Biomedical Engineering Reference
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replaced by a carbonate ion (CO
3
2-
), it is called as type-B carbonate substitution,
which is the most abundant carbonate species in bone. Carbonate ions occupying
hydroxide ion (OH
-
) sites are designated as type-A substitution. Carbonate
ions loosely bound on the crystal surface are called labile carbonate [
16
,
120
,
122
,
127
-
129
,
156
]. Sometimes PO
4
3-
ions are substituted by acid phosphate ions
(HPO
4
2-
), which are found in a labile non-apatitic environment as well [
19
,
22
]. The
mechanisms by which these substitutions occur are not well known. Type-A carbonate
substitution, when attempted synthetically, required high temperature (*1000C),
whereas synthetic type-B carbonation occured within a temperature range of
50-100C[
156
]. Therefore, carbonate ion substitutions do not seem to be favored
thermodynamically and may be driven biologically via proteins. In support of this
concept, Dziak and Akkus demonstrated that carbonation of in vitro crystals grown in
cell cultures can be modified by charged polypeptides; therefore, non-collagenous
proteins may be involved in carbonate substitution [
57
].
Carbonated apatites have a significant role in cellular metabolic activities, such
as, when there is an increase in systemic metabolic acidosis due to declined renal
function or other age-related factors. It was hypothesized that systemic acidity
increases bone resorption, releasing carbonate (CO
3
2-
) and hydrogen phosphate
(HPO
3
2-
) ions to buffer serum pH [
6
,
37
,
38
,
65
]. Other than the metabolic
activities, carbonate ions, particularly type B substitution, distorts the shape of
mineral crystallites [
54
,
120
] inducing microstrains. The changes were such that
crystallite size, inferred from crystallinity parameter, were reported to decrease
along a-axis and increase along c-axis [
81
,
93
-
95
].
Only a few studies have investigated age-related changes in type-B carbonation
of human bone. Akkus et al. used Raman spectroscopy on regions of bone that
survived remodeling for decades. Such isolated locations displayed significantly
less type-B carbonation (measured as a ratio of carbonate to phosphate peak)
compared to younger bone tissue. However, after normalization of the carbonate
peak by the amide (collagen's backbone conformation) peak, they found no
change in type-B carbonate, which led them to conclude that the reduction in type-
B carbonate to phosphate ratio was due to an increase in phosphate content over
time rather than a decline in carbonate substitution [
5
]. Later, a study by
Yerramshetty et al. on human femoral cortical bone revealed a significant increase
with age in mean carbonate to phosphate ratio among an older population
(52-85 years) [
160
]. Additionally, a significant reduction was reported in the
standard deviation of carbonation with age in the medial quadrant of femoral
cortex, and the overall distribution skewed towards higher levels of carbonation.
Higher levels and reduced variation of carbonation imply more immature
crystallites in aged individuals, as carbonation generally reduces with mineral
maturity and perfection. Relative amounts of labile carbonate, measured using
density fractionation technique, obtained from the bone of a single individual
increased with density suggesting that labile carbonates are principally formed
during later stages of mineralization. Similar results were also observed in syn-
thetic apatites [
128
]. Handschin et al. employed chemical analysis to monitor age
related changes in the total carbonate content [
30
,
81
] in human iliac crest samples
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