Creation of replacement tissue to repair articular surface defects remains a challenge. Normal zonal characteristics of articular cartilage throughout its thickness, particularly the superficial tangential zone (STZ), and normal material properties have not been reproduced in vitro in scaffolds nor in vivo in repairing defects. Without sufficient quality, such transplanted scaffolds in vivo may be doomed mechanically from the outset. Removal of the STZ from normal cartilage negatively affects the remaining cartilage’s ability to support axial loads and retain fluids [1–3]. Previous studies have modeled excessive axial deformation of repair cartilage [4–5]. Studies have shown that modeling the STZ of normal cartilage as transversely isotropic provides better agreement with indentation experimental results than isotropic models [6–9]. Others have modeled experimental conditions by incorporating tension and compression nonlinearity [10]. Previous analyses have indicated that strain-dependent permeability within the STZ can positively affect the ability of free-draining normal and repair models to withstand imposed surface loads [11,12]. This finite element study further examined the role of an STZ with strain-dependent permeability on the behavior of normal and repaired articular surfaces under contact loading from rigid permeable and impermeable spheres. Nonlinear geometry permitted finite deformations to occur while the differential stiffness in tension and compression was also represented.

The meniscus performs several functions for the maintenance of knee joint health, such as load transmission and joint stability. Meniscal lesions have been suggested as a precursor to the onset of osteoarthritis (OA)[1]. Such lesions often lead to surgical removal of the torn portion of the meniscus, increasing cartilage to cartilage contact area. Partial meniscectomies have been shown using finite element analysis and histology to lead to altered and increased mechanical loading on the remaining meniscus and underlying articular cartilage[2,3]. Consequently, pathological compressive strains of more than 15% have been shown to increase proteoglycan breakdown and meniscal matrix degradation[4]. Preliminary investigations in our laboratory have demonstrated an increase in interleukin-1α (IL-1α) gene expression of meniscal explants subjected to pathological levels of dynamic compressive strain [6,7]. This inflammatory cytokine has been attributed to apoptosis and matrix degradation[5]. However, gene expression measurements merely suggest possible matrix remodeling mechanisms and do not necessarily result in protein syntheses from which matrix changes occur. Therefore, the purpose of this study was to quantify protein synthesis of IL-1α in porcine meniscal implants after compressive strain exercises. It was hypothesized that, similar to mRNA expression, protein synthesis for pathologically loaded (0 or 20% dynamic strain) samples would be greater than samples loaded to physiological levels (10% strain).

Simplified analytical approaches to estimate hip joint contact pressures using perfectly spherical geometry have been described in the literature (rigid body spring models); however, estimations based on these simulations have not corresponded well with experimental in vitro data. Recent evidence from our laboratory suggests that finite element (FE) models of the hip joint that incorporate detailed geometry for cartilage and bone can predict cartilage pressures in good agreement with experimental data [1]. However, it is unknown whether this degree of model complexity is necessary. The objective of this study was to compare cartilage contact pressure predictions from FE models with varying degrees of simplicity to elucidate which aspects of hip morphology are required to obtain accurate predictions of cartilage contact pressure. Models based on 1) subject-specific (SS) geometry, 2) spheres, and 3) rotational conchoids were analyzed.

Surface topography is an important parameter that affects friction, lubrication and wear of contacting materials. Articular cartilage has a very distinct topography allowing transport of nutritious fluid under elastohydrodynamic lubrication conditions [1]. Topographical measurements are a prerequisite for ex-vivo explant cultures which have become increasingly popular in the field of mechanobiology. Recently, several laboratories established wear tests [2, 3] to evaluate artificial cartilage repair materials intended to articulate against natural cartilage. Stylus profilometry, originally used in 1986 to establish that surface topography is a valuable indicator of cartilage health [1], remains the dominant technique for determining cartilage topography [4] despite significant shortcomings. Contact-based and slow, it can mar the surface and negatively impact tissue viability. However, to understand the underlying damage mechanism(s) it would be of utmost importance to monitor changes in surface topography during wear testing of viable tissue.

Matrix remodeling in articular cartilage is regulated by the elevation and activation of aggrecanases (ADAMTS-4 and ADAMTS-5) and matrix metalloproteinases (MMPs) [2–4, 7–9, 10]. Several recent studies from our and other groups have shown that mechanical loading can counteract interleukin 1 (IL-1) induced pro-inflammatory and catabolic events by down-regulating aggrecanases, MMPs, and pro-inflammatory genes [1, 3, 5, 6], but the molecular mechanism is not clear. Many previous studies have shown that the regulation of pro-inflammatory effect of IL-1 come from several aspects: anti-inflammatory cytokines (TGF-β, IL-10, IL-6 and interferon γ), IL-1 receptor related proteins (IL-1R1, IL-1R2, and IL-1Ra) as well as a family of intracellular inhibitory protein called Suppressor Of Cytokine Signaling (SOCS.) SOCS1 and SOCS3 are especially important, since they can inhibit both MAPK and NF-κB pathways induced by IL-1 [12]. The objective of this study was to determine whether mechanical load affected anti-inflammatory mediators along with anti-catabolic events.

The avascular nature of cartilage results in its limited inability to repair itself upon injury. As a result numerous approaches are being investigated as potential therapies for repair, including tissue engineering strategies. In addition, due to the low density of chondrocytes and the characteristic de-differentiation of the cells when expanded in monolayer [1], other cell types are being investigated as a source for cartilage repair as well. Mesenchymal stem cells (MSCs), which have been shown to differentiate into cells of several lineages including chondrocytes, osteoblasts and adipocytes [2], are being explored as a potential cell type for the regeneration of articular cartilage tissue [3,4].

The meniscus plays an important role in many biomechanical functions of the knee, including shock absorption, load distribution, and joint lubrication. The ability to perform these functions is determined by the interactions of the major tissue constituents (water, proteoglycan, and collagen), although these interactions have not been as thoroughly studied in fibrocartilage as those in other soft tissues. In articular cartilage, electrochemical interactions between negatively charged glycosaminoglycan (GAG) side chains of proteoglycans and the interstitial fluid generate an osmotic swelling pressure that contributes to the compressive stiffness [1], and proteoglycan degradation dramatically decreases compressive and shear moduli [2]. Although the concentration of proteoglycan in the meniscus is substantially lower (<1%) than that in articular cartilage, aggrecan degradation also greatly decreases the compressive and shear moduli of meniscal fibrocartilage [3]. The proteoglycan distribution in meniscal fibrocartilage is macroscopically heterogeneous [4] and is concentrated in the secondary matrix surrounding the circumferential collagen bundles, and the extent to which osmotic interactions explain the influence on fibrocartilage material properties is unknown. Altering the osmotic strength of the bathing solution supplies a means to control osmotic interactions between the GAGs and the environment without degrading the tissue matrix. The objective of this study was to examine the effects of an altered osmotic environment on the dynamic shear modulus of meniscal tissue.

Meniscectomy is a well-known risk factor for osteoarthritis (OA) in humans. It has been reported that total meniscectomy increases the risk of developing osteoarthritis radiographically by 14 times after 21 years [1] and that partial meniscectomy increases the risk of developing radiographic evidence of osteoarthritis within 16 years by a factor of four [2]. Two possible functional measures have been suggested as mechanisms for the development and progression of premature knee osteoarthritis: alterations in either kinematics or kinetics. Changes in kinematics, such as a shift in internal-external (IE) rotation after anterior cruciate ligament injury, have been suggested as a basis for an increased rate of cartilage thinning [3]. The other possible reason for the development of premature OA post-meniscectomy is a change in kinetics, the mechanical loading that occurs, particularly in the medial compartment [4]. The knee adduction moment has been associated with the rate of progression of osteoarthritis [5] and it has thus been suggested that the knee adduction moment is a good surrogate measure for in vivo load on the medial compartment osteoarthritis [6].

Hydrogels are a cross-linked network of polymer swollen with a liquid, and are promising replacements for diseased or damaged load bearing tissues such as articular cartilage [1]. Recently, a linear biphasic model, developed originally for cartilage [2], has been applied to characterize the mechanical behavior of hydrogels [3, 4]. However, the linear elastic assumption for the solid phase ignores the intrinsic viscoelasticity of the polymer network [3, 4]. Some attempts have been made in the literature to simulate hydrogels with a biphasic viscoelastic model using a self-developed finite element code [5]. This study is aimed at simulating hydrogels with a biphasic viscoelastic model and investigating an inverse finite element (FE) technique to identify material parameters of hydrogels via combined creep testing and FE modeling. Creep testing of hydrogels is simulated in the commercial software ABAQUS, which makes this approach easy to adapt to other test geometries. Material parameters are identified by fitting the FE results to the experimental results using an optimization method.

Chondrocytes are responsible for the elaboration and maintenance of the extracellular matrix (ECM) in articular cartilage. Previous studies have demonstrated that mechanical loading modulates the biosynthetic response of chondrocytes in cartilage explants [1,2]. Therefore, the mechanical properties of chondrocytes play a pivotal role in regulating the interaction between the cell and ECM in situ [eg. 3–5]. The goal of this study is to investigate the indentation-dependent stiffness of chondrocytes using atomic force microscopy. Due to the composite nature of cellular organelles and the cytoskeleton, we hypothesize that chondrocytes exhibit a non-linear elastic modulus, requiring a non-Hertzian analysis of the contact and resulting stress-strain response [6,7]. The response of chondrocytes is investigated in a manner to probe the bulk stiffness of the cell (using 10 μm spherical probes), as well as the stiffness arising from composite subcellular structures (utilizing 3 μm spherical probes).

It has been suggested that altered joint loading may be a cause of the cartilage degeneration commonly observed following joint destabilizing events such as anterior cruciate ligament (ACL) injury [1]. Changes in the biochemical environment of the joint accompany these changes in joint loading, though, with acute and chronic increases in synovial concentrations of proinflammatory cytokines such as tumor necrosis factor-alpha (TNFα) [2, 3]. TNFα is a potent catabolic factor associated with increased expression [4] and synthesis [5] of matrix proteases in articular cartilage, and therefore may play in important role in the degenerative events following joint injury.

Tissue-engineering has great potential for treating cartilage pathologies such as osteoarthritis by replacing degraded tissue with newly developed engineered tissue. However proinflammatory cytokines such as interleukin-1α (IL-1α) are a confounding issue as they are often present in high concentrations as part of the chronic pathology or as a result of the surgical intervention itself(1). The catabolic effects of these mediators may be especially pronounced in engineered tissues whose cells are not yet fully embedded in the potentially chondroprotective enclosure of a cartilaginous extracellular matrix(2). One method to protecting initially fragile constructs from degradation may be through the use of non-toxic cross-linking agents. Genipin is a naturally occurring crosslinking agent that reacts with amino acids or amine groups and leads to the formation of stable crosslinked products that are identifiable by a dark blue pigment (Figure 1). Cartilage cross-linked with genipin has been shown to be more resistant to collagenase digestion(3) and to injection of chondroitinase-ABC(4). In this study, we examined whether engineered constructs pre-treated with genipin would better resist IL-1α induced catabolic degradation.

The intervertebral disc (IVD) is the largest cartilaginous structure in human body that contributes to flexibility and load support in the spine. To accomplish these functions, the disc has a unique architecture consisting of a centrally-located nucleus pulposus (NP) surrounded superiorly and inferiorly by cartilage endplates (CEP) and peripherally by the annulus fibrosus (AF). Disc degeneration is strongly linked to low back pain. Poor nutrient supply has been suggested as a potential mechanism for disc degeneration. Previous theoretical studies have shown that the distributions of nutrients and metabolites (e.g., oxygen, glucose, and lactate) within the IVD depended on tissue diffusivities, nutrient supply, and cellular metabolic rates [1, 2]. Based on a multiphasic mechano-electrochemical finite element model of human IVD [3], our recent theoretical study suggested that the mechanical loading has little effect on small solute transport (e.g., glucose), but significantly affects large solute transport (e.g., growth factor). The objective of this study was to further develop the multiphasic finite element model of IVD by including the cartilage endplate and considering the nutrient consumption of disc cells. Using this model, the effects of endplate and mechanical loading on solute transport in IVD were examined.

It has previously been demonstrated that hydrostatic pressure (HP) enhances the biochemical properties of self-assembled articular cartilage constructs [1]. However, studies that systematically assess the effects of HP magnitude and frequency are lacking. Additionally, studies examining the combined effects of hydrostatic pressure and growth factors are limited. To this end, this study sought to test the hypotheses that static HP will have the greatest enhancement of construct biomechanical and biochemical properties, and that there will be additive or synergistic effects when combining growth factors and HP stimulation.

This study was aimed at investigating the relationship between the microstructural responses of healthy (normal) versus mildly osteoarthritic articular cartilage and their associated peak internal pore pressures, when subjected to direct compression.

Impact injury to cartilage, a common consequence of joint trauma, is commonly thought to lead to degenerative osteoarthritis. Many pharmacologic agents have been developed to prevent or forestall this process. Development of a reliable model for the early stages of cartilage injury and osteoarthritis would enable the testing of novel treatments that could delay or even reverse the progression of this debilitating disease.

Osteoarthritis (OA) of the knee affects an estimated 20–40% of individuals over the age of 65 [1], and is nearly 10 times more common in the medial compartment than the lateral compartment [2]. Plain radiography measurements using the Kellgren and Lawrence (KL) scale have been the gold standard for diagnosis of knee osteoarthritis [3]. However, for new treatment interventions for medial compartment knee OA such as load modifying footwear, it would be important to know if the disease-related cartilage loss in the medial compartment occurs initially and predominantly in regions that are weight bearing during walking. Because walking results in highest weight bearing occurring in the anterior and middle regions of the femoral condyle and the anterior region of the tibial plateau, seeing a pattern of thinning in these areas would suggest that walking is an important activity for understanding both the cause and treatment of osteoarthritis.

Intrinsic repair of articular cartilage is poor, and so numerous tissue engineering strategies have been developed for producing functional cartilage replacements. Photopolymerizable methacrylated hyaluronic acid (MeHA) hydrogels have been developed as a potential hydrogel that possesses the distinct advantage of being biologically relevant as well as easily modified to generate a range of hydrogel properties [1]. To date, optimization of this hydrogel has been carried out by adjusting macromer molecular weight, concentration, and extent of methacrylation. Recent studies using MeHA hydrogels with auricular chondrocytes have shown that adjustments in these parameters can have significant impact on cell viability and construct maturation. [1, 2].

Osteoarthritis (OA) is a degenerative disease of articular cartilage that affects millions of people [1]. Local biomechanical factors may severely affect initiation and progression of OA due to changes in loading conditions at the cartilage. The frontal plane tibiofemoral alignment effects the varus/valgus moment which could increase the overall loading at the knee. Biomechanical studies have reported that the varus moment is a key determinant in the load distribution at the knee [2, 3] and has been linked to OA progression [4, 5]. A normal knee will have a tibiofemoral angle approximately 7° valgus [6]. Deviation from this angle leads to a knee joint with a varus or valgus condition. In this investigation, a motion analysis procedure was developed to determine the affect of the frontal plane tibiofemoral angle on the force and moment reactions at the knee. The results of these methods could be utilized in a subject specific finite element model to determine the stress and strain at the knee cartilage and to suggest measures to prevent OA.

Mesenchymal stem cells (MSCs) are a multi-potential cell type that can be induced to differentiate to a variety of tissue-specific cell phenotypes, including cartilage (chondrogenesis) and bone (osteogenesis). Given this multi-potentiality, MSCs are a promising cell source for exploring developmental paradigms and for tissue engineering (TE) applications. For cartilage formation assays, MSCs are collected in high-density pellets and treated with specific biofactors, including TGF-β superfamily members and dexamethasone in a chemically defined medium (CM) [1]. During chondrogenesis, extracellular matrix (ECM) rich in glycosaminoglycan (GAG) and type II collagen is synthesized. While MSC chondrogenesis is well-characterized using existing protocols, the effect of alternative biofactors, their doses and combinations requires laborious combinatorial studies [2]. High-throughput screening (HTS) overcomes this limitation through the simultaneous layout and query of a large number of conditions within a single plate. HTS depends on the use of precise robotic liquid handling systems and on the development of sensitive, validated, and readily quantifiable assays. In a recent study, we optimized cell culture and assay procedures for HTS by minimizing cell number, handling and culture duration [3]. We successfully reduced the time scale from 21 to 7 days and the number of cells required from 225K to 30K cells per pellet. Further, we developed a novel in-well digestion protocol to enable high-throughput analysis and minimize handling. In this study, we have further streamlined these assays for HTS by providing a rapid and robotic approach for layout, culture, and analysis of ECM deposition using ‘micro’ MSC pellets (10K cells per pellet) in a 384-well format. Furthermore, we have carried out an initial screen of the NINDS small molecule library and demonstrated the feasibility of this technology for use in HTS of chondrogenesis.