INTRODUCTION

DENTAL AMALGAM

DENTAL AMALGAM

Seminar by

Dr. SUBHASHINI RPostgraduate Student

DEPARTMENT OF

CONSERVATIVE DENTISTRY &

ENDODONTICS

CONTENTS INTRODUCTION HISTORY CLASSIFICATION COMPOSITION MANUFACTURE OF ALLOY PARTICLES SETTING REACTION PHYSICAL PROPERTIES MANIPULATION AND TECHNICAL CONSIDERATIONS OF DENTAL AMALGAM FACTORS CONTROLLING QUALITY OF DENTAL AMALGAM INDICATIONS/CONTRAINDICATIONS ADVANTAGES/DISADVANTAGES CAUSES OF FAILURE OF DENTAL AMALGAM MERCURY TOXICITY RECENT ADVANCES IN DENTAL AMALGAM CONCLUSION REFERENCES INTRODUCTION

Amalgam is one of the oldest of all materials used for restoring the carious lesion. It has been used more than any other materials in restorative dentistry.

Thus on the basis of frequency of use, one might say that dental amalgam is the most important restorative material used by the dentist.

Amalgam technically means an alloy of mercury with any other metal:

Dental amalgam is an alloy made by mixing mercury with a silver tin alloy. Dental amalgam alloy is a silver tin alloy to which varying amount of copper and small amount of zinc has been added.

According to Skinners, amalgam is a special type of alloy in which one of its constituent is mercury. In dentistry, it is common to use the term amalgam to mean dental amalgam.

To make dental amalgam, mercury is mixed with a powder of amalgam alloy. The powder which consists of silver, tin, copper and small amount of zinc may be in the shape of lathe-cut, spherical or spheroidal. This mixing which is technically known as trituration will form a plastic mass which can be directly forced into the prepared cavity by a process known as condensation. The wetting of the powder particle with mercury will initiate a physicochemical reaction that in time causes hardening onset of the material.

HISTORY:

Dental amalgam is one of the oldest filling materials in use today. Its origin can be traced back to the second century A.D. in China were silver amalgam was developed for filling teeth more than a 100 years before dentists in the west. Silver paste is mentioned in the material medica of Sukung (AD 659).

In 1819, Bell from England advocated the use of a room temperature mixed amalgam as a restorative material. M.Traveau is credited with advocating the first form of amalgam paste in 1826, in France.

In 1833, Amalgam was officially introduced to the United States. Then, Crawcour brothers used a silver coin mercury mixture called "Royal Mineral Succedaneum" and promoted the material as an inexpensive and convenient restorative material. But this technique led to slow setting amalgam that released mercury from unset mass into unprotected dentinal tubules.

First Amalgam War:

In 1845, American Society of Dental Surgeons condemned the use of all filling material other than gold as toxic, thereby igniting "first amalgam war'. The society went further and requested members to sign a pledge refusing to use amalgam.

However, this policy was reconsidered in 1850,and there followed several investigations into amalgam composition and properties in UK,USA and France. The use of amalgam was also promoted by the work of J F Flagg and the final stamp of approval for its clinical use came from G V Black. The improved handling and performance of these materials blocked criticism of amalgam as an inferior restorative material and inspired general confidence in its safety.

Second Amalgam War:

In mid 1920's a German dentist, Professor A. Stock started the so called "second amalgam war". He claimed to have evidence showing that mercury could be absorbed from dental amalgam, which leads to serious health problems. He also expressed concerns over health of dentists, stating that nearly all dentists had excess mercury in their urine.

His writings attracted widespread attention. Charite hospital in Berlin, appointed a committee to investigate the allegations of amalgam toxicity. An account of the committees findings, published in 1930, declared that there was no reason to condemn the newer silver tin amalgams.

Third Amalgam War;

The current controversy, sometimes termed as "Third Amalgam War' began in 1980 primarily through the seminars and writings of Dr.Huggins, a practicing dentist in Colorado.

He was convinced that mercury released from dental amalgam was responsible for human diseases affecting the cardiovascular system and nervous system

He also stated that patients claimed recoveries from multiple sclerosis, Alzheimers disease and other diseases as a result of removing their dental amalgam fillings.

But research in United States and other first World countries demonstrated clearly that there is no cause and effect relationship between dental amalgam restoration and other health problems.Classification (Marzouk):

The amalgam alloy can be classified in the following ways:

I.According to number of alloy metals:

1. Binary alloys (Silver-Tin)

2. Ternary alloys (Silver-Tin-Copper)

3. Quaternary alloys (Silver-Tin-Copper-Indium).

II.According to whether the powder consist of unmixed or admixed alloys.

Certain amalgam powders are only made of one alloy. Others have one or more alloys or metals physically added (blended) to the basic alloy. E.g. Adding copper to a basic binary silver tin alloy

III.According to the shape of the powdered particles.

1. Spherical shape (smooth surfaced spheres).

2. Lathe cut (Irregular ranging from spindles to shavings).

3. Combination of spherical and lathe cut (admixed).

IV.According to Powder particle size.

1. Micro cut

2. Fine cut

3. Coarse cut

V.According to copper content of powder

1. Low copper content alloy - Less than 4%

2. High copper content alloy - more than 10%

VI.According to addition of Nobel metals

1. Platinum

2. Gold

3. Pallidum

VII.According to compositional changes of succeeding generations of amalgam.

1. First generation amalgam was that of G. V Black i.e. 3 parts silver one part tin (peritectic alloy).

2. Second generation amalgam alloys - 3 parts silver, 1 part tin, 4% copper to decrease the plasticity and to increase the hardness and strength. 1 % zinc, acts as a oxygen scavenger and to decrease the brittleness.

3. Third generation: First generation + Spherical amalgam copper eutectic alloy.

4. Fourth generation: Adding copper upto 29% to original silver and tin powder to form ternary alloy. So that tin is bounded to copper.

5. Fifth generation. Quatemary alloy i.e. Silver, tin, copper and indium.

6. Sixth generation (consisting eutectic alloy).

The alloying of palladium (10%), silver (62%) and copper (28%), to form a eutectic al1oy, which is lathe-cut and blended into a first, second or third generation amalgam in a ratio of 1:2.

The set amalgam exhibits the highest nobility of any previous amalgam and has been the most recent (sixth) generation of amalgam to be developed

VIII. According to Presence of zinc.

1. Zinc containing (more than 0.01%).

2. Non zinc containing (less than 0.01%).

ADA Specification No.1, for amalgam alloy includes a requirement for composition. This specification does not states precisely what the composition of alloy shall be; rather it permits some variations in composition. The chemical composition must consist essentially of silver and tin. Copper, zinc, gold, palladium, indium or mercury may be included in lesser amount.Conventional Amalgam Alloys: (G.V. Blacks: Silver- tin alloy or Low copper alloy).

Low copper alloys are available as comminuted particles (Lathe -cut and Pulverized) and spherical particles.

Low copper composition:

Silver: 63-70%

Tin: 26-28%

Copper: 2- 5% Zinc: 0-2%

Silver: It constitutes approximately 2/3rd of conventional amalgam alloy. 1t contributes to strength of finished amalgam restoration. It also decreases flow and creep of amalgam. It also increases expansion on setting and offers resistance to tarnish. To some extent it regulates the setting time.

Tin: Second largest component and contributes th of amalgam alloy. It readily combines with mercury to form gama-2 phase, which is the weakest phase and contributes to failure of amalgam restoration. Tin tends to reduce the expansion but at the same time decreases the strength of amalgam. It also tends to increase the flow. Tin controls the reaction between silver and mercury. Without tin the reaction would be too fast and the setting expansion would be unacceptable. Tin reduces both the rate of the reaction and the expansion to optimal values.

Copper: It contributes mainly hardness and strength. But, it also tends to decrease the flow and increases the setting expansion

Zinc: It acts as Scavenger of foreign substances such as oxides. Zinc also helps in decreasing marginal failure.

The most serious problem with zinc is delayed expansion, because of which zinc free alloys are preferred now a days.

Indium/Palladium: They help to increase the plasticity and the resistance to deformation.

HIGH COPPER AMALGAM ALLOY (COPPER ENRICHED ALLOYS)

To overcome the inferior properties of low copper amalgam alloy,

Youdelis and Innes in 1963 introduced high copper content amalgam alloys. They increased the copper content from earlier used 5% to 12%.

Copper enriched alloys are of two types:

1) Admixed alloy powder.

2) Single composition alloy powder.

I.Admixed alloy powder:

They are also called as blended alloys, these contain 2 parts by weight of conventional composition lathe cut particles plus one part by weight of spheres of a silver copper eutectic alloy (71.9% Ag, 28.1% Cu, approximately).

The admixed alloys are made by mixing particles of silver and tin with particles of silver and copper. The silver tin particle is usually formed by the lathe cut method, whereas the silver copper particle is usually spherical in shape.

Composition:

Silver-69 %

Copper-13 %

Tin-17 %

Zinc-1 %

Amalgam made from these powders are stronger than amalgam made from lathe cut low copper alloys because of strength of Ag-Cu eutectic alloy particles. Ag-Cu particles probably act as strong fillers strengthening the amalgam matrix. Total copper content ranges from 9-20%.

II. Single composition alloy (Unicomposition):It is so called as it contains particles of same composition.

Usually spherical single composition alloys are used. As lathe cut, high copper alloys contain more than 23% copper. A number of different types of single composition alloys are available.

1.Ternary alloy in spherical form, silver 60%, tin 25%, copper 15%.

2.An alloy similar to the first of those detailed in above, but contains particles in spheroidal form.

3.Quaternary alloy in spheroidal form containing Silver: 59%, copper 13%, tin: 24%, indium 4%.

Pre Amalgamated Alloys:

In this, the surface of alloy particles has been introduced to mercury by manufacturer for rapid amalgamation after trituration to make pre-compacted pellets. Pre amalgamated alloy contain upto 35% mercury and may be indirectly responsible for reducing total mercury content to less than 40%.

MANUFACTURE OF ALLOY PARTICLE:

Lathe cut filings:

To produce lathe-cut alloys, ingredients metals are heated, with protection from oxidation, until they are completely melted, and the melt is poured into suitable mold to form an ingot. The ingot may be 3-4 cm in diameter and 20 -30 cm in length. The ingot is cooled relatively slowly. After the ingot is completely cooled, it is heated for various periods of time (frequency 6-8 hours) at 400OC to produce a more homogenous distribution of silver and tin. The ingot is then reduced to filings, cut with a suitable tool on a lathe and ball milled. The particles are filtered through a fine sieve of mesh and larger filings are ball milled again to reduce them to the proper particle size. The particles are typically 50-100 (m in length, 10-60 (m in width and 10-30 (m in thickness. For manufacturing high copper irregularly shaped particles molten alloy is sprayed into water under high pressure.

Ageing of the alloy is desirable to improve the shelf-life of the product. The aging is related to relief of stress in the particles produced during the cutting of the ingot. At room temperature the residual stress is relieved over a period of week or month. Current practice is to age the particles artificially by subjecting them to a controlled temperature of 60-100OC for 1-6 hours. Spherical/ Spheroidal / Atomized Powder:

Today manufacturers can provide a variety of shapes and sizes such as Lathe-cut, Spherical, Spheroidal or combination. If a combination of different particulate shapes is used in the amalgam system it is called a blend.

Atomizing process produces these different shapes. After first liquefying the amalgam alloy, it is sprayed through a jet nozzle under high pressure in a cold atmosphere. If particles are allowed to cool before they contact the surface of chamber, they are spherical in shape. If they are allowed to cool on contact with the surface they are flake shaped.Particle Size:

Initially, lathe-cut particles was used which is no longer used. Modern days, smaller spherical particles are whose size is 15-5 (m. Smaller particle size is chosen because low mercury, rapid hardness, and early compressive strength, produces smoother surfaces on carving and is less susceptible to corrosion.Symbols & Stoichiometry of Phases that are Involved in the Setting of Dental Amalgams:

Ag3Sn

(1

Ag2Hg3(2

Sn7-8Hg

(

Cu3Sn

Cu6Sn5Silver-Copper eutectic Ag-Cu

AMALGAMATION REACTION/ SETTING REACTION

In low copper conventional amalgam alloy.

Amalgamation occurs when mercury comes into contact with the surface of the Ag-Sn alloy particles when a powder is triturated, the silver tin in the outer portion of the particles dissolve into mercury. At the same time, mercury diffuses into the alloy particle.

The mercury has a limited solubility for silver (0.035 wt%) and tin (0.6 wt%). When that solubility is exceeded, crystals of low binary metallic compounds precipitate into the mercury. These are the body- centered cubic Ag2Hg3 compound ((1 phase) and the hexagonal closed packed Sn7-8 Hg compound ((2 phase). Because of solubility of amalgam in mercury is much lower than that of tin, the gamma-1 phase precipitates first, and the gamma-2 phase precipitates later.

Immediately after trituration, the alloy powder coexists with the liquid mercury, giving the mix a plastic consistency. As the remaining mercury dissolves the alloy particles, gamma-1 and gamma-2 crystals grow. As the mercury disappears, the amalgam hardens. As the particle become covered with newly formed crystals mostly gamma-1, the reaction rate decreases. The al1oy is usually mixed with mercury in approximately I: 1 ratio. There is insufficient mercury to completely consume original alloy particles. Consequently, unconsumed particles are present in the set amalgam.

The reaction can be conveniently expressed in terms of phases that form during amalgamations.

Ag3Sn + Hg ( Ag2Hg3 + Sn7-8Hg + Ag3Sn

(() ((l) ((2) (() (unreacted)

The physical properties of the hardened amalgam depend on the relative percentage of each of the micro structural phases. The unconsumed Ag-Sn particles have a strong effect. The more of the phase that is retained in the final structure, the stronger is the amalgam. The weakest component is the (2 phase. The hardness of (2 is approximately 10% of the hardness of (l whereas the ( phase hardness is somewhat higher than that of (1 phase.

(2 phase is also the least stable in a corrosive environment and may experience corrosion attack, especially in "crevices" of the restoration. Generally ( (Ag3Sn) and pure (1 (Ag2Hg3) phases are stable in an oral environment. However (1 phase in amalgam does contain small amounts of tin, which can be lost in a corrosive environment. The interface between the gamma phase and the matrix is important. The high proportion of the unconsumed gamma phase does not strengthen the amalgam unless the particles are bound to the matrix.

The conventional amalgam contains the following component phases and their properties.A. The original Gamma Phase (Ag3Sn) - which has not been completely dissolved in mercury. Strongest phase and it could occupy maximum available space in the volume of restoration.

B. The Gamma -1 Phase (Ag2Hg3) is the noblest phase and most resistant to tarnish and corrosion.C. The Gamma -2 Phase (Sn7-8Hg) it is least resistant to tarnish and corrosion. More prone to corrosion and creep

D. The Mercury Phase unreacted residual mercury will be present within the amalgam mass. Weakest phase and when exceeds a certain volume limit, there will be drastic drop in the strength of the amalgam.

E. The Voids (Pores) Phase- occurs on the process of building amalgam restoration. Traps, air bubbles or voids lead to corrosion and early failure of restoration.

F. The Trace Element Phase in which copper and zinc might be found, only in trace amounts in the final product. Copper can increase strength, brittleness etc. Zinc can increase strength and resistance to oxidation of final product.G. The Inter Phase applies to the inter phases between gamma, gamma -1 and gamma -2. In the final restoration, the more continuous they are and close together, the better in the bonding, the more coherent and more resistant to environment variable the restoration will be.

ADMIXED ALLOYES AND UNICOMPOSITIONAL ALLOYS

The reaction occurs in two phases. The first phase is equivalent to the one as shown in conventional alloys i.e.

Ag3Sn + Hg ( Ag2Hg3 + Sn7-8Hg + Ag3Sn

(() ((l) ((2) (() (unreacted)

The second phase is the eutectic of silver copper phase, which are called (1 and (2 . This type of reaction occurs in admixed typed of high copper alloy. In a eutectic, two metal phases are present, which are separated but closely adjacent to each other. Here (1 is silver rich and (2 is copper rich. These two along with ( (Ag3Sn) react with mercury and again (1 (Ag2Hg3) and (2 (Sn7-8Hg) is formed. All the tin does not react with mercury; part of tin reacts with the eutectic (1 and (2.

After

Ag3Sn + Ag-Cu + Hg ( Ag2Hg3 + Sn7-8Hg + Ag3Sn +Cu3Sn + Cu6Sn5

Mixing

( (1 + (2 (1 (2 (

+ Ag-Cu

(1 + (2

The reaction goes on. After one week, the Sn7-8Hg () phase reacts completely with eutectic. The silver forms (, and the tin forms ( and .After

Ag3Sn + Ag-Cu + Hg ( Ag2Hg3 + Ag-Cu +Cu3Sn + Cu6Sn5 + Ag3Sn.

One week

( (1 + (2 (1 (1 + (2 (

The reaction is not finished yet. After approximately one year, copper tin phase is also vanished. The tin further diffused into the silver copper eutectic slowly and forms more of phase. The final reaction is:

After

Ag3Sn + Ag-Cu + Hg ( Ag3Sn + Ag-Cu + Ag2Hg3 + Cu6Sn5

One year

( (1 + (2 ( (1 + (2 (1

The difference of reaction in an admixed type and the unicomposition type is that the eutectic Ag-Cu is absent and the reaction is directly with silver, tin and copper tin phases.Ag3Sn + Cu3Sn + Hg ( Ag2Hg3 + Cu6Sn5 + unreacted alloy particles

( ( (1 Thus finally the phase formed is Cu6Sn5 () and no (2 phase i.e. Sn7-8Hg is formed. Thus time taken for the reaction is faster in case of unicompositional as compared to admixed typePROPERTIES: ADA specification No.1 for amalgam alloy contains certain requirement that aid significantly to control the qualities of dental amalgam. The specification lists 3 physical properties as a measure of quality of the amalgam.

1. Creep

2. Compressive strength

3. Dimensional changes.

Strength:

A prime requisite for any restorative material is strength sufficient to resist fracture. Fracturing of even a small area, especially at the margins, fastens corrosion, secondary caries, and subsequent clinical failure. It is said that dental amalgam is strongest in compression and weaker in tension and shear. Therefore, the prepared cavity design and manipulation should allow for the restoration to receive compression forces and minimum tension and shear forces.

The compressive strength of a satisfactory amalgam restoration should be atleast 310 MPa. The high copper unicompositional material has the highest early compressive strengths of more than 250 MPa at 1 hour. The compressive strength of 1 hour was lowest for Lathe-cut alloy i.e.145 MPa. and does not meet ADA specifications. The compressive strengths at 7 days, or the final strengths, again are highest for high copper unicompositional alloys.

Comparison of Compressive Strength & Creep of a Low-Copper and High Copper Amalgam:

AmalgamCompressive Strength (MPa)Creep (%)Tensile Strength -24h(MPa)

1 h7 day

Low copper1453432.060

Admix1374310.448

Single Composition2625100.1364

High values of early compressive strength are advantageous for amalgam. Since, they reduce the possibility of fracture by prematurely high contact stresses from the patient before final strength is reached. As amalgam is brittle material, it does not undergo deformation or elongation on loading. Therefore sudden application of excessive forces tends to fracture amalgam restoration.

The tensile strength of both low and high copper amalgam alloy ranges from48-70 MPa. The elastic modulus of amalgam ranges from 11-12 GPa. High copper alloy tend to be stiffer than low copper alloy.

The factors that effect strength of amalgam are:

1. Temperature: Amalgam looses 15% of its strength when its temperature is elevated from room temperature to mouth temperature and looses 50% of room temperature strength when temperature is elevated to 60OC e.g. hot coffee or soup.2. Trituration: The effect of trituration on strength depends on the type of amalgam alloy, the trituration time and the speed of the amalgamator. Either, under trituration or over-trituration decreases the strength for both traditional and high copper amalgams.

More the trituration energy used, more evenly distributed are the matrix crystals over the amalgam mix and consequently more the strength pattern in the restoration. However, excess trituration after formation of matrix crystals will create cracks in the crystals, lead to drop in strength of set amalgam.

3. Mercury Content: Mercury content is a very important factor in control of strength. Sufficient mercury should be mixed with the alloy to coat the alloy particles and allow a thorough amalgamation. Each particle must be wetted by mercury. Low mercury alloy content, contain stronger alloy particles and less of the weaker matrix phase, therefore more strength. If mercury is too less it will result in dry, granular mix, which results in a rough, pitted surface that invites corrosion. Any excess mercury if present can drastically affect the compressive strength. If mercury content of amalgam mix is more than 53-55%, it causes drop of compressive strength by 50%. Increase in the final mercury wi1l cause increase in volume fraction at exposure of alloy particle resulting in amalgam that is weaker.

4. Effect of condensation: Condensation pressure, technique and alloy particle shape affect amalgam properties, when typical condensation technique and lathe-cut alloys are employed, the greater the condensation pressure, the higher the compressive strength, particularly the early strength (e.g., at 1 hour) and good condensation techniques express mercury and result in a smaller volume fraction of matrix phases. Higher condensation pressure is required to minimize porosity and to express mercury from lathe-cut amalgam. On the other hand, spherical amalgams condensed with lighter pressure produce adequate strength.

Effect of Porosity:

Voids and porosity are factors influencing the compressive strength of hardened amalgam. Although, porosity cannot be completely eliminated, it should be minimized. Pores facilitate stress concentration, propagation of cracks, corrosion, and fatigue failure of amalgam restoration. Porosity can be due to under trituration, particle shape, insertion of too large increments into the cavity, delayed insertion after trituration, non-plastic mass of amalgam.

Strength after 1 hour is only 6% of I week strength. Patient should be asked not to bite on that side for at least 8 hours, as by that time a typical amalgam has reached at least 70% of its strength.

Inter particle distance

The closer the particles of amalgam alloys are to each other, the stronger will be the end product. There is increase in compressive strength in 24 hour specimens when the average inter particle distance is 38( or less. In one week specimen, the strength increases, when the average interparticle distance is 32 microns or less.

In terms of tensile strength, 24 hour specimen demonstrated a marked increase, when average inter particle distance is < 28(. For one week specimens, the increase exhibited, when average inter particle distance is