Quantifying the material and structural determinants of bone strength

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M.L. Bouxsein, E. Seeman / Best Practice & Research Clinical Rheumatology 23 (2009) 741–753
Fig. 1. Quantitative computed tomography image of the lumbar spine, showing bone density phantom, and distinct analysis of
cortical and trabecluar bone compartments. Images courtesy of Dr. Thomas Lang, UCSF.
midlife [49]. Effects of drug therapy on cancellous and cortical bone have been documented using QCT
[53–58]. There are many cross-sectional studies showing an association between QCT bone density and
fracture risk [59–68] but there is no consensus on whether QCT performs better than DXA in terms of
predicting fracture risk. One prospective study of risk of hip fracture in men used QCT to show that
bone density and morphology of the femoral neck were independent predictors of hip fracture [69].
Standard QCT generates images with in-plane voxel sizes of 300–500 mm and slice thickness of
1–3 mm and are therefore not adequate to assess trabecular bone microarchitecture, as trabecular
thickness ranges from approximately 100 to 300 mm. High-resolution imaging with multislice spiral CT
scanners (HRCTs) has been used to assess vertebral trabecular architecture (Fig. 2), achieving images
with in-plane voxel size of 150–180 mm and slice thickness of 300–500 mm [70,71]. HRCT provides
superior discrimination of vertebral fracture patients compared with BMD [70]. In monitoring changes
in vertebral trabecular architecture following 1 year of teriparatide therapy, HRCT provided complementary
information to that derived from densitometry [71].
Advantages to QCT are that it can be employed on standard clinical scanners with relatively short
imaging times, providing robust assessment of geometry and volumetric bone density in trabecular
and cortical compartments at sites most prone to fracture, although even the low radiation exposure is
a concern to some. To define the clinical utility of this technique, additional data are needed on the
ability of QCT-based measures to predict fracture risk prospectively and to monitor anti-fracture
efficacy of drug interventions.
High-resolution peripheral computed tomography (HR-pQCT)
HR-pQCT measures bone density and trabecular and cortical microarchitecture, the distal radius
and distal tibia with isotropic voxel size of w80 mm [72–80] (Fig. 3). This technique has excellent
precision for both density (