Dental Amalgam2 / orthodontic courses by Indian dental academy

DENTAL AMALGAMCONTENTS:

1) HISTORY

2) GENERAL CONSIDERATION

3) CLASSIFICATION AND DISCUSSION

4) METALLURGICAL PHASES

5) MANUFACTURE OF ALLOY PARTICLES

6) AMALGAMATION

7) DIMENSIONAL STABILITY

8) STRENGTH

9) CREAGT

10) TARNISH AND CORROSION

11) MANIPULATION

12) AMALGAM ‘PROBLEMS’

13) ALTERNATIVE TO AMALGAM

14) BONDED AMALGAM RESTORATIONS

15) GALLIUM BASED AMALGAM RESTORATION

16) CURRENT STATUS AND CONCLUSION

17) BIBLIOGRAPHY

DENTAL AMALGAM

HISTORY OF DENTAL AMALGAM: The Chinese developed a Silver Amalgam for fillings more than 1000 years before dentists in west. “Silver Paste” is mentioned in the ‘Materia Medica’ of Su Kung (659A.D) and again about 1108 in Ja-Kuan Pen – tsao by T’ang Shen – Wei. During the Ming period in their material medicas, LivWen-t’ai (1505) and lishihchen (1578) discussed its formulation: 100 parts of Mercury to 45 parts of Silver and 900 parts of Tin. Trituration of these ingredients produced a paste said to be as solid as silver.

- The first dental silver amalgam is supposed to have been introduced by Bell of England in 1819 and later used by Traveau in Paris in 1826.

- In 1833, 2 frenchman, by name of Crawcour came to America with what they claimed was a new material for filling teeth. A crude Amalgam, their so called, “Royal Mineral Succedaneum” was produced from shavings of silver cut from coins and mixed with enough Hg to make a sloppy paste. The Crawcour’s blatant advtising and reprehensible habit of leaving carious matter in teeth they filled brought down upon them wrath of many of most prominent members of profession and after a few months were forced to return to France. Nevertheless during their short stay they traveled widely, touting the RMs and placing fillings in a great many mouths. Many American dentists saw in the material an answer to their problems with Au-foil, which was very difficult and time

consuming to use and many began to experiment with silver Amalgams; though leaders of profession did not.

AMALAGAM WARs: Unfortunately it was that the young American society of Dental Surgeons (ASDS) came into being so soon after the crowcour brothers had created such a stir with their new amalgams. Their bombastic advertisement had posed a threat to more ethical and conscientious dentists At the same time many other untrained, often unprincipled practitioners in search of easy money seized upon the new material, which was easier to place in a tooth than AU.

Organised dentistry, which of that time represented only a very tiny percentage of practicing dentists, began a campaign/crusade against the use of Amalgam (~1841) and their drive soon assumed the tone of religious crusade. Proponents of Amalgamwere to be rooted out and to this end every member of ASDS was required to sign a pledge that it was his Opinion and firm conviction that any Amalgam whatever is unfit for plugging of teeth and fangs [retained roots] and I pledge myself never under any circumstance to make use of it in my practice.

Those who refused to sign the pledge were summarily expelled. Meanwhile Dr. Chapi A. Harris, in his opening address to the first class of Balfimore College of Dental Surgery (1839), said Amalgam that, “it is one of the most abominable articles for filling teeth that could be employed. But progress, in whatever form it takes, cannot be so easily implemented. Many dentists –

including a number of highly reputable ones, soon found in Amalgam, answer to certain difficult restorative problems. They also felt compelled to use it to serve the needs of those too poor to pay for Au. and also in order to complete with the quacks, who were, using it widely. As a consequence, so many dentists had refused to sign the pledge by 1850 that the ASDS were forced to rescind it. But their conciliatory action came too late and the annual meeting scheduled for Aug’1856 had to be cancelled. Thus came to an end the 1 st National organization of dentists.

- Dr. Thomas W. Evans of Philadelphia emigrated to France in 1847 and introduced use of silver amalgam for fillings. He was dentist to Emperor Louis Napoleon.

- In 1870s a group of dentists led by prominent J. Foster Flagg forming what they called a “New Departure” group, effectively brought to an end the last hostilities of the great “AmalgamWars”. The basic tenet of the movement was that no single filling material could serve equally well in every case Au had its use, so did Silver Amalgam (which infact was claimed by the group to be more versatile material of the two).

- A number of attempt to improve the resistance of Amalgam to shrinkage had been made since the ‘Crawcorn’ days.

- Dr. Thomas W. Evans, who was chiefly responsible for popularizing the use of silver Amalgam in Europe experimented with mixture of Sn, Cd and Hg. Though he eventually found it necessary to reintroduce Ag into the

mixture, tin and reduces shrinkage has remained an essential ingredient to this day.

- At this time innovation to enhance utility of Amalgam were brought out: Retentive pins that screwed into dentin patented in 1871 and matrix retainers and matrices came into scene in that same year. Johnston brothers advertised the pins.

- In 1895, the great “Greene Vardiman Black” often called the “father of scientific Dentistry”; announced his formula for a truly satisfactory Amalgam. After years of experimentation, using instruments, of his own design to measure hardness, flow and other characteristics, Black hit upon a mixture of metals that remained essentially unchanged: 68% Ag with small amounts of copper, Tin and Zinc with this new alloy, expansion, contraction can be precisely controlled.

- In 1926 – Alfred Stock, Ph.D, a German chemist published an article condemning amalgam restorations. Dr. Stock himselves was exposed to high Hg levels and recognized the danger, posed by the type of Amalgam, in use at that time viz. a tablet had to be heated in a spoon till beads of Hg appeared and then it was transferred to mortar and pestle for trituration. This procedure released significant amount of Hg vapours.

Hence a commission was established to investigate his allegations. In 1930, this commission issued a report that validated the safety of newer amalgam that neednot be heated and it replaced the older formulation.

- In 1959, Dr. Wilmer Eames recommended a 1:1 ratio of Hg to alloy, thus lowering the 8:5 ratio of Hg to alloy that others had been recommending.

- In 1962, a spherical particle dental alloy was introduced.

- In 1963, Innes and Yudelis introduced a high copper-dispersion alloy system that proved to be superior to its low copper predecessors.

- In 1970s, Dr. Hal Huggins began promoting the theory that Amalgam restorations caused wide variety of disease.

- In 1985, Dr. Hal Huggins published a book that detailed his belief about Hg. toxicity. He says that Hg released by Amalgam restorations caused a wide variety of Neurological, CVS, Immunological, collagen, emotional and Allergic disorders. The resulting conditions are said to include Multiple sclerosis, depression, high/low B.P. Tachycardi, Arthritis, lupus, Scleroderma, leukemia, Hodgkins disease, fatigue, Mononucleosis, Gohn’s disease, ulcers and other digestive problems. This and media hype led to some dentists to question safety of Amalga restoration.

- In 1995, survey reported that 8.7% of dentists wanted to ban Amalgam use and 14.3% were undecided about its safety.

- Other physicians like Robert Atkins M.D and Andrew Weil M.D added fuel to the fire through books and T.V programs during 90s.

- American council of science and health, a consumer education and advocacy group has determined that allegations against Amalgam constitute one of the greatest unfounded health scares of recent times.

AMALGAM – An amalgam is an alloy that contains Mercury as one of its constituents. Because Hg is liquid at room temperature, it can be alloyed with solid metals. The process of amalgamation in clinic consists of releasing Hg droplets from sealed chamber within a capsule into another chamber within capsule that contains an alloy powder and then mixing the components together in a device called Amalgamator. Amalgamation process continues while segments of plastic mass are condensed under firm pressure against the walls of prepared teeth; and if present, a matrix band. The reaction continues during the manipulation period in the mouth and decreases within a few minutes as the dental amalgam increases in strt and hardness; although the reaction can continue for several days, the dental amalgam becomes sufficiently strong to support moderate biting forces within the first hour. The general descriptive reaction is as follows: Alloy particles for Amalgam + Mercury –

Dental Amalgam + Non-reacted Alloy

Powder particles.

General considerations for Amalgam Restorations –

Because Dental Amalgam is a direct restorative material the decision is usually a choice between Amalgam and composite. Some of the following information involves a comparative analysis between these two materials:

INDICATIONS –

1) Occlusal factors – Amalgam has somewhat greater wear resistance than composite. It therefore may be indicated in clinical situations that have heavy occlusal functioning. It also may be more appropriate when a restoration restores all of the occlusal contact for a tooth.

2) Isolation factors – unless an Amalgam restoration is to be bonded, the isolation of the operating area is less critical than for a composite restoration. Minor contamination of an Amalgam during insertion may not have as adverse an effect on final restoration as the same would on composite restoration. However, is Amalgam is to be bonded, isolation needs are same as that of composite.

3) Operator Ability and Commitment factors : The tooth preparation for an Amalgam is very exacting. It requires a specific form with uniform depths and precise marginal form. Many failures of Amalgam restoration may be related to inappropriate tooth preparations. The insertion and finishing procedure for Amalgam are much easier than composite. Instead if it is to be bonded, it is as complex as for composite.

Clinical Indications for Direct Amalgam Restoration :

Because of the factors presented above, Amalgam is most appropriately considered for :

1) Moderate to large class I and Class II restorations (especially including those with heavy occlusal loading that cannot be isolated well or that extend onto the root surface)

2) Class V restorations (Including those that are not esthetically critical, cannot be well isolated or are located entirely on root surface)

3) Temporary caries control Restorations (including those teeth which are badly broken down and require a subsequent assessment of pulpal Health before definite treatment).

4) Foundations (including for badly broken down teeth that will require increased retention and resistance form in anticipation of subsequent placement of a crown or metallic Onlay).

Contradiction:

While esthetics is subject to wide variations in personal interpretation, most patients find the appearance of the Amalgam restoration objectionable when compared to composite restoration. Therefore use of Amalgam in prominent esthetic areas of mouth is usually avoided. These areas include anterior teeth, premolars and in some patients – the molars. Because Amalgams requires a larger tooth preparation than composite, most small (even moderate) defects in

posterior teeth should be restored with composite rather than Amalgam which leads to conservation of tooth structure.

Advantages :

1) Ease of use

2) High composite strt.

3) Excellent wear resistance

4) Favourable long term clinical research results

5) Lower cost than for composite restorations

6) Bonded Amalgams have “Bonding benefits:

i. Less microleakage, interfacial staining

ii. Slightly increase strt of remaining tooth structure

iii. Minimal post operative sensitivity

iv. Some retention benefit

v. Esthetic benefit of sealing by not permitting the Amalgam to discolour the adjacent tooth structure.

Disadvantages:

1) Non Insulating

2) Non Esthetic

3) Test conservative

4) Weakens tooth structure (unless bonded)

5) More technique sensitive if bonded.

6) More difficult tooth preparation

7) Initial marginal leakage.

Classification – Major approaches to classifications of Amalgams and Amalgam alloys are in terms of

1) Amalgam Alloy particle geometry and size

2) Copper content, and

3) Zonc content.

A general classification can be as follows:

Lathe cut low/high Cu

Silver Amalgam Admixed

Alloys

Spherical low/high Cu

Composition Morphology Eg:

Traditional

Traditional

High Cu.

High Cu.

High Cu.

Lathe cut

Spherical

Lathe cut (SC)

Spherical (SC)

Admixed

(trad + Ag-Cu Entectic)

Aristaloy

Spheraloy

Epoque 80

Tyti

Disper alloy

ALLOY COMPOSITION: American National Institute (ANSI) American Association (ADA) specification No. 1 requires that Amalgams alloys contain predominantly silver and tin. Unspecified amounts of other elements for eg.: Copper, Zinc Gold and Hg are allowed in concentration less than Ag and Sn.

Alloys that contain in excess of 0.001% Zinc are required to be designated as ‘Zinc containing Alloys for dental amalgam, while alloys containing 0.01% or less of Zn are designated as ‘Zinc free alloys.

Historically amalgam alloys contained at least 65 wt% Ag, 29 wt% tin and less than 6wt% copper, a composition close to that of G.V Black (1896). During the 1970s many amalgam alloys containing between 6wt% and 30wt% copper were developed, many of these being superior in many respects to traditional low-Cu amalgams.

TRADITIONAL AMALGAM ALLOYS:

LATHE CUT: Until 1960s, the chemical composition and microstructure of available amalgam alloys were essentially the same as those of the most successful systems investigated by G.V Black (Black 1895)- Traditional alloys were delivered to the dentist as fillings, which were lathe cut from a cast ingot. Milling and sifting produced the ultimate particle size distribution, as well as the final form of Amalgam Alloy particles.

A commercial alloy evolved into a blend of different particle sizes rather than a unimodel system, in order to optimise packaging efficiency. The length of particles in

commercial lathe cut alloys might range from 60-120µm, their with from 10 to 70µm and their still smaller (<30µm) due to introduction of so called ‘Spherical alloys’.

Traditional alloys contain 66% to 73% of Ag by weight; tin varies from 25 upto 29wt% and amount of copper may be as high as 6wt% and Zn upto 2wt% upto3wt% Hg may also be present. The structure of these traditional alloys are essentially phase mixtures of gamma phase of silver tin system (Ag 3Sn) and the Epsilon phase of copper tin System (Cu 3Sn). Some of these alloys are still available, but represent only a minor component of overall Amalgam market.

SPHERICAL – The spherical alloys were introduced on the market during 1960s. Generally their particle shape is created by means of an Atomizing process. Although alloys produced in this way are classified or spherical, thie r particle shape may be irregular. Generally the maximum particle size in a spherical powder in 40-50µm or less; although there unusually is a particle size distribution. Spherical traditional Amalgam lowered the necessary Mg/alloy ratio and dramatically reduced the condensation pressures.

HIGH COPPER BLENDED AND SINGLE COMPOSITION: During the late 1960s, alloys with a significantly different chemical compositions were introduced on the market. All of these could be characterized by their high copper content.

A first alloy of this type (Dispersalloy; Hohnson and Johnson Dental Care Co.) (innes and Youdelis, 1963) was a mechanical mixture of 2 parts of a traditional lathe cut alloy with one part of spherical alloy. The chemical composition of spherical particle was 72wt% Ag, and 28wt%. Copper, it corresponds to the Eutectic composition of silver copper system. The overall composition of this alloy contained approximately 13% ADA specifications at that time.

Amalgams made from this alloy, however were clinically superior to traditional alloy with respect to marginal integrity (Mahler et. al. 1970), and consequently, other manufacturers developed similar composites featuring some with a copper content greater than that found in traditional Amalgam. At present copper content varies upto approximately 30% by weight in some commercial amalgam alloys.

The structure of several high copper alloys are similar to that of Dispersalloy. They can be classified as “blended” alloys in which traditional and high copper phases are mechanically blended.

Another type of alloy is produced by melting together all components of a high copper systems creating a single composition, spherical or a lathe cut alloy, rather than a mechanical mixture of 2 distinct powders. Depending upon number of components involved, these systems are also referred to as Quaternery alloys or as a ‘single Composition System’.

Some amalgams alloy produces, in an effort to improve clinical handling properties, supply “admixture” types of high copper alloys. In these, the chemical compositions and physical forms of the basic powders )lathe cut/spherical) are varied. This system further differs from those using Dispersalloy, in that both blended components are representatives of copper Enriched alloys. It is important to stress that all of these copper enriched alloys contain > 10% copper by weight in form of either silver copper eutectic or copper tin system.

Although Amalgam alloys containing many other metals have been proposed or one investigated on an experimental basis, at present only Indium, Palladium and Selenium have been utilized as commercial Additives. Palladium improves the corrosion resistance. Selenium has been added to improve biocompatibility of amalgam. Indium has been admixed in larger concentrations (10% by weight) in metallic form to a high copper amalgam in order to reduce the Hg vapour released in mastication process. (Powell et. al., 1988; Youdelis, 1992)

TYPICAL COMPODITIONS OF AMALGAM ALLOYS. CHEM. COMP. (Wt%):

Type Ag Sn Cu Au OthersTLTSHCSHCAd

70.97241-6162-69.7

23.82624-30.515.1-

2.41.512-28.312-22.7

10.50-0.30-0.9

--In 3.4In10

HClG4

4350

18.62926

2515

0.3-

Hg 2.7Pd 9

TL = Traditional Lathe cut, TS= Traditional Spherical, HCS = High Cu Spherical, HCAd = Hi. Cu. Admixed, HCl= Hi Cu lathe cut, GA = Alloy for Gallium Amalgam.

AMALGAM

ALLOYS

CLASSIFIC

ATION

TYPE Ag Sn Cu Zn Hg Other

New true

Dentalloy

Micro II

Dispersalloy

Tytin

Sybraloy

Cupralloy

AristalloyCR

Indiloy

Valiant

Valiant Phd

Low Cu

Low Cu

High Cu

High Cu

High Cu

High Cu

High Cu

High Cu

High Cu

High Cu

Lathe cut

Lathecut

Mixed

Spherical

Spherical

Mixed

Spherical

Lathecut

Lathecut

Mixed

70.8

70.1

69.5

59.2

41.5

62.2

58.7

60.5

49.5

52.7

25.8

1.0

17.7

27.8

30.2

15.1

28.4

24.0

30.0

29.7

24

8.6

11.9

13.0

28.3

22.7

12.9

12.1

20.0

17.4

1.0

0.3

0.9

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

-

-

-

-

-

-

-

3.4 In

0.5 Pd

0.5 Pd

From Osborne Int et al: JDR ’07: 983 to 98, 1978

METALLURGIC PHASES IN DENTAL AMALGAMS:

The setting reactions of alloy for dental amalgam with Hg are usually described by metallurgic phases that are involved. These phases are found in phase diagram for each alloy system.

Symbols and Stoichimetry of phases involved in Amalgam’s setting

Phases in Amalgam

Alloys and set Dent Amalgams

Gamma - √

Gamma 1 - √1

Gamma 2 - √2

Epsilon – ε

Eta – ŋ

Silver Copper Eutectic

Stoichio metric formula

Ag3Sn

Ag2Hg3

Sn7-8Hg

Cu3Sn

Cu6Sn3

Ag-Cu

The Influences of Ag-Sn Phases on Amalgam Properties:

In the range of compositions around the √ phase, increases or decreases in silver influences the amount of B and √ phases and the properties. Because the effect of

these phases is relatively pronounced, their control is essential, if an alloy of uniform quality is to be produced.

If tin concentration exceeds 26.8 wt%, a mixture of √ phase and a tin rich phase is formed. The presence of tin phase increases the amount of tin-Hg phase formed when alloy is amalgamated. The tin-Hg phase lacks corrosion resistance and is the weakest component of dental amalgam. Amalgams of tin rich alloys display less expansion than do silver rich alloys.

Ag-Sn alloys are quite brittle and difficult to comminute uniformly unless a small amount of copper is substituted for Ag. This atomic replacement is limited to about 4wt% - 5wt% above which Cu 3Su. Within limited range of copper solubility, increased copper content hardens and strengthens the Ag-Su alloy.

The use of Zn in an amalgam alloy is a subject of controversy. Zinc is seldom present in an alloy to an extent greater than 1wt%. Alloys Zn are more brittle and amalgam produced tends to be less plastic during condensation and carving. Chief function of Zinc is that of de-oxidiser – Scavenger, viz; it acts as a scavenger during melting, uniting with O2 to minimize formation of other oxides.

Zinc may have some beneficial effects related to early corrosion and marginal integrity as shown in clinical trials. But may even cause abnormal ‘Delayed’ expansion of amalgam if it is condensed in presence of moisture.

MANUFACTURE OF ALLOY POWDER: To make, lathe cut powder, an annealed ingot of alloy is placed in a milling machine or in a lathe and is fed into a cutting tool/bit. The chips are removed are of ten needle like and some manufacturers decrease the ship size by ball milling.

Homonizing Anneal: Because of rap[id cooling conditions from the ascast state, an ingot of an Ag-Su alloy has a cored structure and contains nonhomogeneous grains of varying composition. Hence, a homogenizing Anneal treatment is performed to establish the equilibrium phase relationship.

The ingot is placed in an oven heated at temperature below solidus for sufficient time to allow diffusion of atoms to occur and the phases to reach equilibrium. Time of treatment varies depending on temperature used and size of ingot. A 24 hours time period treatment is not unusual.

After heating, ingot is brought to room temperature very slowly so that the proportion of phases will continue to adjust towards room temperature equilibrium ratio. It is quenched rapidly, phase distribution will essentially unchanged.

For eg.: in a Ag-Su alloy, rapid quenching results in maximum intention of β phase, whereas slow cooling results in formation of maximum amount of the √ phase.

Particle treatment: Once the ingot has been reduced to cuttings many manufacturers perform same type of surface treatment of particles. Although specific treatment are proprietary, treatment of alloy particles

with acid has been a manufacturing practice for many years. Exact function of this step is not known, but is probably related to preferential dissolution of specific components from the alloy. Amalgams made from acid washed powders tend to be more reactive than those from unwashed powders.

The stresses induced into the particle during cutting and ball milling must be relieved or they slowly release over time, causing a change in alloy, particularly in amalgamation rate and dimensional change occurring during hardening. The stress relief process involves an annealing cycle at a moderate temperature; usually for several hours at approximate 1000C. The alloy generally then is stable in reactivity and properties when it is stored for an indefinite period.

Atomised powder: Atomised powder is made by melting together the desired elements. The liquid metal is atomized into fine spherical droplets of metal. If the droplets solidify before hitting a surface, the spherical shape is preserved and these powders are called ‘ Spherical Powders. Like the lathe cut powder, these too are given a heat treatment that coarsens the grains and slows the reaction of these particles with Hg. These are also washed with acid. The tiny drops of alloy are allowed to solidify in an inert gaseous (Argon) or liquid (water) environment.

Lathe cut vs Spherical alloys – Amalgams made from lathe cut or Admixed type tend to resist condensation better than amalgams of spherical alloy because

spherical amalgams are extremely plastic, one cannot rely on condensation press to establish proximal contour. A contoured and wedged matrix band is essential to prevent, flat proximal contours, improper contacts and overhanging cervical margins. Also spherical alloys require less Hg than lathe cut ones, hence do have better properties.

AMALGAMATION AND RESULTING STRUCTURE:

1) Low Copper Alloys – Amalgamation occurs when Hg causes in contact with surface of Ag-Su particles. When a powder is triturated, the Ag and Su in the outer portion of particles dissolve into Hg. At the same time, Hg diffuses into the alloy particles. The Hg has a limited solubility for silver (0.035wt%) and tin (0.6wt%). When that solubility is exceded, crystals of 2 binary metallic compounds precipitate into mercury. These are BCC Ag 2

Hg3 compound (the √ phase) and the hexagonal close packed Sn7-8Hg compound (the √2 phase). Since the solubility of Ag in Hg is much lower than that of tin, the √1 phase ppts first and √2 phase pptr later. Immediately after trituration, the alloy powder coexist liquid Hg, giving the mix a plastic consistency. As remaining Hg dissolves, √1 and √2 crystals grow and finally as Hg is exhausted, amalgam hardens. As the particles become covered with newly formed crystals (mostly √ 1), the reaction rate decreases. Since the alloy is mixed with Hg in a ratio of 1:1 or lower, this amount of Hg is less to

completely conserve all the alloy particles consequently, unconsumed particles are present in set amalgam.

Alloy particles (now smaller, because their surfaces have dissolved in Hg) are surrounded and bound together by solid √1 and √2 crystals Thus a typical low Cu amalgam is a ‘composite’ in which unconsumed alloy particles are embedded in √1 and √2 phases.

The reaction can be expressed as :

Alloy particles (β + √) + Hg - √ 1+ √2 + unconsumed alloy particles (β m+√).

The physical properties of hardened amalgam depend on the relative percentages of each of microstructural phases. The unconsumed Ag3Sn particles have maximum and strong effect. The more of this phase is retained, the stronger is the amalgam. While weakest is √ 2 phase with hardness approximately 10% of √ 1 phase whereas the √ phase hardness is somewhat higher than √ 1.

The interface between √ phase and matrix is important. The maximum amounts of √ phase will not strengthen the alloy unless the particles are bound to the matrix.

2) HIGH COPPER ALLOYS –

a) ADMIXED ALLOYS – These are mixture / blend of spherical silver – copper (Ag-Cu) eutectic alloy (71.9wt%) Ag. And 28.1wt%. copper) particles and lathe cut low copper Amalgam alloy particles.

When Hg reacts with admixed powder, silver dissolves into Hg from, Ag-Cu alloy particles and both silver and tin dissolves into the Hg from the Ag-Su alloy particles; The tin in solution diffuses to the surface of Ag-Cu alloy particles and reacts with copper phase to form the ‘n-phase (Cu3Su5). A layer of n-crystals forms around unconsumed Ag-Cu alloy particles. This n-layer also contains some √1 crystals. The √1 crystals form simultaneously with n phase and surrounds both n covered Ag-Cu particles and Ag-Sn particles.

As in low copper amalgams, √1 is the matrix phase.

The reaction can be expressed as:

Alloy particles (β+√) + Ag-Cu entectic + Hg - √ 1 + n + unconsumed alloy of both type of particles.

Note that √2 phase is formed simultaneously as n phase, but is replaced by the latter. For this the net copper conclusion should be aleast 12% in the alloy powder. Some set admixed amalgams do contain √ 2, although percentage is much smaller than low – Cu amalgams.

b) SINGLE COMPOSITION ALLOYS – number of phases found in each single composition alloy particle includes, β phase (Ag-Sn), (Ag3Sn)̂ال and Є (Cu3Su). Some of alloys may also contain ŋ(Cu6Sn5), Atomised particles have dendritic microstructure with fine lamellar.

When Hg\ is triturated with Hg, Ag and Sn from Ag-Su phases dissolves in Hg, little Cu too dissolves in Hg. The √1crystals forms the matrix with the unconsumed particles ŋ crystals are found as meshes of rod crystals

at the surface of alloy particles as well as in matrix and also are much larger than ŋ- crystals of admixed alloys. The reaction:

Ag-Sn-Cu alloy particles + Hg - √ 1th + uncons. all pont. In single composition, little or none of √ 2 phase is formed. Preferential corrosion of n(Cu6Su5) phase reportedly has been shown to be significant both in vivo (Marshall et. al., 1980) and in vitro (Averette et. al., 1978) studies.

Recently evidence has been presented for presence of an additional tin-Mercury phase, delta – 2 (Sarkar, 1994 a), at the grain boundaries of resulting √ 1- network. This ohase results from lower tin concentration in last Hg solidify. Since it is located a grain boundaries, it will have significant influence indetermining the structure sensitive properties of Amalgam. Since Copper and tin will preferentially combine in dental – amalgam, higher Cu – concentration will also reduce formation of 2 phase. It is also possible that through solid solution, indium may increase the stability of √1phase (Sarkar, 1994)

- In relation to preferential corrosion of n phase, G. M. Grrener and K. Gurgot studied the effect of addition of palladium on enhancement of corrosion resistance of High Cu amalgam and also its effect on its mechanical property. They used low % Pd. They concluded that,

1) Addition of Pd has no effect on mechanical property of High Cu. Amalgation with creep, produced being < 1%

2) Addition of Pd lead to increase in corrosion resistance.

- JDR vol 61 No. 7, 1982, pg 1192-1194

DIMENSIONAL STABILITY – ideally an Amalgam should set with no change in dimensions and their stable for life of restoration. However a variety of factors influence both initial dimensions on setting and long term dimensional stability, as follows;

a) Dimensional change : ADA spe. No. 1 requires that Amalgam should neither contract nor expand more than 20µm/cm measured at 370C between 5 minutes and 24 hours after beginning of trituration, with a device which is accurate to atleast 0.5µm.

Theory of dimensional change: Classic picture of dimensional change is one in which specimen undergoes an initial contraction for about 20 minutes after beginning of trituration and then begins to expand.

a) Contraction: 1) initial dissolution of alloy particles.

2) Growth of √1crystals.

3) Low mercury: Alloy ratio

4) High condensation pressures.

5) Longer trituration time.

6) Smaller particle size alloys

7) Mechanical trituration

b) Expansion: 1) Continued growth of √1 crystals

2) Excess Mercury

3) Hand trituration

4) Larger particle size alloys (used in past)

b) Effect of Moisture Contamination: Occurs in Zinc containing low copper and high copper Amalgam only when they are contaminated by moisture during trituration or condensation. Expansion starts, usually, after 3-5 days and may continue for months, reaching values greater than 400 µm (4%) and is known as Delayed / secondary expansion.

c) Effect of Creep: Amalgam Creeep is plastic deformation principally due to very slow metallurgic phase transformations that involve diffusion controlled reaction and produce volume increases.

STRENGTH

A prime requisite for any restorative material is a strength sufficient to resist #. Fracturing especially at margins hastens corrosion, sec. caries and subsequent failure.

Comparison of Compressive Strength and Creep of a low copp. And high – Cu Amalgam

Amalgam Comp. Strt. (MPa)

Creep (%) Tensile strt (24h) MPa

1h 7 days

Low Copper * 145 343 2.0 60

Admix+

Sing Comp ‡

137

262

431

510

0.4

0.13 48

64

* - fine cut C.D., Caulk Company, Milford

+ - Dispensalloy, Johnson and Johnson Dental Products‡ - Tytin S.S., Whiye Dental Manufaturig Company

Specimen used is comparable with a typical Amalgam restorations dimension wise.

2) Physical Properties of Amalgam – Table in Craig and Powers.

a) Product Hg in Mix (%)

Comp strt (MPa)

1h[0.5mm/min]

Comp. Strt (MPa) 7 day

Creep (%)

0.2mm/min 0.05mm/min

- Low copper

i) fine cut

caulk Co.53.7 45 302 227 6.3

ii) Spherical

- Caulk Sph

- Kerr Sph

- Shofu Sph

- HIGH COPPER

i) Admixed

Dispersalloy

ii) Sing. Comp

- Sybraloy

-Tytin

46.2

48.5

48.0

50.0

46.0

43.0

141

88

132

118

252

292

366

380

364

387

455

516

289

299

305

340

452

443

1.5

1.3

0.5

0.45

0.05

0.09

Product Ten. strt (0.5mm/min) (MPa) Dim change (µm/cm)

15 min 7 days

- LOW COPPER

i) fine cut

caulk Co.

ii) Spherical

- Caulk sph

- Kerr sph

- Shofu sph

- HIGH COPPER

i) Admixed Dispensalloy

ii) Unicompositional

- Sybraloy

- Tytin

3.2

4.7

3.2

4.6

3.0

8.5

8.1

51

55

55

58

43

49

56

-19.7

-10.6

-14.8

-9.6

-1.9

-8.8

-8.1

- Adapted from Malhotra, Asgar, JADA 96: 447, 1978.

- All these figures are recorded as Amalgam is subjected to a particle rate of loading because sudden application of heavy force can # the amalgam.

From both the tables, its clear the amalgam has strong compressive strt and much weaker tensile strength. Hence cavity design should maximize compression and minimize tension or shear forces.

Transverse strength: The values are sometimes referred to as Modulus of Rupture. Since Amalgam are brittle materials they withstand little deformation during Transverse Strength testing. Main factors related to high values of deformation are 1) Slow rates of load application 2) High creep 3) high temperature of testing. Hence, High Cu; Amal with low Creep should be supported by bases with high moduli to minimize deformation and transverse, failure.

- Strength of various phases: This is very important and is studied by initiation and propagation of crack in set Amalgam. The strongest to weakest phases are as follows: (low Copper) 1) Unreacted Alloy Particles (r)

2) √1 phase 3) √2 phase

- Elastic modulus: When determined at low rates of loading. Such as 0.025 mm/min, values in range of 11-20 GPa are obtained. High Copp. Alloys tend to be stiffer than low copper alloys.

- Effect of various factors on strength:

i) Trituration: Under and over trituration decreases strt of both low and high copper amalgams.

ii) Mercury content: Mercury in excess of 54-55% markedly decreases strt of both low and high copper Amal.

iii) Condensation: Greater the condensation pressure, increased is the strt – lathe cut, while for spherical, lighter pressures produces adequate strt.

iv) Porosities: Porosities decreases strt.

v) Amalgam Hardening Rate: Even if a fast hardening Amalgam is used, its strength is likely to be low initially. Patient should be cautioned not to subject the restoration to high biting stresses for atleast hours after placement. By that time, a typical Amalgam has reached atleast 70% of its strt.

- CREEP : Creep rates have been found to correlate with Marginal breakdown of traditional low copper amalgams i.e., higher the creep, greater the degree of marginal breakdown. However for high Cu. Amalgams, creep is not necessarily a good predictor of marginal # It is prudent to select an alloy that has creep rate below level of 3% as in ADA specification No. 1.

The √1 phase has been found to exert a primary influence on low copper Amalgam creep rates. Creep rates increase with larger √1 volume fractions and decrease with larger √1 grain sizes. The presence of √ 2 is associated with higher creep rates. In addition to absence of √2, very low creep rates in single composition high Cu- amalgams may be associated with ŋ rods, which act as barriers to deformation of √ 1 phase.

Also those manipulative factors that maximized strt, minimize creep rates.

- Expansion due to creep: Mercuroscopic Expansion

TARNISH AND CORROSION

Amalgam restoration often tarnish and corrode in oral environment. The degree of tarnish and resulting discolorations appear to be dependent on individuals oral environment and to a certain extent on particular alloy employed. Electrochemical studies indicate some passivation offering protection against further corrosion, occurs as a result of Tarnish.

A tendency towards tarnish, although unaesthetic because of black silver supfide, does not necessarily imply that active corrosion and early failure of restoration will occur.

- Traditional Amalgams and high copper Amalgams show two kinds of corrosion – a) Chemical b) Electrochemical

Traditional Amalgams are susceptible to corrosion with chlorides attacking the gamma – 2 phase. This phase corrodes according to:

8Sn7Hg + 21O2 + 4H2O + 28 Cl - - 14Sn4(OH)6Cl2 + 8Hg

This process then leads to 2 deteriorating effects:

1) The corrosion of interconnected √2 further weakens the Amalgam particularly the tensile strt.

2) Hg liberated by corrosion process can react with remaining unreacted √ to produce additional reaction prod (√1 + √2).

The formation of these 2 phases could produce an additional reaction i.e., additional dimensional change

(Mercuroscopic Expansion), leading to unsupported amalgam at margin which can easily # in tension. The entire mechanism has been associated with phenomenon of Amalgam ditching which was quite prevalent in clinical use of traditional Amalgam. The liberation of Hg has also created additional biocompatibility concerns.

In high copper Alloys, little or none of √ 2 phase is formed due to formation of emphase (Cu 6Sn5) which was more corrosion resistant. However, the eta prime phase is also prover to be susceptible to corrosion in oral cavity;

4Cu6Sn5+19O2+18H2O+12Cl - - 6[CuCl2 3Cu(OH)2]+20SnO

This reaction will not subsequently affect the strength of highcopper Amalgam because eta phase is not an interconnecting phase. However eta phase’s corrosion has raised questions regarding biocompatibility of high copper Amalgams.

However addition of Palladium to high copper Amalgams have produced hope against corrosion of eta phase. It is shown that Palladium is soluble in √ 1 with resultant improvement in behaviour. Recent studies have shown that Hg is released during free corrosion of Amalgam invitro in various salivas. Over short term this Hg burden was found to be in range, of 4-20µg/day or about same value as dietry intake and over longer periods, it was lower than dietry intake. Invivo, however, natural buffering capacity of saliva, along with attendant organic protelus may appreciably lower corrosion kinetics.

- Electro chemical Corrosion : This is an important mechanism of Amalgam corrosion and has potential to occur virtually anywhere on or within a set Amalgam. This occurs whenever chemically different sites act as anode and cathode. Also it requires that the sites be connected by an electrical circuit in of an electrolyte typically saliva. The anode corrodes, producing soluble and insoluble reaction products.

If an amalgam is indirect contact with an adjacent metallic restoration such as Gold crown, Amalgam acts as Anode in circuit . This type of corrosion is called ‘Galvonic Corrosion’ and is associated with presence of macroscopically different electrode sites. A high copper Amalgam is cathode with reference to a conventional Amalgam. These concerns are being expressed that if a high Cu restoration are placed in the mouth with traditional restoration in contact with it, corrosion and failure would be accelerated in latter.

The same process may occur microscopically (local galvanic corrosion or structure selective corrosion) ecause of electrochemical difference of different phases. Residual alloy particles act as strongest cathodes. Su-Hg or Cu-Su reaction product phases are strongest Anodes in low copper and high copper respectively. Local electrochemical cells also may arise whenever a portion of Amalgam is covered by plaque/soft tissue. The covered area has locally lowered O 2 and/or higher hydrogen ion concentration making it behave more Anodically and corrode. Cracks and crevices produce

similar conditions and preferentially corrode (concentration cell corrosion or crevice corrosion) regious within an Amalgam that are under stress also display a greater propensity for corrosion. So in a Cl I dental amalgams, electro chemical corrosion events are as follows;

1) local galvanic corrosion between Amalgam Phases along all surfaces of Amalgam.

2) Stress corrosion during occlusion with opponent tooth surface

3) Concentration cell corrosion within margins with tooth surface.

4) Concentration cell corrosion below plaque on amalgam surface (causing pitting).

In Cl II restoration events are same as Cl I, in addition there is corrosion at interproximal contacts with adjacent metal crowns. In Cl III restoration, events are same as Cl I restoration.

Electro chemical corrosion is not a process of Hg liberation, Hg reacts with unreacted Alloy particles.

MANIPULATION OF AMALGAM

1) Selection of an alloy: This involves a number of factors including setting time, particle size and shape and composition particularly as it relates to elemination of √2 phase and the presence or absence of zinc. It is estimated that more than 90% of dental Amalgam currently placed are high copper alloys. The majority of

alloy selected are unicompositional (spherical) and Admixe with Admixed being favoured slightly.

Also the further processes of Inserting, Condensing, Carving, Finishing also affect the Amalgam alloy selection.

2) Proportions of Alloy to Mercury: Up until the early 60s it was necessary to use an amount of mercury considerably in excess of that desirable in final restorations to achieve smooth, plastic Amalgam mixes because of deleterious effects of Hg on physical and Mechanical properties of Amalgam, Hg was used only to an acceptable level. Excess Hg was removed by squeezing the mixed Amalgam after trituration in a cloth, muslin cloth or guaze piece or by working the excess Hg to the top during condensation of restoration, which was subsequently removed. But there was a considerable chance of error as amount of Hg removed – varied.

However in 1960, Lames described the ‘No squeeze Cloth’ Technique or ‘Minimal Mercury technique’ or after his name, ‘The Lames technique: He suggested that sufficient Hg be present in original mix to provide a coherent and plastic mass after trituration, but be low enough that Hg level of restoration is at acceptable level without need to remove an appreciable amount by condensation.

The amount of alloy and Hg to be used can be described as Mercury: Alloy ratio. The recommended ratio being 1:1 i.e., 50% Hg. However some alloys require

less than 50% Hg, some require more than 50%., the percentage varies between 43-54%.

With spherical alloys, recommended amount of Hg is closer to 42%. Percentage of Hg used depends upon how the alloy particles can be packed together.

Another transitional approach includes redesigning Amalgam to have much less initial Hg. If alloy particle sizes are judiciously chosen to pack together well, it is possible to minimize the Hg required for mixing to 15-25% range. Clinical properties of this alloys are unknown.

The ADA in combination with National Institute on standards and Technology (ADA-NIST) has patented a Hg free direct filling alloy based on Ag-coated Ag-Sn particles that can be self welded by compaction (Hand consolidation) to create an restoration.

Initially Automatic Mechanical dispensers for alloy and Hg have been used in the past. But with recommendation of “No touch” procedures for handling alloy and Hg, the capsules with preproportional amounts of alloy and Hg have been substituted for dispensers.

- Size of Mix : Manufacturers commonly supply capsules containing 400, 600 or 800 mg of alloy with appropriate amount of Hg, colour coded for ease of identification. Also capsule with 100 mg of alloy is available for large amount of amalgam if needed for core building on severely broken tooth.

AMALGAMATION:

1) Hand Trituration : Is done using a Mortar and Pestle. In this it is difficult to follow the manufacturer’s direction explicitly with reference to pressure, rpm, etc. A general recommendation is that mixing should be continued until the Amalgam has a shiny appearance, adheres to the sides of mortar and curls over slightly at the top.

2) Mechanical Trituration : Mechanical Amalgamation saves a lot of time and standarises the procedure. It is carried out by an ‘Amalgamator’. A large number of commercial brands of Amalgamators are available. The basic principle of operation is comparable for most of them. – A capsule serves as a ‘motor’. A cylindrical metal/plastic piston of smaller diameter than capsule is inserted in it and serves as ’pestle’. Reusable capsules also usually contain an appropriate pestle.

The alloy and mercury are dispensed into the capsule or of a disposable capsule system is being used, capsule may require activation. When capsule has been secured in the machine and the machine has been turned on, the arms holding the capsule oscillate at high speed, Hence trituration is accomplished. There is an automatic timer for controlling length of mixing time and most modern amalgamators have two or more operating speeds.

New Amalgamators must have hoods that cover the reciprocating arm holding the capsule, this is to confine Hg that might be sprayed into the room or a capsule that might be thrown from the Amalgamator during trituration. Eg.: of Amalgamators, Promix, Antomix, Varimix, etc. Antomix has plastic cards for particular alloys an size of mix, which when inserted into the slot, sets the amalgamator for correct speed and time.

3) Consistency of Mix: Undermixing, Normal mixing, overmixing can result variations in condition of trituration of alloy and H. These 3 mixes have different appearances and respond differently to subsequent manipulation. The undermixed A,algam appears dull, grainy and appears crumbly, also leaves a rough surface after carving, increasing susceptibility to tarnish. Adequately mixed Amalgam appears shiny and separates in a single mass from capsule. Such a mix is warm (not hot) when removed from capsule, also leaves a smooth surface on carving which will retain its luster after polishing. Over triturated mix appears soupy and tends to stick to the inside of capsule.

These 3 mixes have characteristically different mechanical property of dimensional change. Strength and creep.

INSERTION AND CONDENSATION: The principle objective of Insertion is to condense the amalgam to adapt it to

the preparation walls and matrix and produce a restoration free of voids and have as low as possible Hg content in restoration to improve strength and decrease corrosion. An amalgam carrier is used to insert Amalgam into the cavity. After mixing, condensation should be promptly initiated as longer the time lapse between mixing and condensation, weaker is the Amalgam. In addition Hg content and creep of amalgams are increased. Condensation of partially set material probably #s and break up the matrix that has already formed and can also introduce voids in it or even produce layering.

Condensation can be Hand or Mechanical Condensation.

a) Hand Condesation : i) Procedure. Ii) Condensation pressure.

b) Mechanical Condensation : Procedure is same as that for hand condensation except here condensation is done by Anatomatic device. Various mechanisms are used, some use impact force, while others use rapid vibration, less energy is needed, hence is less fatiguing than hand condensation.

CARVING AND FINISHING:

a) Precarve Burnishing -

b) Carving – The restoration should be carved to reproduce proper tooth anatomy. The objective is to simulate the anatomy rather than to reproduce extremely fine details. If carving is too deep, amalgam bulk at

margins in reduced, and hence may # under masticatory stress.

c) Post carve Burnishing –

- Finishing and Polishing – Most Amalgams do not require finishing and polishing. However, these procedures are occasionally necessary to, i) complete the carving, ii) refine the anatomy iii) refine the marginal integrity iv) Enhance the surface texture of restoration.

Additional finishing and polishing procedures are not attempted within 24 hours of insertion, because crystallization is not complete. An amalgam restoration is less prove to Tarnish and corrosion, if a smooth homogeneous surface is achieved.

Polishing of High Cu. Amalgams is less important than for low copper Amalgams as formers are less prove to tarnish and marginal breakdown.

- Repairing and Amalgam Restoration – If an amalgam restoration # during insertion, the defective area must be reprepared as if were a small restoration.

Appropriate depth and retention form must be generated, sometimes, entirely within existing amalgam restoration . If necessary another matrix must be placed. A new mix of Amalgam can be condensed directly into the defect and will adhere to the amalgam already present, if o intermediary material has been place between 2 amalgams. Therefore sealer material can be placed on any exposed dentin, but should not be place on amalgam preparation walls. If amalgam has been bonded, carefully

condition and apply adhesive to the adhesive to the exposed tooth structure in the preparation.

In such cases, the bond between new and old amalgam is important. The flexural strt. of repaired amalgam is less than 50% of that of unrepaired amalgam. The bond is a source of weakness, factors such as corrosion and saliva contamination at the interface presents formidable barriers that interface with bonding of old and new amalgam.

Repair, of amalgam restorations probably falls into the category of hazardous procedure. Repair should be attempted only if the area involved is small, one that is not subjected to high stresses, capable of adequately supporting and retaining 2 restorations parts.

- C. Leelawat, W. Scherer, J. Chang, A. Scherlman, T Vijayaraghvan, studied the microleakage (invitro) patterns when fresh amalgam was added to existing one. They foun no change in microleakage patterns. But incidence were significantly decreased when fresh Amalgam was bonded to existing one.

- J. Esthetic Dent. Mar-Apr’92 Vol 4 (2), 41-45.

- C. Leelawat, W. Scherer, J. Chang, A. Scherlman, T Vijayaraghvan and J. le Geros studied the invitro shean and flexural strt when fresh amalgam was bonded to existing one. They found a significant improvement in performance of restoration.

- J. Esthetic Dent. Mar-Apr’92 Vol 4 (2), 46-49.

- AMALGAM ‘PROBLEMS’: following are the ‘problems’ that can be encountered when amalgam restorations are evaluated;

1) Amalgam “blues” – Discoloured areas seen through enamel either due to leaching of corrosion products into dentinal tubules or from the colour of underlying Amalgam seen through translucent (undermined) enamel.

2) Proximal overhangs – Are diagnosed visually, tactilely and xgphically. Is a plaque trap and obstacle to maintenance of good oral hygiene and may result in inflammation of adjacent soft tissue.

3) Marginal Ditching – Also called as “Ditched” restoration. May be caused due to a) Improper cavity preparation or finishing. B) Excess Mercury C) Creep.

Shallow ditching less than 0.5mm deep usually is not a reason for restoration. However, if it is too deep, has to be restored.

4) Voids – occurs at margins due to a) Improper condensation b) Material pulling away or breaking from marginal area when carving bonded Amalgam. If the void is 0.3mm deep and is located in gingival 1/3 rds of tooth crown, then restoration is judged as defective and should be repaired / replaced.

5) Fracture lines – May occur across the occlusal surface or in Isthmus reg. May be mistaken with an “Abutted Restoration”.

6) Abnormal Anatomic Contours – should be recontoured or replaced.

7) Marginal Ridge Incompatibility – May cause food impaction, restoration is defective and should be recontoured.

8) Improper Proximal Contact –

9) Recurrent caries –

10) Improper Occlusal contacts –

ALTERNATIVES TO AMALGAM : As given by :

1) B. M. Eley – BDJ July’97 vol 183 No. 1 pg. 11-14.

2) Theodore Croll – Quintissence Int’ Nov’98 vol29 No. 11 Pg. 697 to 703.

Alternatives may be classified as Metal alloys and tooth col. Alternatives.

1) Metal alloys – 1) Gold – Cast gold only real alternative to Amalgam in moderate to larger cavities. Has superior qualities, but is very technique sensitive. Expensive about 7-8 times than equivalent restoration for smaller restorations, gold foil can be used, but is difficult, time consuming, technique sensitive and expensive.

2) Consolidated Silver Alloy : In 1994, another metal alternative was developed which consisted of Ag particles suspended in dilute acid solutions. The acid treatment cleaned the surface by removing oxides or other adsorbed layers silver particles, thus enabling their consolidation and cold welding. This acid assisted consolidation took place at room temperatures under

moderate pressures. Physical properties showed high rupture strt than Amalgam, however composite strt and hardness values were lower than those found for Amalgam. It is still under laboratory studies and is not commercially available.

- S. B. Geiger, D. Gurbator, M. P. Dariel, F. Eichmiller, R. Liberuran, M. Ratzker – Oper Dent Mar-Apr’99 vol 24. No. 2. Pg – 103 to 108.

3) Gallium Alloys : 16 times more expensive than similar amount of mercury-based Amalgam. Both the commercially available brands are much more sticky when mixed and hence are more difficult to pack in a cavity. Needs PTFE (TEFLON) instruments to overcome this ‘mushy and sticky’ problem.

- High levels of corrosion, high levels of expansion leading to even tooth #, unknown toxicology of Ga. Hence all these make it inferior, much more costly, much more difficult to use than Hg based Amalgams.

II) TOOTH COLOURED ALTERNATIVES – Patients demand for Esthetic Restoration has stimulated the development of new tooth coloured restorative materials, which are:

1) GICs 2) Composites 3) Resin modified GICs 4) Compomers 5) Ceramics.

All of these have shorter life span than Amalgam. And only composites and ceramics can be considered to

restore small occlusal cavities in permanent posterior teeth.

i) Composites : Expensive (3 times the cost at insertion), Polymerisation shrinkage, Wear (increases with size), demands total isolation, Acid etch bonding can break causing Microleakage, shorter clinical life than Amalgam.

Indications for posterior cavities are :

Should e small cavity, must not involve