Biomechanical and Optical Behavior of Human Corneas Before and After

Photorefractive Keratectomy

Anna Pandolfi, Politecnico di Milano, ItalyPaolo 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

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

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

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.

Methods

Methods

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).

Results

Results• 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.

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)

Results

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