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V. Teuma, G. Gray, S. Bott, R. Frey; FEA of Limbal Relaxing Incisions Irregularities as a Source of Higher-Order Aberrations. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4220.
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The object of this study is to develop a realistic finite element analysis (FEA) model of the human cornea to simulate the effects of non-ideally shaped limbal relaxing incisions (LRIs) on correction of astigmatism and induction of HOA.
The numerical model was assumed to have non-linear material properties with stiffness and homogeneities varying with five layers of depth. The model was used to simulate several different corneal incision configurations: opposing pairs of straight incisions, representing a departure from ideal curve, compared to ideal arcuate incisions of 30º, 60º and 90º subtended angle of arc length. The resulting geometry of the biomechanical corneal model was analyzed within a 6.0mm optical zone to determine lower and higher-order aberrations (LOAs, HOAs) using a Zernike polynomial fitting algorithm up to 7th order studying irregularities and departures from the cornea’s normal ellipsoidal shape.
Finite element analysis of the cornea after applying relaxing incisions provided a reshaped corneal surface from which refractive power and HOA were computed. Straight incisions, equal length concentric arcuate incisions generated a less rotationally symmetrical corneal and increased spherical aberration. The cornea becomes more oblate as the number and length of incisions increase.Low order aberrations (astigmatism, defocus) and high order abberations (coma, spherical aberration and trefoil) were present in all corneal models with relaxing incisions. HOAs increased with straight incisions compared to equal length concentric arcuate incisions. The HOA increase was more than 64% (coma) and 80% (trefoil)for the straight cuts compared to equal length ideal arcuate incision model.
HOAs were shown to increase significantly if the LRIs were not cut along a circular arc centered at the corneal apex. A more precise system for generating LRIs, such as a computer guided femtosecond laser, may help to reduce induction of HOAs.
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