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

Fig. 2: Percentage of bone volume with a certain error
for different density ranges for the reference
simulation parameters
5. DISCUSSION
Multiple sources cause the discrepancy between the densities acquired from the bone
remodeling simulations and those derived from CT scans. The linear correlations
between the medical image data and real bone properties are an approximation, and
formulations in literature for these vary greatly. This is related to the fact that the scale
of the HU’s varies according to the scanner used. Additional errors arise from the mesh
itself. Since the maximal edge size of the elements used, was bigger than the voxel
resolution, the material properties are calculated from interpolated HU’s instead of the
real values. This results in a blurring of the cortical bone, with a lower density and a
thicker border. Also since some smoothing was performed to provide a better mesh
quality, the outer shell of the mesh will contain volumes that lie outside the bone. The
bone remodeling model will predict cortical bone with a maximal density here, while
the real cortical border starts more inwards.
Only one gait cycle was used to calculate the load cases used. However human gait,
even within one subject, varies a lot. This cycle is not necessarily representative for the
average gait cycle of the subject and the loads that act on the femur. When simulating
bone remodeling with loads coming from an atypical gait cycle, patterns that are far
from reality can be found. Measuring multiple cycles and using the averaged cycle, or a
probabilistic methodology could improve the reliability of the model. Loads that
contribute to the bone remodeling, but that are not part of human walking like stair
walking, should be considered.
An accurate and representative set of muscle and joint forces, will still give bone
densities that deviate from the in vivo values. Factors that are not directly related to the
mechanical stimulus, like genetic predetermination, could skew the bone remodeling