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 (μ_static and μ_static,Neq) and kinetic (〈μ_kinetic,Neq〉 and ) friction coefficients, each normalized to the instantaneous and equilibrium (N_eq) normal loads, respectively.

Results:​ With increasing pre-sliding durations, μ_static and were similar, and increased up to 0.43 ± 0.03 in phosphate buffered saline (PBS) and 0.19 ± 0.01 in SF, whereas 〈μ_kinetic,Neq〉 and were steady. Over a range of sliding velocities of 0.1–1 mm/s and compression levels of 18% and 24%, 〈μ_kinetic〉 was 0.072 ± 0.010 in PBS and 0.014 ± 0.003 in SF, and 〈μ_kinetic,Neq〉 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.