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CHAPTER
16
Dental Materials
37
structure or restorative material at the interface. Such
deterioration would normally increase microleakage.
CAVITY VARNISHES
Cavity varnishes have been used empirically for many
years as a liner for cavity preparations. When varnish is
painted onto the cavity preparation, the solvent evapo-
rates and leaves a thin resin film. The varnish may
reduce microleakage when it is used with certain
restorative materials. Cavity varnish may reduce initial
microleakage around amalgam. Since varnish films are
very thin and do not adhere to tooth structure, some
clinicians have replaced cavity varnish with one of the
dental adhesives currently marketed. In theory, if such
a material adheres to dentin and enamel, any leakage
should occur between the amalgam and the adhesive.
The results of laboratory studies provide limited
support for this theory, although a controlled clinical
study by Mahler et al has shown no reduction in post-
operative sensitivity with the use of dentin adhesives as
liners for amalgam restorations.
CEMENT BASES
The function of the cement base is to promote recovery
of the injured pulp and to protect it against further
insult. The base serves as a thermal insulator and
replaces missing dentin when it is used under the
metallic restoration. The base must be of sufficient
thickness to provide effective insulation. A minimum of
approximately 0.5 mm is required for this purpose.
A base must be able to support the condensation of
the restorative material placed over it. If the strength of
the base is inadequate, it may fracture during conden-
sation and permit amalgam to penetrate and come into
contact with the dentin floor, which thereby compro-
mises the thermal protection afforded by the base. Zinc
phosphate, hard-setting calcium hydroxide, zinc
oxide—eugenol, and glass ionomer cements have suffi-
cient strength to serve effectively. In certain cases, such
as a class II preparation that involves the restoration of
an angle or of a deep depression, it may be necessary to
cover a calcium hydroxide base with a layer of stronger
zinc phosphate or glass ionomer cement.
AMALGAM
Controversy regarding the safety of the dental amalgam
restoration has existed since the material was introduced
to the profession more than 150 years ago. Periodically
this controversy surfaces in the press and other news
media and becomes a matter for public, as well as
professional, debate. As a result, the dentist who uses
dental amalgam can expect questions to be raised by
patients and their guardians and also can expect
requests for replacement of intact amalgam restorations
with other materials.
Amalgam is no longer the most commonly employed
material for restoring posterior carious lesions. Tooth-
colored restorative materials, such as composite resins
and resin-modified glass ionomers, are seeing increased
usage based primarily on considerations. The popularity
of dental amalgam likely will continue to decline as the
longevity of these other materials and their suitability as
general amalgam replacements in the permanent denti-
tion is demonstrated.
The unique clinical success of amalgam during 150
years of use has been associated with many
characteristics. It is likely that its excellent clinical
service, even under adverse conditions, is attributable
to the tendency for its microleakage to decrease as the
restoration ages in the oral cavity. Although amalgam
does not bond to tooth structure and the margins of an
amalgam restoration may appear open, the restoration-
tooth interface immediately below the exposed margin
becomes filled with relatively insoluble corrosion
products that inhibit leakage. Amalgam is unique from
this standpoint. The microleakage around other restora-
tive materials usually increases with time. Amalgam is
the least technique sensitive of all current direct restora-
tive materials. One of the factors slowing the acceptance
of posterior composite resin restorations has been the
very exacting clinical technique and time required for
placement. Another unique property of amalgam as a
direct filling material is its lack of dimensional change
during hardening. The ADA specification for dental
amalgam limits maximum acceptable dimensional
change to ±0.2%. If this is compared with a typical value
of 2.0% or higher for the polymerization shrinkage of a
resin matrix composite material, the potential impact on
microleakage is obvious.
Nevertheless, failures of amalgam restorations are
observed. These may occur in the form of recurrent
caries, fracture (either gross or severe marginal break-
down), dimensional change, or involvement of the pulp
or periodontal membrane. More significant than the
type of failure is its cause. Two factors that lead to such
clinical failures are improper design of the prepared
cavity and faulty manipulation. In other words, the
deterioration of amalgam restorations often can be
associated with neglect in observing the fundamental
principles of cavity design or abuse in preparing and
inserting the material.
One other factor also is involved, and that is the
choice of the alloy used.
SELECTION OF THE ALLOY
Several criteria are involved in the selection of an
amalgam alloy. The first criterion is that the alloy should
meet the requirements of the ADA Specification No. 1
or the corresponding ISO specification for dental
amalgam alloys.
38 Dentistry for the Child and Adolescent
The manipulative characteristics of dental amalgam
are extremely important and a matter of subjective
preference. Rate of hardening, smoothness of the mix,
and ease of condensation and finishing vary with the
alloy. For example, the resistance felt with lathe-cut
amalgams during condensation is entirely different from
that with spherical amalgams. The alloy selected must be
one with which the dentist feels comfortable, because the
operator variable is a major factor influencing the clinical
lifetime of the restoration. Use of alloys and techniques
that encourage standardization in the manipulation and
placement of the amalgam enhances the quality of the
service rendered. Coincident with this is the delivery
system provided by the manufacturer—its convenience,
expediency, and ability to reduce human variables.
Obviously the physical properties should be reviewed
in the light of claims made for the superiority of one
alloy over competing products. Ideally such a list of
properties should be accompanied by documented
clinical performance in the form of well-controlled
clinical studies. Although the cost of the alloy is a factor,
this criterion should not be overemphasized when
balanced against the alloy's ability to render maximum
clinical service. The dentist should always weigh the
fraction that the material cost contributes to the total
charges for a dental procedure when making price com-
parisons between brands, particularly when comparing a
brand with documented clinical performance against a
generic brand of material.
Dental amalgam alloys generally are available as
either small filings called lathe-cut alloys or spherical
particles called
spherical alloys.
Spherical alloys tend to
amalgamate very readily. Therefore amalgamation can
be accomplished with smaller amounts of mercury than
required for lathe-cut alloys, and the material gains
strength more rapidly. Also, the condensation pressure
and technique employed by the dentist in placing the
restoration are somewhat less critical in achieving the
same properties of the amalgam. This is an advantage in
difficult clinical situations in which optimal access for
condensation is limited. Spherical amalgam alloys have
a somewhat different feel during condensation and
require less condensation pressure than lathe-cut alloys.
The dentist and auxiliary should familiarize themselves
with the handling characteristics of a new alloy before
clinical restorations are placed.
HIGH-COPPER ALLOYS
The original dental amalgam alloys were alloys of silver
and tin with a maximum of 6% copper. When signifi-
cantly more copper is available, improved laboratory
properties and clinical performance has been demon-
strated. This improvement has been attributed to the
displacement of the tin-mercury reaction product with a
copper-tin phase during the amalgamation reaction.
Alloys that contain enough copper to eliminate the for-
mation of the tin-mercury phase (11% to 30%) are called
high-copper amalgam alloys. The first such alloy of this
type was an admixed system. Small spherical particles of
a silver-copper alloy were added to filings of a conven-
tional silver-tin alloy. High-copper alloys also can be
made using single composition particles. Each of these
alloy particles has the same chemical composition,
usually silver, copper, and tin. Amalgams made from
high-copper alloys have low creep. Creep is the tendency
of a material to deform continuously under a constant
applied stress. This property has been associated with
the marginal breakdown (ditching) commonly noted
with amalgam restoration. Although the ADA
specification for dental amalgam permits a maxi-mum of
3% creep, a modern high-copper amalgam alloy should
not exceed 1% creep.
Choice of amalgam alloy today should be limited to
high-copper alloy systems.
Regardless of the alloy used, manipulation plays a
vital role in controlling the properties and the clinical
performance of the restoration.
MERCURY/ALLOY RATIO
Most of the properties of amalgam restorations have
been shown to depend on the relative amount of mercury
contained in the finished restoration (the residual
mercury). One of the variables that control the final
mercury content is the amount of mercury used to mix
the amalgam.
Although dental amalgam alloy still may be available
in the form of powder or preweighed compressed pellets
and bulk mercury can be dispensed by volume, most of
the amalgam alloy sold today is in the form of prefilled,
disposable mixing capsules containing the proper
amounts of alloy and mercury. This delivery system
should be used for several reasons. The alloy/mercury
ratio is accurately preproportioned. The need for
disinfection procedures is minimized because the
capsule system is discarded after use. Most importantly,
exposure of dental personnel and environmental
contamination by mercury vapor is minimized. These
prefilled capsules are usually available for different size
mixes, often called single or
double spill capsules.
TRITURATION
The second manipulative variable that controls the
residual mercury content is trituration. Trituration time
can significantly influence both consistency and working
time of the mixed amalgam. These in turn relate to the
ability to bring excess mercury to the surface during
condensation. The correct trituration time varies
depending on the composition of the alloy, the
mercury/alloy ratio, the size of mix, and other factors.
CHAPTER
16
Dental Materials
39
The best practice is to acquire an appreciation for the
appearance of a proper mix and then to adjust the
trituration time accordingly. The most serious error in
amalgamation generally is undertrituration. An under-
triturated mix appears dry and sandy and does not
cohere into a single mass. Such an amalgam will set too
rapidly, which results in a high residual mercury
content, reduced strength, and the increased likelihood
of fracture or marginal breakdown. Properly mixed
amalgam is a shiny, coherent mass that can be readily
removed from the capsule.
MECHANICAL AMALGAMATORS
When first introduced, mechanical amalgamators for
dental amalgam operated at a single speed that was
usually below 3000 cpm. High-copper alloys in prefilled,
self-activating capsules are designed for shorter tritura-
tion times at higher trituration speeds. Failure to acti-
vate these capsules reliably results in undertrituration
and is a common problem with the use of older single-
speed amalgamators. Because amalgamators also
deteriorate with time, replacement of an older unit with
a new high-speed amalgamator is desirable. A unit that
allows multiple speeds of operation should be selected,
because numerous other products such as dental
cements are now marketed in capsules to be mixed in a
dental amalgamator. The trituration times suggested by
the amalgam alloy supplier are starting points.
Amalgamators may vary in operating speed even within
the same brand, and a unit's performance may vary
with line voltage or the number of times it is used in
rapid succession. Trituration speed, as well as time,
significantly influences the rate at which some amalgams
harden, as seen in Fig. 16-1.
CONDENSATION
The purpose of condensation is to adapt the amalgam to
the walls of the cavity preparation as closely as possible,
to minimize the formation of internal voids, and to
express excess mercury from the amalgam. Within
reasonable limits, the greater the condensation pressure,
the lower the amount of residual mercury left in the
restoration and the greater the strength of the restora-
tion. The selection of the condenser and the technique of
"building" the amalgam should be designed to achieve
those objectives, as described in detail in textbooks of
operative dentistry, and should be tailored to the
handling characteristics of the type of amalgam alloy
chosen, as previously discussed.
MOISTURE
Moisture contamination of an amalgam restoration can
promote failure. If zinc is present in the alloy, it will
react with water, and hydrogen gas will be formed. As
this gas builds up within the amalgam, a significant
f
I G 16-1.
The influence of amalgamator speed (low-medium-
high) on the hardening rate of a high-copper amalgam alloy as
measured by the Brinell Hardness (BHN). BHN = 1.0 indicates
the working time and BHN = 4.5 indicates the carving time. (
Redrawn from Brackett W: Master's thesis, Indianapolis, 1986,
Indiana University School of Dentistry.)
delayed expansion can occur and may cause protrusion
of the amalgam from the cavity preparation, which
enhances the possibility of fracture at the margins.
Such moisture contamination can result from failure
to maintain a dry field during the placement of the
restoration. Exposure to saliva after the amalgam has
been completely condensed is not harmful. It is only
moisture incorporated within the amalgam as it is being
prepared or inserted that must be avoided.
Zinc-free alloys are available, and their physical
properties are generally comparable to those of their
counterparts that contain zinc. A zinc-free, high-copper
alloy should be used when the dentist operates in a
field
where moisture control is difficult.
MARGINAL BREAKDOWN AND
BULK FRACTURE
Because dental amalgam is a brittle material, a commonly
observed type of amalgam failure is the restoration in
which the marginal areas have become severely
chipped. The exact mechanisms that produce this break-
down of the amalgam or the adjoining tooth structure
are not established, but it is likely that the deterioration
is precipitated by manipulation and technique of finishing
rather than by dimensional changes during setting.
If the restoration is improperly finished by the dentist,
a thin ledge of amalgam may be left that extends slightly
over the enamel at the margins. These thin edges of such
a brittle material cannot support the forces of mastication.
In time they fracture, leaving an opening at the margins.
Bulk fracture of amalgam is much less common with
high-copper amalgam alloys. Those cases that do occur
likely have one of two causes. Poor cavity design
40 Dentistry for the Child and Adolescent
resulting in an insufficient bulk of material across the
isthmus can lead to failure of even a high-strength alloy,
as illustrated in Fig. 16-2. The other reason for bulk
fracture is premature loading of the restoration. Unlike a
resin matrix composite, amalgam gains strength slowly
over the first 24 hours. Premature loading can result in
minute fractures that are not apparent for weeks or even
months. The use of a rapid-setting amalgam with a high
1-hour compressive strength should be considered when
treating a pediatric patient in whom compliance with
instructions to refrain from biting down hard on the
freshly placed amalgam is in question.
BONDED AMALGAM RESTORATIONS
Because dental amalgam does not adhere to tooth
structure, it must be retained mechanically by the design
of the cavity preparation and/or mechanical devices such
as pins. The placement of an amalgam does not
strengthen the compromised remaining tooth structure
and subsequent fracture may occur, particularly in molar
teeth with relatively large mesiodistocclusal amalgam
restorations. The use of dental adhesive systems, as
described in detail in the section related to resin
composites, as lining materials for amalgam has been
suggested in an attempt to create a "bonded amalgam
restoration." Several products are marketed specifically
for this purpose. In general, they are chemically activated
dentin-bonding systems over which the amalgam is
condensed before the resin adhesive has hardened. This
results in an intermixing of the unset resin and the
plastic amalgam at the interface and forms a mechanical
bond as both materials harden. It is important to distin-
guish this application from the use of a dental adhesive
to seal the dentin surface and reduce early microleakage
f
I G 16-2.
Bulk fracture of an amalgam restoration. Such failure
may occur from improper cavity design or premature occlusal
loading. (From Anusavice KJ:
Phillips'
science of dental
materials, ed 11, St Louis, 2003, WB Saunders.)
as previously discussed. When dental adhesives are
used to seal the dentin surface, the adhesive should be
polymerized before the amalgam is placed. Although
bond strengths reported in laboratory studies between
amalgam and dentin are lower than the maximum
reported for resin composite bonded to dentin, they are
in the same range. In vitro studies also show that teeth
restored with bonded amalgams are more resistant to
fracture than those in which amalgam is placed without a
bonding adhesive.
A caution must be added that these are relatively
short-term laboratory studies. Some longer-term clinical
data are available; however, little is known about the
potential influence of embedding the resin into the bulk
of the amalgam on the long-term properties of the
restoration. At the present time, amalgam bonding
should be considered only as an adjunct for conven-
tional, accepted practices of cavity preparation and
mechanical retention of amalgam.
MERCURY TOXICITY
The amalgam restoration is possible only because of the
unique characteristics of mercury. Mixing this liquid
metal with the alloy powder provides a plastic mass that
can be inserted into the tooth and then hardens rapidly
to a structure that resists the rigors of the oral
environment. As the restoration hardens, mercury
reacts with silver and tin to form stable, intermetallic
compounds. Most of the public controversy about the
safety of dental amalgam has focused on the hazards
associated with elemental mercury and some of its
organic compounds. Many substances commonly
regarded as quite safe contain extremely dangerous ele-
mental ingredients. No one would ever consider human
ingestion of elemental sodium or chlorine, but ordinary
table salt, which is the compound sodium chloride, is an
important dietary substance. From the time of the earli-
est use of amalgam, it has been asked whether mercury
in a dental restoration can produce local or systemic
toxic effects in humans. It is periodically conjectured
that mercury toxicity from dental restorations is the
cause for numerous illnesses of unknown etiology.
The possibility of toxic reactions by the patient to
traces of mercury penetrating the tooth or sensitization
from mercury dissolving from the surface of the amal-
gam is remote, however. The danger has been evaluated
in numerous studies. The patient's encounter with
mercury vapor during insertion of the restoration is
brief, and the total amount of mercury vapor is too small
to be injurious. Furthermore, the amount of mercury
released from the amalgam in service is small compared
with other sources of mercury from air, water, and food.
Metallic mercury in the human digestive track is
apparently not converted to lethal organo-mercury
compounds and is excreted by the body.
CHAPTER
10
Dental Materials
341
Both the National Institutes of Health and the FDA
have examined the evidence for risk of dental restorative
materials to the patient. The conclusion was that, except
for the very small fraction of the population with a true
allergic reaction to mercury or other constituents of
amalgam, the dental amalgam restoration remains a safe
and effective treatment. No evidence was found that
related the presence of amalgam restorations to
disorders such as arthritis, multiple sclerosis, or other
diseases in which amalgam has been implicated. It
should be noted that no currently available restorative
material is completely risk free and that patients should
be informed of the relative risks associated with all
dental treatment alternatives.
The question about the replacement of existing
serviceable amalgams with other materials remains one
of professional judgment. Both the ADA and some state
dental licensing boards have found that a dentist who
recommends replacement of amalgam restorations with
other materials on the claim that this will improve the
physical health of the patient may be acting unethically
and may be subject to sanctions by licensing bodies and
to suits for civil damages. Patients who feel that they
have medical problems related to the presence of any
dental restorative material should be referred to a physi-
cian for diagnosis and treatment recommendations.
What about dental office personnel? Restorative
dentists and their office personnel potentially are
exposed daily to mercury, even in offices in which
amalgam restorations are not being placed. Although
metallic mercury can be absorbed through the skin or by
ingestion, the primary risk to dental personnel is from
inhalation. A potential hazard exists for dentist and staff
from chronic inhalation of mercury vapor in the dental
clinic, although the few actual incidents reported have
been related to poor technique in handling mercury. The
maximum level considered safe for occupational
exposure is 50 p.g of mercury per cubic meter of air
averaged over a standard 8-hour workday. Mercury at
room temperature has a vapor pressure almost 400
times the maximum level considered safe. This vapor has
no color, odor, or taste and cannot be readily detected by
simple means at the level of maximum safe exposure.
Because liquid mercury is almost 14 times more dense
than water, a small spill can be significant. Eliminating
the use of bulk mercury by employing prefilled,
disposable capsules should significantly reduce exposure
to mercury vapor.
The dental operatory should be well ventilated. All
mercury waste and amalgam scrap removed during
placement or removal of amalgam restorations should be
collected and stored in well-sealed containers. When
amalgam is cut, water spray and high-speed evacuation
should be used. More detailed recommendations can be
obtained from the Regulatory Compliance Manual
published by the ADA. The risk to dental personnel from
mercury exposure cannot be ignored. However,
adherence to simple hygienic procedures will ensure a
safe working environment.
Waste materials containing mercury or amalgam
scrap should be disposed of responsibly in accordance
with the regulations of the environmental protection
agency in the area in which the dentist works. These
materials should not be incinerated or subjected to heat
sterilization. Biologically contaminated wastes containing
mercury, including extracted teeth, should be cold-
sterilized with a chemical agent before disposal. The most
significant threat to the continued use of dental amalgam
will likely be from government regulations on
environmental waste discharge. In Japan, use of amal-
gam has been discontinued because it is not feasible for
a dental office using amalgam to meet restrictions on mer
cury discharge into sewers. Amalgam-mercury
separators on dental clinic wastewater discharge lines are
now required in several countries in Europe. Local and
state authorities should be consulted about limitations
on discharge of mercury into wastewater from a dental
practice.
CEMENTS
Dental cements have several functions in restorative
dentistry. One is to serve as a luting agent to fill the space
between a restoration fabricated outside the mouth and
the tooth structure. By flowing into irregularities in both
materials and then hardening, the cement provides
mechanical retention as previously discussed. A second
function is to serve as a filling material for either per-
manent or temporary restorations. Cements are also used
as bases for other restorative materials as previously
described.
Silicate cement was an early tooth-colored filling
material. Although it is no longer used, its ability to
reduce development of secondary caries has made it a
model for the development of caries-resisting dental
materials. Recurrent or secondary caries was seldom
encountered around silicate cement restorations even
when gross disintegration had occurred. Most other
restorative materials have not shown such an ability to
resist recurrent caries, which is today the most common
cause for replacement of restorations.
This beneficial characteristic is attributed to the
presence of fluoride in silicate cement powder, which
typically contains approximately 15% fluoride. After
placement of the silicate restoration, fluoride ion is
released and reacts with the adjoining tooth structure in
much the same manner as does topically applied
fluoride. The enamel solubility is reduced, which builds
up its resistance to acid attack and caries. Because there
is evidence that the fluoride ions are released slowly
throughout the life of the restoration, the protective
mechanism is undoubtedly a continuous one.
42 Dentistry for the Child and Adolescent
LUTING
CEMENTS
Several types of cement may be used as luting agents.
Each has inherent advantages and disadvantages. Thus
the selection of a particular type of cement is governed
by the individual situation presented by the patient.
ZINC PHOSPHATE CEMENT
Formerly, zinc phosphate cement was the most widely
used luting agent. Composed essentially of phosphoric
acid liquid that is mixed with zinc oxide powder, the
cement has excellent handling characteristics such as
setting time, fluidity, and film thickness. Furthermore,
this type of cement has a long history of successful
application for permanent cementation. It does not have
an anticariogenic effect, does not adhere to tooth struc-
ture, and demonstrates a moderate degree of intraoral
solubility.
Because of the phosphoric acid liquid, zinc phosphate
cement is an irritant, and proper pulp protection is
recommended. In those situations in which experience
indicates that sensitivity and pulp response are likely to
be problems, use of a cement that is more biologically
compatible, such as a polycarboxylate cement, is recom-
mended.
POLYCARBOXYLATE CEMENT
Polycarboxylate cement is one of the few dental materi-
als that demonstrate true adhesion to tooth structure.
The powder is primarily zinc oxide, and the liquid is
polyacrylic acid or a copolymer of that acid. Although the
final pH of the set cement is comparable to that of zinc
phosphate cement, its biologic properties are excel-lent.
For this reason, polycarboxylate cement is useful as a
base or as a luting agent, particularly when the cavity
preparation is close to the pulp. In addition, as the
cement sets against the tooth structure, a chemical bond
is formed between the cement liquid and the calcium in
the hydroxyapatite in enamel and dentin.
When the cement is used as a luting agent, several
manipulative factors influence the wetting of the tooth by
the cement and thereby retention of the restoration. After
cavity preparation the enamel and dentin surfaces are
covered with a thin layer of tenacious debris, referred to as
the
smear layer.
Also the preparation may be covered by a
thin film of material, such as zinc oxide—eugenol, if a
provisional restoration was placed. Unless this
contamination is removed, it may inhibit adhesive
bonding of the setting cement to the tooth. One means of
cleaning the surface is a 10- to 15-second swabbing with
10% polyacrylic acid.
As with all types of cement, the liquid should not be
dispensed until just before the mix is to be made. To
slow down the setting reaction and provide longer
working time, a chilled mixing slab may be used. The
powder and liquid should be mixed rapidly, and the mix
should be completed within 30 seconds.
The recommended powder/liquid ratio should be
used. If the mix is too thick, insufficient acid is present
to produce bonding to the tooth. If excess liquid is used,
the intraoral solubility increases significantly. When
properly prepared, the mix has a glossy appearance and
can be extruded into a thin film. It is important that
minimal time elapse between completion of the mix and
placement of the cement; the mix must not have lost its
glossy appearance.
When polycarboxylate cement is used with cast restora-
tions, the inside surface of the casting must be cleaned
thoroughly. After the casting is cleaned in a pickling bath,
the interior should be treated with an air abrasive or a fine
stone. Polycarboxylate cement will not wet a chemically
dirty surface. In time, leakage and loss of retention may
occur along the cement restoration interface.
Although polycarboxylate cement demonstrates
adhesion to tooth structure, it has a relatively low tensile
strength, no significant fluoride release, and modest
intraoral solubility. Good practices of tooth preparation
should be used to insure retention of the restoration.
GLASS IONOMER CEMENT
Another type of cement that is based on polyacrylic acid is
glass ionomer cement (GIC). Because of its biologic kind-
ness, fluoride release, and potential for adherence to the
calcium in the tooth (as with the polycarboxylate system),
glass ionomer cement is used as a restorative material (
type II) for treatment of the eroded area, as a luting agent (
type I), and as a base and liner material (type III).
Like zinc polycarboxylate cement, the glass ionomer
liquid is polyacrylic acid or another alkenoic acid, such
as itaconic or maleic, with tartaric acid added to improve
handling properties. The acid has the potential for
bonding to calcium in the manner described for
polycarboxylate. This chemical bond provides retention
of the cement to the tooth.
The powder is a fluoro-aluminosilicate glass similar to
silicate cement powder and displays fluoride release
patterns similar to that of silicate cement. Data from glass
ionomer restoration of class V erosion lesions for periods
of more than 7 years indicates that GIC shows resistance
to secondary caries. One can immediately see the attrac-
tion of the GIC system: it has a potential for adherence to
tooth structure and possesses anticariogenic potential.
The material is supplied as a powder and liquid and is
commonly preproportioned in a disposable capsule to be
mixed in an amalgamator. With type I GIC, the liquid
acid may be freeze-dried and combined in the powder.
When this powder is mixed with water, the acid
reconstitutes, which results in the same setting reaction.
The freeze-dried products have better shelf life and
CHAPTER
16
Dental Materials
43
somewhat lower viscosity, which are important charac-
teristics for luting cements.
The mix can be made either on a disposable, moisture-
resistant paper pad or on a glass slab. A plastic spatula is
preferred to a metal one to minimize contamination of the
mix from abraded metal. As with polycarboxylate cement,
the polyacrylic acid–based liquid is not dispensed until
just before the start of the mix. The glass ionomer cements
are mixed in a manner like that used for polycarboxylate
cements: large increments of the powder are rapidly
incorporated into the liquid, and the mix should be
completed within 40 seconds. The working time is short,
usually no more than 3 minutes from the start of the mix.
In no instance should the material be used if the mix has
lost its gloss or a skin has formed on the surface.
After setting, the material is more brittle than a poly-
carboxylate cement. It can be trimmed and finished in
much the same manner as zinc phosphate cement.
Before the patient is dismissed, all the accessible mar-
gins should be covered with the varnish or protective
resin supplied by the manufacturer. This protects the
cement from oral fluids and dehydration during the next
few hours as the setting reaction continues.
Instances of postoperative sensitivity have been
reported when GIC is used as a luting agent, particu-
larly in deep preparations with minimal remaining
dentin. This is possibly attributable to the low initial pH
of the cement and its relatively slow set. To guard
against potential irritation, in very deep areas calcium
hydroxide should be placed. The cut dentin surface can
be cleaned mechanically with pumice, but the smear
layer should not be removed. After cleaning, the dentin
should be rinsed and dried but not desiccated. A slightly
damp surface appears to help minimize sensitivity and
does not interfere with the setting reaction.
Glass ionomer luting cements have mechanical prop-
erties similar to those of zinc phosphate cements and
lower intraoral solubility. Because of their potential for
fluoride release and adhesion to tooth structure, they
are becoming the most commonly employed luting
cements for metallic restorations.
In addition to its use as a luting agent for cast restora-
tions, GIC has been employed for bonding orthodontic
brackets to acid-etched enamel. GIC has lower cohesive
strength than do the resin orthodontic adhesives, but
the fluoride release from the GIC should minimize the
white spotting and decalcification sometimes seen
around orthodontic brackets or bands. If orthodontic
bands are employed on posterior teeth, the GIC is the
luting agent of choice.
RESIN-MODIFIED GLASS
IONOMER CEMENTS
The most recent addition to the cement field is the resin-
modified glass ionomer cement. These cements are also
sometimes referred to as
hybrid glass ionomers
or in the
case of type II and III cements as
light-cured glass
ionomers. Disadvantages of conventional glass ionomers
include short working time, slow development of ulti-
mate properties, sensitivity to both moisture exposure
and dehydration during setting, and lower cohesive
strength compared with resin cements. These problems
have been addressed by the development of resin-
modified GIC. Resin monomers or a comonomer of
acrylic acid and a methacrylate such as hydroxyethyl
methacrylate in the same manner as light-activated
restorative resin composites. The resin component
hardens immediately on exposure to the light, which
results in an initial set of the cement. The material then
continues to undergo the acid-base GIC setting reaction
that occurs more slowly than that of a conventional GIC;
this results in a much longer working time for the light-
cured glass ionomer. The rapid set after light exposure
yields a material that is much less sensitive to dehydra-
tion or moisture. Type I resin-modified GIC luting
cements are also available; in this case the resin compo-
nent is either chemically activated or dual (chemical and
light) activated. Resin-modified GIC type II restorative
materials appear to exhibit the advantages of conven-
tional GIC and have received rapid acceptance. The use
of resin-modified GIC luting cements is less well estab-
lished. Fracture of all-ceramic crowns cemented with
resin-modified GIC has been reported, which prompts
concern about their use with ceramic restorations.
ZINC OXIDE—EUGENOL
The acid-base reaction between zinc oxide and eugenol
results in a cement that can be used as both a luting and
restorative material. Because of its low strength and high
oral solubility, zinc oxide–eugenol is not recommended
as a permanent luting cement. However, because of its
exceptionally kind biologic behavior, it is often used as a
base material, as a temporary luting cement, and as a
temporary restorative material. Eugenol is an inhibitor
for additional polymerizing resins and can interfere with
subsequent use of resin cements, restorative materials,
and even impression materials.
RESIN CEMENTS
Resin luting cements are derived from the composite
resin systems used for restorative materials. They may
be viewed as lightly filled composites. The resin matrix
systems used are the same as those employed for
restorative resins. Although these materials are not new to
dentistry, they have only recently become used exten-
sively. Their first major clinical application was in direct
bonding of orthodontic attachments to acid-etched
enamel, for which they quickly became the materials of
choice. Similar formulations were developed into pit and
fissure sealants, which are discussed in Chapter 17.
44 Dentistry for the Child and Adolescent
The resin-bonded bridge such as the "Maryland" bridge is
another application in which resin cements came to the
forefront. The demand for dentistry has resulted in
extensive use of both resin and ceramic veneers. Here,
too, resin cements are the cements of choice. Finally, new
technology for fabricating all-ceramic crowns and inlays
has greatly increased the use of these restorations,
which are normally cemented with resin cements. Resin
cements have high strength, low film thickness, and very
low oral solubility, and can be bonded to etched enamel,
ceramics, resins, and etched or treated metal surfaces.
With the advent of dentin adhesives, resin cements
provide the possibility of bonded, indirect restorations.
Resin cements are usually available in different shades
for color matching beneath translucent restorations, and
opaque cements are made for masking metal
substructure or discolored tooth structure. The first resin
cements were two-component, chemically activated
curing systems. Visible light-activated, single-component
systems are now available and are popular when used
with translucent restorative materials. Dual-activated
materials, which are both chemical and light activated,
are recommended for use beneath thick restorations and
in locations where geometry may limit access to the
curing light.
TEMPORARY AND PERMANENT
RESTORATIONS
The temporary restoration should possess good biologic
characteristics, have minimal solubility, and be rigid,
strong, and resistant to abrasion. The relative impor-
tance of each of these properties depends on the degree of
permanence desired. For example, in the carious mouth
it is often desirable to remove some or all of the caries
immediately and place temporary restorations. These
restorations subsequently are replaced with more
permanent restorative materials. In such situations it
may be necessary for the temporary restoration to serve
for several months or longer. Strength and abrasion
resistance are of paramount importance in these cases.
Usually, temporary restorations need to remain in place
only for days. In the latter instance more emphasis may
be placed on the biologic properties when a material is
selected.
Because of its excellent tissue tolerance and ability to
minimize initial microleakage, zinc oxide-eugenol cement
has been commonly used for holding or inter-mediate
restorations. The strength, rigidity, and resistance to
abrasion of the conventional zinc oxide-eugenol mixture
have been improved by the addition of polymers and by
the surface treatment of the zinc oxide powder.
Type II glass ionomer cements or the newer resin-
modified GICs also are useful as long-term, temporary
restoratives, for example, in restoring eroded areas in
patients when exposed areas of cementum and dentin
are present. Because of its desirable biologic and adhe-
sive characteristics, the GIC can be used to restore these
lesions without the need for a retentive cavity prepara-
tion. If conventional GICs are employed as restorative
materials, they must be protected from exposure to
moisture in the early stages of setting and from dehy-
dration for a very long time, likely the entire time the
restoration will serve. In general, a resin-modified glass
ionomer is a better choice for reasons given previously.
GIC formulations that include fillers to improve their
mechanical properties are marketed. In one product
particles of amalgam alloy are mixed with the GIC
powder; another uses a cermet—silver sintered onto the
GIC glass powder. Although use of these materials for
core buildup and for very conservative class I and class
II restorations in primary dentition has been advocated,
the improvements in mechanical properties of these "
reinforced" GICs are very modest. Whether their
mechanical properties, such as fracture toughness, are
adequate to resist masticatory stress is questionable.
Again, the resin-modified glass ionomer is probably a
better choice.
RESTORATIVE RESINS
CONVENTIONAL COMPOSITES
The term composite material refers to a combination of at
least two chemically different materials with a distinct
interface separating the components. When properly
constructed, such a combination of materials provides
properties that could not be obtained with any of the
components alone. (Examples of natural composites are
bone, tooth enamel, and wood.) In a resin composite
dental restorative material, an inorganic filler has been
added to a resin matrix in such a way that the properties
of the matrix have been improved.
A number of parameters have a pronounced influence
on the properties that are obtained by the addition of
inorganic fillers to a resin matrix. The characteristics of
the dispersed phase in terms of its shape, size, orienta-
tion, concentration, and distribution are very important.
Likewise, the composition of the continuous phase, the
resin matrix, is equally significant.
The resin matrix of many currently available composite
materials is bisphenol A-glycidyldimethacrylate (bis-
GMA) or urethane dimethacrylate resin. Triethylene
glycol dimethacrylate, a lower-viscosity resin, is usually
added as a diluent. Among the materials used for macro
fillers are ground particles of fused silica, crystalline
quartz, and soft glasses such as barium, strontium, and
zirconium silicate glass. These particles, which make up
70% to 80% of the material by weight, resist deformation
of the soft resin matrix. The high filler content and the
chemistry of the resin matrix substantially reduce the
CHAPTER
16
Dental Materials 45
coefficient of thermal expansion compared with an
unfilled acrylic resin. The filler also reduces the poly-
merization shrinkage and increases the hardness.
The filler and the resin matrix must be chemically
bonded together with a coupling agent on the surface of
the filler. If this is not done, the particles may be easily
dislodged, water sorption at the filler-matrix interface
may take place, and stress transfer between matrix and
filler may not occur. The filler particles are coated with a
reactive silane product. Despite use of this coupling
system, the filler particles do become dislodged during
cutting and finishing and under abrasive action such as
tooth brushing or occlusal contact. This abrasive action
likely affects the softer resin matrix, which becomes
worn away and exposes the filler particles. When enough
of the filler particle is exposed, it will break free of the
resin. This process leaves a continual rough surface as
seen in Fig. 16-3. Because the composites are 70% to
80% filled, this surface roughness is clinically noticeable.
Based on clinical experience, there has been a definite
preference for use of smaller filler particles. In the early
resin composites it was common for the particle size to
approach 100 µm; now the coarsest particles would not
exceed 30 µm. The average mean particle size of the
fillers in conventional composites is in the 8- to 12-µm
range, and the trend is to reduce the filler size even
further.
MICROFILLED COMPOSITES
Efforts to improve the surface smoothness and polish-
ability of composite resins led to the development of the
microfilled composite. These composites are based on
the use of an extremely small silica filler particle, whose
size is 0.02 to 0.04 µm, and hence are called microfine,
microfilled, or polishable resins. The particles may be
dispensed directly into the paste, but the amount that
can be added in this manner is very limited. Addition of
amounts in excess of 20% results in a paste too viscous
for the dentist to use. Matrix resin monomer can be
heavily filled with microfine silica and polymerized in the
manufacturing process. The resulting composite is
ground to filler particle sizes comparable to those of the
inorganic filler in conventional composite. This "organic"
filler with additional colloidal silica is then added to the
resin monomer to form the composite resin paste. The
structure of such a resin is illustrated in Fig. 16-4.
The appealing characteristic of these microfilled resins
is their ability to be finished to an extremely smooth
surface, which was a major problem with the
conventional composites. When microfilled resins are
finished, the polymerized resin filler particles cut at the
same rate as the matrix and a much smoother surface
results, as shown in Fig. 16-4. Even if some of the very
small silica particles are dislodged, the surface irregu-
larities cannot be detected by the eye.
Because of the small silica particle size, the filler has a
very large surface area, and the total amount of filler
that can be incorporated is reduced to about 50% by
weight compared with a filler loading of 70% to 80% for
conventional composites. Thus, the microfilled composite
has a higher resin matrix content. As a result, such
resins are softer and have a slightly higher coefficient of
thermal expansion, a higher water absorption, more
polymerization shrinkage, and somewhat lower mechan-
ical properties. Because of this tradeoff in properties,
microfilled resins should be used where esthetics is the
principal consideration and where undue stress will not
be placed on the restoration, such as in class III or class V
restorations. In situations in which the restoration is
subject to stress, such as the incisal margin of a class IV
f
I G . 16-3.
Schematic drawing of a conventional composite
resin with macrofiller (black areas) before and after finishing
or wear. (Redrawn from Phillips RW: Science of dental materials,
ed 9, Philadelphia, 1991, WB Saunders.)
f
I G . 16-4.
Structure of a microfilled resin showing microfiller (
dots) and prepolymerized macrofiller particles before and
after finishing. (Redrawn from Phillips RW Science of dental
materials, ed 9, Philadelphia, 1991, WB Saunders.)
46 Dentistry for the Child and Adolescent
restoration, a composite having better physical properties
is preferred. The development of hybrid and small-
particle composites has significantly reduced the use of
microfilled composites.
SMALL-PARTICLE AND HYBRID COMPOSITES
The conventional macrofilled composite is no longer in
common use. Filler sizes have been continually reduced
in an effort to approach the surface smoothness of the
microfilled resin but retain the filler levels and physical
properties of the conventional composite. Small-particle
composites have an average filler size of 1 to 5 µm, with
a broad distribution of sizes. This permits higher filler
loading than in the conventional composites and results
in the best combination of physical properties of all the
currently available composites. Small-particle composites
are recommended for stress-bearing applications such as
class IV and class II restorations. Their surface finish is
inferior to that of a microfill resin but much better than
that of a conventional composite.
Hybrid composites are the most recent step toward
smaller particle size. They contain radiopaque glass
particles with an average size of 0.6 to 1.0 pm in addition
to 10% to 20% colloidal silica. The total filler level, 70%
to 80%, is lower than in a small-particle composite but
higher than in a microfill resin. Because these combine
two types of fillers, the result is called a hybrid composite.
Although the surface of hybrid resins is not as smooth
as that of microfilled resins, these resins find extensive
anterior use if they are carefully polished. Also, one of
the primary motivations in the development of these
hybrid materials was to find a material that could com-
pare favorably with dental amalgam in wear resistance
in class I and II restorations. The use of composites in
such situations is discussed in the section on posterior
composite restorations. The most recent trend in resin
composites has been the marketing of so-called universal
or all-purpose restorative materials for use in either
anterior or posterior applications.
LIGHT-CURED COMPOSITES
Originally, composite resins were chemically activated,
which required the mechanical mixing of two pastes to
initiate the chemical reaction. Light-cured or light-
activated composites have largely supplanted the chem-
ically activated composites. Light-activated resins do
not differ significantly in composition from the chemi-
cally activated resins except for the polymerization
activation mechanism. However, light curing provides
an advantage in working time and other handling char-
acteristics. The dentist has complete control over the
working time and is not confined to the rather short
working time of the chemically activated systems. This
is particularly beneficial when large restorations such as
class IV restorations are placed.
Most currently available visible light–cured resins
contain the photosensitive initiator camphoroquinone,
which absorbs visible light at wavelengths between 450
and 500 nm (blue light) and forms free radicals that
activate an amine accelerator.
Visible light activation units are simple and relatively
inexpensive, and their output remains fairly constant
throughout the life of the bulb. Visible light is capable of
polymerizing a reasonable thickness of resin (2 mm). It
also will cure the resin through a layer of enamel, a
particular advantage in class III restorations. Although
protective glasses are recommended to shield the
operator's eyes from the glare of the intense blue light,
visible-spectrum curing lights do not pose a significant
safety risk.
One major disadvantage of light-cured composites
must be emphasized. Polymerization will only occur if
the resin is exposed to light of sufficient intensity for an
adequate length of time. The top surface of a restoration
that is nearly in direct contact with the light source will
always be cured if the light and resin are serviceable.
However, the curing of the portion of the restoration
farthest removed from the light is less certain. Normally
this portion of the restoration is not accessible for any
kind of probing to test its hardness. If the cure is incom-
plete on the bottom side of the resin compared with the
top surface, the physical properties will be reduced and a
color shift may occur in time. Likewise, unpolymerized
monomer may increase the potential for pulpal irritation.
Microleakage is another likely scenario. To ensure
maximum polymerization, the end of the light source
should be within 1.0 mm of the surface of the resin. The
curing time should be at least 40 seconds, and the depth
of resin to be cured should not exceed 2.5 mm. Larger
restorations and dark shades of resin require an
incremental placement technique. Dual-activated resins
are available that combine both light and chemical
activation. In situations in which light access to parts of
the restoration is problematic, a dual-activated material
may be preferred.
When visible light curing systems first were intro-
duced into dentistry, considerable emphasis was placed
on the claim that the light output of the visible light
curing unit remained constant with use, unlike the pre-
viously employed ultraviolet light units. Unfortunately,
this statement is only partially true. Numerous factors do
influence the light output of a visible light curing unit
such as power line variations, aging of the filters, aging
of the lamp, damage to the light-conducting pipe or optic
fiber, and resin buildup on the end of the light tip. The
curing light should be tested regularly to ensure adequate
light intensity. Inexpensive meters are avail-able for this
purpose and should be used regularly. Many of the newer
visible light activation units have built-in meters to
verify adequate light intensity. If such