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Coupled numerical-experimental protocol to characterize corneal tissue

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Session Details

Session Title: Presented Poster Session: Cornea I

Venue: Poster Village: Pod 3

First Author: : M.Ariza Gracia SWITZERLAND

Co Author(s): :    A. Ortilles Gonzalo   J. Cristobal   J. Rodriguez Matas   B. Calvo Calzada     

Abstract Details

Purpose:

Current techniques using a single set of experiments to characterize the properties of corneal tissue are said to be ill-posed. A numerical-experimental protocol to determine the material properties of the cornea using inflation (biaxial tension state) and indentation (bending like state of stress) tests is proposed. In this vein, a proper definition of the material provides better insight in the behavior of the tissue, allowing to develop more accurate in silico techniques to help in clinical assessment. To validate the methodology, results from the simulation of an astigmatic keratotomy (AK) performed using the identified material properties were compared with experiments.

Setting:

ESKAS fellow, ISTB, University of Bern (Bern, Switzerland). AMB, Aragón Institute of Engineering Research, University of Zaragoza (Zaragoza, Spain). LaBS, Politecnico di Milano (Milano, Italy). Department of Animal Pathology, University of Zaragoza (Zaragoza, Spain). Department of Ophthalmology, ”Lozano Blesa” University Clinic Hospital of Zaragoza (Zaragoza, Spain).

Methods:

An experimental protocol is developed using animal models (New Zealand white rabbits). Indentation (bending stress) and inflation (biaxial stress) experiments are performed in the laboratory. Apical displacement vs. pressure (inflation test), and force vs. displacement (indentation test) curves are recorded to obtain the mechanical response of the corneal tissue. The experiments are reproduced in silico, using two scenarios: average and patient-specific corneal geometry. By means of an optimization algorithm, the best material parameters matching the experimental curves are determined. The identified material properties are used to simulated an AK, and the predicted post-surgery optical corrections compared with the real surgery.

Results:

The average corneal thickness was 380 microns and the intraocular pressure 14 mmHg. The average radii of curvature were 7.114 mm and 7.038 mm in superior-inferior and nasal-temporal direction, respectively. The sensitivity of the inflation experiments to the material parameters was much higher than the sensitivity of the indentation results, mainly due to the dominance of the corneal thickness on bending. Results from the simulation of an AK surgery using the optimal set of material parameters and patient-specific geometry gave an error of less than 0.4 diopters in cylinder, and 0.1 diopters in sphere with respect to the actual surgery.

Conclusions:

The combined numerical-experimental protocol is able to determine the material properties of the cornea successfully. The optimal set of material parameters allowed reproducing the AK surgery in good agreement with the experiments. The patient-specific geometry is of great importance in terms of optical validation. The main drawback of the methodology is the need of ex vivo inflation tests. Hence, to extend it to humans is essential to build a reliable ex vivo inflation database that can be combined with contact or non-contact tonometry tests, so as to determine the corneal tissue behavior.

Financial Disclosure:

NONE

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