Biomedical Engineering Reference
In-Depth Information
4.4
Wear Behavior of Human Tooth Enamel
in Citric Acid Solution
In this section, we report on our investigation of the wear behavior of human tooth
enamel against titanium alloy in a citric acid solution (pH = 3.20) [
38
]. The wear test
in the artifi cial saliva (pH = 7.00) was used as a control. The main objective was to
understand the wear mechanism of human teeth in an acidic oral environment.
Human tooth enamel specimens were prepared from freshly extracted human
mandibular second permanent molars (M
2
) without caries, which were from indi-
viduals of either gender who were between 18 and 25 years of age. The counterpart
was a titanium alloy (TC4) ball with a hardness of 350 (HV) and having a diameter
of 40 mm. In vitro sliding wear tests with bath lubrication were conducted in a ball-
on- fl at confi guration using a reciprocating horizontal wear machine (Fig.
3.1
)
.
A reciprocating amplitude of 500
m and a frequency of 2 Hz were used for all
tests. Tests up to 2,000 cycles were conducted with an interval of 3 min at every 200
cycles. The normal force was discharged during the period of intermittence. The
total testing time was about 45 min for each specimen. Wear tests were done in two
media, 0.001 M citric acid solution (pH = 3.20) and artifi cial saliva (its composition
is listed in Table
3.1
,
and it was neutralized to pH = 7.00), respectively. Three nor-
mal loads of 10, 20, and 40 N were used.
In general, a low pH value can only stay for a few minutes in the mouth when
people drink an acidic beverage [
39
,
40
], but it can stay longer when chewing acidic
food directly. Tooth wear caused by erosion has been widely reported to be closely
associated with the type of acid [
21
,
41
-
43
]. Citric acid is a common ingredient in
beverages and foods, and its potential
to
erode dental hard tissues is a growing
health concern for dental personnel [
43
]. Therefore, a concentration of 0.001 M
citric acid, a level commonly found in fruit juice drinks, was used as an acidic
medium in this section.
Wear scars on the enamel surfaces were fi rst examined by LCSM; the micro-
graphs of typical worn surfaces are shown in Fig.
4.8
. Distinct differences in the
morphology of the worn surface in different wear media and under different normal
loads existed. Under a low load (10 N), obvious delamination traces appeared on the
worn surface in the citric acid solution. On the worn surface in the artifi cial saliva,
shallow ploughing and local delamination pits were observed. With the load increas-
ing to 20 N, delamination traces were dominant on the wear scar surfaces in two
wear media, but more severe delamination appeared in the citric acid solution. With
the further increase in normal load, signifi cant fl ake delamination happened to the
worn surface of enamel in the citric acid solution. A similar morphology of the worn
surface was observed in the artifi cial saliva.
Typical wear profi les (near the center of the wear scar and perpendicular to the
sliding direction) of each group are shown in Fig.
4.9
. Before measuring, we fi rst
immersed the worn surfaces in distilled water for 7 days, and then they were cleaned
with alcohol. We found that under the same loading level, the area of the wear scar
was bigger in the citric acid solution than in the artifi cial saliva, especially at low
μ
