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Definition Amalgam

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    136 Chapter 4

    Dental AmalgamAmalgam seems first to have been first used for the restoration of teeth in theearly part of the 19th century in Europe.I t was just one type of metallic

    restoration: othe rs included ham mered gold leaf or lead, the latter placed w hilemolten. Right from the start, the use of amalgam was controversial. Mercurywas known to be toxic and the technique of inserting amalgam seemed crudecompared with the meticulous approach needed to place gold foil. Conse-quently the use of amalgam was considered unethical. In America, the disputebetween those dentists who would use amalgam and those who would notbecame extremely polemical, leading to the so-calledumalgam wars. In fact,there was an early professional body, the American Society of DentalSurgeons, whose express purpose was to unite ethical dentists(i .e. those

    refusing to use ama lgam ) against the unethical ones. Later, m any individualswere involved in helping to formulate safe and reliable amalgams for dentalfillings.1 8 7 On e of the most notable wasG.V. Black, whose Manual o OperativeDentistry published in 1896 established the mechanical principles fo r soundcavity design for use with these more satisfactory amalgams. Finally, in 1929,the American Dental Association adopted a specification for dental amalgam,which included the requirement that the material be tested under definedconditions. This was an important step in eliminating unsatisfactory productsfrom the market .

    To prepare dental amalgams, a powdered alloy consisting mainly of silverand tin is mixed with liquid mercury. The powder may be produced either bylathe cutting or by milling a cast ingot of the silver-tin alloy. T he resultingparticles are irregular in shape. Alternatively, the liquid alloy may be atomisedand allowed to condense, a process which results in particles having anessentially spherical morphology. Alloys of both these types are used in clinicalamalgam s, a s also are mixtures of lathe cut an d spherical particles.lX8

    In clinical use, amalgam alloy is mixed with mercury in a process known astrituration. Although formerly do ne by han d, possibly in the han ds themselves

    with a rolling action, modern dental surgeries tend to be equipped withvibratory mixers, and the unmixed am algam is prepared by the m anufa cturersin two chambers of a small capsule. Immediately prior to mixing, the thinmembrane that separates the alloy powder from the liquid mercury is broken,an d the capsule inserted in to the ar m of the mechanical mixer and vibrated forthe required length of time, typically 30 seconds, to bring about thoroughmixing of powder an d liquid. Th e freshly mixed am algam, w hich has a plasticconsistency, is then extruded from the capsule an d into the cavity.

    D urin g the process of tritu ratio n, th e surface layer of the silver-tin alloy

    dissolves in the liquid mercury, and there is a reaction that leads to theformation of new phases. These new phases are solid, and their formationcauses the plastic amalgam paste to solidify. A n um ber of metallurgical phasesare involved in this tr ans form ation , details of which are given in Ta ble 4.13.

    The detailed metallurgy of the phases involved is complex, and changes inthe silver con tent of the initial silver-tin alloy can lead t o the fo rm atio n of

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    Metals 137

    Table 4 13 Phuses involved in the setting of dentulumulgum

    Phase heniical ormula

    different phases which have correspondingly different physical properties.Silver-tin alloys anyw ay are brittle an d difficult to grind uniformly unless asmall amount of copper is included. This is limited to4-5 wt , since abo vethis level the discrete compound Cu3Sn is formed. Below this level, thepresence of cop per hardens and strengthens the Ag-Sn alloy.

    Zinc may also be included in the alloy, typically at levels of around1 wt .The presence of zinc leads to amalgams that are less plastic than zinc-freeamalgams, an important feature during finishing processes for fillings. Themain purpose of adding the zinc, though, is for it to act as a scavenger foroxygen, 89 thereby reducing corrosion through minimising the occurrence ofoth er metal oxides in the finished a ma lgam .

    Th e main setting reaction of dental am alga m is as follows:)

    The final alloy also contains significant amounts of the unreactedy phase,Ag3Sn. M odern amalgams are formulated to include u p to3 wtO opper,Ian d this leads to a subsequent reac tion, as follows:

    Sn7 *Hg Cu- u3Sn HgTh e elemental mercury th at is formed in this reaction is then free to react withfurth er silver-tin alloy, and f orm the desirabley I phase. There are severaladvantages to these reactions occurring in the setting processof amalgams:resulting materials a re less susceptible to creep a nd c orro sion 92 nd they reachtheir final levels of strength quicker than so-called conventional amalgams.9 3The absence of corrosion is regarded as particularly advantageous, because iteliminates the main route by which mercury can be released from the fillingand enter the patient viu the gastro-intestinal tract.As a result, high-copperamalgams are now the material of choice for the clinical repair of cavities,194and in certain countries, e.g. Germany, high-copper is the only type ofamalgam that is perm itted for clinical use.

    Dental amalgam is used within clinical dentistry for a variety ofpermanentrestorations, i.e. those designed t o last several years, rath er th an merely weeksor months .195 The actual survival time varies considerably, depending on the

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    138 Cliupter 4

    brand of material, condition being repaired, and patient factors such as agean d quality of ora l hygiene.'96 Am algams are recomm ended for a range ofcavities, including substantial ones needed to repair the molar teeth.

    Am algam itself has n o adhesion t o either dentine or enamel. This means th at

    there is the potential for marginal leakage, especially with high-copperamalgams, which give rise to less corrosion product that might otherwisefillan y marginal gap. T raditionally, cavity w alls are coated with a layer of copal-ether varnish.197This results in a very thin layerof organic film, about 2 pmdeep, which provides some modest sealingof the gap by flowing to f i l l anysurface irregularities on the prepared wall.

    More recently, amalgam restorations have been bonded in place using abon ding agent especially designed for the pur pos e, Using such materials, quitehigh experimental bond strengths have been recorded between bovine dentineand amalgam; it is, however, too soon to have demonstrated whether suchbonding agents improve the longevity and clinical performance of amalgamresto ratio ns when used in huma n teeth.

    Briefly, ama lgam has the following a dvantages:

    (i) It is inexpensive;(ii) It is stron g and du rabl e in the oral environment, a nd show s excellent

    i. e. minimal) wear;(iii) Its use is relatively insensitive to clinical techn ique ;(iv) It has a proven track record of over150 years of clinical service.

    On the o the r ha nd , it has the following disadvantages:

    i ) Lack of adhesion, which m ay lead t o marginal leakage (see above);(ii) It has to be retained mechanically, which in turn means it is not

    conservative of tooth structure (healthy tooth tissue has to be removedto create the necessary undercut cavity in order to retain the hardenedmaterial);

    (iii) Its aesthetics are po or;(iv) Th ere is patient concern over toxicity.

    Dental amalgam is a space filler, and its placement in the tooth causes aweakening effect. Consequently, techniques that are highly conservative oftooth tissue are generally employed by dentists and cavities are cutso as toavoid sharp angles because they also weaken the tooth by causing stresses tobe conce ntrated a t the corners.

    Amalgam is prepared and placed under the driest possible conditionsbecause if an unset am algam comes into con tact with a wet liquid,i.e. saliva orblood, the cavity margins become difficult to finish properly, and the adapta-tion is poor. In amalgams containing zinc, there is also the possibility ofreaction of the metallic zinc with water to yield zinc oxide a nd hydrogen:

    Zn H2O f Z n O HZ (gas)

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    Metals 139

    Th e latte r causes bubbling a nd expansion of the filling which, in severe cases,will result in pulpal pa in a nd cuspal fra ctur e.

    When used properly, amalgams have extremely good properties as restora-tive materials being durable and showing little in the way of recurrent caries.

    Modern high-copper zinc-containing amalgams have been shown to haveextremely go od survival rates, typically being show n in one stu dy to have over90 surviving for a t least 12 years.'98 T he overall conclusion fr om studies ofdurability is tha t the lifetime of an am alga m restoration depends o n three setsof factors:

    (i) the material (brand , composition, quality of mixingetc.) ;(ii) the dentist (cavity design, con den satio n, moisture contro letc .); and(iii) the patient (oral health and hygiene, diet, occlusal forces applied,

    including possible tooth grinding during sleep, behaviour known clini-cally as bruxism).

    The first two influence performance during the early partof the restoration'slifetime, whereas th e latter emerge as im por tant as the restoration ages.

    Dental Amalgams and HealthMercury is a toxic element, both as the free metal and in chemical combina-t i ~ n . ' ~ ~lemental mercury is relatively soluble in lipids, and is readilyabsorb ed a t the lung surface, where it is oxidised t o H g2+ . It is transpo rtedfrom the lungs by the red blood cellsto other tissues, including the centralnervous system. Mercury is readily methylated in the environment an d, asmethylmercury, easily crosses the blood-brain barrier a nd also the placentainto the foetus. Consequently, it may accumulate in the brain, and may alsoaffect the unborn child. Inorganic and metallic mercury, by contrast, does notcross the blood-brain barrier, an d hence from a ma lgam fillings in these form sit does not pose a threat t o the brain.

    Co rrosion of dental am algam fillings may occur under the conditions foundin the m outh .200 ~2 0' here is som e inhibition of corrosion by the stron gpassivating layer of SnO on the y1 phase which, though soluble in acidsolutions, is not under the relatively mild acidic conditions of active caries,i.e.about 4.9.202Despite this inhibition, some corrosion may occur, and thiscauses mercury to be released as ions which pass into the gastro-intestinaltract. However, the amount is limited, and there is no evidence that it issufficient t o cause a ny adverse e ffects.

    Th ere a re ma ny studies which show me rcury to have negligible effect on thehealth and well-being of patients. For example, in a study in Sweden,'03 thepossible effects of amalgam fillings were examined by evaluating the healthofpatients dra w n from the ongoing Swedish Adoption/Twin Study of Ageing. Th emean age of the subjects was66, and the authors concluded that no negativeeffects on physical or men tal health could be fou nd fro m dental am alga m, evenafter controlling fo r age, gender, education an d num ber of remaining teeth.

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    140 Chupter 4

    The re have been num erous attempts to link the presence of dental am algamwith the disease multiple sclerosis,MS. There are difficulties in that MS ischaracterised by bouts of spontaneous but temporary remission,so thatanec dotal accou nts of improvements in health following removal of am algam

    fillings ar e of n o value in determining whe ther there is any relationship. In fact,what evidence there is shows there to be n o relationship. Clausen204 analysedthe mercury content of brains from those who had suffered fromM S in theirlifetimes an d com par ed the results with those from the brains of deceased non -sufferers. The overall levels showed no significant differences, but the lipid-soluble mercury levels were significantlylower in the M S sufferers. This wasexplained in terms of changes in both the blood-brain barrier a nd in vitaminBIZ metabolism in those affected byMS. Whatever the explanation, it is clearthat M S is no t connected with increased levels of mercury in the bra in, an d an ysuggestion of a connection between denta l amalga ms and the disease seems tohave no basis in fact.

    Dentists and their assistants have a much higher levelof, exposure tomercury in the form of vapo ur th an d o patients,205yet studies have shown tha tthere are no significant differences in health, mortality and morbidity com-pared with the general population. In fact, the only known and scientificallyconfirmed problem with mercury is the very rare instances of mercuryhypersensitivity.206 Studies have also been conducted on th e health of childrenborn to dental personnel, and again there appear to be no risks of abnormal-ities in neonates in this In conclu sion, the scientific evidence suggeststhat the use of amalgam fillings poses no threat to the health of the dentalpersonnel carrying out treatment and that, once placed and set, there issimilarly no evidence that amalgam poses a threa t to the healthof patients.

    8 References

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    1

    K . Bordji, J. Y . Jouzeau, D. Mainard,E. Payan , P. N etter, K. T. Rie, T. Stuckyand M . Hage-Ali, Biornuteriuls, 1996, 17 929.L. Linder and J . Lundskog,Injury, 1975,6,277.H . Enneus and U . Stenram, Actu Orthop. Scund., 1965,36, 115.M . Long and H. J. Rack,Biornuteriuls, 1998, 19. 1621.W. Bonfield, in Bioniuteriuls und Clinicul Applicutions, ed. A. Pizzoferrato,A Ravaglioli and A J. C. Lee, Elsevier, Am sterdam, 1987,pp. 13-19.R . M . Pilliar and G . C. W eatherly, in Criticul Reviews in Biocomputibility,ed. D . F.Williams, CR C Press, Boca Rato n,F L , 1985,pp. 371 -403.A Bartolozzi and J . Black, Biomateriuls, 1985,6 2.L. D Do rr, R . Blocbaum,J . Emmanuel and R. Meldrum,Clin. Orthop. Rel. Res.

    1990,261, 82.R. D eutman, T. J . Rulder, R. Brian and J . P. Nater, J . Bone J t . Surg. 1977, 59A,862.E. M . Evans, M . A R . Freeman, A. J . Miller and B. Vernon-Roberts,J . Bone Jt.Surg. 1974,56B, 626.M. H . Huo ,E. A. Salvati,J . Lieberman, F. Betts and M . Bansal,Clin. Orthop. Rel.Res., 1992,276 157.


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