Abstract
Purpose: The aim of this study was to provide a method to determine corneal nonlinear viscoelastic properties based on the output data of corneal visualization Scheimpflug technology (Corvis ST). Methods: The Corvis ST data from 18 eyes of 12 healthy humans were collected. Based on the air-puff pressure and the corneal displacement from the Corvis ST test of normal human eyes, the work done by the air-puff attaining the whole corneal displacement was obtained. By applying a visco-hyperelastic strain energy density function of the cornea, in which the first-order Prony relaxation function and the first-order Ogden strain energy were employed, the corneal strain energy during the Corvis ST test was calculated. Then the work done by the air-puff attaining the whole corneal displacement was completely regarded as the strain energy of the cornea. The identification of the nonlinear viscoelastic parameters was carried out by optimizing the sum of difference squares of the work and the strain energy using the genetic algorithm. Results: The visco-hyperelastic model gave a good fit to the data of corneal strain energy with time during the Corvis ST test (R 2 > 0.95). The determined Ogden model parameter μ ranged from 0.42 to 0.74 MPa, and α ranged from 32.76 to 55.63. The parameters A and τ in the first-order Prony function were 0.09–0.36 and 1.21–1.95 ms, respectively. Conclusion: It is feasible to determine the corneal nonlinear viscoelastic properties based on the corneal contour information and air-puff pressure of the Corvis ST test.
Highlights
Cornea is a kind of biological soft tissue with nonlinear viscoelasticity
For each subject’s 140-frame Corvis ST image data set, we extracted the contour data of the anterior corneal surface and obtained the data set, where i −287, − 286, /, 286, 287; j 0, 1, /, 139, with r−287 r287 4.29 mm, t0 0, t139 30 ms, and dij is the displacement of the anterior corneal surface at point (i, j)
In order to verify the effectiveness of the displacement results from image extractions, the data set from image extractions was compared with the deflection amplitude (DA)
Summary
Cornea is a kind of biological soft tissue with nonlinear viscoelasticity. The biomechanical property of cornea has been considered as a helpful index in the diagnosis of keratoconus (Wang et al, 2015), selection of refractive surgery (Dupps and Wilson, 2006; Shen et al, 2014), screening before refractive surgery (Piñero and Alcón, 2014), measurement of intraocular pressure (IOP) (Brown et al, 2018), Corneal Biomechanical Properties in Vivo and evaluation of the curative effect of corneal cross-linking surgery (Vinciguerra et al, 2017; Sedaghat et al, 2018).At present, corneal visualization Scheimpflug technology (Corvis ST) has been used in the diagnosis of keratoconus (Chan et al, 2018) and glaucoma (Wang et al, 2015). By applying inverse finite element method and corneal deformation data from Corvis ST, investigators studied the nonlinear mechanical characteristics of the cornea (Lago et al, 2015; Sinha Roy et al, 2015; Bekesi et al, 2016), in which a complicated constitutive equation and a fine geometry model were applied. Our previous study (Qin et al, 2019a) applied a thin spherical shell model to identify corneal elastic modulus based on Corvis ST data. These studies have demonstrated the feasibility of using the output data of Corvis ST to acquire corneal biomechanical parameters, such as elastic modulus. A full description of corneal biomechanical properties based on in vivo tests has been still an issue of great concern to the researchers
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