A Historical Evaluation of Articular Cartilage Lubrication Studies Reveals Distinct Testing Approach Specific Behaviors.

Healthy articular cartilage can sustain near-frictionless motion within the joint; in vivo friction coefficients fall well within the superlubricity regime (<0.01). However, despite nearly a century of study, the mechanisms underpinning these behaviors remain unclear. This uncertainty has been compounded by how cartilage's operating conditions have been defined and subsequently extended to explant testing approaches, resulting in drastically varied tribological responses. To address these discrepancies, we compared friction coefficient results from a comprehensive set of historical cartilage explant tribology studies, and from new data evaluating testing configuration (stationary contact area (SCA) versus convergent SCA (cSCA)) and lubricant (phosphate-buffered saline versus synovial fluid (SF)) choice on friction. These data demonstrate that the SCA, the most common testing configuration utilized, consistently reports friction greater than that of the migrating contact area (MCA) and cSCA configurations. Intriguingly, while the SCA and MCA are almost universally slid at sub-physiological speeds (~1 mm/s), the cSCA has routinely been tested at more physiologically consistent speeds (~60 mm/s). Nevertheless, SF drives noticeable reductions in friction within both SCA and cSCA studies. Importantly, particularly in vivo, only rapidly slid, SF-lubricated cSCA cartilage explants reliably demonstrate biofidelic friction (<0.005), indicating that hydrodynamic-related phenomena must not be discounted in cartilage lubrication. Collectively, these results underscore the need to further probe mechanisms of sustained cartilage superlubricity, a key finding only recently replicated via the cSCA configuration. Such knowledge will be crucial to understanding the true lubrication capacity of articular cartilage.