Lubricant Effects on Articular Cartilage Sliding Biomechanics Under Physiological Fluid Load Support

Abstract

Articular cartilage maintains phenomenally low friction and strains in intact joints during articulation, yet after a century of study, the mechanisms underpinning cartilage’s in vivo functions remain uncertain. We recently re-introduced a unique benchtop testing system, the convergent stationary contact area (cSCA), to investigate frictions and strains in cartilage explants; the cSCA differs from other benchtop testing configurations by enabling sliding-induced hydrodynamic-driven fluid flows to influence interstitial hydration and lubrication during sliding. This study aimed to elucidate several points regarding cartilage tribomechanics and joint lubrication, including the following: (i) if the presence of putative lubricants alters the ability of sliding to drive tribological rehydration (i.e., sliding-driven compression recovery under loading); (ii) how lubricant presence influences the frictional behavior of articular cartilage when tested under physiologically representative sliding conditions (e.g., moderate-to-high hydration and fluid load support, low-to-moderate tissue strains, and fast sliding speeds); and (iii) interpreting these behaviors with respect to lubricant rheology at shear rates reflecting those found in cSCA tests and intact joints (> 104 s−1). We found that synovial fluid and hyaluronic acid solutions (i) were at most 3–10 × the viscosity of saline lubricants at physiological shear rates, (ii) substantially enhanced the rates of fluid and lubrication recovery during sliding, and (iii) minimized the amount of time the explants experienced detrimental ‘pathophysiological’ frictions by fostering tissue hydration recovery. Additionally, we show that when non-lubricating solutions and high sliding speeds are utilized, the cSCA can facilely replicate ‘semi-physiological’ strains and frictions, while the use of lubricant-containing baths fosters ‘physiologically consistent’ tribomechanical behaviors, improving the physiological relevance of long-term benchtop sliding tests.