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D.M. Silver, O. Geyer; Corneal Ecstasia after Laser Refractive Surgery Induced by Intraocular Pressure Rise . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2669.
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Purpose: To explore the hypothesis that corneal ectasia in patients with a history of laser refractive correction surgery depends on the thickness of the residual central corneal zone and the extent of intraocular pressure rise. Methods: Using principles of elasticity and mechanics, differential equations were derived for a mechanical model, consisting of a spherical zone surrounded by a thicker spherical sector, as a representation of a cornea after laser refractive surgery. Literature values of the elastic modulus of corneal tissue span a wide range. The corneal radius of curvature, corneal thickness, depth and radius of corneal ablation, and intraocular pressure provide additional parameter space for numerical experimentation. For each choice of parameters and boundary conditions, deformation of the ablated corneal surface from a spherical shape and position was determined from numerical evaluation of the differential equations, parameterized by a range of intraocular pressures. Results: Deformation of the cornea depends on the parameters explored in this work. For a given set of parameters, the deformation from sphericity (degree of ecstasia) has a strong dependence on the thickness of the unablated residual central corneal layer. The deformations increase with increasing intraocular pressure. Photorefractive keratectomy (PRK) produces an unablated central thickness equal to the original corneal thickness minus the ablation depth. The flap in laser in situ keratomileusis (LASIK) is a structure that doesn't support a mechanical load across the cornea. Therefore, for an equivalent ablation depth, LASIK produces an unablated residual central thickness that is thinner than PRK by the thickness of the flap. Hence, for nearly equivalent circumstances, a greater degree of ecstasia will be associated with LASIK compared with PRK. Conclusions:The residual thickness of the central cornea is an important factor in the integrity of the corneal shape under the influence of increased intraocular pressure. The choice of PRK versus LASIK controls an additional risk factor in the degree of corneal deformation or ecstasia that might accompany elevated intraocular pressures.
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