Figure 1: Locations of rosette strain gauges (n = 10) on the cadaveric pelvis.

* **

**

**

*

**

G

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.

A

B

C

E

F

LOAD

D

3 mm

0 mm

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

600

0

Min

0

-600

Exp

. Min

/ M

ax S

trai

n (s

trai

n)

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

trai

n (s

trai

n)

FE Min / Max Strain (strain)

Subject-SpecificExp. Strain = FE strain