Examining precious free alloys

March 21, 2012

The fabrication of base alloy frameworks are state of the art, but often they receive a “bad reputation” with claims of much of the population suffering from toxic or allergic reactions due to dental metallic materials. It’s not to say that such reactions never occur, but when one compares the number of reported cases to the total number of units manufactured for patients, the ratio is very minimal. The risk of an allergy to food, such as strawberries or nuts, is much greater than the risk of having a reaction against a dental alloy compound. 

The fabrication of base alloy frameworks are state of the art, but often they receive a “bad reputation” with claims of much of the population suffering from toxic or allergic reactions due to dental metallic materials. It’s not to say that such reactions never occur, but when one compares the number of reported cases to the total number of units manufactured for patients, the ratio is very minimal. The risk of an allergy to food, such as strawberries or nuts, is much greater than the risk of having a reaction against a dental alloy compound. 

There are dental alloys on the market today that do have high corrosion rates1 and should be considered as the last possible replacement solution for the patient. For example, aluminum bronzes, which are sometimes used for crown and bridge frameworks in South America or the U.S., show a high polished surface in the patient’s mouth, but eventually they will begin to discolor due to corrosion. Higher discoloration will occur in the areas that are not cleaned thoroughly by brushing. The corrosion process leads to the dissolution of the metallic framework (Fig. A).

Common issues

This behavior isn’t typical of dental alloys, but in general, the ion releases, due to corrosion, are far below nutritional uptake of these same ions.2 Corrosion resistant alloys, such as cobalt chromium alloys including BEGO’s Wirobond C and Wirobond 280, show very favorable results in an immersion test (Fig. B). Even after 5 years of constant immersion, no discoloration or changes of the surfaces are observed. The corrosion solution also shows no discoloration and the surfaces remain highly polished. 

There’s no doubt that all dental alloys, regardless of composition, release ions due to corrosion. Unfortunately, similar investigations for ceramic materials and resin based materials do not exist. The question still remains if these dental materials emit ions and if they are harmful or not. Opinions on this matter differ strongly.

Ceramic components release metal oxides that are soluble, and even quartz releases a metal ion (alkali metal) to a certain extent. In addition to pigments (color giving oxides or similar compounds), aluminum ions are released from ceramic materials used for frameworks and for veneering. This reaction possesses a certain allergic potential.3-7 Even natural feldspar used in veneering ceramics is found to contain considerable amounts of lead.8 Upon further research, ceramic materials may be considered biocompatible materials, but one should judge all materials using the same criteria.

Cobalt in dentistry

Dental cobalt alloys fall into the category of “Stellite” alloys, which are cobalt-chromium alloys designed for their wear resistance. These alloys were initially invented in 1890 for industrial purposes. It wasn’t long thereafter that this alloy was used for hip prostheses and then introduced into dentistry.9,10 Since then, cobalt base alloys have been used successfully for partial dentures. In the early 1980s, cobalt based alloys, veneered with ceramics, came on the market. 

Cobalt chromium (CoCr) dental alloys have many indications. They can be used for crown and bridge frameworks and for partial dentures. In contrast to ceramic materials, they may be fused by brazing or laser welding. Different alloy types are available for different indications, but dental cobalt alloys, such as BEGO’s Wirobond alloys, may be used for all crown and bridge framework possibilities. Ductile metals are more advanced than ceramics on some points. For example, there are no ceramics available for creating clasps. Attachments have historically been made in metal, but even current uses of zirconia with attachments should be weighed heavily because of the history of problems. In any case, where bending the framework is possible, the bonding of ceramics should be considered questionable.

Taking composition out of the equation, dental alloys are distinguished by indication and manufacturing processes. One can classify those into additive or subtractive processes (Fig. C). The advantages of selective laser melting (SLM), an additive process, are price and veneering properties. Advantages of milled restorations, or subtractive processes, have higher precision, which is critical for abutments and similar indications. Both processes produce sufficient dental restorations, and dental cobalt alloys such as the Wirobond materials are used in both additive and subtractive purposes.

Dental technicians have long used cobalt chromium alloys as induction or torch cast alloys. This method has proven to fabricate a precise restoration with wonderful esthetics (Fig. D). With advances in technologies, such as SLM, there are now more options for the dental technician in terms of cobalt chromium alloys.

Adding up advantages

In 2001, BEGO introduced SLM, an additive process, into dentistry.11 In this process, the metal powder is applied layer by layer and is solidified with the help of a laser beam.12 It was the first time in history this rapid prototyping process was used for production in dentistry. Since then it has become a standard production process in dental technology. 

Favorable clinical results are continuously being reported and reveal SLM restorations are at least identical as cast restorations,13,14 and the mechanical and chemical properties are superior to a cast restoration.15 The casting method can displace the mechanical composition over a wide range, thus the composition of cobalt will not be uniform equally throughout the blank. Whereas in powder metallurgy, each grain particle has the same mechanical composition as the grains next to them so when laser melted together the composition is equal throughout the entire framework (Fig. E).

An SLM framework made out of Wirobond C+ has nearly the same chemical composition as the cast alloy Wirobond SG and has the same veneering process. The framework needs only to be blasted with an aluminus oxide such as Korox 250 by BEGO with a pressure of 3-4 bars or 45-60 psi (Fig. F), and degassing is optional. All veneering ceramics with a suitable CTE may be used. 

Another recent technology, called CAD/Cast, is the combination of conventional casting with CAD/CAM.16 CAD/Cast allows the dental technician to use a more economical process of precious alloys and partial dentures.17

Milling, a subtractive fabrication process, is used for ceramics, acrylics and metals. The disadvantage is a high percentage of wasted material. This process is not economical when using precious alloys, but even when using base alloys such as titanium and cobalt chromium, the waste cost is measurable.

Conclusion

Because of different processing techniques and indications used in today’s dentistry, the use of different compositions of alloys is necessary, and using one alloy for all indications is not possible. One reason is the CTEs of conventional ceramics differ, especially in the case of low fusing ceramics (Fig. G). This also is true in the fabrication methods used to process them-SLM may be used for the fabrication of frameworks to be veneered with ceramics, where milling is best when high precision is needed, such as for abutments. Finding the correct alloy and fabrication method are critical in the success of your dental restorations.

Today, finding precious alloy alternatives is easier than in the past, and dental technicians and dentists have new options in achieving optimal results. The technique of veneering ceramics and luting agents doesn’t need to change when using NP alloys, but the benefits for the patient are long lasting, biologically safe and they receive an esthetic non-precious restoration for a reasonable price.