THE INFLUENCE OF THE LOAD AND JOINT CARTILAGE ELASTICITY MODULES ON SYNOVIAL FLUID VISCOSITY

Abstract

During classical journal bearing lubrication the lubricant viscosity is independent of physical properties of cooperating bodies, which is well known by virtue of Hersey-Stribeck (H-S) curve presenting friction coefficient vs. Hersey number = viscosity´velocity/pressure. The result obtained by the H-S is valid for two cooperating bodies with homogeneous, isotropic properties, and for Newtonian oils omitting the elastohydrodynamic effects. In the presented paper, we take into account the two cooperating human joint cartilage surfaces, which, after new AFM measurements, have non-homogeneous hypo- or hyper-elastic properties, and the synovial fluid has non-Newtonian features. Moreover, the cartilage surface during human limb motion and during the squeezing and boosted squeezing effects gains important small deformations. From the abovementioned description, it follows that the H-S result cannot be acceptable in human joint lubrication [L. 1–6]. During human joint hydrodynamic lubrication, we observe the influence of the material coefficients of the hypo- and hyper-elastic cartilage tissue on the apparent viscosity of non-Newtonian synovial fluid occupying the thin joint gap limited by the two cartilage superficial layers. This problem has not been considered in scientific papers describing the hydrodynamic lubrication of the human joint. This problem attains significant meaning because, after numerous AFM laboratory measurements confirmed by the literature achievements, it follows that the joint cartilage tissue with a thin polar membrane made of two lipid molecules has no isotropic but anisotropic properties in general. These membranes are flat sheets that form a continuous barrier around the cartilage cells. Non-homogeneous, anisotropic biological bodies as distinct from classical isotropic materials have the various values of elasticity, hypoelasticity, or hyper-elasticity modules on individual places and directions. These places, loaded by the same forces, tend to various displacements and strains. In consequence, mutually connected physical implications caused by virtue of synovial fluid flow velocity and shear rates changes, indicate to us the conclusion that the dynamic viscosity of synovial fluid gains value variations caused by the cartilage’s physical properties during human joint lubrication.