Biotribology of natural joints

What is it about?

This is Chapter 3 of the book “Biotribology of natural and artificial joints” by Teruo Murakami and composed of 5 sections: 1. Importance of tribology in natural joints, 2. Articular cartilage, 3. Synovial fluid, 4. Lubrication mechanism in natural joints and 5. Wear and degradation mechanism in human joints. First the structure and properties of compliant biphasic articular cartilage composed of the extracellular matrix of proteoglycan reinforced by the collagen fibers and the chondrocytes are described in related to their distribution in superficial, middle and deep zones. The biphasic properties of the articular cartilage as load-bearing material are mentioned. The constituents, viscous properties and roles of the synovial fluid as lubricant are remarked. Next, the various lubrication mechanisms for the fluid film lubrication including soft-elastohydrodynamic lubrication (soft-EHL) and micro-EHL, the biphasic lubrication, the hydration lubrication, the boundary lubrication and the gel film lubrication are described from the viewpoint of the adaptive multimode lubrication with low friction and minimal wear. It is indicated that flattening of surface asperity in micro-EHL extended the ability of fluid film lubrication. The effectiveness of EHL to prevent direct contact between rubbing surfaces is evaluated by the film parameter (lambda ratio). It is shown in biphasic lubrication that the fluid load support maintains with low friction for migrating contact, but the fluid load support gradually decreases for stationary contact on cartilage. Cases with effectiveness of hydration lubrication are shown for several modes. If adequate water molecules are attracted by hydrophilic polymers fixed to the surface, the low friction would be sustained between hydrated layers at low velocity or even under static conditions. In boundary lubrication, adsorbed films of lubricin, phospholipids and serum proteins appear to protect the rubbing surfaces with low friction. In pendulum tests for porcine shoulder joints, the addition of phospholipid and γ-globulin reduced friction in boundary or mixed lubrication regime. In reciprocating tests for articular cartilage against glass plate, the addition of single protein increases friction but the addition of simulated synovial fluid as hyaluronic acid (HA) solution containing 0.01 mass% Dipalmitoyl phosphatidyl-choline (DPPC), 1.4 mass% albumin and 0.7 mass% γ-globulin reduced friction and wear. In more severe reciprocating tests lubricated with saline solution, the changes in transient friction after removal of adsorbed films are observed. The surface observation of atomic force microscopy (AFM) images indicates the protective roles by the proteoglycan gel-like layer as gel film lubrication. In summarization of the various lubrication mechanisms, the adaptive multimode lubrication mechanism is proposed with the synergistic combinations of various lubrication modes in wider operating conditions. Finally, wear and degradation mechanism in human joints are discussed.

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Why is it important?

The sustainability of very low friction and minimal wear in natural synovial joints for longer human life is important for keeping of smooth movement in musculoskeletal system in various daily activities. Low friction is maintained in low shearing layer or low friction plane between rubbing surfaces in various lubrication modes such as soft-EHL, micro-EHL, biphasic, hydration, boundary and gel film lubrication. No surface failure or no wear can be actualized by the full fluid film formation, significant fluid phase load support or appropriate protection by the lubricious cartilage surfaces at local direct contacts. In the cases with local surface failure or wear, the appropriate restoration is expected. In this Chapter, detailed lubrication mechanisms and related phenomena are described in various lubrication modes in the adaptive multimode lubrication mechanism.

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