Ian Wark Research Institute
ARC Special Research Centre
ARC Special Research Centre
Research Area: Materials science
Supervisors: Mr
Colin Hall and Dr Sarbin Ranjitkar (University of Adelaide)
Description: Tooth wear is a complex process that involves
attrition (tooth-to-tooth contact), abrasion (contact between teeth and
food or other abrasive objects or substances) and erosion (non-bacterial
chemical dissolution), or a combination of these factors. There is an
increasing awareness in clinical dentistry of the need to better
understand the aetiology and management of tooth wear as increasing
numbers of elderly patients are retaining their natural teeth to a stage
where they present with extensive tooth wear. A significant number of
younger patients are also presenting with wear of both deciduous (baby)
and permanent (adult) teeth.1
Understanding of the characteristics of tooth wear with various loads
and lubricants is primarily based on in vitro studies, given the
limitations of existing methods (for example, hardness measurement,
profilometry and scanning electron microscopy) to assess tooth wear in
vivo. Although the majority of tooth wear studies have focussed on
dental erosion, we have investigated the simultaneous effects of erosion
and mechanical wear, providing insights into the complex nature of wear
in human enamel,2 between dentine specimens3 and between enamel and
dentine specimens.4 In this context, widely-used methods for the
assessment of tooth wear lack precision to be able to detect early
changes. However, nanotechnology offers an opportunity to overcome this
problem and has been used to detect early changes in mechanical
properties of eroded enamel (such as hardness and elastic modulus).5,6
Our research group has also established an erosion model to detect
erosive demineralization of enamel as well as remineralization of eroded
enamel. Our preliminary findings on five specimens indicate that
baseline nanohardness of enamel (4.70 ± 0.37 GPa, mean ± SD) decreases
after erosion in white wine (at pH 3.5) (3.49 ± 0.28 GPa), but that some
recovery occurs following treatment with 1000ppm fluoride toothpaste
(4.06 ± 0.53 GPa).
As part of ongoing research, we have identified two priority areas on
tooth wear that will (i) elucidate wear mechanism associated with early
changes in enamel, dentine and restorative materials under different
conditions, and (ii) provide an insight into the role of lubrication and
remineralization in tooth wear prevention.
The aim of the current project is to investigate the wear
characteristics of enamel using scratch tests under erosive conditions
at pH 1.2 (simulating an initial stage of gastric regurgitation), pH 3.0
(simulating gastric erosion after some pH recovery) and pH 7.0
(simulating a neutral environment). These findings will improve our
understanding of enamel wear in individuals, who are prone to wear by
simultaneous occurrences of tooth grinding and gastric regurgitation
during sleep. Furthermore, these findings will assist in the development
of a tribological model of tooth wear and will provide further
foundation for the development of strategies to prevent tooth wear
Materials and methods: Thirty intact, human third molar teeth
with no obvious defects will be selected from a pool of extracted teeth
obtained as part of routine dental treatment in South Australia. Only
enamel and dentine will be preserved after discarding pulpal and
periodontal tissues. The Committee for the Ethics of Experimentation on
Humans of the University of Adelaide has granted an exemption to use
de-identified extracted teeth without obtaining patient consent for the
purpose of this study (H/27/90). Buccal sections of these teeth will be
cut and embedded in epoxy resin, followed by polishing flat to a level
of 0.25μm.
Enamel specimens will be randomly allocated into four groups, with each
group containing equal number of specimens (n = 10). A baseline scratch
test will be conducted on all specimens using a spherical tip (with a
20μm radius) in a nano-based indentation system (Ultra-micro Indentation
System, UMIS-2000, CSIRO, Australia), which has been extensively used to
study the mechanical properties of tooth structure by Swain and
colleagues.7-10 Scratches will be placed on enamel surfaces under
various loads from 0.1 to 200mN in 20 linear increments, and the maximum
force will be held for 30 seconds. The distance between two separate
indentations will be no less than 40 microns to avoid the influence of
residual stress from adjacent indentations.7 Profilometric recordings
will then be taken at two loads of 0.1mN and 0.5mN, which will provide
information on the coefficient of friction and surface roughness. As
high load scratches will probe the specimens scratch hardness, a
post-scratch at 0.5mN maximum force will be performed in the same stage
position in order to record the permanent deformation of samples.
After baseline measurements, all specimens will be treated with
artificial saliva for two hours to simulate intra-oral environment.7
Specimens will then be gently blot-dried using paper towels before being
subjected to erosion for two minutes in hydrochloric acid (HCl) at pH
1.2 (group 1), in HCl at 3.0 (group 2) and in deionized water with pH
adjusted to pH 7.0 (group 3). The specimens will then be gently rinsed
off with water followed by blot drying with paper towels. A second set
of scratch tests will be conducted, and the differences in values
related to the coefficient of friction, surface roughness and permanent
deformation of enamel from the baseline will be calculated and compared
between the groups.
Statistical analysis: A power study has indicated that the
proposed sample size is adequate to detect mean difference in scratch
depth and roughness values with an effect size (referring to a ratio of
expected mean differences/standard deviation) of 1.5 between different
groups (alpha = 0.05, power = 90%). Linear mixed modelling will be used
to determine whether there are significant differences in scratch depths
and surface roughness values between groups. Statistical significance
will be set at the 0.05 probability level.
References
1. Lussi A. Erosive tooth wear - a multifactorial condition of growing
concern and increasing knowledge. Monogr Oral Sci 2006;20:1-8.
2. Kaidonis JA, Richards LC, Townsend GC, Tansley GD. Wear of human
enamel: a quantitative in vitro assessment. J Dent Res 1998;77:1983-90.
3. Burak N, Kaidonis JA, Richards LC, Townsend GC. Experimental studies
of human dentine wear. Arch Oral Biol 1999;44:885-7.
4. Ranjitkar S, Kaidonis JA, Townsend GC, Vu AM, Richards LC. An in
vitro assessment of the effect of load and pH on wear between opposing
enamel and dentine surfaces. Arch Oral Biol 2008;53:1011-6.
5. Mahoney E, Beattie J, Swain M, Kilpatrick N. Preliminary in vitro
assessment of erosive potential using the ultra-micro-indentation
system. Caries Res 2003;37:218-24.
6. Barbour ME, Parker DM, Jandt KD. Enamel dissolution as a function of
solution degree of saturation with respect to hydroxyapatite: a
nanoindentation study. J Colloid Interface Sci 2003;265:9-14.
7. He LH, Fusisawa N, Swain MV. Elastic modulus and stress-strain
response of human enamel by nano-indentation. Biomaterials 2006; 27:
4388-98.
8. Mahoney E, Holt A, Swain M, Kilpatrick N. The hardness and modulus of
elasticity of primary molar teeth: an ultra-micro-indentation study. J
Dent 2000;28:589-94.
9. Mahoney E, Beattie J, Swain M, Kilpatrick N. Preliminary in vitro
assessment of erosive potential using the ultra-micro-indentation
system. Caries Res 2003;37:218-24.
10. He LH, Swain MV. Influence of environment on the mechanical
behaviour of mature human enamel. Biomaterials 2007;28:4512-20.
