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Nandor Bekesi, Andres de la Hoz, Sabine Kling, Susana Marcos; The Effects of Cross-linking on the Static and Dynamic Corneal Viscoelastic Properties. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3714.
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© ARVO (1962-2015); The Authors (2016-present)
To characterize the quasi-static and the dynamic viscoelastic properties of the cornea, and their changes produced by UV corneal collagen cross-linking (CXL).
The viscoelastic behavior of the cornea was studied experimentally on freshly enucleated porcine eyes, in 3 conditions: untreated; after 30 min riboflavin-dextran instillation, and after CXL. The quasi-static viscoelasticity was studied by inflation creep tests, where the intraocular pressure (IOP) was increased from 15 to 30 mmHg, and kept constant for 10 min. The experiment was automatically controlled, and corneal images captured using Scheimpflug imaging (Pentacam, Oculus). Changes in corneal geometry versus IOP and time were analyzed. The dynamic viscoelastic properties were assessed from corneal deformation using air-puff high speed Scheimpflug imaging (Corvis, Oculus). The viscoelastic material properties (in a generalized Maxwell model) were determined by reverse modeling using Finite Element Analysis in ANSYS. Dehydration due to the application of riboflavin-dextran was also considered in the modeling. The parametric model of the eye globe was built, and both the static and the dynamic test configuration were simulated on the same model with the same material properties, with different loads: the IOP time curve for the static condition, the IOP and the air puff dynamic pressure distribution for the dynamic one. The parameters of the viscoelastic material model were changed to fit the experimental corneal deformations of the inflation and air-puff experiments.
The resulting elasticity of the virgin cornea was 2.6 MPa with a relative modulus of 0.3 for the dynamic (0.001 s) and 0.15 for the quasi-static (100 s) loads. Applying riboflavin-dextran solution increased both the elastic modulus (x 2.15) and the static viscoelasticity (x 2.3), and decreased the dynamic viscoelastic parameter (x 0.4). Cross-linking increased the elasticity by a factor of 4.75, decreased the dynamic viscoelastic contribution (x 0.57), and increased the quasi-static viscoelastic relative modulus (x 1.67), compared to the virgin cornea.
Riboflavin-dextran and cross-linking change both the elastic and static and dynamic viscoelastic properties of the cornea. The description of the corneal material properties in different time regimes is important for the understanding and modeling of corneal treatments such as CXL.
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