1. Biomechanical and Optical Behavior of Human Corneas Before and After Photorefractive Keratectomy Anna Pandolfi, Politecnico di Milano, Italy Paolo Sanchez, Technical University, Delft, theNetherlands Kyros Moutsouris, Athineum Refractive Center, Athens, Greece Moorfields Eye Hospital, London, United Kingdom The authors have no financial interest to disclose

2. Purpose To validate a numerical code designed a few years ago by Pandolfi and Manganiello (2006); Pandolfi et al. (2009) to model the geometrical changes of the human cornea due to laser ablation in PRK

3. Purpose To setup a numerical model of the cornea, two main ingredients are necessary: First, a geometrical model describing with high fidelity the shape of the organ Second, a material model that combines an accurate and realistic description of the behavior of the corneal tissue with robustness and calculation efficiency

4. Methods 5 patients (10 eyes) were treated with PRK for myopic or myopic compound astigmatism Slit-scan topographic data where gathered pre- operatively see Tab. 1 and at 3 months post- operatively see Tab. 2. Table 3 shows the material properties for the 2 material groups. Table 4 shows the refractive data related to the PRK procedure in the 10 cases. The ablation profiles applied for the numerical procedure were described by biconic surfaces in 6 cases and by ellipsoidal surfaces in the remaining 4 cases.

5. Methods

7. Results These data are used to build the patient specific models of the cornea discretized in finite elements Pandolfi and Manganiello (2006), see Fig. 1(a).

8. Results

9. For each patient, we built a patient specific model of the cornea prior to the refractive surgery and a patient specific model of the cornea after the PRK intervention. As expected, we could not group the 10 eyes within a single set of material constants and had to define two subgroups of material constants. The finite element analyses were able to provide qualitative information about the stress distribution in the cornea, see Fig. 1(b), and across the thickness.

10. Results We evaluated the normal Cauchy stress in the horizontal and vertical meridians, at the center of the anterior and posterior surfaces of the corneas. We observed that the preoperative stress at these locations varies with a distribution characterized by a small standard deviations and average values around 10-15 kPa at the posterior and anterior surface respectively, see Fig. 2(a). The postoperative stresses are characterized by a 30% increase in the average stress and by a larger dispersion in the values, see Fig. 2(b)

11. Results

12. Conclusions Patient specific numerical models of the cornea can provide quantitative information on the changes in refractive power and in the stress fields caused by refractive surgery