Biomedical Engineering Reference
In-Depth Information
of each specimen. After microscopic examinations of the morphology and profi le,
hardness measurement, and nanoscratch testing, about 0.6 mm of the surface was
removed through a grinding and polishing process. A new surface was obtained as
a result, ready for a second erosion test. According to this method, erosion tests
were performed gradually from the outer to the interior enamel in every specimen;
see Fig.
4.23
.
The aim of this study was to investigate the depth dependence of the erosion
property of human tooth enamel at different depths in a whole tooth. Prolonged ero-
sion times were used in most studies probably to observe more signifi cant changes
in the hardness, morphology, and microstructure of human enamel. However, an
erosion time of 3 min was chosen in this work because it is comparable to the clear-
ance time of citric acid in the mouth, and then should have a better physiological
relevance than longer erosion times [
39
,
40
].
Figure
4.24
gives three-dimensional AFM micrographs of the enamel at different
depths. On the control surfaces, the enamel at different depths exhibited different
morphologies. The outer enamel appeared relatively compact (Fig.
4.24a
), while
interstices existed among enamel rods on the control surface of the intermediate
enamel (Fig.
4.24c
). For the interior enamel, we observed an obvious alternate
arrangement of rods and interrod enamel (Fig.
4.24e
). We also found that the ero-
sion morphology of the enamel differed with its depth. For the outer enamel, some
keyhole-like rods (4-8
m in diameter) loomed on the eroded surface after 3-min of
erosion (Fig.
4.24b
). The keyhole-like structure of enamel rods became increasingly
clear and enlarged on the eroded surface of the intermediate enamel (Fig.
4.24d
).
While the enamel layer was thinned continuously, we observed an obvious honey-
comb-like morphology as a result of severe dissolution of enamel rods on the eroded
surface of the interior enamel (Fig.
4.24f
). Surface profi les and erosion depths of the
enamel at different depths are shown in Fig.
4.25
. Only a little fl uctuation on the
profi le lines occurred on the eroded surface of the outer enamel in comparison with
its control surface (Fig.
4.25a
). Greater fl uctuation was observed on the eroded sur-
face of the intermediate enamel, and the eroded surface of the interior enamel
appeared remarkably uneven. Figure
4.25b
shows that the erosion depth increased
signifi cantly from the outer to the interior enamel. One-way analysis of variance
(ANOVA) revealed a signifi cant difference in the erosion depth between different
layers of the enamel (
P
< 0.005). The observations suggested that the erosion resis-
tance of enamel was depth-dependent, and it decreased from the natural surface to
the DEJ in a whole tooth.
The depth-dependent erosion resistance of human enamel may be related closely
to its microstructure. Enamel uniquely consists of aligned keyhole-like rods (4-8
μ
m
in diameter), which are embedded in interrod enamel [
10
,
32
,
51
,
57
,
58
]. Enamel is
mainly composed (above 95 wt%) of hexagonal carbonated hydroxyapatite crystal-
lites with a mean width of 68.3 nm and a mean thickness of 26.3 nm, most of which
are tightly packed in rods. The hydroxyapatite crystallites are covered by enamelin,
a nanometer-thin protein layer. The interrod enamel between rods is rich in protein.
The alternate arrangement of mineral and organic phases can endow enamel with
certain elasticity and a palliative effect against overload or impact wear during
μ
