Biomedical Engineering Reference
In-Depth Information
to fracture under oral conditions; as a result, it shows a high wear rate at lower loads
but a low wear rate at higher loads. The differential wear rate between dentin and
enamel occurring in areas of exposed dentin have been hypothesized to be a cofactor
in the formation of some Class VI lesions [
32
]. Chapter
4
gives a more detailed dis-
cussion of the effect of occlusal load on the friction and wear behavior of enamel.
3.3
Process and Mechanism of Human Tooth Enamel Wear
Tooth enamel is one of those unique natural substances that still cannot be substi-
tuted for effectively by artifi cial restorative materials. The most important feature of
enamel is its excellent wear resistance. Reportedly, sound enamel under friction
from mastication and biting lost only a 10-40-
m-thick layer per year [
33
]. Mass
[
34
] pointed out that the variation in crystallite orientation of prismatic enamels
may contribute to optimal dental function through the property of differential wear
in functionally distinct regions of teeth. In this section, we report on our detailed
study on the wear behavior of human tooth enamel [
35
]. Tests lasting up to 10, 100,
1,000, 2,000, and 5,000 cycles were conducted, respectively. The main objective
was to understand the process and mechanism of enamel wear.
Flat enamel specimens were prepared using the method mentioned in Sect.
3.2
.
Only 0.2-0.3 mm of each specimen was ground and polished off to obtain a surface
similar to the original occlusal surface of enamel in the mouth. Due to its excellent
biocompatibility, pure titanium (TA2) and titanium alloy (TC4) have often been
used as dental materials in recent clinical practice. Compared with TA2 (240 HV
50g
),
the Vickers hardness of TC4 (350 HV
50g
) was closer to that of human tooth enamel
(360 HV
50g
). In terms of the evolution and stable value of the friction coeffi cient and
the worn surface morphology and wear scar depth, the enamel-titanium alloy pair
was similar to the enamel-enamel pair [
36
]. Thus, a TC4 ball with a 40-mm diam-
eter was chosen as a counterpart in this study. The composition of TC4 is C 0.043 %,
Al 6.020 %, H 0.011 %, O 0.160 %, V 4.100 %, Fe 0.168 %, and Ti the balance. The
mechanical properties and surface roughness of specimens are shown in Table
3.2
.
A normal force of 20 N, a reciprocating amplitude of 500
μ
m, and a frequency of
2 Hz were used for all the tests. The composition of artifi cial saliva is listed in
Table
3.1
.
For the wear tests lasting up to 5,000 cycles, typical variations in the friction
coeffi cient, as a function of the number of cycles, are shown in Fig.
3.9
. A lower
friction coeffi cient lasted for about 20 cycles, increased to 0.8 after 100 cycles, and
then increased slowly to a saturation value of 0.91 after 2,000 cycles.
Wear scars on the enamel surface for different cycles were fi rst examined by
laser confocal scanning microscope (LCSM), and the typical micrographs are
shown in Fig.
3.10
. Figure
3.11
gives the typical scanning electronic microscopy
(SEM) micrographs of the central area of wear scars. Energy dispersive X-ray
(EDX) spectra of wear particles on enamel surfaces subjected to different cycles are
shown in Fig.
3.12
.
μ
