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Mengchen Xu, Amy L Lerner, Ashutosh Richhariya, Geunyoung Yoon; Predicting spherical aberration induced by laser refractive surgery by using biomechanical model of cornea. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3730.
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© ARVO (1962-2015); The Authors (2016-present)
To develop a finite element (FE) model of the human cornea to understand biomechanical effects of corneal spherical aberration (SA) induced by laser refractive surgery.
The model used in this study was implemented in the FE code ABAQUS. An axisymmetric model with 7 layers was generated of the intact cornea, and modified to simulate 2, 6 and 10 diopters (D) myopic corrections by removing elements, as calculated by Munnerlyn equation for 6mm optical zone. The biomechanical response of the cornea before and after surgery was evaluated under different intraocular pressure (IOP) acting on the posterior surface of cornea: 10, 15, and 21 mmHg. As in previous literature, we used nonlinear material properties of cornea tissue and fixed boundary conditions to reflect the stiff limbus. We compared cases with uniform and varying material properties along the depth to evaluate the potential effect of relative difference in stiffness of the stroma. Based on calculated deformations in the pressurized cornea models, Zernike SA was calculated using ray-tracing program (Zemax, Radiant Zemax, LLC) to determine the induced amount of SA by the surgical procedure for 6mm optical zone.
Positive SA increased with IOP in both intact and ablated corneas. For intact cornea, the SA increased 25% when IOP changed from 10 to 21mmHg. A larger increase (53% with -6 D correction) in SA was found in ablated cornea and the increase become even larger with increasing amount of refractive correction. Without biomechanical response i.e. no IOP, positive SA is decreased by myopic correction. However, our biomechanical model showed an increase in positive SA when considering corneal biomechanics, 0.10 µm and 0.29 µm under 10 and 21mmHg IOPs for -6D correction, respectively. Relative differences in material properties between corneal layers also had considerable impact on the predicted SA after surgery. For -6D correction, the induced SA is -0.27µm, 0.01µm, 0.29µm and 0.47µm under 21mmHg IOP with uniform and three stiffer material properties of outer anterior stroma.
The axial distribution of material properties within the cornea significantly influences the biomechanical response of the cornea as well as the resulting SA. A biomechanical model of cornea with varied material properties of cornea can improve our ability to predict corneal optical quality after laser refractive surgery.
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