B. PETROVIC et al.: CHARACTERIZATION OF GLASS IONOMER CEMENTS STORED IN VARIOUS SOLUTIONS
285–293
CHARACTERIZATION OF GLASS IONOMER CEMENTS STORED
IN VARIOUS SOLUTIONS
KARAKTERIZACIJA STEKLO-IONOMERSKIH CEMENTOV,
SHRANJENIH V RAZLI^NIH RAZTOPINAH
Bojan Petrovic
1
, Dejan Markovic
2
, Sanja Kojic
3
, Tamara Peric
2
, Georges Dubourg
4
, Mihailo
Drljaca
4
, Goran Stojanovic
3
1
University of Novi Sad, Faculty of Medicine, Department for Dentistry, Hajduk Veljkova 3, Novi Sad, 21000, Serbia
2
Belgrade University, Faculty of Dentistry, Department of Paediatric and Preventive Dentistry, Dr Subotica 11, Belgrade, 11000, Serbia
3
University of Novi Sad, Faculty of Technical Sciences, Trg Dositeja Obradovica 6, Novi Sad, 21000, Serbia
4
University of Novi Sad, BioSense Institute, Dr Zorana Djindjica 1, Novi Sad, 21000, Serbia
Prejem rokopisa – received: 2018-07-18; sprejem za objavo – accepted for publication: 2018-11-29
doi:10.17222/mit.2018.159
The aim of this work was to evaluate the nano-mechanical properties of glass ionomer materials, the ion concentrations at the
surfaces in relation to the storage media and the pH environments using a scanning electron microscope with an
energy-dispersive spectrometer (SEM/EDX). The glass-ionomer-based materials, Fuji Triage (FT), Fuji VIII (FVIII), Fuji IX
GP (FIX), were analyzed. The sample comprised 45 cured cement disks. Five specimens of each tested material were placed in
3 storage solutions (saline, acidic solution with pH of 5.5, NaF solution with 0.05 % of fluoride). Nano-indentations were
performed with a force up to 30 mN, penetration depths of 2500–2700 nm for 1 d and 21 d after setting. The EDX evaluation
was carried out for each experimental disk, identifying the ions: O, Al, Sr, Si, F, Na, P, Ca. The level of significance was placed
at p < 0.05.The highest fluoride proportion at the specimen surface was recorded in the FT material. FT also exhibited the
lowest fluoride ions content when stored in low-pH environments compared with the other tested materials (p < 0.05). The
surface hardness of the tested materials decreased from 1.377 GPa (in saline) to 0.03 GPa (in acid). The Young’s modulus varied
from 14.35 GPa to 0.112 GPa, depending on the material type (Fuji VIII>Fuji IX>FT) (p < 0.001) and the storage media (p <
0.001). Both the mechanical and cariostatic surface properties of commercially available glass ionomer materials are affected by
the storage media.
Keywords: glass ionomer, nano-indentation, SE, EDX
Namen predstavljenega dela je bil ovrednotiti nanomehanske lastnosti ionomerskih steklenih materialov, ki slu`ijo v zobni
protetiki kot razna polnila. Koncentracijo ionov na povr{ini v povezavi z medijem za shranjevanje s specifi~nim pH okoljem so
okarakterizirali tudi z uporabo vrsti~nega elektronskega mikroskopa s prigrajenim energijskim disperzijskim spektroskopom
(SEM/EDX). Analizirali so naslednje materiale na osnovi steklenih ionomerov: Fuji Triage (FT), Fuji VIII (FVIII) in Fuji IX
GP (FIX). Vzorci so vsebovali 45 vulkaniziranih cementnih diskov. Po pet (5) vzorcev vsakega preiskovanega materiala so
vstavili v tri (3) razli~ne raztopine za shranjevanje (slano vodo, kislo raztopino s pH 5,5 in raztopino NaF s 500 ppm fluorida).
Po postavitvi vzorcev so izvedli nanoindentacije (vtiskovanje nanopiramide) na vzorcih s silo do 30 mN in nastala je globina
vtiska od 2500 do 2700 nm. Tako pripravljeni vzorci so se nato nahajali v raztopinah od enega (1) do enaindvajset (21) dni. Z
EDX so dolo~ili ione: O, Al, Sr, Si, F, Na, P in Ca na vseh eksperimentalnih diskih. Nivo pomembnosti so postavili na p<0,05.
Najve~ji dele` fluorida so zaznali na vzorcu iz FT. Ta vzorec je prav tako pokazal najni`jo vsebnost fluoridnih ionov, ko je bil
shranjen v okolju z nizkim pH v primerjavi z drugimi preiskovanimi materiali (p<0,05). Trdota vzorcev na povr{ini je padala od
slanice z 1,377 GPa na 0,03 GPa v kislem mediju. Youngov modul je variiral od 14,35 GPa do 0,112 GPa, odvisno od vrste
materiala (Fuji VIII>Fuji IX>FT) (p<0,001) in medija za shranjevanje (p<0,001). Potrjeno je bilo, da raztopine za shranjevanje
vplivajo tako na mehanske kot tudi kariostati~ne lastnosti (nagnjenost k zobni gnilobi – kariesu) povr{ine.
Klju~ne besede: stekleni ionomeri, nanoindentacija, SE, EDX
1 INTRODUCTION
Glass ionomer cements consist of ion-leachable
glasses, water-soluble polyacrilic acid, water and mono-
mers that are photopolymerizable, for example, hydroxyl
ethyl methacrylate or photopolymerizable side chains
bonded to the polyacrilic acid.
1
Various glasses were
investigated as the constituents of glass ionomer ma-
terials. With the introduction of the modified ratio bet-
ween Al
2
O
3
and SiO
2
in the silicate glass, Wilson and
Kent produced usable cements for dental restorative
materials with polyacrylic acid.
2
Glass ionomer materials show several advantages
compared to other permanent restorative materials, such
as a capacity to chemically bond to a wet dental sub-
strate, long-term fluoride release responsible for a
caries-preventive effect, biocompatibility and an inade-
quate thermal expansion coefficient.
3,4
All these favor-
able properties support their valuable position in every-
day clinical practice, since they meet some important
criteria for the contemporary concept of the prevention,
prophylaxis and treatment of dental caries. However, all
these favorable properties are accompanied by some
disadvantages, such as the inability to achieve a satis-
factory surface polish, significant porosity and relatively
poor physical properties, such as surface wear and
Materiali in tehnologije / Materials and technology 53 (2019) 2, 285–293 285
UDK 620.1:620.3:666.1:616.314-74 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(2)285(2019)
*Corresponding author e-mail:
bojan.petrovic@mf.uns.ac.rs

brittleness when fully matured or set.
5–8
When it comes
to all glass-ionomer-based materials the interrelation-
ships among the structure, surface properties, physical
properties and clinical performance are still not fully
clarified and reported.
The majority of contemporary dental materials
absorb water, even in conditions of low humidity, and
this is emphasized in very humid environments such as
the oral cavity. In general, this water uptake by the resto-
rative material has been considered as detrimental, since
the contact between the cement surfaces in the early
stages of maturation arrests the process of cement
setting, leading to unfavorable surface damage.
9
In addition, the fluctuations of the mechanical cha-
racteristics of glass ionomer cements as a consequence
of water sorption are crucial for assessing their clinical
performance in the oral cavity. Despite reports on the
changes in the mechanical features caused by storage in
aqueous solutions, the effect of the water sorption of
glass-ionomer-based materials is still not fully under-
stood.
1,9,10
It has been shown that during the clinical per-
formance of glass-ionomer-based materials in the oral
environment, the degradation of glass ionomer cements
can be caused by acidic attack, water sorption, erosion,
and wear. All these factors cause the degradation of the
cement matrix, leading to an increased surface roughness
and wear rate.
11,12
It has been reported that conventional glass-iono-
mer-based materials release sodium, fluoride, silica and
calcium ions in aqueous solutions, while metal-rein-
forced glass ionomer cements release ionic silver as
well.
13,14
Ion release has also been confirmed for resin-
modified glass ionomer cements.
15
During the past two decades, the physical charac-
teristics of glass-ionomer-based materials have improved
as a result of an increased proportion of filler-particle
loading, the incorporation of resin monomers into the
cement, or both.
16
The caries-preventive and antibacterial effects of
glass ionomer cements have been primarily credited to
the presence and availability of fluoride, but the actual
fluoride amount as well as a proportion of released fluo-
ride sufficient for achieving cariostatic and prophylactic
efficacy, has not been well documented.
4
The fluoride
contents in glass ionomer cements vary between 10 %
and 23 %. At the same time, it has also been shown that
the ability for extensive fluoride release negatively
affects the physical characteristics of the glass ionomer
material.
17,18
Nano-indentation tests have been reported as effec-
tive methods for the measurement of the mechanical
characteristics of materials, such as nanocomposites,
tooth enamel and bone cement.
19–23
In nano-indentation
testing, the depth of the penetration beneath the analyzed
sample surface is measured as the load is applied to the
indenter. The shape of the load-displacement curves,
which are presented in this paper, are a very useful
source of information, not only for calculating the
modulus and hardness of the analysed specimens, but
also for the discovery of non-linear effects, if any (such
as cracking or phase transformations).
The goals of the present investigation were:
• to determine the nanomechanical properties of glass
ionomer materials (modulus of elasticity and hard-
ness using the nano-indentation method) on the first
day and 21 d after setting
• to assess the ion content of cured glass-ionomer-
cement disks with respect to different storage
solutions and various pH environments using a
scanning electron microscope equipped with an
energy-dispersive spectrometer (SEM/EDX).
2 EXPERIMENTAL PART
Three commercial glass-ionomer-based cements, Fuji
Triage (FT, conventional glass ionomer designed pri-
marily for pits and fissure sealing, GC Int, Tokyo,
Japan), Fuji VIII (FVIII, Fuji VIII GP Capsule, resin-
modified glass ionomer restorative material, GC Int
Tokyo, Japan) and Fuji IX (FIX, Fuji IX GP conven-
tional glass ionomer restorative cement, GC Int Tokyo,
Japan) were evaluated in this paper.
The tested materials were prepared according to the
manufacturers’recommendations, similar to our previous
report.
24
Capsules were activated immediately before
mixing and after that placed into the amalgamator device
for a 10 s period of mixing. Afterwards, the activated
capsules were placed into the original capsule applier.
All three materials were placed into Teflon molds
immediately after mixing. The Teflon molds were
cylindrical with a diameter of 15 mm and a thickness of
10 mm. During the setting of the experimental disks, the
bottom and top of each mold was roofed using glass
plates and hand pressure. Acceleration of the curing
reaction for the FT and FVIII was induced with photo-
activation for a period of 40 s. Regarding the FIX speci-
mens, the disks were stored in the molds, roofed with a
transparent matrix for a period of 10 min. Immediately
upon the complete setting of the materials, the disks
were taken from the molds and polished using Sof-Lex
discs 8691-F (3M ESPE AG, Seefeld, Germany). The
polished specimens were placed into 10 ml of deionized
water and stored at 37 °C for a period no longer than
24 h. The final experimental model comprised 45 disks
(n = 15 specimens of each of three materials). The disks
were placed into 3 different solutions, resulting in
5 disks of each material per storage medium. The first
storage medium was saline, the second was an acidic so-
lution with a pH of 5.5 that was made using 50 mmol/L
of potassium-chloride titrated to pH 5.5 with concen-
trated hydrochloric acid, while medium III was a 0.05 %
solution derived from sodium fluoride.
All the samples were held in three appropriate
storage media at 37 °C. Before the testing the disks were
B. PETROVIC et al.: CHARACTERIZATION OF GLASS IONOMER CEMENTS STORED IN VARIOUS SOLUTIONS
286 Materiali in tehnologije / Materials and technology 53 (2019) 2, 285–293

rinsed using deionized water. A SEM/EDX examination
was performed with a tabletop scanning electron micro-
scope (TM3030, Hitachi, Tokyo, Japan) and an EDX
spectrometer (Figures 2 and 3). The quantitative analysis
of each disk was conducted at three randomly chosen
spots. The ion proportion was recorded for the following
ions, Ca, O, Si, F, Al, Ba, P, and Sr, and the values of
w/%, mass fractions were recorded. After the first set of
measurements, each disk was placed in the same storage
media as before the testing, and the same testing protocol
was repeated after 21 d.
For the indentation testing the prepared samples were
mounted on the sample holder using adhesive tape.
Multiple indentations were performed, fairly distributed
throughout the entire sample surface. These tests were
performed using an Agilent (Keysight) Nano indenter
G200, which provides repeatable and reliable measure-
ments. Multiple indentation (5 indentations per location
and 50 location per sample were made) tests provide
measurement repeatability for the mechanical properties
of the analyzed samples. The system has a resolution of
load and displacement of less than 50 nN and 0.1 nm,
respectively. Nano-indentation tests were conducted with
a Berkovich diamond tip, with a face angle of 65.27°.
The calibration indents were performed on fused silica.
Examination of samples was performed at room tempe-
rature and the Poisson’s ratio for all the samples was set
to 0.27. The indents were located 100 μm apart to avoid
the influence of residual stresses from adjacent im-
B. PETROVIC et al.: CHARACTERIZATION OF GLASS IONOMER CEMENTS STORED IN VARIOUS SOLUTIONS
Materiali in tehnologije / Materials and technology 53 (2019) 2, 285–293 287
Figure 2: SE images of the tested materials’ surfaces in three different storage media
Figure 1: Young’s modulus and hardness of the tested materials for 1
d and for 21d

pressions. All the tests were performed at a max load of
30 mN with 5 indentations per place (with ascending
forces of 1.875 mN, 3.75 mN, 7.5 mN, 15 mN and
30 mN), 10 s peak hold time and time to load of 15 s.
All the statistical analyses was conducted by employ-
ing the Student’s t-test with the level of significance for
intergroup and intragroup differences set at p < 0.05.
3 RESULTS
The results for the Young’s modulus and the surface
hardness determination for all the tested materials in
three different storage media over the same time inter-
vals are shown in Figures 1, 5, 6 and 7. Force-displace-
ment curves for all 3 tested materials are plotted in
Figures 5, 6 and 7.
The Fuji IX samples stored in saline showed signifi-
cantly higher values for both the Young’s modulus and
the surface hardness compared to storage in acidic and
fluoride solutions for 1 day. In addition, the significant
decrease in the Young’s modulus and the hardness in the
Fuji IX was observed in an acidic environment for 21 d,
while a consistent increase regarding both the investi-
gated parameters was observed in the samples stored in
the saline and fluoride solutions (p < 0.001 Student’s
t-test).
The Fuji VIII also showed significantly lower values
for the Young’s modulus and the surface hardness in an
acidic environment compared to the saline and fluoride
solutions for 1 day (p < 0.001 Student’s t-test). In con-
trast to the Fuji IX, a significant increase in the Young’s
modulus and the hardness was observed in the fluoride
solution, while these parameters decreased in the saline
and acidic solutions for 21 d.
The Young’s modulus and the hardness values were
the highest in the Fuji Triage specimens stored in saline
for 1 day (p < 0.001, Student’s t-test). A constant in-
crease in both the investigated parameters was observed
in the Fuji VII stored in all three different media for 21
days with the lowest values in the acidic solution and
without a significant difference in these properties bet-
ween the samples stored in the saline and the samples in
the fluoride solution (p < 0.001, Student’s t-test).
The Fuji VIII stored in saline exhibited significantly
higher values in the Young’s modulus for day 1, com-
pared to the specimens in the other two media and the
Fuji IX and Fuji Triage in all media. In contrast to that,
the highest values for the hardness were recorded in the
Fuji Triage samples stored in saline. After 21 d, signifi-
B. PETROVIC et al.: CHARACTERIZATION OF GLASS IONOMER CEMENTS STORED IN VARIOUS SOLUTIONS
288 Materiali in tehnologije / Materials and technology 53 (2019) 2, 285–293
Figure 4: SE images of indentation marks in 4 different modes
Figure 3: The elements evaluated on the experimental materials, and their amounts with respect to different storage media, in w/%. Same
superscripts indicate average values with statistically significant differences (p < 0.05).

cantly higher values for the hardness were recorded in
the Fuji Triage samples stored in the fluoride solution.
The amounts of calcium, phosphate, oxygen, silicon,
strontium, fluoride, aluminum and barium ions on the
study materials in w/% are shown in Figure 3. Substan-
tially more calcium was observed in the conventional
GIC-based materials immersed in the fluoride solution
than in the resin-modified material. The values for phos-
phorus oscillated the most within all the tested materials
and in all the storage media, with significantly higher
values in the acidic solution. The resin-modified material
showed more silicon, compared with the conventional
glass ionomer materials, but this value significantly
decreased in the acidic environment. On the other hand,
in addition to the strontium decrease in all the materials
in an acidic environment, a significant increase was ob-
served after fluoride immersion for the Fuji Triage.
Regarding the fluoride content, it has been observed that
it significantly increased in all the materials after fluo-
B. PETROVIC et al.: CHARACTERIZATION OF GLASS IONOMER CEMENTS STORED IN VARIOUS SOLUTIONS
Materiali in tehnologije / Materials and technology 53 (2019) 2, 285–293 289
Figure 7: Load-displacement curves for the Fuji IX specimens
Figure 5: Load displacement curves for the Fuji Triage specimens
Figure 8: Profile image of the indentation mark in the glass ionomer
material
Figure 6: Load displacement curves for the Fuji VIII specimens

ride immersion, with significantly higher values in
conventional glass ionomer materials compared to the
resin-modified specimens. The values for aluminum
varied significantly in all the tested materials and in all
the storage media, with the highest values observed in
the Fuji Triage in the acidic solution. In contrast to that,
the content of barium did not show significant variations
in the different storage media.
4 DISCUSSION
This investigation was performed to evaluate the
influence of various storing solutions on the mechanical
and cariostatic properties of commercially available
glass ionomer materials. The property variations of
dental materials during exposure to various media in the
oral cavity are an important parameter when assessing
their clinical performance.
In the first part of the experiment the hardness and
Young’s modulus of two conventional glass ionomers
(FT and FIX), and one resin-modified glass ionomer
(FVIII) were evaluated with respect to different storage
media using the nano-indentation technique.
Large differences in the mechanical properties
between the various conventional and resin-modified
glass ionomer materials have been described.
25–28
How-
ever, the impact of the storage media on both the
physical and chemical properties of glass ionomer ma-
terials is still under discussion. In some reports it has
been clearly emphasized that the obtained results
regarding mechanical properties were valid only for
aqueous solutions.
27
Additionally, it has been shown that
not only the acidity, but the type of the acid, strongly
affected the properties of the glass ionomer materials. In
the oral cavity, bacteria forming dental plaque, in the
initial phases of caries process development, create an
acidic environment that may affect the cement’s physical
and mechanical properties, while the topical use of
fluoride is responsible for fluoride availability at the
material surface.
29,30
The in vitro model employed in the
current experimental study primarily focused on
assessing the quantitative variations of the ion proportion
only at the outer portions of three glass ionomer cements
stored in acidic and neutral environments, as well as in
the fluoride solution.
In the past three decades nano-indentation was used
to test various specimens, including dental materials.
25
Nano-indentation allows the quantification of the me-
chanical performances on the micro-scale. The most
frequently used method to analyse the nano-indentation
data is the method invented by Oliver and Pharr.
31–34
It is
adapted to isotropic elastoplastic materials and allows a
calculation of the modulus of elasticity and the hard-
ness.
32
A nano-indentation method has been explained in
which very small indentations are created using loads of
only several mN.
33–35
Using this information it is possible
to compute the hardness and the Young’s modulus auto-
matically from the load–displacement curve, instead of
using visual measurements of the indentation im-
pression.
The force-displacement response from both the con-
ventional glass ionomers and the resin-modified samples
are shown in Figures 4, 5, 6, 7 and 8, and it is possible to
compare the tested materials directly. It has been re-
corded that both the hardness and the Young’s modulus
decreased in the acidic solution for all three tested
materials, while the fluoride immersion did not affect
these parameters. In a neutral environment the resin-
modified glass ionomer exhibited superior mechanical
properties compared to conventional glass ionomers,
which is not in agreement with some previously pub-
lished reports, where it has been observed that the
hardness of the conventional glass ionomer material is
higher than the resin-modified samples.
26,36
This finding
can be attributed to the different experimental protocols,
storage media and techniques employed. Various mecha-
nical properties of glass ionomer materials have been
determined in numerous experimental studies, and signi-
ficant variations can be observed. The obtained results
are in partial agreement with the available literature data,
and the obtained absolute values for the hardness and
Young’s modulus are lower compared to the results pub-
lished in several experimental studies.
5,37,38
The possible
explanation for these variations is in the technique itself,
because the use of nano-indentation in glass ionomer
materials faces some difficulties. It is well known that
glass ionomer materials consist of two completely diffe-
rent phases, at least on the nano scale, which can lead to
some inaccuracies in the measurement.
Load-displacement curves obtained during nano-
indentation tests for all the analysed samples are shown
in Figures 5, 6 and 7. The curve demonstrates a smooth
shape, and no pop-in could be detected. A maximum
load of 30 mN was applied to all the samples. At this
maximum load, the penetration was around 350 nm for
the Fuji VIII specimens, around 470 nm for the Fuji IX
and around 1450 nm for the Fuji Triage. These results
revealed that the highest hardness is measured for the
Fuji VIII, after that the Fuji IX and finally the Fuji
Triage. No fracture occurred in all the analysed samples.
Considering the open literature, in the paper by Willems
et al. posterior composites, dental ceramics for CAD/
CAM restorations, sintered porcelain, and amalgam were
studied.
20
Applying the indentation method, an inden-
tation depth of 180–350 nm was achieved, depending
upon the material tested, while a maximum load of 10
nN was applied. The glass ionomer – Fuji Triage (FT),
glass carbomer – Glass Seal (CARB), resin – Ultra Seal
XT Hydro (UXT), resin – Teethmate F-1 (TETM) and
resin – Defence Chroma (DEFC) were tested in the study
conducted by Arsenoglu et al.
21
For a maximum applied
force of 6 mN, the insertion range was from 700 nm (for
UXT) to 1100 nm (for TETM), which is in the range of
our samples Fuji IX and Fuji Triage. The authors used
B. PETROVIC et al.: CHARACTERIZATION OF GLASS IONOMER CEMENTS STORED IN VARIOUS SOLUTIONS
290 Materiali in tehnologije / Materials and technology 53 (2019) 2, 285–293

graphite nanopeletes to improve the mechanical proper-
ties of the polymer nanocomposites in their paper.
19
The
normal force of 0.8 mN was used and the indentation
range obtained was from 320 nm to 420 nm. Our sample
Fuji VIII demonstrated almost the same hardness at 0.8
nm (see Figure 6).
In the study conducted by Akashi and coworkers the
changes of the mechanical strength with time were
evaluated as one of the indicators of durability, and all
the analysed restorative resin-modified glass ionomer
cements exhibited a significant deterioration in the com-
pressive and tensile strengths after 12 months of
immersion in water.
1
The ion concentrations at the surfaces of the tested
materials were evaluated with respect to the various
storage solutions in the second part of the experiment.
The presumption was that different chemical composi-
tions at the material’s surface could affect the mechani-
cal properties when submitted to different storage media.
The caries-preventive properties of the glass-ionomer-
based materials are primarily attributed to their ability to
release fluoride, but the exact relationship between the
fluoride ions present at the surface of the material, the
exact amount of fluoride and the fluoride release for
glass ionomer cements is not well documented and the
importance of the understanding of F release in diverse
storage solutions has increased during recent years. In
addition to that, the kinetics of some other ions at the
surface of the glass ionomer materials, such as alumi-
num, calcium and silicon, have gained researchers’
attention, since the glass phase of contemporary glass
ionomer materials contains oxides of calcium, sodium,
phosphorus, and silicon in a quantity that is necessary for
sufficient material surface activity.
27,38,39
The outer portions of the material rather than the
material itself is of particular interest, since it has been
demonstrated that the concentration of fluoride is higher
at the surface of the glass ionomer samples than at the
core of the material.
36
In a comprehensive literature review on fluoride-
releasing dental restorative materials published by
Wiegand et al., two mechanisms have been identified by
which fluoride ions may be released from glass ionomers
into different storage media and diverse environments.
40
The first described is a so-called short-term reaction that
encompasses fast dissolution from the outer surface into
solution, and the second is described as a slower, gradual
process, resulting in the constant diffusion of ions
through the core of the material. It has been uniformly
confirmed that the highest fluoride ion release is found in
acidic environments and the lowest in saliva, and this
finding could be explained by the fact that the acidity
enhances the dissolution of the glass ionomer cement
leading to a higher fluoride level during the acidic attack.
The underlying mechanism of the fluoride kinetics under
conditions with a pH decrease is clarified with the fact
that the ratio between free (uncomplexed) fluoride to
bound (complexed) fluoride was lower in the acidic envi-
ronment in comparison to the neutral environment.
Regarding the fluoride content at the surfaces of the
materials evaluated in the present investigation, it has
been observed that it significantly increased in all the
materials after fluoride immersion with significantly
higher values in conventional glass ionomer materials
compared to the resin-modified specimens. The results
obtained in the present investigation indicate that the
fluoride concentration at the material surface is under
strong influence of the storage medium for all the anal-
ysed materials. Significant fluctuations at the surfaces of
the tested materials were recorded. Both conventional
glass ionomer materials exhibited the lowest amounts of
fluoride in the acidic environment in comparison to the
reference fluoride-ion concentrations recorded in saline.
This is in agreement with previous findings that glass
ionomer materials release more fluoride in an acidic
environment, leaving fewer fluoride ions at the surface of
the material, thus providing the largest amount of fluo-
ride during the acidic attack in order to prevent caries
development.
24
In the present investigation it has been observed that
significantly more calcium was found at the surfaces of
the conventional glass ionomer cements immersed in the
fluoride solution than on the resin-modified material.
This finding is in agreement with the observation that
calcium ions that are present in rather insoluble com-
pounds when exposed to neutral conditions are released
in an acidic environment, and that under acidic condi-
tions, more calcium and strontium is released, too.
41,42
On the other hand, in addition to the strontium decrease
in all the materials in an acidic environment, a significant
increase has been observed after fluoride immersion for
the Fuji Triage. Apart from the release of fluoride, the
strontium release is regarded as particularly important
for the anti-cariogenic properties of glass ionomer mate-
rials, since it has been shown that the re-mineralizing
effect of fluoride is enhanced by the presence of stron-
tium.
43
The fluoride release from the glass ionomer
material is higher where some or all the calcium is re-
placed by strontium.
43
The presence of a large number of different ions in
glass-ionomer-based materials makes them able to
release the majority of these ions, besides fluoride, to the
immediate surroundings. It has been demonstrated that
sodium, aluminum, silicon and phosphorus are released
from glass-ionomer-based materials in all storage condi-
tions, while only small amounts of calcium or strontium
release have been observed under neutral conditions.
44
It
has been suggested that acidic conditions boost the
release of all ions, causing a large proportion of calcium
ions to be released. However, it has been emphasized
that an enhanced release in acid is not uniform for all the
ionic constituents and occurs to different extents for each
ion.
The question about ion release from glass ionomer
cements has been addressed in depth in studies con-
B. PETROVIC et al.: CHARACTERIZATION OF GLASS IONOMER CEMENTS STORED IN VARIOUS SOLUTIONS
Materiali in tehnologije / Materials and technology 53 (2019) 2, 285–293 291

ducted by Nicholson, Wasson and Czarnecka. Based
upon the classic study of Barry et al. that described the
ion releasing stoichiometry of ionomer glasses using the
acetic acid model, and reported that the distribution and
concentration of ions released from the glass ionomer
cement were directly related to the composition of the
glass, particularly to the presence of specific fluoride-
rich calcium fluoride phases, it has been suggested that
acidic attack mainly takes place at these calcium-rich
areas of the set cement surface.
45–47
Like conventional glass ionomer cements, resin-mo-
dified glass ionomers release small amounts of sodium,
aluminum, phosphate and silicon under neutral condi-
tions, which has been confirmed in the present inves-
tigation.
42
The values for phosphorus varied the most
within all the tested materials and in all the storage
media with significantly higher values in the acidic
solution. The significance of the phosphorus in the kine-
tics of ion release from the surface of the glass-iono-
mer-cement-based materials has been identified, together
with its effects on the cement’s working and setting
time.
48
But the mechanism of its effect on the glass
ionomer material surface properties in relation to the
different storage media needs further elucidation.
All glass ionomer materials possess their characte-
ristics thanks to the fact that both alumina and silica are
used in their formulation.
41
In the present investigation
the resin-modified material showed more silica on its
surface than the conventional glass ionomer materials,
but this value significantly decreased in an acidic envi-
ronment. It has been reported that the silicon-rich layer
acts as a template for calcium phosphate precipitation.
26
On the other hand, the values of aluminum varied signi-
ficantly in all the tested materials and in all the storage
media, with the highest values observed for the Fuji
Triage in an acidic solution. In contrast to that, the
content of barium did not show significant variations in
the different storage media.
This study faces some shortcomings that must be
taken into account when the results are compared with
similar studies and particularly when trying to extrapo-
late these results to real clinical conditions. In this paper
the ion content and the mechanical properties were
assessed and evaluated through changes in the tested
variables at the surface of the materials investigated,
while the actual values for the investigated parameters
from glass ionomers into different storage media has not
been obtained. In addition to that, as stated in our
previous report, the surface of the material was the main
point in the study, with no convincing proof that the
changes at the material surface could represent the
changes in the entire restoration.
24
Finally, it is important
to take into consideration that the different methodology
used in the studies, including sample size, the media
used to contain the samples, the quantity of media used
to measure the surface characteristics could be respon-
sible for the large numerical differences found among the
various reports. Thus, comparisons must be made
considering the behaviour of the materials, rather than
the absolute numerical values for the mechanical proper-
ties and the amount of ions in absolute terms present at
the surface of the material.
5 CONCLUSIONS
This study shows that the surface of glass ionomer
materials is strongly influenced by the storage medium.
The hardness and Young’s modulus of both conventional
and resin-modified glass ionomer materials decrease in
an acidic environment, while they remain unchanged
after fluoride immersion. The ion kinetics is also affected
by storage media and all the tested material exhibit a
satisfactory potential for the release of cariostatic
elements in an acidic environment.
Acknowledgment
This work is partly supported by the project III45021
as well as 451-111-2017/1.
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