Numerical study of the effect of corneal layered structure on ocular biomechanics.
OBJECTIVE
The study aimed to improve the accuracy of corneal numerical simulation by adopting a better representation of the corneal layered structure. The study considered both the shear and tensile behavior of the interface surfaces between stromal lamellae, and assessed the effect of modeling the cornea's three main layers-the epithelium, stroma, and endothelium with their respective material properties.


METHODS
Twelve human donor corneas were tested to determine the behavior of the stroma under surface shear. Numerical models were then built to consider the stromal inter-lamellar adhesion, which included the shear behavior determined experimentally and the tensile behavior available in the literature. They also adopted the distinctive material properties of the epithelium, stroma, and endothelium. The numerical models simulated corneal behavior under intraocular pressure elevation, concentric anterior pressure, and the conditions under tonometry with the Goldmann applanation tonometer.


RESULTS
The stress-strain shear behavior of stromal tissue followed an exponential pattern, with an initial low stiffness increasing gradually under higher stresses. This behavior was adopted in the numerical simulation to set the level of adhesion between stromal layers. Considering that the stromal inter-lamellar adhesion and the distinctive material properties of corneal layers had a significant effect in simulating the response to concentrated anterior pressures, which cause bending of corneal tissue, this was almost unnecessary when predicting the effect of intraocular pressure, which put the cornea under membrane tension.


CONCLUSIONS
The corneal layered structure affects the results of numerical simulations especially in problems where the cornea is subjected to bending effects.
Elsheikh
Ahmed
A
Division of Civil Engineering, University of Dundee, Dundee, United Kingdom. a.i.h.elsheikh@dundee.ac.uk
Ross
Stuart
S
Alhasso
Daad
D
Rama
Paolo
P
D000368:Aged; D001696:Biomechanical Phenomena; D003319:Corneal Stroma / Q000502:physiology*; D004548:Elasticity / Q000502:physiology*; D004728:Endothelium, Corneal / Q000502:physiology*; D019573:Epithelium, Corneal / Q000502:physiology*; D006801:Humans; D007429:Intraocular Pressure / Q000502:physiology; D008875:Middle Aged; D008954:Models, Biological; D009716:Numerical Analysis, Computer-Assisted; D013312:Stress, Physiological; D000267:Tissue Adhesions