Biomedical Engineering Reference
In-Depth Information
of the teeth only. Enamel is highly mineralized and acellular, so it is
not a living tissue. Nevertheless, it is sufficiently porous for diffusion
and chemical reactions to occur within its structure, particularly
acidic dissolution (dental caries) and remineralization from saliva
(possible healing of caries lesions). Enamel is the hardest substance
in the body [568] and forms a solid, tough and wear-resistant surface
for malaxation. In the mature state, it contains up to 98% of inorganic
phase (Table 1.2). The crystals of biological apatite of enamel are
much larger as evidenced by higher crystallinity (reflecting greater
crystal size and perfection) demonstrated in their X-ray diffraction
patterns, than those of bone and dentine. Besides, enamel apatite
has fewer ionic substitutions than bone or dentine mineral and more
closely approximates the stoichiometric HA [576]. The organic phase
of enamel does not contain collagen. Instead, enamel has two unique
classes of proteins called amelogenins and enamelins. While the role
of these proteins is not fully understood yet, it is believed that both
classes of proteins aid in the enamel development by serving as a
framework support [676, 677, 686]. The large amount of minerals in
enamel accounts not only for its strength but also for its brittleness.
Dentine, which is less mineralized and less brittle, compensates for
enamel and is necessary as a support [676, 677]. Shark enameloid is
an intermediate form bridging enamel and dentine. It has enamel-
like crystals of fluoridated biological apatite associated with collagen
fibrils [82, 452-457]. Due to the presence of fluorides, biological
apatite of shark enameloid shows both higher crystal sizes and a
more regular hexagonal symmetry if compared to non-fluoridated
biological apatite of bones and teeth [107]. Similar correlation
between the presence of fluorides and crystal dimensions was found
for enamel [687].
Like that for bones, seven levels of structural hierarchy have
been also discovered in human enamel; moreover, the analysis of the
enamel and bone hierarchical structure suggests similarities of the
scale distribution at each level [533, 585, 688]. On the mesoscale
level, there are three main structural components: a rod, an interrod
and aprismatic enamel. Among them, the enamel rod (formerly called
an enamel prism) is the basic unit of enamel. It is a tightly packed
mass of biological apatite in an organized pattern. Each rod traverses
uninterrupted through the thickness of enamel. They number 5 to
12 million rods per crown. The rods increase in diameter (4 up to
8 microns) as they flare outward from the dentine-enamel junction
Search WWH ::
Custom Search