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

distribution, with an average difference of 41 kg/m³ and 14 kg/m³ for resp. five and
seven load cases compared to the reference simulation. Changing the weights however
did have a more pronounced impact on the density. The main difference seems to be a
decrease in the average density, with 455 kg/m³ when lowering the relative weight of
the mid-stance to two compared to the 657 kg/m³ for the reference model. Since the
weight of the body at the beginning and the end of the stance phase is also partially
balance by the other leg, the mechanical stimulus will be lower when a higher weight is
given to these load cases, which eventually leads to a lower density estimate. Reducing
the attachment regions to only the three gluteus medius fibers resulted in similar
average density (618 kg/m³), but with increased bone formation near the muscle
insertion.
A high initial density distribution (500 kg/m³) resulted in a relative high density in the
medullary cavity and the femoral head. The model does not predict enough resorption to
lower the density to realistic values. Note however that the bone remodeling of the
femur in vivo will never start from this (relative high) homogeneous bone distribution.
Further research is needed to evaluate whether this problem also manifests in cases
where resorption does happen, like after total hip arthroplasty. When decreasing or
increasing the amplitude of the muscle and contact forces, the same pattern emerges,
however the apparent density values scale with average densities of 572, 690 and 742
kg/m³ for a scaling factor of resp. 0.8, 1.2 and 1.4.
4.3 Bone density comparison
To compare the simulated result with the bone density estimate from CT scan we
calculate the error level, defined as the minimum of the absolute and relative error. The
absolute error was calculated as the difference between the bone density prediction and
the density computed from HU, both in g/cm³. For the relative error this was normalized
with the image based density estimate. These errors were calculated for four different
ranges of densities (0≤ρ