Validation of Bone Strains and Cartilage Contact Stress in a 3-D
Finite Element Model of the Human Hip
Andrew E. Anderson, Christopher L. Peters, Benjamin J. Ellis, S. Janna Balling, Jeffrey A. Weiss
MRL
Departments of Bioengineering and Orthopedics, & Scientific Computing and Imaging Institute
University of Utah
FE Mesh Generation
• FE model validation by direct comparison with
experimental measurements of bone strains and
cartilage contact pressures has not been performed.
Boundaries of outer cortex, cortical / trabecular bone
interface and cartilage segmented from CT data.
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•
•
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Cortical bone: Shell elements (Fig. 4, left).
Trabecular bone: Tetrahedral elements (Fig. 4, right).
Cartilage: Hexahedral elements (Fig. 4, right).
Rigid Bones
Algorithm was developed to assign a spatially varying
cortical bone shell thickness (Fig. 4, left).
• Develop techniques for subject-specific FE modeling of
hip biomechanics.
0 mm
• Validate FE models using experimental measurements
of cortical bone strain and cartilage contact pressure.
• Perform sensitivity studies for further validation.
Figure 4: Left – position dependent cortical shell thickness.
Right – FE meshes used for the femur, pelvis and cartilage.
Experimental Protocol
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•
•
Kinematic blocks attached to bones
experimental and FE coordinate systems [1].
to
align
Volumetric CT scans with bone density phantom.
Strain gauges attached to one hemipelvis (Fig. 1).
Femoral head of second hip joint fitted with pressure
sensitive film (Fig. 2).
Positions of strain gauges, blocks, and anatomical
reference points on pressure film digitized.
Acetabulum loaded vertically (0.25, 0.5, 0.75, 1 X BW)
via prosthetic femur or cadaveric femur (Fig. 3).
Strain gauge data converted to principal strains;
pressure film transformed to color fringe output.
LOAD
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*
*
*
*
*
•
•
•
•
Cortical bone, trabecular bone, articular cartilage
represented as isotropic elastic [2, 3, 4].
Density-dependent moduli for trabecular bone [3].
LS-DYNA used for FE analyses.
FE-predicted cartilage pressures converted to 2-D
images.
Sensitivity Studies
•
Effects of rigid material assumption for femur and pelvis
on predicted cartilage pressures.
•
Effects of trabecular bone elastic modulus and cortical
bone thickness on predicted cortical strains.
Cartilage Contact Pressure
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A
• Experimental film pressures were 0 - 3 MPa (upper limit
of film detection) (Fig. 5).
B
• FE predicted pressures (0 - 7 MPa) were in good
agreement with experimental results; two distinct
regions of contact present (Fig. 5).
C
Figure 1: Locations of rosette
strain gauges (n = 10) on the
cadaveric pelvis.
D
E
F
0
300
200
Subject-Specific
100
0
-100
-200
-300
-400
Subject-Specific
Exp. Strain = FE strain
-500
-600
-600 -500 -400 -300 -200 -100 0
100 200 300
FE Min / Max Strain (strain)
300
200
100
300
Trabecular E = 45 MPa
Trabecular E = 164 MPa
Trabecular E = 456 MPa
200
100
0
0
-100
-100
-200
-200
-300
-300
-400
-500
Const. thickness -1SD
Const. thickness +1SD
Const. thickness +0SD
Trabecular E = 45 MPa
Trabecular E = 164 MPa
Trabecular E = 456 MPa
Exp. Strain = FE strain
-600
-600 -500 -400 -300 -200 -100 0
-400
Const. Thick. - 1 SD
Const. Thick + 1 SD
Const. Modulus + 0 SD
Exp. Strain = FE strain
-500
-600
100 200 300 -600 -500 -400 -300 -200 -100 0
100 200 300
FE Min / Max Strain (strain)
Figure 7: Top left – min / max principal strain. Top right – FE
predicted vs. experimental cortical bone principal strain. Bottom
left – effect of constant trabecular modulus. Bottom right –
effect of constant cortical thickness.
5. Discussion
4. Results
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600
-600
FE Material Properties & Analysis
Removed all soft tissue except cartilage.
Max
0
Exp. Min / Max Strain (strain)
•
•
•
Figure 6: Left – Rigid bone FE pressures. Right – Deformable
bone FE pressures.
Min
3. Methods
0 MPa
Deformable Bones
Cortical Bone Strains
• FE predicted bone strains were in excellent agreement
with experimental measures (r2 = 0.82) (Fig. 7, top right).
• Changes in trabecular elastic modulus had little effect on
cortical bone strains (Fig. 7, bottom left).
• Changes to cortical thickness had a substantial effect on
strains (Fig. 7, bottom right).
3 mm
2. Objectives
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•
Anterior
Exp. Min / Max Strain (strain)
• Previous hip finite element (FE) models used coarse
geometry and material properties from the literature,
precluding their use for patient-specific modeling.
•
Medial
• Patient-specific modeling of hip biomechanics can aid
diagnosis and surgical treatment.
Posterior
Lateral
1. Introduction
3 MPa
• Contact pattern was not altered substantially when
bones were modeled as rigid; peak pressure was 8%
higher (Fig. 6).
Posterior
3 MPa
• FE predictions of bone strain and cartilage contact stress
were consistent with experimental data.
• Careful estimation of cortical bone thickness provides
more accurate FE predictions of cortical bone strain.
• Cartilage contact pressures not significantly altered
using rigid bones, consistent with [5].
• Well-defined experimental loading configuration allowed
accurate replication of loading in the FE model; models
investigating more complex physiological loading should
be independently validated.
• FE modeling approach has the potential for application
to individual patients using CT image data.
2005 Summer Bioengineering Conference, June 22-26, Vail, Colorado
6. References
Medial
Figure 2: Pressure film cut into
a rosette pattern to prevent
crinkle artifact and placed on
the femoral head between sheets
of polyethylene.
Figure 3: Fixture for loading
the pelvis (A) actuator, (B)
load cell, (C) ball joint, (D)
femoral
component,
(E)
pelvis, (F) mounting pan for
embedding pelvis, and (G)
lockable X-Y translation
table.
Lateral
G
Anterior
Experimental
FE
0 MPa
Figure 5: Left – pressure film contact pressure. Right – FE
predictions of contact pressure.
[1] Fischer et al., J Biomech Eng, 2001; [2] Dalstra et al.,
J Biomech Eng, 1995; [3] Dalstra et al., J Biomech, 1993;
[4] Shepherd et al., Rheumatology, 1999; [5] HautDonahue et al., J Biomech Eng, 2002
7. Acknowledgments
U. Utah Seed Grant and OREF Grant #51001435.
Ph.D. Poster Competition #II-51