Abstract
Purpose :
LIRIC (Laser Induced Refractive Index Change) is a novel approach to vision correction which uses a femtosecond laser to modify stromal refractive index in a thin layer (∼20um). While biomechanical properties of the treated tissue are still unknown, this work investigates the potential effects (biomechanically and optically) of thin stromal layers modified to be either softer or stiffer than surrounding native tissue.
Methods :
A linear isotropic finite element model of the cornea was used to simulate the effect of changes in cornea mechanical properties on shape and optical performance. The size and location of the treated zone were varied systematically. The radius was varied from 2.5 to 3.5 cm, the location varied between 80 and 120 μm below the anterior surface, and the number of modified layers varied from 1 to 5. The intensity of the softening/stiffening effect was varied as well. A sensitivity analysis was performed to identify the key determinants of mechanical and optical behavior among the different combinations of factors.
Results :
Stiffer tissue (up to 3.5x native tissue) produced a flattening effect on the cornea surface. This effect introduced a negative shift up to 0.8 diopters for some sizes and locations of the LIRIC-treated pattern. Spherical aberration and defocus are modified over clinically meaningful thresholds in some cases. When the treated tissue is simulated as softer, the cornea bulges, producing a positive diopter shift. Extreme stiffening and softening effects are shown in Fig. 1. Besides the intensity of the stiffening/softening effect, the radius of the LIRIC pattern and the number of layers have the strongest optical influence (Fig. 2).
Conclusions :
Modeling showed how the geometry and position of a modified layer in the stroma can affect shape and optical performance. Moreover, different optical corrections (e.g. refractive power, spherical aberration) are found to be sensitive to different aspects of biomechanical modification (e.g. diameter, number of layers), providing a potential pathway to control/tune these effects. Further study is needed to accurately characterize the material properties of treated tissue.
This is a 2020 ARVO Annual Meeting abstract.