Boundary lubrication of articular cartilage: Role of synovial fluid constituents

Abstract

Objective. To determine whether the synovial fluid (SF) constituents hyaluronan (HA), proteoglycan 4 (PRG4), and surface-active phospholipids (SAPL) contribute to boundary lubrication, either independently or additively, at an articular cartilage-cartilage interface.Methods. Cartilage boundary lubrication tests were performed with fresh bovine osteochondral samples. Tests were performed using graded concentrations of SF, HA, and PRG4 alone, a physiologic concentration of SAPL, and various combinations of HA, PRG4, and SAPL at physiologic concentrations. Static ( static, Neq ) and kinetic (< kinetic, Neq >) friction coefficients were calculated.Results. Normal SF functioned as an effective boundary lubricant both at a concentration of 100% (< kinetic, Neq > ‫؍‬ 0.025) and at a 3-fold dilution (< kinetic, Neq > ‫؍‬ 0.029). Both HA and PRG4 contributed independently to a low in a dose-dependent manner. Values of < kinetic, Neq > decreased from ϳ0.24 in phosphate buffered saline to 0.12 in 3,300 g/ml HA and 0.11 in 450 g/ml PRG4. HA and PRG4 in combination lowered further at the high concentrations, attaining a < kinetic, Neq > value of 0.066. SAPL at 200 g/ml did not significantly lower , either independently or in combination with HA and PRG4. Conclusion. The results described here indicate that SF constituents contribute, individually and in combination, both at physiologic and pathophysiologic concentrations, to the boundary lubrication of apposing articular cartilage surfaces. These results provide insight into the nature of the boundary lubrication of articular cartilage by SF and its constituents. They therefore provide insight regarding both the homeostatic maintenance of healthy joints and pathogenic processes in arthritic disease.

Alteration of articular cartilage frictional properties by transforming growth factor β, interleukin‐1β, and oncostatin M

“…However, the in vitro test environment only approximates in vivo conditions, due to changes in cartilage from resection and articulation against a rigid nonpermeable glass surface. It is important to note that despite these differences, changes in due to loss of lubricin in this system testing a cartilage-glass interface are similar to those in systems testing cartilage-cartilage interfaces (18,60). Further, the present study was conducted with immature bovine tissue, which may differ from adult tissue in the response of lubricin metabolism to exposure to cytokines.…”

“…Lubricin has been shown to play several important roles in the maintenance of joint metabolism, including boundary lubrication of cartilage (17,18) and chondroprotection, by contributing to the elastic absorption and energy dissipation of synovial fluid (19) and inhibiting synovial cell overgrowth (20). Lubricin has been observed to lubricate a range of bearings, from latex-glass to cartilage-cartilage, in the boundary mode regime (18,(21)(22)(23), although the mechanism of lubrication is not fully understood.…”

“…Lubricin has been observed to lubricate a range of bearings, from latex-glass to cartilage-cartilage, in the boundary mode regime (18,(21)(22)(23), although the mechanism of lubrication is not fully understood. The investigation of the role of lubricin in cartilage lubrication is further complicated by a biphasic or pressure-mediated lubrication mecha-nism, which occurs when cartilage tissue is pressurized (24,25).…”

PRG4 exchange between the articular cartilage surface and synovial fluid

“…Alternatively, this result might be attributed to possible alterations of the molecular structure of PRG4 resulting from the purification process, which could disrupt the binding mechanism. However, PRG4 purified in this manner remains functional in terms of its contribution to boundary lubrication, 11 suggesting that it has also retained its ability to bind to the articular surface. In addition, the PRG4 in SF bathing fluid had not been subjected to any purification, and should therefore be expected to behave as it does in vivo.…”

“…In these experiments, cartilage-on-cartilage sliding in phosphate-buffered saline (PBS) resulted in removal of PRG4 from the surface while sliding in SF resulted in the deposition of additional PRG4 to the surface in the region of cartilage-cartilage contact. 11 However, a definitive and quantitative relationship between PRG4 in SF and that bound to the articular surface remains to be established.…”