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.

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The tribological functions of cartilage are governed primarily by its interstitial fluid content, but the means by which cartilage recovers and retains interstitial fluid during articulation following periods of static loading remain unclear. Recently, we demonstrated a phenomenon in which articular cartilage recovers fluid at the loaded contact interface; we refer to this as 'tribological rehydration'. Our findings were consistent with two competing hypotheses: (1) that hydrodynamic pressures exceeded local interstitial pressures; (2) that fluid from the trailing wedge squeezed into the porous surface during direction reversals (reciprocal wedging). In this paper, we use unidirectional sliding to eliminate the potential effects of reciprocal wedging on tribological rehydration. We observed the same tribological rehydration effects from speed as in our previous reciprocating studies; thus, any effects of reciprocation are secondary to those of sliding itself. Tribological rehydration was enhanced by increased speeds, decreased loads, increased lubricant viscosity, and increased sample diameter. Although our results are generally consistent with a hydrodynamic interpretation of tribological rehydration, our attempts to eliminate the convergence zone by systematically decreasing the sample diameter failed to extinguish tribological rehydration. This final observation reveals an extremely robust mechanism for preventing tissue collapse and highlights some of the remaining challenges in modeling cartilage as a mechanical and tribological material.

Articular cartilage is an avascular tissue; diffusive transport is critical for its homeostasis. While numerous techniques have been used to quantify diffusivity within porous, hydrated tissues and tissue engineered constructs, these techniques have suffered from issues regarding invasiveness and spatial resolution. In the present study, we implemented and compared two separate correlation spectroscopy techniques, fluorescence correlation spectroscopy (FCS) and raster image correlation spectroscopy (RICS), for the direct, and minimally-invasively quantification of fluorescent solute diffusion in agarose and articular cartilage. Specifically, we quantified the diffusional properties of fluorescein and Alexa Fluor 488-conjugated dextrans (3k & 10k) in aqueous solutions, agarose gels of varying concentration (i.e. 1%, 3%, 5%), and in different zones of juvenile bovine articular cartilage explants (i.e. superficial, middle, and deep). In agarose, properties of solute diffusion obtained via FCS and RICS were inversely related to molecule size, gel concentration, and applied strain. In cartilage, the diffusional properties of solutes were similarly dependent upon solute size, cartilage zone, and compressive strain; findings that agree with work utilizing other quantification techniques. In conclusion, this study established the utility of FCS and RICS as simple and minimally invasive techniques for quantifying microscale solute diffusivity within agarose constructs and articular cartilage explants.

Osteoarthritis is a chronic joint disease characterized by articular cartilage degeneration, pain, and disability. As an avascular tissue, the movement of water and solutes through the tissue is critical to cartilage health and function, and early changes in solute diffusivity due to micro-scale changes in the properties of cartilage's extracellular matrix might precede clinical symptoms. A diagnostic technique for quantifying alteration to the diffusive environment of cartilage that precedes macroscopic changes may allow for the earlier identification of osteoarthritic disease, facilitating earlier intervention strategies. Toward this end, we used two confocal microscopy-based correlation spectroscopy techniques, fluorescence correlation spectroscopy and raster image correlation spectroscopy, to quantify the diffusion of two small solutes, fluorescein and 3k dextran, within human osteoarthritic articular cartilage. Our goal was to determine if these relatively simple optical correlation spectroscopy techniques could detect changes in solute diffusivity associated with increasing cartilage damage as assessed by International Cartilage Repair Society scoring guidelines, and if these measures are correlated with mechanical and compositional measures of cartilage health. Our data show a modest, yet significant increase in solute diffusivity and cartilage permeability with increasing osteoarthritis score (grades 0-2), with a strong correlation between diffusion coefficients, permeability, and cartilage composition. The described correlation spectroscopy techniques are quick, simple, and easily adapted to existing laboratory workflow and equipment. Furthermore, the minimal solute concentrations and laser powers required for analysis, combined with recent advances in arthroscopic microscopy, suggest correlation spectroscopy techniques as translational candidates for development into early OA diagnosis tools. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.