Friday, April 19, 2013
Modeling the Biomechanics of Human Cornea
Anna Pandolfi, Associate Professor, Department of Structural Engineering, Politecnico di Milano
The cornea is a thin concave-convex lens which provides 70% of total refractive power of the eye. Anterior and posterior surfaces of the cornea are similar to spherical segments, and the cornea reaches its maximum refractive power (43 diopters) in equilibrium configuration under the action of physiological intraocular pressure (15-18 mmHg). Refractive errors resulting in myopic, hyperopic or astigmatic vision can be partially or totally corrected with laser refractive surgery (PKR, LASIK, LASEK). On the basis of geometry of the original cornea and of desired power correction, refractive surgery removes a thin layer of the cornea, modifies external curvature, and changes refractive power. We've developed an accurate three-dimensional finite element model of the human cornea. A discretized geometry is created by an automatic procedure based on some geometrical data available from standard clinical measurements. The material is modeled as a distributed two-fiber reinforced hyperelastic medium, describing the well organized collagen structure embedded in isotropic matrix of the cornea. In order to simulate the surgical correction of myopic, hyperopic and astigmatic eyes, the code is equipped with a reshaping procedure based on standard and personalized ablation profiles. Besides the postoperative shape of the cornea and the final effective refractive power, numerical results provide the pre-postoperative stress distribution, of primary importance in refractive surgery planning. We used the numerical model to carry on a quantitative study on influence of the cornea geometry on mechanical response by direct comparison with laboratory tests; and to estimate the increase on local stress induced by PRK reshaping through use of accurate patient specific corneal geometries. We observe that the use of simple mathematical models based on linearized thin shell theory applied to short-term postoperative measurements can be useful in identifying corneal tissues prone to develop post-operative ectasia.