1. Master Thesis presentation BioMechanical Engineering
By: Joris Moerkerken Supervisors: Prof. dr. ir. Fred van Keulen Prof. dr. Huib Simonsz Dr. ir. Matthijs Langelaar
Welcome to my thesis presentation. I performed a thesis study for my master in biomechanical engineering and I will present my findings for you today. In Biomechanics we investigate the mechanical behavior of biological systems. For example, the mechanics of a knee or a shoulder.
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26/12/2018
By Joris Moerkerken

2. This movie shows the rotation of the human eye
This movie shows the rotation of the human eye. The eye is suspended in the orbit (oogkas). Unlike other body parts were bones, muscles and cartilage regulate motion, the eye is suspended on fat. Point: muscles, nerve, eye, orbit and fat. The soft tissue interaction in the orbit is able to move the eye very accurately up to 900°/second. My study was devoted to the mechanical behavior of this complex system.
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By Joris Moerkerken

3. Organization Background Problem statement Methods Results
A three dimensional model
A two dimensional model approach
Results
Critical areas
Conclusions
This presentation will take approximately one half an hour. First i will provide background information on the motivation of this research. I will define my problem statemenent. In the methods i will talk about how i started to solve the problem. Then i will tell how the first approached failed and that a different was needed. Then i will talk about the results of this new approach, point out the critical areas of the problem. Finally i will end this presentation with the conclusions.
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By Joris Moerkerken

4. Background
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5. Background
As you might have seen in the movie, the eye slides on the fat. This sliding is possible because of a fluid interface between the fat and the sclera, the hard outer shell of the eye.
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By Joris Moerkerken

6. Background Why model these structures?
Insight in functionality of anatomical structures.
Mechanical properties of these structures could help in surgical interventions.
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7. Background
Mechanical models of the human eye were created in the past.
This was always done in systems with 3 degrees of freedom.
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8. Background In reality eye rotation is not about a single point.
The eye can translate and rotate on the fat, this gives the eye 6 degrees of freedom.
Modeling such a complex mechanical structure can be done with the finite element (FE) method.
FE is used to predict deformation and stresses in complex structures in many fields.
FE is used in the car industry, in the modeling of ships, planes. And more recently it also used in biomechanics, to model for example bone mechanics.
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9. Background How to built an FE model in 5 steps:
1: Obtain structure surface
2: Subdivide surface into cubes or elements
The whole system of elements is called: mesh
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10. Background 3: Each element needs material properties
4: The whole system of elements needs boundary conditions (BC’s).
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11. Background 5: Apply conditions for a simulation 26/12/2018
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12. Background This was first done in 2001 by van den Bedem and Schutte.
limited rotation (15°) real eye can go up to 50°.
Later attempts were done to improve the model:
Beerepoot et al. 2006, muscle model.
Goudsmit et al. 2009, detailed anatomical structures.
No improvement on rotation performance, still maximum 15°. It was not clear what caused this limited rotation.
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By Joris Moerkerken

13. Summary so far Eye is suspended on the orbital fat.
Modeling the mechanics of the eye in the orbit shows functionality of anatomical structures.
A mechanical model of the orbit was made in 2001 with the FE method.
This model and later models did allow for maximum 15° rotation of the eye.
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By Joris Moerkerken

14. Problem statement
The goal of this study is to identify and analyze the problems in the finite element model of orbital mechanics in order to be able to simulate full eye rotations.
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By Joris Moerkerken

15. Methods Find causes of the limited rotation in the 3D model.
How to do this in a 3D model?
Later in this presentation it will become clear why this model is so complex.
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By Joris Moerkerken

16. Problem: what is happening?
I can just click now and show you the 3D model, however keep in mind that this model took former researchers about one year to create. I was able to recreate it with their help and was then faced with the task to make it work. You can see that the eye rotates (point) because the muscle contracts (point).
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26/12/2018
By Joris Moerkerken

17. Problem: what is happening?
What stops the simulation?
In FE a simulation conists of several calculation steps.
A solution is approximated, no exact solution exists
If a simulation step convergece, the simulation can proceed with the next step.
If a step does not converge, the simulation is stopped.
In this model the simulation stopped because convergence could not be reached.
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By Joris Moerkerken

18. A new modeling approach
Three dimensional analysis is extremely time consuming
A lack of insight and overview of the model leads to an uncontrollable situation
A controllable situation is needed to solve the problems
Back to basic! Comparable with air crash investigation.
After a struggle of a few months, you can imagine that this situation is very depresing ;)
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By Joris Moerkerken

19. Methods One way to simplify the situation is to go back to 2D.
Faster simulations
One plane to analyse instead of 3D environment
A more insightful situation is formed.
Plak uit het oog. Twee spieren voor de rotatie, de zenuw en het vet.
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By Joris Moerkerken

20. How to built a 2D model The 5 steps: 1: Obtain structure surface
MRI slice
Overlay curves on the surfaces
Leg mri plak onder
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21. How to built a 2D model 2: Create mesh 3: Assign material properties
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22. How to built a 2D model 4: Apply boundary conditions
Orbital wall, muscle back ends and the optic nerve back end are fixed.
Important determinant in model: contact between structures.
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23. How to built a 2D model Sliding between structures Fat and eye
Fat and orbital wall
Fat and muscles
Fat and optic nerve
Muscles and eye
Muscles and orbital wall
We can model this with a contact algorithm which I will explain in a later slide
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24. How to built a 2D model 5: simulate eye rotation Contract one muscle
Relax the antagonist muscle
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25. Recap The 3D FE model was unsuited to identify problems
A 2D FE approach was thought to create a controllable base to find problems
A 2D FE model was built
A rotation can now be simulated
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26. 2D simulations Rule out causes of problems:
First rotate the eye about a fixed point excluding the fat
Add structures to see where problems start to occur
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27. 2D model: Muscle contraction
Simulate eye rotation about a fixed point.
No problems were found in this simulation.
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28. 2D model: eye rotation with two muscles
Simulate eye rotation about a fixed point.
No problems here either.
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29. 2D model: eye rotation on fat
Simulate eye rotation on the fat.
To rule out problems with the fat.
A finer mesh was chosen, adequate for this problem?
Rotation on the fat seems problematic.
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30. 2D model: eye rotation on fat including the optic nerve
Problems were observed.
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31. Problems in modeling the fat
Fat is almost incompressible
Fat is highly inhomogeneous
Mechanical behavior is different in each region
80% water
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32. Problems in modeling the fat
Incompressible material:
No change in volume (isovolumetric) when compressed
Results in overly stiff response of the element
Results in too small displacement
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33. Results: four main problems
Distorted mesh
Contact between structures is not simulated correctly
Penetration of elements
Loss of contact
How to choose a correct mesh size
Incompressible behavior of elements
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34. Results 2D model
These problems cannot be solved one at the time, they relate to each other.
This 2D model shows problems, the causes of these problems are still not found.
For example: the contact algorithm that simulates the contact between structures uses information on the mesh size.
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35. Results 2D model
Now it is time to look at the areas where the problems occur in order to find causes and possible solutions.
Three critical areas
Simulate focussed subtasks
Even uitzoomen, dus eerst 3d toen 2d en nu inzoomen op de 2d gebieden
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By Joris Moerkerken

36. Critical areas 1: Muscle attachment on the eye
2: Optic nerve attachment on the eye
3: Fat directly behind the eye
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37. Critical area (2) Optic nerve attachment on the eye
What happens in this area?
Fat flows around the optic nerve as the eye rotates (Schaafsma 2010).
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By Joris Moerkerken

38. Critical area (2) Optic nerve attachment on the eye
Is this flow behavior observed in the model?
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By Joris Moerkerken

39. Critical area (2) Optic nerve attachment on the eye
Flow behavior not observed in the simulation.
We do observe:
1: Highly distorted mesh: large deformations.
2: Penetration of elements occurs.
3: Bad incompressible behavior.
4: A gap is formed: lack of pressure gradients.
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40. Critical area (2) Optic nerve attachment on the eye
1: Highly distorted mesh, what happens here? Aspect ratio is negative, the simulation does not converge
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41. Critical area (2) Optic nerve attachment on the eye
1: Highly distorted mesh
How to prevent bad aspect ratios?
Remesh algorithm is employed:
Creates a new mesh in the calculation step where a distorted mesh is formed.
Remesh criteria:
Based upon distorted mesh
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By Joris Moerkerken

42. Critical area (2) Optic nerve attachment on the eye
2: Penetration problems, what happens here?
Contact algorithm
Searches for contact in an area around the elements.
Creates ties between nodes.
Looses contact if the seperation treshold is exceeded (e.g. positive reaction force)
Show contact movie!
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By Joris Moerkerken

43. Critical area (2) Optic nerve attachment on the eye
2: Penetration problems
Sensible for stepsize!
Nodes are searched at a certain distance of the element side.
This distance is smaller than 5% of the smallest element side.
If an element has a larger displacement in one step compared to this distance, no contact is detected: penetration occurs.
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By Joris Moerkerken

44. Results 2D model
3: Bad incompressible behavior: It was found that a specialized group of elements called “Herrmann elements” can cope with incompressible material behavior.
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45. Results 2D model 4: A gap is formed No adequate solution was found.
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By Joris Moerkerken

46. Critical area (2) Optic nerve attachment on the eye
Proposed solutions:
Contact problem:
Adaptive stepsize: link stepsize to deformation fields, this prevents penetration
Distorted mesh problem:
Remesh the original mesh when the mesh gets distorted, this prevents distortion
Herrmann elements for incompressible behavior
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By Joris Moerkerken

47. Critical area (2) Optic nerve attachment on the eye
Performance improved.
No mesh distortion
No penetration
Still a gap...
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By Joris Moerkerken

48. Back to our original 2D model
Incorperate solutions that were found in the analyses of the critical areas shows:
Improved simulation:
No distorted mesh
No penetration
Allow for rotations up to 30°
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By Joris Moerkerken

49. Conclusions 3D model unsuited to identify problems
Problems could be found in the 2D model
Split up this complex modeling task in more focused subtasks shows causes of problems
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By Joris Moerkerken

50. Conclusions Better performance of the model is expected if
Adaptive stepping procedure is used
Remeshing of distorted mesh is used
Herrmann elements for incompressible behavior are used
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By Joris Moerkerken

51. Conclusions
There is however still the problem of lack of pressure gradient. The problem of gaps could not be solved adequately. FE simulations are specially created to model solid structures. Fat seems to behave more like a fluid.
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By Joris Moerkerken

52. Conclusions
The elements representing the orbital fat impeed the large deformations occuring in the orbit and leave gaps in the mesh: This raises the question whether the orbital fat could be modeled properly with FE.
26/12/2018
By Joris Moerkerken