Chapter 14
Glass-Ionomer Restorative Materials
I.
Introduction
Glass-ionomer cements are used as:
1.
restorative materials
2.
luting agents
3.
adhesive bases
4.
crown build-up materials
Conventional
glass-ionomer
This chapter will examine glass-ionomer
cements and resin-modified glass-ionomers
cements used as restorative materials. Other
chapters will describe glass-ionomers designed
for use as luting cements or as adhesive liners.
Glass-ionomers cements are water-based
cements that set by an reaction between an
acid and a base. An improved material gradually evolved during the 1980s. This material,
called a resin-modified glass-ionomer
(RMGI) cement, sets by a combination of an
acid-base reaction and a polymerization reaction. RMGI cements have some properties
that are similar to conventional glaessionomer cements and other properties that are
similer to those of resin composite restorative
materials.
A third type of material also became available during the 1980s. These restorative
materials, called a polyacid-modified resin
composites, are similar to glass-ionomer
cements in that they contain water-soluble
polymeric acids. They are different from glassionomers in that their setting reaction is
almoat totally by polymerization. The spectrum in Fig.1 illustrates the relationship
among these four types of materials. In the
following we will first describe glass-ionmer
cements, then resin-modified glass-ionomer
cements, and finally, polyacid-modified resin
composites.
Conventional
Resin-composite
Resin-modified GI
Polyacid-modified
resin composite
Fig. 1. A spectrum showing the relationship
between modified materials conventional
glass-ionomers on one pole and conventional
resen composites on the other pole.
II.
Glass-ionomer Cements
History
Glass-ionomer cements were introduced by
Kent and Wilson in 1972. The crux of their
thinking seems to have been to combine the
desirable features of zinc polyacrylate cements
and of silicate cements. These features are: (1)
the adhesion to dentin and enamel of the zinc
polyacrylate cements and (2) the anticariogenic properties of the silicate cements. The
adhesion was initially thought to be between
calcium ions in enamel and dentin and carboxylate anions in the zinc polyacrylate salt.
The anticariogencity is the result of fluoride
ions that are leached from silicate cements.
Thus, Kent and Wilson produced the glassionomer cements by combining a polyacrylic
acid liquid, like that of the zinc polyacrylate
cements, with acid-soluble glass particles, like
those used in silicate cements.
Powder
Acid-soluble glasses. During the mid
20th century, the only tooth-colored material
available to dentists was silicate cement.
These cements were made by mixing phosphoric acid with a powder made up of acid-soluble glass particles. Silicate cement restorations had several desirable particles including:
white color, and resistance to recurrent caries.
1
Glass-Ionomer Restorative Materials
Oral Biomaterials - Chapter 14
Unfortunately,
in some
patients’
mouths, silicate
cement restorations proved
excessively soluble.
Table 1
The composition of Powder and Liqu id
of a Typical Glass-ion omer Cemen t
(wt. %)
Po w der
SiO 2
Liqui d
29.0
As will be
seen, the glass
that is used to
Al 2O 3
16.6
make the pow34.2
Ca 2F
der for glassiionomer
Na 3AlF 6
4.9
cements is simiAlF 3
5.3
lar to that which
AlPO 4
9.9
was used to
make silicate
cement. This type of glass is unusual. Most
glasses resist dissolution in acids. Silica glass
is composed of a three dimensional network of
SiO4 that are connect to one another by Si-OSi bridges. These Si-O bridges are very strong
and will not be broken by acid-supplied protons (H+).
Soluble glasses used in silicate
cements. A silca glass will become soluble in
acids if aluminum cations are fused into the
glass structure. If aluminum cations (Al3+)
replace some of the silicon cations (Si4+), an
effective negative charge is imparted to the
network. The negative charge is localized at
Al3+ ions. Protons (H+) are attracted to these
ions and break the weak O-Al bonds. The
attack releases Al3+ ions and other weakly
bound ions such as Ca2+, Na+, and F-. The
last of these ions, the fluorine, originate from
crystalline fluorides, such are fluorite (Ca2F)
that are incorporated in the glass particles.
Soluble glasses used in silicate
cements. The powder is an aluminosilicate
glass much like that used in silicate cements.
The ingredients in the glass are given in Table
1. Since the polyacrylic acid is weaker than
2
Copoly mer of polyacrylic acid
and/or polymaleic acid and/or
itaconic acid
50.0
H 2O
40.0
Tartaric Acid
10.0
the phosphoric acid used with silicate cements,
high aluminum to silicon ratios are maintained to enhance the solubility of the glass.
The glass is fused at 1150oC and quenched in
water to produce a thoroughly crazed glass,
which is milled and passed through a sieve so
that no particle is larger than 40 μm. Typical
cements have an average particle size of 10
μm.
The glass powders des-cribed in the preceding do not produce radio-paque glass-ionomer
cements. However, the glasses can be made
radiopaque by adding strontium (Sr), barium
(Ba), or lanthanum (La) to the glass. Other
commercial cements are made radiopaque by
mixing zinc oxide or zirconium oxide powder
with the glass powder.
Liquid
The acids that are reacted with glasses to
form glass-ionomer cements, include: polyacrylic, itaconic, and polymaleic acids. This
class of acids is called poly(alkenoic) acids. All
are polymer acids (e.g., polyacids) that have
numerous carboxyl groups (-COOH) along each
carbon chain backbone. The International
Standards Organization (ISO) has declared
Glass-Ionomer Restorative Materials
Oral Biomaterials - Chapter 14
that the official name for this type of cement is
glass polyalkenoate cements.
uid, the liquid tends to polymerize during storage.
The original liquid used by Kent and
Wilson was 50% polyacrylic acid (Fig. 2) in
Both polyacrylic acid and polymaleic acid
based cement liquids include about 10% tartaric acid. This addition increases the work-
C OC OC OC OC OC O
OH OH OH OH OH OH
Fig. 2. Polyacrylic acid (PAA) is a water soluble acid that has carboxyl groups coming off of
every other carbon atom along its carbon backbone
OH OH OH OH OH OH OH
C OC OC OC OC OC OC O
C OC OC OC OC OC O
OH OH OH OH OH OH
Fig. 3. Polymaleic acid is a water-soluble acid
that has carboxyl groups coming off of each
carbon atom along the carbon chain that
makes up the backbone of the polymer molecule.
aqueous solution. The average molecular
weight of this polymer was 23,000. An average molecule with this molecular weight would
be about 0.1 μm long if it could be untangled
and stretched out. Higher molecular weight
polyacrylic acids produce stronger cements.
Unfortunately, the liquid's viscosity increases
with average molecular weight, such that
higher molecular weight polyacids cannot be
successfully mixed with the powder.
In later formulations of the cement, copolymers of polyacrylic and itaconic acid have been
used. The latter is another polyalkenoate acid
similar to polyacrylic acid. The copolymer has
a lower viscosity and improved shelf-life. If
polyacrylic acid is the only polyacid in the liq-
HO
O
OH H
O
C
C
C
C
H
OH
OH
Fig. 4. Tartaric acid.
ing time, promotes a rapid set (closer to a
"snap set"), and increases the ultimate
strength of the ce ment. Some workers have
suggested that tartaric acid may increase the
rate at which ions are leached from the glass.
That tartaric acid improves the working qualities of glass-ionomer restoratives is undisputed; why it does so is not understood.
Some manufacturers are marketing glassionomer restoratives whose liquids contain no
polyacrylic acid. Instead, these formulations
contain polymaleic acid. This polymeric acid
is similar to polyacrylic acid except that it has
twice the number of carboxyl groups (-COOH)
along the polymer chain.
Some commercial formulations of glassionomer cement are mixed with water. The
manufacturer places the polyacid under vacuum. The vacuum removes the water (e.g.,
dehydrates) in which the polyacid is dissolved,
turning the liquid into a powder. This powder
is then mixed with the glass powder to form a
single powder. This latter powder can then be
mixed with water to form the glass-ionomer
cement. Several such water-activated glassionomer restoratives are currently on the market. Such systems have longer shelf lives than
conventional systems.
The liquid acids have been found to become
less reactive after several years. Cements
mixes made from aged liquids show decreased
3
Glass-Ionomer Restorative Materials
Oral Biomaterials - Chapter 14
diametral tensile strengths. All these liquids
should be stored in cool locations.
The Setting Reaction
As shown in Fig. 5, glass-ionomer cements
are the product of the reaction between an
1.
dissolution
2.
gelation
3.
initial hardening
4.
long-term strengthening
CH
2
CH
COOH
CH2
CH
Calcium
Aluminosilicate
Glass
+
COOH
Polyacrylic acid
(PAA)
2+
Ca
CH
2
CH
CH
CH
CH
2
COO
F
OOC CH
Al
2
COO
F
3+
CH
OOC CH
Ca, Al polysalt hydrogel
2
Dissolution. When the
powder and liquid are mixed,
the carboxyl groups are ionized
forming carboxylate ions
(COOH to COO-) and hydronium cations (hydrated protons H3O+) attack the surface of the
glass particles. Calcium ions
(Ca++), sodium ions, complex
aluminate anions, silicate
anions, and fluoride ions (F-)
are leached from the glass surface. That is, the ions dissolve
into the water-polyacid solution.
Gelation. Some of the ions
that enter the water-polyacid
solution react with the multiacid and a base. Fig. 5. Glass-ionomer cements set do to a
ple carboxylate ions that reside
reaction between an acid (PAA or a cousin)
The products of
along the length of each polyand a base (the glass).
acid-base reacacid molecule, forming calcium
tions are a salt and water. A salt is produced
polyacrylate and aluminum polyacrylate salts
when a metal ions replace hydrogen ions in an
(i.e., polysalts). Because the aluminum ions
acid. We shall see that, in glass-ionomer
are released as ionic complexes, the complexes
cements, polyacids become calcium or alumust be further broken down before aluminum
minum polysalts.
cations are available to form aluminum polyacrylate salts. Consequently, calcium polyThe actual reaction involves only the suracrylate salts form first and aluminum polyfaces of the glass particles. The surfaces of
acrylate salts form later.
particles dissolve into the water-acid solution.
As calcium and aluminum ions from the glass
Note, however, that at first, only a few of
react with the acid, the water-acid solution
the ions released by the glass react with cargels around the partially dissolved glass partiboxylate groups. Most of the ions remain in
cles. The resulting cement is a glass- reinthe water that surrounds the polyacid moleforced composite in which glass filler particles
cules. As the ions react with the carboxylate
are bound together by a matrix that may be
groups, they reduce the acidity of the of the
described as a polysalt hydrogel.
water (increase its pH). Unlike the polyacids,
the calcium and aluminum polyacrylate salts
The setting reaction is can be thought of as
are insoluble in water. Consequently, as the
consisting of four stages:
4
Glass-Ionomer Restorative Materials
Oral Biomaterials - Chapter 14
ions react with the carboxylate groups, the
polyacrylate molecules begin to precipitate.
The resulting solid material, consisting of a
tangle of thousands of long polyacrylate molecules (mostly calcium polyacrylate) with water
trapped in and between each molecule, is
called a hydrogel.
The initial set of glass-ionomer cements is
due to this gelation. The gelation, in turn, is
due to the insolubility of calcium and aluminum polyacrylate salts in water. Although
mechanisms have been suggested to explain
this insolubility, they are not well enough
established to be presented here (see Wilson
AD, McLean JW. Glass-Ionomer Cement
Quintessence Publishing Co, Chicago, 1988,
47-49).
Initial hardening. Gelation occurs minutes after mixing. If the glass- ionomer is to
reach its maximum strength and hardness, the
next 24 hours are critical. (Here 24 hours is
fairly arbitrary; some currently marketed
cements may achieve initial hardening sooner
than this.) During the initial hardening period, the cement is soluble in oral fluids and
must be protected from dissolution by a layer
of light-cured resin. If the cement undergoes
dissolution, it will not develop its full strength.
Initial hardening apparently involves two
processes:
1)
continued reaction of calcium and aluminum ions with carboxylate ions some of these ions cross-link polyacid
chains, and
2) hydration of polyacid chains.
As the calcium and aluminum ions continue
to react with carboxylate ions, some of these
ions cross-link polyacrylate salt molecules,
thereby, increasing the rigidity of the entangled matrix of polyacrylate molecules. (Note
that early explanations of the setting reaction
asserted that cross-linking was responsible for
gelation. Current thinking, however, is that
few cross-links form at gelation. Most of the
cross-linking occurs subsequently.) If ions
embedded in the hydrogel dissolve before
cross-links can form, the cement will never
develop its full strength and hardness.
The second process that occurs during initial hardening is hydration. Water surrounding the precipitated polyacrylate salt molecules becomes hydrogen bonded to the polysalt
molecules. That is, unbound water molecules
Fig. 6. Set glass-ionomer cements are composites. Partially dissolved glass particles
reinforce a hydrogel matrix.
are converted to bound water molecules. The
ions dissolved in the loosely bound water (calcium, aluminum, fluorides, silicon, and sodium) are bound to the polyacrylates along with
the water molecules. After hydration, the
cement is no longer susceptible to disintegration.
Long-term hardening. Glass-ionomer
cements, however, do not reach their full
strength until weeks or months after mixing.
The mechanism for this long-term hardening
is controversial. One view is that the hardening is caused by further hydration of the polyacrylate molecules. Another view is that the
bound water molecules hydrate silicon ions,
forming a silica gel. Recent transmission electron microscopy of glass-ionomer cements supports the latter view - it shows that silicon is a
part of the cement matrix.
The Set Cement
In summary, set glass-ionomer cement is a
composite. Aluminosilicate glass particles are
embedded in a matrix that is an amorphous
hydrogel of calcium and aluminum polyacrylate salts and, possibly, an amorphous silica
gel.
5
Glass-Ionomer Restorative Materials
Oral Biomaterials - Chapter 14
Absorption of water. Freshly mixed
glass-ionomer cements absorb water. It is
believed that the extra water interferes with
cross-linking of the polyacids. The result is a
cement that is weak and highly soluble in oral
fluids. As will be seen in the following, coating
hydrogel
matrix
Partially
dissolved
glass
particle
of the a restoration's surface to protect against
water absorption is an essential step in placing a glass- ionomer restoration.
Solubility and Erosion. If glass-ionomer
restoratives are protected from early exposure
to water, they are extremely resistant to subsequent dissolution. Unlike silicate cements,
glass-ionomer restoratives do not disintegrate
in organic acids. This is surprising since the
water solubility of fully set glass-ionomers is
only slightly lower than that of the silicates.
After initial hardening, fluorides, aluminum,
sodium and silicon continue to dissolve from
the cement (albeit at a continuously decreasing rates), but the cement does not disintegrate.
The exceptional resistant of glass-ionomers
cements to dissolution is due to the long polymer molecules that make up its matrix. The
polyacrylate molecules are highly entangled.
Consequently, even if a cross-linking cation
dissolves, it is unlikely that the polyacrylate
molecule will become disentangled from other
polysalt molecules. Moreover, loss of one
cross-link is unimportant. A given molecule is
likely to be cross-linked at multiple sites.
6
Some of these cross-links will be particularly
stable: they are formed by calcium ions, which
do not dissolve from set glass-ionomer restoratives. Note that loss of ions such as fluorides,
sodium, and silicon is even less important
than loss of aluminum. These do not participate in cross-linking and, consequently, don't
have major structural importance.
There is a difference in the solubilities of
glass-ionomer restoratives made with polyacrylic acid-based liquids and polymaleic acidbased liquids. The polyacrylic-acid-based liquids suffer less loss of substance when eroded
by a weak organic acid. It is not known
whether this result has clinical significance.
Fluoride release. Like the silicate
cements, glass-ionomer restoratives slowly
release fluoride ions. Unlike the silicate
cements, the fluoride release from glass-ionmer cements occurs without loss of strength
and disintegration of the cement. The release
of fluoride is initially high but decreases to a
constant value which is maintained for a very
long time (see Fig. 7).
The fluoride originates from the cement's
matrix where it is bound, probably with other
ions and water, polyacrylate salt molecules.
4
flu o rid e release (PPM )
Properties
3
2
1
0
0
20
40
60
80
100
m o n th s
Fig. 7. Fluoride ion release from a glass
ionomer cement continues at a constant reate
that continues for at least eight years. [From
Fortsen, Biomaterials 1998;19:503-508.]
Glass-Ionomer Restorative Materials
Oral Biomaterials - Chapter 14
The fluoride in unconsumed glass particles is
not available for dissolution. Laboratory tests
have shown that the fluoride released by
glass-ionomer luting agents and restorations is
retained by enamel and that this enamel
exhibits increased resistance to artificially
induced demineralization. In spite of laboratory data that suggests that glass-ionomers
cements may be anticariogenic, clinical trials
have yet to find a decrease in recurrent caries
in teeth restored with glass-ionomer restorative materials.
Acidity and Compatibility with the
Pulp. The initial pH of glass-ionomer restoratives is 2.5. By 24 hours, the pH is 5.3.
Tests of glass-ionomer restoratives in cultures of human pulp cells (Kawahara et al.: J
Dent Res 1979:58:1080-1086.) show that set
glass-ionomer cement have no effect on the
density of cells in the culture. Freshly mixed
glass- ionomer cement slightly reduces the
density of cells in culture, but the reduction is
less than is observed for other dental cements.
In the chapter on luting cements, reports of
pupal sensitivity to glass-ionomer luting
cements will be discussed. There have been no
reports of pulpal sensitivity to glass-ionomer
restorative materials.
Strength. As is the case for the other dental cements, the strength, solubility, and setting time of glass-ionomer restoratives is
affected by the powder-liquid ratio of the
cement. At 24 hours after mixing, compressive
strength of glass-ionomer restoratives are
around 170 MPa for powder-to- liquid ratios
(P/Ls) near 3.15, which are typical glassionomer restoratives materials.
If the glass-ionomer cement is stored in an
environment where ion loss is prevented, the
compressive strength will continue to rise to
275 MPa at one week and 380 MPa at one
year. This continuous increase in strength is
not seen when the cement is stored in water.
After 24 hours there may be a gradual
increase in strength such that the compressive
strength is 214 MPa at seven days. However,
this is about as high as the compressive
strength gets.
The strength of glass-ionomer restorative
materials is lower than that of composites and
amalgams. Moreover, the tensile strength
(diametral tensile strength) of glass-ionomer
restorative materials is relatively low - 13-14
MPa. Note that the tensile strength of amalgam and composite resins is around 60 MPa.
This means the cement is extremely brittle
and that it should not be used in restorations
that are subject to occlusal loads.
Chemical bonding. Glass-ionomer
restoratives contain many free carboxylate
ions. In the freshly mixed cement, these carboxylate ions are available for bonding. Some
of these ions react with calcium and aluminum
cations that are leached from the glass.
However, there are other carboxyl groups
available to form hydrogen bonds with the
tooth or restoration. The hydrogen bonds are
thought to gradually convert to ionic bonds
with multivalent cations such as calcium or
silver.
Bonding to enamel and dentin. The
strength of the bond to enamel is difficult to
estimate because the cement itself often fails
before the bond with the tooth fails. In any
case, specimens bonded to enamel tested dry
24 hours after bonding yield tensile strength of
at least 4.5 MPa. Specimens bonded to dentin
exhibit lower tensile bond strength - 2.4 MPa.
Cleaning dentin with polyacrylic or other
polyalkenoic acid before the glass- ionomer
cement is applied will nearly double the bond
strength. This dentin conditioning treatment
will remove the organic biofilm and any smear
layer (from instrumentation) that may be present. Unfortunately, such acid pretreatment
7
Glass-Ionomer Restorative Materials
Oral Biomaterials - Chapter 14
also opens dentinal tubules and may increase
pulpal sensitivity.
now being marketed. These may be more
acceptable aesthetically.
There is some evidence that such etching
demineralizes the dentin. Prior to setting, this
hydrophilic cement is able to penetrate this
dentin and to form a shallow hybrid layer.
Interpenetration of cement into tooth structure
may add a mechanical component to the bond.
Manipulation
The bond of these cements to dentin is only
a small fraction of the bond strength of the
some dentin bonding agents. The best of the
latter, have bond strengths close to that of
resin to acid etched enamel (18 MPa). Bond
strengths in the 3 to 8 MPa range are typical
for glass-ionomer restoratives.
The adhesion of glass-ionomer restoratives
to dentin and enamel has one clear advantage.
When the glass-ionomer cements are used as
restorative materials, little marginal leakage
is observed. The cement forms a tight seal.
Considering the cement's relatively weak bond
strength, the tight seal may strike one as surprising.
The cement's thermal expansion coefficient
may provide an explanation for the good sealing ability. Unlike resin cements, the glassionomer's coefficient of thermal expansion is
close to that of dentin and enamel. Consequently, temperature extremes produce smaller stress concentrations at margins between
tooth structure and glass-ionomers restorations than at margins between tooth structure
and resin composite restorations.
Color. The first generation glass-ionomer
restoratives were fairly opaque due to the fluoride crystals in the glass particle. Aesthetically, resin composite restorations are superior to restorations made with glass-ionomer
restoratives. It has been reported that the
color match with dentin is better than that
with enamel. More translucent cements are
8
Preparation of the tooth. Glass-ionomer
restorative materials are recommended for
restoration of medium to large Class 5 cervical
lesions. Because of glass-ionomer's extreme
brittleness, restoration of small and shallow
lesions is not recommended.
Because of the high initial solubility of
glass-ionomer restorative materials it is essential that the area to be restored be isolated
with a rubber dam or with cotton rolls.
Retraction cord can be used to protect the
preparation from crevicular fluid.
For cervical erosion/abrasion lesions, no
preparation is necessary. When caries are
present, remove only the carious material.
Small or shallow lesions should be enlarged to
make room for a sufficient thickness of the
glass- ionomer restorative material.
Conditioning the tooth. If tooth structure is removed, dentin and enamel will be
covered with smear layers. To remove the
smear layers and improve the retention of the
glass-ionomer, condition the preparation with
a 5-40% aqueous solution of polyacrylic acid.
Etch for at least 20 seconds when using a
dilute acid (5-20%). A 5-10 second etch is sufficient when the concentrated acids are used.
Use a passive etching technique; do not rub
the tooth surface.
It is suggested that Class V erosion/abrasion lesions not be etched. These eroded areas
contain calcium salts deposited by saliva - consequently, they present an enhanced surface
for bonding to the cement.
Hydrating the tooth's surface. It is
thought that very dry ("desiccated") dentin will
absorb water from the cement. Since glassionomer restoratives are weakened when they
Glass-Ionomer Restorative Materials
Oral Biomaterials - Chapter 14
become dehydrated, it is possible that cement
adjacent to the dentin will be weaker. When
placed under stress, this weak cement may
fracture.
To avoid desiccating the dentin, moisten the
surface with a section of a wet cotton roll.
While mixing the cement, blot the visible moisture with a dry cotton pellet. For best bonding, the dentin needs to be moist, but free of
visible fluid. Do not use an air syringe to dry
the dentin.
Mixing - Predosed Capsules. Glassionomer restorative materials are available as
either a powder and liquid that is mixed on a
mixing pad or in predosed capsules that are
mixed using an amalgamator. The capsules
make is possible to consistently make mixes
that include high powder-to-liquid ratios.
High ratios are essential if maximum strength
and low solubility is to be attained. Capsules
usually include a dispensing device that allows
the mix to be squeezed from the capsule directly into the preparation.
Mixing - Powder and Liquid on a
Mixing Pad. Mixing powder and liquid does
have one advantage: one can mix differently
colored glasses to improve the restorations
aesthetics. The manufacturer will suggest an
optimal powder-liquid ratio. For glassionomer restorative materials, this is usually
about 3:1. Mixing on a cool slab will prolong
the working time. One tries to incorporate the
powder into the liquid quickly; mixing should
be complete in 45 seconds.
Placing the restorative material. The
cement should be placed in position while it is
glossy. That is, it must be placed quickly.
Working times are approximately 2 minutes
from the beginning of mixing. If the cement is
dull, it will not bond to enamel or dentin.
Inject the mix into a plastic matrix if possible.
Otherwise, manipulate the cement with a plastic instrument.
Initial finishing of the restoration.
Immediately, coat the surface of the restoration with a water-proof varnish or with a lightcurable unfilled resin. If a resin is used, do
not cure it. Remove any excess cement with a
sharp knife or carbide bur. Then reapply the
resin or varnish. This time light-cure the
resin.
Final finishing. For best results, delay
final finishing for 24 hours. However, most
current glass-ionomer restorative materials
can be finished 15 minutes after initial set.
After finishing, recoat the restoration's surface
with varnish or with a light-curable unfilled
resin. Cure the resin. Even 24 hours after
initial set, the cement's properties can still be
degraded by absorption of water.
Dehydration of restorations. Glassionomer restorations should also be protected
against dehydration. Such dehydration can
occur when one is using a rubber dam to isolate a nearby tooth. Dehydration (sometimes
called desiccation) of the restoration's surface
will lead to a finely cracked (crazed) and weakened cement. The water which is part of the
hydrated gel matrix is apparently essential to
its integrity. A glass-ionomer restoration that
is in jeopardy of drying out can be protected by
coating its surface with a varnish or with a
light-cured resin.
II.
Resin-Modified Glass-Ionomer
Cements
History
During the 1980s researchers put considerable
effort into improving glass-ionomer cements.
It seemed to many that glass-ionmers had the
potential to be an ideal restorative material.
Glass-ionomer was a material that released
fluoride continuously without exhibiting a
degradation in mechanical properties. It was
also a self-adhesive material. No separate
adhesive was needed! Moreover, investigators
9
Glass-Ionomer Restorative Materials
Oral Biomaterials - Chapter 14
suspected that the bond to tooth structure
might be stronger than indicated by bond
strength measurements. A number of investigators had noticed that glass-ionomer specimens were failing cohesively, not adhesively.
That is, the glass-ionomer would break before
the bond between the glass-ionomer and the
tooth broke.
The trick was to preserve glass-ionomer
cement’s good qualities (fluoride release and
adhesion to tooth structure) while simultaneously fixing its major deficits. The resin-modified glass-ionomers fixed three deficits:
1.
the properties of RMGIs did not
degrade when the cement was exposed
to water or saliva soon after setting.
2.
the properties of RMGIs did not
degrade when the restoration is allowed
to dehydrate.
3.
the RMGIs were stronger and tougher
than glass-ionomers.
Polymerizable Liquids
Simple Liquids. The goal is to find polymers that can go into aqueous solution with
polyacrylic acid. One such polymer is HEMA
(hydroxyethylmethacrylate, see Fig. 4, Chapter 13). HEMA contains a double bonded carbon atom, which in the presence of either
chemically-activated or light-activated initiator can polymerize as shown in Fig. 8. The
trick is to control the rate of polymerization so
that both polymerization and the acid-base
reaction go forward. For the material to act
like a glass-ionomer there must be enough
water available for the carboxyl groups to ionize to carboxylate ions. It is these ions that
react with surfaces to produce bonding. It is
these ions that supply the protons that attack
the glass and, therefore, dissolve the glass constituents into the hydrogel. Without ionization there will be no fluoride release by the
10
H
H
....
H
H
C
C
C
C
C
C
C
C
C
H
C
H
C
H
C
H
O
O
O
...
O
H C HH C HH C H H C H
H C HH C HH C H H C H
OH
OH
OH
OH
Fig. 8. PolyHEMA. With the appropriate initator system, HEMA can polymerize in aqueous solution.
glass. Without ionization the material is just
another polymer.
The polyacrylic acid and the HEMA in
these simple system interact through being
entangled with one another. There is no
copolymerization between the HEMA and
PAA. The resulting matrix has not been
described, but it is presumably an intimately
mixed two phase mixture of polyHEMA and
polysalt. The polyHEMA provides a strong
and insoluble grid that evidently protects the
hydogel.
Liquids that utilize polymerizable
polyacids. In the late 1980s, Sumita Mitra of
the 3M Co. successfully grafted a polymerizable acrylic group onto a polyacid molecule,
thereby making it possible for the cause the
same molecule to take part in both the acidbase reaction and the polymerization reaction.
Fig. 9 shows an example of such a molecule
and Fig. 10 illustrates that ability of such a
molecule to undergo both acid-base and polymerization reactions.
When polymerizable polyacids are used, the
acid-base and polymerization reactions occur
along the same molecules. The two chemical
processes are tightly linked. There is no possibility for the two systems to unentangle themselves.
Glass-Ionomer Restorative Materials
Oral Biomaterials - Chapter 14
C OC OC OC OC OC O
OH OH OH
OH OH O
3.
the solvent in which the polyacids are dissolved.
4.
the element that is added to silica glass to
make the glass soluble in acids.
5.
the two salts that form within polyacidwater solution of glass-ionomer restoratives and cause this solution to gel; correct
statements about the relative solubility of
polyacids and polysalts in water on the
gelation to form the matrix of glassionomers cements.
6.
the chemical group that reacts with calcium ions to form polyacid salts during the
setting of glass-ionomer restoratives.
7.
the relative strength of the adhesion of
glass-ionomer cements to enamel and
dentin.
8.
four uses for glass-ionomer cements; correct statements about the type of restoration in which glass-ionomer restorative
materials are most frequently used.
9.
the three major improvements of resinmodified glass-ionomer cements as compared to glass-ionmer cements.
R C CH2
CH3
Fig. 9. Polyacrylic acid with a methacrylate
graft.
O
C
O
_
+2
_
M
O
O
O
C
R
O
O
H
H
C
C
CH3 H
H
CH
C
_
_
O
C
O
C
O
HC
R
C
CH3
Fig. 10. Two polyacrylic acid molecules that
each have methacrylate groups grafted to
them have reacted with one another in aqueous solution In the acid-base reaction, they
react with metal ions leached from the glass.
The polysalts formed precipitate in the water
causing gellation. Simultaeously, crosslink
formation via polymerization could also cause
the matrix to harden.
IV.
Behavioral Objectives
From a list of choices you will be able
to select:
1.
the polyacid present in the largest percentages in the first generation of glassionomer cement liquids and is still a component of many currently marketed glassionomer liquids.
2.
the polyacid that is the largest part of
other currently marketed glass-ionomer
liquids.
10. the approximate pH of the set glassionomer restorative materials and correct
statements about the compatibility of
glass-ionomer restorative materials with
the pulp.
11. correct statements comparing the compressive strength and tensile strength of
glass-ionomer restorative materials.
Correct statements comparing the tensile
strength of these cements to that of composite resins and amalgam. Correct statements about how low tensile strength limits the use of glass-ionomer restorative
materials.
12. statements explaining why the surfaces of
glass-ionomer restorations are coated with
11
Glass-Ionomer Restorative Materials
Oral Biomaterials - Chapter 14
varnish or resin immediately after placement; correct statements about effects of
failure to protect surfaces.
13. correct statements about the translucency
of glass-ionomer restorative materials.
14. correct statements about finishing procedures for glass-ionomer restorations.
15. the official name (designated by the
International Standards Organization) for
glass-ionomer cements.
16. two dental cements that release fluorides.
17. correct reason for the need to isolate a
lesion prior to restoring with a glassionomer cement.
18. the correct reason for not completely drying (desiccating) and slightly moistening
dentin prior to placing a glass-ionomer
restoration.
19. correct statements about the effect of tartaric acid on the rate of setting of glassionomer restorative materials.
You will be able to select the correct definition or description of each of the following terms, or, given the definition or
description, select the correct term.
carboxyl groups
carboxylate ions
dissolution
gelation
glass polyalkenoate cements
glass-ionomer restoratives
hydrogel
initial hardening
long-term hardening
maleic
polyacids
polyacrylates
polyacrylic acid
polymaleic acids
12
polysalts
salts
silicate cements
tartaric acid