Finite Element Comparison of Biconvex and Plano-Convex Lenticule Designs for Myopic Correction

Abstract

Purpose : The ELITA^TM Femtosecond Laser System is indicated for the creation of corneal flaps for LASIK refractive surgery in the United States and for the lenticule removal procedure (SILK^TM) outside the U.S. The SILK^TM procedure uses a unique biconvex (BC) lenticule shape compared to the traditional plano-convex (PC) (Fig 1A) for myopia correction. Finite element analysis was used to determine cornea shape change following lenticule removal using BC and PC designs.

Methods : A generic model of a human cornea was built in Comsol Multiphysics v5.3. The anterior and posterior radius of curvature were 7.8 mm and 6.8 mm and central thickness was 550 um. The stroma was modeled as a hyper-elastic fiber-based material with properties fit to match experimental test data. An iterative pre-stressing routine was used to determine the initial strain in the model prior to lenticule creation (Fig 1B). The deformation between the unstressed and stressed state of the intact cornea was subtracted from the treated cornea model (Fig 1C), and this was used as the stress-free geometry. An IOP rise of 15 mmHg was simulated and the radius of curvature of the corneal surface was analyzed.

Results : PC and BC profiles intended to treat a manifest sphere of -6.00D were simulated (Fig 2). The intended anterior surface mean curvature was 8.82 mm and both simulations predict an undercorrection (PC = 8.26 mm, BC = 8.42 mm). The BC model had 44% less induction of spherical aberration compared to PC. Both models show similar levels of induced vertical coma due to the entry incision. The 1^st principal strain maps (Fig 2B) show that the BC design reduces displacement of the posterior apex, suggesting it better preserves corneal strength. Undercorrection is due to 4 factors: 1) Curvature change of the posterior cornea; 2) Thinning of the residual bed; 3) Thickening of the anterior cap; 4) Bending effects at the lenticule edge.

Conclusions : The BC design minimizes the induction of spherical aberration and better preserves the biomechanical strength of the cornea compared to the PC design.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.

 

Fig 1. A. Diagram of the two lenticule designs (Shown applanated). B. The pre-treatment models show initial strain of 2.7% under 15mmHg IOP. C. Geometry of the treatment models. Lenticule surfaces modeled as contact surfaces.

 

Fig 2. A. Anterior surface mean radius of curvature. B. 1^st Principal strain, wire frame shows the initial position of the cornea.