Biomedical Engineering Reference
In-Depth Information
wear mechanism of enamel was dominant by brittle delamination. Hence, both the
wear morphology and the wear loss of enamel in the two wear media were similar
at the load of 40 N.
In general, it would be expected that mechanical action in combination with acid
might cause an increase in the wear rate when the tooth wear process occurs in an
acidic solution. Shabanian et al.and Richards conducted an in vitro study to com-
pare the wear rates of human tooth enamel under different loads (3.2, 6.7, and
9.95 kg) and pH levels (1.2, 3.3, and 7.0) [
33
]. Their results showed that the wear
rates increased with the reducing pH values regardless of the load. However, more
wear was not always seen with decreasing pH in the previous studies [
4
]. An unex-
pected trend was fi rst found by Kaidonis et al. [
35
]. Their in vitro wear-testing
results showed that under the normal loads of 9.95, 13.2, 14.2, and 16.2 kg, less
enamel wear occurred with an acidic lubricant (pH = 3) than with an neutral lubri-
cant. Similar phenomena were reported by Eisenburger and Addy [
8
,
13
]. They
found that enamel surfaces appeared fl at and smooth at low pH, and thus less enamel
wear in acidic conditions was attributed to a smoothing effect of erosion on contact-
ing surfaces. Considering that the magnitude of masticatory force in the oral cavity
ranges from 3 to 36 N during the human chewing process [
47
], three normal loads
of 10, 20, and 40 N were used in this study. Compared with previous studies [
8
,
13
,
33
,
34
], the applied contact stress at the interface between the enamel and the coun-
terpart was lower in this study. Within the used normal loading range, it seems that
a competitive mechanism should exist between the mechanical action and the chem-
ical action when tooth wear occurred in an acidic medium. Under a lower normal
loading level, the surface softening of enamel caused by erosion dissolution played
a signifi cant role in its wear behavior, and thus the wear volume was much higher in
the citric acid solution (pH = 3.20) than in the artifi cial saliva (pH = 7.00). With the
load increasing, enamel wear was gradually characterized by mechanical removal
because of its inherent brittleness. As a result, the wear loss of enamel in the citric
acid solution was similar to that in the artifi cial saliva at a high normal load of 40 N.
4.5
Erosion and Remineralization of Human Tooth Enamel
In this section, we investigate the erosion behavior of human tooth enamel in vitro
using 0.001 M citric acid solution (pH = 3.20). We paid particular attention to the
effect of erosion time and location on the tribo-erosive properties of enamel.
Furthermore, we investigated the remineralization repair behavior of acid-eroded
human tooth enamel in vitro using artifi cial saliva. We gave special attention to the
repair effect of remineralization on the antiwear properties of the acid-eroded enamel.
It should be noted in this section that we examined the tribological behavior of
enamel using a nanoscratch tester. The indenter is a diamond with a radius of 2
m.
The length of each scratch was 0.3 mm. Given that the thickness of the softened
layer caused by a few minutes' erosion should be a few hundred nanometers on the
surface of enamel [
46
], the friction and wear behavior of enamel, either eroded or
μ
