Alloys and Base Metal notespdf
Alloys and Base Metal notespdf
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  1. Crowns and extra-coronal restorations:Materials selectionR. W. Wassell1 A. W. G. Walls2 and J. G. Steele3; “Crowns and Extra-Coronal Restorations: Materials Selection;” British Dental Journal;March 2002; DOI: 10.1038/sj.bdj.4801334a · Source: PubMed; https://www.researchgate.net/publication/11433136In the early 1970s the gold price increasedfour fold resulting in the introduction of lessexpensive alloys of lower Karat.1 Palladium,which had been used since the 1930s to producea cheaper white gold (Au 30%, Pa 10–35%, Ag35-60%, Cu 6–25%), became the noble constituentof many of the new precious alloys.These alloys include:• Silver-palladium• Palladium-silver-gold• Gold-silver-copper-palladium (with a goldcontent greater than 40%)• Palladium-copper• Palladium-tinPalladium has a strong whitening effectwhich means that most of these alloys will havea silvery appearance unless the gold content isgreater than 40% and the palladium less than6%.2 Unfortunately, both palladium and silverabsorb oxygen when molten, which can resultin porous castings especially if casting buttonsare reused. Also, reducing the gold contentlowers the specific gravity (density), which maymake casting less reliable than with high goldalloys. This is because less dense alloys have2lower kinetic energy during casting which inturn reduces the penetration of molten metalinto the mould. Nevertheless, some authoritiesargue that with ideal conditions almost allcommercial alloys result in acceptable castings,3 but experience indicates that techniciansmay find such conditions difficult to achieve.Tarnishing of some alloys is noticeable incertain patients, particularly around the marginsof their restorations. This observation isborn out by a five-year clinical study comparingtwo palladium silver alloys to a type III gold
  2. alloy.4 In the UK the National Health Servicehas encouraged the use of alloys containingonly 30% gold. The potential for corrosionproblems to result from this change is currentlyunknown.Allergies to gold, palladium and platinumare extremely rare.5 In-vitro studies6 show thathigh gold alloys have excellent corrosionresistance, which implies that few metal ionsare eluted from restorations. Metal ions areeluted more easily from alloys of low noblemetal content, including those of copper andsilver. Copper ions have been implicated inproducing lichenoid reactions.7 However,lichenoid reactions to metal ions from crownsare not as well described as for amalgam wheremany lichen planus series show up to a third ofpatients to be sensitive to mercury salts.8 Thisraises the possibility that some cases of lichenplanus adjacent to crowns may be linked to theunderlying amalgam core. Nevertheless, withcurrent trends to use more easily corrodiblecasting alloys we should be alert to the possibilityof more lichen reactions in the future.Base metal alloysBase metal alloys used to make indirect restorationsinclude:• Nickel-chromium• Nickel-chromium-beryllium• Titanium• ‘Progold’Nickel-chromium alloysThe most commonly used base alloys are nickelchromiumand nickel-chromium-beryllium.Beryllium is added to improve the alloy’s physicalproperties; it is used as a hardener, grainstructure refiner and to reduce the alloy’s fusiontemperature.9 As a result of health concerns (seebelow) some alloys contain molybdenuminstead of beryllium.10 All of these alloys have ahigher modulus of elasticity than noble alloys.This means that they are more rigidA worrying trend, however, is forsome laboratories to substitute a precious metalinstead of nickel chromium without telling thedentist. Clearly, this could reduce the expectedbond strength.10
  3. Casting conditions need to be carefullycontrolled to obtain well fitting restorationsand any technician will tell you that thesealloys are hard to adjust and difficult to finish.This in turn can be a nuisance clinically whenfitting restorations.beryllium grindings and casting fumes areextremely hazardous unless controlled by anadequate exhaust and filtration system. Acuteproblems include conjunctivitis, dermatitisand bronchitis. Chronic beryllium disease maynot express itself for several years after exposure.9 Similarly, nickel can cause techniciansdermatological and lung problems withhigh levels of nickel or nickel compoundsbeing carcinogenic.Titanium and its alloys are well known forbiocompatibility. Restorations can be either castor electro-formed. Casting requires hightemperatures (1650°C) and a special magnesiuminvestment.16In 1985, Ida et al.17reported that the fit of cast titanium crowns wasintermediate between those made from a highnoble alloy and nickel chromium. Electroformingwas introduced in 198918 and involvesthe milling of a titanium blank by sparkerosion. Two year clinical follow-up of electroformedcopings veneered with composite haveshown encouraging results.19‘Progold’Do not be misled by alloys such as ‘Progold’. Likebrass they consist largelyof copper and zinc,and tarnish easily.Metal composite copingsThe Captek system was developed over a
  4. decade ago for producing composite metalframeworks for metal-ceramic crowns,bridges, inlays and onlays(Fig. 5). The technique,which involves the fabrication of ametal composite (Fig. 6) coping (88% Au, 4%Pt, 4% Pa) is comparatively simple but doesrequire some technician training.75The sheer bond strength of porcelainto Captek was almost twice as high whilstcrowns of both materials loaded at the incisaledge showed similar load bearing characteristics.Marginal fit was also reported to be excellentand significantly better than conventionalmetal ceramic crowns.Cast copingsPorcelain fused to metal (metal-ceramic) technologywas first described in 195662 and patentedin 1962.63 Alloys were produced with meltingpoints sufficiently high to resist the firing ofporcelain.With such a high gold content the originalalloys were extremely expensive, resulting inmany laboratories preferring high palladiumlow gold alloys –although paradoxically, palladiumprices have recently been so high that thehigh gold alloys are sometimes the more affordablealternative! High palladium alloys have theadvantage of having a high modulus of elasticityand are therefore more rigid allowing slightlythinner copings to be made. This rigidity is particularlyuseful in bridgework where flexion ofthe pontics under load can result in fracture ofthe overlying porcelain.Some silver palladium alloys can cause a greenishhue to appear due to diffusion of silver compoundsinto the porcelain. Titanium oxidiseseasily and a thick nonadherent oxide layer canform under regular feldspathic porcelains. Thuslow fusing porcelains (eg Procera or Duceratin)are used to avoid problems of the oxide layercompromising strength.64 Porcelain bondstrengths to titanium are in any case not as high
  5. as with other alloys which may explain the higherrisk of metal-ceramic failure seen in a 6-yearclinical follow-up.65Cast copings are the most commonly usedmethod of strengthening porcelain and haveserved us well, but consistently good aestheticsare difficult to achieve because the metal has tobe covered by an opaque layer which in turnlimits the thickness of an adequate overlyinglayer of porcelain. However, proper tooth reductionand excellent technical support will largelyovercome this problem.One of the main advantages of cast copings isthat the coping can be waxed to create a metalocclusal surface —a facility that is either absentor more difficult to achieve in the following twosystems.Foil copingsIn 1976 McClean 67 reported a technique of fusingplatinum foil to the fit surface of an aluminousPJC. The foil was made adhesive to theporcelain by electroplating with tin and subsequentoxidisation. The crown was made usingtwo layers of foil with...............................................................................................................................Academy of Dental Learning and Osha TrainingSpiller, Martin; “Castable Metal Alloysin DentistryThe Academy of Dental Learning and OSHA Training, LLC, designates this activity for 2continuing education credits(2CEs).Martin S. Spiller, DMDHealth Science Editor: Megan Wright, RDH, MSPublication Date: October 2012UpdatedDate: July2017Expiration Date: August2020Spiller, Martin S, DMD and Wright, Megan RDH, MS; “Castable Metal Alloys in Dentistry;” Academy of Dental Learning and OSHA Training; Pub October 2012; revised July 2017; https://www.dentallearning.org/course/DentalAlloys/CastableMetalAlloys.pdf
  6. 26Base-metal alloys have been around since the 1970's. They contain less than 25% noble metal, but in actuality, most contain no noble metal at all. They can be used for full-cast or PFM restorations as well as for partial denture frameworks. As a group, they are much harder, stronger and have twice the elasticity of the high-noble and noble-metal alloys. Castings can be made thinner and still retain the rigidity needed to support porcelain. They have excellent sag resistance and are great for long span porcelainbridges. They appear to be the ideal metal for cast-dental restorations and were heavilyused for PFM frameworks due to their low cost and high strength characteristics.Unfortunately, nickel and beryllium, two of the most commonly used constituents of base-metal alloys can cause allergic reactions when in intimate contact with the gingiva. Since many women and men have been sensitized to these metals by wearing inexpensive skin piercing jewelry, crowns and bridges made from these alloys have been known to cause gingival discoloration, swelling, and redness in susceptible individuals.Base-metal alloys also have other disadvantages for lab technicians and dentists. Base-metal alloys have a very high melting temperature which makes them more difficult to cast. They exhibit a high casting shrinkage (about 2.3%) which must be compensated for. Their hardness makes them difficult to burnish and polish. Their high melting temperature makes them difficult to solder. These alloys are more prone to corrosion under acidic conditionsIn 1962, Dr. Abraham Weinstein patented the first gold-based alloy upon which porcelain could be baked. The metal substructure reinforced the porcelain and gave it the durability and the strength to resist fracturing in the mouth.20Palladium raises the melting temperature, increases hardness and whitens gold even in very small concentrations. Palladium prevents tarnish and corrosion and acts to absorb hydrogen gas which may be released during casting.Zinc acts as an oxygen scavenger and
  7. prevents the formation of porosity in the finished alloy. Zinc also increases fluidity and reduces surface tension in the molten state improving the casting characteristics of the alloy.Porcelain AlloysUntil the mid-20th century, gold and amalgam were the only materials available for the restoration and replacement of posterior teeth. Porcelain jacket crowns were available for front teeth, but they did not fit well and were prone to fracture easily. In 1962, Dr. Abraham Weinstein patented the first gold-based alloy upon which porcelain could be baked. The metal substructure reinforced the porcelain and gave it the durability and the strength to resist fracturing. For the first time, it was possible to replace missing teeth with natural looking, tooth colored, fixed bridgework. Due to the accuracy of the lost wax technique, these appliances could fit tooth preparations exactly.Porcelain will not chemically bond with gold alone. Trace elements need to be in the alloy composition to form an oxide layer on the surface which then bonds the porcelain to gold.Three oxide-forming elements are:•Iron•Indium•TinPorcelainis made of metal oxides. It will bind with oxides on the surface of gold frameworks.Dipping Waxis melted to a liquid state in a heated dipping pot at a preset temperature. It is used to apply a uniform layer of wax in a given thickness desired by the technician to the surface of a die that is dipped into and quickly removed from the liquid wax. [https://www.dentalventures.com/dip-n-cast-wax/]
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