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A. Sinha Roy, W. J. Dupps, Jr.; Patient-Specific Characterization of Myopic PRK Outcomes Using Finite Element Modeling (FEM). Invest. Ophthalmol. Vis. Sci. 2009;50(13):1759.
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Analyze topographic outcomes after myopic PRK using patient-specific FEM model
A 3-D FEM model of the cornea was constructed from Atlas patient topography and Visante OCT maps. Myopic PRK with mitomycin C 0.02% and programmed correction of -8.05D sphere change was simulated using Munnerlynn’s equation. The cornea was modeled as hyperelastic tissue with collagen fibers oriented orthogonally. The assumed elasticity of the native cornea and IOP was also perturbed in post-PRK model as the IOP was elevated post-PRK due to topical steroids. Axial power difference maps were constructed to compare pre- with post-PRK FEM and in vivo outcomes.
The pre- and post-PRK IOP was normal. At 1 month follow-up, the IOP nearly doubled due to topical steroids. Once steroids were discontinued and topical antihypertensives added, the IOP and axial power returned to normal (2 month follow-up). The axial power differences are similar though the magnitudes differ from pre to post-PRK after IOP spike at 1 month follow-up (see figure). In vivo, the average power difference in the central 3 mm zone was -6.65D. If FEM elasticity was assumed to be unchanged from pre to post-PRK, the power difference was -7.98D (20% difference from in vivo). If the tissue was made less stiff than pre-PRK tissue stiffness values by 25%, the power difference was -6.75D (1.5% difference from in vivo). After post-PRK IOP returned to normal, the cornea flattened by 0.82D in vivo. When the elasticity was left unchanged after PRK, the flattening was 1.24D. When the elasticity was reduced by 25%, the flattening predicted by FEM was 1.03D. Thus, a less stiff cornea and steepening with IOP elevation after PRK were predicted by FEM.
FEM can be useful to predict refractive change after PRK using patient specific topography. Iterative modeling to match clinical outcomes predicts a weaker cornea after high myopic PRK. This highlights the need to measure elastic properties in vivo to improve patient-specific FEM results.
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