Effects of the LASIK flap thickness on corneal biomechanical behavior: a finite element analysis

Abstract

Background: It is well known that the biomechanical properties change after LASIK refractive surgery. One reason is the impact of flap creation on the residual stroma. The results have revealed that the change is closely related with the flap thickness in several studies. However, the quantitative relationships between the distributions of displacement and stress on the corneal surface and flap thickness have not been studied. The aim of the study was to quantify evaluate the biomechanical change caused by the LASIK flap. Methods: By building a finite element model of the cornea, the displacement, the stress and the strain on the corneal surface were analyzed. Results: The results showed that the corneal flap could obviously cause the deformation of the anterior corneal surface. For example, the displacement of the corneal vertex achieved 15 μm more than that without corneal flap, when the thickness of corneal flap was 120 μm thick. This displacement was enough to cause the change of aberrations in the human eyes. In the central part of the cornea, the stress on the anterior corneal surface increased with flap thickness. But the change in the stress on the posterior corneal surface was significantly less than that on the anterior surface. In addition, the stress in the central part of the anterior corneal surface increased significantly as the intra-ocular pressure (IOP) increase. Furthermore the increase of IOP had a clearly less effect on stress distribution at the edge of the cornea. Distributions of strain on the corneal surface were similar to those of stress. Conclusions: The changes in the biomechanical properties of cornea after refractive surgery should not be ignored.

Effects of Laser In Situ Keratomileusis and Small-Incision Lenticule Extraction on Corneal Biomechanical Behavior: A Finite Element Analysis

“…With the same refractive surgery and myopic correction, the displacement and Von Mises stress of the residual stromal bed vertex increased linearly with the IOP. Fang et al also presented similar findings ( Fang et al, 2020 ). The results indicated that the IOP had a direct influence on the residual stromal bed.…”

The Biomechanical Response of the Cornea in Orthokeratology

“…A finite element analysis (FEA), a commonly used numerical method, is an effective way to study and analyze the mechanical behavior and functional mechanism of the cornea, which has been extensively implemented to study corneal mechanical problems ( Kling et al, 2014 ; Li ZD. et al, 2016 ; Fang et al, 2020 ; Meng et al, 2020 ). Not only has the FEA been widely recognized for its potential in simulating and analyzing the effects of corneal surgery, it has also been used in corneal diseases and ocular trauma because of its predictive feature.…”

Time-varying regularity of changes in biomechanical properties of the corneas after removal of anterior corneal tissue

“…If we had a fully understanding of the corneal biomechanical properties in different periods before and after operation, then we can get a good prediction of the postoperative refractive power based on the FEM, and then we may give a solution to the problem of postoperative overcorrection or undercorrection through comprehensive consideration of postoperative corneal biomechanical properties, residual bed thickness of the cornea, and the removed actual thickness of the stroma. Furthermore, we noticed that the displacement of posterior corneal surface, an important index after corneal refractive surgery [ 27 ], of HE3 was about 0.74 times that of HC3 (Fig. 9 b) when IOP was 15 mmHg.…”

“…Therefore, it is better to consider the changes of corneal biomechanical parameters in the preoperative design of refractive surgery and prediction of postoperative refraction. As we know, finite element method is often used to simulate and analyze the changes of corneal morphology and stress postoperatively [ 27 , 28 ]. However, some finite element method simulations were performed by using preoperative or (partial) fixed corneal material parameters [ 27 , 29 , 30 ].…”

“…As we know, finite element method is often used to simulate and analyze the changes of corneal morphology and stress postoperatively [ 27 , 28 ]. However, some finite element method simulations were performed by using preoperative or (partial) fixed corneal material parameters [ 27 , 29 , 30 ]. This may cause a large deviation between the simulation analysis results and the actual situation.…”