What is it about?
This is Chapter 5 of the book “Biotribology of natural and artificial joints” by Teruo Murakami (2023) and composed of 6 sections: 5.1 Effectiveness of compliant artificial cartilage, 5.2 Effectiveness of hydrogel artificial cartilage, 5.3 Role of biphasic lubrication in hydrogel artificial cartilage, 5.4 Roles of boundary lubrication, 5.5 Influence of hydrogel wear debris and 5.6 Bionic design with biomimetic artificial cartilage. First, the effectiveness of compliant artificial cartilage is mentioned on better lubrication mechanism and low friction property compared with the exiting joint prostheses composed of ultra-high molecular weight polyethylene (UHMWPE) and hard materials. It is indicated in these cushion form bearings as joint prostheses with compliant articular layer, the fluid film lubrication appears to be attained by effective soft-elastohydrodynamic lubrication (soft-EHL) and micro-EHL mechanism during normal walking condition. Second, the effectiveness of hydrogel artificial cartilage is discussed. Hydrogels are three-dimensional crosslinked hydrophilic polymer networks swollen by water. It is shown in knee joint simulator test that the knee prosthesis with artificial cartilage of biocompatible and hydrophilic poly(vinyl alcohol) (PVA) hydrogels of high water content exhibited significantly low friction in hyaluronate (HA) solution containing serum protein, while the knee prosthesis with polyurethane layer showed more than tenfold higher friction. The interstitial fluid pressure in reciprocating tests for the cylindrical PVA hydrogel layer as a biphasic material moving specimen on the acrylic flat specimen was measured using microelectromechanical system (MEMS) pressure sensor. For PVA hydrogels different in manufacturing methods as repeated freezing-thawing (FT), cast-drying (CD) and hybrid (a layered structure of FT and CD) gels, the reciprocating tests and biphasic finite element (FE) analysis were performed. CD gel has lower permeability then FT gel. It is shown that hybrid (CD on FT) exhibits lowest friction and minimal wear. In addition, the effectiveness of fiber-reinforcement in PVA FT hydrogel on interstitial fluid pressurization and friction reduction is shown by sliding biphasic FE simulation and reciprocating ball-on-plate test. It is indicated that fiber-reinforcement in parallel to the surface is effective to enhance the stiffness to compressive load and interstitial fluid pressurization. For rubbing compliant artificial cartilage surfaces in mixed or boundary lubrication regime, some direct contacts occur. To reduce friction and protect the compliant surfaces, adsorbed films or some lubricating layers should be formed. At appropriate total protein concentration of 2.1 mass% as 1/2 or 2/1 ratio of albumin and γ-globulin (A/G ratio) in HA solution, the wear of PVA was remarkably reduced. Next, the influence of PVA hydrogel wear particles on the immune response of macrophages by assaying two cytokines IL-1β and TNF-α was examined. Finally, bionic design with biomimetic artificial cartilage is discussed. Various biocompatible compliant artificial hydrogels have been developed to mimic natural articular cartilage.
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Why is it important?
In total hip and knee prostheses as Hard(metal or ceramic)-on-UHMWPE, the minimum fluid film thickness is estimated to be thinner than 0.1 μm during stance phase of walking, because UHMWPE has Young’s modulus of 0.5 to 1.5 GPa, which is stiffer at 2 or 3 order than that of articular cartilage. The fluid film thickness is usually smaller than the surface roughness of UHMWPE. Therefore, the main lubrication mode is the mixed or boundary lubrication mode and thus local direct contact occurs, resulting in problems on friction and wear. Considerable wear debris resulted in joint loosening due to osteolysis. To improve the durability of existing joint prostheses, the reduction of friction and minimization of wear are required. Therefore, it is important that the effectiveness of application of compliant artificial cartilage including biphasic hydrogels is evaluated for improvement of lubrication modes, friction and wear properties. First, the effectiveness of compliant artificial cartilage is confirmed by soft-EHL and micro-EHL mechanism during normal walking condition. However, under adverse lubrication conditions at slower motion, at shorter stroke length, at little movement, or starting and stopping, the fluid film may not be sufficiently formed. So, the compliant artificial cartilage of hydrogel with high water content is applied to mimic the natural articular cartilage and to improve lubrication by combined effect of soft-EHL, micro-EHL, biphasic lubrication, hydration lubrication and boundary lubrication. For hydrogels, effectiveness of biphasic lubrication is clarified by measurement of the interstitial fluid pressurization and friction. For PVA hydrogels different in manufacturing methods (different in structures and material properties), it is shown that friction and wear characteristics are different. The effectiveness of fiber-reinforcement in PVA FT hydrogel on interstitial fluid pressurization and friction reduction is important. For rubbing compliant artificial cartilage surfaces in mixed or boundary lubrication regime, it is indicated that the lubricant constituents and protein concentration gave important influence on wear property of PVA hydrogel. In addition, it is important to evaluate the influence of PVA hydrogel wear particles on the immune response of macrophages, although it was suggested that PVA hydrogel wear particles do not affect the macrophage immune response.
Perspectives
Natural articular cartilage with superior lubricity and load-carrying capacity is natural hydrogel which contains plenty water in extracellular matrix composed of proteoglycan reinforced by collagen network with lubricious surface layer. Therefore, various biocompatible artificial hydrogels as complex hydrophilic polymer network has been developed to mimic natural articular cartilage. Both the repair device for focal chondral defects and the application as rubbing surface layer in total joint replacements or hemi-arthroplasty are considered as possible clinical applications of compliant artificial hydrogel cartilage. The improvement of geometrical congruity with appropriate layer thickness is capable to reduce the contact stress and friction shear stress with enhancement of fluid film formation or lubricating abilities, which induce longer durability. For hydrogel materials with biphasic property, the translation of loading conjunction area specified by geometric design and motion modes can control the sustaining efficient interstitial pressurization for various daily activities. In PVA hybrid (CD on FT) gel composed of CD gel with low permeability and FT gel with high permeability, the layered structure can be optimized by preparation conditions, and can improve the interstitial fluid pressurization. And, the hybridization method compositing cross-linking network of FT and CD gels in single layer gel reduced friction and wear at high contact pressure. The PVA FT gel reinforced by PVA fibre showed the improvement of friction, probably due to increasing fluid load support in biphasic lubrication mechanism. The extent of water content in hydrogel usually controls biphasic properties. For physically cross-linked PVA gels having hydrogen bonds without using a cross-linking agent, the gamma ray sterilization for clinical applications usually causes additional chemical cross-linking and changes physical properties of gels. The laminated CD on FT hybrid gels irradiated at appropriate irradiation condition retained excellent friction and minimum wear. Effective hydration lubrication and adsorbed film formation are beneficial. As an effective method to suppress friction level for slow speed, the polymer brushes on gel surfaces with repulsive interaction with the sliding substrate can reduce surface friction. In lubricants, phospholipid DPPC and albumin are likely to act as low shearing layer, and γ-globulin acts as the protective film as strongly adsorbed molecules on PVA surface. HA has ability to thicken the lubricating fluid film and form some lubricating gel-like layer. It is noteworthy that the coexistence of DPPC, albumin, γ-globulin and HA at appropriate concentration is expected to bring very low friction and minimal wear of PVA hydrogels. For the extensive development of clinical application of biocompatible hydrogels, durability with appropriate mechanical and tribological properties, synergistic interaction with lubricants, ease of production, formability, appropriate fixation methods with stability, optimum geometric design with surface texture and less damage to the opposing cartilage or rubbing surface are required.
Professor Emeritus Teruo Murakami
Kyushu University
Read the Original
This page is a summary of: Biotribology of artificial joints with artificial cartilage, January 2023, Elsevier,
DOI: 10.1016/b978-0-12-823669-7.00004-1.
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