Mechanical Contributions of the Cortical and Trabecular
Compartments Contribute to Differences in Age-Related
Changes in Vertebral Body Strength in Men and Women
Assessed by QCT-Based Finite Element Analysis
Blaine A Christiansen,1,2David L Kopperdahl,3Douglas P Kiel,4Tony M Keaveny,3,5and Mary L Bouxsein1
1Center for Advanced Orthopedic Studies, Beth Israel Deaconess Medical Center, Boston, MA, USA
2Department of Orthopaedics, University of California, Davis, CA, USA
3O.N. Diagnostics, Berkeley, CA, USA
4Institute for Aging Research, Hebrew Senior Life, Harvard Medical School, Department of Medicine Beth Israel Deaconess Medical
Center, Boston, MA, USA
5Departments of Mechanical Engineering and Bioengineering, University of California, Berkeley, CA, USA
The biomechanical mechanisms underlying sex-specific differences in age-related vertebral fracture rates are ill defined. To gain insight
and old men and women to assess age- and sex-related differences in the strength of the whole vertebra, the trabecular compartment,
and the peripheral compartment (the outer 2mm of vertebral bone, including the thin cortical shell). We sought to determine whether
structural and geometric changes with age differ in men and women, making women more susceptible to vertebral fractures. As
expected, we found that vertebral strength decreased with age 2-fold more in women than in men. The strength of the trabecular
compartment declined significantly with age for both sexes, whereas the strength of the peripheral compartment decreased with age in
with age in both sexes and at both vertebral levels. Taken together, these results indicate that men and women lose vertebral bone
differently with age, particularly in the peripheral (cortical) compartment. This differential bone loss explains, in part, a greater decline in
bone strength in women and may contribute to the higher incidence of vertebral fractures among women than men. ? 2011 American
Society for Bone and Mineral Research.
KEY WORDS: VERTEBRAL FRACTURE; FINITE ELEMENT ANALYSIS; QUANTITATIVE COMPUTED TOMOGRAPHY; BONE LOSS; VERTEBRAL STRENGTH;
BONE STRENGTH; BIOMECHANICS
Despite the high rate of occurrence and the significant
personal and societal costs, the biomechanical mechanisms
underlying vertebral fractures remain largely unknown.(2,3)It is
possible that in addition to a decline in bone density, there
are structural and/or geometric changes to the cortical and
trabecular compartments with age that differentially affect men
and women, making women more susceptible to vertebral
omen have a higher incidence of osteoporotic fractures
than men, over 25% of which are vertebral fractures.(1)
With age, vertebral trabecular bone begins to deteriorate,
starting in the center of the vertebral body and progressing
superiorly and inferiorly, with thinning of the endplates and
cortical shell duetoendosteal bone resorption.(4)Meanwhile, the
cross-sectional area of the vertebral body increases with age in
both men and women because of periosteal bone formation.(5,6)
It is likely that these age-related changes in bone structure alter
the mechanical contributions of the cortical and trabecular
compartments of vertebral bodies, with the cortical compart-
ment assuming a proportionally higher contribution in older
subjects than in young subjects.(7,8)To date, several studies have
used quantitative computed tomography (QCT)–based finite
Received in original form July 12, 2010; revised form September 21, 2010; accepted November 5, 2010. Published online November 18, 2011.
Address correspondence to: Mary L Bouxsein, Beth Israel Deaconess Medical Center, Center for Advanced Orthopedic Studies, 330 Brookline Avenue, RN 115,
Boston, MA 02215, USA. E-mail: firstname.lastname@example.org
Journal of Bone and Mineral Research, Vol. 26, No. 5, May 2011, pp 974–983
? 2011 American Society for Bone and Mineral Research
element analysis (FEA) to determine the contributions of cortical
and trabecular bone to the strength of the distal radius,(9)
proximal femur,(10–12)and vertebral body.(13,14)However, no
studies have investigated the mechanical contributions of the
bone compartments in subjects taken from a community-based
study or have investigated how age and sex influence the
mechanical role of trabecular and cortical bone in the thoracic
and lumbar spine. Improved understanding of cortical and
trabecular bone contributions to vertebral strength may guide
efforts at diagnosing vertebral fragility and may enhance our
Conventional assessment of BMD in the spine typically
analyzes only vertebrae of the lumbar region (typically L2–L4
or L1–L4), yet many fractures occur in the thoracic spine. How
vertebrae from different regions of the spine lose bone with age
is not well defined. Heterogeneity of age-related bone loss along
the spine may contribute to higher incidence of vertebral
fracture at some vertebral levels; therefore, it is possible that
clinical fracture risk assessment can be improved by assessing
vertebral levels in both the thoracic and lumbar spine.
In this study we used QCT-based FEA of lumbar (L3) and
thoracic (T10) vertebrae of young men and women and old men
and women to estimate vertebral body strength and its
determinants (ie, bone density and morphology). We quantified
age-related differences in the mechanical strength, bone
strength, and bone density of cortical and trabecular bone
compartments and determined whether these age-related
differences are similar in vertebrae from the thoracic and
lumbar spine and for men and women.
Subjects and Scan Parameters
Subjects were chosen from participants in the community-based
Framingham Heart Study Offspring and Third Generation
Multidetector CT Study.(15–18)The sample consisted of 30 men
aged35to42 years,30 womenaged 36to41 years,30 menaged
73 to 82 years, and 30 women aged 74 to 83 years (Table 1). The
study protocol was approved by the Boston University School of
Medicine and Hebrew Senior Life, and all subjects gave written
informedconsent.Thestudy isoverseenby anindependentdata
safety and monitoring board. For each subject, finite element
models were created for the vertebral bodies of the T10 and L3
T10 or L3 vertebral body was fractured or missing from the QCT
scan volume, an adjacent vertebral body was analyzed instead
eight-detector helical QCT scanner (Lightspeed Plus, General
Electric, Milwaukee, WI, USA) at 120 kVp, 100–360 mAs. A chest
scan imaged the area from the tracheal bifurcation to the base of
the heart (approximately vertebral levels T7–T11), while an
abdominal scan imaged a 150-mm-long volume superior to the
upper endplate of S1 (approximately vertebral levels L2–L5).
Scans had a nominal in-plane voxel size of 0.68mm and a slice
thickness of 2.5mm. A multichambered hydroxyapatite phan-
tom (Image Analysis, Columbia, KY, USA) was included in each
scan to allow conversion of Hounsfield units to bone density
Finite Element Models
QCT-based finite element models of T10 and L3 vertebrae were
generated for each patient using previously published meth-
ods.(19–21)Briefly, each vertebra (excluding posterior elements)
was segmented from the image, rotated into a standard
coordinate system, and resampled into 1-mm cube-shaped
voxels. The finite element mesh was created by converting each
voxel into an 8-noded brick element (Fig. 1). Elastically
anisotropic(21)and elastic-perfectly plastic material properties
were assigned to each element using the QCT mineral density of
the voxel along with the empirical correlations between
mechanical properties and calibrated BMD for human vertebral
trabecular bone.(22)Material failure of the bone was modeled by
Table 1. Subject Characteristics (mean?standard deviation)
nAge (yrs) Height (cm)Mass (kg)
L2 was analyzed in one man (age 75) and one woman (age 77), T9 was
analyzed in six women (ages 39, 40, 40, 41, 77, and 81), and T11 was
analyzed in one woman (age 77). One woman (age 41) had no lumbar
scan available, so only T10 was analyzed.
Fig. 1. QCT-based finite element models of L3 vertebral bodies from a
38-year-old man (top left), 75-year-old man (top right), 40-year-old
woman (bottom left), and 79-year-old woman (bottom right). Each ver-
tebra (excluding posterior elements) was segmented from the QCT
image, rotated into a standard coordinate system, and resampled into
1-mm cube-shaped voxels. The finite element mesh was created by
converting each voxel into an 8-noded brick element. Elastic-perfectly
plastic material properties were assigned to each element using the
mineral density derived from the brightness of the voxel along with the
empirical correlations between mechanical properties and calibrated
BMD for human vertebral trabecular bone.(22)Images are representative
of the means for peripheral bone mass.
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Journal of Bone and Mineral Research
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