were performed on fully intact cartilage to obtain data that essentially represent continuous distributions of cartilage properties in the knee. Machine learning was then introduced to approximate the tissue inhomogeneity with several regions via clusters of indentation locations, and finite element modeling was used to obtain poromechanical properties for each region using indentation-relaxation and thickness data. Significant region dependence was established from the full time-dependent mechanical response. Seventeen regions, or clusters, were found to best approximate the site-specific poromechanical properties of articular cartilage for femoral groove, lateral and medial condyles and tibial plateaus, after up to eight clusters were tested for each of the five cartilage sections. The region partitions recommended, and tissue properties acquired would facilitate implementation of tissue inhomogeneity in future applications, e.g., contact modeling of the knee joint. The results obtained from 14 porcine knees revealed interesting region differences, for example, the two condyles have the same effective stiffness when responding to slowly applied mechanical loadings but substantially lower stiffness in the medial condyle when responding to fast loadings. This mechanical behavior may be associated with the fact that medial femoral cartilage is more prone to focal lesions than the lateral one.

Full Regional Creep Displacement Map of Articular Cartilage Based on Nanoindentation Array.

The elucidation of the mechanisms underlying articular cartilage lesions poses a formidable challenge in the field of cartilage repair. Despite significant strides in cartilage mechanics research, the region-dependent creep properties of articular cartilage remain elusive. In this study, we employ depth-sensing indentation tests to experimentally determine the creep properties of four distinct regions of articular cartilage, thereby unveiling a region-dependent full map of creep parameters. The measured creep displacement-time response curves indicate that the creep properties of the articular cartilage exhibit a clear regional correlation. Accordingly, the full regional creep map of articular cartilage is experimentally constructed for the first time. The correlation between the microstructures and the creep properties of cartilage in different regions is revealed. A three-parameter model is established to describe the creep velocity-displacement response of cartilage. Raman spectra reveal that the proteoglycan content is positively correlated with creep properties. The Raman shift directly indicates diverse residual stresses in different microregions. The obtained data facilitate a deep understanding of the potential creep dependent damage mechanism of cartilage and the further development of artificial cartilage materials.

Immunohistochemical characterization of the mandibular condyle for type I and II collagen, aggrecan, MMP-9, and MMP-13 in MMP-2-deficient mice.

Mice devoid of matrix metalloproteinase (MMP)-2 due to gene targeting have been reported to show articular cartilage destruction in the knee joint; however, the phenotype of the mandibular condylar cartilage remains unknown. Thus, in the present study, we investigated the mandibular condyle in Mmp2-/- mice. We obtained and bred Mmp2-/- mice from the same source as the previous study, and performed genotyping using genomic DNA extracted from finger snips. The mandibular condyle of Mmp2-/- mice and wild-type (WT) mice was immunohistochemically examined for the localization of extracellular matrix (ECM) proteins (type I and II collagen, and aggrecan), and MMP-9 and MMP-13. No cartilage destruction was observed in the mandibular condyle of Mmp2-/- mice, and no difference was found in the localization of the ECM proteins between the Mmp2-/- mice and WT mice. However, the bone marrow cavity in the subchondral bone of the mandibular condyle was more distinct in Mmp2-/- mice than in WT mice at the age of 50 weeks. Of note, MMP-9 characteristically localized in multinucleated cells in the mandibular condyle in 50-week-old Mmp2-/- mice. MMP-2 may be involved in the regulation of osteoclast differentiation and the formation of the bone marrow cavity in aged mice.

A novel device for investigating structure-function relationships and mechanoadaptation of biological tissues.

Exploring the structure-function relationships of cartilage on a microstructural level is crucial for tissue engineering approaches aiming to restore function. Therefore, a combination of mechanical testing with cell and tissue-level imaging would allow for longitudinal studying loading mechanisms, biological responses and mechanoadaptation of tissues at a microstructural level. This paper describes the design and validation of FELIX, a custom-built device for non-destructive image-guided micromechanical evaluation of biological tissues and tissue-engineered constructs. It combines multiphoton microscopy with non-destructive mechanical testing of native soft tissues. Ten silicone samples of the same size were mechanically tested with FELIX by different users to assess the repeatability and reproducibility. The results indicate that FELIX can successfully substitute mechanical testing protocols with a commercial device without compromising precision. Furthermore, FELIX demonstrated consistent results across repeated measurements, with very small deviations. Therefore, FELIX can be used to accurately measure biomechanical properties by different users for separate studies. Additionally, cell nuclei and collagen of porcine articular cartilage were successfully imaged under compression. Cell viability remained high in chondrocytes cultured in agarose over 21 days. Furthermore, there were no signs of contamination indicating a cell friendly, sterile environment for longitudinal studies. In conclusion, this work demonstrates that FELIX can consistently quantify mechanical measures without compromising precision. Furthermore, it is biocompatible allowing for longitudinal measurements.

Design of a double acting pneumatic cartilage loading device for magnetic resonance imaging.

Studies of osteoarthritis initiation and progression that measure strain in cartilage require physiological loading levels. Many studies use magnetic resonance (MR) imaging, which necessitates a MR-compatible loading device. In this study, the design and validation of a new device, the cartilage compressive actuator (CCA), is presented. The CCA is designed for high-field (e.g., 9.4 T) small-bore MR scanners, and meets a number of design criteria. These criteria include capability for testing bone-cartilage samples, MR compatibility, constant load and incremental strain application, a water-tight specimen chamber, remote control, and real time displacement feedback. The mechanical components in the final design include an actuating piston, a connecting chamber, and a sealed specimen chamber. An electro-pneumatic system applies compression, and an optical Fibre-Bragg grating (FBG) sensor provides live displacement feedback. A logarithmic relationship was observed between force exerted by the CCA and pressure (R2 = 0.99), with a peak output force of 653 ± 2 N. The relationship between FBG sensor wavelength and displacement was linear when calibrated both outside (R2 = 0.99) and inside (R2 = 0.98) the MR scanner. Average slope was similar between the two validation tests, with a slope of -4.2 nm/mm observed inside the MR scanner and -4.3 to -4.5 nm/mm observed outside the MR scanner. This device meets all design criteria and represents an improvement over published designs. Future work should incorporate a closed feedback loop to allow for cyclical loading of specimens.

Correlation analysis of cartilage wear with biochemical composition, viscoelastic properties and friction.

Healthy articular cartilage exhibits remarkable resistance to wear, sustaining mechanical loads and relative motion for decades. However, tissues that replace or repair cartilage defects are much less long lasting. Better information on the compositional and material characteristics that contribute to the wear resistance of healthy cartilage could help guide strategies to replace and repair degenerated tissue. The main objective of this study was to assess the relationship between wear of healthy articular cartilage, its biochemical composition, and its viscoelastic material properties. The correlation of these factors with the coefficient of friction during the wear test was also evaluated. Viscoelastic properties of healthy bovine cartilage were determined via stress relaxation indentation. The same specimens underwent an accelerated, in vitro wear test, and the amount of glycosaminoglycans (GAGs) and collagen released during the wear test were considered measures of wear. The frictional response during the wear test was also recorded. The GAG, collagen and water content and the concentration of the enzymatic collagen crosslink pyridinoline were quantified in tissue that was adjacent to each wear test specimen. Finally, correlation analysis was performed to identify potential relationships between wear characteristics of healthy articular cartilage with its composition, viscoelastic material properties and friction. The findings suggest that stiffer cartilage with higher GAG, collagen and water content has a higher wear resistance. Enzymatic collagen crosslinks also enhance the wear resistance of the collagen network. The parameters of wear, composition, and mechanical stiffness of cartilage were all correlated with one another, suggesting that they are interrelated. However, friction was largely independent of these in this study. The results identify characteristics of healthy articular cartilage that contribute to its remarkable wear resistance. These data may be useful for guiding techniques to restore, regenerate, and stabilize cartilage tissue.

Toward defining the role of the synovium in mitigating normal articular cartilage wear and tear.

Cartilage repair has been studied extensively in the context of injury and disease, but the joint's management of regular sub-injurious damage to cartilage, or 'wear and tear,' which occurs due to normal activity, is poorly understood. We hypothesize that this cartilage maintenance is mediated in part by cells derived from the synovium that migrate to the worn articular surface. Here, we demonstrate in vitro that the early steps required for such a process can occur. First, we show that under physiologic mechanical loads, chondrocyte death occurs in the cartilage superficial zone along with changes to the cartilage surface topography. Second, we show that synoviocytes are released from the synovial lining under physiologic loads and attach to worn cartilage. Third, we show that synoviocytes parachuted onto a simulated or native cartilage surface will modify their behavior. Specifically, we show that synoviocyte interactions with chondrocytes lead to changes in synoviocyte mechanosensitivity, and we demonstrate that cartilage-attached synoviocytes can express COL2A1, a hallmark of the chondrogenic phenotype. Our findings suggest that synoviocyte-mediated repair of cartilage 'wear and tear' as a component of joint homeostasis is feasible and is deserving of future study.

Effects of dexamethasone and dynamic loading on cartilage of human osteochondral explants challenged with inflammatory cytokines.

Post-traumatic osteoarthritis (PTOA), characterized by articular cartilage degradation initiated in an inflammatory environment after traumatic joint injury, can lead to alterations in cartilage biomechanical properties. Low dose dexamethasone (Dex) shows chondroprotection in cartilage challenged with inflammatory cytokines, but little is known about the structural biomechanical response of human cartilage to Dex in such a diseased state. This study examined changes in the biomechanical properties and biochemical composition of the cartilage within human osteochondral explants in response to treatment with exogenous cytokines, Dex, and a regimen of cyclic loading at the start and end of culture. Osteochondral explants were harvested from five pairs of human ankle talocrural joints (Collins grade 0-1) and cultured for 10 days with/without exogenous cytokines (100 ng/mL TNFα, 50 ng/mL IL-6, 250 ng/mL sIL-6R) ± Dex (100 nM). Biomechanical testing on day-0 and day-10 enabled estimation of the unconfined compression equilibrium modulus (Ey), dynamic stiffness (Ed) and hydraulic permeability (kp) of cartilage excised from bone, accompanied by biochemical assessment of media and cartilage tissue. Dex preserved chondrocyte cell viability and decreased sulfated glycosaminoglycan (sGAG) loss and nitric oxide release, but did not alter Ey, Ed and kp (before or after loading) on day-10. In the cytokine/cytokine+Dex treated groups, sGAG content exhibited a weaker correlation with Ey and Ed than at baseline, suggesting an important role for structural rather than biochemical changes in producing biomechanical alterations in response to cytokines and Dex. These findings aid in forming a more complete profile of potential clinical effects of Dex for use in OA/PTOA treatment regimens.

Creation of a Knee Joint-on-a-Chip for Modeling Joint Diseases and Testing Drugs.

The high prevalence of debilitating joint diseases like osteoarthritis (OA) poses a high socioeconomic burden. Currently, the available drugs that target joint disorders are mostly palliative. The unmet need for effective disease-modifying OA drugs (DMOADs) has been primarily caused by the absence of appropriate models for studying the disease mechanisms and testing potential DMOADs. Herein, we describe the establishment of a miniature synovial joint-mimicking microphysiological system (miniJoint) comprising adipose, fibrous, and osteochondral tissue components derived from human mesenchymal stem cells (MSCs). To obtain the three-dimensional (3D) microtissues, MSCs were encapsulated in photocrosslinkable methacrylated gelatin before or following differentiation. The cell-laden tissue constructs were then integrated into a 3D-printed bioreactor, forming the miniJoint. Separate flows of osteogenic, fibrogenic, and adipogenic media were introduced to maintain the respective tissue phenotypes. A commonly shared stream was perfused through the cartilage, synovial, and adipose tissues to enable tissue crosstalk. This flow pattern allows the induction of perturbations in one or more of the tissue components for mechanistic studies. Furthermore, potential DMOADs can be tested via either "systemic administration" through all the medium streams or "intraarticular administration" by adding the drugs to only the shared "synovial fluid"-simulating flow. Thus, the miniJoint can serve as a versatile in vitro platform for efficiently studying disease mechanisms and testing drugs in personalized medicine.