Biomedical Engineering Reference
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from about 4% at 40 years of age to more than 10% at age 80, whereas the ''true
mineral density'', i.e., the density of the solid bone phase, does not vary signifi-
cantly with age [
108
]. Similar findings were reported for human femoral cortical
bone [
109
]. Thus, the main factor in age-related intracortical bone loss is the
increased porosity. Other two-dimensional morphological studies have indicated that
it is the increased pore area (i.e., Haversian canal size) rather than pore number that
explains most of the age-related increase in cortical porosity [
158
-
160
]. A recent
three-dimensional lCT study of human cadaveric femurs indicated that Haversian
canal volume fraction (porosity) increases with age (18-92 years); the number of
canals increased until the 6th decade then decreased [
110
].
4.1.2 Mineralization
Data on mineralization of cortical bone are mixed with respect to age and anatomic
site-dependence. Here we define mineralization as the percent of the solid phase of
bone that is mineral; a related measure is tissue mineral density (TMD) which is
mineral content per volume of solid tissue. These are not the same as ''bone
mineral density (BMD)'' which is defined as mineral content per total area
(aBMD) or volume (vBMD), and is thus influenced by porosity in bone as well as
tissue mineralization. Based on samples of femoral cortical bone from men, tissue
mineral density becomes less uniform with age, changing from predominantly low
density (*2.0 g/cc) in young adult (20-25 year old) to an increased fraction of
high density (2.2-2.3 g/cc) in elderly (80-85 years old) [
111
]. Age-dependent
increases in average mineralization of bone were also noted in cancellous bone
samples from women and men (18-96 years) [
112
]. Moreover, a backscattered
electron imaging study showed that the portion of hypermineralized tissue
increases significantly with age (in men and women), although the change was
greater in femoral neck and intertrochanteric cortices than in the femoral diaphysis
[
105
]. Consistent with the above studies, quantitative microradiography indicated
greater heterogeneity of mineralization with increasing age, although only in
women [
100
]. On the other hand, this same study reported that the average degree
of mineralization of femoral cortical bone decreased with age in women and did
not change in men [
100
]. Another study reported no age-related increase in cal-
cium concentration of human femoral bone (male and female donors). Taken
together, the data indicate no strong effect of age on the average mineralization of
cortical bone, although there appears to be greater heterogeneity with age.
Mineralization of bone is not uniform within the tissue and reflects a dynamic
process. Raman microscopy analysis of cortical bone samples from male donors
ranging between 17 and 73 years old demonstrated that the compartments of
primary lamellar bone (i.e., not replaced through remodeling process) grow older
through continuous maturation and growth of mineral crystals that may persist as
long as two decades [
113
]. Bone remodeling can remove such ''elderly'' tissues,
thus impeding the tissue aging process and maintaining the average mineralization
of tissue approximately constant over time. However, reduced bone remodeling
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