The Role of Hyaluronic Acid in the Synergistic Lubrication of Articular Cartilage

Introduction:: Articular cartilage is an unmatched lubricating material, supporting remarkably low friction coefficients in vivo (μ≤0.005).¹ Only recently has such biofidelic lubricity been replicated on the benchtop, through our use of the convergent stationary contact area (cSCA) testing configuration.^2,3 In the cSCA, sliding promotes speed-dependent, hydrodynamically-mediated recovery and maintenance of fluid load support (FLS), interstitial lubrication (ISL), and low friction values (μ~0.03 in PBS), via a mechanism termed ‘tribological rehydration’.² When synovial fluid (SF) is introduced into high-speed cSCA studies, truly in vivo-like equilibrium frictions are observed (μ< 0.004).³ We’ve termed this frictional benefit, arising from the presence of SF within a sliding environment where ISL recovery is driven by tribological rehydration, ‘synergistic lubrication’.

The unanticipated discovery of SF-mediated synergistic lubrication in the cSCA raises an important question: what aspect(s) of SF mediate lubrication synergy? SF contains numerous molecules posited to influence cartilage lubrication.^3,4 One of these is hyaluronic acid (HA), a high-molecular weight glycosaminoglycan found at concentrations of 1-3mg/mL in SF.⁵ While HA contributes to SF’s viscosity^5-7 and has been postulated to aid boundary lubrication^5,8,9, HA’s influence on synergistic lubrication is unknown. Here, we explored friction mitigating interactions among hydrodynamics and ISL recovery (titrated through sliding speed dependent tribological rehydration in the cSCA), and HA presence (titrated by varying HA concentration and molecular weight [MW]).


Materials and Methods:: F19mm osteochondral explants (n=9/study) were extracted from skeletally mature bovine stifle femoral condyles^2,3 and stored in 1X PBS + sucrose to approximate SF’s osmolarity (400mOsm¹⁰; hereon referred to simply as PBS).

The effects of HA concentration and MW on cSCA lubrication were investigated using a custom reciprocating tribometer.² All explants underwent loading and sliding preconditioning before tribological characterization.

In a first study, sliding “speed sweep” tests were conducted (at 5N) to investigate the effect of HA concentration on cSCA tribology. Explants were tested, sequentially, at 0, 0.1, 1.5, & 5.0mg/mL HA (1.4MDa) in PBS (osmolarity changed by no more than 4mOsm). Sliding conditions (e.g., sliding speed or bath solution) were only changed upon achieving deformation & friction equilibria. In a second study, “speed sweep” tests were used to explore the effect of HA MW (~30kDa, 375kDa, & 1.4MDa; 3.0mg/mL; < 0.1mOsm change) on cSCA lubrication. Equilibrium compressions (d_eq) and kinetic friction coefficients (m_eq) at each condition were recorded in LABVIEW and analyzed in MATLAB.


Results, Conclusions, and Discussions:: In a manner unique to the cSCA configuration, the lowest observed m_eq always occurred at sliding speeds >40mm/s, while peak m_eq occurred at <5mm/s regardless of HA concentration (Fig 1A). Both peak and minimal m_eq decreased with increasing HA concentration (study 1, Fig 1B,C), with the lubrication benefit associated with high speed sliding (80mm/s) increasing from 8.8-fold in PBS to 25-fold in 5mg/mL HA in the cSCA (relative to peak m_eq). Finally, increasing HA concentration reduced the sliding speeds at which equivalent levels of lubrication – defined as the matched m_eq at 80mm/s in PBS–was achieved (Fig 1D; e.g., for 5mg/ml 1.4MDa HA, this speed was 20mm/s).

When HA M.W. was varied from ~30kDa to ~1.4MDa, a further effect of HA M.W. on lubrication synergy was observed, with both peak and minimal m_eq, as well as equivalency speed, decreasing with increasing M.W (not shown).

The cSCA testing configuration, in being able to replicate biofidelic cartilage tribology conditions on the benchtop, through harnessing sliding speed-dependent tribological rehydration to titrate FLS and ISL², represents a novel tool for studying the role of HA and SF in cartilage lubrication.

In the absence of tribological rehydration (e.g., at slow sliding speeds) and ISL recovery, HA functions as a modest, concentration- and M.W.-dependent, boundary lubricant (reducing m_eq by ~50%). In contrast, with increasing sliding-speeds and tribological rehydration, HA’s effect on friction evolves dramatically. Upon sliding-dependent recovery of FLS and ISL, HA acts as powerful synergistic lubrication partner (reducing m_eq by >95%). These synergistic benefits are HA concentration and M.W. dependent. Most importantly, upon ISL recovery/maintenance by articulation, high concentration, high M.W. HA supports vanishingly small m_eq values, approaching both our limit of our detection (m_eq ~0.002) and values comparable to those seen with SF in the cSCA³ and in vivo¹.

Overall, the present study highlights a crucial synergy between interstitial hydration and HA-presence in regulating cartilage lubrication under physiologically-informative sliding conditions (e.g., high FLS and high sliding speeds). These insights continue to advance a unifying framework to explain cartilage’s unmatched lubricity in vivo and to better understand how cartilage naturally works.


Acknowledgements (Optional): :

References (Optional): : [1] Linn J Biomech 1968; [2] Moore OA&C 2017; [3] Farnham Trib Let 2021; [4] Bonnevie J Biomech Eng 2020; [5] Fam Biorheol 2007; [6] Krause Biomacromol 2001; [7] Miyazaki J Appl Polym Sci 1998; [8] Ogsten J Physiol 1953; [9] Bonnevie PloS One 2015; [10] Baumgarten J Bo Join Surg 1985.