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

The main goal of this study is to combine the bone remodeling scheme with a subjectspecific
musculoskeletal model for estimating the loads, and to determine the model
parameters that have the biggest influence on the bone remodeling outcome when
estimating bone remodeling for a 3D geometry with 3D loading cases. Secondly to get
an insight which aspects of a patient-specific load-based bone remodeling model are
important to accurately predict the bone density distribution in the proximal femur by
comparing the results with image based material properties.
3. MATERIALS AND METHODS
3.1 Bone remodeling model
The anisotropic bone remodeling model used is based on the principles of continuum
damage mechanics [2]. It uses a fabric tensor and the apparent density as internal
variables to model both the change in bone density distribution as well as the
directionality of the microstructure. Starting from isotropic material properties, the
material will adjust and align its principal directions with those of the stress tensor,
while also resorbing bone where it is not contributing to the load-supporting structure of
the bone. The bone remodeling algorithm was implemented and executed for hundred
increments of analysis, after which changes in bone density became negligible.
3.2 FE-model
A semi-automatic segmented triangulated surface mesh was constructed from CTimages
using Mimics (Materialise NV, Leuven, Belgium). Starting from a bone
threshold of 226 HU, followed by a cleaning operation including manual editing and
morphological operations, Standard Triangulation Language (STL) files were generated.
A linear tetrahedral volume mesh was constructed using an automatic meshing in
MSC.Patran after smoothing and homogenizing the surface triangles. To limit
calculation time, the volume mesh was reduced to only contain the most proximal third,
and was then distally fixed in three vertices around the border of the cut. The density
profile of the proximal part was very similar whether or not a larger part of the femur
was used. Only near the elements that were clamped, was there a visual difference.
After relative positioning of the femoral head with respect to the pelvic bone, the
maximal contact surface was determined by thresholding the maximum distance
between the contact surfaces. Contact forces on this surface were chosen pointing
towards the hip joint center, with amplitudes proportional to the projected vertex surface
area, diminishing in amplitude away from a pole [8]. The magnitude and location of this
pole was determined iteratively starting from the direction of the total contact force until
the required total contact force was acquired. When no relative positional data of the hip
bones was available, as is the case when using generic loading conditions, the maximal
contact surface was chosen by a conical cut with an opening angle of 45 degrees from
the direction of the force.
The FE-meshes were imported into Mimics again and material properties were assigned
by relating the HU from the CT-image with bone mineral density. In literature different