The elasticity of a tissue, while the ground substance

The function of the cornea can be summarized
succinctly: Protection, transmission, and
refraction. It provides mechanical stability to the eye and acts as a
protective layer to the interior of the eye. Most of that refraction in the eye
takes place at the cornea. About 80% of the refraction occurs in the cornea
1. To achieve the good quality vision,
the cornea has an intricate structure of collagen fibers arranged in lamellae and interwoven with a cellular matrix, this structure helps to
maintain a specific curvature. Thus, the
biomechanical status of the cornea plays a key role in maintaining quality
vision 2.

Corneal biomechanics is a branch of science that
studies deformation and equilibrium of corneal tissue under the application of
any force. The mechanical properties of a tissue depend on how the fibers, cells, and ground substance are
organized into a structure 3. Collagen and elastin are responsible for the
strength and elasticity of a tissue, while the ground substance is responsible
for the viscoelastic properties. All these terms are important because the
cornea is considered a viscoelastic material and some devices try to measure
and even differentiate between the different components of the biomechanical
behavior of the living corneal tissue.

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The biomechanical characterization of in vivo tissue
is a growing field of surgical planning.
Being able to estimate in vivo the mechanical response of the cornea can allow
the detection of some pathology whose symptoms may change the stiffness of the
cornea. Furthermore, the in vivo estimation of the mechanical behavior of the
cornea is the core of the analysis of the real response during surgical
interventions 4.

There are several studies identify the biomechanical
properties either in vivo or ex vivo that have been done. Newer technologies
may provide the in vivo quantification. Shear wave speed imaging using
ultrasound can measure shear and Young’s modulus of the corneal tissue in vivo.
The technique can resolve depth and radial variation in the modulus of the
cornea. However, it approximates tissue behavior as a linear, elastic material
and requires contact with the tissue during measurement 56.

Optical coherence elastography (OCE) is another
technique that can resolve depth-dependent
strain variation in corneal tissue subject to contact stress ex vivo. However,
the correlation between OCE mechanical
strain and linear modulus of the cornea has not been reported yet. A modified
non-contact OCE method can measure shear wave speed in the cornea but this method assumes the cornea as a
linear, isotropic material 7.

Brillouin scattering uses an optical method to
quantify the corneal biomechanical properties. By measuring the spectral shift
in the wavelength of the scattered light, Brillouin modulus can be measured in
normal and crosslinked corneal tissue ex vivo and recently in vivo. The
Brillouin modulus may correlate with Young’s modulus, though the correlation
with in vivo properties requires further study. However, none of these
investigational techniques described the stress stiffening effect or non-linear
biomechanical properties of corneal stroma observed ex vivo 8.

Air puff tonometers use non-contact techniques. The
advantages of non-contact tonometers over contact tonometers include their
relative ease of use and less-invasive operation. In
air puff non-contact tonometers, force is
a column of air which is emitted with gradually increasing intensity. At the
point of corneal flattening, the air column is shut off and the force at that
moment is recorded and converted into mmHg. The dynamic deformation
following an air-puff has been proposed in the biomedical
cornea. The degree of deformation of the sample is empirically related to
mechanical parameters, and the inherent mechanical parameters of the tissue
were rarely retrieved.

There are two devices designed for
clinical use to provide corneal biomechanical data the Ocular Response Analyzer a dynamic bi-directional applanation device, the Corvis ST, and a dynamic
Scheimpflug analyzer device.

The Ocular Response Analyzer uses
an air puff to deform the cornea into a slight concavity
and measures the intensity of the infrared
beam of light reflected off from the anterior corneal surface during
deformation.

Recently, the Corvis ST combines a high-speed camera with air-puff.
This camera records the movement of the eye with more than 4000 images per
second. These images provide detailed insight into a corneal biomechanical response. Then the simulation of the cornea
deformation was performed using the Finite Element Method (FEM), which is a
numerical method to obtain approximate solutions for partial differential
equations involving physical behavior for space and time dependent problems and
is able to predict its response to external loading.

 

Therefore, this project aimed to create 2-D cornea
model using ABAQUSE FINITE ELEMENT SOFTWARE. A biomechanical model of the
cornea describes the response of the cornea in terms of displacements to the
application of loads.