The document discusses the development of a microstructure-enhanced material model using ANSYS for predicting bone failure, particularly in patients with osteoporosis. It highlights the limitations of current diagnostic approaches and emphasizes the importance of using continuum finite element models that integrate high-resolution microstructural data. The study demonstrates that database-derived finite element models can more accurately reproduce the mechanical behavior of bone compared to traditional isotropic models.

1. / Department of Biomedical Engineering

2. Implementing a microstructure-enhanced
material model using ANSYS USERMAT
for the prediction of bone failure
Javad Hazrati, Bert van Rietbergen, Keita Ito
Materials Technology
Eindhoven University of Technology

4. Osteoporosis
• Osteoporosis is the most epidemic bone disease in elderly
populations.
• It is characterized by:
• low bone mass
• deterioration of bone micro-architecture
• compromised bone strength
• It leads to bone fragility and increased risk of fracture under
low loads.
Healthy Osteoporotic
PAGE 4

5. Motivation
Current diagnostic approach (DXA) is not
an accurate predictor of osteoporosis or
PAGE 5
bone strength
There is a need to have a better predictor
of bone strength
Image based patient specific mechanical
approaches using continuum finite
elements (FE) method are better predictor
of bone strength
The goal is to build micro-structure
enhanced continuum FE models
Dual Energy X-ray Absorptiometry (DXA)
Neck BMD
Age
g/cm2
1.22
0.98
0.74
0.50
*
20 40 60 80 100
Density
Image based patient specific FE modeling

6. Bone
• From a material point of view, bone tissue can be considered a
porous material with a varying degree of porosity:
1. Cortical bone: tube-like structure of high-density bone
2. Trabecular bone: porous type of bone
PAGE 6
• Trabecular bone can be
highly anisotropic:

7. Continuum FE models
• Building continuum FE models of bone relies on two essential
tools:
• ANSYS Mesh morphing tool
• Morphology analysis tool to measure local micro-structural
parameters.
PAGE 7

8. Mesh morphing
ANSYS Mesh morphing tool:
• Automation of subject-specific
FE model generation while
keeping surface regularity.
• Comparison of results sets from
two or more meshes (subjects)
since morphing generates
isotopological meshes.
PAGE 8
Lorenzo Grassi et al., Medical Engineering & Physics, 2011

9. Morphology analysis
Morphology analysis tool:
• Bone density distribution (ρ) and
Fabric tensor (M) are used to
calculate compliance tensor.
 
3 3
 M M 
M M
E E
 1 ,  1;

3
, 1;
1
,M
( ) ( )
1
( )
ij
i j i j
i i s i j i j i s
i j
i j i j ij s
M M
G


 

 
 




C
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(a)
(b)
a) Bone density distribution
b) Major fabric direction (major eigenvector)

10. Material behavior
• A validated anisotropic elastic-plastic damage model is used to
define bone mechanical behavior.
• The material model requires specific constitutive equations
which do not exist in ANSYS standard material library.
• Therefore, we need to use ANSYS user’s programmable
features to implement the material model in ANSYS (UserMat).
PAGE 10
Rheological model
Algorithmic stiffness

11. UserMat
• The UserMat subroutine is a tool for advanced users and allows
us to write our own material constitutive equations within a
general material framework.
• We use Fortran programming language to implement our
material model in ANSYS.
PAGE 11
Elastic-plastic damage model

12. A case study using ANSYS UserMat
A novel approach to estimate trabecular bone anisotropy using a
database approach.
FEM Database website: www.bmt.tue.nl/nl/fe_database/
doi:10.4121/uuid:4ae59365-92f0-480b-a899-ade34bc84a00
PAGE 12

13. A case study using ANSYS UserMat
• Problem: we can’t measure bone fabric information from
clinical CT scans because of the lack of resolution, which is
needed for such anisotropic fabric-elasticity relationships.
Database
Isotropic model Orthotropic model
PAGE 13
High resolution
Femur
low resolution
Human can’t be exposed to high
X-ray doses to obtain high
resolution images.
• Proposed solution:
Anisotropy is derived
from a database of
high resolution
images.
Mapping
anisotropy from
best matched
model in DB
Mesh Template

14. Study design
• To investigate if FE models with DB-derived anisotropy
produce similar stress and stiffness results as models based
on the actually measured bone fabric.
• To investigate if DB-derived models can provide more accurate
results than isotropic models.
PAGE 14

15. Materials
• 10 test femurs were used in this study.
• Loading conditions applied to the models represented a fall
PAGE 15
to the side configuration.

16. Results
Isotropic model DB-derived model
Gold standard
a) Von Mises stress distribution
b) Damage parameter distribution (user defined parameter in UserMat)
PAGE 16

17. Summary
• DB-derived FE models well reproduce the mechanical behavior
of models with anisotropy based on the actually measured
bone fabric.
• DB-derived FE models can provide more accurate results than
isotropic models.
• Database approach can be also used to obtain more
parameters than fabric such as Tb.Th, Tb.N and etc. for the
patient’s models.
PAGE 17

18. Acknowledgement
Funding from the European Union for
the osteoporotic virtual physiological
human project (VPHOP FP7-ICT2008-
223865) is gratefully acknowledged.
PAGE 18

19. Thanks for your attention!
Comments, questions or more informations?
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j.marangalou@gmail.nl
b.v.rietbergen@tue.nl