1 Department of Mechanical Engineering Develop an FEA model of a scoliosis child’s spine Student Name: Xiyuan Wang Student No: X00107250 Date of Submission:16/12/2015 Supervisor: Tony Tansey Declaration of Originality I hereby certify, that this report, submitted as a part of the BEng (Hons) Mechanical Engineering, is entirely the work of the author and that any resources used for the Completion of the work done are fully referenced within the text of this report. Signature: XIYUAN WANG Date: 15/12/2015
Transcript
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Department of Mechanical Engineering
Develop an FEA model of a scoliosis child’s spine
Student Name: Xiyuan Wang
Student No: X00107250
Date of Submission:16/12/2015
Supervisor: Tony Tansey
Declaration of Originality
I hereby certify, that this report, submitted as a part of the BEng (Hons) Mechanical Engineering, is entirely the work of the author and that any resources used for the Completion of the work done are fully referenced within the text of this report.Signature: XIYUAN WANG Date: 15/12/2015
1.1 Introduction...................................................................................................................41.2 Detailed Aims & Objectives.........................................................................................41.3 Objectives for Literature Review..................................................................................5
1.3.1 List of Questions...............................................................................................6
1.4 Sources of Information.................................................................................................61.5 Methodology.................................................................................................................61.6 Analysis........................................................................................................................7
Chapter2 Literature Review..........................................................................................................82.1 Introduction of this chapter...........................................................................................82.2 Introduction of spine.....................................................................................................82.3 The Basis, Characteristic and Range of Spine............................................................10
2.3.1 The Motion Segment of Spine........................................................................10
2.3.2 The Characteristic of Movement in Spine.......................................................10
2.3.3 The Range of Movement in Spine...................................................................12
2.4 Introduction of Vertebrae............................................................................................122.5 Introduction of a Scoliosis Spine................................................................................12
2.5.1 The Definition of Scoliosis.............................................................................13
2.5.2 The Pathogeny of Scoliosis.............................................................................14
2.6 Using computer on Scoliosis.......................................................................................152.6.1 The difficulties of design FE model................................................................15
2.7 The Instability of the Spine.........................................................................................152.7.1 The function of Muscle...................................................................................16
2.7.2 The movement of trunk and the stability of vertebrae.....................................16
2.8 C.T. Scan.....................................................................................................................172.9 STL File......................................................................................................................182.10 The method of using Mimics......................................................................................192.11 The introduction IA-FEmesh......................................................................................20
3.3.3 Create Model by Segmentation.......................................................................29
3.4 Summary of using Mimics..........................................................................................29Chapter4 Finite Element Mesh Creation....................................................................................30
4.2.5 Assigning User Defined Material Properties: 2 Material................................34
4.3 The Unfinished Step in Using IA-FEmesh.................................................................35Chapter5 Conclusion..................................................................................................................36Chapter6 Reference....................................................................................................................36
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Chapter1 Introduction/Aims& Objectives
1.1 Introduction
Scoliosis, curvature of the spine, develops during growth spurts and affects many
adolescents. The present study combines clinical and engineering know-how
confirming that spinal growth can be mechanically modified. Surgeons achieve an
intra-operative reduction in the deformity by applying compressive forces across the
inter-vertebral disc spaces while they secure the rods to vertebra.
This project is to use software package to simulate the spine and its components.
However, this method has some limitation, because the selection of optimum outcome
is limited. The advantage of this method is multiple tests can be investigated on the
solid model and do not need to care whether the experiment would affect the patient.
Using computer to test human spines also rise the selection of different kind of
treatment and methodologies, this advantage can be employed in order to different
biomechanically treatments.
Due to the requirement and limitation of this project, this project could use the CT
scan images to create solid model and load the solid model into ANSYS to analysis.
Creating an analysis mesh to simulate is very complex and need a long time to finish.
However, using FEA model to investigate is the best method to address spinal
problems.
To research method to create a solid model and a finite element analysis mesh from a
CT scan image is one of the aims in this project. Another aim is use different kinds of
load to the model by using ANSYS, and collect the data from this experiment which
can be used to compare and modify the material properties.
1.2 Detailed Aims & Objectives
Firstly, understand the key concepts about three important aspects of the project
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contains the information of developing STL file from CT scan images, creating
meshed model to simplify the ANSYS process and analyzing the solid model by using
FEA model.
These are the detailed aims and objectives to be focused on:
Research the structure of human spine, disc and vertebrae.
Research the basis ,characteristic and range of the spine
Research the scoliosis spine (e.g.: the definition and pathogeny of scoliosis)
Research instability of spine (e.g.: the function of muscle, the movement of trunk
and the stability of vertebrae)
Research the method of using Mimics, IA-FEmesh and ANSYS software
packages.
Import DICOM data from a child’s scoliosis spine.
Load the CT data into Mimics to create solid model.
Load the STL file into IA-FEmesh and create a meshed model containing the
images of different components.
Import meshed model into ANSYS to analysis.
Simulate using ANSYS software package to compare different kinds of load to
the meshed model.
1.3 Objectives for Literature Review
1.3.1 Introduction
The literature review can help to make a theoretical framework. The professional
information like formula, key terms, definitions and terminology can be found in the
books, journals and websites. It can necessary find different parts to complete the
project are carried out, so it should obtained on a sufficient amount understanding
about the project. There are some questions which need to be asked about previous
work which has been done in the area. So doing literature review can be a great help
to improve the understanding of the whole process of project. Also it can accelerate
the speed of doing the project later.
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1.3.1 List of Questions
What is structure of the spine, disc and vertebrae?
What the definition of basis, characteristic and range of spine?
What is the definition of scoliosis spine?
What is the pathogeny of scoliosis?
What is the function of muscle?
What methods are be used on Mimics, IA-FEmesh and ANSYS?
What methods are currently used to convert CT scan image to STL file?
What is FEA model matching and how does it contain theoretical?
How to make load on the model in ANSYS?
How to compare different kinds of load in the experiment?
1.4 Sources of Information
Books from supervisor---Reliable.
Books from the school library or city library---Reliable.
PDF files of relevant project information in Science Direct or school library---
Reliable.
Internet searches like Google, Wikipedia---Not very reliable, someone could
change the content.
Conversations with supervisor to get guide or examples---Reliable
Tutorial videos for software packages from “YouTube”---Reliable
1.5 Methodology
In order to complete the project successfully, a completed plan and particular
methodology guide are very necessary.
In this project, there are a lot of process related to medical knowledge. From this,
a particular literature review can prove the project to be completed successfully.
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This project need to use three different software, they are Mimics, IA-FEmesh
and ANSYS. In order to carry out the project, getting familiar with the software is
very important.
The original data is CT scan. To convert the DICOM data into STL file, it need to
use Mimics software package.
A finite element analysis mesh was create by IA-FEmesh software package.
ANSYS software package is used to do analysis on the meshed model. Using
different kinds of load to test the model.
The data is produced from the simulated experiment by using ANSYS software
packages. The data were plotted on graph by using EXCEL.
1.6 Analysis
It is necessary to determine a STL file from the CT data. And use the STL file to make
a meshed model by IA-FEmesh. After that it will analysis the model by ANSYS
package. Then it would begin the endeavor to assign a range of load to simulate
variation of bone. Developing load sets for the spine. Demonstrate ability to simulate
loads and to validate results by alternate calculation methods.
It is necessary to use the (1) Solid model (2) Meshed model (3) FEA model
It is necessary to make conclusion by analysis the data from the FEA model. It can be
helpful to make simulate loads and to validate results by alternate calculation
methods.
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Chapter2 Literature Review
2.1 Introduction of this chapter
The aim of this chapter is to introduce a scoliosis child’s spine, highlighting the most
curved part of a whole spine, in order to investigate the vertebrae in the region of
most deformation. This chapter also introduce the software that could be used in this
project. The process of cornering a CT scan into a 3D model needed to use a
simulation package, and there are also some other processes to concentrate on
developing a simulation model to replicate a child’s spine. As is the aim of project,
after creating a FEA model, the next step is to calculate the density of the bone and
material properties.
2.2 Introduction of spine
In the human body, maybe the spine is one of the most important and obligatory part.
The spine is fixed by ligaments and small spinal joint. The spine has an essential role
in human body which can allow a lot of motional activities. Through the spine, the
human body can keep a balance from these activities, and have a stable and protected
condition.
A whole spine is made up by a lot of parts which is illustrated in (Figure 2-1). There
are usually five big regions, and a whole spine has thirty three bones from cervical to
pelvis. On the top of spine called Cervical, it has seven vertebrae, and the vertebrae in
this part is very compact. The part under Cervical called Thoracic, it has twelve
vertebrae. The thoracic occupies the biggest area in the whole spine, around one third
of the whole spine. The middle of spine is called Lumbar spine, it has five vertebrae.
The diameter of vertebrae in this part increases gradually from top to the end. The
other two regions do not have any vertebrae, they both look like a triangle. They
called Sacrum and Coccyx. In each region the vertebrae are similar to each other,
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however, they can be found change considerably from region to region.
A very essential characteristic of spine is flexibility. Due to this characteristic, it
arrange to separate the vertebrae and the inter-vertebral discs. Annulus fibrosis (tough
core) and nucleus pulposus (soft core) make up the inter-vertebral discs. When people
do some sport, the spine could have some little change. Then the inter-vertebral could
do the job of absorbing and decreasing the shock to prevent hurting spine.
The central nervous system is almost the most important thing to a human being and
is located in the spine. So the spine has another important duty which is to protect the
central nervous system.
To ensure stability and rigidity of spine and vertebrae, ligaments can help to connect
vertebrae and spine. The ligament is usually attached to the surface of bone or merge
with the outer layer of joint capsule.
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Figure 2-1a complete spine with two views (1)
2.3 The Basis, Characteristic and Range of Spine
2.3.1 The Motion Segment of Spine
Motion segments is the fundamental structural and functional unit. This project is use
a mechanical model of lumbar motion segment, and this model is developed with 3D
finite element method.
Motion segments consist of two vertebrae which are connected and some soft tissue
between these two vertebrae. This is also the smallest element to explain
biomechanical characteristics.
Motion segments have two parts, they are the forward and rearward components. The
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forward part has two vertebrae, disc, posterior longitudinal ligament and anterior
longitudinal ligament. The rearward part has vertebral arch, zygapophyseal joint,
transverse process, spinous process and the ligament between these processes.
A comprehensive suite of these motion segments can make a six degree of freedom
movement in three dimensions. Three dimensions mean three axis of motion, it
include coronal axis, sagittal axis and vertical axis. Six degree of freedom movement
means three angular translations and three linear translations.
Three linear translation include the lateral translation within the coronal axis, forward-
rearward translations within sagittal axis and compression-tension translation within
vertical axis.
The angular translation includes flex translation, extension translation, and orbital
translation.
2.3.2 The Characteristic of Movement in Spine
The spine has a translation motion across an axis, and also do some rotational motion
in the same axis.
The direction of the cervical vertebrae approach the horizontal direction, so the
cervical vertebrae can do flex movement, extension and lateral movement easily.
The articular surface of thoracic vertebrae has a shape like frontal plane. This part of
the spine could also be affected by the thorax, so it could have a limitation to its
movement.
The articular surface of the lumbar vertebrae has a 45 degree angle with coronal
plane. The articular surface is vertical to the cross section in lumbar vertebrae. The
range of flex movement could grow bigger from the top to down. Unfortunately, the
extension could be limited by the vertebrae.
In one word, the movement of spine can be called conjugate phenomenon. The
movement includes Forward Bending (flection), Backward Bending (extension),
Lateral Bending right left and Rotation right left. They are illustrated in Figure 2-2.
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Figure 2-2 the motion segment of spine (2)
2.3.3 The Range of Movement in Spine
The range and subjective nature of movement of every component in the spine is
depend on the thickness of the vertebrae and the shape of the vertebrae. The tightness
of the vertebrae and the muscle around the spine can also affect the movement in the
spine.
To sum up: if the thickness of vertebrae is large, the performance of movement can be
enhanced easily.
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2.4 Introduction of Vertebrae
Vertebrae in Cervical Thoracic and Lumbar are similar which in these regions
vertebrae can move independently. However, the vertebrae located in Sacrum and
Coccyx cannot do like the other three kinds.
Usually the vertebrae can be divided into two parts: vertebral body or block and ring
or vertebral arch. The vertebral body is packaged by a thin cortical which can help to
protect the body. The inside of body is cancellous bone which has different density in
different region. Most of vertebral body look like a half circle. The Pedicle is used to
connect the body and the arch. Between the two Pedicles there is a void called the
vertebral foramen. The whole human vertebrae is illustrated in Figure 2-3.
There are five bulges in a vertebral arch, they are two Transverse processes, two
Mammillary processes and one Spinous process. The Spinous process located in the
Lumbar and Cervical regions is also called neural spine, because they can be felt
through the skin.
2.5 Introduction of a Scoliosis Spine
The aim of this project is to research the adolescent idiopathic scoliosis which has a
3D deformity in the spinal column. Scoliosis is a common illness in teenagers, and
this project use a fifteen years old teenager’s spine to do research.
There are limited ways to treat this sickness. For example, the surgeons could use
screws to attach a rod into the curving part of the spine, most time is used on the
vertebral bodies. Unfortunately, to face some severe deformity spine, the surgeons
need to get a correct spinal curvature then to start a surgical treatment. The stiffness of
the spine is the precondition of correct spinal curvature.
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Figure 2-3 a whole human vertebrae with top view (3)
Another thing to be noted is if the treatment is applied on teenagers, the spinal
stiffness could change with the growing age. Then the degree of spinal curvature
would be changed with this stiffness.
On the other hand, the vertebral bodies and discs can be implemented with height
growth. And many other data of the curving spine could be changed or grow up, for
example, the facets, ligaments could be changed and could be hard to model.
2.5.1 The Definition of Scoliosis
A part of the spine has some degree of deviation from the axis line of human body
lastingly. And the scoliosis part always has a shape like ‘S’.
Generally, the scoliosis together with the rotation of spine. The number of protrusion
and posterior process on the vertical plane could also be increased or reduced. The rib
could also be affected by scoliosis, and cause a movement from left to right which is
not in level of the rib.
Generally, scoliosis happens in cervical vertebra, thoracic vertebra and the spine
between the cervical vertebra and lumbar vertebra. However, scoliosis can also
happen in lumbar vertebra individually. In this project, scoliosis in lumbar vertebra
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could be investigated. The image of scoliosis is illustrated in Figure 2-4.
2.5.2 The Pathogeny of Scoliosis
Malnutrition: The lack of vitamin D could cause rickets. Due to this disease,
adolescents can have scoliosis.
Spinal injury: It could cause the mechanical forces been changed in spine.
Poor posture of spine: The wrong posture of standing, sitting and sleeping could
cause scoliosis and distortion in pelvic.
Genetic factors
Lack of physical exercise: The strength of the muscle and ligament which around
the spine is very poor, this issue could cause an instability spine.
Nervous system disease: Poliomyelitis, the disease of neurofibroma and
syringomyelia could cause the mechanical forces been changed in spine. The age
of patient is younger and the condition of scoliosis could be worst.
Figure 2-4 the back view of scoliosis (1)
2.6 Using computer on Scoliosis
In nowadays, Surgeons can use computer modeling and simulated software to do
some bio-mechanical research on spinal curving deformity. In general process is use
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modeling software to create a 3D model, then put the model into finite element
analysis software to do investigation.
To understand the bio-mechanical of curving spine, finite element analysis is very
common software to use. The software is also can be used to design and related tools.
2.6.1 The difficulties of design FE model
The spine is complex to design, the spine deformation with FEA is hard to use.
The function of vertebrae and inter-vertebrae discs has non linearity.
The model could has a long time to analysis kinematic problem.
2.7 The Instability of the Spine
In general, if two vertebrae and a disc have an abnormal movement, this movement is
called spinal instability. If a disc cannot generate, the disc could lose its tension. Due
to this issue, the two vertebrae between the disc which can make extrusion with each
other and the disc cannot achieve in order inside the vertebrae. According to the disc
losing tension, the disc could also be reduced its height. If the height been smaller, the
facet joints could displace and cover their original location and also can cause
generate bone spurs and deteriorate the spine.
The strength of spinal stability provide by bone and ligaments is very small, only take
about twenty percent. Most of the strength is very provided by the muscle which
attached around the spine.
Every components around the spine has a close relation with each other. For example,
if a disc in spine which is injured, the muscle which attach around the spine could be
inhibited quickly. Due to this issue, this disease can decrease in a large amount of
muscle, and it could cause the spine without any strength to support its balance then
the spine could become instability.
The picture below display two vertebrae of an instability spine. It can be found both
of vertebrae has a different direction, also the shape and location of the disc has been
changed either. The image is illustrated in Figure 2-5.
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Figure 2-5 an instability spine (4)
2.7.1 The function of Muscle
Muscle has a great impact on spinal loads, and also affect the stability of spine. In this
project, the duty is to do some mechanical studies and biomechanical studies. Due to
this condition, investigate the load of spine has an essential factor.
2.7.2 The movement of trunk and the stability of vertebrae
The main function of transversospinales and erector spinae is to keep stable when the
spine has a backward extension. In the closed-chain kinetic exercise, Psoas major
muscle is the main phasic muscle. Phasic muscle has an essential function which is to
immobilize the spine, pelvic and thoracic. Phasic muscle also can keep stable of the
muscle of shoulder, neck and marrow when the human do some sports.
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2.8 C.T. Scan
Surgeons use an X-ray to take images from human’s body, this can help to investigate
bone and soft tissue. This medical process called computerized tomography (C.T).
The images which product by C.T. can use on medical and mechanical areas. People
can use computer software to analysis and create model the complex construction.
In this project is to use a C.T Scan from a fifteen years old teenager. Next step is to
use a software to create a STL file. The image of CT scan is illustrated in Figure 2-6.
Figure 2-6 Screenshot from Mimics illustrating of three views of CT scan
2.9 STL File
This file format is created by 3D systems, however, STL file can be supported by
many software. For example, the file can be used to rapid prototyping and computer-
aided manufacturing in many areas.
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STL file is a format which can only describe surface of a 3D object. The surface is full
of a row unstructured triangle in gray color and the density of the model can be
changed by changing its max size.
The coordinate in this format must be positive number, and the units in this format are
optional. The image of STL file is illustrated in Figure 2-7.
Figure 2-7 STL file from IA-FEMmesh
2.10 The method of using Mimics
The aim of this project is to investigate the most curve part of a scoliotic spine, so the
first step is to use Mimics to create a useful model from a CT scan image. According
to this aim and this part is to introduce how to use Mimics.
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Mimics is a software which can help to create a model for medical segmentation
processing and 3D model creation. It also has a function to turn the 2D file into 3D
file. The screenshot of using Mimics was illustrated in Figure 2-8.
Figure 2-8 Screenshot from Mimics illustrating of three views in mimics
This graph illustrate a process of putting CT scan file into Mimics. There are three
views: side view, front view and top view. There also a 3D model located in the lower
right corner.
Figure 2-9 Screenshot from Mimics illustrating of the main toolbar in mimics
This figure is the main toolbar (Figure 2-9) for segmentation the image. The image of
one pencil is a tool to select image which is useful or useless. Using mouse to click
this image, it could show erase and paint, and these two thing are the main solution to
choose surface.
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The second icon from left to right is a tool to create a new mask, then the next step is
to calculate a 3D model.
This figure is a process which illustrate how turn the model into STL file. First, it
need to select the mask which is useful, and add this mask into empty frame below.
After these two steps, the STL file could be proved.
2.11 The introduction IA-FEmesh
IA-FEmesh has a good performance to do a multi block mesh, and help to create mesh
at hexahedral mesh generation. In this project, this software is to turn STL file into
mesh file, then put into ANSYS. According to this condition, IA-FEmesh provide an
efficient and reliable method to develop the model, and evaluate the mesh quality.
2.11.1 The step of using IA-FEmesh
The following points are illustrated the step of this process.
1. Load surface. (The surface is the STL file)
2. Use surface to create block.
3. Use mesh panel to mesh the model from the block.
4. Use quality panel to evaluate or display mesh quality.
5. Use material panel to improve the model by User-Defined or Image-Based.
Chapter3 Design/Modelling
3.1 Introduction
This chapter is to illustrate the methods used to create a solid model of the two
vertebrae and the disc between them from the CT scan images. This chapter is to deal
with the process of segmenting the vertebrae and disc to create a solid model using
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Mimics step by step. In the end of this chapter is the summary of using Mimics and
the difficulties encountered using in Mimics.
3.2 Mimics steps
The flowchart (Figure 3.1) which describes the process of segmenting the vertebrae
and disc to create a solid model using Mimics step by step.
Figure 3-1 flowchart of using Mimics
3.2.1 Loading DICOM Data
Use shortcut key (Ctrl+O) to find the project which is useful. To open the dialog box
can be found the all files which are suit to Mimics in the working directory, then use
double-click to open the DICOM data. Saving the DICOM data on hard drive can
help to open the data without any issues. The image of initial interface is illustrated in
Figure 3-2.
Figure 3-2 Screenshot from Mimics illustrating of the initial interface of Mimics
Load DICOM Files from CT
scan
Volume Rendering
Create Model by
Segmentation
Save as an STL File
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3.2.2 Volume Rendering
After loading the DICOM data in to Mimics, the C.T scan images can be produced by
three views on the software interface. The three views are top view, back view and
front view. Due to the aim of project, the next step is to find the most curve part of the
spine. It can be found two deformed vertebrae and disc which are located in the
lumbar spine, and this part is the section which need to be segmented.
The first step is to change the contrast with a suitable degree, because a suitable
contrast can help to distinguish the muscle and bone and choose different components
which have different density. Clicking the threshold icon in the project navigator can
change contrast in any time. The image of the threshold icon is illustrated in Figure 3-
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Figure 3-3 Screenshot from Mimics illustrating of the threshold icon
The next step is to create a 3D model. The icon in the upper right corner is to create
3D model. Use right-click to the mask and choose the button (Calculate 3D), then the
dialog box can be illustrated the component that need to be created. The quality can
choose Optimal and this quality is just the effect of display on the screen, it cannot
affect the quality of model. Figure 3-4 is the image of mask. Figure 3-5 is the image
of the icon of 3D object. Figure 3-5 is the view of 3D model.
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Figure 3-4 Screenshot from Mimics illustrating of the image of mask
Figure 3-5 Screenshot from Mimics illustrating of the image of 3D object
Figure 3-6 Screenshot from Mimics illustrating of the image of 3D model
Using the dialog box of contrast to adjust the density of the vertebrae which not only
removes or adds material. The image of the contrast picture is illustrated in Figure 3-
7. Changing the contrast, it can be distinguished the useful component and
unnecessary component easily. The region of interest placed in dialog box, and can be
views in each slicer. After adjusting the region of interest the vertebrae model can be
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displayed by 3D view. Using 3D view is helpful to find and adjust the boundaries of
vertebrae and can distinguish the different parts of the vertebrae.
3.2.3 Create Model by Segmentation
After the step of volume rendering, the next step is to create the maps within the
region of interest slicer by slicer. From the module upward tab the editing module can
be selected. The editor tool has an essential role in Mimics and is illustrated in Figure
3-8.
Figure 3-8 Screenshot from Mimics illustrating of the editor tool
In order to create a useful model need to wipe off the useless part. Due to this
condition, it needs to select a mask which located between useful and useless slicers
and use editor tool to wipe it off.
Firstly, to select a slicer and investigate the structure of the vertebrae from three
views. Changing the thresholding values can help to distinguish easily. Secondly, in
Figure 4-7 Screenshot from Mimics illustrating of the image of contrast picture
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the selected slice view the paint brush tool was used to wipe off the useless part and
leave the useful part. This process need use the eraser to wipe off and it include three
views. The top view is illustrated in Figure 3-9.
Note: The difficulties of this process need to get familiar of the structure of the
vertebrae and disc. Also this process needs a lot of patient to finish. The further work
with this software package is illustrated on page 28
Figure 3-9 Screenshot from Mimics illustrating of top view of the vertebrae
The view of front view and back view are illustrated in Figure 3-10.
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Figure 3-10 Screenshot from Mimics illustrating of front view and back view of the vertebrae
3.2.4 Save as an STL File
STL file is a type of file to describe 3D file, and the abbreviation of Stereo
Lithography. In this software, there are three types of file can be transfer from Mimics
to STL+. They are .3dd file, mask and 3D file. The dialog box is illustrated in Figure
3-11. Using left-click to the Next icon, the dialog box of STL can be produced. The
parameters can be changed easily from this dialog box.
Figure 5-11 Screenshot from Mimics illustrating the dialog box of STL+
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3.3 Difficulties Encountered
3.3.1 Loading DICOM Files
There are a lot of data of the CT scan, and the software does not have a memory key.
Due to this condition, it needs to load the data and save them on the hard drive. This
can help to load the file without any issues.
3.3.2 Volume Rendering
In this process, it needs to choose a suitable contrast by the dialog box of threshold.
This presented difficulties when use the editor tool to choose slicer and do the
segmentation for the vertebrae. Because the density of muscle and bone is difficult to
distinguish.
3.3.3 Create Model by Segmentation
This process is the most difficult step in using Mimics, because the model is consisted
by many numerous models and every numerous model has different slicer. The most
essential part in vertebrae are the upper and lower plates. Creating a completion
vertebrae need to get familiar with the structure, and this structure can not only be
found in the view of 3D model clearly, however, it also can be found in three different
views. Another detail needs to be cared is smoothing, this process need pay attention
because after model been smoothed to a certain level it is saved as to a brand new
model.
3.4 Summary of using Mimics
Creating a solid model from a CT scan by using Mimics is useful to the project.
Though it is difficult to learn how to use this software at the beginning, however, this
software is easy to get familiar with. To the opposite, this software also has some
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defect associated with it. For example, smoothing the solid model need a lot of time,
and a smooth solid model is very important to the next process which needs to use IA-
FEmesh. This issue may affects the structure of the solid model, because after model
been smoothed to a certain level it is saved as to a brand new model.
To sum up Mimics is an excellent software to create solid model, and it is also very
easy to use it.
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Chapter4 Finite Element Mesh Creation
4.1 Introduction
This chapter describes two software packages which are been used in the project, and
also explain the reason why the IA-FEmesh is the most suitable software to this
project. This chapter also introduce the steps of using this software and the details of
methods which employed using IA-FEmesh to create a finite element analysis mesh
model of the two vertebrae and the disc. At the last, this chapter would document the
difficulties during using IA-FEmesh and illustrate the way of how to solve these
difficulties. (5)
4.2 IA-FEmesh
The solid model is created by Mimics, and the solid model is consisted by three
components, they are upper vertebrae, lower vertebrae and the disc between them.
This software is used to make mesh creation simpler and also reduce computation
time when put the model into ANSYS. The segmented CT scan image was imported
into this software, the aim is to compare the material properties with the solid model.
Due to this condition, the solid model created by Mimics also needs to be imported
into IA-FEmesh. The details of the process are introduced below step by step.
4.2.1 Import Surface
The STL file was loaded into IA-FEmesh by selecting the “Surface Tab” on the main
menu and then use left-click the “Load” in the dropdown menu, and select the STL
file. At last the surface of the material can be illustrated by image. The results of this
process is illustrated by image below. The surface is illustrated in Figure4-1.
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4.2.2 Import Image
Following the step of importing surface, the next step is to import image by load the
CT scan images. The CT scan images include the segmentation vertebrae. From the
main menu, select the tab “Image” and click the “Load” in the dropdown menu. At
last the image can be illustrated in the screen. The image of “Image” is almost the
same with “Surface”.
Figure 4-1 Screenshot from IA-FEmesh illustrating of importing surface
4.2.3 Create Building Blocks
Building blocks is the next step after loading surface. First, the image should be
clicked off by tick box, the icon is located in the top left hand corner of the screen.
Second, click the “Block(s)” tab and select the dropdown menu within “Create”. After
that step, the option of “Create blocks from surface bounds” needs to be selected. At
last, the model could be surrounded by a hexahedron. The image is illustrated in
Figure 4-2 below.
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Figure 4-2 Screenshot from IA-FEmesh illustrating of importing blocks
The dropdown menu “Build/Edit” tab has many editor tool, using the split tool can
help to divide the initial model into many smaller components. This process is
illustrated in Figure 4-3.
Figure 4-3 Screenshot from IA-FEmesh illustrating of splitting the blocks
The move tab is used to select the point attach the surface of the model. The red point
is used to reposition the edges and faces of the disc body to create a shape. This
process need to drag and reposition the red point at the corner of each blocks until
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every blocks have been attached. The image is illustrated in Figure 4-4.
Figure 4-4 Screenshot from IA-FEmesh illustrating the using red ball to attach the surface
4.2.4 Create Mesh
This process is to create a meshed model after the creation of building block structure.
The first step is to select the “Assign Mesh Seeds” from the dropdown menu.
Select the tab of “Mesh”, after that find and click the tab of “Create” in the dropdown
menu. There are two choice in the menu of “Create”, they are “Volumetric Mesh” and
“Surface Mesh”. Select the “Volumetric Mesh” and choose the smoothing box. The
nodes and elements of the disc also can be given. After clicking the apply button, the
meshed model can be produced in the screen. The images is illustrated in Figure 4-5.
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Figure 4-6 Screenshot from IA-FEmesh illustrating the image of creating mesh
4.2.5 Assigning User Defined Material Properties: 2 Material
Both of the mesh created and smoothed are very necessary to the process of using IA-
FEmesh. The next step is to investigate the material type and the material properties in
this model. To select the “Material” tab and click the “Option-Defined” option. After
that, the “Element Set Definitions” could be illustrated in Figure 4-6.
After the “Surface Element” was been clicked, then hold the “Ctrl” whilst clicking the
left mouse and drag the elastic box until the surface of the model turn into green. The
model which was surrounded by green mesh is illustrated in Figure4-7. After release
click and use right-click the green highlighted model could be changed to red
highlighted model. The red highlighted mesh means inner elements, and green
highlighted model means outer elements. The red highlight model is illustrated in
Figure 4-8.
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Figure 4-6 Screenshot from IA-FEmesh illustrating of Element Set Definition
Figure 4-7 Screenshot from IA-FEmesh illustrating of outer elements
Figure 4-8 Screenshot from IA-FEmesh illustrating of inner elements
4.3 The Unfinished Step in Using IA-FEmesh
After the step of “Defined Material Properties”, the next step is to assign the image
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material properties. This step is to complete the creation of this solid model, it can
help to simplify the process when the model been loaded into ANSYS.
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Chapter5 Testing
5.1 Introduction
The aim of this chapter is to describe the methods which used to test the meshed
model. ANSYS 15.0 is the main software package which used to test the meshed
model. The details of using workbench is the aim to illustrate to the readers which
includes how the simulated model was been applied force and the how to obtain the
results.
5.2 Loading the model
First, open the toolbox and find “Static Structure” from “Analysis System” in ANSYS
15.0. Second, input the GIES file in Geometry and define the material property in
“Engineering Data”. The GIES file is created in IA-FEMesh. The meshed model
which been input into ANSYS is shown below.
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Figure 5-1 meshed model in ANSYS
The material of this simulated model has four different kinds of property. The material
can be defined by Young’s Modulus (MPa) and Poisson’s Ratio. The vertebrae and
disk can be simplified as isotropic elastic material. The outer face of vertebrae can be
defined as cortical bone; the inner part of vertebrae can be defined as cancellous bone.
The matrix of annulus fibrosus can also be simplified as isotropic elastic material. The
nucleus pulposus is defined as a material that cannot be compressed. The definition of
material property is illustrated below.
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Figure 5-2 the definition of material property
5.2.1 Loading condition
Fixing the bottom surface of the L5 vertebrae to make sure the nodes which located
on the surface cannot make translation or make rotation. Generally speaking, is to fix
six degrees of freedom on the bottom surface.
Coupling the bottom end plate of L4 vertebrae and the upper surface of disk, and use
the same method to make the upper end plate of L5 vertebrae and bottom surface of
disk to be linked together. This method is to ensure the nodes which located these
three components can have the same degree of freedom.
The method of compression test is to load a vertical force (1KN) on the upper end
plate of the L4 vertebrae. To simulate four physiological movements, this project try
to force a torque (10 N.m) on three axises.
5.2.2 Compression Test
The FE module containing the model was then linked to a static structural module. In
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the static structural module, the test load a force on the upper end plate of the L4
vertebrae which setting as a loading surface. And the surface of the lower end plate of
L5 vertebrae set as a fixed support.
Figure 5-3 Screenshot from ANSYS illustrating the result of defining the upper end plate as the loading face
The simulated model been tested 5 different kinds of load, the result of deformations
and stresses which were recorded into Excel spreadsheet. The test data is used to
analysis and to compare with the previous report.
The Screenshot from ANSYS illustrating the result of defining the lower end plate as
a fixed support is illustrate in the figure below.
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Figure 5-4 Screenshot from ANSYS illustrating the result of defining the loweer end plate as a fixed support
After setting these two stress conditions of the simulated model. The next step is to
use ANSYS to test the model by five different kinds of load, the vertical load are 0.5,
1.0, 1.5 and 2.0, and the unit is KN.
The simulated model which been tested by a 2.0 KN vertical load is illustrated in the
figure below.
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Figure 5-5 the model been tested by a 2.0 vertical load
5.3 Force moment to test the simulated model
In this test, the simulated model is tested by a moment (10N.m) in three different
directions. First step, the test load a force on the upper end plate of the L4 vertebrae
which setting as a loading surface in the static structural module. And the surface of
the lower end plate of L5 vertebrae set as a fixed support.
The second step is to force the moment on X, Y, Z axis. The moment which force on
X axis is simulated the physical movement of front bending and back banding. The
moment which force on Y axis is simulated the physical movement of lateral bending.
And the moment which force on Z axis is simulated the physical movement of
rotation.
To compare with back bending, the range of motion of front bending is larger. The
range of motion of lateral bending is the largest. To the opposite, the range of motion
of rotation is the smallest.
The figure below which illustrates the simulated model which been forced a moment
on the Y axis.
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Figure 5-5 the model been forced a moment on Y axis
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Chapter6 Result
The chart below which illustrates the comparison between finite element results and
results from experimental studies. The maximum displacement of the vertebrae under
different axial loads which is illustrated in the chart below.
