A Century of Cartilage Tribology Research Is Informing Lubrication Therapies

Grafting of cationic molecules to hyaluronic acid improves adsorption and cartilage lubrication

Synovial fluid lubricates articular joints by forming a hydrated layer between the cartilage surfaces. In degenerative joint diseases like osteoarthritis (OA), the synovial fluid is compromised, which leads to less effective innate lubrication and exacerbated cartilage degeneration. Studies over the years have led to the development of partially or fully synthetic biolubricants to reduce the coefficient of friction with cartilage in knee joints. Cartilage-adhering, hydrated lubricants are particularly important to provide cartilage lubrication and chondroprotection under high normal load and slow speed. Here, we report the development of a hyaluronic acid (HA)-based lubricant functionalized with cationic branched poly-L-lysine (BPL) molecules that bind to cartilage via electrostatic interactions. We surmised that the electrostatic interactions between the BPL-modified HA molecules (HA-BPL) and the cartilage facilitate localization of the HA molecules to the cartilage surface. The number of BPL molecules on the HA backbone was varied to determine the optimal grafting density for cartilage binding and HA localization. Collectively, our results show that our HA-BPL molecules adhered readily to cartilage and were effective as a lubricant in cartilage-on-cartilage shear measurements where the modified HA molecules significantly reduce the coefficient of friction compared to phosphate-buffered saline or HA alone. This proof-of-concept study shows how the incorporation of cartilage adhering moieties, such as cationic molecules, can be used to enhance cartilage binding and lubrication properties of HA.

Synovium friction properties are influenced by proteoglycan content

The synovium plays a crucial role in diarthrodial joint health, and its study has garnered appreciation as synovitis has been linked to osteoarthritis symptoms and progression. Quantitative synovium structure-function data, however, remain sparse. In the present study, we hypothesized that tissue glycosaminoglycan (GAG) content contributes to the low friction properties of the synovium. Bovine and human synovium tribological properties were evaluated using a custom friction testing device in two different cases: (1) proteoglycan depletion to isolate the influence of tissue GAGs in the synovium friction response and (2) interleukin-1 (IL) treatment to observe inflammation-induced structural and functional changes. Following proteoglycan depletion, synovium friction coefficients increased while GAG content decreased. Conversely, synovium explants treated with the proinflammatory cytokine IL exhibited elevated GAG concentrations and decreased friction coefficients. For the first time, a relationship between synovium friction coefficient and GAG concentration is demonstrated. The study of synovium tribology is necessary to fully understand the mechanical environment of the healthy and diseased joint.

Cartilage extracellular matrix polymers: hierarchical structure, osmotic properties, and function

Proteoglycans are hierarchically organized structures that play an important role in the hydration and the compression resistance of cartilage matrix. In this study, the static and dynamic properties relevant to the biomechanical function of cartilage are determined at different levels of the hierarchical structure, using complementary osmotic pressure, neutron scattering (SANS) and light scattering (DLS) measurements. In cartilage proteoglycans (PGs), two levels of bottlebrush structures can be distinguished: the aggrecan monomer, which consists of a core protein to which are tethered charged glycosaminoglycan (GAG) chains, and complexes formed of the aggrecan monomers attached around a linear hyaluronic acid backbone. The principal component of GAG, chondroitin sulfate (CS), is used as a baseline in this comparison. The osmotic modulus, measured as a function of the proteoglycan concentration, follows the order CS < aggrecan < aggrecan-HA complex. This order underlines the benefit of the increasing complexity at each level of the molecular architecture. The hierarchical bottlebrush configuration, which prevents interpenetration among the bristles of the aggrecan monomers, enhances both the mechanical properties and the osmotic resistance. The osmotic pressure of the collagen solution is notably smaller than in the proteoglycan systems. This is consistent with its known primary role to provide tensile strength to the cartilage and to confine the aggrecan-HA complexes, as opposed to load bearing. The collective diffusion coefficient D governs the rate of recovery of biological tissue after compressive load. In CS solutions the diffusion process is fast, D ≈ 3 × 10-6 cm2 s-1 at concentrations comparable with that of the GAG chains inside the aggrecan molecule. In CS solutions D is a weakly decreasing function of calcium ion concentration, while in aggrecan and its complexes with HA, the relaxation rate is insensitive to the presence of calcium.

Advances in viscosupplementation and tribosupplementation for early-stage osteoarthritis therapy

Joint kinematic instability, arising from congenital or acquired musculoskeletal pathoanatomy or from imbalances in anabolism and catabolism induced by pathophysiological factors, leads to deterioration of the composition, structure and function of cartilage and, ultimately, progression to osteoarthritis (OA). Alongside articular cartilage degeneration, synovial fluid lubricity decreases in OA owing to a reduction in the concentration and molecular weight of hyaluronic acid and surface-active mucinous glycoproteins that form a lubricating film over the articulating joint surfaces. Minimizing friction between articulating joint surfaces by lubrication is fundamental for decreasing hyaline cartilage wear and for maintaining the function of synovial joints. Augmentation with highly viscous supplements (that is, viscosupplementation) offers one approach to re-establishing the rheological and tribological properties of synovial fluid in OA. However, this approach has varied clinical outcomes owing to limited intra-articular residence time and ineffective mechanisms of chondroprotection. This Review discusses normal hyaline cartilage function and lubrication and examines the advantages and disadvantages of various strategies for restoring normal joint lubrication. These strategies include contemporary viscosupplements that contain antioxidants, anti-inflammatory drugs or platelet-rich plasma and new synthetic synovial fluid additives and cartilage matrix enhancers. Advanced biomimetic tribosupplements offer promise for mitigating cartilage wear, restoring joint function and, ultimately, improving patient care.

How Do Cartilage Lubrication Mechanisms Fail in Osteoarthritis? A Comprehensive Review

Cartilage degeneration is a characteristic of osteoarthritis (OA), which is often observed in aging populations. This degeneration is due to the breakdown of articular cartilage (AC) mechanical and tribological properties primarily attributed to lubrication failure. Understanding the reasons behind these failures and identifying potential solutions could have significant economic and societal implications, ultimately enhancing quality of life. This review provides an overview of developments in the field of AC, focusing on its mechanical and tribological properties. The emphasis is on the role of lubrication in degraded AC, offering insights into its structure and function relationship. Further, it explores the fundamental connection between AC mechano-tribological properties and the advancement of its degradation and puts forth recommendations for strategies to boost its lubrication efficiency.

A missed opportunity: A scoping review of the effect of sex and age on osteoarthritis using large animal models

OBJECTIVE

The objective was to critically analyze the published literature accounting for sex differences and skeletal age (open vs. closed physis) in preclinical animal models of OA, including the disaggregation of data by sex and skeletal maturity when data is generated from combined sex and/or multi-aged cohorts without proper confounding.

METHOD

A scoping literature review of PubMed, Web of Science, EMBASE, and SCOPUS was performed for studies evaluating the effect of sex and age in experimental studies and clinical trials utilizing preclinical large animal models of OA.

RESULTS

A total of 9727 papers were identified in large animal (dog, pig, sheep, goat, horse) models for preclinical OA research, of which 238 ex vivo and/or in vivo studies disclosed model type, animal species, sex, and skeletal age sufficient to analyze their effect on outcomes. Dogs, followed by pigs, sheep, and horses, were the most commonly used models. A paucity of preclinical studies evaluated the effect of sex and age in large animal models of naturally occurring or experimentally induced OA: 26 total studies reported some kind of analysis of the effects of sex or age, with 4 studies discussing the effects of sex only, 11 studies discussing the effects of age only, and 11 studies analyzing both the effects of age and sex.

CONCLUSION

Fundamental to translational research, OARSI is uniquely positioned to develop recommendations for conducting preclinical studies using large animal models of OA that consider biological mechanisms linked to sex chromosomes, skeletal age, castration, and gonadal hormones affecting OA pathophysiology and treatment response.

Impact of hyaluronic acid injection on the knee joint friction

PURPOSE

The purpose of this in vitro study was to investigate whether or not hyaluronic acid supplementation improves knee joint friction during osteoarthritis progression under gait-like loading conditions.

METHODS

Twelve human cadaveric knee joints were equally divided into mild and moderate osteoarthritic groups. After initial conservative preparation, a passive pendulum setup was used to test the whole joints under gait-like conditions before and after hyaluronic acid supplementation. The friction-related damping properties given by the coefficient of friction µ and the damping coefficient c (in kg m2/s) were calculated from the decaying flexion-extension motion of the knee. Subsequently, tibial and femoral cartilage and meniscus samples were extracted from the joints and tested in an established dynamic pin-on-plate tribometer using synthetic synovial fluid followed by synthetic synovial fluid supplemented with hyaluronic acid as lubricant. Friction was quantified by calculating the coefficient of friction.

RESULTS

In the pendulum tests, the moderate OA group indicated significantly lower c0 values (p < 0.05) under stance phase conditions and significantly lower µ0 (p = 0.01) values under swing phase conditions. No degeneration-related statistical differences were found for µend or cend. Friction was not significantly different (p > 0.05) with regard to mild and moderate osteoarthritis in the pin-on-plate tests. Additionally, hyaluronic acid did not affect friction in both, the pendulum (p > 0.05) and pin-on-plate friction tests (p > 0.05).

CONCLUSION

The results of this in vitro study suggested that the friction of cadaveric knee joint tissues does not increase with progressing degeneration. Moreover, hyaluronic acid viscosupplementation does not lead to an initial decrease in knee joint friction.

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

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

Aggrecan and Hyaluronan: The Infamous Cartilage Polyelectrolytes - Then and Now

Cartilages are unique in the family of connective tissues in that they contain a high concentration of the glycosaminoglycans, chondroitin sulfate and keratan sulfate attached to the core protein of the proteoglycan, aggrecan. Multiple aggrecan molecules are organized in the extracellular matrix via a domain-specific molecular interaction with hyaluronan and a link protein, and these high molecular weight aggregates are immobilized within the collagen and glycoprotein network. The high negative charge density of glycosaminoglycans provides hydrophilicity, high osmotic swelling pressure and conformational flexibility, which together function to absorb fluctuations in biomechanical stresses on cartilage during movement of an articular joint. We have summarized information on the history and current knowledge obtained by biochemical and genetic approaches, on cell-mediated regulation of aggrecan metabolism and its role in skeletal development, growth as well as during the development of joint disease. In addition, we describe the pathways for hyaluronan metabolism, with particular focus on the role as a "metabolic rheostat" during chondrocyte responses in cartilage remodeling in growth and disease.Future advances in effective therapeutic targeting of cartilage loss during osteoarthritic diseases of the joint as an organ as well as in cartilage tissue engineering would benefit from 'big data' approaches and bioinformatics, to uncover novel feed-forward and feed-back mechanisms for regulating transcription and translation of genes and their integration into cell-specific pathways.

Recent Advances in Understanding the Role of Cartilage Lubrication in Osteoarthritis

The remarkable lubrication properties of normal articular cartilage play an essential role in daily life, providing almost frictionless movements of joints. Alterations of cartilage surface or degradation of biomacromolecules within synovial fluid increase the wear and tear of the cartilage and hence determining the onset of the most common joint disease, osteoarthritis (OA). The irreversible and progressive degradation of articular cartilage is the hallmark of OA. Considering the absence of effective options to treat OA, the mechanosensitivity of chondrocytes has captured attention. As the only embedded cells in cartilage, the metabolism of chondrocytes is essential in maintaining homeostasis of cartilage, which triggers motivations to understand what is behind the low friction of cartilage and develop biolubrication-based strategies to postpone or even possibly heal OA. This review firstly focuses on the mechanism of cartilage lubrication, particularly on boundary lubrication. Then the mechanotransduction (especially shear stress) of chondrocytes is discussed. The following summarizes the recent development of cartilage-inspired biolubricants to highlight the correlation between cartilage lubrication and OA. One might expect that the restoration of cartilage lubrication at the early stage of OA could potentially promote the regeneration of cartilage and reverse its pathology to cure OA.

Squeeze-film properties of synovial fluid and hyaluronate-based viscosupplements

The rheological properties of synovial fluid and hyaluronate (HA) solutions have been studied using a variety of viscometers and rheometers. These devices measure the viscosity of the fluid's resistance to shearing forces, which is useful when studying the lubrication and frictional properties of movable joints. Less commonly used is a squeeze-film fluid test, mechanistically similar to when two joint surfaces squeeze interposed fluid. In our study, we used squeeze-film tests to determine the rheological response of normal bovine synovial fluid and 10 mg/ml HA-based solutions, Hyalgan/Hyalovet, commercially available 500-700 kDa HA viscosupplements, and a 1000 kDa sodium hyaluronate (NaHy) solution. We found similar rheological responses (fluid thickness, viscosity, viscosity-pressure relationship) for all three fluids, though synovial fluid's minimum squeeze-film thickness was slightly thicker. Squeeze-film loading speed did not affect these results. Different HA concentrations and molecular weights also did not have a significant or consistent effect on the squeeze-film responses. An unexpected result for the HA-solutions was a linear increase in minimum fluid-film thickness with increasing initial fluid-film thickness. This result was attributed to faster gelling of thicker HA-solutions, which formed at a lower squeeze-film strain and higher squeeze-film strain rate compared to thinner layers. Also included is a review of the literature on viscosity measurements of synovial fluid and HA solutions.