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P.M. Pinsky, D. van der Heide, D. Chernyak, S. Somani; Biomechanical Response of the Cornea to Microkeratome Incisions . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2739.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Disturbances of the stromal microstructure, common in refractive surgical procedures, may create unexpected and undesired changes to the vision quality of the eye. The mechanical properties of the cornea and sclera derive primarily from the specialized architectural arrangement of their collagen and it is hypothesized that anisotropy in fibril orientation across the cornea will result in anisotropy of the mechanical properties. Recently, the preferred orientations of stromal collagen fibrils throughout the cornea, limbus, and adjacent sclera have been measured using synchrotron X–ray scattering. A tissue material model based on the fibril microstructure was developed to determine biomechanical deformation of the cornea resulting from tissue cutting and removal. Methods: The tissue model, with a position–dependent probability density function for the corneal collagen fibril orientations and the IOP–induced corneal pre–stress, was implemented in a finite element program. The elastic constants for the corneal fibrils and extra–fibrillar matrix were determined by matching numerical results for RK against published data. The model was also validated on an independent subset of the RK data and provided agreement with reported changes in spherical equivalent as a function of IOP. Results:The calibrated model was employed to simulate a tunnel incision in the sclera made for cataract extraction and intraocular lens implantation. Simulation predicted induced astigmatism as a function of incision orientation. A symmetrical cut for a LASIK cap of 8 mm diameter and 150 µm thickness was simulated for a typical cornea. The results indicate that cutting a LASIK cap induces central flattening and peripheral steepening compared to the intact cornea. The average shift of refractive power for the central 4 mm zone is about 0.25D towards hyperopia. For flap thicknesses in the 300 – 400 µm range, nomograms relating applanation lens diameters and resection depths for lamellar keratectomy were used to verify results which indicate steepening of the thinner stromal bed under influence of the IOP. The transition between flattening and steepening as a function of flap diameter and thickness has been established Conclusions: The ability to validate RK data indicates that the material model accurately captures corneal biomechanics. The proposed model for the cornea enables the accurate simulation of several different surgical procedures to the cornea without need to re–calibrate the model parameters. Understanding the biomechanical changes may lead to refinement of surgical techniques to minimize undesired induced aberrations.
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