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Opto-Mechanical Model of Arcuates for Astigmatism Correction. Low and High Order Aberrations

AuthorsNavarro, Rafael; Palos, Fernando; Lanchares, Elena; Calvo, Begoña; Cristóbal, José Angel
Astigmatism correction
Finite element model
Ray tracing
High order aberrations
Issue Date2011
CitationJournal of Cataract & Refractive Surgery
AbstractTo develop a realistic model of the opto-mechanical behaviour of the cornea after curved relaxing incisions, and compare the astigmatism correction predicted by the model with that of the Lindstrom's nomogram. Methods: A three-dimensional finite element model of the anterior hemisphere of the ocular surface was generated, considering three parts: cornea, limbus and sclera. The corneal tissue was modeled as a quasiincompressible, anisotropic hyperelastic constitutive behaviour strongly dependent on the physiological collagen fibril distribution. Similar models were used for limbus and sclera. With this model we simulated the effects of curved corneal incisions. The resulting geometry of the optical zone was analyzed and finite ray tracing performed to compute refractive power and high order aberrations (HOA). Results: The finite element simulation provides the local displacements of the corneal tissue, and from that we obtain the resulting elevation topographies of the surfaces. Results of finite ray tracing show a close agreement between the model and the Lindstrom's nomogram. However, paraxial computations would yield significantly different results (undercorrection of astigmatism). In addition, arcuates induce important amounts of HOA, mainly coma, trefoil and quadrafoil. Conclusions: Finite element models together with finite ray tracing computations permit to perform realistic simulations of the biomechanical and optical changes induced by relaxing incisions. The close agreement found between model and nomogram supports the validity of this approach. This can be a powerful tool for planning the surgery or designing new incisional techniques, whereas commonly used paraxial formulas fail to predict the resulting astigmatism and HOA.
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