Chapter 9
Dental Amalgam
I.
Introduction
An amalgam is a special type of alloy in
that one of its constituents must be mercury.
Since mercury is liquid at room temperature,
it can be combined with metals that are solid
at room temperature to form an alloy. This
type of alloying is known as amalgamation.
Before these alloys combine with mercury
they are known as dental amalgam alloys.
Historically, amalgam alloys were required by
ADA Specification No. 1 to contain at least 65
wt. % silver and 29 wt. % tin. This composition is close to that which was first recommended by G.V. Black in 1896. Copper, if present in alloys, was limited to less than 6 wt. %.
During the l970s, many amalgam alloys containing between 6 wt. % and 30 wt. % copper
were developed. Many of these high-copper
alloys produce amalgams (high-copper
amalgams), which are superior in many respects to low-copper amalgams. In 1977, the
composition limitations of ADA Specification
No. 1 were modified to take in compositions
including much more copper.
To make amalgam, mercury is mixed with a
powder of amalgam alloy. The powder may be
produced by milling or lathe cutting a cast
ingot of the amalgam alloy. The particles of
this lathe-cut powder are irregularly
shaped. Alternately, the powder may be produced by atomizing liquid alloy. The particles
of this atomized powder are spherical.
The procedure in which alloy and mercury
are mixed is called trituration. The product
of the trituration is a plastic mass. The plastic
mass is forced into the prepared cavity; this
process is condensation.
During trituration of an alloy powder with
mercury, the mercury dissolves the surface of
alloy particles (dissolution) and then new
phases form (precipitation). These new
phases have melting points well above any
temperature which might occur in the mouth
under normal conditions. The transformation
of the mercury- powder mixture to a precipitate-powder mixture (a composite) brings
about the setting and hardening of the amalgam. In its set condition dental amalgam is
best described as a metal-matrix composite.
The filler particles are partially dissolved
alloy particles. The filler particles are bound
together by a metal matrix which is made up
by the new phases that are formed by precipitation.
II.
Clinical Performance of Amalgam
Restorations
Dental amalgams are used more than any
other material for the restoration of lost tooth
structure. More than 80 million amalgam
restorations are placed annually in the United
States.
Reduction of marginal leakage. One of
the reasons for dental amalgam's exceptionally
fine record of clinical performance may be the
tendency of the amalgam restoration to minimize marginal leakage. After a few months in
the mouth, corrosion products will seal gaps
between the tooth and restoration. However,
immediately after an amalgam restoration is
placed, there is leakage between the cavity
walls and the restoration. Even the best condensed amalgam restorations provide only
close adaptation to the walls of the prepared
cavity. Since leakage can lead to secondary
caries, cavity varnishes or liners that bond to
both amalgam and tooth structure are recommended to reduce the initial leakage that
occurs around the new restoration.
The ability to gradually reduce leakage as
the restoration ages is a unique characteristic
of amalgam restorations. Note, however, that
this property depends on proper insertion of
Revised 5/24/07
1
Dental Amalgam
Oral Biomaterials - Chapter 9
the restoration so that a small initial marginal
gap is achieved. As the restoration ages, corrosion products will form in the interface. If
the initial marginal gap is small enough, the
corrosion products and the plaque that forms
on the corrosion products will block the interface and slowly choke off leakage. This ability
to limit leakage via interfacial corrosion is
shared by both the older low-copper amalgams
and newer high-copper amalgams.
Durability of amalgam restorations. In
spite of this desirable property, amalgam restorations frequently fail. Common problems
include recurrent and marginal caries, fracture, ditched or fractured margins, and excessive tarnish and corrosion. When amalgam
restorations are followed in settings typical of
general clinical practice, the half of all amalgam restorations are replaced after 7 to 15
years in service.
Amalgam longevity increases signifciantly
when amalgam restorations are followed in
more ideal settings. For example, many clinical trials are done in dental schools where
patients are often selected from among students and school staff. The exceptional oral
hygiene of such patients may bias the study.
In addition, clinicians may be working to higher standards and be less hurried. Under such
ideal conditions, the typical amalgam restoration is found to have a half-life of somewhere
between 55 and 70 years. These studies show
that the potential durability of amalgam
restorations is much greater than is realized
in general practice settings.
Factors leading to the failure of amalgam restorations. The factor that is primarily responsible for recurrence of caries and
fracture of restorations is improper design of
the prepared cavity. One clinical survey
(which included only low- copper amalgams)
has shown that at least 56 per cent of all
amalgam failures may be attributed to violation of the fundamental principles of amalgam
2
cavity preparation, i.e., insufficient provision
for bulk, inadequate retentive form, and failure to extend the margins to relatively
immune areas. Forty per cent of all failures
were attributed to faulty manipulation of the
amalgam.
III.
Alloy Powder for Dental Amalgam
Types of Alloy Powders - Particle Shapes
Lathe-Cut Powder. To make lathe-cut
powder, an ingot of alloy (either low- copper or
high-copper alloy) is placed in a milling
machine or in a lathe and is fed into a cutting
tool or bit. Since the chips removed are often
needle-like, some manufacturers reduced the
chip size by ball-milling. The resulting particles are very irregular.
Atomized Powder. Atomized powder is
made by melting together the desired elements, which may give either a low-copper or
high-copper composition. The liquid metal is
then sprayed into a fine mist consisting of
spherical droplets of metal. If the droplets
solidify before hitting a surface, the spherical
shape is preserved. Whether or not they are
truly spherical, atomized powders are frequently called spherical powders.
Particle Size. Maximum particle size and
the distribution of sizes within an alloy powder are controlled by the manufacturer. The
average particle size of modern powders
ranges between 25 and 35 μm.
The most significant influence on amalgam
properties is not the average particles size, but
rather the distribution of sizes around the
average. For example, very small particles
(less than 3 μm) greatly increase the surface
area per unit volume of the powder. As will be
explained in Section IV, a powder containing
tiny particles requires a greater amount of
mercury to form acceptable amalgam.
In producing lathe-cut alloys, the cutting
rate is precisely controlled to maintain the
Dental Ceramics
Oral Biomaterials - Chapter 8
desired average particle size and size distribution. Similarly, parameters of the atomizing
process are controlled to produce desired particle size of spherical alloys. In addition, the
particles may be graded according to size and
the graded particles remixed to produce a powder with a optimum size distribution.
Amalgams from lathe-cut powders vs.
amalgams from spherical particles. Amalgams made from lathe-cut powders or admixed
powders of a blend of lathe-cut and spherical
powders tend to resist condensation better
than amalgams made entirely from spherical
powders. Since amalgams of spherical powders are very plastic, one can not rely upon the
pressure of condensation to establish proximal
contour. A contoured and wedged matrix band
is essential to prevent flat proximal contours,
improper contacts and overhanging cervical
margins.
Spherical alloys require less mercury than
typical lathe-cut alloys because spherical
alloys have a smaller surface area per volume
than do the lathe-cut alloys. As will become
apparent, lower mercury content amalgams
have better properties.
Lathe-cut amalgams require higher condensation pressures than spherical amalgams.
One study compared the tensile strength of
amalgams of lathe-cut and spherical low-copper powders. Low pressure condensation of
amalgams of the spherical powder produced
good tensile strength. To obtain the same
strength level in amalgams of lathe-cut powder required pressures ten times higher.
Low-copper Alloys - Compositions
Powders of low-copper alloys are used to
make low-copper amalgams. Although most
clinicians now use high-copper amalgams,
description of these alloys is useful for two reasons. First, the phases present in these alloys
are also present in the powders of high-copper
alloys. Second, one of the types of particles is
made of the same low-copper alloy that is used
to make powders for low-copper amalgams.
Phases within alloy particles. Powders
for low-copper amalgams are made from Ag-Sn
alloys. Most of these alloys contain two Ag-Sn
phases - the gamma phase (γ Ag-Sn ), which
is the intermetallic compound Ag3Sn (e.g., containing three silver atoms for every tin atom),
and the beta phase (β Ag-Sn). Silver-tin
alloys containing between 19 wt. % tin and 25
wt. % tin will be mixtures of β Ag-Sn and γ
Ag-Sn. Ag-Sn alloys containing 19 wt.% tin
will be 100% β. As the tin concentration
increases, the amount of γ Ag-Sn increases.
That is, at 19.6 wt. % tin, there is 90% β and
10% γ; at 22.5 wt. % tin, there is 50% β and
50% γ; and at 24.4 wt. % tin, there is 10% β
and 90% γ. At 25 wt. % tin, the alloy contains
only γ Ag-Sn.
Most dental alloys are formulated so that
they contain more γ Ag-Sn than β Ag-Sn, and,
consequently, the properties of the brittle
gamma phase dominate the properties of the
particles. Intermetallic compounds, such
as γ Ag-Sn are ordered alloys. Like other
intermetallic compounds, Ag3Sn acts more like
a covalently bonded compound than a metal.
That is, the interatomic bonds are highly
directional. Consequently, Ag3Sn lacks the
ductility one often finds in alloys. Unlike γ AgSn, β Ag-Sn is a disordered alloy. Consequently, by itself, it is more ductile than γ AgSn. In the section on amalgamation, it will be
seen that the ratio of β Ag-Sn to γ Ag-Sn in
alloy particles influences an amalgam's hardening rate and its expansion or contraction
during setting.
Other elements. ADA SCDP Specification
No. 1 allows amalgam alloys to contain elements other than silver and tin. These include
copper, zinc, indium, gold, platinum, palladium, and mercury. However, these metals
must be present in concentrations much less
than either silver or tin. The ADA
3
Dental Amalgam
Oral Biomaterials - Chapter 9
Specification does not define "low" or "high"
copper alloys. Low-copper amalgams often
contain some copper. For the purposes of this
chapter, alloy powders containing more than 6
wt. % Cu are classified as "high" copper alloys.
Zinc. Zinc is a component of some alloy
powders. When silver-tin alloys containing
zinc are melted, the zinc reacts with oxygen in
the melt. The zinc oxides float to the surface
and therefore can be excluded from the castings that are used to make lathe-cut or atomized powders. A small amount of the zinc
(usually less than 1.0 wt. %) is unreacted and
remains as solute within the alloy.
Zinc-containing amalgams have some
advantages. Restorations made from low-copper alloy powders that contain zinc have been
shown to tarnish less than restorations made
from zinc-free alloy powders.
Zinc also has a major disadvantage. When
zinc-containing low-copper amalgams are contaminated with water or saliva during condensation, they undergo a slow expansion that can
continue for months after the restoration is
placed. This expansion is sufficient to cause
extrusion of restorations out the preparation
and, in other cases, pain and/or tooth fracture.
To alert clinicians that they must avoid moisture contamination, international standards
require that alloys containing zinc in excess of
0.01 wt. % be labeled as "zinc-containing."
Those alloys containing zinc equal to or less
than 0.01 wt. % are labeled as "non-zinc".
Delayed expansion is discussed in more detail
in the section to follow.
High-copper Alloys - Compositions
Like the low-copper alloys, high-copper
alloys contain silver and tin. Once again, silver is the element present in the greatest concentration. Unlike low-copper alloys, high-copper alloys also contain between 6 and 30 wt. %
copper. High-copper alloys may also contain
4
small amounts of zinc, indium, and/or palladium.
Ag-Sn-Cu particles
Ag-S n particles
Ag-Cu particles
a.
b.
admixed alloy
powder
single-composition
alloy powder
Fig. 1. a) Admixed alloy particles. Note
that the spherical particles in some admixed
alloys contain some tin. b) Single-composition particles. Although most single-composition powders consist of only spherical particles, some consist of only irregularly shaped
powders.
Two different types of high-copper alloy
powders are available. The first type is an
admixed alloy powder and the second type
is a single-composition alloy powder. Fig.
1 is a schematic representation of these powders.
Admixed Alloys. The first admixed highcopper amalgam was made in 1963. Innes and
Yondelis added spherical silver-copper alloy
(71.9 wt. % Ag and 28.1 wt. % Cu) particles to
lathe-cut low-copper particles. Amalgam made
from the resulting powder was the first new
amalgam to exhibit significantly improved
clinical performance, since Black announced
his amalgam formulations.
The Ag-Sn alloy particles in admixed powders are identical to the low-copper alloy particles described in the preceding. Each particle
contains γ Ag-Sn and β Ag-Sn. γ Ag-Sn is present in the largest percentage.
Admixed alloy powders contain between 3055 wt. % of the spherical silver-copper particles. Some admixed alloys also contain a
Dental Amalgam
Oral Biomaterials - Chapter 9
small amount of tin in the copper- containing
particles. In presently marketed admixed
amalgams, all the copper- containing powders
are made by atomization and are spherical.
The total copper content in admixed alloys
ranges from approximately 9-20 wt. %.
Each spherical particle is a Ag-Cu alloy.
These alloys are made of mixtures of nearly
pure copper that has a small amount of silver
dissolved within it (α Cu) and silver that contains at small amount of copper dissolved
within it (called α Ag).
Single-composition Alloys. Success of
the admixed amalgams led to the development
of another type of high-copper alloy. Unlike
admixed alloy powders, each alloy particle of
these alloy powders has the same chemical
composition. Therefore, they are called single-composition alloys. These powders are
shown schematically in Fig. 1.
The major components of the particles are
silver, copper, and tin. For example, the first
alloy of this type contained 60 wt. % silver, 27
wt. % tin, and 13 wt. % copper. The copper
content in these alloy particles ranges from 13
to 30 wt. % depending on the brand. In addition, small amounts of indium or palladium
are also found in some of the currently marketed single-composition alloys.
Each alloy particle contains three phases:
γ Ag-Sn, β Ag-Sn, and ε Cu-Sn. The last
phase is the intermetallic compound Cu3Sn.
IV.
Amalgamation of Low-copper Alloys
Mercury Requirements
Amalgamation occurs when mercury comes
into contact with the surface of silver- tin alloy
particles. Low-copper alloy powders are typically mixed so that the mercury content of typical amalgams contain between 45 and 55 wt.
% mercury. As will be explained in the following, the amount of mercury that is required to
make a usable mix depends on how fast the
powder reacts with mercury. If the powder
reacts rapidly with mercury, more mercury is
needed in order to produce an amalgam that
remains plastic long enough to permit condensation into the prepared cavity.
Dissolution
When a powder is triturated, the silver and
tin in the outer portion of the particles dissolve into mercury (Fig. 2a). At the same
time, mercury diffuses into alloy particles.
Since diffusion in a solid is very much slower
than diffusion in a liquid, the reaction in the
liquid (the dissolution of Ag and Sn into Hg) is
the dominant reaction.
Precipitation
The mercury has a limited solubility for silver (0.035 wt. %) and tin (0.6 wt. %). When
the mercury is supersaturated, crystals of two
binary intermetallic compounds precipitate
(Fig. 2b). These are the body-centered cubic
silver-mercury compound Ag2Hg3 and the
hexagonal tin-mercury compound (approximately Sn8Hg). The former, called γ1 Ag-Hg
("gamma-one"), precipitates first and the latter, called γ2 Sn-Hg (gamma-two), precipitates
subsequently.
The effect of particle size and shape on the
rate of precipitation. Considering this precipitation reaction can help one understand why
alloy powders that have a small average particle size will set faster than alloy powders that
have a larger average particle size. For the
same weight of powder, the powder with the
smaller particle size will have a much larger
total surface area. Note that particle shape
will also affect surface area. Since the surfaces of lathe-cut particles are irregular, they
have a higher surface area per unit volume
than do spherical particles.
Increasing the surface area of alloy particles accelerates the rates of two processes: 1)
addition of silver and tin to the liquid mercury
5
Dental Amalgam
Oral Biomaterials - Chapter 9
cles becomes significant.
(Until the dissolution slows,
the surface of particles dissolve before much mercury
can diffuse into the surface.)
The amount of mercury
that diffuses into the powder
is also proportional to the
total surface area of its particles. Since the powder with a
small average particle size
has a larger total surface
area than a powder with a
larger average particle size,
the fine-particle powder will
"absorb" more mercury. As
mercury is removed from the
liquid, the silver concentration in the mercury increases,
causing the silver to become
supersaturated within the
liquid mercury. Consequently, a mercury and alloy powder mixture with a fine-partiFig 2. The amalgamation reaction. The phases that are found in
cle powder will reach supervarious types of dental amalgam are listed in Table 1. a.) Silver
and tin dissolve from γ Ag-Sn particles into the mercury. b.) When saturation faster than a mixture made with a coarse-parthe mercury is saturated with silver, crystals of γ1 Ag-Hg precipitate. c.) The γ1 crystals grow and new γ1 crystals form. When the ticle powder.
mercury is saturated with tin, γ2 Sn-Hg crystals precipitate and
When silver becomes
grow. d.) The remaining mercury is consumed by additional
supersaturated in the liquid
growth of the γ1 and γ2 crystals. Space not occupied by the γ1 and
mercury, γ1 Ag-Hg crystals
γ2 becomes voids in the final microstructure.
nucleate. Once γ1 nucleation
has occurred, rapid dissoluby dissolution of alloy particles and 2) loss of
tion resumes, driven now by the reduction in
mercury from the liquid through diffusion of
chemical potential that occurs when silver
mercury into alloy particles. The effect of surfrom the alloy particle is converted to γ1 crysface area on silver and tin concentration in the
tals.
mercury is simple: the larger the surface area
of dissolving particles, the faster the silver and
tin concentrations increase. As silver concentration in mercury approaches saturation, the
rate of dissolution slows (there is no room in
the liquid mercury for additional silver) and
diffusion of mercury into the surface of parti6
Effect of alloy phases on precipitation
rate. Changing the composition of Ag-Sn alloy
particles so that the ratio of β Ag-Sn to γ AgSn in the alloy particle increases will also
increase the rate of precipitation. β Ag-Sn
absorbs mercury more rapidly than γ Ag-Sn.
Dental Amalgam
Oral Biomaterials - Chapter 9
Consequently, increasing the amount of ∃
affects the rate of precipitation (and, consequently, also the rate of hardening) in the
same manner that increasing the total surface
area.
Hardening
Immediately after trituration, the alloy
powder coexists with the liquid mercury, giving the mix a plastic consistency. γ1 and γ2
crystals grow as the remaining mercury partially dissolves the alloy particles (Fig. 2c). As
the mercury disappears, the amalgam hardens. As the particles become covered with
newly formed crystals, the reaction rate
decreases.
The alloy is usually mixed with mercury in
about a 1:1 ratio. This is insufficient mercury
to completely consume the alloy particle; consequently, unconsumed particles are present
in the set amalgam. The particles are smaller
now because their surface has dissolved in
mercury. Thus, a typical low-copper amalgam
Table 1
Phases Found in
Amalgam Restorations
Greek
Letter
Composition
Gamma-one
γ1
Ag2Hg3
Beta-one
β1
~Ag3Hg4
Gamma-two
γ2
Sn7Hg - Sn8Hg
Eta Prime
η´
Cu6Sn5
Gamma
γ
Ag3Sn
Epsilon
ε
Cu3Sn
Beta
β
~Ag5Sn
Phase
matrix phases
particle phases
is a composite in which the unconsumed particles (about 50 vol. %) are embedded in a metal
matrix of γ1 Ag-Hg and γ2 Sn-Hg phases. Note
that γ1 is the predominant matrix phase - γ2 is
never more that 10 vol. % of the structure.
Note also that voids are always present when
amalgam is condensed by hand.
In terms of reactants and products the
amalgamation reaction can be summarized as
follows:
Amalgam alloy particles + Hg
→
γ 1 Ag-Hg + γ 2 Sn-Hg
+ partially consumed alloy particles
Expansion or contraction during hardening. For most of amalgam's history, the
factors that produced expanding or contracting
amalgams have not been well understood. In
the mid-1980s Abbott et al. (J Biomed Mater
Res 1986:20:1391- 1400) showed that 1) amalgams made from pure β Ag-Sn powders expanded during setting, and 2) that amalgams
made from pure γ Ag-Sn powders contracted
during setting. Consequently, by changing
the alloy particle composition to increase
the γ Ag-Sn to β Ag-Sn ratio, one can
reduces an amalgam's setting contraction.
The mechanisms underlying amalgam's
expansion or contraction are still not well
understood. Regardless of the mechanism,
when the dimensional change is excessive,
it may affect the restoration's performance.
Excessive expansion of an intracoronal
restoration might lead to tooth fracture.
Excessive contraction might lead to leakage. International specifications for dental
amalgam allow amalgams to contract or
expand a limited amount. There is no evidence that small amounts of expansion or
contraction increase the risk of leakage or
tooth fracture.
Delayed expansion. If a zinc-containing amalgam is contaminated by water or
7
Dental Amalgam
Oral Biomaterials - Chapter 9
saliva before condensation is complete, a large
expansion can take place. This expansion usually starts after three to five days and may
continue for months, reaching values greater
than 400 micrometers per centimeter (4 per
cent). This type of expansion is known as
delayed expansion or secondary expansion.
One of the products of the reaction between
water and zinc is hydrogen gas. It is produced
by electrochemical action between the zinc, the
electrolyte, and the anodic elements present.
This hydrogen does not combine with the
amalgam constituents, but rather collects
within the restoration. It has been shown that
the internal pressure of the hydrogen may
build up to levels high enough to cause the
amalgam to flow, thus producing the observed
expansion.
If the zinc is not present, the expansion
does not occur. When alloys do contain zinc,
contamination is only a problem when it
occurs before the completion of condensation.
After the amalgam has set, the external surface may come in contact with water or oral
saliva without subsequent delayed expansion.
Contamination of the amalgam can occur at
almost any time during its manipulation and
insertion into the cavity. A no-touch technique
should be use. Body oil can produce delayed
expansion. Furthermore, use of rubber dam
isolation to prevent saliva contamination is
recommended. When isolation is not possible,
non-zinc alloys should be used.
V.
Amalgamation of High-copper Amalgams
Mercury Requirements
Like low-copper amalgams, high-copper
alloy powders are mixed with mercury to produce amalgams that contain between 45 and
55 wt. % mercury. Many high- copper amalgams contain less mercury than the typical
8
low-copper amalgams. Some single-composition amalgams contain as little as 41 Wt. %
mercury.
Admixed Amalgams
When mercury is reacted with an admixed
powder, silver enters the mercury from the silver-copper alloy particles and both silver and
tin enter the mercury from the silver-tin alloy
particles. The tin diffuses in the mercury to
the surfaces of silver- copper alloy particles
and reacts with the copper phase to form η´
Cu-Sn (Cu6Sn5). A layer of η´ crystals forms
around unconsumed silver-copper alloy particles. The γ1 Ag-Hg phase forms simultaneously with the η´ phase and surrounds both the
η´- covered silver-copper alloy particles and
the silver-tin alloy particles. As in the lowcopper amalgams, γ1 is the matrix phase; this
is the phase which binds the unconsumed alloy
particles together. Gamma-one makes up over
40% of the volume of the amalgam.
The reaction of the admixed alloy powder
with mercury can be summarized as follows:
(Ag-Sn alloy particles) +
(Ag-Cu alloy particles) + Hg
γ 1 Ag-Hg + η ′ Cu-Sn +
→
(unconsumed alloy
particles of both types)
Some admixed amalgams also contain γ2
Sn-Hg; however, the percentage of γ2 is typically less than found in low-copper amalgams.
The effectiveness of the copper- containing
particles in preventing γ2 formation depends
on availability of Cu in the copper-containing
particles. The η´ Cu-Sn phase apparently
forms in preference to the γ2 phase. If there is
insufficient copper to react with the tin dissolved in mercury, the excess tin reacts with
mercury to form γ2. Note that admixed amalgams are predominantly intermetallic compounds: γ Ag-Sn and γ1. η´ Cu-Sn is also an
intermetallic compound.
Dental Amalgam
Oral Biomaterials - Chapter 9
Single-composition High-copper
Amalgams
Silver and tin from γ and β Ag-Sn areas in
the Ag-Sn-Cu alloy particles dissolve into the
mercury. Cu-rich phases apparently do not
dissolve. The tin diffuses to ε Cu-Sn areas on
the alloy particle where it reacts to form η´
Cu-Sn crystals. Subsequently, silver reaches
supersaturation in the mercury and γ1 crystals
nucleate and grow. They grow around η´ CuSn and form the matrix that binds together
the partially dissolved alloy particles.
The η´ Cu-Sn crystals often form as meshes
of rod crystals at the surface of alloy particles.
Individual rod crystals may be 2 to 3 μm long.
These are much larger than the η´ crystals
found in the reaction layers surrounding AgCu particles in admixed amalgams. In one
admixed amalgam, the average cross sectional
thickness of η´ crystals has been estimated to
be less than 0.05 μm.
The reaction of the single-composition alloy
powder with mercury can be summarized as
follows:
(Ag-Sn-Cu alloy particles) + Hg
γ 1 Ag-Hg + η ′ Cu-Sn +
→
(unconsumed alloy particles)
The undesirable γ2 Sn-Hg phase is also
found in some single-composition amalgams.
Nevertheless, in most single-composition amalgams little or no γ2 Sn-Hg forms.
Hardening of High-copper Amalgams
Rate of hardening. Like the low-copper
amalgams, high-copper amalgams harden by a
dissolution-precipitation mechanism. Factors
influencing the rate of hardening are not completely understood. The lathe-cut Ag-Sn alloy
particles contained in admixed powders probably influence the rate of hardening. As in lowcopper amalgams, increasing percentages of β
Ag-Sn in these particles should accelerate
hardening. Similarly, in single-composition
powders, increases in the percentage of β AgSn in the Ag-Sn-Cu alloy particles should
accelerate hardening. There is little data on
the role of α Ag, α Cu, or ε Cu-Sn on the rate
of hardening.
Setting contraction and expansion.
There is little information on the factors that
affect setting expansion or contraction of highcopper amalgams. As the percentage of β AgSn in alloy powders increases, one would
expect less contraction. However, the effect of
other phases is unknown.
Delayed expansion. The data on delayed
expansion of zinc-containing high- copper
amalgams is conflicting. The results of some
studies suggest that when contaminated with
water or saliva, restorations made with such
amalgams undergo expansion that continues
for months. Other studies do not detect
delayed expansion. To be safe, clinicians
should maintain dry fields when placing zinccontaining high-copper amalgams.
VI.
Mechanical Properties
Strength
Sufficient strength to resist fracture is a
prime requisite for any restorative material.
Fracture of even a small area, especially at the
margins, hastens corrosion, recurrence of
decay, and subsequent clinical failure.
Traditionally, the strength of dental amalgam has been measured under compressive
stress, using cylindrical specimens of dimensions comparable to the volume of typical
amalgam restorations. When measured in this
manner, the compressive strength of a satisfactory amalgam should be at least 310 MPa
(megapascals). Note that one MPa is 145 psi
(pounds per square inch). When manipulated
properly, most amalgams will exhibit a compressive strength in excess of this value.
9
Dental Amalgam
Oral Biomaterials - Chapter 9
Table 2
Amalgam Type
Mechanical Properties of Dental Amalgam
Compressive
Compressive
Strength
Strength
at one hour
at 7 days
Low-copper 1
149
353
5.70
Admixed 2
137
432
0.46
Single-composition 3
262
509
0.13
1
Optaloy, L.D. Caulk Co.;
2
Dispersalloy, L.D. Caulk Co.;
In Table 2, typical compressive strengths at
one hour and seven days after preparation are
given for a low-copper amalgam and two highcopper amalgams. After seven days the compressive strengths of high-copper amalgams
are generally higher than those of low-copper
amalgams. In addition, note that the one-hour
compressive strength of the single-composition
amalgam is almost double that of the other
two amalgams. This trend is generally true
for other single-composition amalgams.
The significance of the seven-day compressive strength to amalgam performance has
been questioned. The strength of amalgam is
more than adequate to withstand potential
compressive loads.
Unfortunately, amalgam is much weaker in
tension than in compression. Both low- and
high-copper amalgams have tensile strengths
which range between 48 and 70 MPa. Tensile
stresses can easily occur in amalgam restorations. For example, corrosion at the margins
of amalgam restorations can leave wedges of
unsupported amalgam, which will fail in tension when subjected to occlusal forces.
It is important to remember that amalgam
cannot withstand high tensile stresses. Restoration designs should include supporting
structures whenever there is danger that the
restoration will be bent or pulled in tension.
10
ADA Creep
(%)
3
Tytin, Kerr Mfg. Co.
Switching to high-copper amalgams does not
help, since tensile strengths of high-copper
amalgams are not different from those of lowcopper amalgams.
The Influence of Microstructure on
Strength
The physical properties of hardened amalgam depend upon the relative percentages of
each of the microstructural features. The
unconsumed Ag-Sn particles have a very
strong effect. The more of this phase which is
retained in the final structure, the stronger
will be the amalgam. The physically and
mechanically weakest component is the γ2 SnHg phase. The hardness of γ2 is approximately
10% of the hardness of γ1; the hardness of the γ
Ag-Sn phase is somewhat greater than that of
γ1. Elimination of γ2 is probably the major reason for the increased compressive strength of
high-copper amalgams.
All dental amalgams contain two intermetallic compounds: γ Ag-Sn (in partially consumed alloy particles) and γ1 Ag-Hg (in the
matrix). Like most intermetallic compounds,
these phases are brittle. That is, they fracture
without flowing plastically. They are strong
when compressed, but will be very weak when
bent (subjected to shear and tensile stresses)
or stretched (tensile stresses only). Since
amalgams are predominantly made of two
Dental Amalgam
Oral Biomaterials - Chapter 9
brittle phases, γ Ag-Sn and γ1 Ag-Hg, it should
not be surprising that amalgam is also brittle.
Effect of Trituration on Strength
Over-trituration and under-trituration can
effect the strength of a dental amalgam. Proper trituration time for a given amalgamator
is usually provided by the manufacturer of the
dental amalgam. The proper time will depend
on the vibration frequency of the amalgamator. In a given amalgamator, different dental
amalgams will require different trituration
times, depending on the reaction rate of the
amalgam.
In an under-triturated amalgam, the alloy
particles will be inadequately wetted by mercury. Badly under-triturated amalgams will
fail to form a coherent plastic mass. Slightly
under-triturated amalgams may form a plastic
mass, but on a microstructural scale will contain excess voids and porosities. These voids
will weaken the amalgam.
Over-triturated amalgam will begin to set
too quickly. The amalgam mass will lack plasticity and may be difficult to condense into the
cavity preparation. In addition, increments of
amalgam will not stick to one another. There
will be strings of porosity where the increments join. The net effect will be a very much
weakened dental amalgam restoration.
Effect of Mercury Content on Strength
A very important factor in the control of
strength is the mercury content of the restoration. Sufficient mercury should be mixed with
the alloy to coat the alloy particles and to
allow a thorough amalgamation. Each particle
of the alloy must be wetted by the mercury;
otherwise, a dry, granular mix results. Such a
mix results in a rough, pitted surface that
invites corrosion.
Above approximately 55 per cent mercury
content, the strength of low-copper amalgam
decreases markedly. In amalgams containing
59 wt. per cent mercury, the compressive
strengths is 125 MPa. At 54 wt.% compressive
strengths of more than 280 MPa are typical.
Similar decreases in strength with increased
final mercury content have been observed for
spherical low-copper amalgams and for highcopper amalgams.
The strength of an amalgam is a function of
the relative volume fractions of unconsumed
alloy particles and matrix phases γ1 Ag-Hg and
γ2 Sn-Hg). Low mercury content amalgams
contain more of the alloy particles, which are
stronger, and less of the matrix phases, which
are weaker. Increasing the final mercury content increases the volume fraction of the
matrix phases at the expense of alloy particles.
As a result, amalgams containing higher
amounts of final mercury content increases the
volume fraction of the matrix phases at the
expense of alloy particles. As a result, amalgams containing higher amounts of final mercury are weaker.
The amount of γ2 Sn-Hg that forms affects
the strength of high-copper amalgams. As
mentioned earlier, γ2 Sn-Hg will form if a highcopper alloy powder is mixed with too much
mercury. The γ2 Sn-Hg is the weakest phase
within dental amalgam. Even a very small
volume fraction of γ2 Sn-Hg will weaken a
high-copper amalgam.
Effect of Condensation
Both condensation pressure and technique
affect the strength. When typical condensation techniques and lathe-cut alloys are employed, higher condensation pressures produce
greater compressive strengths. The early
strength, e.g., at one hour, is particularly
influenced by the condensation pressure. Good
condensation techniques may express mercury,
and thereby result in a greater volume fraction
of residual alloy particles and a smaller volume fraction of matrix phases. Because of
11
their irregular shapes, lathe-cut amalgams
resist condensation more than spherical amalgams. For the latter, higher condensation
pressures are required to minimize porosity.
On the other hand, spherical amalgams condensed with lighter pressures have adequate
strength.
Effect of Porosity
Voids and porosity influence the compressive strength of hardened amalgam. One per
cent increase in porosity reduces the compressive strength 10 times as much as a 1 per cent
increase in final mercury content. Therefore,
porosity is as important in regulating the
strength of amalgam as is the final mercury
content.
The porosity is affected by the plasticity of
the mix. Plasticity of amalgam mixes decreases with increased time from the end of
trituration and to the beginning of condensation (delayed condensation). It could be anticipated that porosities would thereby be greater,
and strength lower, under such conditions.
Rate of Hardening
Since patients are often dismissed within
20 minutes after placement of an amalgam
restoration, one might ask whether the amalgam is sufficiently strong for its function. It is
possible that many restorations fracture shortly after insertion. Clinical evidence of fracture
may not be evident for several months, but the
initial crack within the restoration may develop within the first few hours.
The low-copper amalgams do not gain
strength as rapidly as might be desired. For
example, at the end of 20 minutes, compressive strength may be as low as 6 per cent of
the strength at one week. After six-months,
both low- and high-copper amalgams may still
be increasing in strength. Such observations
suggest that the reactions between the matrix
12
phases and the alloy particles continue indefinitely.
One-hour compressive strengths. A good
index of the rate of hardening is the one-hour
compressive strength test required in ADA
SCDP Specification No. 1. The test is conducted on specimens only one hour old. The
specification stipulates a minimum strength of
80 MPa.
The one-hour compressive strengths of
high-copper single-composition amalgams are
exceptionally high. (See Fig. 3 and Table 2).
This strength may have some advantages clinically. These amalgams may be strong enough
9
8
no. of brands
Dental Amalgam
Oral Biomaterials - Chapter 9
high Cu
low Cu
7
6
5
4
3
2
1
0
0
40
80
120
160
200
240
one-hour compressive strength (MPa)
Fig. 3. One-hour compressive strength of a
number of amalgam brands. (Adapted from
Beech & Brockhurst. Aust Dent J 1982;
27:306-309.)
shortly after placement to permit amalgam
cores to be constructed. Good early strength is
needed so that the amalgam core can survive
condensation around pins or posts, finishing
by high speed grinding, and the stresses of
removing impression material that has set
around it.
Unless a fast hardening amalgam is used,
the initial strength of the amalgam restoration
is likely to be low. For example, after three
hours, typical admixed high-copper amalgams
have reached only 40 - 50 % of their 24 hour
transverse strength. Some single-composition
high-copper amalgams do better, reaching over
88% of their 24 hour strength by three hours.
But other single-composition amalgams reach
only about 70% of their 24-hour strength by
three hours (Cruickshanks-Boyd DW. J Dent
1983;11:214). Patients should be cautioned
not to bite down on their restoration for at
least 8 hours. Advising patients to consume
"liquids only" in the period following placement is probably a sound precaution.
Early polishing of amalgam restorations. Another potential advantage of fast
hardening amalgams is that they may be polished soon after placement. Recent research
has compared the polish of single-composition
amalgam restorations polished eight minutes
after trituration and restorations polished 24
hours after trituration (Corpron RE et al.
Pediatr Dent 1983: 5:126-130). After 36
months, the restorations polished at eight
minutes were very similar to those polished
conventionally.
Creep
The Significance of Creep to
Amalgam Performance. During the 1970s,
dental researchers discovered laboratory tests
for amalgam that seemed to correlate with
data on long-term clinical performance (see
Fig. 4). All these tests measured amalgam's
resistance to slow strain rate deformation.
Dental Amalgam
Oral Biomaterials - Chapter 9
One such test measures the static creep of
amalgam. An amalgam specimen is placed
under a constant load which is less than that
needed to produce either instantaneous plastic
deformation or fracture (that is, the load is
less than its yield strength). When subjected
to such a load, a specimen will slowly deform.
The rate of deformation will depend on the
magnitude of the stress in the specimen and
on the temperature at which the test is conducted.
Dental amalgam is a material whose performance is influenced by its viscoelastic
properties. It exhibits both elastic deformation (determined only by the applied load) and
viscous flow (determined by both the applied
load and the length of time the load is held).
Creep is an example of a viscoelastic property.
Creep rate correlates with the extent of
marginal fracture exhibited by low-copper
amalgam restorations (see Fig. 5). As mentioned earlier in this chapter, marginal fracture is the most commonly observed defect in
amalgam restorations. For low-copper amalgams, higher creep values are found to predict
wide marginal defects. Most high-copper
8
A
N
m arg in al fractu re categ o ry
4.5
M
D
Category
6
4
2
0
0
1
2
3
4
5
6
Time (years)
Fig 4. Mean marginal fracture category rating (high number = more fracture) for Class I
and II amalgam restorations as a function of
time. M, A, and N are low-copper amalgams.
D is a high-copper amalgam. D exhibits the
slowest development of marginal fracture and
also has the slowest creep rate. (From:Mahler
et al. J Oral Rehab 1979;6:391-398.)
3.5
2.5
low copper
high copper
1.5
1
3
5
7
9
creep rate ( 10
11
-6
13
15
17
/ sec )
Fig. 5. Scatter diagram of marginal fracture
rating versus creep rate. For low-copper
amalgams, but not high-copper amalgams,
marginal fracture increases with creep rate.
(From: Vrijhoef & Letzel J Oral Rehab
1986;13:299 - 303.)
13
amalgams exhibit lower creep values and better resistance to marginal fracture than lowcopper amalgams. Unfortunately, the correlation between creep and extent of marginal
fracture does not hold for high-copper amalgams. Lower creep values do not mean that
an amalgam restorations will exhibit less marginal fracture than another high-copper amalgam with higher creep values. Once the creep
rate is below a certain level, the extent of marginal fracture is no longer strictly proportional
to creep rate.
Measurement of Creep. ADA SCDP
Specification No. 1 contains a test for creep.
This test subjects an amalgam cylinder, 8 mm
long and 4 mm in diameter, to a compressive
load of 36 MPa in a 37oC environment (see
Fig. 6). The change in length of the cylinder
which occurs between the first and fourth hour
of testing is divided by the original length and
multiplied by 100. This gives the percent
strain during this three-hour period. This
values for high-copper amalgams range between approximately 0.05 and 2.50%. The
high end of this range includes high-copper
amalgams that contain some γ2. If these products are excluded, "most" high-copper amalgams are found to have ADA creep values
between 0.01 and 1.00% (see Fig. 7). The best
admixed amalgams have ADA creep values as
low a approximately 0.25%. Some single-composition amalgams have ADA creep values as
low as 0.05%. See, for example, Table 2.
8
7
no. of brands
Dental Amalgam
Oral Biomaterials - Chapter 9
high Cu
low Cu
6
5
4
3
2
1
0
0
0.4
0.8
1.2
1.6
2
2.4
2.8
3.2
3.6
4
ADA Static Creep (%)
Fig. 7. ADA creep of amalgam brands. Highcopper amalgams usually have lower ADA
creep percentages. From: Beech & Brockhurst. Aust Dent J 1982; 27:306-309
Nevertheless, there is no data available which
suggests that reducing the ADA creep value
below approximately 0.50% will influence marginal fracture.
VII. Manipulation of Amalgam
Proportioning
Fig. 6. The ADA static creep test. The cylinder on the left is subjected to a constant load.
After four hours, the cylinder has deformed as
shown on the right.
"percent creep" is proportional to the creep
rate.
ADA creep values of low-copper amalgams
range between 0.80 and 8.00%. ADA creep
14
An amalgam's strength increases with the
percentage of unconsumed alloy particles in
the structure. Consequently, only as much
mercury as is required to make an acceptable
plastic mass is mixed with the powder. Typically mercury makes up less than 50 wt.% of
the amalgam's mass. Since mercury is often
expressed during condensation, the final alloymercury ratio may be less than 1:1 in the
hardened restoration. (Note that alloy to mercury ratio means the weight of alloy divided by
the weight of mercury). Thirty-five years ago
typical low-copper amalgams were mixed with
alloy mercury ratios of 5:7 or 5:8, corresponding to initial mercury contents of greater
than 60 wt.%.
Most amalgams currently being marketed
are mixed with alloy-mercury ratios near 1:1.
Some have initial mercury content as low as
42 wt.%.
Today most amalgams are available in preproportioned capsules. The amalgam alloy
and the mercury are in two compartments of
the capsule, separated by a thin membrane.
In some systems, the capsule must be "activated" before trituration. In others, the motion of
the amalgamator breaks the membrane. In
either case, activation permits the powder and
mercury to mix. The preproportioned capsules
eliminate the need to weight out alloy and
mercury. Moreover, there is ample evidence
(Chopp GF & Kaufman EG. Oper Dent
1983:8:23-27) that use of preproportioned capsules significantly reduces the probability that
mercury levels in operatory air exceeds OSHA
limits (0.05 mg/m3 for an eight hour day and a
five day week).
Trituration
The alloy powder (or tablet) is mixed with
the liquid mercury until all the particles of the
powder are wetted with mercury. This process
is called trituration. At one time trituration
was done by hand using a mortar and pestle.
To obtain consistent amalgam properties mechanical amalgamators now are recommended.
Such amalgamators reciprocate the capsule at
frequencies between 2000 and 5000 rpm and
with amplitudes around 50 mm.
As has already been discussed, the amalgam should be triturated for the proper time
to produce optimum properties. This time will
depend on the kind of amalgamator, the size of
the capsule, the size and weight of the pestle,
the kind of alloy powder, and the alloy to mercury ratio. For a given alloy, faster amalgamator frequencies, lower alloy to mercury ratios
(e.g., more mercury) and use of a pestle all
decrease the trituration time needed to pro-
Dental Amalgam
Oral Biomaterials - Chapter 9
duce an optimum amalgam mix. For best
results, use the amalgam manufacturer's recommended trituration time for the particular
capsule, pestle, and amalgamator being used.
As has been discussed in Section VI, either
under trituration or over trituration can profoundly affect strength. A properly triturated
mix will be a homogeneous mass with a shiny,
wet look. It may be slightly warm to the touch
and, if triturated with a pestle, it may cling to
the pestle. Severely under triturated mixes
can be distinguished by their dry, granular
appearance. Such mixes will crumble when
worked. Severely over triturated mixes may
still be wet looking, but will lack plasticity.
They may also be excessively warm. Slightly
over or under triturated mixes may be difficult
to detect. Dentists should take care to note
the normal plasticity of amalgam mixes and be
alert for small changes in plasticity.
Condensation
Condensation is the process of packing
triturated amalgam into a cavity. The force
used should be sufficient to (1) express excess
mercury, (2) adapt the amalgam to the walls of
the cavity and (3) pack together voids and
gaps between increments of amalgam. Note
that mercury is not expressed from most modern amalgams. Modern amalgams contain less
mercury (typically less than 50 wt. % mercury), so there is less mercury to express.
The interval between the end of trituration
and condensation should be as short as possible. During the interval, mercury continues to
react with alloy; consequently, less unreacted
mercury is expressed during condensation. As
discussed earlier, amalgams containing more
mercury exhibit undesirable properties: higher ADA creep and lower compressive strength.
A delay between trituration and condensation
will also produce a drier mix. Increments of
dry mixes adhere poorly to one another and
are difficult to condense. As a result, porosity
15
Dental Amalgam
Oral Biomaterials - Chapter 9
especially if their oral hygiene is poor. Some
of the high-copper amalgams form a tarnish
layer of copper sulfide.
and the number of voids within the restoration
increases.
Corrosion
VIII. Tarnish and Corrosion
There is some disagreement in the scientific
literature on the distinguishing features of
"tarnish" and "corrosion". For this discussion,
"tarnish" will be considered surface discoloration of a metal that may be caused by
chemical or electrochemical interactions of the
metal with its environment. Tarnish, as
defined here, does not affect the underlying
metal. Corrosion, on the other hand, is a
chemical or electrochemical interaction of the
metal with its environment which affects the
properties of the underlying metal.
Tarnish
Tarnish on amalgam has been found to be
tin sulfide (Sn2S3). Thus, individuals whose
diet contains large amounts of sulfur may
have badly tarnished amalgam restorations,
16
Crevice Corrosion. In normal saliva the
phases of dental amalgam are in a state of
passivity and consequently undergo little corrosion. This situation is changed in crevices
between the amalgam restoration and the
tooth; in these crevices that amalgam will
undergo crevice corrosion (see Fig. 8).
O2 O2 OH- e- O2 + 2H2O + 4e4OHO2 OH- eO2
O2 OH- e
O2
Alloy
OH
eInsoluble metal
oxides or
hydroxides
M++
M
M2+ + 2eTissue / bone / tooth
M++
M++
++
M
Crevice
Corrosion
O2
cathode
When condensing spherical amalgams (and
in particular the single-composition high-copper amalgams), their more "mushy" texture
may lead to poorer adaptation to the walls of
the preparation. This poor adaptation has
lead to reports of patient sensitivity to some
single-composition amalgam restorations.
However, adaptation of these amalgams can
be improved. If the condenser is pushed laterally toward the walls of the preparation, better
adaptation is achieved and the observed marginal leakage is no different than that found
for lathe-cut amalgams.
Galvanic Corrosion. Corrosion driven by
electrical potential differences between different metals is called galvanic corrosion. The
many different phases in dental amalgam provide many potential anodes and cathodes. In
addition, amalgam restorations can form cells
with other restorations in the mouth. In particular, some phases in amalgam restorations
are very anodic with respect to the high and
medium gold alloys frequently used to make
crowns and fixed partial dentures. The γ2
phase is especially anodic; it tends to form galvanic cells with respect to each of the other
phases in amalgam. The galvanic dissolution
of γ2 is a principal contributor to the weakening of amalgam restorations during service.
anode
Small increments are easier to condense
than large increments. It is difficult to
express mercury from large increments of
amalgam. Larger mixes of amalgam may dry
before the last increments can be condensed.
This is especially true if condensation takes
more than 3 to 4 minutes. In such a case, it is
better to complete the restoration with a fresh
amalgam mix.
Fig. 8. Crevice corrosion in dental amalgam.
Saliva in crevices will contain less oxygen than
saliva on the surface of the restoration.
Crevice corrosion is a special case of a general
type of galvanic corrosion called "concentration cell corrosion", occurs. Areas of the
restoration in contact with oxygen-rich electrolyte become cathodic, consuming electrons
by the reaction:An electrical potential arises
between these oxygen-rich areas and oxygenpoor areas within the crevice. Thus, amalgam
within the crevice becomes anodic and dissolves. The formation of the hydroxide ions
O2 + 2H2O + 4e → 4OH −
sets off a series of events that can accelerate
the corrosion. The hydroxides react with
metal ions or with chlorine ions from the saliva to form the reaction products, tin oxide and
tin oxychloride. Consequently, in the crevices
of low-copper amalgam restorations, one finds
the corrosion products tin oxide and tin oxychloride. Moreover, in the amalgam adjacent
to the crevice and in the amalgam near the
occlusal surface, one finds that the γ2 has disappeared, leaving pores that are partially
filled with tin oxides and/or tin oxychlorides.
These products fill the crevice, blocking diffusion of new oxygen to the anodic site, thereby increasing the potential difference between
the oxygen-rich and oxygen- poor areas. In
addition, removal of hydroxides from the saliva lowers the pH of the saliva. Such acidic
saliva aggressively attacks the restoration,
increasing the crevice corrosion rate.
Crevice corrosion may be more beneficial
than detrimental. Examination of retrieved
amalgam restorations reveals no correlation
between the extent of corrosion and the extent
of marginal fracture. Moreover, the corrosion
products that fill marginal crevices help to
seal the margins of amalgam restorations.
Resin composites lack this self-sealing feature.
In both high and low-copper phases, the tincontaining phases that the most susceptible to
corrosion. Comparatively, both γ1 Ag-Hg and
γ Ag-Sn are relatively resistant to corrosion.
The η´ Cu-Sn found in high-copper amalgams
is less susceptible to corrosion than γ2. Nevertheless, when one examines corroded highcopper amalgams one finds that the η´ has disappeared. At sites that contained η´, one once
again finds tin oxides and tin oxychlorides. No
copper is found. In the laboratory, the copper
Dental Amalgam
Oral Biomaterials - Chapter 9
is found to form the reaction products as Cu2O
or CuCl2 . Cu (OH)2. However, copper oxides
and copper hydoxychlorides have not been
found in corroded high-copper amalgam restorations that have been retrieved after service.
Polishing of dental amalgams is one way to
slow the corrosion of amalgam restorations.
Polishing reduces the tendency of plaque to
adhere to the restoration's surface. Oxygenpoor areas, which can lead to concentration
cell corrosion, arise under plaque and within
surface irregularities of an unpolished restoration.
Galvanic Shock
Patients may report pain or sensitivity after
the placement of an amalgam restoration.
Possible sources of this pain are galvanic currents induced between two different kinds of
metal (for example, a gold crown and the
amalgam). If these currents are conducted
through the bone, soft tissues, or pulp, galvanic shock may result.
Galvanic shock may occur when two dissimilar restorations come into contact or when
saliva provides a conducting pathway between
two restorations. Pain is most commonly associated with freshly placed amalgam restorations, which produce galvanic cells with respect to crowns or with respect to an older
amalgam restoration. Trauma to the pulp
during cavity preparation may magnify the
sensitivity. As the pulp recovers, the pain
usually disappears, even though the galvanic
current remains.
Sensitivity to galvanic currents varies from
one individual to another. Some individuals
are insensitive to currents which cause other
individuals great pain. Temporary relief from
galvanic shock can be obtained by coating the
17
Dental Amalgam
Oral Biomaterials - Chapter 9
surface of the restoration with an insulating
layer of cavity varnish.
IX.
Objectives
You will be able to select from a list of
choices:
1.
the two elements usually present in the
greatest percentages in alloy powders for
low-copper amalgams and the three elements present in the greatest percentages
in alloy powders for high-copper amalgams; the element present in the greatest
percentage in both types of alloy powder.
2.
the reaction products which form when
low-copper (e.g., γ Ag-Sn based alloys),
admixed high-copper and single-composition high-copper alloy powders react with
mercury.
3.
the time range at which half of all amalgam restorations fail when placed by dentists practicing in general practice environments; correct factors contributing to
the longer half-life of amalgam restorations placed in more ideal settings.
4.
5.
the factor primarily responsible for recurrent caries and bulk fracture of amalgam
restorations.
the two phases found in the Ag-Sn particles used in low-copper amalgams and in
admixed high-copper amalgams; the AgSn phase that is present in the greatest
percentage; the effect of increasing the
percentage of β Ag-Sn on the amalgam's
rate of hardening and setting contraction.
6.
the factor primarily responsible for recurrent caries and bulk fracture of amalgam
restorations.
7.
the phase in dental amalgam which is the
weakest and has the lowest corrosion
resistance.
18
8.
the phase in γ2-free high-copper amalgams
that has the lowest corrosion resistance.
9.
correct statements about the corrosion
resistance of γ1 Ag-Hg.
10. correct statements about the effect of particle size and particle shape on the
amount of mercury needed to make an
acceptable amalgam; correct reasons why
particle size affects the setting rate of dental amalgam.
11. statements which correctly compare the
resistance to condensation of amalgams
containing lathe-cut and spherical powders.
12. the approximate volume percent unconsumed alloy particles present when a 1:1
mercury alloy ratio is used; the approximate weight percent mercury in typical
dental amalgams.
13. the set of conditions which will lead to
delayed expansion of amalgam restorations.
14. correct statements about the relative tensile strengths of low-copper, admixed, and
single-composition amalgams; the typical
tensile strength in MPa of these amalgams.
15. the relative one-hour compressive strength
of single-composition amalgams, compared
to that of low-copper and admixed amalgams.
16. the influence of unconsumed alloy particles and the matrix phases, γ1 Ag-Hg and
γ2 Sn-Hg, on the strength of dental amalgam.
17. the effects of small amounts of γ2 Sn-Hg on
the strength and creep rate of high-copper
amalgams.
18. correct statements about the correlation
between marginal fracture and creep rate
for low-copper amalgams and high-copper
amalgams.
19. the approximate range of ADA creep values found for low-copper amalgams and
the range of ADA creep values exhibited
by most high-copper amalgams; also, the
type of high-copper amalgam that has
yielded the lowest ADA creep values.
20. the influence of ADA creep values below
0.50% on marginal fracture.
21. the phase in dental amalgam which exerts
primary influence on creep rate.
22. correct statements about the effect of mercury-alloy ratio on compressive strength,
creep rate, gamma-two volume fraction,
and gamma volume fraction.
23. the composition of tarnish formed on dental amalgam.
24. the chemical names of two corrosion products formed by corrosion of dental amalgam.
25. the reason cleaning plaque from amalgam
and polishing the amalgam inhibit corrosion.
26. the cause of galvanic shock.
Dental Amalgam
Oral Biomaterials - Chapter 9
32. correct statements about the effect of
"light" and "heavy" condensation pressure
on the strength of amalgams made from 1)
powders that contain are mostly lathe-cut
particles (over 45% by weight) and 2) powders made totally from spherical particles.
33. correct statements about the percentage of
their 24-hour transverse strength that
typical admixed high-copper amalgam
obtain in three hours; correct statements
about the minimum percentage their 24hour transverse strength that single-composition high-copper amalgams obtain in
three house.
You will be able to identify or select the
correct definition for each of the following terms. Alternatively, given an identifying description or definition, you will
be able to select the correct term.
admixed alloy
admixed amalgam
alloy-mercury ratio
27. correct statements about why properly
inserted amalgam restorations exhibit less
leakage as they age.
amalgam
28. the steps in crevice corrosion that lead to
corrosion in the margins of amalgam
restorations.
concentration cell corrosion
29. effects of a delay between trituration and
condensation on compressive strength,
ADA creep, and porosity.
delayed expansion
30. correct statements about the presence of
gamma-two in high-copper amalgam.
galvanic corrosion
31. factors that will reduce the trituration
time required to make an amalgam mix
with optimum plasticity.
gamma-one
amalgamation
atomized powder
condensation
crevice corrosion
dental amalgam alloy
eta prime
gamma silver-tin
gamma-two
high-copper amalgams
high-copper alloys
lathe-cut powder
low-copper alloys
low-copper amalgams
19
Dental Amalgam
Oral Biomaterials - Chapter 9
marginal fracture
marginal fracture
metal matrix
metal-matrix composite
single-composition amalgam
single-composition alloy
spherical powder
tarnish
trituration
20
Revised 5/24/07