ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

3. MATERIALS & METHODS
FE-models of human cadaver femurs were developed based on qCT scans of the
specimens. Three previously established density-elasticity relationships (Eqs. (i)-(iii))
were retrieved from the literature and were implemented in the FE-models (4, 9, 10).
The accuracy of the FE-models in predicting the bones’ mechanical response to load
was validated by biomechanical experiments. 23 human femur specimens, harvested
fresh, were obtained from 14 donors (mean age ± standard deviation 61.5 ± 9.6 years,
range 48 to 90 years, nine pairs, five single specimens, 10 male, 4 female). QCT-scans
were conducted with 1.0 mm slice thickness and a pixel size of 0.547 or 0.488 mm
respectively. The specimens were scanned in air with a mineral density calibration
phantom to determine the relation between Houndsfield units (HU) and radiological
density (ρQCT) in gK2HPO4/cm³.
Quasi-static axial compression tests up to 1000 N with
elastic deformation of the specimens were conducted on
the 23 human cadaver femurs to determine strains, local
displacements, and the axial bone stiffness for validation
purposes. The specimens were mounted in a testing
setup that was free of any constraining forces or
moments and allowed a physiological bending of the
femur (Fig. 1) (11). 17 specimens were supplied with
five uniaxial strain gauges (SGs) and six specimens
were supplied with eight SGs. The SGs were distributed
over the lateral and the medial side of the femoral shaft,
and the superior and the inferior side of the femoral
neck, respectively. The axes of the SGs were aligned
with the femoral neck axis or the femoral shaft axis,
respectively, to measure compressive or tensile strains
during axial compression.
Optical markers were glued to the bone surface to
measure local displacements. Eight to 14 optical
markers per specimen were distributed over the frontal
plane of the bones (Fig. 1). Further optical markers were
applied to the proximal adapter of the testing machine
and the frontal part of the cardan joint (Fig. 1). An
Fig. 1 Mechanical test setup for
axial compression test of femur
specimen.
optical measurement system tracked the markers and calculated coordinates and
displacements using stereo-image based evaluation techniques.
Subject-specific FE-models were generated based on the qCT-scans of the 23 human
femurs. The FE-models were developed in four steps: (1) The CT data was segmented
to obtain a 3D reconstruction of the bone surface. The reconstructed CT data was then
converted into a solid model of the bone specimen. (2) The solid model was spatially
registered and processed to match the boundary conditions with the experimental setup.
(3) The solid model was meshed with finite elements and the boundary conditions were
applied. (4) The material properties were mapped onto the FE mesh and the solution
was obtained. Finally, the computational models were verified by mesh-convergence
analysis.