Finite-element modeling of intrastromal ring segment implantation into a hyperelastic cornea.

PURPOSE Intrastromal corneal-ring segments (ICRSs) are applied to improve highly distorted vision in keratoconic, myopic, and astigmatic patients. Selections of ICRS geometry and position are primarily based on empirical nomograms. We developed a finite-element model (FEM) predicting the corneal response to different ICRS geometries in normal and keratoconic corneas. METHODS A two-dimensional FEM was built in proprietary software (ANSYS-APDL), consisting of hyperelastic ocular tissues (cornea, limbus, sclera) and a triangular/hexagonal ICRS of poly(methyl methacrylate). An incrustation model was developed considering the local material addition and rigidity increase at the ICRS position and also for the triangular ICRS the geometric difference between its base (plane) and corneal tunnel (parallel to corneal surface). Different ICRS heights (150-350 μm) and optical zones (4.4-6.6 mm) were simulated. An axis-symmetric model of keratoconus was studied, where corneal elasticity was decreased locally. RESULTS ICRS GEOMETRY (HEIGHT AND OPTICAL ZONE) HAD A SIGNIFICANT INFLUENCE ON CORNEAL POWER: changes from 4.08 to -17.7 diopters (D) (healthy)/3.31 to -20.5 D (keratoconic) were observed. Central corneal thickness was predicted to increase by up to 38.5 μm (healthy)/97.9 μm (keratoconic). Spherical aberration also changed upon ICRS implantation. The protrusion of the posterior cornea behind the rings was well predicted. The model confirmed the clinically reported trends on the effect of ring geometry. CONCLUSIONS FEM is a powerful tool to study the corneal response to ICRS implantation. The combination of FEM with individual biomechanical properties and geometry of patients holds promise to increase the predictability of ICRS surgery.