Fluoride release and recharge characteristics of denture base resins ...

Dental Materials Journal 2009; 28(2): 227-233
Original Paper
Fluoride release and recharge characteristics of denture base resins
containing surface pre-reacted glass-ionomer filler
Kazuko KAMIJO 1 , Yoshiharu MUKAI 2 , Takatoshi TOMINAGA 2 , Izumi IWAYA 2 , Fukue FUJINO 3 , Yukio HIRATA 1 and
Toshio TERANAKA 2
1
Department of Dental Sociology, Division of Sociological Approach in Dentistry, Kanagawa Dental College, 82 Inaoka-cho, Yokosuka, Kanagawa
238-8580, Japan.
2
Department of Oral medicine, Division of Restorative Dentistry, Kanagawa Dental College, 82 Inaoka-cho, Yokosuka, Kanagawa 238-8580, Japan.
3
Department of Dental Hygiene, Shonan Junior College, 82 Inaoka-cho, Yokosuka, Kanagawa 238-8580, Japan
Corresponding author, Toshio Teranaka; E-mail: teranaka@kdcnet.ac.jp
The flexural strength, flexural modulus, and the amount of fluoride released from four experimental denture base resins
containing 5, 10, 20 and 30 wt% surface pre-reacted glass-ionomer (S-PRG) filler added to the powder were evaluated.
The mean flexural strength of the experimental resins, except the 30 wt%, and the flexural modulus of all the resins,
complied with ISO 1567 requirements.
In the 20 wt% resin, the amount of fluoride released in the initial phase was 1.88 μg/cm 2 /day, after which the level
decreased. After recharging in a 9,000 ppm fluoride solution for eight hours, the level of released fluoride increased
markedly to 40.21 μg/cm 2 /16hrs. Our results show that fluoride levels increased as a function of the S-PRG filler content.
After the almost completely discharged resins were recharged, similar fluoride release occurred again.
These results suggest that denture base resins containing S-PRG filler have great recharge and release capabilities
which may assist in preventing root caries of abutment teeth.
Key words: Fluoride releasing and recharging, Denture base resin, Surface pre-reacted glass-ionomer filler
Received Mar 9, 2008: Accepted Nov 1, 2008
INTRODUCTION
The proportion of geriatric patients wearing
removable partial dentures is increasing 1) . At the
same time, the prevalence of root caries accompanied
by gingival recession is increasing 2,3) . Abutment teeth
in particular are more likely to be affected by caries
and periodontal disease than any other teeth 4) .
Because abutment teeth anchoring removable partial
dentures tend to be inadequately cleaned, preventing
root caries in these teeth is crucial. A variety of
vehicles can deliver fluoride into the oral cavity,
including fluoride mouth-rinse, fluoride dentifrice,
topical fluoride, and fluoride-releasing restorative
materials, all of which effectively prevent root caries
and suppress recurrent caries 5-9) .
A recently developed resin composite containing
a filler of surface pre-reacted glass-ionomer (S-PRG)
has the valuable property of being fluoride
rechargeable. Recent studies of this resin composite
have revealed some aspects of its clinical value 9-12) .
The purpose of our study was to examine an
experimental heat-polymerizing polymethylmethacrylate
(PMMA) resin containing S-PRG filler in terms
of both the initial fluoride release and the fluoride
release after recharging with fluoride solution. We
suggest that the development of a fluoride
rechargeable denture base resin will make a
significant contribution to reducing root caries in the
abutment teeth of people who wear partial dentures.
MATERIALS AND METHODS
Materials
White S-PRG filler with average particle size of
approximately 4.1 μm (kindly supplied by Shofu Inc.
Kyoto, Japan) was added to PMMA powder (URBAN,
clear pink, Lot # 0804413, Shofu Inc.) at 5, 10, 20
and 30 % by weight and dispersed with a mixing
machine for 15 minutes. The resin polymers
containing well dispersed S-PRG filler were
polymerized with monomer according with the manufacturer’s
instructions (powder/liquid ratio of 10
g/4.5 ml) at 100°C for 10 min under pressure of 6.1
MPa and dry conditions. Using metal molds, we
prepared rectangular specimens (64×10×3.3 mm,
n=5/group) to measure flexural strength and flexural
modulus, and disk specimens (φ15×3 mm, n=8/
group) to measure fluoride released. The specimens
were polished with 1,500-grit waterproof abrasive
paper (Sankyo Rikagaku Co. Ltd., Saitama, Japan)
under dry conditions, and the polishing dust was
removed with compressed air. The accuracy of the
dimensions (width and depth) was verified with a

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Dent Mater J 2009; 28(2): 227-233
micrometer (MDC-MJ/PJ, Mitutoyo, Tokyo, Japan)
to within 1 μm at three locations in each dimension
with a 0.02 mm tolerance. As a control group, we
used the same resin without S-PRG filler (0 wt%).
Methods
Flexural strength and flexural modulus
The specimens were tested for flexural strength and
flexural modulus according to ISO 1567 13) . Before
testing, specimens were stored in deionized water
(DW) for 50 hours at 37 °C. To determine flexural
strength, five specimens were subjected to threepoint
bending tests using a universal testing machine
(AGS-1000, Shimadzu Corp., Kyoto, Japan) at a
crosshead speed of 1 mm/min. The flexural modulus
was calculated from the linear portion of the loadtime
curve up to the proportional limit obtained by
the bending test.
According to ISO 1567 13) the minimum flexural
strength of Type 1 denture base materials (heatpolymerized
polymers) should not be less than 65
MPa, and the minimum flexural modulus should not
be less than 2 GPa.
Measurement of released fluoride
Phase 1 (phase of initial fluoride release; day 1 to day
15)
Each specimen of denture base resin was placed in
3.0 ml of DW for 15 days at 37 °C. The fluoride-ion
concentration of the resulting solutions was measured
daily, except on days 5, 6, 8 and 13, and the resulting
solutions were discarded. A 0.3 ml total ionic
strength adjustment buffer III (Catalogue number
940911, Thermo Orion, Beverly, MA, USA) was
added to the solution, then a combination fluoride
electrode (Orion 9609BN, Thermo Orion) was
inserted, connected to a 720A plus fluoride-ion meter
(Thermo Orion). Fluoride concentration was
measured while the solution was stirred at room
temperature, and the amount of fluoride released per
unit surface area (cm 2 ) of the resin disk was
computed. After each measurement the resulting
solution was discarded and replaced with fresh DW.
Phase 2 (phase of fluoride release after recharging;
day16 to day 49)
At the end of phase 1, each sample was stored in 3.0
ml of sodium fluoride solution (9,000 ppm fluoride)
for eight hours. The specimens were then rinsed with
DW for five seconds prior to storing in 3.0 ml DW for
16 hours, followed by another rinse and storing in
fresh DW for a further 24 hours. This sequence was
repeated five times during the next 10 days. The
fluoride-ion concentration of each solution after the
storage period of either 16 hours or 24 hours was
measured in the same manner as in phase 1. This
procedure simulates a common pattern among
denture wearers in which they store their dentures
in a fluoride solution during periods of sleep every
other day.
Phase 3 (phase of confirmation of fluoride recharging;
day 50 to day 60)
At the end of the phase 2, the specimens of denture
base resin were immersed in DW which was
discarded and replaced every day for 24 days until
the release of fluoride had diminished to trace levels.
To confirm the fluoride recharging ability, we
immersed the specimens in 3.0 ml of 9,000 ppm
fluoride solution for eight hours to recharge. The
specimens were rinsed with DW for five seconds and
then immersed in 3.0 ml DW for 16 hours. We then
measured the fluoride-ion concentration and replaced
it with fresh DW. This procedure was repeated over
the next 10 days except on days 55 and 57, using the
same procedure as in phase 1. The amount of fluoride
released into the immersing solution was determined
using the same procedure as in phases 1 and 2.
STATISTICAL ANALYSIS
There were five specimens for the flexural strength
and flexural modulus determination, and eight
specimens for the measurement of fluoride concentration
in each of five groups. We statistically analyzed
fluoride release with two-way repeated-measures
ANOVA, followed by Tukey’s HSD multiple
comparison test at the release peak selected date in
phases 1, 2 and 3 ( Dr. SPSS II, Nankodo Inc., Tokyo,
Japan). Differences with p values

Dent Mater J 2009; 28(2): 227-233 229
Fig. 1
Flexural strength of experimental denture base
resins
Mean ± SD, n=5
Resin powder contained 5, 10, 20 and 30 wt% S-
PRG filler. Control was conventional denture base
resin without S-PRG filler. The minimum flexural
strength should not be less than 65 MPa (ISO
1567).
Fig. 2
Flexural modulus of experimental denture base
resins
Mean ± SD, n=5
Resin powder contained 5, 10, 20 and 30 wt% S-
PRG filler. Control was conventional denture base
resin without S-PRG filler. The minimum flexural
modulus should not be less than 2GPa (ISO 1567).
Fig. 3
Amount of fluoride released in each phase
Unit indications without circle : μg/cm 2 /day, with circle : μg/cm 2 /16hrs.
Phase 1: Initial fluoride release from the denture base resins. Fluoride concentration rapidly decreased with an
increase in the immersion period. Phase 2: Fluoride release after fluoride recharging. All experimental resins
released approximately 20 times the amount of fluoride of phase 1. Phase 3: The amount of fluoride released from
the specimens after fluoride was expended and the specimens were recharged. The fluoride release returned as
with phase 2.

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Dent Mater J 2009; 28(2): 227-233
Table 1
Amount of fluoride release in phase 1 (μg/cm 2 /day)
day
1
2
3
4
7
9
10
11
12
14
15
(μg/cm 2 /day) (μg/cm 2 /day) (μg/cm 2 /day) (μg/cm 2 /day) (μg/cm 2 /day) (μg/cm 2 /day) (μg/cm 2 /day) (μg/cm 2 /day) (μg/cm 2 /day) (μg/cm 2 /day) (μg/cm 2 /day)
control ND ND ND ND ND ND ND ND ND ND ND
5wt% 0.70(0.07)a 0.29(0.03)a + 0.20(0.03)a + 0.21(0.03)a + 0.10(0.01)a+ tr tr tr tr tr tr
10wt% 1.00(0.09)b 0.40(0.04)b + 0.32(0.03)b + 0.33(0.03)b + 0.15(0.02)b + 0.11(0.01)a + 0.14(0.01)a + 0.15(0.02)a + 0.18(0.02)a + 0.22(0.03)a + 0.11(0.03)a +
20wt% 1.88(0.32)c 0.85(0.12)c + 0.73(0.06)c + 0.70(0.04)c + 0.36(0.04)c + 0.27(0.02)b + 0.34(0.03)b + 0.34(0.03)b + 0.34(0.02)b + 0.31(0.03)b + 0.15(0.03)b +
30wt% 2.91(0.16)d 1.37(0.12)d + 1.10(0.06)d + 1.10(0.08)d + 0.64(0.06)d + 0.47(0.03)c + 0.60(0.08)c + 0.60(0.07)c + 0.62(0.07)c + 0.53(0.05)c + 0.26(0.03)c +
n=8, mean (SD)
ND: not detectable (less than 0.022 μg/cm 2 ), tr: trace (less than 0.10 μg/cm 2 ), +: significantly different from day 1 (p0.05) on the same experimental day.
Significant differences were found between the groups on day 1 (p0.05).
All groups maintained fluoride release following recharging during the whole experimental period.
Fluoride release
Figure 3 shows the amount of fluoride released in
phases 1, 2 and 3. Tables 1, 2 and 3 summarize the
results of the determined fluoride amounts in phases
1, 2 and 3 respectively.
Phase 1
On day 1, fluoride ions were released into the DW
from the experimental denture base resins at levels
of 0.70, 1.00, 1.88 and 2.91 μg/cm 2 /day for
specimens with 5, 10, 20 and 30 wt% of S-PRG,
respectively. Significant differences were found
between the groups (p

Dent Mater J 2009; 28(2): 227-233 231
Table 3 Amount of fluoride release in phase 3
day
50
(μg/cm 2 /16hrs)
51
(μg/cm 2 /day)
52
(μg/cm 2 /day)
53
(μg/cm 2 /day)
54
(μg/cm 2 /day)
56
(μg/cm 2 /day)
58
(μg/cm 2 /day)
59
(μg/cm 2 /day)
60
(μg/cm 2 /day)
control 0.96(0.58)a 0.03(0.02)a tr ND ND ND ND ND ND
5wt% 3.02(0.56)ab 0.17(0.02)ab tr tr tr tr ND ND ND
10wt% 10.02(1.90)bc 1.07(0.29)b 0.50(0.30)a 0.25(0.14)a 0.19(0.07)a 0.15(0.05)a tr tr 0.13(0.13)a
20wt% 45.34(3.56)d 5.62(0.99)b 2.64(1.20)b 1.79(0.60)b 1.12(0.22)b 0.84(0.19)b 0.63(0.15)a 0.44(0.11)a 0.56(0.42)a
30wt% 98.14(11.19)e 15.51(3.93)c 6.83(1.30)c 4.66(0.88)c 3.41(0.50)c 3.00(1.44)c 1.87(0.35)b 2.51(2.87)b 1.45(0.32)b
n=8, mean (SD)
ND: not detectable (less than 0.022 μg/cm 2 ), tr: trace (less than 0.10 μg/cm 2 )
Values with the same letters are not significantly different (p>0.05) on the same experimental day.
The fluoride release returned as with phase 2 on day 50.
ing value on day 1 in phase 1 (Figure 3 and Table 2).
These values increased markedly in proportion to the
loading of S-PRG filler. On day 17, the amount of
fluoride released decreased significantly (p

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Dent Mater J 2009; 28(2): 227-233
were the easiest to recharge. According to Shaw et
al. 22) , the initial amount of fluoride release by a
conventional glass ionomer was 105 μg/cm 2 on day 1
and 33 μg/cm 2 on day 10, after which it gradually
decreased. In compomers, fluoride release decreased
from 8 μg/cm 2 on day 1 to 5 μg/cm 2 on day 10 in
the same study. In the present study, even the
highest values obtained from the 30 wt% group were
2.91 μg/cm 2 /day on day 1 and 0.60 μg/cm 2 /day on
day 10. These values were almost 1/3 and 1/6 of the
values of conventional glass ionomers at the same
time intervals. In the present study, even the highest
values obtained from the 30 wt% group were 2.91
μg/cm 2 /day on day 1 and 0.60 μg/cm 2 /day on day
11. These values were extremely low in comparison
with Shaw’s study and were approximately 1/35 and
1/55 of the values obtained with conventional glass
ionomers on day 1 and day 10 respectively.
Nevertheless, the amount of fluoride released after
recharging with a solution of sodium fluoride was
12.50, 22.00, 40.21 and 67.81 μg/cm 2 /16hrs from the
5, 10, 20 and 30 wt% specimens respectively on day
16. These values were approximately 12–70% of the
amount of fluoride released by glass ionomers on day
1. According to ten Cate et al. 26,27) , demineralization
inhibition depends on fluoride concentration, and
microradiographic data in vitro showed that 2 ppm
fluoride in artificial saliva containing calcium and
phosphate at pH 4.5 inhibited demineralization of
enamel lesions, while dentin demineralization was
inhibited in clinically relevant percentages (40%) at
fluoride levels above 1 ppm. However, these fluoride
concentrations would be toxic in clinical situations 28) .
In our study, the total amounts of fluoride released
were 5.61, 9.86, 18.1 and 30.4 ppm/3 ml/ day of the
5, 10, 20 and 30 wt% specimens respectively on day
16. These levels are sufficient to prevent dentin
lesion formation in the narrow space between
dentures and abutment teeth without causing
toxicity.
Some reports show that fluoride may induce
corrosion of alloys. Nakagawa et al. 29) reported that
pure titanium and titanium alloys were corroded by
fluoride at half or less than half of the concentration
currently found in commercial dentifrices less than
453 ppm fluoride. Watanabe et al. 30) reported that
several applications of acidulated fluoride agents
may cause discoloration of β-titanium alloy wires.
However, these problems occurred with excessive
fluoride concentrations and at low pH; these
conditions are unlikely to arise during clinical use 31) .
The experimental denture base resin containing
S-PRG filler used in the present study releases
fluoride ions on contact with water. The amount of
fluoride released increases as a function of filler
content. In addition, when this denture base resin is
recharged with fluoride solution, it retains excellent
fluoride releasing ability. However, because an
increase in filler content correlates with a decrease
in the durability and strength of the denture base
resin, it is important to determine the optimal
percentage of filler that satisfies the need for fluoride
release while maintaining acceptable strength. Silane
coupling agents, which we did not use in this study,
would help to give strength to resin mixtures that
have a greater fraction of filler.
Our results show that this experimental denture
base resin can be recharged with fluoride by soaking
dentures during non-wearing hours in a fluoridecontaining
denture cleaner or solution for preserving
dentures. Wearing a fluoride-recharged denture in
the oral cavity will provide enough fluoride to inhibit
demineralization and enhance remineralization
around abutment teeth, thereby preventing caries. In
addition to preventing caries among patients who
require care and who have poor oral hygiene, such as
the geriatric and person with special needs, this
fluoride-releasing resin can be incorporated into
bleaching trays for at-home bleaching, orthodontic
retainers, and night guards. It thus has the potential
to improve the oral health of people of all ages.
CONCLUSION
Experimental heat-polymerizing PMMA denture base
resin containing S-PRG filler releases significant
amounts of fluoride after recharging and can act as a
fluoride reservoir in the oral cavity.
ACKNOWLEDGEMENTS
The authors gratefully thank Mr. Toshiyuki
Nakatsuka of Shofu Inc., for his help and discussion
during this study. Shofu Inc. subsidized the
manufacture of the S-PRG filler. This work was
supported in part by a Grant-in-Aid for Young
Scientists (B) from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT) of
Japan (# 18791624).
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