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 # Discussion 6. References 7. Acknowledgments 4. Results 3. Methods 2. Objectives 1. Introduction 2005 Summer Bioengineering Conference, June 22-26, Vail, ColoradoPh.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. Weiss Departments 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 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 MPa ExperimentalFE Anterior 3 MPa Cortical Bone Strains FE predicted bone strains were in excellent agreement with experimental measures (r 2 = 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 Min Exp. Min / Max Strain (  strain) FE Min / Max Strain (  strain) Trabecular E = 45 MPa Trabecular E = 164 MPa Trabecular E = 456 MPa Exp. Strain = FE strain Const. Thick. - 1 SD Const. Thick + 1 SD Const. Modulus + 0 SD Exp. Strain = FE strain Exp. Min / Max Strain (  strain) FE Min / Max Strain (  strain) Subject-Specific Exp. Strain = FE strain