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

It is well established that trabecular bone is an anisotropic, inhomogeneous material.
Some efforts have been made to account for the heterogeneity of bone using empirical
relationships between apparent density and mechanical properties [12, 13]. However,
very few studies take anisotropy into account, and those which do, do so indirectly [14,
15, 16]. In fact, anisotropy may be much more significant than inhomogeneity in such
models. This neglect can be attributed to the lack of available data on anisotropy of
bone at various locations in the human body.
This paper presents the determination of bone elastic properties within the femoral head,
their inhomogeneity and anisotropy using the FE method and µ-CT. Trabecular bone is
analysed at different locations in each of the femoral heads of osteoporotic and
osteoarthritic condition. The bone elastic properties are correlated with location within
the femoral head and micro-structural indices as determined by a morphometric
analysis.
3. VARIATION OF BONE PROPERTIES IN THE FEMORAL HEAD
The femoral head consists of a thin layer of dense, cortical bone surrounding the
considerably more porous trabecular bone. It is generally accepted that trabeculae tend
to align in directions that optimise bone function including its load carrying
mechanisms. Within the femoral head, this manifests itself in an alignment of
compressive trabeculae approximately along the axis of the femoral head in the main
trabecular direction (MTD), with perpendicular tensile trabeculae that coalesce in the
centre, where lies the stiffest region.
Many studies have found that bone volume fraction (BV/TV) is the strongest predictor
of trabecular bone apparent elastic stiffness [17, 18]. Some studies [15, 19, 20] have
examined the variation of bone density with location in the femoral head. Issever et al
[19] found that the superior pole is significantly denser in comparison with the inferior,
anterior and posterior poles. On the other hand, trabecular thickness showed less depthdependence.
Tassani et al. [21] and Ohman et al. [22] demonstrate a clear dependance of trabecular
bone strength in the femoral head upon loading angle in cancellous bone within the
femoral head. In fact, Ohman et al. [22] showed that a 20° misalignment of testing
direction results in a 40% decrease in elastic modulus on average. Previous studies have
not examined anisotropy in detail.
4. MATERIALS AND METHOD
4.1 Samples
Twelve femoral heads from 10 female and 2 male donors (age 71.25 ± 7.61 years) were
obtained from the Tissue Bank of the Scottish National Blood Transfusion service.
5x5x5mm cubic regions of interest were extracted from 10 locations in each head as
shown in Figure 1, and scanned using µ-CT (Skyscan 1172; Kontich, Belgium). Six
femoral heads were from osteoarthritic (OA) donors, and the remaining six were