Sliding enhances fluid and solute transport into buried articular cartilage contacts

Abstract

Objective: Solutes and interstitial water are naturally transported from cartilage by load-induced interstitial fluid pressures. Fluid and solute recovery during joint articulation have been primarily attributed to passive diffusion and mechanical 'pumping' from dynamic loading. This paper tests if the sliding action of articulation is a significant and independent driver of fluid and solute transport in cartilage. Design: The large osteochondral samples utilized in the present study preserve the convergent wedges necessary for physiological hydrodynamics. Following static load-induced fluid exudation and prior to sliding, a fluorescent solute (AlexaFluor 633) was added to the lubricant bath. In situ confocal microscopy was used to quantify the transport of solute from the bath into the buried stationary contact area (SCA) during sliding. Results: Following static exudation, significant reductions in friction and strain during sliding at 60 mm/s were accompanied by significant solute transport into the inaccessible center of the buried contact; no such transport was detected for the 0-or 1 mm/s sliding conditions. The results suggest that external hydrodynamic pressures from sliding induced advective flows that carried solutes from the bath toward the center of contact. Conclusions: These results provide the first direct evidence that the action of sliding is a significant contributor to fluid and solute recovery by cartilage. Furthermore, they indicate that the sliding-induced transport of solutes into the buried interface was orders of magnitude greater than that attributable to diffusion alone, a result with critical implications for disease prevention and tissue engineering.

How Sliding and Hydrodynamics Contribute to Articular Cartilage Fluid and Lubrication Recovery

“…This study isolated how sliding contributes to tribological rehydration by eliminating potential contribution from reciprocal wedging. During unidirectional sliding, we observed the same qualitative relationship between rehydration and sliding speed that we observed in our previous reciprocal sliding studies [29][30][31]39]; thus, we can reasonably conclude that the tribological rehydration effects observed therein were primarily caused by sliding and less so (if at all) by effects from reciprocation. Our results were generally consistent with the hydrodynamic hypothesis presented previously [29,39].…”

“…The steady-state compression and friction responses to increased sliding speed are illustrated in Fig. 2; these effects are qualitatively the same as those reported in our previous studies with reciprocal sliding [29,31]. From these results, we can conclude that tribological rehydration is sliding-induced with or without contributions from reciprocal wedging.…”

“…This idea has emerged in our minds based primarily on three observations. First, cursory modeling results, which are currently unpublished, have suggested to us that the slopes necessary to support hydrodynamic pressure-induced tribological rehydration must be far shallower than the steep slopes we have observed in the macro-wedge [30,31]. Secondly, we find that significant deformations are necessary before we begin detecting tribological rehydration, even at high sliding speeds [29].…”

“…Finally, we feel obliged to comment on why tribological rehydration is not more sensitive to sample curvature and diameter. One possibility we have considered is that the unique mechanics of this fibrous tissue (e.g., high tensile stiffness, low shear stiffness [57]) helps create a microwedge at the leading edge of contact with little connection to the observable geometry of the macro-wedge [31]. This idea has emerged in our minds based primarily on three observations.…”

“…A few years ago, however, we used in situ strain and solute transport measurements to demonstrate that cartilage can actively recover interstitial hydration during sliding within a constantly loaded stationary contact area that precludes any possible contribution from contact migration [29][30][31]; we called the phenomenon 'tribological rehydration' to reflect the fact that recovery was induced by sliding. While existing theories fail to explain the tribological rehydration phenomenon, our results are consistent with hydrodynamic origins.…”