Dental Amalgam PDF

Chapter 21 Dental Amalgam 21.1 Introduction compositional limit specified in the earlier version of the ISO Standard represented an attempt to An amalgam consists of a mixture of two or more...

Chapter 21 Dental Amalgam 21.1 Introduction compositional limit specified in the earlier version of the ISO Standard represented an attempt to An amalgam consists of a mixture of two or more control properties such as corrosion and setting metals, one of which is mercury. Dental amalgam expansion in the absence of any real understand- consists, essentially, of mercury combined with a ing of the structure of amalgam. Materials having powdered silver–tin alloy. Mercury is a liquid at a composition which is in line with the pre-1986 room temperature and is able to form a ‘work- standard are referred to as ‘conventional’ amalgam able’ mass when mixed with the alloy. This behav- alloys. The change in the compositional limits iour renders the material suitable for use in specified in the current standard (post-1986) dentistry. reflects a marked improvement in the understand- The reaction between mercury and alloy which ing of structure–property relationships for the follows mixing is termed an amalgamation reac- materials. tion. It results in the formation of a hard restor- The quantities of silver and tin specified ensure ative material of silvery-grey appearance. The a preponderance of the silver/tin intermetallic colour generally limits its use to those cavities compound Ag3Sn. This compound, known as the where appearance is not of primary concern (see γ (gamma) phase of the silver–tin system, is formed Fig. 21.1). over only a small composition range and is par- Dental amalgam has been used for many years ticularly advantageous since it readily undergoes with a large measure of success. For many years an amalgamation reaction with mercury. Most it was the most widely used of all filling materials. conventional alloys contain around 5% copper, For various reasons, including the development of which has a significant strengthening effect on the viable alternatives based upon resins and ceramics set amalgam. and perceptions of a dubious and frequently ques- The role of zinc is as a scavenger during the tioned level of safety, its popularity has declined. production of the alloy. The alloy is formed by melting all the constituent metals together. At the elevated temperatures required for this purpose 21.2 Composition there is a tendency for oxidation to occur. Oxida- Mercury used in dental amalgam is purified by tion of tin, copper or silver would seriously affect distillation. This ensures the elimination of impu- the properties of the alloy and amalgam. Zinc rities which would adversely affect the setting reacts rapidly and preferentially with the available characteristics and physical properties of the set oxygen, forming a slag of zinc oxide which is amalgam. easily removed. Many alloys contain no zinc. The composition of the alloy powder is con- They are described as zinc-free alloys and oxida- trolled by the ISO Standard for dental amalgam tion during melting is prevented by carrying out alloy (ISO 1559). The compositional limits the procedure in an inert atmosphere. allowed by the standard are given in Table 21.1. The majority of alloy powders contain no It can be seen that the major components of the mercury. Those products containing up to 3% alloy are silver, tin and copper. Small quantities mercury are called pre-amalgamated alloys. They of zinc, mercury and other metals such as indium are said to react more rapidly when mixed with or palladium may be present in some alloys. The mercury. 181 182 Chapter 21 (a) (b) Fig. 21.1 This shows an occlusal amalgam filling which has been contoured and polished. Table 21.1 Compositional limits of dental amalgam alloys specified in ISO 1559. Weight (%) Limits prior to 1986 Metal (‘conventional’ alloys) Current limits Silver 65 (min) 40 (min) Tin 29 (max) 32 (max) Fig. 21.2 Dental amalgam alloys. (a) Lathe-cut alloy Copper 6 (max) 30 (max) particles (×100). (b) Spherical alloy particles (×500). Zinc 2 (max) 2 (max) Mercury 3 (max) 3 (max) The shape and size of the alloy powder particles separation to occur and for a cored grain structure vary from one product to another. Two methods to be formed. The heat treatment involves heating are commonly used to produce the particles. to about 420ºC for several hours. The resulting Firstly, filings of alloy may be cut from a pre- alloy contains relatively large grains of γ phase homogenized ingot of alloy. These lathe-cut alloy material. The second heat treatment is carried out powders are irregular in shape (Fig. 21.2a) and after lathe-cutting. This is a lower temperature are graded according to size, being described as treatment typically involving heating the alloy fine-grain or coarse-grain. Secondly, particles may powder to approximately 100ºC for about 1 hour. be produced by atomization. Here, molten alloy This treatment is referred to as alloy ageing; it is is sprayed into a column filled with inert gas. The thought to remove residual stresses introduced droplets of alloy solidify as they fall down the during cutting and ensures that the alloy remains column. Particles produced in this way are either stable during future storage. spherical or spheroidal in nature (Fig. 21.2b). For spherical alloys the method of manufacture Lathe-cut alloys are normally subjected to two dictates that each small sphere is like an individual heat treating procedures. The first of these is a ingot. Thus homogenization is normally carried homogenization heat treatment (see Section 6.5) out for the reasons outlined above. normally carried out on the alloy ingot before Many alloy powders are formulated by mixing lathe-cutting and designed to produce homoge- particles of varying size or even shape in order to neous grains in which the Ag3Sn intermetallic increase the packing efficiency of the alloy and compound predominates. During the formation of reduce the amount of mercury required to produce the ingot of alloy there is a tendency for phase a workable mix.  Dental Amalgam 183 After the discovery in the 1960s that some of the properties of ‘conventional’ amalgam materi- als could be improved by the inclusion of great quantities of copper (in place of silver) a new class of materials was developed and became available for use by the dentist. The ISO Standard finally recognized this change in composition when the 1986 version of ISO 1559 was published. As shown in Table 21.1, these newer alloy powders have the same basic ingredients as the conven- tional products but they contain much greater concentrations of copper, typically 10–30% compared with less than 6% in the conventional materials. These newer alloys are referred to as Fig. 21.3 Dispersion-modified alloy powder. Lathe-cut copper-enriched alloys. In addition to the increased particles of conventional alloy and spherical particles of copper levels some alloys also contain small quan- silver-copper eutectic alloy (×500). tities of other metals such as palladium. Higher copper levels in alloy powders may be produced by the manufacturer in one of several ways. Lathe- cores of alloy particles remain embedded in a cut, spherical or spheroidal powders can be pro- matrix of reaction products. duced in which the manufacturer alters the ratio In simplified terms, the reaction for conven- of metals at the melting stage. Hence the resulting tional amalgam alloys may be given by the follow- alloy particles are similar in shape and size to ing unbalanced equation: conventional alloys but simply contain a higher 123 2 possibilities Ag3Sn + Hg → Ag2Hg3 + SnxHg + Ag3Sn copper content. These are single-composition, or γ + Hg → γ1 + γ2 + γ copper-enriched alloys. An alternative approach is to blend particles of conventional alloy with those The primary reaction products are a silver– of, for example, a silver–copper alloy in order to mercury phase (the γ1 phase) and a tin–mercury achieve a higher overall copper content. Such phase (the γ2 phase). The γ2 phase has a rather blends are called dispersion-modified, copper- imprecise structure and the value of x in the enriched alloys and one widely used product con- formula SnxHg may vary from seven to eight. The tains two parts by weight of a lathe-cut alloy of equation emphasizes the fact that considerable conventional composition (less than 6% copper) quantities of unreacted alloy (γ phase) remain and one part by weight of spherical silver–copper unconsumed. eutectic particles (Fig. 21.3). The latter particles For copper-enriched alloys the reaction may be contain 72 parts silver and 28 parts copper and represented by: the overall copper content in the blended alloy 123 Cu mass = 63.5 Ag3Sn + Cu + Hg → Ag2Hg3 + Cu6Sn5 + Ag3Sn is 12%. or γ + Cu + Hg → γ1 + Cu6Sn5 + γ The essential difference between this and the 21.3 Setting reactions reaction for conventional alloys is the replacement The reaction which takes place when alloy powder of the tin–mercury, γ2 phase in the reaction product and mercury are mixed is complex. Mercury dif- with a copper–tin phase. The copper–tin phase fuses into the alloy particles; very small particles may exist in the form of Cu6 Sn5 (η phase) or Cu3 may become totally dissolved in mercury. The Sn (ε phase) depending on the precise formulation alloy structure of the surface layers is broken of the alloy. In either case, the elimination of the down and the constituent metals undergo amalga- γ2 phase has a profound effect on the properties mation with mercury. The reaction products crys- of the set material. tallize to give new phases in the set amalgam. A In the case of the dispersion-modified, copper- considerable quantity of the initial alloy remains enriched materials, it is believed that the particles unreacted at the completion of setting. The struc- of conventional lathe-cut alloy initially react to ture of the set material is such that the unreacted form γ1 and γ2 phases. The γ2 phase then reacts  184 Chapter 21 with copper from the silver–copper eutectic when crystallization of new phases becomes the spheres to form the copper-tin phase. Thus, in predominant feature of the setting reaction. The these materials, the γ2 phase exists as an intermedi- outward thrust of growing crystals causes the ate reaction product for a short time during expansion. The overall effect may cause a slight setting. The reaction rate is quite slow and some- final expansion as shown in curve (a) or a slight times takes several days or even weeks to reach final contraction as in curve (b). Factors which completion. This is reflected in the rate of develop- affect the amount of expansion of contraction ment of mechanical properties. include the type of alloy used, the particle size and shape and, most significantly, manipulative vari- ables such as the pressure used to condense the 21.4 Properties amalgam into the cavity. It is important that the Some of the important physical and mechanical final set filling should not have dimensions which properties of amalgam are specified as tests and are very different from that of the cavity. A large requirements in the ISO specification for dental contraction would result in a marginal-gap down amalgam alloy (ISO 1559). The requirements are which fluids could penetrate. A large expansion given in Table 21.2. may result in the material protruding from the Dimensional changes: The setting reaction for amalgam involves a dimensional change. If cylin- Table 21.2 Physical and mechanical properties of dental drical specimens of material are prepared and amalgam specified in ISO 1559. allowed to set in unrestrained conditions, plots of dimensional change versus time are akin to those Property Required value shown in Fig. 21.4. Curves (a) and (b) are typical of results obtained for commonly used materials. Dimensional change (%) −0.1 to +0.2 Compressive strength (MPa) 1) A small contraction takes place during the first at 1 hour 50 (minimum) half hour or so. This corresponds to the stage at 24 hours 300 (minimum) during which mercury is still diffusing into the Creep (%) 3.0 (maximum) 2) alloy particles. The upturn in the curve begins Fig. 21.4 Dimensional change versus time for dental amalgam. Measurements started soon after mixing. (a) and (b) Examples of normal behaviour. (c) Example of moisture-contaminated zinc containing material. (note: logarithmic time scale)  Dental Amalgam 185 Strength: The strength of dental amalgam is devel- oped slowly. It may take up to 24 hours to reach a reasonably high value and continues to increase slightly for some time after that. At the time when the patient is dismissed from the surgery, typically some 15–20 minutes after placing the filling, the amalgam is relatively weak. It is necessary, there- fore, to instruct patients not to apply undue stress to their freshly placed amalgam fillings. The requirements of the ISO Standard (Table 21.2) reflect the slow development of the strength which can occur with dental amalgam. The requirement for strength at 24 hours is six times the require- ment at 1 hour. 50-300 Spherical particle alloys and copper-enriched Fig. 21.5 An occlusal amalgam filling which has caused alloys develop strength more rapidly than conven- the tooth to crack. The most likely cause of this cracking is tional lathe-cut materials. Fine-grain, lathe-cut the expansion of the amalgam during or shortly after products develop strength more rapidly than setting. coarse-grain products (Fig. 21.6). There is little difference in the ultimate compressive strength values of the materials – all being adequate in this respect. surface of the cavity or even in the fracture of The tensile strength and transverse strength the tooth (see Fig. 21.5). Hence, standard values of amalgam are very much lower than the specification tests for dental amalgam permit only compressive strength. The material is weak in thin a small expansion (typically 0.1% maximum) or sections and unsupported edges of amalgam are a small contraction (typically 0.1% maximum). readily fractured under occlusal loads. Due regard A far greater expansion than the maximum must be paid to the mechanical properties of value given above may result if a zinc-containing amalgam when considering cavity preparation. amalgam is contaminated with moisture during The material should be considered essentially condensation. Zinc reacts readily with water brittle in nature, requiring adequate support from producing hydrogen: surrounding structures. Technique may play an important part in determining the final strength of Zn + H2O → ZnO + H2 amalgam. There is good correlation between The liberation of hydrogen causes a considerable strength and mercury content. Optimum proper- delayed expansion as illustrated by curve (c) in ties are produced for amalgams containing 44– Fig. 21.4. This confirms the need for adequate 48% mercury. Since most materials are initially moisture control when using these materials. proportioned at more than 50% mercury it is nec- In order to facilitate a good marginal seal essary to reduce this level during manipulation. between amalgam fillings and the cavity wall it is Table 21.3 gives mechanical properties of a suggested that a cavity varnish is used. Such var- typical lathe-cut amalgam along with those of nishes consists of solutions of natural or synthetic enamel and dentine for comparison. It can be seen resins in a volatile solvent such as ether. The that in many respects the material is a relatively varnish is applied to the cavity walls and after good replacement for the natural tooth substance. evaporation of the solvent a thin layer of resin Values of modulus of elasticity, tensile strength covers the dentine. The amalgam is condensed and hardness lie between those of the materials against the varnish which helps to seal the cavity being replaced. The hardness of amalgam is some- walls and to take up some of the strain if the what lower than that of enamel, a factor that may amalgam expands. In order to effectively seal the be responsible for amalgam restorations develop- cavity the varnish should be water resistant, a ing surface facets when they make contact with property which is not achieved in some of the cusps of opposing teeth. Despite having a surface natural resin varnishes. hardness which is over three times lower than that  186 Chapter 21 Fig. 21.6 Graph showing increase in compressive strength as a function of time. (a) Coarse-grain, lathe-cut material. (b) Fine-grain, lathe-cut material. (c) Spherical particle material. (note: logarithmic time scale) Table 21.3 Mechanical properties of a lathe-cut amalgam compared with tooth substance. Property Enamel Dentine Amalgam Modulus of elasticity (GPa) 50 12 30 Compressive strength at 7 days (MPa) 250* 280 350 Tensile strength at 7 days 35† 40–260‡ 60† Vickers hardness 350 60 100 * Value for enamel cusp. † Diammetral test. ‡ Higher values calculated from flexural test. of enamel, amalgam appears to have adequate and the creep is calculated as the change in length resistance to intra-oral abrasion and rarely fails between 1 hour and 4 hours as a percentage of by this mechanism. the original length. The significance of creep can be explained by Plastic deformation (creep): Amalgam undergoes reference to Fig. 21.7. Creep causes the amalgam a certain amount of plastic deformation or creep to flow, such that unsupported amalgam pro- when subjected to dynamic intra-oral stresses. The trudes from the margin of the cavity (Fig. 21.7b). tendency for a material to creep is, however, nor- These unsupported edges are weak and may be mally measured in the laboratory using a static further weakened by corrosion. Fracture causes creep test. (See p. 18.) Creep is determined by the formation of a ‘ditch’ around the margins applying an axial compressive stress of 36 MPa to of the amalgam restoration. The phenomenon is two 3's a cylinder of amalgam 6 mm long and 4 mm in often referred to as the ditching of amalgam. The two 4's diameter. The specimen is stored at 37ºC for 7 γ2 phase of amalgam is primarily responsible for two 6's days before testing. After loading, the change in the relatively high values of creep exhibited by two 7's length of the specimen is monitored for 4 hours some materials. The copper-enriched amalgams,  Dental Amalgam 187 Fig. 21.7 Diagram showing how creep of amalgam causes the formation of unsupported edges which can fracture. (a) Initial restoration. (b) Following creep. (a) (b) (c) (c) Following marginal fracture. Table 21.4 Values of static creep for amalgam. The γ2 phase of a conventional amalgam is the most electrochemically reactive and readily forms Material type Creep (%)* the anode in an electrolytic cell. The γ2 phase breaks down to give tin-containing corrosion Conventional lathe-cut 2.5 Dispersion-modified, copper-enriched 0.2 products and mercury which may be able to Copper-enriched, containing 0.5% 0.06 combine with unreacted alloy (γ phase). Not all palladium the mercury formed during corrosion is able to combine rapidly with unreacted alloy and small * Creep after 7 days, stress of 37 MPa applied for 4 hours. quantities inevitably become ingested. This source of ingested mercury is a worry to those concerned about the cumulative toxic effects of mercury in the body. The proposed mechanism for the release which contain little or no γ2 in the set material, of mercury during corrosion would suggest that have significantly lower creep values and clinical this problem should be less acute for the copper- trials show they are less prone to ditching. Amal- enriched, γ2 free materials. For these products the gams produced from copper-enriched alloys con- most reactive phase, and the one most likely to taining small quantities of metals such as palladium form the anode in a corrosive couple, is the Cu–Sn or indium have lower values still. This suggests phase. The rate of corrosion is accelerated if the that although the γ2 phase may be implicated as amalgam filling contacts a gold restoration. The being responsible for high creep it is not the only large difference in potential results in a significant factor involved. Typical values of static creep for corrosion current being established. three types of amalgam are given in Table 21.4. Corrosion produces a restoration with poor These values can be compared with the maximum appearance and may significantly affect mechani- value accepted in standards (Table 21.2). cal properties. The chances of ditching are increased, particularly if creep has also occurred. Corrosion: The term corrosion should be distin- The level of corrosion can be minimized by polish- guished from the often misused term tarnish. Tar- ing the surfaces of restorations. Smooth surfaces nishing simply involves the loss of lustre from the are less prone to concentration cell corrosion. surface of a metal or alloy due to the formation The corrosion products are thought to produce of a surface coating. The integrity of the alloy is one beneficial result. They are thought to gather not affected and no change in mechanical proper- at the restoration-tooth interface and to eventu- ties would be expected. Amalgam readily tarnishes ally form a seal which prevents microleakage. This due to the formation of a sulphide layer on the proposed mechanism is supported by the fact that, surface. in laboratory tests, microleakage is observed to Corrosion is a more serious matter which may decrease with time if amalgam restored teeth are significantly affect the structure and mechanical stored in a corrosive environment. properties. Copper-enriched amalgams contain little or no The heterogeneous, multiphase structure of γ2 phase. The copper–tin phase, which replaces γ2 dental amalgam makes it prone to corrosion. Elec- in these materials, is still the most corrosion-prone trolytic cells are readily set up in which different phase in the amalgam. The corrosion currents phases form the anode and cathode and saliva produced, however, are lower than those for con- provides the electrolytes (p. 29). ventional amalgams. 188 Chapter 21 It is now generally accepted that copper-enriched nervous system. The patient is briefly subjected to amalgams perform better than conventional mate- relatively high doses of mercury during placement, rials in terms of corrosion and that this may be a contouring and removal of amalgam fillings. A factor involved in the lower incidence of ditching lower, but continuing, dose results from ingestion reported for these materials. There are no reports of corrosion products. Some studies have shown of increased marginal leakage for the copper- a higher concentration of mercury in the blood enriched materials, indicating that sufficient quan- and urine of patients with amalgam fillings than tities of corrosion product are produced to seal those without. Levels of mercury were generally the margins. within acceptable limits however, and some studies have been unable to demonstrate a difference Thermal properties: Amalgam has a relatively between patients with amalgam fillings and those high value of thermal diffusivity, as would be without. Despite this there have been reports expected for a metallic restorative material. Thus, linking mercury from dental amalgams with a in constructing an amalgam restoration, an insu- variety of ailments, ranging from fairly mild lating material, dentine, is replaced by a good behavioural problems to major psychiatric distur- thermal conductor (Table 21.5). In large cavities bances and multiple sclerosis. Many of the claims it is necessary to line the base of the cavity with are unsubstantiated and are often based on unsci- an insulating, cavity lining material prior to con- entific evidence. There are some documented densing the amalgam. This reduces the harmful cases, however, in which symptoms appear to effects of thermal stimuli on the pulp. subside after a patient’s amalgam fillings are The coefficient of thermal expansion value for removed. This is a surprising observation that amalgam is about three times greater than that for goes a long way towards proving that the original dentine (Table 21.5). This, coupled with the symptoms had little or nothing to do with mercury greater diffusivity of amalgam, results in consider- since the removal of amalgam fillings would ably more expansion and contraction in the res- ensure a marked increase in the body burden of toration than in the surrounding tooth when a mercury. This is caused by release of mercury patient takes hot or cold food or drink. Such a vapour during the grinding of amalgam. mismatch of thermal expansion behaviour may Another concern has been associated with cause microleakage around the filling since there reports that mercury can be concentrated in the is no adhesion between amalgam and tooth sub- placenta and then be passed from mother to fetus, stance. However, one must take care not to potentially causing spontaneous abortion or overstate the effects of thermal expansion and abnormalities in the new-born child. Scientific evi- contraction since the transient nature of intra-oral dence suggests that there are no grounds for such thermal stimuli indicates that only the surface concerns. Despite the wealth of scientific evidence layers of exposed materials will be affected as some authorities have advocated avoidance of the suggested in Section 20.5. The occurrence of decay exposure of pregnant women to dental amalgam. in the dentine which surrounds an amalgam filling Concerns over mercury release from amalgam fill- is the major cause for replacement of such restora- ings should have become less of an issue since the tions. It is likely that microleakage plays an impor- introduction and widespread use of non-γ2 alloys. tant part in initiating such lesions. These alloys have significantly better corrosion resistance and the corrosion process involves Biological properties: Certain mercury compounds the liberation of far less mercury than the γ2- are known to have a harmful effect on the central containing products. There are global variations in perceptions of mercury toxicity and its use in dentistry. Often, Table 21.5 Thermal properties of amalgam and dentine. concerns are focused upon unwanted environmen- tal effects related to contamination of water by Thermal diffusivity Coefficient of thermal waste amalgam products. Some countries have a ×10−3 cm2 s−1 expansion ×10−6 ºC−1 history of environmental and human health prob- lems related to contamination of water or food Amalgam 78 25 Dentine 2 8 with industrial mercury. Such a background not surprisingly effects perceptions of safety for the  Dental Amalgam 189 use of mercury in dentistry. These specific prob- of a suitable type is chosen such that accidental lems have led to the situation in which amalgam spillages can be readily dealt with. Excess, waste is rarely used in certain countries. In other coun- or scrap amalgam should be stored, under water tries its use is restricted to certain groups of ‘low or chemical fixative solution, in a sealed container risk’ patients. Where amalgam use is deprecated, in order to prevent another possible source of the potential hazardous effects of alternative contamination. Mercury or freshly mixed amalgam materials are often conveniently neglected. Whilst should never be touched by hand. Mercury is the potential hazards of mercury are frequently readily absorbed by the skin, a fact which was highlighted there is far less scrutiny of the poten- obviously not appreciated in the days when it was tial harmful effects of resin matrix composites, normal practice to ‘mull’ the material in the hand including the cytotoxicity of various components, before condensation. In addition to being hazard- the oestrogenicity of some commonly used resin ous this practice leads to contamination of the precursors and the potentially tumour-inducing amalgam. aerosols of fine glass particles produced during Despite the increased exposure of dental per- polishing. Whilst the evidence for each of these sonnel to mercury vapour, examinations of the problems may be tenuous, it is no more tenuous health, mortality and morbidity rates for dentists than the body of evidence on the toxicity of have shown that they are not significantly differ- mercury in dental amalgam. ent from those of the general population, a fact Another potential problem concerns allergic which should go a long way towards reassuring reactions to mercury in dental amalgam. Such those who harbour fears over mercury toxicity. allergic reactions, usually manifested as a con- tact dermatitis or lichenoid reaction, are well 21.5 Clinical handling notes for documented and can normally be explained by dental amalgam previous sensitization of the patient with mercury- containing medicaments. Despite the vast numbers Cavity design: Many designs of cavity have been of amalgam fillings placed every year the number used for amalgam restorations, starting with mod- of reported allergic reactions is very small and will ification of Black’s design for cavities for gold presumably decrease further if the use of mercury- restorations. Over the years the cavity design has containing sensitizing agents (e.g. certain eye oint- been refined to minimize destruction of sound ments) declines. tooth tissue and to give an appropriate form to Whilst it is generally agreed that amalgam fill- the restoration to ensure that the physical proper- ings cause little damage to patients, concern has ties of the material are optimized in the end been expressed over the possible effects of long- product. term exposure of dentists and assistants to mercury Amalgam has no intrinsic ability to bond to vapour. Mercury vapour may be released into the enamel and dentine, hence cavities have to be used atmosphere during trituration, condensation or which are undercut, i.e., the cavity is wider within during the removal of old amalgam restorations. the structure of the tooth than at its surface, in In addition, spillages of mercury in the surgery can order that the material should be mechanically cause long-term contamination of the atmosphere. retained. At all times the cavity should be no It should be remembered that the vapour pressure wider than is compatible with removal of caries of mercury increases markedly with temperature. from the dentine, removal of any unsupported The levels of atmospheric mercury will increase enamel and adequate access to pack the amalgam if an attempt is made to sterilize instruments into the cavity. All internal line angles should contaminated with mercury or dental amalgam. be rounded to minimize internal stresses within Mercury-containing material should always be the restoration and to facilitate adaptation of stored well away from any heat source. Spillages the material to the cavity walls. The floor of the of mercury which occur near any source of heat, cavity, both that overlying the pulp and at the such as radiator or oven, will cause a marked gingival extent of any box, should be flat to permit increase in the concentration of mercury in the condensation of amalgam. atmosphere. The cavo-surface margin is of particular impor- Serious problems can be avoided by ensuring tance for amalgam restorations. Amalgam is weak that the surgery is well ventilated and that flooring in thin section and hence a cavo-surface angle of 190 Chapter 21 approaching 90º is desirable. This can be difficult and grooves in the remaining dentine into which to achieve, particularly on a cusp slope, whilst the amalgam can be condensed. These act as retaining a reasonable quantity of tooth tissue. retentive features if positioned correctly in rela- Local modifications to the cavity margin, in tion to the remaining tooth tissues. Alternatively enamel, may help to surmount this problem. dentine pins can be used. A pin hole is prepared It is always necessary to remove unsupported in the dentine and a pin is cemented, pressed or enamel once any carious dentine has been removed. threaded into place. Nowadays, the most common This is relatively easy to achieve on the clearly form of pin is the self-threading pin in which a visible cavity surface, but it should be remembered thread on the pin acts as its own tap to cut a that the enamel prism orientation close to the thread into the dentine. In practice the quality of gingival margins is apical. Hence this area of the the thread cut into the dentine is poor and such tooth needs to be finished using a gingival margin pins are retained by tightly packed dentine chips. trimmer. Failure to remove unsupported enamel Pins which have a shoulder which engages the will result in an intrinsic weakness at the margins tooth tissue before the threaded shaft of the pin of the restoration. The unsupported tissue could contacts the base of the pin hole cause less damage fail either during function or under the pressure to the tooth. Pins need to be placed with care to applied by a steel matrix band whilst the restora- avoid the pulp and the periodontium. At the same tion is being packed (Fig. 21.8). Such failure time adequate space needs to be available between would result in very rapid marginal ditch forma- the pin and the location of the surface of the res- tion and probable early failure of the restoration toration to permit the condensation of an appro- through recurrent decay. priate bulk of amalgam. Finally pins should not Small cavities rely upon the undercut between be placed too close together. All dentine pins opposing walls of the tooth for retention. If one weaken the restoration in which they are placed or more cusps has fractured off a tooth it may be so they should be used sparingly. The most recent necessary to use an alternative form of retention innovation for retention of amalgam is the use of for the amalgam. One method is to prepare pits chemically-active adhesive resins as an adhesive between tooth structure and the restoration. These materials are covered in Sections 23.9 and 27.2. Matrices: If an external wall of a tooth is breached by a cavity a steel matrix band needs to be applied to the tooth to provide a surface against which the amalgam can be condensed. In addition to forming the external wall of the cavity the matrix should adapt very closely to the gingival margin of the cavity to prevent the production of ledges of amalgam outside the cavity during packing. Matrices either come with some form of holder or can be made from stainless steel tape held in place using impression compound. It is important when rebuilding the proximal surfaces of any tooth to restore its contact rela- tionship with any adjacent tooth. Obviously the use of a matrix may compromise this objective as Fig. 21.8 Preparation of cavity margins with round- the thickness of the matrix is interposed between tipped cutting instruments can lead to the production of a the filling material and the tooth. This problem is marginal lip of unsupported enamel. If this lip is not surmounted when using amalgam in two ways. removed prior to adaptation of the matrix the band will First, having adapted the matrix to the tooth it is apply considerable pressure to the enamel which will tend burnished outward to try to achieve a contact to fracture. The fractured portion will be held in place by the matrix, but will be lost relatively rapidly after matrix with the adjacent tooth. Second, a wooden or removal, resulting in a marginal defect at the base of the metal wedge should be inserted between the teeth box. if possible. This has a dual benefit in that it helps  Dental Amalgam 191 to maintain adaptation of the band to the tooth bind the unreacted γ cores together. This would surface cervically and it separates the teeth slightly. result in porosity being present in the set Once the wedge is in place the matrix can be material. loosened slightly to facilitate burnishing against Various methods of dispensation are available. the adjacent tooth. The most accurate method is to weigh the mercury and alloy components using a balance. This method is rarely used however, and both are com- 21.6 Manipulative variables monly proportioned using volume dispensers. The manipulation of amalgam involves the fol- The simplest type of volume dispenser consists lowing sequence of events. of a glass bottle with a plastic, screw-top cap. The cap has a spring-loaded plunger which releases a (1) Proportioning and dispensing; known volume of either mercury or alloy when (2) Trituration; depressed. This method of dispensation is rela- (3) Condensation; tively accurate and reproducible for mercury but (4) Carving; less so for the powdered alloys since the amount (5) Polishing. of alloy released depends on the way in which the The way in which each of these operations is particles are packed together in the container. carried out has an effect on the properties of the An alternative method of dispensation for the final restoration. alloy is preproportioned as a powder in a small sachet or envelope or as a tablet in which the Proportioning and dispensing: Alloy/mercury powder particles are compressed together. Mixing ratios vary between 5 : 8 and 10 : 8. Those mixes involves the use of either the contents of one containing greater quantities of mercury are envelope or one tablet with a given volume of ‘wetter’ and are generally used with hand mixing. mercury. Those mixes containing smaller quantities of Perhaps the most commonly used method of mercury are ‘drier’ and are generally used with dispensation involves the use of semi-automatic mechanical mixing. For any given alloy/mercury dispensers which also carry out the mixing or ratio, the nature of the mix may vary depending trituration. These devices typically have two upon the size and shape of the alloy particles. hoppers. One is filled with alloy, the other with Spherical particle alloys, for example, require less mercury. The alloy/mercury ratio can be set by the mercury to produce a workable mix. operator and the required amount of each com- For optimum properties the final set amalgam ponent is released into a mixing chamber on the should contain less than 50% mercury. Those throw of a switch or the press of a button. materials used at alloy/mercury ratios at or Another convenient method of dispensation approaching 5 : 8 require the removal of excess involves the use of encapsulated materials. Each mercury following trituration and during capsule contains both alloy and mercury in pro- condensation. portions which have been determined by the The optimal final mercury content ranges from manufacturer. The two components are initially an average of 45% for lathe-cut materials to an separated by an impermeable membrane which is average of 40% for spherical materials. Hence, readily shattered using a purpose-built capsule the amount of mercury required to produce a press or on starting to vibrate the capsule in a workable plastic mass of material is generally mechanical mixer. The capsules are similar to greater than that required to produce optimal those for some dental cements (Fig. 24.2) and are properties in the set material. If too much mercury mixed using devices such as that shown in Fig. is present in the final set amalgam it is likely that 24.3. Capsules which do not require the use of a too much of the relatively hard γ phase will be press are called self-activating capsules. converted into relatively weak and soft γ2 phase and that a considerable amount of mercury will Trituration: The mixing or trituration of amalgam remain unreacted. If, on the other hand, an attempt may be carried out by hand, using a mortar and is made to use too little mercury there may not be pestle, or in an electrically powered machine enough to wet the surface of the alloy particles which vibrates a capsule containing the mercury and produce sufficient matrix phase material to and alloy. 192 Chapter 21 For hand trituration, a glass mortar and pestle risk of atmospheric mercury contamination. In with roughened surfaces are normally used. A low order for this potential advantage to be realized it alloy/mercury ratio (around 5 : 8) is often required is essential that the capsules do not release mercury to produce a workable mix and care must be during trituration. If the capsule is not properly taken not to use excessive pressure during tritura- sealed, considerable amounts of mercury may tion in order to prevent splintering of alloy parti- escape as the temperature within the capsule cles which may change the character of the mix. increases during mechanical mixing. A further The trituration time may have an effect on the precaution is recommended when trituration is properties of the final set amalgam. Some prod- complete and the capsule is opened. The contents ucts require at least 40 seconds trituration in order remain warm at this stage and the capsule should to achieve full ‘wetting’ of alloy particles by be opened away from the face in well-ventilated mercury and optimal properties in the amalgam. conditions. Following trituration it is necessary to reduce the mercury content of the mix before condensing. Condensation: Following trituration, the material This is normally done by placing the amalgam is packed or condensed into the prepared cavity. into a strip of gauze or chamois leather and A variety of methods have been suggested to con- squeezing to express excess mercury which appears dense amalgam including ultrasonic vibration and as droplets on the outside. mechanical condensing tools. The mechanical Trituration by hand is not extensively practised tools apply quite high loads with a reasonably in developed countries nowadays. Mechanical large amplitude of movement of the condensing mixing is far more widely used. There are three tool. As a consequence they may be associated levels of sophistication which may be employed. with damage to teeth, notably cuspal fracture Following proportioning, the mercury and alloy during condensation. may be placed in a capsule which is vibrated on Ultrasonic condensers tend to produce local a purpose-built machine, often referred to as an heating of the amalgam with detrimental effects amalgamator. Alternatively, mechanical mixing in both in terms of mercury vapour release and mod- a semi-automatic machine, which also propor- ification in the setting reaction of the material. tions mercury and alloy, is possible. The use of The most widely used method of condensation encapsulated, preproportioned materials is prob- is with a hand instrument called an amalgam con- ably the most convenient, although also the most denser. These are flat-ended and come in a variety expensive option. For all three options, trituration of styles. The shape and size of the condenser times of 5–20 seconds are normal. The trituration should be chosen with the size of the cavity in time will vary according to the nature of the alloy mind. The condenser must be able to fit within the and the alloy : mercury ratio. For uncapsulated cavity outline and should be able to get reasonably products the mercury and alloy are separated by close to the peripheral margin of the restoration. a diaphragm which commonly has to be broken This can be a problem with boxes on the surface mechanically before the material can be triturated. of a tooth as a large round condenser will not Manufacturer’s instructions should be followed at pack the amalgam well into the box walls close to all times for length of trituration. the matrix. It is often better to use a smaller diam- The advantages of mechanical trituration are as eter round condenser or an ovoid instrument to follows. facilitate this first stage of packing. The amalgam is packed in increments, each increment being (1) A uniform and reproducible mix is equivalent to the volume of material which can be produced. carried in an amalgam ‘gun’. This is the device (2) A shorter trituration time can be used. used to transfer the material from the mixing (3) A greater alloy/mercury ration can be used. vessel to the prepared cavity. During condensa- This negates the requirement to express excess tion, a fluid, mercury-rich layer is formed on the mercury before condensing. Encapsulated materi- surface of each incremental layer. The cavity is als have the extra advantage that they are propor- overfilled and the mercury-rich layer carved away tioned by the manufacturer. from the surface. This effectively reduces the Another potential advantage of using encapsu- mercury content of the filling thus improving its lated materials is that they may help to reduce the mechanical properties.  Dental Amalgam 193 The technique chosen for condensation must is a danger of ‘dragging out’ significant amounts ensure the following. of material from the surface. If carving is delayed too long the material may become too hard to (1) Adequate adaptation of the material to all carve and there is a danger of chipping at the parts of the cavity base and walls. margins. It is useful to try to retain a mental (2) Good bonding between the incremental picture of the extent of the cavity when carving. layers of amalgam. The amalgam needs to be cut back to the cavity (3) Optimal mechanical properties in the set margins. If this is not done, long fine-tapered amalgam by minimizing porosity and achiev- extensions of amalgam will lie on top of the ing a final mercury content of 44–48%. enamel surface. These will be poorly supported There should be a minimal time delay between and will fracture rapidly if under occlusal load trituration and condensation. If condensation is producing a positive margin (the amalgam stands commenced too late, the amalgam will have proud of the tooth structure). achieved a certain degree of set and adaptation, It will be necessary to check the pattern of bonding of increments and final mechanical prop- occlusal contacts whilst carving a restoration. erties are all adversely affected, When the amalgam is still soft, rubbing the surface There is good correlation between the quality with cotton wool produces a matt finish, if the of an amalgam restoration and the energy subject then gently taps their teeth together or expended by the operator who condenses it. For moves them from side to side in contact, any areas lathe-cut amalgam alloys best results are achieved of contact will show up as bright burnished spots by using a high condensing force at a rapid con- and can be removed/reduced as required. If carving densing frequency and continuing to condense is not complete by the time that the amalgam has until the amalgam feels hard and a mercury-rich become hard, occlusal contacts need to be marked layer has been formed at the surface. Amalgams using thin articulating tape. High areas can be made from spherical alloy particles have very dif- removed using steel instruments or ultimately a ferent condensation characteristics to their lathe- bur, usually a steel bur in a slow hand piece. cut counter parts. They require lower condensation Spherical amalgams are easier to carve than pressures to achieve the same degree of homoge- lathe-cut materials and fine-grain products easier neity and physical strength. Indeed, there is a risk than coarse-grain. when condensing spherical alloys of using too great a condensation pressure. If this occurs the Removal of the matrix: If a matrix has been used alloy particles roll over each other and are dis- to help form a large amalgam restoration this placed by the condenser rather than being con- should be removed during the carving process. densed into an homogenous mass. Recently some The amalgam must be sufficiently set that removal manufacturers have started to sand blast the of the band will not result in bulk failure of the spherical alloy particles. This has the effect of restoration. Equally, the materials should be suf- giving the material a similar handling characteris- ficiently soft that any marginal excesses can be tic to lathe-cut alloys but still retaining their ease removed easily once the band has been removed. of condensing with low pressure. It is important to check that excess amalgam has not been forced beyond the matrix band gingivaly Carving: The objectives of carving an amalgam during condensation. This would otherwise result restoration are to remove the mercury-rich layer in a marginal ledge formation which could lead to on the amalgam surface and to rebuild the anatomy periodontal disease as a consequence of inade- of the tooth, re-establishing contact with the quate cleansibility. opposing dentition. Obviously a knowledge of normal tooth anatomy is necessary for this Polishing: Polishing is carried out in order to purpose. achieve a lustrous surface having a more accept- Soon after condensing the amalgam, the surface able appearance and better corrosion resistance. layer, which is rich in mercury, is carved away The fillings should not be polished until the mate- with a sharp instrument. Carving should be carried rial has achieved a certain level of mechanical out when the material has reached a certain degree strength, otherwise there is a danger of fracture, of set. If attempts are made to carve too soon there particularly at the margins. The strength which 194 Chapter 21 should be attained before polishing is commenced to burnish the surface of the restoration, improv- is not certain but many products require a delay ing its marginal adaptation. Care is required when of 24 hours between placing and polishing. It is using impregnated rubber points as the amalgam claimed that some faster setting materials can be can be heated significantly as a result of friction polished soon after placing. between the material and the rotating point. They Gross irregularities in the surface are reduced should be used with intermittant contact pressures using multi-bladed steel burs in a slow hand piece. rather than continuous loods. This stage should result in a smooth surface contour. Fine polishing to produce a lustre is then 21.7 Suggested further reading undertaken using graded abrasives, either flours Eley, B.M. (1997) The future of dental amalgam: A of pumice followed by zinc or ceric oxides with review of the literature. Br. Dent. J. 182, 7 parts. water as a carrier or by using abrasive impreg- Horsted-Bindlsev, P. (2004) Amalgam toxicity – nated rubber points and wheels. Pastes need to be environmental and occupational hazards. J. Dent. 32, applied using a rubber cup or brush, but are less 359. frequently used now that impregnated points are ISO 1559 Alloys for dental amalgam. available. One beneficial effect of using a rubber Jones, D.W. (1993) The enigma of amalgam in den- carrier for the abrasive (either a cup or a point) is tistry. J. Can. Dent. Assoc. 59, 155.