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Tiron O.I., Todorova A.V.
The
types of enamel rods pathways and their contribution to the mechanical behavior
of tooth enamel
Human enamel is
a unique biocomposite material, whose mechanical
properties enable it to protect underlying soft dentin and pulp and survive
millions masticatory cycles without fracturing [1,2]. Being hard and crack
resistant at the same time it caused vast interest in both morphologists and
material scientists [3]. Inorganic material comprises more than 96% of the human enamel and previously this fact was believed to be the
only one which is responsible for the unique mechanical properties of this
material. It, undoubtedly, explained the phenomenon of the hardness of enamel.
However, the question regarding its crack resistance had been remaining unclear
for a long period of time. But with the development of research methods it was
estimated that the complex hierarchical structure of tooth enamel plays crucial role in its
so-called “mechanical behavior” [4].
Enamel is
considered to have six levels of organization. The second level is a level of multiple
crystals or enamel rods (prisms) and it appears to be the most important in
terms of its contribution to the mechanical properties [5]. Enamel prism is a rod-like structure
and it is known as the structural and functional unit of the human enamel. It
has a key-hole shape, consisting of head, tail and rod sheath. The head of
enamel rod is composed of ribbon-like hydroxyapatite (HAP) crystals which are
arranged roughly parallel to the rod long axis.
The rod sheaths and tails are the sites of higher organic content and,
therefore, in these areas the HAP crystals fan out from center towards the
edges. In human, unlike other species, the enamel does not exhibit the interrod
material itself, so the tails of enamel rods act as the interrod substance. The
rod sheaths and the tails of enamel rods, since they contain higher
concentration of the organic protein remnants, are responsible for stress
absorption and its equal distribution within the enamel, thereby preventing it
form fracturing [6].
Taking all the
abovementioned into account, it should be acknowledged that the patterns of
enamel rods arrangement within the human enamel do contribute to its mechanical
properties. Although enamel rod is known to extend continuously from the
dentin-enamel junction (DEJ) to the enamel surface, its length is higher than
the enamel thickness due to its irregular pathway [7].
In our previous research, on the
example of human permanent first molar tooth, we estimated that there are two
types of the enamel rod pathways occurring in human enamel – straight-lined and
arc-shaped. Moreover, the direction of
the arc peak (in case of the arc-shaped pathways) might be towards the enamel
surface or, oppositely, towards the tooth root. The combination of these three types of enamel
rods pathways differs among different surface of one tooth [8].
The principal aim of the
current research was to estimate whether the mechanical properties in different
parts of the tooth crown differ depending on the type of the enamel rod pathway
characteristic for each of them.
For this purpose the hardness of the
tooth enamel was measured at three different sites of the molar crown, each
exhibiting one of the three variants of enamel rods pathway (straight-lined,
arc-shaped directed towards the enamel surface, and arc-shaped directed towards
the tooth root).
The obtained results
show that different types of enamel rods pathways do contribute to different
hardness values and this can be used for the prediction of the mechanical
response of different parts of the tooth crown to applied force.
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and compositional characteristics : thesis. Ph.D. / . – Sydney, Australia, 2008. – 187.
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to flaws nanoscale: lessons from
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Gao, B. Ji , I.L. Jager ,
7. Áûêîâ
Â.Ë. Ãèñòîëîãèÿ è ýìáðèîëîãèÿ îðãàíîâ ïîëîñòè ðòà ÷åëîâåêà. / Â.Ë. Áûêîâ // – Ñ-Ï.: Ñïåöèàëüíàÿ ëèòåðàòóðà, 1996.
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features of enamel rods arrangement at the different surfaces of human
permanent molar teeth. / A.V. Todorova, V.