Numerical and Experimental Study of Collagen Crosslinking

Treatment for Keratoconus – FEA Study

2010 RET Final Report

Prepared by

Matthew RanftLyons Township High School, Lagrange IL

Laboratory for Product for Process DesignDirector: Professor Andreas Linninger

_____________________________ ___________________________Matthew Ranft Dr. Craig FosterRET Fellow RET Mentor

University of Illinois at Chicago, Chicago ILDate: 08/06/2010

Image Reconstruction of Brain Structures

Matthew RanftMath & Science Teacher

Lyons Township High School

Advisors: Dr. Craig Foster University of Illinois at Chicago

Director: Dr. Andreas Linninger University of Illinois at Chicago

Abstract

The purpose of this project is to seek out various Finite Element Analysis (FEA) software

programs that are capable of performing specific functions to assist in the overall analysis

of the human cornea. Better understanding of the structure of the human cornea will

hopefully lead to better understanding and treatment of eye defects such as the

degenerative eye disease Keratoconus. The human cornea is complex both in geometry

and material [1]. The proper FEA software will allow for complex analysis of cornea to

be performed that otherwise would be impossible. Requirements for an acceptable FEA

software for this project is that it has to be able to export information in text or excel

formats, mesh triangles quadrilaterals, tetrahedral and hexahedral, produce a

3dimensional mesh, can post process, user friendly and low cost. Of the FEA software I

looked into I found ANSYS and ADINA best meet these criteria.

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Table of Contents

Abstract................................................................................................................................2

Table of Contents.................................................................................................................3

Introduction..........................................................................................................................4

Material & Methods.............................................................................................................6

Results..................................................................................................................................7

Conclusion.........................................................................................................................10

Discussion and Future Results...........................................................................................11

Acknowledgements............................................................................................................12

References..........................................................................................................................13

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Introduction

Keratoconus is a degenerative eye disease that effects one in two thousand people

primarily between the ages of 16 to 30. The disease causes the cornea to lose its

spherical shape and bulge out to form a shape more similar to a cone (hence the name

Keratoconus, derived from the Greek word for cornea (‘kerato’) and cone shaped

(‘conus’)[2]. Figure 1 shows a drawing on a normal cornea compared to one

that has been affected with Keratoconus.

These irregularities of the cornea severely distorts vision.

The exact cause of Keratoconus is not known;

however it is believed to have some genetic ties. Several

treatment options do exist for Keratoconus. Contact

lenses are effective in the early stages; however as the disease progresses surgery is

usually required. With surgery, such as corneal transplants, the cornea does not always

respond well to incisions and it fails to stop the progressive loss of stiffness and strength

of the cornea. An alternative treatment is Collagen Crosslinking. This non invasive

procedure is relatively inexpensive. A riboflavin solution (vitamin B-12) is applied to the

stroma layer of the cornea. Once the riboflavin has been applied for the required time the

eye is then exposed to UVA radiation. This procedure allows for the collagen fibrils to

crosslink (bond) which restores strength and stiffness to cornea. While this treatment

shows much promise the results are inconsistent in the amount of reversal experienced by

patients. It is not known how variations in riboflavin concentration and/or UVA

exposure affect the outcomes of treatment. Therefore further research of this treatment is

required.

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A tool that can be used to further evaluate this treatment is Finite Element Analysis

(FEA). By using FEA software a computer generated model of the cornea can be

created. Because the cornea is mechanically and geometrically complex, stress and

deformation analysis become extremely difficult and time consuming. FEA will allow

for an approximate analysis to be performed fairly easily. Additionally the graphics

capability of FEA software will allow for a clear visual depiction of the cornea as well in

both normal and Keratoconic states.

FEA allows analysis of objects to be performed that would otherwise be impossible.

While the analysis of the object is an approximate solution, the accuracy is kept to within

a given tolerance. Additionally, FEA software allows for a more detailed analysis of the

object. Instead of having to analyze a complex geometry and or material as a whole,

FEA allows for the object to be broken down into much smaller pieces called elements.

These elements come in the shapes of triangles, quadrilaterals, tetrahedral and hexahedral

(Figure 2). Additionally these elements are comprised of nodes which are specific points

located on the element. Analysis of the object is performed at these node locations within

the object. The connecting of all the nodes and elements within an object is called a

mesh (Figure 3).

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Figure 2Example of elements used within FEA. The points located on the elements are called nodes.

Figure 3Example of a mesh applied over a simulated cornea shape using FEA software.

The goal of the research done during this study was to find an acceptable FEA software

to use for this project. The requirements for the software are that it must be able to:

Export information in text or excel. The desired information is nodal coordinates,

elements connectivity, element connectivity arrays, node sets, sides sets.

Can pre-process by producing duce a mesh using triangles, quadrilaterals,

tetrahedral, hexahedral.

Can post process – analyze data

User friendly

Low cost

Material & Methods

Analyzing FEA software essentially consisted of using a ClientPro 385 PC with

Windows XP operating system. Using this PC I was able to visit various FEA software

sites and request free software trials. Additionally a website

(http://www.andrew.cmu.edu/user/sowen/softsurv.html) created by Steve Owen PhD was

used. This website was a survey of approximately 80 different FEA software programs

available. While the survey is not a complete list, it did serve as a useful tool to focus

and narrow the search of FEA software on the market today. Finally, some other

popular FEA software programs were looked at as well because of their use at UIC

campus or known popularity in the market today

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Results

The following FEA software programs were evaluated using free trial versions of the

software as supplied by developer, ADINA, ABAQUS, Cubit and GID. Additionally,

ANSYS 11.0 was also evaluated as it is widely available in the UIC Engineering

Department. The following is a summary of the findings for each of the above listed

Software

.

ANSYS 11.0

Cost – Free. ANSYS 11.0 is made widely available within the UIC Engineering

Department.

Exportability – The Desired output data is easily available. Through the list

option on the top utility menu node connectivity and sets are available and

exportable to a txt file (lists -> nodes select coordinates only then select OK. This

will create a data table in a .lis format which needs to be converted .txt. In the

data file choose file -> saveas -> choose name. Then go to the newly saved file

right click -> rename -> choose name with .txt at the end of it) Additionally,

element connectivity is available through the same means (lists -> elements ->

nodes + attributes then covert to .txt as mentioned above). Additionally, a “pick”

options exists under the list option that allows using the mouse to click on the

geometry and select attributes to be displayed in tabular exportable form.

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Meshing – ANSYS has the ability to mesh various different elements such as

triangles, quadrilaterals, tetrahedral, and hexahedral (type solid186)

Post Processing – is easily performed in ANSYS although only simple

mathematical models were used. More complex or project specific codes was not

tested.

User Friendly – This is the area of ANSYS that was found to be of the lowest

quality. When compared with other FEA software the interface of actually

learning to draw three dimensional geometries was cumbersome and time

consuming. Creating a new model was not as intuitive and required much help

from the tutorial. Additionally, the controls to create and manipulate computer

generated models required a great deal of effort to learn and become familiar

with. Finally, ANSYS was found to “freeze” quite often and required the saving

of data to be performed regularly to avoid losing work.

Note: There is a new version ANSYS 12.0 available. This version was not evaluated although it does appear to be more user friendly.

ADINA

Cost - $1200.00

Exportability - Nodal coordinates and element connectivity are easily available

through Adina. Under the meshing option of the top utility menu tabular data is

easily generated and put into and exportable to a .txt file. Additionally nodes sets

and surface sets can be generated using the meshing option however this can only

be performed on surfaces that were created during the preprocessing phase.

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Meshing - ADINA has the ability to mesh various different elements such as

triangles, quadrilaterals, tetrahedral, and hexahedral.

Post Processing - is easily performed in ADINA although only simple

mathematical models were used. More complex or project specific codes were

not tested.

User Friendly – ADINA was found to be user friendly. Majority of operations

were windows based and creating/manipulating the actual geometry was simple.

Points within the geometry were defined, then using the line function to connect

points to create a surface and then rotating the surface about a desired axis to

CUBIT

Cost - $300.00. Free 30 day trial version was used for this analysis.

Exportability – Not able to fully evaluate this aspect. While it does appear that

node connectivity and element and sides sets can be produced, I was not able to

figure out how to get this data into an exportable format.

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Meshing – CUBIT has the ability to mesh various different elements such as

triangles, quadrilaterals, tetrahedral, and hexahedral

Post Processing - is easily performed in CUBIT although only simple

mathematical models were used. More complex or project specific codes were

not tested.

User Friendly – Aside from the exportability issue CUBIT was found to be

extremely user friendly. Creating the geometry was easy. This only required

making a simple sketch, putting the correct dimension on the sketch and the

software created an accurate shape. Additionally, in the tutorial, actual video are

supplied that show step by step how to pre and post process.

ABAQUS

Cost - $1000.00

Exportability – Nodal connectivity as well as elements sets are available through the query option. However I was not able to determine how to export this data during the duration of this project.

Meshing – ABAQUS has the ability to mesh various different elements such as

triangles, quadrilaterals, tetrahedral, and hexahedral

Post Processing - is easily performed in ABAQUS although only simple

mathematical models were used. More complex or project specific codes was not

tested.

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User Friendly – Through the use of the tutorial ABAQUS was found to be user

friendly. Creating a geometry was found to be fairly simple as well as creating

and analyzing a mesh. However the ease or capability of exporting data remains

unknown at this time.

GID

Although this software is windows based it was found to be the most difficult to

use software. The time required to learn to use this software was high and

frustration level of following tutorial made creating a quality working geometry

impractical

Conclusion

The use of FEA software to analyze the human cornea for this project is crucial. Based on

the analysis of various different FEA software, it appears that ANSYS and ADINA would

best serve this project. ANSYS is capable to perform all of the required parameters.

Although the user may have to spend time necessary to become more familiar with the

aspects of operating ANSYS once this time is invested ANSYS it appears to be very

capable. While ADINA does appear to be more user friendly and should require less time to

create and analyze models, the cost of $1200.00 needs to be considered.

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Discussion and Future Results

The overall objective of this project is to use available tools to gain a better

understanding of the complicated human cornea. Hopefully by gaining this better

understanding improved treatments for disorders such as Keratoconus will result. Once

an acceptable FEA software is chosen for this project the next step will be to develop and

incorporate FEA software a complex mathematical equation that models the cornea’s

behavior in both the normal and Keratoconic states. Eventually the developed model will

need to be compared to actual human corneas obtained from the Illinois Eye Bank.

Variations in Collagen Crosslinking procedure will be performed on the human corneas

and results will be compared to the mathematical/FEA model.

Ideally what may be developed is an eye model that is patient specific. A patient specific

model may prove to be useful in developing individualized treatment of patients on a case

by case basis.

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Acknowledgements

Financial support by NSF RET Grant – EEC 0743068 (Andreas Linninger, PI) is

gratefully acknowledged

Andreas Linninger

Dr. Craig Foster

Dipika Gongal

Brain J. Sweetman

Sukhraaj S. Basati

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References

1. Dr. Craig Foster, Numerical and Experimental Study of Collagen Treatment for Keratoconus – Research Proposal, 2010

2. Yaron S. Rabinowitz,M.D., http://keratoconus.com/Keratocus.com, accessed August 5, 2010

3. http://en.wikipedia.org/wiki/Keratoconus , accessed August 5 2010.

4. Owen, Steven J., "A Survey of Unstructured Mesh Generation Technology", Proceedings 7th International Meshing Roundtable, Dearborn, MI, October 1998

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