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

Because of the orthotropic assumption, the intersection of trabeculae in the region
below the epiphysis can be seen to be orthogonal. Although this agrees with Wolff’s
trajectorial theory, other studies have measured the angle of decussation to be acute [21,
23]. This is the limitation of assuming bone to be a continuum with local orthotropic
symmetry. The inclusion of a shear modulus adaptation algorithm may overcome the
limitation of modelling bone’s anisotropy using orthotropic symmetry. Nevertheless,
orthotropy has been shown to be a closer approximation to bone’s anisotropy than
isotropy [13]. Modelling of different frames for each of the activities allows for a more
accurate prediction of the distribution of the material properties and associated
orientations.
6. CONCLUSION
This study combined two developed approaches: considering bone to be a strainadaptive
continuum material with local orthotropic symmetry and inclusion of free
boundary conditions in the model. Multiple activity load cycles were shown to be
critical in bone optimisation simulation. The stiffness distribution and structure
directionality for the whole femur can be correctly assessed, with results approaching
those observed in vivo. Further developments to this method will be the inclusion of
more daily activities, including sit to stand, thus creating a loading envelope more
representative of the loading environment bone is subjected to in vivo. Nonetheless, the
models obtained from this approach can be applied in structure and directionality
dependent research areas such as fracture mechanics (with applications in impact
protection devices and blast biomechanics) and implant design improvement.
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