Cartilage mechanical properties

The ECM has a very wide variety of functions it establishes mechanical connections between cells it creates structures with special mechanical properties (as in bone, cartilage, tendons, and joints) it creates filters (e. g., in the basal membrane in the renal corpuscles see p.322) it separates cells and tissues from each other (e.g., to allow the Joints to move freely) and it provides pathways to guide migratory cells (important for embryonic development). The chemical composition of the ECM is just as diverse as its functions. 

An even more feasible approach seems to be that of Moutos et al., who used 104-pm sized, commercially available PGA multifilaments to weave into 3D structures [210], As shown in Fig. 11, interlocking of multiple layers led to porous scaffolds with 70-75% porosity. For cell growth, a composite structure was fabricated by vacuum-assisted infusion of a hydrogel. The resulting scaffolds had mechanical properties similar to those of artricular cartilage [210],... 

The remarkable mechanical properties of articular cartilage result from the structure of collagen and proteoglycans, and from the properties of the fibrous composite formed by the interactions of collagen and proteoglycans in intercellular matrix. 

It is commonly accepted that without the PG/HA interaction, the PGs would not be retained within cartilage. Since the prime task of the negatively charged polymers of the proteoglycans is the binding of water, absence of HA would result in a loss of water and subsequently in a loss of the mechanical properties of cartilage [65]. The interaction with collagen is, however, primarily mediated by the keratan and chondroitin sulfate of the individual PGs. 

The importance of lysyl hydroxylation is seen in patients with the type VI variant of Ehlers-Danlos syndrome (Table 25-5). The collagen in these individuals has a decreased fibril diameter and profound changes in mechanical properties. Skin fibroblasts show virtually no lysyl hydroxylase activity. Furthermore, hydroxylysine formation can be severely affected in some tissues, mildly affected in others, and unaffected in still others (e.g., cartilage). These observations suggest the presence of tissue-specific lysyl hydroxylases. 

Biocompatibility and Mechanical Properties. Currently, their are no suitable artificial materials for the prosthetic replacement of articular cartilage. The biocompatibility is considered the primary criterion in the selection of such a material. In a recent study, Furst and co-workers(10) compared the biocompatibility of the polyurethane to the well known medical grade silicone polymer. The tissue reactions to small polymer discs, inserted in an articulating space—the suprapatellar bursa of rabbits, was examined. The foreign body reaction of the tissue at the implantation site was evaluated at intervals of 7 days,... 

Compatibility and Mechanical Properties of a Candidate for Articular Cartilage Replacement, Trans, of the 4th Annual Meeting Society of Biomaterials, 1978, 2, 159. ... 

Keinpson, G.E. Mechanical Properties of Articular Cartilage, "Adult Articular Cartilage" Edited by M.A.R. Freeman,... 

Hyaluronan is continuously synthesized and secreted by fibroblasts, keratino-cytes, chondrocytes and other specialized cells in the extracellular matrix (ECMs) throughout the body. It is synthesized by HA synthase (see also Chapter 9) at the inner face of the plasma membrane [98]. The level of HA synthesis is very high in skin and cartilage [99]. Hyaluronic acid is not one of the major components of the ECMs of the connective tissues, but it is found in various locations such as synovial fluid, vitreous humor, and umbilical cords [100]. Its biological functions include the maintenance of mechanical properties such as swelling in connective tissues and control of tissue hydration, providing lubricating properties in synovial fluid and heart valves. 

The Porous Platens. Since the Interstitial fluid of cartilage is exuded during compression (5), its measured mechanical properties will vary if any Impediments to flow are Imposed by the experimental apparatus (i.e. a resistance above that inherent to the tissue). Measurement of the tissue s real mechanical properties at significant strains, would require that it be compressed against completely free draining platens, (i.e. ones that conceivably have zero or negligible resistance to flow) that cause no distortion to its surface during compression. Then the confined compression of the tissue would presumably result in predominantly uniaxial flow fields with little lateral flow of... 

Since the obtaining of a platen that approached ideal properties was not possible, it was necessary to assess the variance of the measured mechanical properties of the cartilage as a function of the porosity and size of holes of the platens. 

It is suggested that the unique mechanical properties of cartilage in its normal environment may be due to an anisotropic nature of the resistance to flow. 

The physical and mechanical properties of cartilage are certainly important for the physiological role of this tissue. Proteoglycan in connection with collagen and other constituents of this tissue may affect its physical properties. Cartilage proteoglycan is a complex species. To a protein core, many chondroitin sulfate and keratan sulfate chains are attached. 

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