Biotribology of artificial joints

What is it about?

This is Chapter 4 of the book “Biotribology of natural and artificial joints” by Teruo Murakami and composed of 5 sections: 4.1 Importance of tribology in artificial joints, 4.2 Lubrication modes in artificial joints, 4.3 Boundary lubrication in artificial joints, 4.4 Wear in artificial joints and 4.5 Improvement of tribological properties in artificial joints. First, historic development of artificial joints with various materials and design are described. In all artificial joints, low friction and minimum wear are required to improve the function and the durability in various daily activities, where tribology can play important roles. It is indicated that the actual lubrication modes which control the friction and wear behavior in artificial joints should be clarified. Here, lubrication modes in hip (hard-on-hard, hard-on-UHMWPE) and knee (Hard-on-UHMWPE) joint replacements are discussed on the basis of experimental evaluation and numerical analysis on elastohydrodynamic lubrication. The lubrication mode is fundamentally discriminated as “fluid film lubrication”, “mixed lubrication” or “boundary lubrication” according to the range of the film parameter (lambda ratio) as the ratio of theoretical minimum fluid film thickness to the composite surface roughness of both rubbing surfaces. It is shown in experimental evaluation in hip joint simulator that frictional behaviors in Stribeck curve indicate the corresponding lubrication mode. In friction tests for most of artificial joints which operate in mixed or boundary lubrication mode, it should be noticed that the proteinaceous constituents in lubricant can generate considerable frictional resistance or act as protective adsorbed films. In mixed lubrication regime, the combination of protein-based boundary lubrication and a super-imposed fluid film action with elastohydrodynamic lubrication appears to control the frictional behaviors. On wear in artificial joints, wear mechanisms such as adhesive wear, abrasive wear, fatigue wear/delamination and corrosive wear are discriminated. It is noted that the surface roughness influence wear mechanisms. The influences of multidirectional motion, oxidation and lubricants on wear of UHMWPE are described. Serious problems of pseudotumor formation as a result of the production of very fine wear debris and the associated production of metallic ions of Co and Cr for metal-on-metal hip replacement and hip resurfacing replacement are mentioned. Stripe wear in ceramic-on-ceramic hip joint is discussed with micro-separation. Elastoplastic contact analysis of UHMWPE tibial components is conducted in related the delamination and microscopic wear in knee prostheses. Wear estimation by simplified multidirectional wear tests, simulator tests and computer simulation are mentioned. Finally, examples for improvement of tribological properties in artificial joints are shown as cross-linking for UHMWPE, addition of vitamin E and surface modification by grafting of biocompatible phospholipid polymer on cross-linked polyethylene as a fail-safe design.

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Why is it important?

High friction in artificial joint increases the severity of rubbing to induce wear and joint loosening and interferes the smooth movement. Minimization of wear in artificial joints is regarded as an important issue in prevention of not only surface damage but also generation of wear debris, because wear debris can induce osteolysis and the eventual loosening of joint prostheses. Therefore, both reduction of friction and minimization of wear are required in artificial joints. To reduce wear, it is required to identify the predominant wear mechanism from the various mechanisms such as adhesive, abrasive, fatigue, corrosive wear and so on. The improvement of surface finish or surface texture and wear resistance of rubbing material becomes effective in wear reduction with beneficial changes in lubrication modes. It is indicated for wear of UHMWPE that the considerations of the multi-directional motion, oxidation of UHMWPE and influence of lubricants become important. Application of highly cross-linked polyethylene is considered to have remarkably improved wear resistance of UHMWPE. Advantage of Ceramic-on-UHMWPE to metal-on-UHMWPE, and the prevention of the stripe wear in ceramic-on-ceramic with better alignment to avoid the edge loading, application of improved composite ceramic materials and optimum geometric cup design are described. In total knee prostheses, the delamination and pitting as fatigue wear are discussed on the basis of elasto-plastic analyses, which indicate the importance of the reduction of the severity at contact area with optimized geometric design.

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