Quantifying the material and structural determinants of bone strength

748
M.L. Bouxsein, E. Seeman / Best Practice & Research Clinical Rheumatology 23 (2009) 741–753
properties and loading conditions influence the strength predictions. Nonetheless, this state-of-the-art
technique is among the most promising tools available today for clinical assessment of bone strength
and fracture risk [112].
3D-QCT (or HR-pQCT) scans are directly converted, voxel by voxel, into a finite element model,
which accurately represents the 3D geometry and heterogeneous density distribution of the bone of
interest (Fig. 6). Forces simulating activities that cause fractures are applied to this model, and the
stiffness and strength of the bone in response to the applied forces are computed. These analyses can
incorporate different in vivo loading conditions, such as compressive and bending loading for the
vertebrae, or stance and fall loading for the femur. Several studies in human cadaveric femurs,
vertebrae and forearms have shown that bone strength is better predicted by FE analyses than by BMD
alone [113–116].
Over 15 years ago, Faulkner and colleagues reported that QCT-based FE analysis of vertebral
strength discriminated between women with and without vertebral fracture, with less overlap
between the two groups than observed using QCT [117]. More recently, QCT-based FE analyses have
been shown to discriminate women with vertebral fractures from controls[118] and to explore the
mechanisms underlying increased bone strength following anabolic and/or anti-catabolic treatment[57,119,120,121].
The deleterious effects of glucocorticoids on femoral strength have also been
explored by in vivo QCT-based FE analyses [122]. In postmenopausal women matched for age, weight
and history of hormone therapy, FE analyses showed that femoral strength in women with a history of
glucocorticoid use was w15% lower than controls for both fall-loading and stance configurations.
The feasibility of in vivo micro-FEA using both MR and HR-pQCT has also been demonstrated in
clinical studies [78,82,123,124]. High-resolution MR images of trabecular bone in the distal radius were
subjected to FE analysis to determine effects of trabecular microarchitecture on bone mechanical
properties in normal and osteopenic postmenopausal women [123]. MR images of trabecular architecture
in the calcaneus were also assessed using FE to quantify the response to 1 year idoxifene [124].
In both cases, the use of FE analysis provided additional information regarding bone structure and
strength to that available from structure measurements alone. Thus, this approach warrants further
investigation. However, because this technique presently requires extensive computational resources
and specialised software, widespread evaluation will likely be limited. To date, there are few data on
the precision of the technique and its ability to reflect disease- and age-related changes, and no
prospective fracture studies exist. However, cross-sectional studies using HR-pQCT have shown the
FEA-based strength estimates discriminate women with prior history of distal radius fracture [76,82].
Improved imaging and computational methods have made subject-specific FE analyses more feasible,
and further technological advances will continue.
Fig. 6. Finite element model of the proximal femur in a sideways fall configuration. Colors represent plastic strain, showing likely
regions of failure. Image courtesy of Dr. David Kopperdahl, O.N. Diagnostics.