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Validation of Bone Strains and Cartilage Contact Stress in a 3-D Finite Element Model of the Human Hip. 3 mm. 0 mm. Andrew E. Anderson, Christopher L. Peters, Benjamin J. Ellis, S. Janna Balling, Jeffrey A. Weiss - PowerPoint PPT Presentation
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Figure 1: Locations of rosette strain gauges (n = 10) on the cadaveric pelvis.
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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.
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Figure 4: Left – position dependent cortical shell thickness.Right – FE meshes used for the femur, pelvis and cartilage.
Figure 2: Pressure film cut into a rosette pattern to prevent crinkle artifact and placed on the femoral head between sheets of polyethylene.
[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] Haut-Donahue et al., J Biomech Eng, 2002
U. Utah Seed Grant and OREF Grant #51001435.
5. Discussion
6. References
7. Acknowledgments
4. Results
3. Methods
2. Objectives
1. Introduction
2005 Summer Bioengineering Conference, June 22-26, Vail, Colorado Ph.D. Poster Competition #II-51
MRL
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. WeissDepartments of Bioengineering and Orthopedics, & Scientific Computing and Imaging Institute
University of Utah
Posterior
Lateral
Medial
Rigid Bones Deformable Bones
Anterior
0 MPa
3 MPa
Figure 6: Left – Rigid bone FE pressures. Right – Deformable bone FE pressures.
• 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.
• Patient-specific modeling of hip biomechanics can aid diagnosis and surgical treatment.
• Previous hip finite element (FE) models used coarse geometry and material properties from the literature, precluding their use for patient-specific modeling.
• FE model validation by direct comparison with experimental measurements of bone strains and cartilage contact pressures has not been performed.
• Develop techniques for subject-specific FE modeling of hip biomechanics.
• Validate FE models using experimental measurements of cortical bone strain and cartilage contact pressure.
• Perform sensitivity studies for further validation.
Experimental Protocol
• Removed all soft tissue except cartilage.
• Kinematic blocks attached to bones to align experimental and FE coordinate systems [1].
• 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.
FE Mesh Generation
• Boundaries of outer cortex, cortical / trabecular bone interface and cartilage segmented from CT data.
• Cortical bone: Shell elements (Fig. 4, left).
• Trabecular bone: Tetrahedral elements (Fig. 4, right).
• Cartilage: Hexahedral elements (Fig. 4, right).
• Algorithm was developed to assign a spatially varying cortical bone shell thickness (Fig. 4, left).
FE Material Properties & Analysis
• 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
• Experimental film pressures were 0 - 3 MPa (upper limit of film detection) (Fig. 5).
• FE predicted pressures (0 - 7 MPa) were in good agreement with experimental results; two distinct regions of contact present (Fig. 5).
• Contact pattern was not altered substantially when bones were modeled as rigid; peak pressure was 8% higher (Fig. 6).
Figure 5: Left – pressure film contact pressure. Right – FE predictions of contact pressure.
Posterior
Lateral
Medial
0 MPaExperimental FE
Anterior
3 MPa
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).
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.
Max
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Exp
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ax S
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FE Min / Max Strain (strain)
Trabecular E = 45 MPaTrabecular E = 164 MPa Trabecular E = 456 MPaExp. Strain = FE strain
Const. Thick. - 1 SDConst. Thick + 1 SDConst. Modulus + 0 SDExp. Strain = FE strain
Exp
. Min
/ M
ax S
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FE Min / Max Strain (strain)
Subject-SpecificExp. Strain = FE strain