Age-Related Changes in Whole-Bone Structure and Strength - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 14

3.1 Cross-Sectional Geometry and Bone Mineral Density
Similar to the diaphyses, the metaphyses of long bones and the vertebral bodies
exhibit age-related periosteal and endosteal expansion. The average rates of
periosteal expansion are comparable at metaphyseal and diaphyseal sites (1-2 %/
decade). Because metaphyseal and vertebral sites have trabecular bone in the
medullary cavity, endosteal expansion corresponds to increased trabecular area.
The average rates of endosteal expansion are notably less at metaphyseal sites
(1-2 %/decade) than diaphyseal sites (5-15 %/decade).
The changes in morphology at metaphyseal and vertebral sites occur concur-
rently with dramatic reductions in bone density, as quantified by volumetric bone
mineral density (vBMD). vBMD values are often reported separately for trabecular
and total (trabecular plus cortical) regions. Cortical regions can be difficult to
reliably isolate because of the thin cortical shell and partial volume averaging
effects that are problematic for clinical QCT. Because we are presenting com-
parisons in terms of percent changes per decade rather than absolute changes, it is
difficult to compare the relative impact of cortical and trabecular bone loss.
Nonetheless, it is clear that both cortical and trabecular bone loss contribute
importantly to age-related bone loss at the femoral neck, distal tibia, distal radius,
and spine.
A longitudinal study by Riggs et al. [ 49 ] gives particular insight into the dif-
ferences in trabecular and cortical bone loss, and into the variable rates of change
during aging. They followed *1100 subjects (aged 20-97 years at baseline) for 3
years, obtaining QCT scans at the distal tibia, distal radius and lumbar spine.
Importantly, their results challenge some of the conventional wisdom about when
bone loss begins. Trabecular vBMD declines throughout life in women and men,
such that approximately 40 % of the lifetime loss of trabecular bone occurs before
age 50. By contrast, only 6 and 15 % of the lifetime loss of cortical bone occurs
before age 50 in women and men, respectively. Notable decreases in cortical
vBMD do not begin until around the time of menopause in women and even later
in life in men.
3.1.1 Lower Extremity: Femoral Neck and Distal Tibia
The femoral neck is not accessible to pQCT, but because of the clinical sig-
nificance of hip fractures, age effects at the femoral neck have been examined
using DXA and clinical QCT (Table 3 ). The size (total area) of the femoral neck
is 25-30 % smaller in women than men [ 8 , 50 ]. In both sexes, average increases
in total area are 1.5 %/decade, with increases in medullary area of 2.5 %/decade.
As at other sites, these changes result in cortical bone loss (i.e., decreased
cortical area) that is somewhat greater in women (-5 %/decade) than men
(-3 %/decade). Trabecular bone loss at this site is dramatic, and is greater in
women than men; average declines in trabecular vBMD based on two reports are