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Which decision criteria apply when choosing an alloy?
Choosing an alloy is not easy, especially because of the many different types available.
Sometimes the choice is regulated by law or through agreements with health insurers. In principle, other factors are decisive (Fig. 1).
Fig 1 Summary of decision criteria when choosing an alloy
Within the European Union (EU), the responsibility for prescribing an alloy lies with the dentist and is regulated by the Medical Devices Directive. In dentistry, when a dentist orders a restoration from the dental technician, it is a prescription. Once fitted, complete responsibility for the restoration then lies with the dentist. He has to ensure that anamnesis and diagnosis lead to the correct indication. He must also determine the principle compatibility. The technician must be made aware of any known allergies so as to avoid corresponding materials.
For example, nickel may not be used in the case of nickel allergies. At first glance simply avoiding nickel-based alloys could be the solution. However, some so-called steel solders contain nickel and release it in relatively high concentrations2. It is important therefore that both dentist and technician are well informed of possible allergies.
As a matter of principle the dentist should communicate to the technician which materials are to be used as he bears the final responsibility. But it makes no sense to force a new material upon the technician, which he may not be able to process.
Further criteria for the dentist are aesthetics and price. The importance of aesthetics is to be weighed against functionality. Aesthetics in the anterior region are of course of higher priority than in the posterior region where greater biting strength abounds. Finally, the restoration’s price must be acceptable if the patient is to be able to afford it.
The processing of the material is of most importance to the technician. Only when this is present can the technician deliver a restoration worthy of all technological, material, and economic demands.
For example, the addition of aluminium or beryllium leads to strong oxide layers during casting, making modern induction casting machines unusable for such alloys. The oxide layer described prevents the reliable recognition of casting moment. This may cause many errors (miscast, overheating…).
Light-coloured oxides are advantageous to aesthetics because they do not affect the veneering porcelain. Dark-coloured oxides can be an advantage after casting because they allow a better control during finishing. Small, dark areas show insufficient oxide removal. Overall, lighter oxides are more of an advantage.
Fig. 2 The oxide shading of Wiron light/BEGO after devesting in comparison to a common nickel chrome alloy.
Another important factor is the availability of the alloy. To secure high quality processes within the laboratory structure, it is imperative that the alloy is available at all times because each alloy has certain individual casting or processing characteristics. If varied too much (for example because any one supplier has a short-term promotion running) the slight difference between alloys can cause an additional workload. The additional cost of work must then be calculated against the savings made on the material but this is usually unfavourable.
For the technician, the price has to be evaluated differently. For one, there is the pure material price. It is small in relation to the alloy’s end price and/or is a transit item (as the case with precious alloys). More important for manufacturing costs is the reliability of the whole system. If a miscast occurs, it is hard to work economically altogether. This also applies to things such as unfavourable characteristics or large casting residue.
During the development of an alloy, mechanical, chemical, and biological characteristics are determined. At BEGO, alloys are tried and tested for their mechanical values in the company’s own casting units to determine possible effects of varying melting systems (open flame, induction) and casting systems (centrifuge, vacuum/pressure). Moreover, soldered and laser-welded test bodies are subjected to tensile tests. Also, the CTE (coefficient of thermal expansion) and hardness are defined.
Once application technology and basic mechanical characteristics are approved, the alloy is tested for veneering quality, if need be. This is done in various ways:
• Flaking test: The porcelain on a veneered bridge is hit with a hammer. This subjective test provides initial evaluation of bonding.
• 3-point bending test (Schwickerath) (ISO 96933): a test body is pressured under standardised conditions until the porcelain flakes off or cracks.
• Quenching test: Veneered crowns are heated to various temperatures and then quenched in cold water. The number and degree of cracking is the proportion of inner tension.
• Testing real bridges: real bridges of varying geometry are veneered with many common types of porcelain by different technicians.
All these tests together with a field test by commercial dental laboratories provide the technician with a high level of reliability. The results are passed on to service and sales department, as are all other results.
Once the application and mechanical demands are satisfied, the chemical characteristics (corrosion and discolouration) are tested. An ISO 102714 standardised immersion test is used to define the corrosion rate (ions/time). Test bodies are placed in a corrosive solution and the displaced ions in the solution measured over time. The BEGO testing method is slightly different from the DIN standard. Firstly, testing is carried out more often and for longer. Secondly, different surface conditions are tested (polished, sand blasted like the inside of a crown after firing). Test bodies made from re-used material are also tested.
An example can be found in Fig. 3 of Wirobond SG/BEGO, a metal-ceramic cobalt chrome alloy. Even under the most unfavourable circumstances, this alloy shows good corrosive resistance. Due to the low ion-count, the risk of unwanted biological reaction is classed as low.
Discolorations are hard to reproduce in the laboratory. There is the possibility of placing test bodies in sulphide and/or fluoride solutions and measuring the change over time. Both Wiron light and Wirobond SG showed no reaction. This corroborates clinical tests.
Fig.3 Results of corrosion tests on Wirobond SG/BEGO. This shows the total ion count within a week depending on different processing stages. It shows a very good corrosive resistance for this cobalt chrome alloy.
A further important marker for the quality of dental alloys is biocompatibility. The fundamental difference made here is between toxic and allergic reactions. Unlike other manufacturers, apart from comprehensive research of literature both cytotoxicity (as measure of potential local toxic reaction) and sensitisation (as measure of risk of allergic reaction) are tested by an external institute. The results are published in so-called Bio-certificates (www.bego.com). These can be used as advertising media by a technician for his dentists, or by the dentist for his patients, thus providing safety.
The data for BEGO alloys are made public and comprehensible for anyone through training events and publications. This additional effort justifies the sometimes higher price of these alloys.
Overall, quality management is very important. In the EU, the medical devices directive requires retain samples of alloys be kept. Internal delivery and outgoing materials inspections are important factors for high-quality materials. In any imaginable case of damage, the dentist or technician can fall back on such a system and rely upon the manufacturer’s support. A company only recently established is not in a position to do this.
WATAHA published an article5 in 2001 outlaying which criteria are of importance when choosing an alloy manufacturer. These five criteria can be used as a checklist.
1. Company with a dental division and long-term experience in dentistry (over 20 years)
2. Company with its own development department
3. The company must provide biological data
4. The company has its own service department
5. The company readily provides information
In conclusion, there should be a few words on standards, the CE mark of conformity, and certificates. There is mostly a lack of comprehension here. Standards represent the state of technology. They have no legal force, but are quoted in case of discrepancies. There are standards which describe systems (e.g. quality systems), products (e.g. alloys, porcelains), appliances (e.g. porcelain firing furnaces), or tests.
The CE mark of conformity can only be obtained from certain authorities. In the case of medical devices, there is a four number code next to the CE mark. This code indicates the so-called “notified body”. The number 0197 indicates therefore the Technical Control Board (TÜV) Rheinland, Germany. This body controls BEGO, for example, to check if all legal and standard requirements for medical devices are upheld.
ISO 9001 a “must” which describes the demands made regarding quality management systems. It has nothing to do with any specific product. It is the minimum requirement to sell medical devices in the EU.
ISO 13845 a “nice to have”. In this case, the company can apply its own CE mark.
CE 0197 a “must”. Without the CE mark of conformity with the four figure code, no medical device (e.g. alloy) can be sold within the EU.
Bio-certificate completely optional, a “nice to have” which shows the manufacturer actually measured the biological characteristics of the product themselves, not just evaluated through literature.
We can conclude that the securing of dental and material technology in alloys is a complex and time consuming process.
Literature
1. Schorn GH. Medizinproduktegesetz: Gesetzestext mit amtlicher Begründungund einer Einführung von Gert H. Schorn. Stuttgart: Wissenschaftliche Verlagsgesellschaft mbH, 1994.
2. Buch D, Strietzel R. Korrosion von gelöteten Kobalt-Chrom-Legierungen. dent lab 1996;44:403 - 409.
3. DIN_EN_ISO_9693. Metall-Keramik-Systeme für zahnärztliche Restaurationen. Berlin: Beuth Verlag, 2001.
4. DIN_EN_ISO_10271. Dentale metallische Werkstoffe Korrosionsprüfverfahren. Berlin: Beuth Verlag, 2001.
5. Wataha J. Selecting a manufacturer for dental casting alloys. Can J Dent Technol 2001:60-61.
