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H. Studer, P. Büchler; Corneal Material Property Identification from Directional Dependent Strip Extensiometry Data. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1751.
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© ARVO (1962-2015); The Authors (2016-present)
Corneal tissue shows a high degree of material anisotropy, mainly caused by the unique arrangement of its collagen fibers. The objective of the present work is to use experimental data to develop a numerical model of the cornea that accounts for the directional dependent nature of corneal tissue.
In 2008 Elsheikh et. al. [Elsheikh et. al. 2008, J Refract Surg, 24(2):178-87, Feb 2008] published experimental results of corneal tissue extensiometry, performed on vertical, horizontal and diagonal strips cut from the cornea. The corneal stress-strain relationship showed that the tissue was significantly stiffer for the horizontal and vertical strips, than for the diagonal strips. This result can be explained by the different number of fibers being active along each direction. These data are therefore ideal for the characterization of corneal biomechanics, and allow the distinction between the contribution of the matrix and collagen fiber to the tissue properties.Based on continuum mechanics, we developed a material constitutive equation, which consists of a sum of the mechanical contributions of collagen fibers, volume change and the surrounding matrix tissue. Numerical models of the strip experiments were used to optimize the mechanical properties used in the model. The tissue strips in Elsheikh's experiments are cut from intact corneas, thus many fibers in the samples do not contribute to the reported behavior. This effect is included in the model with a novel fiber distribution function, which is able to model active and inactive fiber directions at any point on the specimen.
Results showed that the numerical model was able to reproduce the experimentally measured force/displacement relationships. The model was further validated by comparison with published inflation experiments.
Strip extensiometry data can be used to identify the directional dependent material properties of corneal tissue if severed fibers are taken into account. One limitation of the current approach is that collagen cross-linking is not taken into account.
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