Dental amalgam

Dr. Waseem Bahjat Mushtaha

Specialized in prosthodontics

Terminology

Amalgam : an alloy of mercury.Amalgamation : the process of mixing liquid mercury with one or more metals or alloys to form an amalgam.

Creep : the time-dependent strain or deformation that is produced by a stress. The creep process can cause an amalgam restoration to extend out of the cavity preparation, thereby increasing its susceptibility to marginal breakdown.

Delayed expansion: the gradual expansion of a zinc-containing amalgam over a period of weeks to months that is associated with hydrogen gas development caused by contamination of the plastic mass with moisture during its manipulation in a cavity preparation.

Dental amalgam: an alloys of mercury, silver, copper, tin, which may also contain palladium, zinc, and other elements to improve handling characteristics and

clinical performance .

Dental amalgam alloy : an alloy of silver, copper, tin and other elements that is formulated and processed in the form of powder particles or as a compressed pellet.

Trituration : the process of grinding powder, especially within a liquid. In dentistry, the term is used to describe the process of mixing the amalgam alloy particles with mercury in an amalgamator.

Trituration

Alloys composition

American dental associated (ADA) specification No. 1 requires that amalgam alloys be predominantly silver and tin. Unspecified amount of other elements, such as copper, zinc, gold, and mercury, are allowed in concentrations less than the silver or tin content. Alloys containing zinc in excess of 0.01% or less of zinc are designated as nonzinc.

Metallurgic phase in dental amalgam

The setting reactions of alloys for dental amalgam with mercury are usually described by the metallurgic phases that are involved. These phase are named with Greek letters that correspond with the symbols found in phase diagram for each alloy system

Symbols and stoichiometry of phases that are involved in the setting of dental amalgam

Phases in amalgam stoichiometric formula Alloys and set dentalAmalgam.

γ Ag3Sn γ1 Ag2Hg3 γ2 Sn7-8Hg

ε Cu3Snή Cu6Sn5Silver-copper eutectic Ag-Cu

The Greek letter named as follows: γ(gamma);ε(epsilon);ή (eta) .

Manufacture of alloys powder

Lathe-cut powder: to make lathe-cut powder, an annealed ingot is placed in a milling machine or in lathe and is fed in to cutting tool or bit. The chips removed are often needlelike. (irregular particles).

Homogenizing Anneal: because of the rapid cooling conditions form the as-cast state, an ingot of an Ag-Sn alloy has a cord structure and contains nonhomogeneous grains of vary composition. A homogenizing heat treatment is performed to re-establish the equilibrium phase relationship. The ingot is placed in an oven and heated at a temperature below the solidus for sufficient time to allow diffusion of the atoms to occur and the phases to reach equilibrium.

Particle treatments: once the alloy ingot has been reduced to cuttings, many manufactures performed some type of surface treatment of the particles. Treatment of the alloys particles with acid has been a manufacturing practice for many years. The exact function of this treatment is not entirely understood, but it is probably related to the preferential dissolution of specific components from the alloy. Amalgams made from acid-washed powder tend to be more reactive than those made from unwashed powder.

Atomized powder: atomized powder is made by melting together the desired elements. The liquid metal is atomized into fine spherical droplets of metal. (spherical powder).

Particle size: maximum particle size and the distribution of sizes within an alloy powder are controlled by the manufacturer. The average particle sizes of modern powders range between 15 and 35 µm. The most significant influence on amalgam properties is the distribution of sizes around the average. For example very small particles (< 3 µm) greatly increase surface area per unit volume of the powder. A powder containing tiny particles requires a greater amount of mercury to form an acceptable amalgam.

Lathe-cut compared with atomized alloys:

Amalgams made from lathe-cut powders, or admix powder of blend of lathe-cut and spherical powders, tend to resist condensation better than amalgams made entirely from spherical powder.

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. Amalgam with a low mercury content

generally have better properties .

Amalgamation and resulting structure

Low-copper alloys:

Amalgamation occurs when the mercury comes into contact with the surface of the Ag-Sn alloy particles. When the powder is triturated, the silver and tin in the outer portion of the particles dissolve in to mercury. At the same time, mercury diffuse into alloy particles. The mercury has a limited solubility for silver (0.035 wt%) and tin

(0.6 wt%) .

When that solubility is exceeded, crystal of two binary metallic compounds precipitate in to the mercury. These are the body-centered cubic Ag2Hg3 compound (the γ phase) and the hexagonal closed packed Sn7-8Hg compound (the γ2 phase). Because the solubility of silver in mercury is much lower than of tin, the γ1 phase precipitates first, and the γ2 phase precipitates later. Immediately after trituration, the alloy powder coexists with the liquid mercury, giving the mix a plastic consistency. As the remaining

mercury disappears, the amalgam hardens .

As the particles become covered with newly formed crystals, mostly γ1, the reaction rate decreases. The alloy is usually mixed with mercury in approximately a 1:1 ratio. This is insufficient mercury to completely consume original alloy particles; consequently, unconsumed particles are present in the set amalgam. Alloys particles ( smaller now, because their surfaces have dissolved in mercury) are surrounded and bound together by solid γ1 and γ2 crystals.

Thus, atypical low-copper amalgam is a composite in which the unconsumed particles are embedded in γ1 and γ2 phases. The reaction can be conveniently expressed in terms of the phases that form during amalgamation:

Alloy particles (β +γ) + Hg γ1+ γ2 + unconsumed alloy particles (β +γ).

The physical properties of the hardened amalgam depend on the relative percentages of each of the microstructural phases. The unconsumed Ag- Sn particles have a strong effect. The more of this phase that is retained in the final structure, the stronger is the amalgam. The weakest component is the γ 2 phase. The hardness of γ2 is approximately 10% of the hardness of γ1 .

γ2 phase is the least stable in a corrosive environment and experience corrosion attack, especially in “cervices” of the restorations. Pure γ1 phases are stable in an oral environment. However, γ1 in amalgam does contain small amounts of tin, which can be lost in a corrosive environment .

High –copper alloys :

High –copper alloys have become the material the materials of choice because of their improved mechanical properties, corrosion characteristics, and better marginal integrity and performance in clinical trials, as compared with traditional low-copper alloys. Two different types of high-copper alloy powders are available:

1 )Admix alloy powder.

2 )Single composition alloy powder.

Both types contain more than 6 wt% copper.

1 )Admix alloy powder

In 1963, Innes and Youdelis added apherical silver-copper (Ag-Cu) eutectic alloy (71.9 wt% silver and 28.1 wt% copper) particles to lathe-cut low-copper amalgam alloy particles. This was the first major change in the composition of alloy for dental amalgam since Black’s work. Theses alloys are often termed admix alloys because the final powder is a mixture of at least two kinds of particles. An admix powder, showing lathe-cut low-copper alloy particles and spherical Ag-Cu alloy

particles .

Amalgam made from these powder is stronger than amalgam made from lathe-cut low-copper (composite materials {materials that consist of a matrix and filler} can be strengthened by the addition of strong fillers) and the Ag-Cu particles probably act as strong fillers, strengthening the

amalgam matrix .Several classic studies have shown that restorations made with this prototype admixed amalgam were clinically superior to low-copper amalgam restorations when they were evaluated for resistance to marginal improved clinical performance.

Admix alloy powders usually contain 30 wt% to 55 wt% spherical high-copper powder. The total copper content in admixed alloys ranges from approximately 9 wt% to 20 wt%. The phases present in the copper-containing particles depend on their composition. The Ag-Cu alloy consists of mixtures of two phases-silver rich and copper rich-with the crystal structures of pure silver and pure copper, respectively. Each phase contains a small amount of the other element. In the atomized powder (which is fast cooled), the eutectic two-phase mixture from very fine lamellae. Compositions on either side of the eutectic from relatively large drains of copper-rich phase or silver-rich phase amide the eutectic mixture.

When the mercury reacts with an admixed powder, silver dissolves into the mercury from Ag-Cu alloy particles, and both silver and tin dissolve into the mercury from Ag- Sn alloy particles. The tin in solution diffuses to the surfaces of the Ag-Cu alloy particles and reacts with the copper phase to form the ή phase (Cu6Sn5). A layer of ή crystals form around unconsumed Ag-Cu alloy particles. The ή layer on Ag-Cu alloy particles also contains some γ1 crystals. the γ1 phase form simultaneously with the ή phase and surrounds both the ή covered Ag-Cu alloy particles and the Ag- Sn alloy particles. As in the low-copper amalgams, γ1 is the matrix phase, that is, the phase that binds the unconsumed alloy particles together

The reaction of the admixed alloy powder with mercury can be summarized as follows:

Alloy particles (β + γ) + Ag-Cu eutectic + Hg

γ1 + ή + unconsumed alloy of both types of particles.

N.B γ2 has been eliminated in this reaction.

Single composition alloys

Unlike admixed alloy powders, each particle of these alloy powders has the same chemical composition. Therefore, they are called single-composition alloys. The major components of the particles are usually silver, copper, and tin. The first alloy of this type contained 60 wt% silver, 27 wt% tin, and 13 wt% copper. The copper content in various single composition alloys rang from 13 wt% to 30 wt%. In addition, small amounts of indium or palladium are also found in some of the currently

marked single-composition alloys .

A number of phases are found in each single-composition alloy particle, including β (Ag- Sn) γ(Ag3Sn), and ε (Cu3Sn). Some of the alloys may also contain some ή phase (Cu6Sn5) .

When triturated with mercury, silver and tin from the Ag-Sn phases dissolve in mercury, little copper dissolves in mercury. The γ1 crystals grow, forming a matrix that bind together the partially dissolved alloy particles. the ή crystals are found as meshes of rod crystals at the surface of alloy particles, as well as, dispersed in the matrix. Theses are much larger than the ή crystals found in the reaction layers surrounding Ag-Cu particles in admix amalgams .

To summarized the reaction of the single-composition alloy powder with mercury is as follows:

Ag-Sn-Cu alloy particles + Hg

γ1 + ή + unconsumed alloy particles.

Manipulation

1 )proportioning

2 )Trituration

3 )Condensation

4 )Trimming and carving

5 )Polishing

6 )Some precautions

Proportioning

a) Mercury : the required quantity can be obtained by weighing or by using a volume dispenser. Clearly the latter method is quicker. It is important to use pure clean mercury.

b) Alloy : this can be proportional by:1 )Weighing

2 )Using tables of alloy, particularly with mechanical mixing.

3 )Having envelopes with pr-weighed quantities.4 )Using a volume dispenser.

Two disadvantages of a volume dispenser are: 1 )It is difficult to measure any powder accurately by

volume, as the weight of material per volume depends on the efficiency with which the particles are packed

together .2 )Alloy can cling to the walls of the dispenser.

c) Alloy/mercury ratio. In the final set amalgam it is desirable to have less than 50% mercury. Two techniques have been recommended:

1 )The use of an alloy/mercury ratio of 5/7 or 5/8. the excess mercury makes the trituration easier, giving a smooth plastic mix of material. Before insertion into the cavity, excess mercury is removed from the mix by squeezing it in a dental napkin.

2 )Minimal mercury techniques, where about equal weights of alloy and mercury are used and no mercury is

squeezed out of the mix before condensation . d) Many materials are supplied in capsules with per-proportioned alloy and mercury.

Trituration

a) Hand mixing by mortar and pestle. A glass mortar and pestle are used. The mortar has its inner surface roughened to increase the friction between the amalgam and the surface. A rough surface can be maintained by occasionally grinding with carborandium paste. The pestle is a glass rod with a rounded end.

b) Mechanical mixing : the proportioned alloy and mercury can be mixed mechanically in a capsule, either with or without a stainless steel or plastic pestle. A pestle, which should be of considerably smaller diameter than the capsule, should be used with tablet alloys, to help break up the material. The mechanical amalgamators have time switches to ensure a correct mixing time. A number of these materials are available in an encapsulated form, each capsule containing a controlled weight of alloy, and having the right quantity of mercury sealed in its lid. The choice of trituration time is important, and will depend both on the type of alloy and the speed of mixer. In particular rich copper alloys require precise control of trituration conditions. Some products require high energy mixing to break up the oxide coating which forms on copper rich particles.

Condensation

1 )Each portion is properly adapted by a condenser of suitable size.

2 )A load of up to 4-5 kg is applied to each increment.

3 )As the mix is condensed, some mercury- rich material rises to the surface. Some of this can be removed, to reduce the final mercury content, and improve the mechanical properties. The remainder will assist bonding with the next increment, to avoid the production of a weak laminated restoration

A material should be condensed as soon as possible after mixing. If it is left too long, and has begun to set:

1 )Proper adaptation to the cavity will be impossible

2 )Elimination of excess mercury will be difficult

3 )Bonding between increments will be poor

4 )Lower strength values will result.

Mechanical condenser are also available, which apply vibration to pack the amalgam.

Amalgams prepared from spherical alloys need less packing pressure than those prepared from lath cut alloys. However, condensation can be more difficult for spherical alloys, as the condenser point tends to pass right through the material instead of developing pressure. Generally spheroidal particles amalgams condense with more resistance than spherical materials.

Trimming and curving

When the cavity is overfilled, the top mercury-rich layer can be trimmed away and the filling carved to the correct contours. The amalgam prepared from a coarser grain alloy may be more difficult to carve, as the instrument may pull out large pieces of alloy from the surface. Spherical alloys are used where ease of carving is desired.

Polishing

Conventional amalgams are polished not less than 24 hours after insertion that is, when the material has gianed considerable strength. Since amalgams from rich copper

alloys gain strength rapidly.

Some precautions

a) Mercury is toxic, so free mercury should not be allowed to enter the atmosphere. This hazard can arise during trituration, and condensation and finishing of restorations, and also during the removal of old restorations at high speed.

b) Skin contact with mercury should be avoided, as it can be absorbed by the skin.

c) Any excess mercury should not be allowed to get into sinks, as it can react with some of the alloys used in plumbing.

d) Contamination of the amalgam by moisture must be avoided.

Properties

1 )Toxicity

2 )Corrosion reactions

3 )Marginal leakage

4 )Strength

5 )Marginal failure

6 )Thermal diffusivity

7 )Dimensional changes

Toxicity

a) The wisdom of using a restorative material containing mercury has often been questioned .

b) The potential danger of any form of mercury is related to:

1 -The form in witch the mercury is present

2 -The quantity and frequency of exposure

c) There is no evidence of harmful effect of the amalgam.

Corrosion reaction

a) Tarnish : amalgam can tarnish in the presence of sulpher, to give layer of sulphides on the surface of restoration.

b) Corrosion of conventional amalgams: the set material is heterogeneous stimulate corrosion. Of the three phases present, the γ2 is the most active electrochemically, being anodic in relation to both the γ and γ1

phases .

As γ2 corrodes, essentially two products result :1 (Ionic tin produced: in the presence of saliva,

corrosion products such as SnO2 and Sn(OH)6Cl are found.

2 (Hg is produced, which can react with some of the remaining hitherto unreacted γ phase.c) Corrosion of rich copper amalgam:

1 )No γ2 is present, and Cu6Sn5 is phase most prone to corrosion.

2 (However, the corrosion currents associated with those with conventional amalgam .

3 )The volume of corrosion products is less than with conventional amalgam.

4 )No mercury is produced as a result of the corrosion.

d) Practical considerations:

1 )Corrosion resistance is greatly improved if the amalgam is polished. This process removes pits and voids on the surface, which aid concentration cell corrosion

2 )If amalgam comes in contact with a gold restoration, an electrolytic cell may be set up leading to corrosion of the amalgam and incorporation of mercury on the gold restoration.

3 )Corrosion of conventional amalgam can have a significant effect on long-term mechanical properties. It has been shown, fore example, the tensile strength is reduced by 30% when the network of γ2 has

corroded.

Marginal leakage

The initial marginal leakage of an amalgam restoration, reduces with time, because of sealing of the micro-fissures by products of corrosion breakdown.

Strength

The following factors can lead to the production of a weak restoration:

1 )undertrituration

2 )Too high a mercury content

3 )Too low condensation pressure

4 )Slow rate of packing

5 )Corrosion

The rate of development of strength of an amalgam is of importance. With amalgams which develop strength slowly, there is danger of early fracture of such a restoration. Generally, spherical and rich copper amalgams have high early strengths. Of the phases present in conventional amalgams, the γ2 is the weakest and softest.

Marginal failure

Ditching of the margins of amalgam is a common occurrence. Clinical trials have shown that higher copper alloy formulations show much less marginal breakdown than conventional materials. The following observations are relevant:

a) Poor technique can cause breakdown, e.g. an unsupported ledge of amalgam extending over the enamel may fracture during mastication

b) In general, materials with high creep values have been shown to have poorer durability of margins. This does not, however, necessarily prove that there is a cause and

effect relationship between the two phenomena .

c) A theory has been propounded which links marginal breakdown with corrosion characteristics. It has been suggested that the mercury corrosion product (from conventional materials) reacts to form more γ1 and γ2 material, with associated expansion, termed “mercuroscopic expansion”. The expanded material, weakened by corrosion, protrudes away from the supporting tooth structure, and fractures.

Thermal diffusivity

Dental amalgam is a conductor of heat, whereas the enamel and dentin it replaces is a thermal insulator. Consequently, large amalgam restorations are usually lined with a thermal insulating cement to product the pulp from temperature changes in the mouth caused by hot and cold foods and liquids.

Dimensional changes

Ideally there should be little or no contraction on setting of a dental amalgam, otherwise a gap between filling and cavity walls may result, enhancing the possibility of further decay. Too great an expansion should also be avoided, as this will cause the filling to protrude from the cavity.

In laboratory experiments, where free expansion is measured, it has been shown that a greater

expansion on setting will result if :

1 )A higher alloy/mercury ratio is used.

2)There is a shorter trituration time

3 )Lower pressure during condensation is used

4)The alloy has a larger particle size

5 )There is contamination by water before setting in zinc-containing materials.

An electrolytic reaction between zinc (the anode) and the other metals which are cathodic and the water as an electrolyte.

Hydrogen is evolved as a result of this reaction.

The pressure of the evolved hydrogen may cause the amalgam to flow.

This causes an expansion, which may not appear within the first 24 hours, but may become evidence some days after insertion of the restoration.