Oral Presentation ANZBMS-MEPSA-ANZORS 2022

Introduction: The Stribeck curve is a plot of the coefficient-of-friction (COF) versus the Hersey number.  The latter is a function of the dynamic viscosity (Pa.s) of the fluid, multiplied by the shearing speed (Hz), and divided by the compressive stress (Pa).  When the Hersey number is small, the friction mechanism is dominated by ‘solid’ material-to-material contact Boundary Lubrication and the COF is relatively large. When the Hersey number is large, fluid interface mechanics dominate and the COF is small – and this phase is known as Hydrodynamic Lubrication. Articular cartilage covering the bone ends in weight-bearing joints allow ultra-low friction during activities of daily living while withstanding compressive stresses in the mega Pascal range. The aim of this study was to determine the Stribeck curves for cartilage-cartilage shearing, and how pre-osteoarthritic collagen fibrillar matrix changes^1,2 affect the surface shear response.

Methods and Results: A dynamic shear test (refer to figure) was applied to eight pairs of cartilage obtained from bovine patellae of varying matrix health ¹, confirmed using post-test microstructural imaging (Differential Interference Contrast Microscopy). The ratio of shear force and compressive force was used to obtain the COF. The dynamic viscosity of saline was used and the different speeds from 1Hz to 8Hz. The following Stribeck curves were obtained (below right).

Conclusion: It appears that collagen matrix ‘destructuring’, occurring at the sub-micron level, results in more boundary lubrication mechanics, which may indicate a vulnerability to cartilage wear at high-velocities.