Gold in Dentistry: Introduction Gold is the oldest dental restorative material, having been used for dental repairs for more than 4000 years. These early Alloys, Uses and dental applications were based on aesthetics, rather than masticatory ability. The early Phoenicians used gold wire to Performance bind teeth, and subsequently, the Etruscans and then the Romans introduced the art of making fixed bridges from gold strip. During the Middle Ages these techniques were lost, and only rediscovered in a modified form in the middle of the Helmut Knosp, nineteenth century. Consultant, Pforzheim, Germany The use of gold in dentistry remains significant today, with Richard J Holliday, annual consumption typically estimated to be approximately World Gold Council, London, UK 70 tonnes worldwide (1). However, with an increasingly wide Christopher W. Corti, range of alternative materials available for dental repairs, it is World Gold Council, London, UK considered appropriate to review the current gold based technology available today and thereby highlight the The current uses of gold in dental applications are exceptional performance that competing materials must reviewed and the major gold-based dental alloys are demonstrate if they are to displace gold from current uses. described with reference to current International New gold-based dental technologies are also highlighted. Standards. Newer techniques, such as electroforming, are highlighted with suggestions for potential future areas for research and development. The future role of Uses of Gold in Dentistry gold in restorative and conservative dentistry is also discussed in the light of increasing competition from In conservative and restorative dentistry, as well as in alternative materials. It is clear that if longevity, orthodontics, gold is used either as a pure metal, or alloyed functionality, aesthetics and biocompatibility, together together with noble metals and base metals. This use of pure with ease of manufacture are considered as the most gold is limited to direct filling of small occlusal cavities and no important selection criteria, the optimum material for standard exists for the application and properties of direct dental restorations is still an approved gold alloy. filling gold. However, pure gold used in this application is very soft (HV 25), has a very low 0.2% proof stress (30 MPa) and a large elongation (45%). As a result it can be very easily cold worked, a necessary requirement for precisely filling a cavity. Since gold fillings do not have high mechanical resistance against masticatory forces, they are only suitable for very small cavities. In recent years pure gold has also been used through the electroforming process. Electroformed inlays and onlays are suitable to be cemented into cavities after they have been veneered with porcelain. Tooth restorations such as porcelain veneered copings for crowns and bridgework can be electroformed with pure gold. Unfortunately, no standard yet exists for this process, which is rapidly becoming an established mainstream technique in modern dentistry. A more common technique in conservative dentistry is cementing investment cast gold alloy inlays and onlays into cavities. Mostly commonly used alloys are based on 65 - 75% gold, fulfilling the international standard EN ISO 1562:1995, Table1. Alloys with lower gold contents corresponding to EN ISO 8891:2000 can also be used. The standards EN ISO 1562 and 8891 are valid for both cast gold alloys, with different contents of gold and platinum group metals, Table 2. Inlays, onlays and partial crowns from high gold alloys can Gold Bulletin 2003 • 36/3 93 (a) (b) (c) Figure 1 Castings of inlay, onlay and partial crown respectively as examples of conservative dental treatment. (a) Gold inlay in molar 47 (b) Gold onlay on molar 15 (c) Partial crown for molar 36 on the model die. (Pictures courtesy of Hans H. Caesar, Zahntechnikermeister, Ludwigsburg) Table 1 Dental gold alloys - compositions and indications (uses) Alloy Group Gold content Others % (m/m) ISO EN Indications* % (m/m) Standards 1 99.9 - 99.99 —— —— A, M 2 97.9 - 98.3 1.7 Ti, Ir, Rh, Nb 9693 C, D, E 3 75 - 90 10 - 20 PGMs, In Sn, Fe, Re, Ag, Cu, Zn, Ta, Ti, Mn 9693 C, E 4 60 - 75 approx. 10 PGMs, 10 - 25 Ag, Cu, In, Sn, Zn 1562 B, C, D 9693 5 65 - 75 5 - 10 PGMs, 5 - 20 Ag, Cu, Zn 1562 B, D, E, F, G, H, L 6 60 40 PGMs —— H, I, N 7 40 - 60 5 - 10 Pd, 10 - 30 Ag, Pt, Cu, Zn, Ir 8891 B, D, L 8 40 - 60 20 - 40 Pd, 0 - 20 Ag, In, Sn, Cu, Ga, Ir, Re, Ru 9693 C 9 approx. 15 approx. 50 Pd, approx. 20 Ag, In, Sn, Ga, Ir, Re, Ru 9693 C 10 2 - 12 30 - 60 Ag, 20 - 45 PGMs, Cu, Zn, Ir, 8891 B, F, L 11 0.1 - 2 50 - 60 Pd, 25 - 40 Ag, In, Sn, Ga, Ir, Re, Ru 9693 C 12 0.1 - 5 70 - 80 Pd, Sn, Cu, Co, Ga, Ir, Re, Ru, Pt 9693 C 13 10 - 80 0 - 20 PGMs, 0 - 70 Ag, Cu, Sn, In, Zn, Co, Ni, Mn 9333 K * A Electroforming, B Inlays, Onlays, Fixed Crown- and Bridgework as Full Cast and for Polymer Veneers, C Fixed Crown- and Bridgework for Porcelain Veneers, D Telescopic Crowns and Milling Work for Removable Dentures, E CAD/CAM, F Orthodontic Wire, G Clasp Wire, H Prefabricated Attachments, I Root Canal Posts, K Solders, L Partial Denture Framework, Cast Attachments, Plates, Connecting Bars, Saddles and Splints, M Gold Foil for Direct Fillings, N Cast-On Technique PGMs = Platinum Group Metals be precisely fabricated in the dental laboratory with an alloys with special properties, including high sag resistance excellent marginal fit and longevity of many years, Figure 1. and compatible coefficients of thermal expansion to Full cast or partially veneered crowns are mostly used for porcelains, have been developed fulfilling the necessary posterior teeth because gold (as well as other metallic requirements for fixed crowns and long span bridges with materials) should not be visible in the anterior area. porcelain veneers. The standard EN ISO 9693:2000 describes Four types of alloys are defined in EN ISO 1562 and 8891 the requirements for these alloys, which are not divided into respectively, covering a wide range of uses in conservative and different types nor have a prescribed gold content. This restorative dentistry and orthodontics. The most widely used is standard is valid for noble and base metal alloys as well as for the Type 4 alloy, which is suitable for all kinds of extra-high- the suitable porcelains, Table 3. strength crown- and bridgework as full cast and/or polymer For all groups of dental gold alloys, lower melting point veneered. Orthodontic and clasp wires, attachments, partial gold solders should be available with a composition as similar denture framework, plates, bars, saddles and splints can be as possible to the alloy to be soldered in order to get a high cast or prefabricated from Type 4 alloys. In addition, telescopic bond strength. The current standard for all kinds of dental crowns and milling work are appropriate uses of this alloy. solders is EN 29333/ISO 9333:1992 - ‘Dental Brazing Materials’ Porcelain-fused-to-metal restorations have become a - which requires a minimum bond strength of the soldered widespread technology in the last 20 to 30 years (2). Gold joint of 350 MPa. An increasingly used joining method is laser 94 Gold Bulletin 2003 • 36/3 Table 2 Requirements for dental casting gold alloys with minimum gold content of 65%(m/m) and minimum content of gold and PGM’s of 75%(m/m) according to EN ISO 1562 and for dental casting alloys with noble metal content of at least 25%(m/m) but less than 75%(m/m) according to EN ISO 8891 Type of alloy 0.2% Proof stress (MPa) Elongation after fracture (%) softened hardened softened hardened min. max. min. min. min. 1 80 180 —- 18 —- 2 180 240 —- 12 —- 3 240 —- —- 12 —- 4 300 —- 450 10 3 Type 1: low-strength – for castings subject to very slight stress, e.g. inlays Type 2: medium-strength – for castings subject to moderate stress, e.g. inlays and onlays Type 3: high-strength – for castings subject to very high stress, e.g. onlays, thin cast backings, pontics, full crowns and saddles Type 4: extra-high-strength – for castings subject to very high stress and thin cross-section, e.g. saddles, bars, clasps, thimbles, unit castings and partial frameworks Table 3 Gold Alloys and Properties Requirements for metal-ceramic dental restorative systems according to EN ISO 9693 In order to examine the gold alloys specified for each application in Table 1, a characteristic alloy has been 0,2% Proof Elongation Metal-ceramic stress (MPa) after fracture bond strength extracted from each group and presented with its total min. (%) min. (MPa) min. composition, together with the most important physical 250 3 25 properties, Table 4. Due to the stress generated by masticatory forces, alloys welding, for which a German standard DIN 13972-2:2002 for dental restoration must have adequate mechanical ‘Suitability of identical and similar metallic dental materials properties. Alloys for indication B, Type 4 as well as C, D, E and for laser welding’ now exists. This standard is valid for all L (see table 1) must have a high proof stress, and a suitable dental alloys and also requires a minimum bond strength of elongation in order to prevent cracks. Alloys for indications F 350 MPa for the welded joint. and G gain additional strength through work hardening One further application for gold in dentistry is in plating or during the wire drawing process.