Aging and degeneration of the intervertebral discs are cell mediated processes that include biochemical, mechanical and structural changes. Although these processes are similar, disc degeneration is defined as an accelerated aging process that results in a detriment in the function of the disc. Biochemical changes include protein cross-linking, proteoglycan depletion and changes on collagen type. These compositional changes are related to changes in the mechanical properties of the disc and its tissues. For instance, it has been shown that an increase of protein cross-linking by glycation or genipin treatment causes an increase of the stiffness in disc tissues [1,2]. On the other hand, a decrease on the amount of proteoglycan has been shown to cause a decrease on tissue stiffness due to a reduction of the osmotic pressure [3,4]. However, during aging and degeneration, these two processes occur simultaneously with opposing effects on the mechanical properties of the tissue. Consequently, it is important to analyze these effects separately. Additionally, many multiphasic models for soft charged tissues, such articular cartilage and intervertebral disc, also consider the ionic phases separately from non-charged solids. Although multiphasic models for the disc have been used in the past, the mechanical properties of the non-charged extra-fibrillar matrix (EFM) have not been measured directly.

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