Transgenic mice offer a novel way to probe structure function relationships in healthy and osteoarthritic cartilage. Indentation is a convenient method to measure mechanical properties of cartilage in the mouse. In order to reduce test data to material properties, test model geometry along with a material model needs to be assumed. Most recent developments support the use of a poroviscoelastic (PVE) model for cartilage. However, using this model makes separation of the flow-dependent and flow-independent viscoelastic parameters challenging. For cartilage from larger animals, Huang  showed that tensile tests have negligible flow-dependent response and hence can identify the flow-independent material parameters. A compression experiment can then be used to find only the flow-dependent parameters. However, limited cartilage volumes in mouse do not allow for tension tests, so mouse cartilage is primarily tested by indentation. Mak  has shown that fluid flow occurs mainly for times comparable to the gel diffusion time T = a2/Hκ where a is the tip size, H is the aggregate modulus and κ is permeability. Consequently, we propose use of two different sized indenters to separate flow-independent and dependent effects in mouse cartilage. One tip is small enough to make T negligible (say <0.1 s), then relaxation data will probe only the flow-independent response, whereas a second considerably larger tip will probe both flow-dependent and fluid flow effects. The data from the small indenter can be used to fit the flow-independent parameters; the data from the large indenter, in conjunction with parameters from the first fit, can be used to fit the flow-dependent parameters.
Poroviscoelastic Properties of Mouse Cartilage From Inverse Finite Elements and Indentation
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Chiravarambath, S, Simha, NK, & Lewis, JL. "Poroviscoelastic Properties of Mouse Cartilage From Inverse Finite Elements and Indentation." Proceedings of the ASME 2007 Summer Bioengineering Conference. ASME 2007 Summer Bioengineering Conference. Keystone, Colorado, USA. June 20–24, 2007. pp. 963-964. ASME. https://doi.org/10.1115/SBC2007-176688
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