CONTENTS 1. INTRODUCTION 2. PAST – HISTORY 3. PRESENT i) CLASSIFICATION ii) DIFFERENT PHASES OF AMALGAMATION iii) FUNCTION OF EACH CONSTITUENT iv) COMPOSITION AND REACTION a) LOW COPPER ALLOYS b) HIGH COPPER ALLOYS 1. ADMIXED ALLOYS 2. UNICOMPOSITIONAL ALLOYS c) DIFFERENCES BETWEEN HIGH COPPER AND LOW COPPER ALLOYS v) MANUFACTURE OF ALLOY PARTICLES a) LATHE CUT ALLOY PARTICLES b) SPHERICAL ALLOY PARTICLES vi) PROPERTIES OF AMALGAM a) DIMENSIONAL STABILITY b) STRENGTH c) CREEP d) CORROSION vii) TECHNICAL CONSIDERATIONS a) SELECTION OF ALLOYS b) MERCURY ALLOY RATIO c) PROPORTIONING OF ALLOY 1. HAND 2. AMALGAMATORS d) TRITURATION 1. OBJECTIVES 2. MOVEMENTS 3. FACTORS 4. TIME e) MULLING 1. TYPES f) CONDENSATION 1. OBJECTIVE 2. HAND CONDENSATION 3. MECHANICAL CONDENSATION 4. CONDENSATION PRESSURE
g) BURNISHING 1. OBJECTIVES 2. PRECARVE BURNISHING 3. POST CARVE BURNISHING h) CARVING 1. DEFINITION 2. OBJECTIVES 3. METHODS i) FINISHING AND POLISHING viii) ix) x) xi) xii)
MERCURY TOXICITY INDICATIONS LIMITATIONS FAILURES OF AMALGAM RECENT ADVANCES a) FLUORIDE RELEASING AMALGAM 1. RESULTS / ADVANTAGES 2. DISADVANTAGES
b) INDIUM ALLOYS 1. FACTORS RESPONSIBLE 2. PROPERTIES c) GALLIUM ALLOYS 1. INTRODUCTION 2. PROPERTIES 3. COMPOSITION 4. REACTION 5. BIOLOGICAL CONSIDERATIONS d) BONDED AMALGAM RESTORATION 1. INTRODUCTION 2. INDICATIONS 3. ADVANTAGES 4. DISADVANTAGES 5. MANIPULATION – PROCEDURE 6. BONDING INTERFACE 4. REFERENCES 5. CONCLUSION / FUTURE
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. It has been estimated that more than 160 million amalgam restoration are placed each year all over the world. 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 of mercury, silver, tin, copper which may also contain palladium, zinc and other metals. 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. The Average life span of silver amalgam is 8-10 years, though the failure have also been found. 50% of failures of amalgam restorations are due to faulty cavity preparation. American National Standard Institute (ANSI) and American Dental Association (ADA) specification No. 1 is given to amalgam alloys predominantly containing silver and tin. PAST: HISTORY: Dental amalgam is one of the oldest filling materials in use today. It is available to dental profession for over 150 years. It’s origin can be traced back to the second century A.D. in China where 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). Formulation consisted of mixing 100 parts of mercury to 45 parts of silver and 55 parts of tin producing a paste which was solid as silver. Amalgam as a restorative material has sparkled controversies several times in its history of 200 years. The material used in early 1800’s is “mineral cement” also called as D’Arcets cements. This alloy of bismuth, lead, tin and mercury was plasticized to 100oC and poured directly into the cavity. However in 1818 Regnart, perhaps out of some kindness for the long suffering patients, suggested a lowering of the fusing temperature of the D’Arcets cement by increasing the concentration of mercury, he lowered the temperature to 68oC and eased the patients discomfort considerably. 3
And so to Regnart we give the title father of Amalgam. In 1819 Bell from England advocated the use of a room temperature mixed amalgam as a restorative material. 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 1843 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. Then during later half of 1800’s the improved amalgam by G.V.Black came into widespread use. 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 lead to serious health problems. He also expressed concerns over health of dentists, starting that nearly all dentists had excess mercury in their urine. 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 Colarado. 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, Alzhemer’s 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. However the controversy has intestified again recently, particularly as a result of the “60 minute” television programme on December 6, 1991, that created considerable public alarm with a sensationalistic treatment of the issue. But National Institute of Health (NIM), National Institute for Dental Research (NDR), Food and Drug Administration claimed that there were no basis for claims that amalgam was a significant health hazard. This controversy will probably never be resolved became there will always be a certain percentage of 4
patients seeking a miracle cure for their problems. Now it should be concluded that fears of amalgam are not a basis for amalgam removal. But the greatest contribution was probably by Dr. G.V. Black at North Western University Dental School, Chicago. Black conducted a number of experiments and developed an amalgam alloy of superior physical and mechanical properties. Black's publication in 1985 in Dental COSMOS, concluded that an alloy containing 72.5% Silver, 27.5% Tin, 5% copper gave the best results when amalgamated with mercury. PRESENT: 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. Other have one or more alloys or metals physically added (blended) to the basic alloy. eg. adding copper to a basic binary silver tin alloy. III. According to the shape of the powdered articles. 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. Microcut 2. Fine cut 3. Coarse cut I. According to copper content of powder 1. Low copper content alloy – Less than 4% 2. High copper content alloy – more than 10% V. According to addition of Nobel metals. - Platinum - Gold - Pallidum VI. 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 plasticity and increase hardness and strength. 1% zinc, as oxygen scavenger and decrease 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 : Quaternary alloy ie.. silver tin and copper and indium. 6. 6th generation (Eutectic alloy consisting). The alloying of palladium (10%), silver (62%) and copper (28%), to form a eutectic alloy, 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. VII. According to Presence of zinc. 1. Zinc containing (more than 0.01%). 2. Non zinc containing (less than 0.01%). ANSI/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, selenium or mercury may be included in lesser amount. CLASSIFICATION ACCORDING TO STURDAVENT According to Sturdavent amalgam is classified into : 1) Amalgam alloy particle according to geometry and size. 2) Copper content 3) Zinc content 1) Amalgam alloy particles according to Geometry and size : A) Irregular powder particles (lathe cut) : In these more mercury is needed to fill the spaces between the particles. Mercury is later removed by wringing the mass in a squeeze cloth. B) Spheroidal alloy particles. 2) Copper content : A) High Copper (More than 11.9% to 28.3%) B) Low copper (2.4% to 8.6%) 3) Zinc content A) Zinc containing B) Zinc free DIFFERENT PHASES OF AMALGAMATION : 1) Original Gamma Phase 2) Gamma â€“ 1 Phase (Silver Mercury Phase) 3) Gamma â€“ 2 Phase (Tin Mercury Phase) 4) Mercury phase 5) Voids (Pores) phase 6) Trace Element Phase 6
7) Interphases 1) Original Gamma Phase (γ) Ag3Sn Silver tin as alloy powder are not completely dissolved in mercury. It is strongest phase. For this reason it should occupy maximum available space. 2) Gamma -1-Phase (γ1) Ag2Hg3 Silver mercury phase. It is noblest phase. This phase is most resistant to tarnish and corrosion and every effort is done so that these phase stays for maximum time in final product. 3) Gamma-2-Phase (γ2) Sn7Hg8 It is a product of amalgamation reaction. It is least resistant to tarnish and corrosion and every effort should be made to minimize this phase. Most of amalgam failures are due to this phase which is prone to corrosion and creep. 4) Mercury Phase : Unreacted or residual mercury is present in some areas of the mass. Although unreacted mercury will keep on reacting with alloy particles in other phases. This is the weakest phase and when it exceeds to certain volume there is drastic drop in the strength and hardness of amalgam. 5) Void (Pores) Phase : Occurs during the process of building the amalgam restoration there is trapment of an bubbles a voids. Such voids act as N1D1 not only for internal corrosion but also decrease the stress concentration. Both of these leads to earlier failure of restoration. 6) Trace Element Phase : This is a phase in which copper and zinc are found. These elements plays very important role in final preparations as copper increases hardness, strength and brittleness whereas zinc increases strength and resistance to oxidation. 7) Interphase : The interphase is specially between three phase – Gamma, Gamma-1 and Gamma-02. More continuous these phases and less dimensions (closer together) there is better binding between these phases. Therefore the final restoration will be more resistant. FUNCTION OF EACH CONSTITUENT : 1) Silver : Whitens the alloy Decreases creep Increases strength and this increase in strength is because of γ phase. Increases expansion on setting Increases resistance to tarnish and corrosion Decrease flow 2) Tin : Tin controls reaction between silver and mercury. Without tin reaction would be too fast and setting expansion is unexceptable. So tin reduces both setting reaction and rate of expansion.
Reduces stress because with more tin AgHg is reduced and SnHg is increased which is weaker phase. Increases setting time. 3) Copper : Increases hardness Increases setting expansion Reduces flow 4) Zinc : Zinc acts as "Scavanger" and "Deoxidizer". When various metals are melted together during manufacture of alloy zinc acts rapidly with impurities and oxygen present in the metal thus protecting other metals like silver, copper and tin from getting oxidized. Alloys without zinc are more brittle. Amalgam is less plastic. Zinc causes delayed expansion if contaminated with moisture during condensation and penetration. 5) Mercury : In some case very less amount of mercury is added i.e. 3% to alloy to cover the surface of alloy particles. These are called preamalgated alloys. Preamalgamation produces more rapid reaction. 6) Platinum : Hardens the alloy Increases resistance to corrosion 7) Palladium : Hardens and whitening the alloy. COMPOSITION AND AMALGAMATION : According to ANSI and ADA Specification No.1 amalgam alloys contains predominantly silver and tin. Unspecific amount of other elements such as Copper, zinc, Gold and Mercury can be added but less than Silver or Tin content. LOW COPPER ALLOYS : (G.V. Blacks Silver Tin Alloy or Conventional Amalgam Alloy) Silver : 63 – 70% Tin : 26 – 28% Copper : 2- 5% Zinc : 0-2% Reaction : Amalgmation occurs when mercury comes in contact with surface of Ag-Sn alloy particles. When powder is triturated Ag and Sn in outer portion of particles dissolve into mercury and at the same time mercury diffuses with alloy particles. Mercury has low solubility for silver 0.035% by weight and greater for Sn i.e. 0.6% by weight. When the solubility increases the crystals of two binary metallic compounds precipitates into mercury. There are body centered cubic Ag 2Hg3 (γ1) phase and hexagonal closed packs Sn7Hg (γ2) phase. Because of solubility of silver in mercury is much more lower than tin, γ1 ppts first and γ2 ppts later.
Immediately after triturating both alloys dissolve in mercury and gives plastic consistency. As remaining mercury dissolves the γ 1 and γ2 crystals grows and as mercury disappears amalgam becomes harder and harder. Alloy is mixed with mercury to the ratio of 1:1. This percentage of mercury is less to dissolve the alloy particles and so unconsumed alloy particles are also present in the set amalgam. These alloy particles which are small are mixed and surrounded by solid γ1 and γ2 crystals. Thus a typical low copper is that which contains mixture of unconsumed particles embedded in γ1 and γ2 phases. Ag3Sn + Hg Ag2Hg3 + Sn7Hg8 + Ag3Sn (γ) (γ1) (γ2) Unreacted Physical properties of set amalgam depends upon relative percentage of each phase. Unconsumed Ag2Sn have strong effect. The more the phase in final structure the more will be the strength. Weakest phase γ2 phase. Hardness of γ2 is 10% of hardness of γ1. Whereas γ phase hardens is higher than that of γ1 phase. γ2 phase is also least stable in corrosive environment and can be easily attacked by corrosion. So γ and γ1 phases are stable. However γ1 phase contains small amount of tin that can be lost in corrosive environment. HIGH COPPER ALLOYS : To overcome the inferior properties of low copper amalgam alloy. Youdelis and Innes in 1963 introduced high copper content amalgam alloy. They increased the copper content from earlier used 5% to 12%. Composition : These are 2 types : A) Admixed Alloys B) Single Compositional (Unicompositional) Alloys A) ADMIXED ALLOYS : These are also called as blended alloys, sometimes also referred as "dispersion modified 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 (70% Ag = 30% 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. Particle Shape Lathe Cut Spherical Silver 40-70% 40-65% Tin 26-30% 0-30% Copper 2-30% 20-40% Zinc 0-2% 0%
High copper alloys have become material of choice because of Improved mechanical properties Improved corrosion characteristics Improved marginal integrity Overall Composition : Amalgam : 69% Silver : 17% Copper : 13% Zinc : 15 Discovery or Invention of Admixed Alloys : In 1963 Innes and Youdelis added spherical silver copper eutectic alloy (Alloy we shows complete liquid solubility by limited solid solubility) to lathe cut low copper alloy particles. AgCu + Lathe cut Admixed (Spherical) Low Cu alloy This was first major change in history of amalgam after Black's work. These are of admixed alloys because final result is mixture of two kind of particles. These admixed powder shown presence of lathe cut and spherical particles. The amalgam made from these alloys is stronger than the amalgam made from lathe cut particles this is because of strength of Ag-cu particles. It is suggested that materials can be strengthened by addition of strong fillers and Ag-Cu and AgSn acts as a strong fillers. CONTENTS : Admixed alloy, powder contains 30 wt% - 55 wt%, spherical high copper powder. Total content of copper ranges from 9 wt% - 20 wt%. The phases in copper containing particles depends upon these composition. REACTION : Ag-Cu alloys consist of mixture of 2 phases. Silver rich and copper rich with crystal structure of pure silver and pure copper. Each phase contain small amount of other phase. In atomized powder two phases mixture formed very fine lamellae. When mercury reacts with admixed powder. Silver dissolve within mercury from Ag – Cu alloy particles and both AgSn dissolves into mercury from Ag-Sn alloys particle. Ag Sn + Ag-Cu + Hg Tin in the solution diffuses to surface of Ag-Cu alloys. Then the Sn reacts with copper to form η phases i.e. Cu6Sn5 The layer of ή crystals forms around silver-copper alloy particles. Thus η layer on Ag-Cu particles also contains some γ1 crystals. Thus γ1 forms simultaneously with η phase and surrounds both η covered silver Cu alloys and silver tin alloy particles. γ1 phase binds the unconsumed alloy particles together. Reaction : 10
1st Reaction : Ag3Sn + Ag2 Cu + Hg Ag3Hg + Sn8Hg + Ag3Sn + Ag3Cu γ γ1 γ2 γ nd 2 Reaction : Sn8Hg + Ag3Cu Cu6Sn5 + Ag2 Hg3 ή γ1 Note that γ2 has been eliminated in this relation γ2 is formed at a same time as η but is later replaced by it. To accomplish this elimination of γ2 phase it is necessary to have copper concentration o atleast 12% in alloy powder. B) SINGLE COMPOSITION ALLOY : Composition : Silver : 40-60% Tin : 22-30% Copper : 13-30% Zn : 0-4% After success of admixed alloys there is development of unicompositional alloy. Unlike admixed alloy, each particles of these alloy powder has same chemical composition therefore these are of a single composition alloy. Major components of these particles are usually Silver, Copper and Tin. First single compositional alloys contain Copper : 13-30% Silver : 27 wt% Tin : 60% Zinc :1 Recently some amount of indium or palladium is also added. Phases : Number of phases are found in each single compositional alloy particles. Including β, γ, E β (Ag-Sn) γ (Ag3-Sn) η (Cu6Sn5) E (Cu3Sn) Some of the phases may also contain η phases. Atomized molecules have dendritic microstructures consisting of fine lamellae. Reaction : When triturated with mercury, silver and tin from Ag-Sn phase dissolves in mercury. Very little copper dissolves in mercury. Ag-Hg γ1 crystal grow forming a matrix that binds together partially dissolved alloy particles. β η-Cu-Sn crystals are formed on the surface of alloy particles. Ag-Sn-Cu + Hg Ag3Hg + Cu6Sn5 + Ag3Sn γ1 ή γ or Ag3Sn + Cu3Sn + Hg Ag3Hg + Cu6Sn5 + Ag3Sn Difference between the elimination of γ2 phase in an admixed and uncompositional alloy is that in the admixed alloys, γ2 forms around silver tin (lathe cut ) and is eliminated around silver copper particles (spherical). 11
In uncompositioned particles at the beginning of reaction function like Ag-Sn particles of admixed type and later same particles functions like the Silver – Cu particles of admixed alloys eliminating γ2 phase. The Major Differences between Low Copper and High Copper : Low Copper More mercury is required for amalgamation (53.37%). Solubility of mercury in tin is 170 times more than in copper and 17 times more than in silver.
High Copper Less mercury is required
Dominant phase is AgHg i.e. γ1
Dominant phase I sCu6Sn5 i.e. ή
Corrosion due to γ2 phase is due to formation of tin oxychloride from tin. Dissolution of this oxide or chloride leads to porosity.
Cu6Sn5 (ή) phase is the least corrosion resistant phase. Corrosion occurs in the form of CuCl2 3 Cu (OH)2. Order of corrosion of different phase is : AgHg < AgSn < Ag3Cu2 < Cu3 Sn γ1 γ Eutectic ε
Surface tarnish is associated with γ γ1
Surface tarnish is due to copper rich phases.
Lower copper alloys can be amalgamated in slow speed and low energy amalgamation.
High copper alloys require high speed and high energy amalgamation because copper has low solubility in mercury as compared to silver and tin.
Setting reaction is slow. burnishing and finishing recommended.
Setting reaction is fast. It can be burnished at first appointment.
Low copper amalgam has higher value of creep. The range is between 1-8%.
Creep is much less (Mostly less than 1% and might be as low as 0.1%)
Compressive strength between 1 hour and 7 days is 150-350 MPa.
Compressive strength varies between 250-500 MPa for one hour and 7 days especially with unicomposition alloys.
Tensile strength in 24 hours is 60 MPa.
Tensile strength is composition alloys.
10. Dimensional change are more in low copper alloy varies from 10 to 20 µm/cm.
Much less with high copper alloy, varies from 1 to 9 µm/cm.
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 a suitable mold to form an ingot. The ingot may be 3.8 cm in width and 20 to 25 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 tine. 12
This process is known as thermodynamic equilibrium. The ingot is then reduced to filings being cut with a suitable tool on a lathe and ball milled. The particles are passed through a fine sieve of 100 mesh or more and then are ball milled to form the proper particle size. Control of the particle size and its general dimensions are important. The particles are typically 60-120 µm in length, 10-70 µm in width, and 10-35 µm in thickness. Most products are labeled as fine-cut. Irregularly shaped high-copper particles are made by spraying the molten alloy into water under high pressure. In general, fresh cut alloys amalgamate and set more properly than aged particles, but some aging 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 o to 100o c for 1-6 hours. This process of releasing stresses is called annealing. Spherical / Spheroidal / Atomised 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. These different shapes are produced by atomizing process. 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 particle shape was used which is no longer used. Modern day smaller spherical particles are used whose size is 15-35 µm. Smaller particle size is chosen because low mercury, rapid hardness, early compressive strength produces smoother surface or carving and is less susceptible to corrosion. PROPERTIES OF AMALGAM (BEHAVIOUR OF AMALGAM) : There are 3 main properties of amalgam. A. Dimensional stability B. Strength properties C. Creep D. Corrosion A. DIMENSIONAL STABILITY Ideally amalgam should set with no dimensional changes and should remain stable for whole life of restoration. a) Dimensional changes b) Effect of moisture contamination
a) Dimensional Changes : Introduction : Amalgam can either expand or contrast depending upon its manipulation. However these dimensional change should be small. Severe contraction and can lead to – Microleakage, plaque accumulation and secondary caries. Excessive Expansion : Excessive expansion can lead to pressure on pulp and post operative sensitivity. Protrusion of a restoration from excessive expansion. Demineralized change depends upon how much amalgam is constrained. ADA Specification No.1 Required that amalgam should neither contract nor expand. More than 20 µm/cm at 37oC between 5 minutes – 24 hours after beginning of trituration. STAGES OF DIMENSIONAL CHANGES : Dimensional Changes occurs in 3 stages : 1) Stage I : Called initial contraction for approximately 20 minutes after beginning of trituration. . It results from absorption of mercury, into interparticular spaces of alloy powder. 2) Stage II : Called as expansion stage. This is due to formation and growth of matrix crystals. 3) Stage III : Called as limited delayed contraction. This occurs due to absorption of unreacted mercury. Factors Affecting Dimensional Changes : a) Constituents : More the gamma phase the greater is possibility of expansion. More traces of Tin – less expansion. b) Mercury: More amount of mercury in amalgam, produces prolonged 2 nd stage of amalgamation (expansion). > Hg > Expansion. More amount of crystals i.e. γ1 and γ2 more will e expansion. > γ1 γ2 > Expansion. c) Particle Size : Smaller size of particle has more surface area / unit volume. So when they are mixed with mercury than 1 st stage will occur more rapidly and more efficiently. Thus leading to marked contraction. 2nd Stage also occurs very fast which may neutralize original contraction. Expansion pleatue is achieved too early (before cavity is filled). Therefore contraction of stage 3 is more noticeable. Smaller size of particle ↓ Greater surface area / unit volume 14
↓ First stage more rapidly 2nd stage more early. 3rd Stage contraction 2nd Stage occurs earlier (To compensate contraction) more noticeable d) Trituration : More trituration energy, the more smaller particles are made and if more force is there more mercury is pushed into the particles. Both of these discourage expansion. More forces of trituration more will be the distribution of matrix crystal over the mix, this will prevent outward growth hence Expansion of second stage. More energy of trituration faster amalgation Limited contraction No apparent expansion So pleatue of expansion is Reached much earlier of 2nd stage e) Condensation : Greater the energy use in condensing the amalgam into cavity, more closer are the particles brought together. More condensation energy Squeezes more mercury out of mix. Both of the conditions will lead to less formation of growing matrix crystals More contraction. f) Shape of Particles : More regular shape of the particles and smoother surface Mercury will wet the particles more easily and faster Thus leads to faster amalgamation in all the stages More expansion occurs before filling the cavity And not much expansion afterwards. g) Moisture Contamination : Dimensional change occur after 24 hours but it is studied that in admixed alloy expansion use to occur even after 2 years. This expansion may be due to disappearance of some of all of γ2 in these high Copper. If zinc containing low copper or high copper amalgam is contaminated by moisture during trituration or condensation a large expansion occurs. This expansion usually starts after 3-5 days and may continue for months reaching the value greater than 400 µm. This type of expansion is called as delayed expansion or secondary expansion. Delayed expansion is associated with zinc. In amalgam this effect is caused by reaction of Zn with water and is absent in non zinc containing amalgams. Hydrogen is produced by the reaction of zinc with water. This hydrogen does not combine with amalgam but rather collects within the restoration and thus increasing internal pressure to levels high enough to cause amalgam to creep thus producing the expansion. STRENGTH PROPERTIES : 15
Set amalgam has weak tensile and very high compressive strength. If it is properly fabricated then strength of amalgam is adequate for normal situations within the oral cavity. However strength can be reduced during manipulative procedures. Compressive strength of satisfactory amalgam is 310 MPa (45,000 psi). Unicompositional material have highest compressive strength after 1 hour i.e. 250 MPa. Lathe cut have lowest compressive strength after this ie. 140 MPa. Tensile strength of both high copper and low copper ranges from 4870MPa. Following are the factors which effects strength of the amalgam : a) Temperature : Amalgam loses about 15% of its strength when temperature is elevated from room temperature to mouth temperature and looses 50% of its strength (room temperature) when temperature elevated upto 60oC. Trituration : More trituration energy is used more continuous phases between amalgam matrix. Crystals and original particles and even distribution of matrix crystals. This leads to greater strength. If further trituration is continued after crystal formation Excess energy will create cracks between crystals. Drop in the strength of set amalgam. c) Condensation : More energy during condensation less will be residual mercury, which will result in strong original particles More continuous interphases between original particle and primary matrix More strength. Condensation of amalgam after formation of matrix crystals does not decreased strength. d) Porosity : Total elimination of porosity is not possible but it is important to minimize number of pores and size of pores. And pores should be kept away from the margins and the surface. Porosity of only 1% can reduce strength upto 10%. Porosity results because of different phases of amalgam does not completely wet each other. Porosity is increased by : Under trituration, under condensation Insertion of too large increments Delayed insertion after trituration. Non plastic mass of amalgam. e) Mercury : Because it is the weakest phase in amalgam so mercury effects strength of the restoration very much. Mercury is fluid at room temperature and mouth temperature so it cannot resist any slip or dislocation while condensation. It is estimated that by increasing mercury content in amalgam upto 50-55% will decrease compressive strength of amalgam upto 50%.
f) Particle Shape : Particles of regular shape will combine more effective hence strengthening the mass. g) Interparticle Distance : Closer the original particles of amalgam alloy are to each other the stronger the end product will be. It has been studied that when the interparticle distance is 38 µ or less after 24 hours this gives maximum compressive strength. h) Particle Size : Smaller the diameter of original particles greater will be the strength. i) Dispersion : By addition of copper, silver-copper, eutectic alloy, silver-Cu-Palladium alloy. All of these are capable of Solid state modifications and Preparations of new phases. Without changing dimensions this will lead to increased strength. j) Gamma-2 Phase : Mechanically γ2 phase is the second weakest phase. So increase in γ2 will reduce strength and decrease in γ2 phase will increase strength. k) Corrosion : Decreasing the corrosion activity will protect joining of different phases thus increasing strength. CREEP : Creep is time dependent plastic deformation. Creep rate leads to marginal breakdown of low copper amalgams. That is greater the creep greater is the marginal breakdown. However in case of high copper Creep is not a noticeable cause of marginal fracture. ADA Specification No.1 : Creep rate should be lower than 3%. Low Copper Amalgam : 0.85 – 8% Higher Copper : 0.4 less than .1% Microstructure of Creep : γ1 phase is the major cause of creep in low copper alloys. Creep rate increases with larger γ1 volume fraction. Decreases with larger γ1 grain size. γ2 presence of γ2 is also associated with higher creep rates. Low creep rate is seen in higher copper alloys as this is related to η rods. These η rods acts as a barrier to deformation of γ1 phase. Factor Effecting Creep : More the strength lesser is the creep. Therefore mercury alloys ratio should be minimize and condensation pressure should be maximized. But careful attention should be paid for timing of condensation and trituration. Excessive Mercury: Will increase the creep. It is estimated that creep of 53% mercury in mix and 1.5 time more than 43% mercury in the mix.
How to Measure a Creep : On removal of load creep recovery curve can be obtained. - After load is removed there is instantaneous drop in strain. - This instantaneous drop in strain represents recovery of elastic strain. - Slower recovery represents an elastic strain and remaining permanent strain represents viscous strain. Creep can be calculated by creep compliance Creep compliance : Is defined as strain divided by stress at a given time (Jt). Once creep curve is obtained, creep compliance curve can be calculated. Creep compliance can be made by the equation Jt = JO + JR + (t/n) JO = Is instanceous elastic compliance JR = Is retarded elastic compliance t = Time n = Viscosity - The strain associated with JO is completely recoverable after the load is removed. - The strain associated with JR is not recovered immediately but requires some finite time. - Strain associated with t/n is not recovered and represents, permanent deformation. CORROSION : The general corrosion is the destructive attack of a meal by chemical or electrochemical reaction with its environment. Excessive corrosions can lead to increased porosity, reduced marginal integrity, loss of strength and the release of metallic products into the oral environment. Electrochemical measurement on pure phases have shown that the Ag2Hg3 phase has the highest corrosion resistance, followed by A 3Sn, Ag3Cu2, Cu3Sn, Cu6Sn5 and Sn7-8Hg. The presence of a relatively high percentage of tin in low copper alloys reduces the corrosion resistance of their γ1 phase so that it is lower than their γ phase. This is not true for high copper alloys. In general the tin content of the γ1 phase is higher for low copper alloy then for high copper alloys. The average depth of corrosion for most amalgam alloys is 100-500 um. In the low copper amalgam system, the most corrodible phase is tinmercury or γ2 phase. Even though a relatively small portion (1% to 13%) of the amalgam mass consists of the γ2 phase, in time and in an oral environment the structure of such an amalgam will contain a higher percentage of corroded phase. On other hand, neither the γ nor the γ1 phase is corroded as easily. The low copper alloy the corrosion results in the formation of tin oxychloride, from the tin in the γ2 and also liberates Hg. Sn7-8Hg + 1/2O2 + H2O + Cl- Sn4 (OH)6 Cl2 + Hg Tin oxychloride.
The reaction of the liberated mercury with unreacted γ can produce additional γ1 and γ2. It is proposed that the dissolution of the tin oxide or tin chloride and the production of additional γ1 and γ2 result in porosity and lower strength. The high copper admixed and unicomposition alloy do not have any γ2 phase in the final set mass. The η’ phase formed with high copper alloys is not an interconnected phase such as the r 2 phase, and it has better corrosion resistance. However η’ is the least corrosion resistant phase in high copper amalgam and a corrosion product CuCl2.3Cu (OH)2 has been associated with storage of amalgams in synthetic saliva. Cu6Sn + 1/2O2 +H2O + Cl- CuCl2 3Cu (OH)2 + SnO. Phosphate buffer solutions inhibit the corrosion process; thus saliva may provide some protection of dental amalgams from corrosion. Types of Electrochemical Corrosion : Galvanic corrosion : If dental amalgam is in direct contact with an adjacent metallic restoration such as gold crown, the dental amalgam is the anode in the circuit. Crevice Corrosion : Local electrochemical cells may also arise whenever a portion of amalgam is covered by plaque on soft tissue. The covered area has a lowered oxygen and/or higher hydrogen ion concentration making it behave anodically and corrode. If these occur in cracks or crevice it is called crevice corrosion. Stress Corrosion : Region within the dental amalgam that are under stress display a greater propensity for corrosion called stress corrosion. For occlusal dental amalgam greatest combination of stress and corrosion occurs along the margins. DENTAL AMALGAM : TECHNICAL CONSIDERATIONS : I) SELECTION OF ALLOY : The selection of alloy mainly depends on the operator. Generally speaking, the basic lathe cut alloy Ag 3Sn without any modifications are rarely used these days and have given way to dispersed alloys, ternary and quaternary as well as noble phase alloys. The choice between spherical and lathe cut depends on type of population being treated. It is estimated that 90% of amalgams used are high Cu alloys basically for elimination of γ2 phases. A high copper alloy is selected because of no γ2 phase and high early strength, low creep, good corrosion resistance, good resistance to marginal fracture. Finer particle size are used for ease of handling and dispensing and also they produce a smoother surface for carving and finishing. Shape of particles are also important. Lathe cut alloys exhibit rough, irregular surfaces and require 50% or more mercury to obtain adequate plasticity as compared to spherical particles which have more regular surfaces and require less mercury for trituration. Lathe cut and spherical alloy react differently to condensation pressures.
These differences result from frictional forces within the amalgam mass that offer higher resistance to face of condenser in lathe cut alloys than in spherical alloys. Another criteria is presence or absence of zinc. If an alloy contain more than 0.01% zinc. It must be mentioned in packaging as amalgam from such material will show excessive corrosion and expansion if allowed moisture contamination. Alloy not containing zinc will be less plastic, less workable and more susceptible to oxidation. So non zinc containing alloys should be chosen only for the cases where it is clinically impossible to eliminate moisture. Indium containing alloys are exception to this rule as Indium performs the same function of zinc. II. MERCURY: ALLOY RATIO : There are two mercury concentration techniques ; i) High mercury technique (increased dryness technique) ii)
Minimal mercury technique (Eames technique)
i) High Mercury Technique or Increased Dryness Technique : In this the initial amalgam mix contains little more mercury than needed for the powder (52~ 53% Hg), producing a very plastic mix. It is necessary to squeeze the mercury out of the increments being introduced during build up of the restoration so that each increment is drier than the previous one. Because of deleterious effect of high mercury content on physical and mechanical properties of amalgam, it is not used these days. Special indication may be pin amalgam restoration, or very large restoration where more wetting of amalgam is required. But with advent of amalgam bond this can be eliminated. ii) Minimal Mercury Technique or EAMe's Technique : In 1960, Eames was the first to promote a low mercury : alloy ratio. This method reduces the mercury content upto 43% for high Cu single composition alloy as compared to 55% for lathe cut low Cu alloys. The excellence of clinical restoration placed by this technique depends on proper manipulation including proportioning of mercury and alloy. Trituration and condensation should be done with equal care. The recommended mercury : alloy ratio is 1:1. Again to choose the technique, will depend on type of alloy being used. Excellence of clinical restoration depends upon proper manipulation including proportions of mercury and alloy. III. PROPORTIONING : The amount of alloy and mercury to be used can be described as mercury / alloy ratio. For Example, Mercury/ alloy ratio of 6/5 indicates 6 parts of mercury and 5 parts of alloy by weight. This recommended ratio varies from different alloys to different alloys, particle size, particle shape and heat treatment. Recommended mercury/ alloys ratio for most of modern lathe cut alloy is 1:1 or 50% of mercury.
For spherical alloys the recommendations for mercury is closer to 42%. Because spherical particles have lower surface/ volume ratios requiring less mercury to completely wet the particles. For proper proportioning wide variety of dispenses are available. During the early part of the twentieth century alloy powder and mercury were proportioned crudely and mixed manually and excess squeezed with a cloth. To proportion and mix dental amalgam more carefully, manufacturers later recommended use of alloy pellets, mercury dispensers, reusable mixing capsules and pesttes pastes and amalgamator.In amalgamators, it is preferable to proportion by weight and not by volume. Initially preweighed pellets or tablets were used for dispensing the alloy. The individual pellets are uniform in weight provided care is exercised in handling to avoid chipping of pellet. Disposable capsule containing proportioned aliquots of mercury and alloy are now widely used. These preproportioned capsules containing alloy particles and mercury in compartments separated by a disk or membrane are available. Before use, the membrane is ruptured by compression of capsule and then capsule is placed in an amalgamator. A typical reusable capsule is a hollow tube with rounded ends constructed as two pieces that can be friction fit or screwed together. Amalgam alloy is dispensed into the capsule as a pellet of pressed powder of standard weight. Mercury is dispersed from automatic dropper bottle. A small metal or plastic pate was added to the capsule and it was closed. The capsule and its contents are automatically mixed using an amalgamator. The amalgamator has been designed to oscillate in figure of eight pattern. On electric amalgamator, the trituration speed and trituration time are manually set in front of the equipment. Disposable precapsulated dental amalgam are advantageous for convenience, saving of proportion time, minimize contamination, mercury hygiene is also maintained. For reusable capsules, threaded ones are better than friction fit as latter may create a mercury aerosol in office atmosphere. Size of the Mix : Manufacturer's commonly supply capsules containing 600, 900, 800 mg of alloy and the appropriate amount of mercury. Clinical usage results have shown that these amounts are sufficient for most restorations. It is usually suggested that if larger amount are required that several smaller mixes be made at staggered time so the consistency of the mixed amalgam remains reasonably constant during the preparation of the restoration. Capsule containing 1200 mg are also available, if amalgam is needed in large amount. IV. MECHANICAL TRITURATION : Definition : It is defined as the process of mixing the amalgam alloy particles with mercury in an amalgamator. Originally the alloy and mercury well mixed or
triturated by hand with mortar and pestle. Today however mechanical amalgamator saves time and standardizes the procedure. The objectives of trituration process are : 1. To achieve a workable mass of amalgam within a minimum time, leaving sufficient time for insertion into cavity. 2. To remove oxides from the powder particle surface, facilitating direct contact between particles and mercury. 3. To pulverize pellets into particles that can be easily attacked by mercury. 4. To reduce particle size so as to increase surface area of alloy particles/unit vol. 5. To keep the amount of Îł1 or Îł2 matrix crystals as minimal as possible, yet evenly distributed throughout the mass. Alloy and mercury are dispensed into the capsule, or a disposable capsule system is being used. 1. When the capsule is secured into the machine and machine is turned on, the arm holding the capsule ossilates and thus trituration is accomplished. 2. Automatic timer is there for controlling the mixing time and most of modern amalgamators have 2 or more mixing speeds. 3. New amalgamators have hoods that cover the reciprocating arms holding the capsule. The purpose of this hood is to prevent the scattering of Mercury due to accidental escape of mercury from amalgamator. 4. An amalgamator should be used at the recommended speed by alloy manufacturer. The three basic movement of mechanical trituration are : 1. Mixing arm carrying a capsule moves back and forth in a straight line. 2. 2. Mixing arm travel back and forth in a figure of 8. 3. Mixing arm travels in a centrifugal fashion. Factors that affect trituration are : 1. Speed or number of movements / unit volume. 2. Thrust of the movement. 3. Weight of the capsule and/or pestle eg. more the weight more is the energy required. 4. Time involved in trituration. 5. Difference in size between pestle and the encasing capsule. Time of trituration or amalgamation ranges from 3-30 seconds. Variation in 2-3 seconds can also produce a under or over mixed mass. For mechanical amalgamators. Work (Trituration) : time X Motor Speed X Capsule-pestle action. Speed varies from 100-300 alterations/minutes with an increase from 400-1200 mg of amalgam in the capsule. V. MULLING:
Milling is actually a continuation of trituration. Mulling is mainly done to improve the homogenicity of the mass and to assure a consistent mix with improved texture.It is mainly done when mechanical amalgamation with pestle is accomplished. It can be done in two ways. 1. Mix is placed in a dry piece of rubber dam and vigorously rubbed between the first finger and the thumb. This process should not exceed 2-5 seconds. After trituration pestle can be removed from the capsule and mix triturated at a low speed for 2-3 seconds. This process also allows cleaning of capsule. VI. CONDENSATION : The objectives of condensation are : 1. To squeeze the unreacted mercury out of the increments building up the restoration, there by preventing entrapment of mercury. 2. To bring the strongest phases of amalgam closer together, thereby increasing the final strength of the restoration. 3. To adapt the plastic amalgam mix to cavity walls margins thereby increasing retention and minimizing microleakage. 4. To reduce the number of voids and keep matrix crystals to minimal dimensions and continuous. Condensation should start immediately after trituration. Usually 33.5 minutes can be given for condensation of amalgam mix. Further condensation can create cracks in already formed matrix. Field of operation must be kept absolutely dry during condensation. The incorporation of slightest moisture in a zinc containing amalgam at this stage can result in delayed expansion and finally corrosion and loss of strength. The ultimate result of moisture contamination is premature failure of the restoration. Condensation can be done in two ways : 1) Hand condensation 2) Mechanical condensation. 1) Hand Condensation : Amalgam mixture should be never touched with bare hands because freshly mixed amalgam contains free mercury and also moisture on the skin is source of contamination of the amalgam. a) Increments of alloy should be called to and inserted in prepared cavity by the help of amalgam carrier. b) One increment of amalgam is inserted into the cavity preparation, it should be immediately condensed with sufficient pressure for proper removal of voids. c) Condensation is usually started at the central and then the condenser point is stepped little by little towards the cavity walls. Force required depends upon the shape of alloy particles. d) After condensation of an increment, the surface should have shiny appearance. This indicates that there is sufficient mercury present at the 23
e) f) g)
surface to diffuse into the next increment. If this is not done and increments do not bond and the restoration is laminated. Such restoration may fracture probably when matrix is removed. Even with minimum mercury technique one should remove some of the soft or meshy material that is brought to the surface of each increment. This procedure of adding the increments is continued till the cavity is overfilled. After this the amalgam mix on surface is condensed heavily over the restoration using largest condensers possible. This is called blotting mix. It serves to blot excess mercury from marginal and surface area of restoration and to adapt amalgam more intimately to cavosurface anatomy. Small size increments should be carried into the cavity. The large increments make it more difficult to reduce voids and adapt alloy to the cavity walls.
Mechanical Condensation : Mechanical condensers are more useful and more popular for condensing lathe-cut alloys when high condensation forces are required. With the development of spherical alloys, the need for mechanical condensers was eliminated.Ultrasonic condensers are not recommended because during condensation they increase the mercury vapour level to values above the safety standards for mercury in dental office. Condensation Pressure : The area of condenser point or, face and force exerted on it by the operator govern the condensation pressure (force/unit area). Smaller condenser will produce greater pressure on amalgam. For Example, â€˘ 2 mm dia condenser result sin condensation pressure of 13.8 MPa when thrust of 44 neutons is exerted. â€˘ Now when 3.5 mm dia condenser results in condensation pressure of 4.6 MPa when same thrust is applied. Forces of 66.7 N (15 Lb) are recommended for condensation but it is doubtful that forces of this magnitude are used. Study has shown that 30 practitioners shoed the forces in range of 13.3 â€“ 17.8 N. With irregular shaped alloys, one should use condensers with a relatively small tip, 1-2 mm and apply high condensation forces in a vertical directions. During condensation as much mercury rich mass as possible should be removed from the restoration. When condensers with small tips are used with high condensation forces on spherical amalgams, the particle tend to roll over the another, the tip adapts well to the cavity walls, with spherical alloys one should use condensers with large tips, almost as large as the cavity permit. Because of the spherical shape of the particles, a lateral direction of condensation provides better adaptations of amalgam to cavity walls than of condensation towards the pulpal floor.
With high copper spherical amalgams, a vertical and lateral direction of condensation with vibration is recommended. Small to medium diameter condensers are advocated with admixed high copper alloys with a medium to high force and vertical and lateral direction of condensation with vibration is recommended. Condensation is usually started at entire or spherical alloy and 45 o to walls and floors for non spherical amalgam. Subsequently condensation should be done at 90o to prevent displacement of primary increment. Each portion must be condensed from centre to periphery.Shape of condenser points should conform to the area under condensation for example, a round condenser point is ineffective adjacent to corner of angle of prepared cavity. In such area triangular or rectangular point is indicated. VII. BURNISHING : Burnishing is a process sin which smooth, rigid instrument is used for smoothening restoration surface which has become rough by carving. There is a conflict between that what should be carried out first burnishing or carving. If carving is done before burnishing the effect of carving is lost. If burnishing is don before carving the carving leads to production of rough surfaces. This has lead to concept of precarve burnishing and post carve burnishing. Objectives of Burnishing : 1. It is continuation of condensation, in that it will further reduce the size and number of voids on the critical surface and marginal area of the amalgam. 2. It brings any excess mercury to the surface, to be discarded during carving. 3. It will adapt the amalgam further to cavosurface anatomy. 4. It conditions the surface amalgam to the carving step. Pre Carve Burnishing : Is carried out before carving. It smoothens margins and shapes the contour and curvatures Post carve Burnishing : After carving the rough surface which is produced is later smoothed by final burnishing. At this stage the mass is hard enough to prevent any disturbance of anatomy formed by carving. VIII. CARVING : Definition : Carving is anatomical sculpturing of amalgam. The objectives of carving are : 1. To produce a restoration with no underhangs. 2. To produce a restoration with proper physiologic contours. 3. To produce a restoration with minimal flash or overhanging. 4. To produce a restoration with functional non-interfering occlusal anatomy 5. To produce a restoration with adequate marginal ridge, proper size, location of contact areas and embrasures. 6. To produce a restoration which will not interfere with the periodontium. 25
Carving is begun soon after condensation but the amalgam should be hard enough to offer resistance to carving instrument. A scraping or "ringing" sound should be heard. If carving is started too soon amalgam will pull away from margins. First carve the embrasures with Hollenbeck carvers, then triangular fossa with discoid/cleoid carvers which will enhance marginal ridge. Remove marginal flash. Then inclined planes as well as occlusal fossae and grooves are carved. Occlusal contours are checked during centric occlusion and during lateral mandibular movements. Carving is done by moving the instrument laterally and cutting the amalgam while guided by intact tooth. Post carve burnishing is done to remove scratches irregularities on the amalgam surface, facilitating easier and efficient finishing and polishing. FINISHING AND POLISHING OF AMALGAM RESTORATIONS : Finishing and polishing of amalgam is most important for proper success of restoration but most often these step is neglected because it requires final appointment. Finishing and polishing reduces plaque accumulation and decreases secondary caries and gingival inflammation. This decreases Fatigue failure under masticatory load. Fatigue failure is that we occurs by joining of surface cracks towards inside of restoration which join together to form a fracture line. Vens (1982) showed that he Vickers hardness of amalgam improved from 75-90 after polishing and transverse strength also increased. Procedure: 1) Cavity is overfilled during amalgam condensation. 2) Mercury rich layer is removed during carving after 3-5 minutes. Carving can be initiated. Amalgam CRY is the absolute time for carving and carving time depends upon different silver alloy (setting). 3) Burnishing is done both prior and after the carving to smoothen the surface as it prepares the surface both for finishing and polishing. 4) Restoration is then left undertubed for 24 hours. This time period is recommended to allow for the setting hardening and dimensional changes of amalgam to take place which continues for 24 hours. Patient is cautioned that heavy biting force should not be applied to the filling for 7-8 hours. After 24 hours : Surface of restoration is rough due to heterogeneous structure of amalgam on setting. A) Finishing is now begun with use of steel finishing burs or stones. This includes - Trimming excess and over extending margins. - Creating contour - Correcting occlusal disharmonies.
High point appears as a shiny point WC is later reduced by carborundum stones as finishing burs. Common errors during finishing of amalgam is that of carving fins or flashes on margins. B) For Proximo-oclcusal restoration : Finishing begins at cervical margins and buccal and lingual proximal margins and then occlusal margins. There may be some overhangs. These overhangs are reduced by using â€“ Thin trimmers. a) Back and parallel knife b) Periodontal film c) Gold foil knifes C) Finishing of cervical areas is done by inserting fine water resistant strips. Cervical to the contact area through interdental space and then moving to and fro. D) Facial and lingual proximal margins are marginated by cuttle fish sandpaper discs. E) Superficial Scratches and Irregularities : These are also removed simultaneously one thing should along be keep tin mind that always apply abrasures in descending order i.e. coarse, medium, fine ultra fine. The final polish or metallic luster is obtained by application of polishing agent taken â€“ Tin oxide, Zinc oxide, Chalk, Pumice, Extra fine silex extra. For polishing in cervical areas : Polishing steeps, and dental tapers are used. During polishing the surface is kept moist and at only speed and low pressure is used otherwise it will lead to overheating and at temperature above 60 o there is irreversible damage to pulp and also there is formation of mercury vapours wc are injurious both to individual and the patient. With high copper amalgam one can finish the restoration much more earlier i.e. they can be polished 15-30 minutes after the condensation. MERCURY TOXICITY : Like all other materials, mercury has the potential for being hazardous if not used properly. Infact when dental amalgam was first introduce in the United States, the first amalgam war was because of release of potentially harmful mercury as compared to gold foil. Though mercury present in amalgam always been under controversy, the contribution of mercury to overall body burden has been relatively low. Mercury is ubiquitous in the environment and is taken into the body in one form or another via, water, air diet etc and excreted through urine. Forms of Mercury : Mercury has many forms, including inorganic and organic compounds. Both inorganic and organic compounds are potentially toxic. Mercury is normally mixed as an inorganic sulfide ore which is heated in air to oxidize and drive off sulphur. The most toxic organic compounds are methyl and ethyl mercury. The conversion of mercury from dental amalgam into methyl mercury invivo is impossible. Mercury released in dental office is in forms of mercury vapours. Mercury vapours are released during all procedures such as 27
mixing, setting, polishing and removal, mercury vapours has also been reported to be released during mastication on drinking of hot beverages. Concentration of Mercury : The occupational safety and health administration has set a threshold limit value (TLV) of 0.01 mg/m3 as maximum amount of mercury vapour allowed in the work lace. Mercury concentration of 2 mg/m 3 showed no ill effects. Sources of Mercury Exposure in Dental Office : 1. Dental amalgam raw materials being stored for use. 2. Mixed but unhardened dental amalgam during triturations, insertion and intraoral setting. 3. Dental amalgam scrap that has insufficient alloy to completely consume the mercury present. 4. Dental amalgam undergoing finishing and polishing procedure. 5. Dental amalgam restoration being removed. Mercury Management : 1. Symptoms : Know the potential hazards and symptoms of mercury exposure such as development of sensitivity and neuropathy. 2. Hazards : Know the potential spruce of mercury vapours such as (a) Spills, (b) Leavy dispensers, (c) Polishing amalgam, (d) Removing amalgam. 3. Ventilation provide sufficient ventilation by ensuring that the air flow is reasonably high and that fresh air is brought into the office in path from waiting room, through the outer office and expelled to the outer building area without contaminating other building areas. 4. Monitor office : Monitor the mercury vapour level in the office periodically (this may be done using dosimeter badges) as recommended by OSHA. 5. Monitor personnel : By periodic urine analysis (the average mercury level in urine is 6.1 Âľg/lt for dental office personnel. 6. Mercury spills : On the floor covering can be decontaminated by replacing them. There is no effective treatment for removing liquid mercury from carpet, reaction with sulphur to form sulphide (cinnabar) is slow and inefficient. 7. Precapsulated alloy : Use precapsulated alloy to eliminate possibility of bulk spills. 8. Amalgamator cover : Use amalgamator fitted with cover to contain aerosol produced during trituration. 9. Handling care : Use care by avoiding stress contact with mercury of freshly mixed amalgam. 10. Evacuation system : Use high volume evacuation when finishing and removing amalgam. Use rubber dam. 11. Recycling : Scarp dental amalgam should be collected and stored under water, glycerine or spent x-ray fixer in a tightly capped jar. Spent x-ray
fixer has an advantage of controlling mercury because it is a source of both silver and sulfide ions for reaction to solid product. 12. Contaminated items : Dispose of mercury contaminated items in sealed bags according to applicable regulations. 13. Clothing : Wear professional clothing only in the dental operatory. INDICATIONS FOR USE OF AMALGAM : 1. Pit and fissure caries : Amalgam is appropriate for conservative cavity preparation, when pit and fissure caries are not extensive especially bicuspids and molars. 2. In Class I, Class II, Class V, Class III on distal of cuspids â€“ especially when involve the permanent teeth. 3. Cemental caries : They can be used as treatment restoration for root caries. 4. Short life expectancy of tooth : Patient with large cavity preparation when life expectancy of tooth is questionable eg. Medically compromised patients. In such cases amalgam is the material of choice regardless of age of the patient. 5. Rampant caries : In primary teeth, where children have rampant caries in such cases amalgam can be the material of choice. 6. High prevalence of dental caries : In permanent teeth where patient is highly potential to have caries. 7. Preventive procedure : Hyatt had recommended a preventive on prophylactic procedure, in which pits and fissures may be minimally prepared and restored with amalgam before visible attack by caries. He referred to this procedure as "prophylactic odontomy". 8. Core build-up : Amalgam may be considered a core built up material prior to complete crown restoration. It can also be used for endodontically treated teeth. 9. Age of the patient : Although amalgam can be used regardless of age of the patient, they can be used for invalid and aged patients, where physical conditions of patient warrants such a restoration, as they can be completed in one sitting and are less time consuming. 10. Galvanism : In certain cases complete rehabilitation of posterior teeth with amalgam may become necessary if patient complains of sensitivity due to dissimilar metals. 11. Size and position of carious lesions : Amalgam can be used on distal surface of canine if lesion is mall and has not involved the facial surface and has not undermined the incisal corner. 12. Complex restoration : Amalgam can be used where large amount of tooth structure is undermixed by preparing slots and using pins to enhance the retention form. 13. Control restoration : Amalgam can be used as control restoration in teeth which are either symptomatic preoperatively and are poor periodontally. These restoration help to (1) Isolate pulp from oral fluids, (2) Provide an
anatomic contour against which gingival \tissue may heal, (3) Facilitate control of caries and plaque. Limitations of Dental Amalgam : The subject of dental amalgam has been the subject of considerable discussion since the introduction of new resin composition and glass ionomer cements. The short comings of dental amalgam are ; 1) Poor aesthetics : Being metallic restoration they are not visually attractive. Also polished finish is lost with time due to tarnishing. With advent of glass ionomer cement and composites use of amalgam in anterior restorations has been eliminated. 2) Mercury Toxicity : Mercury vapours released into the dental office can be potentially hazardous and management is extremely essential. Mercury hygiene procedures must be followed. 3) High thermal conductivity : Being a metallic material thermal conductivity of amalgam is very high, problems presented are pulp sensitivity due to hydrodynamic effect of pumping fluid through marginal gap. 4) Galvanic effects : When 2 dissimilar metallic restorations are in contact in the mouth, there may be a galvanic cell set up which can result in patient discomfort in form of strong metallic taste and can accelerate corrosive break down of the more electronegative metal. 5) Lack of adhesion : The need for use of retentive cavity designs with dental amalgam imposes secrete constraint. Large amounts of sound enamel and dentine are removed under banner of "extension for prevention". 6) Secondary caries : One of the hazardous with amalgam restoration are microleakage. Formation of corrosion products decreases microleakage, but this is usually slower in high copper alloys, resulting in passage of oral fluids and secondary caries. 7) Marginal integrity : Marginal breakdown of amalgam restoration ranging from ditching to complete break down of marginal restoration can also result in many secondary discrepancies. CAUSES OF FAILURE OF AMALGAM RESTORATIONS : The different types of failure in an amalgam restoration are ; 1. At visual level • Secondary caries • Marginal fracture • Bulk fracture • Tooth fracture • Dimensional change 2. At the microstructural level • Corrosion and tarnish • Stresses associated with masticatory forces 30
3. Pain following amalgam restoration 4. Pulp and / or periodontal involvement. Failures in an amalgam restoration can be studied in detail under three main headings : I. Failures due to faulty cavity preparation a) Inadequate occlusion extension b) Inadequate extension of the proximal box c) Overextension of the cavity preparation walls d) Minimum depth of cavity preparation e) Curved pulpal floor f) Incorrect cavosurface angle g) Sharp internal line angle h) Less or more isthmus width i) Non retentive proximal box II. Poor matrix adaptation III. Faulty amalgam manipulation a) Mercury alloy ratio b) Trituration c) Condensation d) Contamination e) Burnishing f) Carving g) Finishing and polishing RECENT ADVANCES IN DENTIN AMALGAM : Fluoride Containing Amalgam : Secondary caries is one of the most important cause of failure in amalgam restoration. This was considerably low in case of silicate cement which was associated with the high fluoride content of that material. The addition of fluoride to amalgam was therefore attractive way to stimulate the anticariogenic properties of silicate cement. Stannous fluoride was added Results / Advantages : Studies showed that there was reduced solubility of enamel adjacent to fluoride containing amalgam. One study has shown that there was lower incidence of secondary caries around the fluoride containing amalgam restoration. Exact mechanism played in fluoride uptake in the fluoride containing amalgam restoration is unknown. Disadvantages : Invitro studies have shown that there is reduction in mechanical properties such as compressive strength and corrosion resistance when stannous fluoride is added to the amalgam. INDIUM :
The possibility of adverse effects caused by exposure to mercury vapour caused researchers to experiment with alternative materials. In one such series, indium was incorporated into the amalgam structure to minimize the vaporization of mercury of mercury from the amalgam surface. Powell et al in 1989 first reported that the addition of pure indium powder to a high copper amalgam alloy decreases mercury vaporization. This type of amalgam is currently marketed by Indisperse (Indisperse Inc, Canada). Okare et al (1994) conducted a study which showed that amount of indium incorporated into amalgam was less than reported by Powell in 1989. Several effects of the incorporation of indium into the mercury releasing phase may be responsible for the reduction of mercury evaporation from amalgam. Effects : These Are : i. Total reduction in the amount of mercury present. ii. More efficient oxidation of the surface of mercury releasing phase. A reduction of vapour pressure of mercury in mercury releasing phase. iii. It is good wetting agent and adapts well to tooth surface. The use of admixed indium is dispersed phase alloy permits a significantly higher amount of dispersed phase that can be used for preparation of amalgam restoration. Johnson reports that indisperse (5% indium) is higher in compressive strength by 16% (24 hours and 79,800 psi), lower in creep by 40% (0.17%) and has a lower dimensional change on setting when compared to Disperse alloy. Creep rate is important as marginal breakdown is directly related to creep rate. GALLIUM ALLOYS : Silver amalgam, though an accepted restorative material, yet the mercury controversy limits it use. The toxic effects of mercury coupled with problems of mercury hygiene, led the researchers think of mercury free alloys. Gallium alloy is the first of its kind and was suggested by Putt Kammer as early as in 1928. However, it was used for dental purposes a couple of decades later. Properties : The melting point of Gallium is 24.78oC and the boiling point is 1983oC. The density of the gallium is 5.90 gm/cm3. It has the property of wetting many materials including tooth structure. The alloys of gallium are mixed and condensed as silver amalgam using almost the same instruments. It sets in reasonable time and possesses strength, diametrical stability and corrosion resistance equal to or even greater than silver amalgam. The compressive and tensile strength increases with time comparable with silver amalgam. Creep values are as low as 0.09%. It sets early so polishing can be carried out the same day. They expand after mixing therefore provides better marginal seal.
The physical properties of high copper silver alloys and gallium alloys are depicted in following table. Table Alloy Creep % Compressive Setting Strength (After 6 Contraction / hours) Expansion (%) Silver Alloy 1.04 ± 0.06 370 MPa -0.05 (High Copper) Gallium Alloy 0.09 ± 0.03 350 MPa +0.39 Composition : The recent gallium alloys has the following composition : Alloy : Silver (Ag) 60% Tin (Sn) 25% Copper (Cu) 13% Palladium (Pd) 20% Liquid : Gallium (Ga) 62% Iridium (Ir) 25% Tin (Sn) 25% The early gallium alloys had wide variations in the composition. Variation in alloy composition : Tin (Sn) 5 – 38% Palladium (Pd) 1-40% Copper (Cu) 3-30% Zinc (Zn) 1-12% Silver (Ag) 6-80% Variation in liquid composition : Gallium (Ga) 47-83% Iridium 7-38% Tin (Sn) 3-30% Reaction : The alloy and the liquid are mixed as usual. The structure of the gallium alloy resembles that of silver amalgam. The reaction between AgSn particles and liquid Gallium involves the formation of GaAg phase and a pure tin phase. AgSn + Ga AgGa + Sn The basic reaction remains the same, however, the composition of the alloy gallium varies considerably. After mixing, the alloy tends to adhere to the walls of the capsule and thus reported to be more difficult to handle. However, as per manufacturer's instructions, by adding a few drops of alcohol, the problem of sticking can be minimized. Biological Considerations : Biologically, the results are not promising. In early gallium alloys, surface roughness, marginal discoloration and fracture were reported. With the 33
improvement in composition these defects were significantly reduced but not totally eliminated. The problem of setting expansion though considered good initially later proved to be deleterious. The gallium alloys could not be used in larger restorations as the expansion lead to fracture of the weakened cusps. The expansion may lead to post-operative sensitivity. The creep values are best suited for gallium alloys. Even high copper alloys which are Îł2 free still exhibit significant creep value. Gallium alloys exhibit negligible creep value, which is beneficial for the life of the restoration. The compressive strength is adequate; therefore, it can be given in stress bearing areas. However, manipulation of these is difficult. Since these alloys are sticky, their condensation into the cavity is time consuming. This also creates problem with removal of matrix bands. The cleaning of instrument tips and carriers is also difficult and time consuming. It is also less popular because of its cost, which is approximately 16 times that of silver alloys. BONDED AMALGAM RESTORATIONS : To overcome one of the major disadvantage of silver i.e. it does not adhere properly to cavity walls, adhesive systems designed to bond amalgam to enamel and dentin have been introduced. It also improve its adhesion, inability to strengthen remaining tooth structure and the need for removal of healthy tooth structure for gaining retention. One of the earliest methods to bond and hence improve retention of amalgam restorations to the cavity surface relied on painting the walls of the cavity with a layer of zinc phosphate cement and then condensing amalgam over the wet surface. Later, the 'selective interfacial amalgamation liner' was tried. This liner was developed by combining the components of polycarboxylate cement and amalgam alloy particles. Though these techniques improve bond strength by 2.3 MPa but were not sufficient and desirable. Further improvement in amalgam bonding became possible with the introduction of adhesive resins meant for the "Maryland bridge; technique. Two Japanese manufacturers marketed special adhesive resin systems, Superbond (Sun Medical) and Panavia (Kurary) which contained monomers and enhanced bonding to metal surfaces after air abrading or tin plating these surfaces. Superbond was based on 4-META/MMA-TBB resins while Panavia was based on a BisGMA phosphonated ester. The use of Panavia Ex to reduce microleakage of amalgam restorations with or without a glass ionomer base has not been documented well in literature. Shear bond strengths for Panavia Ex to etched enamel and dentin were reported to be 10 MPa and 6.4 MPa respectively. Panavia used in combination with glass ionomer cement was more effective than Panavia used alone and Panavia in combination with both fluoride and glass ionomer cement was even more effective. Since then not only resin cements but dentin bonding agents have also been a subject of bonding amalgam to dentin in a number of studies. Various agents that have been tried are amalgam bond, amalgam bond with HPA 34
(Parkell), All Bond 2 (Bisco), Optibond 2 (Kerr), Panavia 21 (Kurary), Clearfil Linerbond 2 (Kuraray), Scotchbond MP (3M) and etc. Indications : These are indicated in situations that warrant auxillary retention, reinforcement of remaining tooth structure, conservative preparations and improvement of marginal seal. Boned amalgam restorations are specially indicated for extensively carious posterior teeth where the more expensive cast metal restorations and metal ceramic crowns cannot be afforded by the patient. It also allows use of amalgam in teeth with low gingivo-occlusal height where conventional amalgam, pin retained amalgam, inlays, onlays and complete cast crown restorations are difficult to place. Bonded amalgam restorations may be used as a temporary restoration, which later can be reduced to a core under a cast crown. Can be used as amalgam sealants. Advantages : It permits more conservative cavity preparations because it does not require extensive undercuts and additional mechanical retention unlike conventional amalgam, pin retained amalgam, inlays, onlays and complete cast crown restorations. It reinforces tooth structure weakened by caries and cavity preparation. It eliminates the use of retention pins and their associated problems like periodontal perforation and pulp exposure etc. It decreases the incidence of marginal fracture. It provides a bond at the tooth restoration interface and hence minimizes microleakage, recurrent caries and post-operative sensitivity. It allows biologic sealing of the pulpodentinal complex. It permits restoration of a tooth in one appointment compared to cast or ceramic restorations that may require more than one appointment. It is a cost effective treatment compared to the more expensive cast metal restorations or metal ceramic crowns. Limitations and Disadvantages : It is technique sensitive, as it requires the amalgam to be introduced into the cavity while the adhesive resin is still wet. It requires time to adapt to the new technique. Long term results of its clinical performance are not yet documented to prove its success. Experiments have shown no sustained effects of amalgam bonding when subjected to thermocycling. Hydrolytic stability of the bond is questionable over a prolonged time period. It increases the cost of an amalgam restoration. Manipulation : The design of cavity for bonded amalgam restorations does not require the traditional form of cavity preparation. The unnecessary removal of healthy tooth structure is avoided as bonded amalgam technique preserves the 35
remaining tooth structure. The cavity form is conservative yet an adequate form of resistance should be provided, as the bonding agent does not remove the need for parallel walls, grooves and box forms. A high copper single composition alloy is selected as it offers excellent strength and immediate mechanical qualities that permit early polishability. The bonding agent to be chosen under amalgam should preferably be chemically cured to dual cured. The most commonly used amalgam adhesives are based on the 4- META system or 10 MDP (Methacryloxy decyl dihydrogen phosphate) system. Amalgam bond, which utilizes the 4 META system, uses a solution of 10% citric acid and 3% ferric chloride to remove the smear layer and demineralize the dentin surface. A primer is subsequently applied on the conditioned dentin followed by a self curing 4 META system. High performance additive powder (HPA) in Amalgam bond plus contains polymethyl methacrylate fibers, which may cross the interface between the amalgam and the bonding resin producing a reinforced connection between the two materials. Panavia resin is a chemically activated. Bisphenol glycydyl methacrylate based resin cement. The addition of 10 MDP in the formulation contributes to the adhesive properties. Panavia Ex is a powder liquid system while Panavia 21 is a paste system. Both products are chemically cured and will not polymerize when oxygen is present i.e. are anaerobic in nature. Panavia 21 includes application of a self etching primer following by the application of the resin. A rubber dam is applied to isolate the concerned tooth. The carious lesion is removed with a slow speed round carbide bur. The unsupported enamel is removed and finished. The cavity preparation is gently rinsed with water and dried. If the depth of the cavity so dicates, a protective base of chemically cured or light cured glass ionomer cement can be placed. Remember fragile cusps need not be sacrificed. Properly fitted auto matrices and wedges are applied. Automatrix is preferred as it is convenient to place and does not hinder manipulation. Wedges may have been inserted earlier prior to cavity preparation if there are chances of damaging the rubber dam during cavity cutting. Enamel and dentin are etched with a 10% phosphoric acid gel for 15 seconds after which the acid gel is removed with an air water spray. The vital dentin and enamel are dried with absorbent paper or gently with air through chip syringe. Properly etched enamel will have a dull white frosted appearance. Adhesive primer (Primer A + Primer B, All Bond 2 System) is applied thoroughly throughout the cavity surface. The enamel dentin bonding agent (All Bond Liner F) is applied with a disposable brush. Freshly initiated amalgam which has been triturated by an assistant is condensed immediately into the cavity while the resin is still wet i.e. has not polymerized. 36
ď‚§ The restoration is carved, finished and polished. The Bonding Interface : a) It may include tag formation. b) Formation of precipitates on pretreated dentin surfaces to which an adhesive resin mechanically or chemically binds. c) Chemical binding to the inorganic and /or organic components of dentin, or d) Diffusion and impregnation of monomers into the substrate of pretreated dentin and subsequent polymerization resulting in a hybrid layer of reinforced dentin. CONCLUSION : Amalgam has been used in clinical dentistry for over 100 years.Till today in India it is used as the most common restoration ,however if one continues use of amalgam, strict mercury and amalgam hygiene should be maintained. REFERENCES : 1. D.B. Mahler : High Copper Dental Amalgam Alloys. Journal of Dental Research. 76 (1) : 1997 : 537-541. 2. B.M. Eley : The future of dental amalgam : A review of the literature. Part 7 : Possible alternative materials to amalgam for the restoration of posterior teeth. Br Dent J. Vol. 183 : 12, 1997. 3. Alton M. Lacy and Michal A. Staninec : The bonded amalgam restoration. Quintessence International. Vol.20 : No.7 : 1989. 521-541. 4. Simonsen R.J. : Move over amalgam at last. Quint. Int. 26 : 157 ; 1995.
DEPARTMENT OF CONSERVATIVE DENTISTRY AND ENDODONTICS COLLEGE OF DENTAL SCIENCES DAVANGERE.
PAST, PRESENT AND FUTURE OF DENTAL AMALGAM
Presented by :
Dr. Dr BHAWANPREET SINGH