Restoration of Tooth Enamel Using a Flexible Hydroxyapatite

Bioceramics Development
and Applications
Yamamoto et al., Bioceram Dev Appl 2013, S:1
http://dx.doi.org/10.4172/2090-5025.S1-006
Research Article
Open Access
Restoration of Tooth Enamel Using a Flexible Hydroxyapatite Sheet
Coated with Tricalcium Phosphate
Ei Yamamoto1*, Nobuhiro Kato1, Arata Isai1, Hiroaki Nishikawa1, Masanobu Kusunoki1, Kazushi Yoshikawa2 and Shigeki Hontsu1
1
2
Department of Biomedical Engineering, Faculty of Biology-Oriented Science and Technology, Kinki University, Kinokawa, Wakayama 649-6493, Japan
Department of Operative Dentistry, Osaka Dental University, Hirakata, Osaka 573-1121, Japan
Abstract
Restoration and protection of tooth enamel are of great importance in operative and conservative dentistry. Using
a Pulsed Laser Deposition (PLD) method, we have successfully created a freestanding flexible double-layered sheet
composed of a Hydroxyapatite (HAp) layer (4 m in thickness) coated with a Tricalcium Phosphate (TCP) layer (500 nm).
In order to apply the newly developed HAp/TCP sheet to the restoration and protection of tooth enamel, the adhesive
characteristics of the HAp/TCP sheet on the enamel were evaluated mechanically and microstructurally in the present
study. The adhesive strength (5.7 MPa) between the HAp/TCP sheet and enamel was markedly higher than that (1.9
MPa) between the mono-layered HAp sheet and enamel. The electron microscopic observation revealed that HAp/TCP
sheet was widely fused with the enamel. Therefore, the double-layered HAp/TCP sheet can be used as a material to
promote the repair of tooth eruption and to maintain healthy dentine.
Keywords: Hydroxyapatite sheet; Tricalcium phosphate; Pulsed
laser deposition; Artificial enamel; Tooth restoration
Introduction
The primary function of teeth is to prepare food for digestion by
mastication. Therefore, dental integrity is significantly important for
overall health. For a long while, dental materials have been essential
components of the effective treatments for dental diseases. As the
demand for healthy teeth increases, new materials in restorative
dentistry have been advanced progressively. Metals, alloys, composite,
and ceramics have been extensively used for tooth restoration and
protection [1]. However, most dental materials need to be replaced
due to the time-related deterioration. More adequate therapeutic
methods by means of useful biomaterials are strongly required in
restorative dentistry. Dental enamel is a rigid and hard substance that
covers the tooth crown and protects the tooth from decay. Harmful
chemicals and temperatures are insulated by the layer of the enamel. In
contrast with bone fractures that can be naturally repaired in human
body in most cases, the extreme damages of tooth enamel are hardly
restored or regenerated. The enamel has no living cells, and therefore
its healing capabilities are inferior to the other biological tissues. The
cause of dental hyperesthesia is loss of the enamel on tooth crowns. In
the hyperesthetic condition, a sharp pain is generated from exposed
tooth dentine in response to stimuli. A well-accepted mechanism for
the hyperesthesia is the hydrodynamic theory which is related to a
rapid outward flow of fluid contained in dentinal tubules. This fluid
movement in the tubules creates a pressure change that stimulates
pulpal nerves and results in pain. Dental hyperesthesia is a common
disease experienced in clinical dentistry. To cure a hyperesthetic
tooth, exposed dentinal tubules were covered by organic compounds
in order to achieve an immediate nerve blockage of pain-producing
stimuli. Previous clinical studies [2,3] demonstrated the effectiveness
of the covering of resin materials in the short term management of
the hyperesthesia. However, it is difficult to intrinsically regenerate
tooth enamel by attaching the resin cement. This cement is frequently
detached from tooth surface. In addition, the resin materials contain
a substance that causes an allergic response. To overcome such
disadvantages, the development of novel materials is important for a
new strategy of pain management in hyperesthesia. Hydroxyapatite
(HAp) is one of the most attractive materials for the reconstructing of
biological hard tissues, because its chemical composition is resemble to
Bioceram Dev Appl
that of natural bones and teeth in mammals. This resemblance leads to
an excellent and bioactivity and biocompatibility. Therefore, synthetic
HAp in forms such as powders, beads, or blocks is extensively used for
various biomedical applications [4,5]. In dentistry, metallic implants
are often coated with HAp to alter the surface properties. Furthermore,
tooth defects or cavities are filled with HAp materials. However, it
is impossible to repair tooth enamel by attaching Hap powders or
particles. The surface quality of tooth inhibits to unify the materials
into the enamel. Thus, to date, there have been no efficient biomaterials
for the treatment of hyperesthesia.
In our previous study, a freestanding Hap sheet has successfully
developed by a pulsed laser deposition (PLD) method and a film isolation
technique [6]. Then, we have been doing a series of experimental
studies on the development and application of HAp ultrathin films
and sheets [7-9]. In these previous studies, the HAp sheet was directly
attached to the surface of human tooth enamel. Biomechanical and
microstructural evaluations showed that the sheet and enamel were
not adhered completely at the interface between them. Thus, the
enhancement of adhesiveness of the HAp sheet is essential for the
initial mechanical integrity as well as the long-term remineralization
at the sheet-enamel interface. The repair of tooth enamel is of great
importance, in particular for the development of a new treatment for
hyperesthesia in dentistry. Using a pulsed laser deposition method
and thin film isolation techniques, we have successfully created a
freestanding flexible double-layered sheet composed of a HAp layer
coated with a tricalcium phosphate (TCP) layer. Our final goal was
to apply the newly developed double-layered HAp/TCP sheet to the
restoration and protection of tooth enamel. In the present study,
*Corresponding author: Ei Yamamoto, Department of Biomedical Engineering,
Faculty of Biology-Oriented Science and Technology, Kinki University, Kinokawa,
Wakayama 649-6493, Japan, E-mail: ei@waka.kindai.ac.jp
Received June 08, 2013; Accepted July 23, 2013; Published August 23, 2013
Citation: Yamamoto E, Kato N, Isai A, Nishikawa H, Kusunoki M, et al. (2013)
Restoration of Tooth Enamel Using a Flexible Hydroxyapatite Sheet Coated with
Tricalcium Phosphate. Bioceram Dev Appl S1: 006. doi: 10.4172/2090-5025.S1-006
Copyright: © 2013 Yamamoto E, et al. This is an open-access article distributed
under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the
original author and source are credited.
Conferences Proceedings: ISACB-6
ISSN: 2090-5025 BDA, an open access journal
Citation: Yamamoto E, Kato N, Isai A, Nishikawa H, Kusunoki M, et al. (2013) Restoration of Tooth Enamel Using a Flexible Hydroxyapatite Sheet
Coated with Tricalcium Phosphate. Bioceram Dev Appl S1: 006. doi: 10.4172/2090-5025.S1-006
Page 2 of 4
the adhesive properties of the HAp/TCP sheet on the enamel were
evaluated crystallographically, mechanically, and microstructurally,
and were compared to those of the mono-layered HAp sheet.
Materials and Methods
Preparation of HAp and HAp/TCP Sheets
The freestanding hydroxyapatite sheet was obtained by a thin film
fabrication process using the PLD method and the thin film isolation
technique reported in our previous paper [9]. In the present study,
a HAp sheet coated with TCP thin layer and a mono-layered HAp
sheet were produced by the PLD method using a KrF excimer laser.
The TCP layer was used as adhesion promoter in order to reduce the
period required to fuse with tooth enamel. First, the HAp sheet of 4
m in thickness was obtained in the same conditions as that reported
previously [9]. The sheet was crystallized by post-annealing at 800°C
for ten hours. Then, a TCP thin layer of 500 nm in thickness was
deposited on the HAp sheet under an oxygen gas atmosphere at 0.1
Pa at room temperature. Figure 1 shows the obtained HAp/TCP sheet.
Adhesion of HAp and HAp/TCP sheets on tooth enamel
mm/min until failure occurs. The tooth specimens attached the HAp
and HAp/TCP sheets were embedded in a polymethylmethacrylate
resin. Then, the specimens were polished with water-resistant sand
papers until the interface between the sheet and enamel was exposed.
Observation of the interface structure was performed by a low vacuum
scanning electron microscope (Miniscope TM-1000, Hitachi HighTechnology Co.).
Results and Discussion
The phase and crystallographic features analyzed by XDR
demonstrated that the Hap sheet was highly crystalized (Figure 4).
Moreover, the remarkable peaks of calcium oxide were observed in the
XRD pattern of the sheet. There were high intensities of the (002) and
(004) peaks in the XRD pattern of tooth enamel (Figure 5), indicative
of the c-axis orientation. The XRD patterns at 5 days after attachment
are given in Figures 6 and 7 separately for HAp and HAp/TCP sheets,
respectively. There were tendencies to moderate the peaks of calcium
oxide in the both sheets at this point. These results suggest that the HAp
and HAp/TCP sheets were structurally unified in part to tooth enamel
within a few days.
An extracted human tooth was sectioned in the tooth root crown
area, and the enamel was exposed. After the section was polished with
water-resistant sand papers, we adhered the HAp and HAp/TCP sheets
to the enamel surface using a calcium phosphate aqueous solution
with a pH of 5.5. After drying the sheet, artificial saliva (Saliveht, Teijin
Pharma Co.) was sprayed on it. This process was repeated every day.
The HAp and HAp/TCP sheets on the enamel surface are shown in
Figure 2.
Crystallographic,
mechanical,
and
microstructural
characterization of HAp and HAp/TCP sheets attached to
tooth enamel
Figure 2: HAp (left) and HAp/TCP (right) sheets on tooth enamel at 5 days
after attachment.
The crystallinities of the HAp and HAp/TCP sheets attached to tooth
enamel were evaluated using an X-ray diffraction (XRD) instrument
(Ultima IV, Rigaku Co.) at 5 days after attachment. The XRD analyses
were also conducted for the HAp sheet itself and tooth enamel before
attachment. At 5 days after attachment, quasi-static tensile tests were
carried out to quantify the adhesive strength between the sheet and
tooth enamel. A stainless steel rod of a diameter of 3 mm was glued
on the sheet using epoxy adhesive. The rod was attached to the jig of a
tensile tester (EZ-test, Shimadzu Co.) having a universal joint (Figure
3). Tensile load was applied to the specimen at the tensile rate of 0.5
Figure 3: Tensile test of HAp and HAp/TCP sheets adhered to tooth enamel.
Figure 1: Freestanding HAp/TCP sheet.
Bioceram Dev Appl
Figure 4: XRD pattern of HAp sheet before attachment to tooth enamel.
Conferences Proceedings: ISACB-6
ISSN: 2090-5025 BDA, an open access journal
Citation: Yamamoto E, Kato N, Isai A, Nishikawa H, Kusunoki M, et al. (2013) Restoration of Tooth Enamel Using a Flexible Hydroxyapatite Sheet
Coated with Tricalcium Phosphate. Bioceram Dev Appl S1: 006. doi: 10.4172/2090-5025.S1-006
Page 3 of 4
Figure 5: XRD pattern of tooth enamel.
Figure 8: Stress-displacement curves of the HAp and HAp/TCP sheets
adhered to tooth enamel at 5 days after attachment.
Figure 9: SEM micrographs of the interface between the sheet and tooth
enamel (left: Hap sheet, right: HAp/TCP sheet).
Figure 6: XRD pattern of HAp sheet on tooth enamel at 5 days after
attachment.
Figure 7: XRD pattern of HAp/TCP sheet on tooth enamel at 5 days after
attachment.
Figure 8 shows the stress-displacement relation of the HAp and
HAp/TCP sheets adhered to tooth enamel. The adhesive strength
between the HAp/TCP sheet and tooth enamel and that between the
HAp sheet and the enamel were 5.7 MPa and 1.9 MPa, respectively.
A previous report has shown that the bonding strength between HAp
coated titanium and bone tissues is approximately 3.5 MPa [10]. This
value is comparable to the adhesive strength between the sheet and
enamel in the present study. Therefore, we can say that the HAp and
HAp/TCP sheets are attached tightly to tooth enamel. In a previous
study [11], the adhesive strength of HAp films deposited on titanium
alloy can be improved by increasing the roughness of the underlying
substrate surface. Therefore, the tooth surface morphology may be
important for the adhesive strength of the HAp and HAp/TCP sheets
attached to the enamel. The effects of tooth surface morphology on the
adhesive strength should be investigated to improve the mechanical
integrity of the sheets for dental applications. The adhesive strength
of the HAp/TCP sheet was markedly higher than that of the HAp sheet
Bioceram Dev Appl
(Figure 8). This result indicates that the thin TCP layer on the HAp
sheet is efficacious in order to increase the adhesive strength in a short
bonding duration. Figure 9 shows scanning electron micrographs of the
interface structure between the sheet and enamel. There were adhesion
layers at the interface for both Hap and HAp sheets. It is likely that the
adhesion layers have important roles in the adhesive properties of the
sheets. In addition, the adhesion layer of the HAp/TCP sheet was wider
than that of the HAp sheet, indicative of the strong adhesion at the
interface between the HAp/TCP sheet and enamel. Previous reports
[12-14] demonstrated that TCP possesses the high degradability in
biological environment. Therefore, TCP may provide an ideal condition
for the repair of tooth enamel. Biochemical and ultrastructural studies
should be conducted to know the mechanisms of the remineralization
process at the adhesion layer of the HAp/TCP sheet attached to tooth
enamel.
Conclusion
We newly developed a flexible HAp sheet coated with TCP using a
pulsed laser deposition method. The ultrathin layer of TCP on the Hap
sheet has an important role in the mechanical integrity at the sheetenamel interface. The double-layered HAp/TCP sheet can be used as
a material to promote the repair of tooth eruption and to maintain
healthy dentine.
Acknowledgement
This work was partially supported by the Ministry of Education, Culture, Sports,
Science, and Technology, Japan - Supported Program for the Strategic Research
Foundation at Private University (S0801074).
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Conferences Proceedings: ISACB-6
ISSN: 2090-5025 BDA, an open access journal
Citation: Yamamoto E, Kato N, Isai A, Nishikawa H, Kusunoki M, et al. (2013) Restoration of Tooth Enamel Using a Flexible Hydroxyapatite Sheet
Coated with Tricalcium Phosphate. Bioceram Dev Appl S1: 006. doi: 10.4172/2090-5025.S1-006
Page 4 of 4
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Citation: Yamamoto E, Kato N, Isai A, Nishikawa H, Kusunoki M, et al. (2013)
Restoration of Tooth Enamel Using a Flexible Hydroxyapatite Sheet Coated
with Tricalcium Phosphate. Bioceram Dev Appl S1: 006. doi: 10.4172/2090-5025.
S1-006
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