Refractive change induced by the LASIK flap in a biomechanical finite element model

PURPOSE: To study the effect of varying four parameters on the refractive change induced by the LASIK flap. METHODS: Using a variety of patient-specific data such as topography, pachymetry, and axial length, a finite element model is built. The model is used in a non-linear finite element analysis to determine the response and change in optical power of the cornea as a function of a material property of the cornea (corneal elasticity), flap diameter and thickness, and intraocular pressure. RESULTS: The central flattening or hyperopic shift occurred atop the flap in all four of the simulated eyes tested with the creation of the LASIK flap. Of the four parameters tested, modulus of elasticity (Young's modulus) had the most profound effect on the results of hyperopic shift, varying from <0.5 diopters (D) in the least elastic (stiffest) cornea to >2.0 D of hyperopic shift in the most elastic cornea. The depth of the lenticular cut was the second-most significant parameter tested varying from 0.24 D at 100 microns to 1.25 D at 275 microns of depth. Varying intraocular pressure demonstrated less difference, and varying corneal flap diameter demonstrated the least difference in induced refractive change on the model. The hyperopic shift was noted to be greater in hyperopic than in myopic eyes (simulated) tested. CONCLUSIONS: Three-dimensional finite element analysis modeling of actual patient data could lead to a better understanding of the biomechanical response of comeal tissue to the lenticular flap creation and potentially for ablation patterns produced by the excimer laser. Understanding these biomechanical responses may lead to greater predictability and improvement of visual outcomes.