Atomic force microscope investigation of the boundary-lubricant layer in articular cartilage

Fundamental, mechanism and development of hydration lubrication: From bio-inspiration to artificial manufacturing

Friction and lubrication are ubiquitous in all kinds of movements and play a vital role in the smooth operation of production machinery. Water is indispensable both in the lubrication systems of natural organisms and in hydration lubrication systems. There exists a high degree of similarity between these systems, which has driven the development of hydration lubrication from biomimetic to artificial manufacturing. In particular, significant advancements have been made in the understanding of the mechanisms of hydration lubrication over the past 30 years. This enhanced understanding has further stimulated the exploration of biomimetic inspiration from natural hydration lubrication systems, to develop novel artificial hydration lubrication systems that are cost-effective, easily transportable, and possess excellent capability. This review summarizes the recent experimental and theoretical advances in the understanding of hydration-lubrication processes. The entire paper is divided into three parts. Firstly, surface interactions relevant to hydration lubrication are discussed, encompassing topics such as hydrogen bonding, hydration layer, electric double layer force, hydration force, and Stribeck curve. The second part begins with an introduction to articular cartilage in biomaterial lubrication, discussing its compositional structure and lubrication mechanisms. Subsequently, three major categories of bio-inspired artificial manufacturing lubricating material systems are presented, including hydrogels, polymer brushes (e.g., neutral, positive, negative and zwitterionic brushes), hydration lubricant additives (e.g., nano-particles, polymers, ionic liquids), and their related lubrication mechanism is also described. Finally, the challenges and perspectives for hydration lubrication research and materials development are presented.

Insight into the assembly of lipid-hyaluronan complexes in osteoarthritic conditions

Interactions between molecules in the synovial fluid and the cartilage surface may play a vital role in the formation of adsorbed films that contribute to the low friction of cartilage boundary lubrication. Osteoarthritis (OA) is the most common degenerative joint disease. Previous studies have shown that in OA-diseased joints, hyaluronan (HA) not only breaks down resulting in a much lower molecular weight (MW), but also its concentration is reduced ten times. Here, we have investigated the structural changes of lipid-HA complexes as a function of HA concentration and MW to simulate the physiologically relevant conditions that exist in healthy and diseased joints. Small angle neutron scattering and dynamic light scattering were used to determine the structure of HA-lipid vesicles in bulk solution, while a combination of atomic force microscopy and quartz crystal microbalance was applied to study their assembly on a gold surface. We infer a significant influence of both MW and HA concentrations on the structure of HA-lipid complexes in bulk and assembled on a gold surface. Our results suggest that low MW HA cannot form an amorphous layer on the gold surface, which is expected to negatively impact the mechanical integrity and longevity of the boundary layer and could contribute to the increased wear of the cartilage that has been reported in joints diseased with OA.

Perspective in Achieving Stratified Articular Cartilage Repair Using Zonal Chondrocytes

Articular cartilage is composed of superficial, medial, and deep zones, which endow the tissue with biphasic mechanical properties to withstand shearing force and compressional loading. The tissue has very limited self-healing capacity once it is damaged due to its avascular nature. To prevent the early onset of osteoarthritis, surgical intervention is often needed to repair the injured cartilage. Current noncell-based and cell-based treatments focus on the regeneration of homogeneous cartilage to achieve bulk compressional properties without recapitulating the zonal matrix and mechanical properties, and often oversight in aiding cartilage integration between host and repair cartilage. It is hypothesized that achieving zonal architecture in articular cartilage tissue repair could improve the structural and mechanical integrity and thus the life span of the regenerated tissue. Engineering stratified cartilage constructs using zonal chondrocytes have been hypothesized to improve the functionality and life span of the regenerated tissues. However, stratified articular cartilage repair has yet to be realized to date due to the lack of an efficient zonal chondrocyte isolation method and an expansion platform that would allow both cell propagation and phenotype maintenance. Various attempts and challenges in achieving stratified articular cartilage repair in a clinical setting are evaluated. In this review, different perspectives on achieving stratified articular cartilage repair using zonal chondrocytes are described. The effectiveness of different zonal chondrocyte isolation and zonal chondrocyte phenotype maintenance methodologies during expansion are compared, with the focus on recent advancements in zonal chondrocyte isolation and expansion that could present a possible strategy to overcome the limitation of applying zonal chondrocytes to facilitate zonal architecture development in articular cartilage regeneration. Impact Statement The zonal properties of articular cartilage contribute to the biphasic mechanical properties of the tissues. Recapitulation of the zonal architecture in regenerated articular cartilage has been hypothesized to improve the mechanical integrity and life span of the regenerated tissue. This review provides a comprehensive discussion on the current state of research relevant to achieving stratified articular cartilage repair using zonal chondrocytes from different perspectives. This review further elaborates on a zonal chondrocyte production pipeline that can potentially overcome the current clinical challenges and future work needed to realize stratified zonal chondrocyte implantation in a clinical setting.

Hyperelastic characterization reveals proteoglycans drive the nanoscale strain-stiffening response in hyaline cartilage

Degenerative diseases such as osteoarthritis (OA) result in deterioration of cartilage extracellular matrix (ECM) components, significantly compromising tissue function. For measurement of mechanical properties at micron resolution, atomic force microscopy (AFM) is a leading technique in biomaterials research, including in the study of OA. It is common practice to determine material properties by applying classical Hertzian contact theory to AFM data. However, errors are consequential because the application of a linear elastic contact model to tissue ignores the fact that soft materials exhibit nonlinear properties even at small strains, influencing the biological conclusions of clinically-relevant studies. Additionally, nonlinear material properties are not well characterized, limiting physiological relevance of Young's modulus. Here, we probe the ECM of hyaline cartilage with AFM and explore the application of Hertzian theory in comparison to five hyperelastic models: NeoHookean, Mooney-Rivlin, Arruda-Boyce, Fung, and Ogden. The Fung and Ogden models achieved the best fits of the data, but the Fung model demonstrated robust sensitivity during model validation, demonstrating its ideal application to cartilage ECM and potentially other connective tissues. To develop a biological understanding of the Fung nonlinear parameter, we selectively degraded ECM components to target collagens (purified collagenase), hyaluronan (bacterial hyaluronidase), and glycosaminoglycans (chondroitinase ABC). We found significant differences in both Fung parameters in response to enzymatic treatment, indicating that proteoglycans drive the nonlinear response of cartilage ECM, and validating biological relevance of these phenomenological parameters. Our findings add value to the biomechanics community of using two-parameter material models for microindentation of soft biomaterials.

Intra-Articular Mesenchymal Stem Cell Injection for Knee Osteoarthritis: Mechanisms and Clinical Evidence

Knee osteoarthritis presents higher incidences than other joints, with increased prevalence during aging. It is a progressive process and may eventually lead to disability. Mesenchymal stem cells (MSCs) are expected to repair damaged issues due to trilineage potential, trophic effects, and immunomodulatory properties of MSCs. Intra-articular MSC injection was reported to treat knee osteoarthritis in many studies. This review focuses on several issues of intra-articular MSC injection for knee osteoarthritis, including doses of MSCs applied for injection and the possibility of cartilage regeneration following MSC injection. Intra-articular MSC injection induced hyaline-like cartilage regeneration, which could be seen by arthroscopy in several studies. Additionally, anatomical, biomechanical, and biochemical changes during aging and other causes participate in the development of knee osteoarthritis. Conversely, appropriate intervention based on these anatomical, biomechanical, biochemical, and functional properties and their interactions may postpone the progress of knee OA and facilitate cartilage repair induced by MSC injection. Hence, post-injection rehabilitation programs and related mechanisms are discussed.

Dopamine-conjugated hyaluronic acid delivered via intra-articular injection provides articular cartilage lubrication and protection

Due to its high molecular weight and viscosity, hyaluronic acid (HA) is widely used for viscosupplementation to provide joint pain relief in osteoarthritis. However, this benefit is temporary due to poor adhesion of HA on articular surfaces. In this study, we therefore conjugated HA with dopamine to form HADN, which made the HA adhesive while retaining its viscosity enhancement capacity. We hypothesized that HADN could enhance cartilage lubrication through adsorption onto the exposed collagen type II network and repair the lamina splendens. HADN was synthesized by carbodiimide chemistry between hyaluronic acid and dopamine. Analysis of Magnetic Resonance (NMR) and Ultraviolet spectrophotometry (Uv-vis) showed that HADN was successfully synthesized. Adsorption of HADN on collagen was demonstrated using Quartz crystal microbalance with dissipation (QCM-D). Ex vivo tribological tests including measurement of coefficient of friction (COF), dynamic creep, in stance (40 N) and swing (4 N) phases of gait cycle indicated adequate protection of cartilage by HADN with higher lubrication compared to HA alone. HADN solution at the cartilage-glass sliding interface not only retains the same viscosity as HA and provides fluid film lubrication, but also ensures better boundary lubrication through adsorption. To confirm the cartilage surface protection of HADN, we visualized cartilage wear using optical coherence tomography (OCT) and atomic force microscopy (AFM).

Particulate ECM biomaterial ink is 3D printed and naturally crosslinked to form structurally-layered and lubricated cartilage tissue mimics

Articular cartilage is a layered tissue with a complex, heterogenous structure and lubricated surface which is challenging to reproduce using traditional tissue engineering methods. 3D printing techniques have enabled engineering of complex scaffolds for cartilage regeneration, but constructs fail to replicate the unique zonal layers, and limited cytocompatible crosslinkers exist. To address the need for mechanically robust, layered scaffolds, we developed an extracellular matrix particle-based biomaterial ink (pECM biomaterial ink) which can be extruded, polymerizes via disulfide bonding, and restores surface lubrication. Our cartilage pECM biomaterial ink utilizes functionalized hyaluronan, a naturally occurring glycosaminoglycan, crosslinked directly to decellularized tissue particles (ø 40-100 µm). We experimentally determined that hyaluronan functionalized with thiol groups (t-HA) forms disulfide bonds with the ECM particles to form a 3D network. We show that two inks can be co-printed to create a layered cartilage scaffold with bulk compressive and surface (friction coefficient, adhesion, and roughness) mechanics approaching values measured on native cartilage. We demonstrate that our printing process enables the addition of macropores throughout the construct, increasing the viability of introduced cells by 10%. The delivery of these 3D printed scaffolds to a defect is straightforward, customizable to any shape, and adheres to surrounding tissue.

Articular Chondrocyte Phenotype Regulation through the Cytoskeleton and the Signaling Processes That Originate from or Converge on the Cytoskeleton: Towards a Novel Understanding of the Intersection between Actin Dynamics and Chondrogenic Function

Numerous studies have assembled a complex picture, in which extracellular stimuli and intracellular signaling pathways modulate the chondrocyte phenotype. Because many diseases are mechanobiology-related, this review asked to what extent phenotype regulators control chondrocyte function through the cytoskeleton and cytoskeleton-regulating signaling processes. Such information would generate leverage for advanced articular cartilage repair. Serial passaging, pro-inflammatory cytokine signaling (TNF-α, IL-1α, IL-1β, IL-6, and IL-8), growth factors (TGF-α), and osteoarthritis not only induce dedifferentiation but also converge on RhoA/ROCK/Rac1/mDia1/mDia2/Cdc42 to promote actin polymerization/crosslinking for stress fiber (SF) formation. SF formation takes center stage in phenotype control, as both SF formation and SOX9 phosphorylation for COL2 expression are ROCK activity-dependent. Explaining how it is molecularly possible that dedifferentiation induces low COL2 expression but high SF formation, this review theorized that, in chondrocyte SOX9, phosphorylation by ROCK might effectively be sidelined in favor of other SF-promoting ROCK substrates, based on a differential ROCK affinity. In turn, actin depolymerization for redifferentiation would “free-up” ROCK to increase COL2 expression. Moreover, the actin cytoskeleton regulates COL1 expression, modulates COL2/aggrecan fragment generation, and mediates a fibrogenic/catabolic expression profile, highlighting that actin dynamics-regulating processes decisively control the chondrocyte phenotype. This suggests modulating the balance between actin polymerization/depolymerization for therapeutically controlling the chondrocyte phenotype.

Triborheological Study under Physiological Conditions of PVA Hydrogel/HA Lubricant as Synthetic System for Soft Tissue Replacement

In soft tissue replacement, hydrophilic, flexible, and biocompatible materials are used to reduce wear and coefficient of friction. This study aims to develop and evaluate a solid/liquid triborheological system, polyvinyl alcohol (PVA)/hyaluronic acid (HA), to mimic conditions in human synovial joints. Hydrogel specimens prepared via the freeze–thawing technique from a 10% (w/v) PVA aqueous solution were cut into disc shapes (5 ± 0.5 mm thickness). Compression tests of PVA hydrogels presented a Young’s modulus of 2.26 ± 0.52 MPa. Friction tests were performed on a Discovery Hybrid Rheometer DHR-3 under physiological conditions using 4 mg/mL HA solution as lubricant at 37 °C. Contact force was applied between 1 and 20 N, highlighting a coefficient of friction change of 0.11 to 0.31 between lubricated and dry states at 3 N load (angular velocity: 40 rad/s). Thermal behavior was evaluated by differential scanning calorimetry (DSC) in the range of 25–250 °C (5 °C/min rate), showing an endothermic behavior with a melting temperature (Tm) around 231.15 °C. Scanning Electron Microscopy (SEM) tests showed a microporous network that enhanced water content absorption to 82.99 ± 1.5%. Hydrogel achieved solid/liquid lubrication, exhibiting a trapped lubricant pool that supported loads, keeping low coefficient of friction during lubricated tests. In dry tests, interstitial water evaporates continuously without countering sliding movement friction.

Effect of Loading on the Adhesion and Frictional Characteristics of Top Layer Articular Cartilage Nanoscale Contact: A Molecular Dynamics Study

Articular cartilage is a water-lubricated naturally occurring biological interface imparting unique mechanical and ultralow frictional properties in bone joints. Although the material of cartilage, synovial fluid composition, and their lubricating modes and properties have been extensively investigated at various scales experimentally, there is still a lack of understanding of load bearing, adhesion, and friction mechanisms of the cartilage-cartilage interface from an atomistic perspective under heavy loads. In this study, the effect of loading on adhesion and frictional behavior in articular cartilage is investigated with a proposed atomistic model for top layer cartilage-cartilage contact in unhydrated conditions using molecular dynamics (MD) simulations. Pull-off tests reveal that cohesive interactions occur at the interface due to formation of heavily interpenetrated atomistic sites leading to stretching and localized pulling of fragments during sliding. Sliding tests show that friction is load- and direction-dependent with the coefficient of friction (COF) obtained in the range of 0.20-0.75 at the interface for sliding in parallel and perpendicular directions to the collagen axis. These values are in good agreement with earlier nanoscale experimental results reported for the top layer cartilage-cartilage interface. The COF reduces with an increase in load and tends to be higher for the parallel sliding case than for the perpendicular case owing to the presence of the constant number of H-bonds. Overall, this work contributes toward understanding sliding in unhydrated biointerfaces, which is the precursor of wear, and provides insights into implant research.

Cryopreserved, Thin, Laser-Etched Osteochondral Allograft maintains the functional components of articular cartilage after 2 years of storage

BACKGROUND

Despite improvements in treatment options and techniques, articular cartilage repair continues to be a challenge for orthopedic surgeons. This study provides data to support that the 2-year Cryopreserved, Thin, Laser-Etched Osteochondral Allograft (T-LE Allograft) embodies the necessary viable cells, protein signaling, and extracellular matrix (ECM) scaffold found in fresh cartilage in order to facilitate a positive clinical outcome for cartilage defect replacement and repair.

METHODS

Viability testing was performed by digestion of the graft, and cells were counted using a trypan blue assay. Growth factor and ECM protein content was quantified using biochemical assays. A fixation model was introduced to assess tissue outgrowth capability and cellular metabolic activity in vitro. Histological and immunofluorescence staining were employed to confirm tissue architecture, cellular outgrowth, and presence of ECM. The effects of the T-LE Allograft to signal bone marrow-derived mesenchymal stem cell (BM-MSC) migration and chondrogenic differentiation were evaluated using in vitro co-culture assays. Immunogenicity testing was completed using flow cytometry analysis of cells obtained from digested T-LE Allografts and fresh articular cartilage.

RESULTS

Average viability of the T-LE Allograft post-thaw was found to be 94.97 ± 3.38%, compared to 98.83 ± 0.43% for fresh articular cartilage. Explant studies from the in vitro fixation model confirmed the long-term viability and proliferative capacity of these chondrocytes. Growth factor and ECM proteins were quantified for the T-LE Allograft revealing similar profiles to fresh articular cartilage. Cellular signaling of the T-LE Allograft and fresh articular cartilage both exhibited similar outcomes in co-culture for migration and differentiation of BM-MSCs. Flow cytometry testing confirmed the T-LE Allograft is immune-privileged as it is negative for immunogenic markers and positive for chondrogenic markers.

CONCLUSIONS

Using our novel, proprietary cryopreservation method, the T-LE Allograft, retains excellent cellular viability, with native-like growth factor and ECM composition of healthy cartilage after 2 years of storage at - 80 °C. The successful cryopreservation of the T-LE Allograft alleviates the limited availably of conventionally used fresh osteochondral allograft (OCA), by providing a readily available and simple to use allograft solution. The results presented in this paper supports clinical data that the T-LE Allograft can be a successful option for repairing chondral defects.

The tribology of cartilage: Mechanisms, experimental techniques, and relevance to translational tissue engineering

Diarthrodial joints, found at the ends of long bones, function to dissipate load and allow for effortless articulation. Essential to these functions are cartilages, soft hydrated tissues such as hyaline articular cartilage and the knee meniscus, as well as lubricating synovial fluid. Maintaining adequate lubrication protects cartilages from wear, but a decrease in this function leads to tissue degeneration and pathologies such as osteoarthritis. To study cartilage physiology, articular cartilage researchers have employed tribology, the study of lubrication and wear between two opposing surfaces, to characterize both native and engineered tissues. The biochemical components of synovial fluid allow it to function as an effective lubricant that exhibits shear-thinning behavior. Although tribological properties are recognized to be essential to native tissue function and a critical characteristic for translational tissue engineering, tribology is vastly understudied when compared to other mechanical properties such as compressive moduli. Further, tribometer configurations and testing modalities vary greatly across laboratories. This review aims to define commonly examined tribological characteristics and discuss the structure-function relationships of biochemical constituents known to contribute to tribological properties in native tissue, address the variations in experimental set-ups by suggesting a move toward standard testing practices, and describe how tissue-engineered cartilages may be augmented to improve their tribological properties.

Biocompatible implant mimicking cartilage: A new horizon for reconstructive facial field

Cartilage is avascular with limited to no regenerative capacity, so its loss could be a challenge for reconstructive surgery. Current treatment options for damaged cartilage are also limited. In this aspect there is a tremendous need to develop an ideal cartilage-mimicking biomaterial that could repair maxillofacial defects. Considering this fact in this study we have prepared twelve silicone-based materials (using Silicone 40, 60, and 80) reinforced with hydroxyapatite, tri-calcium phosphate, and titanium dioxide which itself has proven their efficacy in several studies and able to complement the shortcomings of using silicones. Among the mechanical properties (Young's modulus, tensile strength, percent elongation, and hardness), hardness of Silicone-40 showed similarities with goat ear (P > .05). Silicone peaks have been detected in FTIR. Both AFM morphology and SEM images of the samples confirmed more roughed surfaces. All the materials were nonhemolytic in hemocompatibility tests, but among the twelve materials S2, S3, S5, and S6 showed the least hemolysis. For all tested bacterial strains, adherence was lower on each material than that grown on the plain industrial silicone material which was used as a positive control. S2, S3, S5, and S6 samples were selected as the best based on mechanical characterizations, surface characterizations, in vitro hemocompatibility tests and bacterial adherence activity. So, outcomes of this present study would be promising when developing ideal cartilage-mimicking biocomposites and their emerging applications to treat maxillofacial defects due to cartilage damage.

Biotribology of Synovial Cartilage: A New Method for Visualization of Lubricating Film and Simultaneous Measurement of the Friction Coefficient

A healthy natural synovial joint is very important for painless active movement of the natural musculoskeletal system. The right functioning of natural synovial joints ensures well lubricated contact surfaces with a very low friction coefficient and wear of cartilage tissue. The present paper deals with a new method for visualization of lubricating film with simultaneous measurements of the friction coefficient. This can contribute to better understanding of lubricating film formation in a natural synovial joint. A newly developed device, a reciprocating tribometer, is used to allow for simultaneous measurement of friction forces with contact visualization by fluorescence microscopy. The software allowing for snaps processing and subsequent evaluation of fluorescence records is developed. The evaluation software and the follow-up evaluation procedure are also described. The experiments with cartilage samples and model synovial fluid are carried out, and the new software is applied to provide their evaluation. The primary results explaining a connection between lubrication and friction are presented. The results show a more significant impact of albumin proteins on the lubrication process, whereas its clusters create a more stable lubrication layer. A decreasing trend of protein cluster count, which corresponds to a decrease in the thickness of the lubrication film, is found in all experiments. The results highlight a deeper connection between the cartilage friction and the lubrication film formation, which allows for better understanding of the cartilage lubrication mechanism.

Metal-based micro and nanosized pollutant in marine organisms: What can we learn from a combined atomic force microscopy-scanning electron microscopy study

Atomic force microscopy (AFM) is a consolidated technique for the study of biological systems, usually ex vivo or in culture, under different experimental conditions. Yet, the diffusion of the technique in the scientific context of histology is still rather slow and limited. In the present work, we demonstrate the potential of AFM, in terms of morphological and nanomechanical imaging, to study the effects of nano- and micro-sized metallic pollutants in living biological systems. As a model, we investigated marine molluscs (Mytilus galloprovincialis) grown in the Adriatic Sea. We characterized histological sections from two organs (gonads and digestive glands) of molluscs collected during several surveys at different growth time and distance from gas extraction platforms. We evaluated the effects of nano-pollutants mostly on the local tissue structure by combining AFM microscopy with scanning electron microscopy (SEM). Furthermore, the AFM images allowed evidencing the presence of nano- or micro-sized structures that exhibit different nanomechanical properties compared to the rest of the tissue. The results demonstrate how coupling AFM and SEM analysis can provide an effective procedure to evaluate the morphological alterations produced by the exposure to exogenous nano-pollutants in tissue and constitute a promising way to reveal basic mechanisms mediating the cytotoxicity of specific exogenous pollutants ingested by edible organisms.

Probing the frictional properties of soft materials at the nanoscale

The understanding of friction in soft materials is of increasing importance due to the demands of industries such as healthcare, biomedical, food and personal care, the incorporation of soft materials into technology, and in the study of interacting biological interfaces. Many of these processes occur at the nanoscale, but even at micrometer length scales there are fundamental aspects of tribology that remain poorly understood. With the advent of Friction Force Microscopy (FFM), there have been many fundamental insights into tribological phenomena at the atomic scale, such as 'stick-slip' and 'super-lubricity'. This review examines the growing field of soft tribology, the experimental aspects of FFM and its underlying theory. Moving to the nanoscale changes the contact mechanics which govern adhesive forces, which in turn play a pivotal role in friction, along with the deformation of the soft interface and dissipative phenomena. We examine recent progress and future prospects in soft nanotribology.

Proteoglycan 4 predicts tribological properties of repaired cartilage tissue

Purpose: One of the essential requirements in maintaining the normal joint motor function is the perfect tribological property of the articular cartilage. Many cartilage regeneration strategies have been developed for treatment in early stages of osteoarthritis, but there is little information on how repaired articular cartilage regains durability. The identification of biomarkers that can predict wear resistant property is critical to advancing the success of cartilage regeneration therapies. Proteoglycan 4 (PRG4) is a macromolecule distributing on the chondrocyte surface that contributes to lubrication. In this study, we investigate if PRG4 expression is associated with tribological properties of regenerated cartilage, and is able to predict its wear resistant status.

Methods: Two different strategies including bone marrow enrichment plus microfracture (B/BME-MFX) and microfracture alone (B-MFX) of cartilage repair in sheep were used. PRG4 expression and a series of tribological parameters on regenerated cartilage were rigorously examined and compared.

Results: Highly and continuously expression of PRG4 in regenerated cartilage surface was negatively correlated with each tribological parameter (P<0.0001, respectively). Multivariate analysis showed that PRG4 expression was the key predictor that contributed to the promotion of cartilage wear resistance.

Conclusion: Higher PRG4 expression in regenerated cartilage is significantly associated with wear resistance improvement. PRG4 may be useful for predicting the wear resistant status of regenerated cartilage and determining the optimal cartilage repair strategy.

Dynamics of Synovial Fluid Aggregation under Shear

The synovial fluid (SF) that lubricates articular joints exhibits complex rheological and tribological properties due to the interactions and behaviors of its various molecular components. Under shear, SF films abruptly thicken by more than 300% and large, dense aggregates form within the fluid. In this study, we used the Surface Force Apparatus to elucidate which SF components are involved in this shear-induced transformation by (i) determining which (if any) of all major SF components replicate the behavior of SF under shear and (ii) observing the effect of removing implicated components from SF by enzymatic digestion. While most previous studies of SF have focused on the tribological roles of lubricin or hyaluronic acid, our results indicate that albumin is a key contributor to the formation of aggregates in SF under shear. Our results also suggest that SF aggregation is associated with efficient surface protection against wear. As our findings are based on experiments involving rigid, nonporous surfaces, they may be used to investigate shear-mediated aggregation mechanisms occurring during the lubrication of artificial joints, ultimately advancing our current vision of implant design.

Mechanics of Brain Tissues Studied by Atomic Force Microscopy: A Perspective

Tissue morphology and mechanics are crucial to the regulation of organ function. Investigating the exceptionally complex tissue of the brain at the sub-micron scale is challenging due to the complex structure and softness of this tissue, despite the large interest of biologists, medical engineers, biophysicists, and others in this topic. Atomic force microscopy (AFM) both as an imaging and as a mechanical tool provides an excellent opportunity to study soft biological samples such as live brain tissues. Here we review the principles of AFM, the performance of AFM in tissue imaging and mechanical mapping of cells and tissues, and finally opening the prospects and challenges of probing the biophysical properties of brain tissue using AFM.

Chronic inflammation deteriorates structure and function of collagen fibril in rat temporomandibular joint disc

Collagen is the building component of temporomandibular joint (TMJ) discs and is often affected by inflammation in temporomandibular disorders. The macromechanical properties of collagen are deteriorated by chronic inflammation. However, the mechanism by which inflammation influences disc function remains unknown. The relationship between the ultrastructure and nanomechanical properties of collagen in inflamed discs should be clarified. Seven-week-old female Sprague-Dawley rats were randomly divided into two groups. Chronic TMJ inflammation was induced by intra-articular injection of complete Freund's adjuvant, and samples were harvested after 5 weeks. Picrosirius staining revealed multiple colours under polarized light, which represented alternative collagen bundles in inflamed discs. Using atomic force microscopy scanning, the magnitude of Young's modulus was reduced significantly accompanied with disordered collagen fibril arrangement with porous architecture of inflamed discs. Transmission electron microscopy scanning revealed a non-uniform distribution of collagen fibres, and oversized collagen fibrils were observed in inflamed discs. Fourier transform infrared microspectroscopy revealed a decrease in 1 338 cm^-1/amide II area ratio of collagen in different regions. The peak positions of amide I and amide II bands were altered in inflamed discs, indicating collagen unfolding. Our results suggest that sustained inflammation deteriorates collagen structures, resulting in the deterioration of the ultrastructure and nanomechanical properties of rat TMJ discs.

Biomimetic proteoglycans can molecularly engineer early osteoarthritic cartilage in vivo

Biomimetic proteoglycans (BPGs) have the potential to treat osteoarthritis (OA) given that these molecules mimic the structure and properties of natural proteoglycans, which are significantly reduced in OA. We examined the effects of BPGs injected into the intra-articular space in an in vivo OA rabbit knee model and evaluated the effect on histological response, joint friction, and BPG distribution and retention. Rabbits underwent ACL transection to create an arthritic state after 5 weeks. OA rabbits were treated with BPGs or Euflexxa® (hyaluronic acid) intra-articular injections. Non-OA rabbits were injected similarly with BPGs; contralateral joints served as controls. The progression of OA and response to injections were evaluated using Mankin and gross grading systems indicating that mild OA was achieved in operated joints. The coefficient of friction (COF) of the intact knee joints were measured using a custom pendulum friction apparatus, showing that OA joints and OA + Euflexxa® joints demonstrated increased COF than non-operated controls, while BPG-injected non-OA and OA + BPGs were not significantly different from non-OA controls. Injected fluorescently labeled BPGs demonstrated that BPGs diffused into cartilage with localization in the pericellular region. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:403-411, 2019.

Molecularly Engineered Biolubricants for Articular Cartilage

Lubrication within articular joints plays a crucial role in daily life, providing an extremely low coefficient of friction and preventing wear at the surface of the articular cartilage. Natural biomacromolecules responsible for lubrication are part of the synovial fluid and their degradation is associated with the onset of degenerative diseases, such as osteoarthritis (OA). The current absence of effective treatments for OA has captured the attention of chemists and material scientists over the last two decades, triggering the development of partially or fully synthetic biolubricants aimed to reduce friction within the joints and restore cartilage functions. Although there is still a long way to go before synthetic replacements of natural biolubricants can be applied clinically, this review highlights those formulations that meet the fundamental requirements for being efficient lubricants for articular cartilage.

Detection of depth-depend changes in porcine cartilage after wear test using Raman spectroscopy

Cartilage damage and wear can lead to severe diseases, such as osteoarthritis, thus, many studies on the cartilage wear process have already been performed to better understand the cartilage wear mechanism. However, most characterization methods focus on the cartilage surface or the total wear extent. With the advantages of high spatial resolution and easy characterization, Raman microspectroscopy was employed for the first time to characterize full-depth changes in the cartilage extracellular matrix (ECM) after wear test. Sections from the cartilage samples after wear were compared with sections from the control group. Univariate and multivariate analyses both indicated that collagen content loss at certain depths (20%-30% relative to the cartilage surface) is possibly the dominating alteration during wear rather than changes in collagen fiber orientation or proteoglycan content. These findings are consistent with the observations obtained by scanning electron microscopy and histological staining. This study successfully used Raman microspectroscopy efficiently assess full-depth changes in cartilage ECM after wear test, thus providing new insight into cartilage damage and wear.

Functional properties of chondrocytes and articular cartilage using optical imaging to scanning probe microscopy

Mature chondrocytes in adult articular cartilage vary in number, size, and shape, depending on their depth in the tissue, location in the joint, and source species. Chondrocytes are the primary structural, functional, and metabolic unit in articular cartilage, the loss of which will induce fatigue to the extracellular matrix (ECM), eventually leading to failure of the cartilage and impairment of the joint as a whole. This brief review focuses on the functional and biomechanical studies of chondrocytes and articular cartilage, using microscopic imaging from optical microscopies to scanning probe microscopy. Three topics are covered in this review, including the functional studies of chondrons by optical imaging (unpolarized and polarized light and infrared light, two-photon excitation microscopy), the probing of chondrocytes and cartilage directly using microscale measurement techniques, and different imaging approaches that can measure chondrocyte mechanics and chondrocyte biological signaling under in situ and in vivo environments. Technical advancement in chondrocyte research during recent years has enabled new ways to study the biomechanical and functional properties of these cells and cartilage. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:620-631, 2018.

In Vivo Cellular Infiltration and Remodeling in a Decellularized Ovine Osteochondral Allograft

Interest in decellularized tissues has steadily gained as potential solutions for degenerative diseases and traumatic events, replacing sites of missing tissue, and providing the relevant biochemistry and microstructure for tissue ingrowth and regeneration. Osteoarthritis, a progressive and debilitating disease, is often initiated with the formation of a focal defect in the otherwise smooth surface of articular cartilage. Decellularized cartilage tissue, which maintains the structural complexity of the native extracellular matrix, has the potential to provide a clinically relevant solution to focal defects or large tissue damage, possibly even circumventing or complementing current techniques such as microfracture and mosaicplasty. However, it is currently unclear whether implantation of decellularized cartilage in vivo may provide a mechanically and biochemically relevant platform to promote cell remodeling and repair. We examined whole decellularized osteochondral allografts implanted in the ovine trochlear groove to investigate cellular remodeling and repair tissue quality compared to empty defects and contralateral controls (healthy cartilage). At 3 months postsurgery, cells were observed in both the decellularized tissue and empty defects, although both at significantly lower levels than healthy cartilage. Qualitative and quantitative histological analysis demonstrated maintenance of cartilage features of the decellularized implant similar to healthy cartilage groups. Noninvasive analysis by quantitative magnetic resonance imaging showed no difference in T_1ρ and T₂* between all groups. Investigation of the mechanical properties of repair tissue showed significantly lower elasticity in decellularized implants and empty defects compared to healthy cartilage, but similar tribological quantities. Overall, this study suggests that decellularized cartilage implants are subject to cellular remodeling in an in vivo environment and may provide a potential tissue engineering solution to cartilage defect interventions.

Modulation of Superficial Zone Protein/Lubricin/PRG4 by Kartogenin and Transforming Growth Factor-β1 in Surface Zone Chondrocytes in Bovine Articular Cartilage

OBJECTIVE

Superficial zone protein (SZP)/lubricin/PRG4 functions as a boundary lubricant in articular cartilage to decrease friction and wear. As articular cartilage lubrication is critical for normal joint function, the accumulation of SZP at the surface of cartilage is important for joint homeostasis. Recently, a heterocyclic compound called kartogenin (KGN) was found to induce chondrogenic differentiation and enhance mRNA expression of lubricin. The objective of this study was to determine whether KGN can stimulate synthesis of SZP in superficial zone, articular chondrocytes.

DESIGN

We investigated the effects of KGN and transforming growth factor-β1 (TGF-β1) on articular cartilage and synovium of the bovine knee joint by evaluating SZP secretion by enzyme-linked immunosorbent assay analysis. Monolayer, micromass, and explant cultures of articular cartilage, and monolayer culture of synoviocytes, were treated with KGN. SZP accumulation in the medium was evaluated and mRNA expression was measured through quantitative polymerase chain reaction.

RESULTS

TGF-β1 stimulated SZP secretion by superficial zone chondrocytes in monolayer, explant, and micromass cultures as expected. In addition, SZP secretion was inhibited by IL-1β in explant cultures, and enhanced by TGF-β1 in synoviocyte monolayer cultures. Although KGN elicited a 1.2-fold increase in SZP mRNA expression in combination with TGF-β1, KGN neither stimulated any significant increases in SZP synthesis nor prevented catabolic decreases in SZP production from IL-1β.

CONCLUSIONS

These data suggest that the chondrogenic effects of KGN depend on cellular phenotype and differentiation status, as KGN did not alter SZP synthesis in differentiated, superficial zone articular chondrocytes.

Mapping the Nonreciprocal Micromechanics of Individual Cells and the Surrounding Matrix Within Living Tissues

The biomechanical properties of the extracellular matrix (ECM) play an important role in cell migration, gene expression, and differentiation. Biomechanics measurements of ECM are usually performed on cryotomed tissue sections. However, studies on cell/matrix interplay are impossible to perform due to disruptions in cell viability and tissue architecture from freeze-thaw cycling. We developed a technique to map the stiffness of living cells and surrounding matrix by atomic force microscopy and use fluorescence microscopy to relate those properties to changes in matrix and cell structure in embryonic and adult tissues in situ. Stiffness mapping revealed significant differences between vibratomed (living) and cryotomed tissues. Isolated cells are softer than those in native matrix, suggesting that cell mechanics are profoundly influenced by their three-dimensional environment and processing state. Viable tissues treated by hyaluronidase and cytochalasin D displayed targeted disruption of matrix and cytoskeletal networks, respectively. While matrix stiffness affected cellular stiffness, changes in cell mechanics did not reciprocally influence matrix stiffness.

Quantitative OCT and MRI biomarkers for the differentiation of cartilage degeneration

OBJECTIVE

To evaluate the usefulness of quantitative parameters obtained by optical coherence tomography (OCT) and magnetic resonance imaging (MRI) in the comprehensive assessment of human articular cartilage degeneration.

MATERIALS AND METHODS

Human osteochondral samples of variable degeneration (n = 45) were obtained from total knee replacements and assessed by MRI sequences measuring T1, T1ρ, T2 and T2* relaxivity and by OCT-based quantification of irregularity (OII, optical irregularity index), homogeneity (OHI, optical homogeneity index]) and attenuation (OAI, optical attenuation index]). Samples were also assessed macroscopically (Outerbridge classification) and histologically (Mankin classification) as grade-0 (Mankin scores 0-4)/grade-I (scores 5-8)/grade-II (scores 9-10)/grade-III (score 11-14). After data normalisation, differences between Mankin grades and correlations between imaging parameters were assessed using ANOVA and Tukey's post-hoc test and Spearman's correlation coefficients, respectively. Sensitivities and specificities in the detection of Mankin grade-0 were calculated.

RESULTS

Significant degeneration-related increases were found for T2 and OII and decreases for OAI, while T1, T1ρ, T2* or OHI did not reveal significant changes in relation to degeneration. A number of significant correlations between imaging parameters and histological (sub)scores were found, in particular for T2 and OII. Sensitivities and specificities in the detection of Mankin grade-0 were highest for OHI/T1 and OII/T1ρ, respectively.

CONCLUSION

Quantitative OCT and MRI techniques seem to complement each other in the comprehensive assessment of cartilage degeneration. Sufficiently large structural and compositional changes in the extracellular matrix may thus be parameterized and quantified, while the detection of early degeneration remains challenging.

The synergistic effects of shear stress and cyclic hydrostatic pressure modulate chondrogenic induction of human mesenchymal stem cells

PURPOSE

In this study, we examined chondrogenic regulation of 2 types of mesenchymal stem cells seeded on the bioengineered substrate in monolayer cultures under mechanically defined conditions to mimic the in vivo microenvironment of chondrocytes within articular cartilage tissues.

METHODS

Human adipose-derived mesenchymal stem cells (ASCs) and bone marrow mesenchymal stem cells (BSCs) were exposed to 0.2 Pa shear stress, 3 MPa cyclic hydrostatic pressure, and combined loading with different sequences on chemically designed medical-grade silicone rubber, while no soluble growth factors were added to the culture medium. The expression levels of chondrogenic-specific genes of SOX9, aggrecan, and type II collagen (Col II) were measured. Results were compared to those of cells treated by biological growth factor.

RESULTS

Gene expression patterns were dependent on the loading regime. Moreover, the source of mesenchymal stem cells (adipose or bone marrow) was influential in gene expression. Overall, enhanced expression of chondrogenic markers was found through application of mechanical stimuli. The response was generally found to be significantly promoted when the 2 loading regimes were superimposed.

CONCLUSIONS

Differentiation of ASCs was shown by a modest increase in gene expression profiles. In general, BSCs expressed higher levels of chondrogenic gene expression than ASCs after 3 weeks. A greater effect on Col II and SOX9 mRNA expression was observed when combined loadings were applied. Results may be applied in determining the proper loading sequence for obtaining functional target cells in cartilage engineering applications.

Surface zone articular chondrocytes modulate the bulk and surface mechanical properties of the tissue-engineered cartilage

The central hypothesis of functional tissue engineering is that an engineered construct can serve as a viable replacement tissue in vivo by replicating the structure and function of native tissue. In the case of articular cartilage, this requires the reproduction of the bulk mechanical and surface lubrication properties of native hyaline cartilage. Cartilage tissue engineering has primarily focused on achieving the bulk mechanical properties of native cartilage such as the compressive aggregate modulus and tensile strength. A scaffold-free self-assembling process has been developed that produces engineered cartilage with compressive properties approaching native tissue levels. Thus, the next step in this process is to begin addressing the friction coefficient and wear properties of these engineered constructs. The superficial zone protein (SZP), also known as lubricin or PRG4, is a boundary mode lubricant that is synthesized by surface zone (SZ) articular chondrocytes. Under conditions of high loading and low sliding speeds, SZP reduces friction and wear at the articular surface. The objective of this investigation was to determine whether increasing the proportion of SZ chondrocytes in cartilage constructs, in the absence of external stimuli such as growth factors and mechanical loading, would enhance the secretion of SZP and improve their frictional properties. In this study, cartilage constructs were engineered through a self-assembling process with varying ratios of SZ and middle zone (MZ) chondrocytes (SZ:MZ): 0:100, 25:75, 50:50, 75:25, and 100:0. Constructs containing different ratios of SZ and MZ chondrocytes did not significantly differ in the glycosaminoglycan composition or compressive aggregate modulus. In contrast, tensile properties and collagen content were enhanced in nearly all constructs containing greater amounts of SZ chondrocytes. Increasing the proportion of SZ chondrocytes had the hypothesized effect of improving the synthesis and secretion of SZP. However, increasing the SZ chondrocyte fraction did not significantly reduce the friction coefficient. These results demonstrate that additional factors, such as SZP-binding macromolecules, surface roughness, and adhesion, need to be examined to modulate the lubrication properties of engineered cartilage.

Synthesis and characterization of a lubricin mimic (mLub) to reduce friction and adhesion on the articular cartilage surface

The lubricating proteoglycan, lubricin, facilitates the remarkable low friction and wear properties of articular cartilage in the synovial joints of the body. Lubricin lines the joint surfaces and plays a protective role as a boundary lubricant in sliding contact; decreased expression of lubricin is associated with cartilage degradation and the pathogenesis of osteoarthritis. An unmet need for early osteoarthritis treatment is the development of therapeutic molecules that mimic lubricin function and yet are also resistant to enzymatic degradation common in the damaged joint. Here, we engineered a lubricin mimic (mLub) that is less susceptible to enzymatic degradation and binds to the articular surface to reduce friction. mLub was synthesized using a chondroitin sulfate backbone with type II collagen and hyaluronic acid (HA) binding peptides to promote interaction with the articular surface and synovial fluid constituents. In vitro and in vivo characterization confirmed the binding ability of mLub to isolated type II collagen and HA, and to the cartilage surface. Following trypsin treatment to the cartilage surface, application of mLub, in combination with purified or commercially available hyaluronan, reduced the coefficient of friction, and adhesion, to control levels as assessed over macro-to micro-scales by rheometry and atomic force microscopy. In vivo studies demonstrate an mLub residency time of less than 1 week. Enhanced lubrication by mLub reduces surface friction and adhesion, which may suppress the progression of degradation and cartilage loss in the joint. mLub therefore shows potential for treatment in early osteoarthritis following injury.

Lamellar slippage of bilayers--a hypothesis on low friction of natural joints

The cartilage's amphoteric surface behavior is a physical phenomenon in biological lubrication. However, there is a lack of knowledge on amphoteric phospholipids bilayers and in overcoming friction in cartilage joints. In this paper, friction experiments were conducted, and the cartilage's surface was characterized using pH and wettability, while the interfacial energy and coefficients were determined. The lamellar slippage of bilayers and a short-range repulsion between the interfaces of negatively charged (-PO4 (-)) cartilage surfaces resulted in low frictional properties of the joint.

The distribution of superficial zone protein (SZP)/lubricin/PRG4 and boundary mode frictional properties of the bovine diarthrodial joint

The diarthrodial, knee joint is a remarkably efficient bearing system; articulating cartilage surfaces provide nearly frictionless performance with minimal wear. The low friction properties of the cartilage surfaces are due in part to the boundary lubricant, superficial zone protein (SZP); also known as lubricin or proteoglycan 4 (PRG4). In previous work, SZP localization and cartilage friction were examined across the femoral condyles. Studies in the literature have also individually investigated the other tissues that comprise the human knee and four-legged animal stifle joint, such as the meniscus or patella. However, comparisons between individual studies are limited due to the variable testing conditions employed. Friction is a system property that is dependent on the opposing articulating surface, entraining speed, and loading. A cross-comparison of the frictional properties and SZP localization across the knee/stifle joint tissues utilizing a common testing configuration is therefore needed. The objective of this investigation was to determine the friction coefficient and SZP localization of the tissues comprising the three compartments of the bovine stifle joint: patella, patellofemoral groove, femoral condyles, meniscus, tibial plateau, and anterior cruciate ligament. The boundary mode coefficient of friction was greater in tissues of the patellofemoral compartment than the lateral and medial tibiofemoral compartments. SZP immunolocalization followed this trend with reduced depth of staining and intensity in the patella and patellofemoral groove compared to the femoral condyles and tibial plateau. These results illustrate the important role of SZP in reducing friction in the tissues and compartments of the knee/stifle joint.

Role of hyaluronic acid and phospholipid in the lubrication of a cobalt-chromium head for total hip arthroplasty

The tribological performance of total hip arthroplasty has an important influence on its success rate. This study examined the concentration-dependent role of hyaluronic acid (HA) and phospholipid (dipalmitoylphosphatidylcholine, DPPC) in the boundary lubricating ability of retrieved cobalt-chromium femoral heads. The microscale frictional coefficients (μ) were measured by atomic force microscopy using a rectangular silicon cantilever integrated with sharp silicon tips. In the case of HA lubricant, the frictional coefficients decreased significantly at concentrations of 2.0 (0.16 ± 0.03) and 3.5 mg/ml (0.11 ± 0.01) while increased at 5.0 mg/ml (0.15 ± 0.01), compared to that with phosphate buffer saline (0.25 ± 0.03). The concentration-dependent lubrication behavior of DPPC was most effective when DPPC was in the physiological concentration range, showing μ = 0.16 ± 0.01 in polypropylene glycol, and 0.05 ± 0.01, 0.02 ± 0.01, and 0.03 ± 0.01 at a DPPC concentration of 0.05, 0.2, and 3.0 mg/ml, respectively. Results obtained show significant differences between the DPPC concentration groups. Conclusively, the microscale frictional response of the retrieved CoCr femoral head has a significant dependence on the concentrations of HA and DPPC. Moreover, observed optimal concentration of HA and DPPC for effective lubrication is similar to that observed in normal human synovial fluid. Therefore, a retrieval of the synovia may be considered during total hip replacement surgeries in an effort for reduction of friction between head and liner of total hip replacement implants.

Investigations of micron and submicron wear features of diseased human cartilage surfaces

Osteoarthritis is a common disease. However, its causes and morphological features of diseased cartilage surfaces are not well understood. The purposes of this research were (a) to develop quantitative surface characterization techniques to study human cartilages at a micron and submicron scale and (b) to investigate distinctive changes in the surface morphologies and biomechanical properties of the cartilages in different osteoarthritis grades. Diseased cartilage samples collected from osteoarthritis patients were prepared for image acquisition using two different techniques, that is, laser scanning microscopy at a micrometer scale and atomic force microscopy at a nanometer scale. Three-dimensional, digital images of human cartilages were processed and analyzed quantitatively. This study has demonstrated that high-quality three-dimensional images of human cartilage surfaces could be obtained in a hydrated condition using laser scanning microscopy and atomic force microscopy. Based on the numerical data extracted from improved image quality and quantity, it has been found that osteoarthritis evolution can be identified by specific surface features at the micrometer scale, and these features are amplitude and functional property related. At the submicron level, the spatial features of the surfaces were revealed to differ between early and advanced osteoarthritis grades. The effective indentation moduli of human cartilages effectively revealed the cartilage deterioration. The imaging acquisition and numerical analysis methods established allow quantitative studies of distinctive changes in cartilage surface characteristics and better understanding of the cartilage degradation process.

Effects of enzymatic degradation after loading in temporomandibular joint

Synovial fluid of the joint decreases friction between the cartilage surfaces and reduces cartilage wear during articulation. Characteristic changes of synovial fluid have been shown in patients with osteoarthritis (OA) in the temporomandibular joint (TMJ). OA is generally considered to be induced by excessive mechanical stress. However, whether the changes in synovial fluid precede the mechanical overloading or vice versa remains unclear. In the present study, our purpose was to examine if the breakdown of joint lubrication affects the frictional properties of mandibular condylar cartilage and leads to subsequent degenerative changes in TMJ. We measured the frictional coefficient in porcine TMJ by a pendulum device after digestion with hyaluronidase (HAase) or trypsin. Gene expressions of interleukin-1β (IL-1β), cyclooxygenase-2 (COX-2), matrix metalloproteinases (MMPs), type II collagen, and histology were examined after prolonged cyclic loading by an active pendulum system. The results showed that the frictional coefficient increased significantly after HAase (35%) or trypsin (74%) treatment. Gene expression of IL-1β, COX-2, and MMPs-1, -3, and -9 increased significantly in enzyme-treated TMJs after cyclic loading. The increase in the trypsin-treated group was greater than that in the HAase-treated group. Type II collagen expression was reduced in both enzyme-treated groups. Histology revealed surface fibrillation and increased MMP-1 in the trypsin-treated group, as well as increased IL-1β in both enzyme-treated groups after cyclic loading. The findings demonstrated that the compromised lubrication in TMJ is associated with altered frictional properties and surface wear of condylar cartilage, accompanied by release of pro-inflammatory and matrix degradation mediators under mechanical loading.

Both hyaluronan and collagen type II keep proteoglycan 4 (lubricin) at the cartilage surface in a condition that provides low friction during boundary lubrication

Wear resistant and ultralow friction in synovial joints is the outcome of a sophisticated synergy between the major macromolecules of the synovial fluid, e.g., hyaluronan (HA) and proteoglycan 4 (PRG4), with collagen type II fibrils and other non-collagenous macromolecules of the cartilage superficial zone (SZ). This study aimed at better understanding the mechanism of PRG4 localization at the cartilage surface. We show direct interactions between surface bound HA and freely floating PRG4 using the quartz crystal microbalance with dissipation (QCM-D). Freely floating PRG4 was also shown to bind with surface bound collagen type II fibrils. Albumin, the most abundant protein of the synovial fluid, effectively blocked the adsorption of PRG4 with HA, through interaction with C and N terminals on PRG4, but not that of PRG4 with collagen type II fibrils. The above results indicate that collagen type II fibrils strongly contribute in keeping PRG4 in the SZ during cartilage articulation in situ. Furthermore, PRG4 molecules adsorbed very well on mimicked SZ of absorbed HA molecules with entangled collagen type II fibrils and albumin was not able to block this interaction. In this last condition PRG4 adsorption resulted in a coefficient of friction (COF) of the same order of magnitude as the COF of natural cartilage, measured with an atomic force microscope in lateral mode.

Effects of hyaluronic acid and γ-globulin concentrations on the frictional response of human osteoarthritic articular cartilage

Synovial fluid plays an important role in lubricating synovial joints. Its main constituents are hyaluronic acid (HA) and γ-globulin, acting as boundary lubricants for articular cartilage. The aim of the study was to demonstrate the concentration-dependent effect of HA and γ-globulin on the boundary-lubricating ability of human osteoarthritis (OA) cartilage. Normal, early and advance stage articular cartilage samples were obtained from human femoral heads and in presence of either HA or γ-globulin, cartilage frictional coefficient (µ) was measured by atomic force microscopy (AFM). In advanced stage OA, the cartilage superficial layer was observed to be completely removed and the damaged cartilage surface showed a higher µ value (∼ 0.409) than the normal cartilage surface (∼ 0.119) in PBS. Adsorbed HA and γ-globulin molecules significantly improved the frictional behavior of advanced OA cartilage, while they were ineffective for normal and early OA cartilage. In advanced-stage OA, the concentration-dependent frictional response of articular cartilage was observed with γ-globulin, but not with HA. Our result suggested that HA and γ-globulin may play a significant role in improving frictional behavior of advanced OA cartilage. During early-stage OA, though HA and γ-globulin had no effect on improving frictional behavior of cartilage, however, they might contribute to disease modifying effects of synovial fluid as observed in clinical settings.

Functional imaging in OA: role of imaging in the evaluation of tissue biomechanics

Functional imaging refers broadly to the visualization of organ or tissue physiology using medical image modalities. In load-bearing tissues of the body, including articular cartilage lining the bony ends of joints, changes in strain, stress, and material properties occur in osteoarthritis (OA), providing an opportunity to probe tissue function through the progression of the disease. Here, biomechanical measures in cartilage and related joint tissues are discussed as key imaging biomarkers in the evaluation of OA. Emphasis will be placed on the (1) potential of radiography, ultrasound, and magnetic resonance imaging to assess early tissue pathomechanics in OA, (2) relative utility of kinematic, structural, morphological, and biomechanical measures as functional imaging biomarkers, and (3) improved diagnostic specificity through the combination of multiple imaging biomarkers with unique contrasts, including elastography and quantitative assessments of tissue biochemistry. In comparison to other modalities, magnetic resonance imaging provides an extensive range of functional measures at the tissue level, with conventional and emerging techniques available to potentially to assess the spectrum of preclinical to advance OA.

Toward regeneration of articular cartilage

Articular cartilage is classified as permanent hyaline cartilage and has significant differences in structure, extracelluar matrix components, gene expression profile, and mechanical property from transient hyaline cartilage found in the epiphyseal growth plate. In the process of synovial joint development, articular cartilage originates from the interzone, developing at the edge of the cartilaginous anlagen, and establishes zonal structure over time and supports smooth movement of the synovial joint through life. The cascade actions of key regulators, such as Wnts, GDF5, Erg, and PTHLH, coordinate sequential steps of articular cartilage formation. Articular chondrocytes are restrictedly controlled not to differentiate into a hypertrophic stage by autocrine and paracrine factors and extracellular matrix microenvironment, but retain potential to undergo hypertrophy. The basal calcified zone of articular cartilage is connected with subchondral bone, but not invaded by blood vessels nor replaced by bone, which is highly contrasted with the growth plate. Articular cartilage has limited regenerative capacity, but likely possesses and potentially uses intrinsic stem cell source in the superficial layer, Ranvier's groove, the intra-articular tissues such as synovium and fat pad, and marrow below the subchondral bone. Considering the biological views on articular cartilage, several important points are raised for regeneration of articular cartilage. We should evaluate the nature of regenerated cartilage as permanent hyaline cartilage and not just hyaline cartilage. We should study how a hypertrophic phenotype of transplanted cells can be lastingly suppressed in regenerating tissue. Furthermore, we should develop the methods and reagents to activate recruitment of intrinsic stem/progenitor cells into the damaged site.

Molecular adhesion between cartilage extracellular matrix macromolecules

In this study, we investigated the molecular adhesion between the major constituents of cartilage extracellular matrix, namely, the highly negatively charged proteoglycan aggrecan and the type II/IX/XI fibrillar collagen network, in simulated physiological conditions. Colloidal force spectroscopy was applied to measure the maximum adhesion force and total adhesion energy between aggrecan end-attached spherical tips (end radius R ≈ 2.5 μm) and trypsin-treated cartilage disks with undamaged collagen networks. Studies were carried out in various aqueous solutions to reveal the physical factors that govern aggrecan–collagen adhesion. Increasing both ionic strength and [Ca²⁺] significantly increased adhesion, highlighting the importance of electrostatic repulsion and Ca²⁺-mediated ion bridging effects. In addition, we probed how partial enzymatic degradation of the collagen network, which simulates osteoarthritic conditions, affects the aggrecan–collagen interactions. Interestingly, we found a significant increase in aggrecan–collagen adhesion even when there were no detectable changes at the macro- or microscales. It is hypothesized that the aggrecan–collagen adhesion, together with aggrecan–aggrecan self-adhesion, works synergistically to determine the local molecular deformability and energy dissipation of the cartilage matrix, in turn, affecting its macroscopic tissue properties.

Boundary cartilage lubrication: review of current concepts

Effective lubrication of synovial joints is important to prevent cartilage degeneration and to keep the joints healthy. This paper sets out the basics of engineering lubrication with respect to the composition and properties of synovial fluid constituents. Two basic types of boundary lubrication are discussed: the presence of highly hydrophilic proteoglycans that provide a water liquid film, and the existence of multilamellar phospholipids lubricating layers at the surface ofarticular cartilage. Based on current knowledge, we may conclude that no single mechanism of boundary lubrication exists, and that effective boundary lubrication of synovial joints is maintained by the synergic effect of all synovial fluid constituents.

Frictional response of normal and osteoarthritic articular cartilage in human femoral head

We evaluated the microscale frictional response of human articular cartilage in different osteoarthritis stages using an atomic force microscope. Four human femoral heads (60-80 years old) with different osteoarthritis stages were explanted, and two cylindrical cartilage samples were sectioned from each femoral head. The microscale frictional coefficient mu of human cartilage in phosphate-buffered saline increased with increasing osteoarthritis stages, resulting in mu = 0.119 +/- 0.036 for stage 0 (normal cartilage), 0.151 +/- 0.039 for stage I, 0.158 +/- 0.041 for stage 2, and 0.409 +/- 0.119 for stage 3. Statistically significant differences of mu values for different osteoarthritis stages were detected only between stage 3 and other stages (p < 0.0001). The average surface roughness Rq significantly increased with increasing osteoarthritis stages, ranging from 137 +/- 25 nm for stage 0 to 533 +/- 196 nm for stage 3. A significant correlation between mu and Rq for different osteoarthritis stages was observed (R2 = 0.981). These results demonstrate a positive correlation between the osteoarthritis stages and cartilage surface roughness, and the dependence of the human cartilage frictional response, on osteoarthritis progression. The results could be due to a decrease in the superficial zone protein concentration during the natural progression of osteoarthritis.

Microscale surface friction of articular cartilage in early osteoarthritis

Articular cartilage forms the articulating surface of long bones and facilitates energy dissipation upon loading as well as joint lubrication and wear resistance. In normal cartilage, boundary lubrication between thin films at the cartilage surface reduces friction in the absence of interstitial fluid pressurization and fluid film lubrication by synovial fluid. Inadequate boundary lubrication is associated with degenerative joint conditions such as osteoarthritis (OA), but relations between OA and surface friction, lubrication and wear in boundary lubrication are not well defined. The purpose of the present study was to measure microscale boundary mode friction of the articular cartilage surface in an in vivo experimental model to better understand changes in cartilage surface friction in early OA. Cartilage friction was measured on the articular surface by atomic force microscopy (AFM) under applied loads ranging from 0.5 to 5 μN. Microscale AFM friction analyses revealed depth dependent changes within the top-most few microns of the cartilage surface in this model of early OA. A significant increase of nearly 50% was observed in the mean engineering friction coefficient for OA cartilage at the 0.5 μN load level; no significant differences in friction coefficients were found under higher applied loads. Changes in cartilage surface morphology observed by scanning electron microscopy included cracking and roughening of the surface indicative of disruption and wear accompanied by an apparent disintegration of the thin surface lamina from the underlying matrix. Immunohistochemical staining of lubricin - an important cartilage surface boundary lubricant - did not reveal differences in spatial distribution near the cartilage surface in OA compared to controls. The increase in friction at the 0.5 μN force level is interpreted to reflect changes in the interfacial mechanics of the thin surface lamina of articular cartilage: increased friction implies reduced lubrication efficiency and a higher potential for cartilage surface wear in OA. The effects of mechanical or biochemical changes or loss of the thin surface lamina on the remaining tissue with respect to OA progression is unknown and requires further study, but preservation of the surface lamina seems an important early target for the maintenance of cartilage health and prevention of OA.

Matrix molecule influence on chondrocyte phenotype and proteoglycan 4 expression by alginate-embedded zonal chondrocytes and mesenchymal stem cells

Articular cartilage resists load and provides frictionless movement at joint surfaces. The tissue is organized into the superficial, middle, deep, and calcified zones throughout its depth, each which serve distinct functions. Proteoglycan 4 (PRG4), found in the superficial zone, is a critical component of the joint's lubricating mechanisms. Maintenance of both the chondrocyte and zonal chondrocyte phenotype remain challenges for in vitro culture and tissue engineering. Here we investigate the expression of PRG4 mRNA and protein by primary bovine superficial zone chondrocytes, middle/deep zone chondrocytes, and mesenchymal stem cells encapsulated in alginate hydrogels with hyaluronic acid (HA) and chondroitin sulfate (CS) additives. Chondrogenic phenotype and differentiation markers are evaluated by mRNA expression, histochemical, and immunohistochemical staining. Results show middle/deep cells express no measurable PRG4 mRNA by day 7. In contrast, superficial zone cells express elevated PRG4 mRNA throughout culture time. This expression can be significantly enhanced up to 15-fold by addition of both HA and CS to scaffolds. Conversely, PRG4 mRNA expression is downregulated (up to 5-fold) by CS and HA in differentiating MSCs, possibly due to build up of entrapped protein. HA and CS demonstrate favorable effects on chondrogenesis by upregulating transcription factor Sox9 mRNA (up to 4.6-fold) and downregulating type I collagen mRNA (up to 18-fold). Results highlight the important relationship between matrix components and expression of critical lubricating proteins in an engineered cartilage scaffold.