Biomedical Engineering Reference
In-Depth Information
mineralization of the matrix, and accordingly the mean tissue age, is mostly
determined by the rate of remodeling. Unfortunately, there is limited insight on
how the rate of local remodeling changes with age, especially via formal dynamic
bone histomorphometry. The most reliable information was reported by Harold
Frost (refer to
Sect. 3
for a more detailed discussion) using human ribs which
showed that the remodeling rate declines between birth and the fourth decade, then
increases up to sixth decade and decreases again thereafter [
68
]. Rehman et al.
characterized the activation frequency from histomorphometry in iliac crest bone
(including the cortical envelope), where they found an insignificant increasing
trend with age in both males and females [
125
]. Other studies which analyzed
remodeling related parameters (osteoid surfaces, mineralizing surfaces, bone
formation and resorption rates), mostly reported increase in bone resorption,
especially for females, and a decrease in formation indices [
42
,
47
]. Apart from the
ribs and iliac crest, remodeling rate is not well-investigated other locations, par-
ticularly fracture prone regions such as the femoral neck and the vertebral bodies.
There is no consensus on the progression of the mean DOM with age. Some
reported a decline in the amount of primary lamellar bone and the overall minerali-
zation with age [
34
,
75
,
147
,
158
]. It has been proposed that accumulation of partially
mineralized osteons due to diminished rate of mineralization might result in regions of
less mineralization in elderly [
27
,
99
,
117
]. In contrast, others reported an overall
increase in mineralization in elderly, at least until fifth to seventh decades, followed by
a shallow increase [
51
,
79
,
113
,
132
,
160
]. While the design of these studies do not
include quantification of remodeling dynamics, increasing mineralization with age is
attributed to slower remodeling which in turn provides longer time for mineral growth
and increased mineralization of the osteoid. It must be stated that contradictions on
age-related trends in DOM may be a simple matter of statistical power. Probably the
most comprehensive sample size in investigating DOM changes with age was
included in a study by McCalden et al. who observed no change in mineralization with
age in the femurs of males and females [
106
]. This is further supported by Yeni et al.,
who also found no change in mineralization in individuals aged 60 years and above,
however tibias in the same study had shown lower mineralization with age [
158
].
Therefore, the third group of studies imply a homeostasis of mean tissue minerali-
zation with age in which the new bone formation balance against the aging old
compartments, maintaining a constant mean tissue age after the third decade [
5
].
Techniques such as density fractionation, microradiography and electron
backscattering imaging allow for inferring the distribution of mineralization, i.e.,
assessment of high and low mineralization fractions [
18
,
25
,
126
,
132
,
140
,
150
].
High density bone was reported to increase with age at the expense of low density
bone for which the researchers provided diverse interpretations that ranged from
reduced remodeling at specific sites [
124
,
126
] to densely calcified fibrocartilage at
ligament attachment sites [
140
] and lack of lamellar regions at sites possibly
caused by the formation of woven bone [
25
] and/or fracture callus due to acci-
dental overloads [
36
,
101
].
Mean mineralization does not reflect the heterogeneity of bone's composition.
For instance, a set of individuals may have the same mean mineralization, yet the
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