Biomedical Engineering Reference
In-Depth Information
further transformed into platelets of crystalline apatite within the
collagen matrix [654].
Furthermore, to investigate how apatite crystals form inside
collagen fibrils, researchers carried out a time-resolved study
starting from the earliest stages of mineral formation [655]. After
24 h of mineralization calcium orthophosphate particles were
found outside the fibril, associated with the overlap region, in
close proximity to the gap zone. Cryogenic energy-dispersive X-ray
spectroscopy confirmed that these precipitates were composed of
calcium orthophosphate, while a low-dose selected-area electron
diffraction technique showed a diffuse band characteristic of ACP.
After 48 h, apatite crystals started to develop within a bed of ACP and
after 72 h, elongated electron-dense crystals were abundant within
the fibril, in many cases still embedded within a less dense matrix. A
low-dose selected-area electron diffraction technique demonstrated
that the mineral phase consisted of both ACP and oriented apatite, the
latter identical to bone apatite [655]. This process is schematically
shown in Fig. 1.12 [656]. The modern points of view on the bone
formation mechanisms have been summarized in a recent excellent
review [575], where the interested readers are referred.
The maturation mechanism of bone minerals is not well
established, mainly because of the difficulty involved in the
nanostructural analyses of bone minerals [575, 657]. Only indirect
evidences for the
bone mineral maturation are available. For
example, X-ray diffraction patterns of bones from animals of different
age show that the reflections become sharper with age increasing [98,
658]. This effect is more pronounced in the crystallographic
in vivo
a
-axis
[(310) reflections] as compared to the
-axis [(002) reflections] [659,
660]. The most comprehensive report describing how normal human
bone mineral changes in composition and crystal size as a function
of age was based on X-ray diffraction analyses by Hanschin and Stern
[661], who examined 117 homogenized iliac crest biopsies from
patients aged 0-95 years. They found that the bone mineral crystal
size and perfection increased during the first 25-30 years and then
decreased thereafter, slightly increasing in the oldest individuals.
The same 117 homogenized biopsy samples were analyzed by
wavelength-dispersive X-ray fluorescence to quantify the carbonate
substitution in biological apatite as a function of age. Although the
changes observed in carbonate substitution were relatively slight (at
most 10%), there was a general increase from 0 to 90 years that is
c
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