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Three-Dimensional Modeling and Computational Analysis of the Human Cornea
Anna Pandolfi, April 3, 2008 Three-Dimensional Modeling and Computational Analysis of the Human Cornea Anna Pandolfi Dipartimento di Ingegneria Strutturale Politecnico di Milano, Italy The cornea is a thin concave-convex lens, which supplies about 70% of the total refractive power of the eye. The external and internal surfaces are very close to spherical segments. The refractive power of a lens is dependent strongly on the surface curvatures. In the normal configuration, in equilibrium under the physiological intraocular pressure (15-18 mmHg), the cornea reaches its maximum refractive power (43 D). Refractive errors resulting in myopic, hyperopic or astigmatic vision can be partially or totally corrected with laser surgery. Refractive surgery removes a thin layer of the cornea, modifies its external curvature, and changes its refractive power. The profile of the thin layer removed by the laser is planned on the basis of the geometry of the original cornea and of the desired power correction. We developed a realistic three-dimensional numerical model of the human cornea. The geometry is created by an automatic procedure, which needs a few geometrical data (in plane radius, internal and external curvatures, and thickness) provided by standard measurement tests. The solid model is then discretized in finite elements. The cornea is modeled as two-fiber reinforced hyperelastic material, able to describe a well organized collagen structure. The model has been validated with remarkable correspondence against experimental results. An accurate parametric analysis has been carried out to assess the sensitivity of the optical response of the cornea to the material parameters. For the simulation of surgical correction of myopic, hyperopic and astigmatic eyes, the code includes a cornea reshaping procedure based on personalized ablation profiles. Numerical results provide the postoperative shape of the cornea, the final effective refractive power, and the preoperative/postoperative stress distribution. The availability of the stress fields is of primary importance in refractive surgery planning of problematic or degenerated corneas.
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