INTRODUCTION

The cornea is the transparent, anterior part of the eye that refracts light onto the
lens and is essential to vision. The curvature and thickness of the cornea play an
important role in focusing the light to the lens; therefore, they are largely
responsible for its optical powers^{(}^{1}^{)}.

In humans and many other mammals, the cornea consists of five layers: the epithelium,
Bowman's membrane, the stroma, Descemet's membrane, and the endothelium^{(}^{2}^{)}. Of these, the middle layer, the
stroma, accounts for 90% of the corneal thickness in humans and is the major layer
contributing to the mechanical strength and stiffness of the cornea^{(}^{3}^{)}.

The stroma is a composite material comprising matrix embedded with a complex network of
collagen fibers. The matrix is a viscoelastic and nearly incompressible
material^{(}^{2}^{)}. The
collagen fibrils are bundled together to form fibers^{(}^{4}^{)}, which are stacked in parallel layers called
lamellae^{(}^{5}^{)}. In the
human cornea, the stroma consists of approximately 300 lamellae at the center and
approximately 500 lamellae at the limbus^{(}^{6}^{)}. The fibers reinforce the tissue and give it mechanical
strength along their orientation. The arrangement of collagen fibrils is important to
determine the mechanical strength of the cornea. X-ray scatter intensity distribution
data has indicated two preferred directions of collagen orientation at the center of the
cornea: along the nasal-temporal and superior-inferior meridians^{(}^{7}^{-}^{8}^{)}. Closer to the limbus, the fibers tend to run in a
circumferential direction^{(}^{8}^{)}. Approximately two-thirds of the fibrils are preferentially
oriented in the cornea, with the remaining third exhibiting a more or less random
orientation^{(}^{9}^{-}^{10}^{)}. This arrangement of collagen fibrils
results in anisotropy of the corneal material.

Biomechanical experiments on corneal tissue have been performed in both *in
vitro* and *in vivo* studies. Recently, the use of
computational models based on the finite element method has proven to be an effective
way for studying corneal mechanics and related diseases^{(}^{2}^{,}^{6}^{,}^{11}^{-}^{12}^{)}. These models have been successful in
predicting the pre- and postoperative response to eye surgery for myopia, hyperopia,
astigmatism, and keratoconus^{(}^{5}^{,}^{13}^{-}^{16}^{)}.

The finite element method (FEM) is a numerical technique to obtain approximate solutions for partial differential equations involving physical, thermal, chemical, and other phenomena. Generally, these differential equations are impossible to solve analytically because of the complex geometries and properties of the materials. The approach for solving the equations is to simplify the form of the equation using simple interpolation functions and integrate the entire solution to obtain the final results. This method is very useful for solving problems involving complex material properties and irregular geometries.

To conduct a finite element analysis (FEA), a given body or system is divided into small units called elements, which are interconnected at points called nodes. The nodes and elements create a network referred to as a mesh. Each element is assigned specific material and structural properties and the body is analyzed under certain boundary conditions. These boundary conditions can include forces, displacements, and temperatures. The analyzed solution of each element is assembled together to give the global response of the system.

The accuracy of this method, in general, depends upon the number of elements used for analysis. A larger number leads to a more precise solution. However, over-refinement of the mesh can lead to longer computational time and memory use in the computer. The element shape and type also play an important role in the accuracy of analysis.

The focus of this article will be the use of FEA for mechanical analysis of the cornea.
We will review how this method has been applied to examine the cornea under different
loads such as intraocular pressure (IOP), impact from a foreign object, or incisions.
The method can also be used to examine how the shape is affected by changes in material
properties, such as those occurring during keratoconus. It is worth mentioning briefly,
though, that the method can be applied to a wide variety of problems. It has been used
to study thermal, electrical, and other physical responses as well as ionic transport.
Shafahi and Vafai^{(}^{17}^{)} used
a thermal finite element model of the eye to study its response to thermal disturbances.
Papaioannou and Samaras^{(}^{18}^{)}
used a rabbit eye model to simulate the temperature distribution and velocity field
generated under exposure to millimeter wave radiation. Jo and Aksan^{(}^{19}^{) }performed a simulation of conductive
keratoplasty, a thermal treatment for hyperopia and presbyopia, to predict thermal
damage to the corneal tissue. Mandel et al.^{(}^{20}^{)} used a three-dimensional cornea model exposed to an electrical
field to evaluate the electrical properties of the endothelial layer. Guimera et
al.^{(}^{21}^{)} also developed a
noninvasive FEM to predict the electrical properties of the endothelium and study
variations in epithelial permeability. Li and Pinsky^{(}^{22}^{) }created a multiphasic mathematical model describing
the transport of ionic solution and ionic species in human corneal tissue. We refer the
reader to those papers and references therein for further details on these topics of
study.

Section 2 discusses the modeling assumptions made in many finite element models of the cornea. The purpose of this section is to give modelers an understanding of the different assumptions that have been used, so that they may choose an appropriate model for their application of interest. Section 3 discusses different applications in corneal mechanics. Finite elements have been used to model the response to corneal surgery, disease, and treatments such as collagen cross-linking (CXL); trauma; and basic research applications. Section 4 summarizes the work and gives some suggestions on the future of such modeling in the practice of ophthalmology and eye research.

Modeling basics

The structural model of the cornea can be approached by creating a continuum model or
shell model. Continuum models explicitly model the geometry of the cornea in three
dimensions, although two-dimensional (2D) approximations are sometimes used.
Shell-based models use the theory of thin shell behavior to decrease the geometrical
complexity. The modeled geometry is a surface, with the thickness of the shell as a
defined parameter. Tensile and bending loads on the shell create deformations in the
shell in accordance with shell theory. While this approach decreases geometrical
complexity and increases efficiency of the formulation, it is difficult to extend to
complex constitutive models. Anderson et al.^{(}^{23}^{)} proposed a corneal model by using shell analysis
to develop a nonlinear finite element model to study the mechanical behavior of the
cornea. Howland et al.^{(}^{24}^{)}, Pinsky and Datye^{(}^{25}^{)}, Li and Tighe^{(}^{26}^{)}, and Elsheikh and Wang^{(}^{27}^{)} also based their models on this
method.

Geometry

The structure of the human cornea has a nonuniform curvature with variable thickness
throughout^{(}^{27}^{)}. It is
thinner at the center and thickens towards the edges. Dubbelman et
al.^{(}^{28}^{) }recorded the
corneal geometry for the internal and external surfaces of the human cornea using
Scheimpflug photography. Roy et al.^{(}^{14}^{)} used magnetic resonance imaging to determine the geometric
profile of the eye.

Some early models of the cornea, e.g.^{(}^{29}^{)}, use a 2D axisymmetric approximation of the cornea. This
models the geometry as a surface of revolution and is efficient compared with
three-dimensional models (3D). However, it cannot capture the subtle variations in
the geometry of the cornea from a surface of revolution, prevalent anisotropies in
the material, or any loads that occur off-center, such as an impact or incision.
Figure 1 illustrates common shell elements
and 3D elements used for FEA of the cornea.

Pandolfi and Holzapfel^{(}^{2}^{)} used the corneal geometry data published by Dubbelman et
al.^{(}^{28}^{)} to create
biconic interior and exterior surfaces for a 3D corneal model. Pandolfi and
Manganiello^{(}^{10}^{)} used
an ellipsoidal shape to define the anterior and posterior corneal surfaces. Nguyen et
al.^{(}^{30}^{)} and Nguyen
and Boyce^{(}^{31}^{)} also
approximated the corneal surface to be ellipsoidal according to digital image
correlation (DIC) measurements of the corneal surface profile.
Buzard^{(}^{32}^{)} and
Howland et al.^{(}^{24}^{)} used
axisymmetry to create their corneal models.

Salimi et al.^{(}^{33}^{)} meshed
the corneal model into triangular shell elements. For 3D models, hexahedral elements
are more commonly used^{(}^{2}^{,}^{6}^{)},
although tetrahedral elements may also be used. Linear tetrahedrals and standard
trilinear hexahedrals are known to perform poorly for nearly incompressible materials
such as soft tissues. Trilinear hexahedral elements can be modified using techniques
such as selective reduced integration^{(}^{34}^{)}; alternatively, higher order elements may be employed.
Figure 2 shows an example of an FE mesh of
the cornea.

Boundary conditions

The viscous fluid filled inside the anterior chamber of the eye exerts pressure known
as IOP on the cornea. Under physiological conditions, the IOP inflates the cornea and
gives it shape. A model developed for the cornea using the entire eyeball allows
appropriate realistic displacement at the limbus. Uchio et al.^{(}^{35}^{)} and Amini and
Barocas^{(}^{36}^{)} modeled
the entire eye globe for the analysis. However, the whole eye model may not be
efficient because it is highly time consuming and not economical for development and
analysis.

Alastrue et al.^{(}^{5}^{)
}analyzed the corneal model with the limbus constrained against displacement.
Anderson et al.^{(}^{23}^{)}
compared the deformation pattern of the entire eye model to the rigid cornea-limbus
boundary. They proposed an approximate boundary condition, with roller support at the
edges inclined at 40° with respect to the horizontal axis, to represent the
cornea-limbus behavior. The results obtained from their boundary assumption were
similar to the results obtained for the whole eye. Other studies^{(}^{11}^{,}^{37}^{,}^{38}^{)}
adopted similar boundary conditions for their corneal models. Roy et
al.^{(}^{14}^{)} created the
whole eye model and compared the displacement results with those for the corneal
model with fixed sclera to show that the corneoscleral limbus plays an important part
in predicting the response to refractive surgery. Pandolfi and
Holzapfel^{(}^{2}^{) }proposed
a corneal model allowing rotation at the limbus and restricting displacement at the
edges. Rotation at the limbus plays a role in changing the curvature of the cornea.
Figure 3 illustrates these boundary
conditions.

Figure 4 illustrates displacement results of FE simulation of a cornea subjected to IOP, with rotation at the limbus.

Material models

For creating a material model, the corneal material is often approximated as the
stroma, which is the major contributor to mechanical strength^{(}^{2}^{,}^{6}^{,}^{31}^{)}. The
stromal layer may be modeled as a nonlinear, anisotropic, viscoelastic material that
undergoes large deformation. Finite deformation theory is incorporated in the corneal
model, which successfully captures its true nonlinear response under
deformation^{(}^{2}^{,}^{31}^{)}.

Earlier finite element models were simple linear elastic models. One of the earlier
models developed by Vito et al.^{(}^{39}^{)} considered the stroma as a linear, elastic, homogeneous,
and isotropic material. Similarly, Bryant et al.^{(}^{40}^{)}, Hanna et al.^{(}^{41}^{)}, Gefen et al.^{(}^{12}^{)}, and Velinsky and Bryant^{(42) }also
assumed that the cornea would undergo a small deformation and used linear elasticity
for their analysis. Pinsky and Datye^{(}^{25}^{)} developed a linear material corneal model based on the
anisotropic constitutive model using the predominant fiber directions in the cornea.

The first geometrically nonlinear models represented the stroma as a nearly
incompressible, isotropic, hyperelastic material. The Neo-Hookean model is widely
used for modeling tissues^{(}^{2}^{,}^{6}^{)}. Other
hyperelastic models have also been used. Niroomandi et al.^{(}^{43}^{)} used a simpler Saint
Venant-Kirchhoff hyperelastic model. Bryant and McDonnell^{(}^{44}^{)} created a 2D axisymmetric model to
compare various isotropic and a couple of transversely isotropic constitutive models
of the cornea. Of the models they compared, an exponential nonlinear model best fit
the inflation experiments.

Many researchers used a base isotropic model for the matrix of the stroma and
additional terms for the effects of the fibers, as discussed below. Alastrue et
al.^{(}^{5}^{)}, Hanna et
al.^{(}^{41}^{)}, and Pandolfi
et al.^{(}^{13}^{)} considered
the matrix as a Mooney-Rivlin model. Nguyen et al.^{(}^{30}^{)} and Pinsky et al.^{(6) }used a Neo-Hookean
model as their matrix model.

Anisotropic nonlinear hyperelastic models with embedded collagen fibers have been
frequently used in recent corneal models^{(}^{2}^{,}^{5}^{,}^{6}^{,}^{31}^{)}. Alastrue et al.^{(}^{5}^{)} used a discrete fiber model with embedded collagen fibers in
two preferred orientations, the nasal-temporal and superior-inferior directions. Most
other recent models do not simulate each fiber explicitly, which can be
computationally expensive; however, they include a smeared effect of oriented fibers.
Cristobal et al.^{(}^{45}^{)} and
Pandolfi and Manganiello^{(}^{10}^{)} developed their models with two preferred directions at the
center and circumferential at the limbus region. Pandolfi and
Holzapfel^{(}^{2}^{)} used a
distributed model with two preferred orientations. Pinsky et al.^{(}^{6}^{) }and Nguyen et al.^{(}^{30}^{) }developed a continuous distributed
fiber model with preferred orientation.

Nguyen et al.^{(}^{30}^{)}
created an anisotropic corneal model, taking into consideration the viscoelastic
properties. Yoo et al.^{(}^{46}^{)} also used a viscoelastic ocular tissue model for their
study.

Parameter measurement and optimization

Corneal tissue material properties are determined by various laboratory tests of
corneal specimens. The material parameters used for the cornea model are based on
these studies. Wollensak et al.^{(}^{47}^{)}, Hoeltzel et al.^{(}^{29}^{)}, and Zeng et al.^{(}^{48}^{)} performed uniaxial tension tests on corneal tissue to
record stress-strain relation. Kohlhaas et al.^{(}^{49}^{)} and Wollensak et al.^{(}^{47}^{)} extended these tests to study the
effects of CXL on the mechanical response of the cornea. Bryant et
al.^{(}^{44}^{)}, Elsheikh et
al.^{(}^{50}^{)}, and Anderson
et al.^{(}^{23}^{)} conducted
inflation tests on the entire cornea to determine the apical displacement at
different pressure values. Petsche et al.^{(51) }examined the depth-dependent
material properties of the stroma. However, further experimental work to completely
determine the mechanical behavior of the cornea, particularly *in
vivo,* is required.

Inverse simulation is a mathematical approach to determine unknown model parameters to match observed or assumed physical responses. In inverse FEM, an optimization algorithm is coupled with an FEM to find a set of optimal parameters for a given model to be used in the FE simulation. Such methods are important for determining the physical properties of actual corneas.

Nguyen and Boyce^{(}^{31}^{)}
presented an inverse FEM to estimate bovine corneal material properties using an
*in vitro* inflation experiment and investigate the influence of
variations in these properties on the bovine inflation response. They determined a
set of anisotropic material properties, minimizing the error between simulation
results and experimental measurements of the surface displacement field.

Summary of finite element modeling choices

Different choices in finite element models of the cornea can change the results. 2D or shell models are efficient, but they may not be able to accurately capture the complex material behavior of some applications. While modeling the entire eye may avoid approximate boundary conditions at the limbus, the computational time increases significantly. However, modeling the changes in IOP during an impact may be difficult without a full model of the eye.

Material models are also increasingly accurate and, at the same time, computationally more expensive. While linear models are very efficient, they may be inaccurate over large changes in deformation. Isotropic models may not accurately capture the behavior of the complex material in the stroma, but they may be adequate for problems that only require a rough idea of the displacements. The depth of knowledge of the properties of corneal tissue is improving. In applications with rapid load, the viscoelastic and dynamic properties of the cornea may be important. For other analyses, such as those of the long-term results of surgery, a quasistatic model is acceptable. It is important to select a model that can simulate an application with the necessary accuracy. However, a model with too much detail may computationally become very expensive.

APPLICATIONS

FEA is a useful tool for studying corneal mechanical behavior and the mechanisms underlying its functions. FEA has been widely used for modeling surgical effects on the cornea and for studying corneal diseases and eye trauma. In this section, different applications of FEA in simulation of corneal surgery, disorder, and impact, among others, are discussed.

Surgery

The curvature of the internal and external surfaces of the cornea significantly
affect its refractive power. Refractive eye surgery can improve the visual acuity in
patients with common refractive maladies such as astigmatism, hyperopia, and myopia.
In several surgical techniques, the dioptric power of the cornea is improved by
removing thin layers of biological tissues and adjusting the curvature of the cornea
using excimer lasers^{(}^{13}^{)}. Laser-assisted in-situ keratomileusis (LASIK),
photorefractive keratectomy (PRK), and intrastromal photorefractive keratectomy
(ISPRK) are the 3 main types of refractive surgeries. Refractive surgeries on the
periphery of the cornea, such as conductive keratoplasty (CK) and intracorneal rings
(ICR), have recently attracted more interest. These surgeries are primarily performed
on hyperopic eyes that require steepening rather than flattening of the corneal
surface^{(}^{38}^{)}.

FEA has potential usefulness as a simulation tool for refractive surgery planning
using accurate *in vivo* corneal material properties and geometry
data^{(}^{14}^{)}. FEA has
been extensively used to model deformation of the cornea for corneal surgery
simulation and predict mechanical and refractive effects of surgery^{(}^{24}^{)}.

Pandolfi et al.^{(}^{13}^{)
}implemented an FEM in astigmatic and myopic corneas to evaluate the mechanical
and refractive outcomes of laser refractive surgery. They computed the change in the
curvature and thickness of the human cornea exposed to laser ablation and evaluated
the corrected dioptric power in individual patients.

Roy et al.^{(}^{14}^{) }proposed
an FE model of the whole eye to give insight into refractive surgery planning. They
later presented an inverse FEM to determine the undeformed state of the
cornea^{(}^{16}^{)}. They also
developed a 3D, patient-specific, corneoscleral FE model to estimate the surgical
impact on corneal shape variations and changes in corneal elastic properties.

Niroomandi et al.^{(}^{43}^{)
}presented a novel numerical technique for actual surgery simulation using an
extended FEM (X-FEM). The X-FEM enhances standard FE by defining additional degrees
of freedom, in this case, for fracture opening (those interested in further theory of
the X-FEM can refer to the work of Belytschko et al.^{(}^{52}^{)}, among others). In their model, fracture opening
induced by the incision placed during refractive surgery was reproduced using the
X-FEM method. Their presented FEM was able to simulate the complex constitutive model
of the cornea and reproduce the discontinuities of the scalpel incision for actual
surgery simulations.

Nomograms in ophthalmology are sets of tables of corrected values or graphics used to
plan the surgical procedure, particularly incisional surgeries for astigmatism and
similar diseases^{(}^{45}^{)}.
Nomograms can manage the incision effect by revising the cut parameters such as
depth, length, and optical zone in incisional surgery. Cristóbal et
al.^{(}^{45}^{) }proposed an
FE model of a limbal incision to predict patient-specific optical power that can be
induced during surgery. The nomogram outcomes were compared to simulation results,
and, in some cases, the nomograms were revised accordingly.

CK is a subablative thermal treatment wherein thin electrodes are inserted into the
stroma to apply radiofrequency (RF) heating. The heat causes the peripheral corneal
tissue to shrink. Jo et al.^{(}^{19}^{) }developed a 3D FE model, including the cornea, aqueous
humor, and RF electrodes, to investigate the resultant thermal damage during RF
heating in the CK procedure. Fraldi et al.^{(}^{38}^{) }also presented a viscoelastic FEA to study the mechanical
response of the hyperopic cornea to CK and assess the postsurgical stability of the
applied refractive correction. Hameed et al.^{(}^{53}^{)} created a 3D FE model to simulate the ISPRK
technique for correcting myopic corneas and predicting treatment outcomes. The
simulation outcomes were compared with the results from an earlier 2D FE model
developed by Bryant et al.^{(}^{40}^{)
}for the same purpose. The model outputs were more strongly correlated with the
clinical data when the 3D model was used than when the 2-D model was used.

Corneal diseases

Finite element analysis has been widely used to study biomechanical interactions in corneal maladies (e.g. keratoconus) and better understand the etiology and treatment of the diseases. FEM can be used to investigate the effects of variations in corneal material properties on vision power in patients with corneal diseases.

Keratoconus is a degenerative corneal disease characterized by irregular thinning and
bulging of the corneal structure and progressive topographic irregularities in the
cornea. This shape distortion leads to an optical aberration that can be corrected
with the use of hard contact lenses or glasses in mild cases or corneal
transplantation in more severe cases^{(}^{12}^{)}. CXL is a new method for treating keratoconus. In this
technique, the epithelium is usually removed from the central zone of the cornea, and
riboflavin is absorbed into the stroma^{(}^{16}^{)}. Under ultraviolet (UV) radiation, the riboflavin creates
additional cross-linking in the collagen fibrils, stiffening the cornea.

Gefen et al.^{(}^{12}^{)}
presented a 3D linear anisotropic FE model of normal and keratoconic corneas to
investigate the mechanical behavior and optical performance. The results of analysis
revealed that IOP had a considerable influence on the optical power of the
keratoconic cornea, while having little effect on the refractive power of the normal
cornea. Roy et al.^{(}^{16}^{)}
presented a 3D patient-specific FE model of the whole eye to study keratoconus
progression. The model was able to demonstrate a patient-specific procedure for
investigating the locally decreased corneal elastic properties during keratoconus
development.

Carvalho et al.^{(}^{11}^{)}
presented an FEM of a keratoconic cornea to predict the biomechanical behavior and
evolution of the cornea in keratoconus. Using shell elements, they investigated how
variations in IOP and material properties of the cornea (i.e., decrease in rigidity
of the cornea) can lead to a localized increase in corneal curvature. Foster et
al.^{(}^{54}^{) }examined the
stiffening effect of CXL in corneal tissue, fitting the material parameters to
experiments on corneal strips.

Impact and trauma

Ocular trauma can cause long-term vision disorders and can be expensive to
treat^{(}^{55}^{)}. FEM is a
useful tool for simulating ocular trauma. It may offer possible solutions for
decreasing eye impact injuries and designing protective tools against globe trauma.
The impact of different projectiles on the eye has been simulated using FEA to
investigate the injury potential.

Stitzel et al.^{(}^{55}^{)}
offered a nonlinear FEM of the human eye to predict eye rupture injuries caused by
high-speed blunt impact. The numerical model was verified with experiments to predict
the injury due to different types of impacting projectiles and loading conditions.

Weaver et al.^{(}^{56}^{)
}implemented a numerical model of eye impact that considered a variety of blunt
projectiles and loading conditions and was based on many experimental impact tests.
They investigated the effects of different projectile mass, size, material
properties, and velocity on the response of the eye to impact. The study results gave
insights for predicting eye rupture in various loading conditions and designing eye
safety equipment.

Gray et al.^{(}^{57}^{) }studied
the impact of paintballs on the cornea, with the aim of designing safer paintballs.
To this end, they implemented a numerical model of the human eye, orbit, and
paintball into the CTH computer code (a 3D, finite-volume, and large deformation
numerical hydrocode developed by Sandia National Laboratory) to study the physical
mechanisms causing impact eye injuries. By varying material properties, they
concluded that paintballs with less mass can be safer for the eye.

Uchio et al.^{(}^{35}^{) }also
presented a 3D FEA supercomputer simulation of traumatic impacts on the eyeball. They
investigated the threshold of impact eye injuries caused by bodies of different sizes
and velocities.

Other applications

FEM can be used as a potential noninvasive tool in clinical applications for investigating the biomechanical behavior of the cornea and obtaining characteristic measurements of the tissue. FEM may be a critical method for assessing corneal mechanical and electrical properties and for obtaining IOP measurements.

Applanation tonometry is among the most widely used contact tonometry techniques for
IOP measurement. FEM has been used to examine applanation tonometry for measuring
IOP^{(}^{33}^{)}. Ghaboussi et
al.^{(}^{58}^{)} developed a
computationally efficient numerical method to accurately measure IOP in the cornea
using a modified applanation tonometer. They used the combination of a neural network
and genetic algorithm to fit clinical applanation tonometry outcomes (force and
displacement history data) to the outcomes achieved from the FE model.

As the heart beats, IOP behind the cornea changes; the vibration characteristics of
the eye also vary with IOP. Salimi et al.^{(}^{33}^{)} developed a coupled fluid-structure FEM of the eye and
investigated the dynamic response of the eye to changing IOPs. They validated the
vibrational characteristics from the simulation model with experimental modal
analysis (EMA) of a water-filled spherical shell.

Rhee et al.^{(}^{59}^{)
}developed a nonlinear, anisotropic FE model of the mouse cornea to study the
formation of spiral patterns on the epithelium. They compared the obtained numerical
curves of maximum shear strain with actual spiral patterns observed on X-ray images
of mouse cornea. The presented work helped in providing insight into the possible
development of the cornea.

CONCLUSIONS

FEA is a powerful numerical technique for finding approximate solutions to partial differential equations in complex structures. FEA may be used to simulate a wide range of biological structures and body organs. In this work, the different applications of FEA in studying mechanical behavior of the cornea were reviewed. This review included information about a wide variety of applications, ranging from surgery to disease and impact simulation.

On the basis of the articles reviewed, FEA has been used to develop a better understanding of the mechanics of the cornea in a variety of settings. It can be a noninvasive, predictive method for assessing corneal function and properties and studying related diseases. The FE models can be used in clinical applications to investigate corneal malfunctions and present possible treatments. FEA can predict the pre- and postoperative responses of the cornea to refractive surgeries and may be used as a patient-specific simulation tool in clinical applications for surgical planning.

In the future, FE models may become important tools for planning treatments. Initially, the models will be used to improve procedural parameters such as incision locations and depths. However, it will soon be possible to provide patient-specific corneal models that can be used to plan individual surgeries and predict outcomes, such as optimal treatment times and concentrations for CXL.

Obtaining the parameters for the stiffness of individual corneas, particularly
*in vivo*, remains a challenge. A combination of investigations of
excised corneas and simple tests of *in vivo* corneas will provide
reasonable data to approximate patient-specific corneal properties. Ongoing
investigations on the complex mechanical properties of the cornea will continue to
create more realistic and accurate models for all these applications.