Effect of synovial fluid on boundary lubrication of articular cartilage

OBJECTIVES: The lubrication of articulating cartilage surfaces in joints occurs through several distinct modes. In the boundary mode of lubrication, load is supported by surface-to-surface contact, a feature that makes this mode particularly important for maintenance of the normally pristine articular surface. A boundary mode of lubrication is indicated by a kinetic friction coefficient being invariant with factors that influence formation of a fluid film, including sliding velocity and axial load. The objectives of this study were to (1) implement and extend an in vitro articular cartilage-on-cartilage lubrication test to elucidate the dependence of the friction properties on sliding velocity, axial load, and time, and establish conditions where a boundary mode of lubrication is dominant, and (2) determine the effects of synovial fluid (SF) on boundary lubrication using this test.

METHODS: Fresh bovine osteochondral samples were analyzed in an annulus-on-disk rotational configuration, maintaining apposed articular surfaces in contact, to determine static (mu(static) and mu(static),(N(eq)) and kinetic ([mu(kinetic)] and [mu(kinetic),(N(eq))]) friction coefficients, each normalized to the instantaneous and equilibrium (N(eq)) normal loads, respectively.

RESULTS: With increasing pre-sliding durations, mu(static) and mu(static),(N(eq)) were similar, and increased up to 0.43 +/- 0.03 in phosphate buffered saline (PBS) and 0.19 +/- 0.01 in SF, whereas [mu(kinetic)] and [mu(kinetic),(N(eq))] were steady. Over a range of sliding velocities of 0.1-1 mm/s and compression levels of 18% and 24%, [mu(kinetic)] was 0.072 +/- 0.010 in PBS and 0.014 +/- 0.003 in SF, and [mu(kinetic),(N(eq))] was 0.093 +/- 0.005 in PBS and 0.018 +/- 0.002 in SF.

CONCLUSIONS: A boundary mode of lubrication was achieved in a cartilage-on-cartilage test configuration. SF functioned as an effective friction-lowering boundary lubricant for native articular cartilage surfaces.