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Alloys are and will continue to be the gold standard when it comes to providing artistic, esthetically pleasing and lasting restorations to clinicians and their patients, according to a group of experts. While on the surface it may seem alloys have remained the same over time, new technology, engineering and blending of metals have brought about significant changes and improvements.
“There are different ways that alloys have evolved,” explained Paul Cascone, Senior Vice President of Research and Development at the Argen Corp., San Diego, where more than 600 different alloy compositions are produced. “The actual manufacturing process internally has changed over the years which has resulted in new and improved products. The manufacturing of alloys is a very intricate process.”
Cascone said over time, various innovations have been introduced to improve the performance of dental alloys. In the 1960s grain refiners were added allowing for the fabrication of PFMs. Specialized ceramic molds were developed in the 1970s enabling the fabrication of palladium based alloys. Next in the 1980s, continuous casting expanded the possibilities of alloy development, and in the 1990s exotic elements were added to yellow alloys to improve their strength and color. Most recently, in the 2000s powdered metals were employed for use with selective laser melting machines.
“There has been a history of innovation in dental materials which has accelerated over the last decade,” Cascone said. “Throughout the history of Argen alloy developments, we have responded to volatile commodity prices with lower intrinsic cost alloys that provide the same characteristics.”
Over the expanse of his career, Chuck Cook, CDT, owner of Cook Dental Studio, in Manchester, N.H., has seen and worked with those innovations. He worked as a lab technician at various facilities for many years before starting his own company 25 years ago. And he’s not just a technician, he’s a customer. Eleven of the crowns in his mouth, which were placed quite a while ago, are 74% gold.
“All ceramics are the future and we need to embrace it, but alloys will not go away for a long, long time,” Cook said. “For a molar, I choose all gold because it is indestructible.”
David Novak, president of Chain O’Lakes Dental Lab in Lake Villa, Ill., has a staff of 15 and over the past 30 years has been both a technician and lab owner. He’s also training one of his sons, Nick, 18, to be a lab technician.
“Escalation of gold market prices required development of lower gold content, lower cost alloys by dental alloy manufacturers,” Novak said. “Some of these alloys have high sliver contents. Many are palladium based. Some porcelains on the market are marginally compatible with the newer alloys. This situation necessitated changes in techniques by the technician, often resulting in more time consuming procedures.”
Younger dentists and students are more likely to be exposed to non-metal materials, so they tend to gravitate to those materials more than metal, said Donald Morris, owner of Eclipse Dental Laboratory in Mansfield, Ohio. He’s been in the industry for 17 years and has operated his lab for 12 years.
“That being said, dental laboratories can create an esthetic porcelain restoration with metal under it,” he explained. “In reality a porcelain fused to metal restoration is extremely esthetic, has the structural support of the metal and can be fabricated without the use of expensive equipment.”
As technical advisors Aurident, Inc., Fullerton, Calif., manufacturer of alloys since 1974, Cook, Morris and Novak regularly help other lab technicians better understand how to work with alloys.
For each case, the lab technician should consider the physical properties of the alloy before making a selection, Cascone advised. Strength, color and marginal elasticity are all important factors. Tissue response—how well the patient’s tissue will react to the material—is another important property.
“Elasticity determines how stiff the material is and how thin you can make the wall of the coping, providing more room for the porcelain,” he said. “Depending on the application you are using the alloy for, you have to weigh all the characteristics and consider the porcelain compatibility.”
Howard Hoffman, President of Aurident, Inc., explained that, “the coefficient of thermal expansion (CTE) of an alloy and porcelain is a major factor determining the compatibility of that metal/porcelain system. In general, the CTE of the alloy should be slightly higher than that of the porcelain in order to minimize interfacial stress that could induce porcelain cracking or spalling during the cool down period. But other factors also affect compatibility such as thermal conductivity and heat capacity of both materials as well as modulus of elasticity of the alloy.”
Alloys with low modulus of elasticity, typical of those with high gold content, may be compatible with a porcelain having a gross CTE mismatch. The only certain test of alloy/porcelain compatibility is a trial of the system in question, Hoffman added.
Being aware of the alloy’s properties is essential, Morris said. You need to know if the alloy is compatible with the type of porcelain being applied.
Coefficient of expansion between the porcelain and the alloy is critical, if they are not coordinated, fractures and delamination of the porcelain are possible. Burnout temperatures are just as key.
“Incomplete castings, short margins and rough appearance are just some of the symptoms of improper burnout temperatures,” he said.
Keep your cool
Both over- and underheating when casting can have detrimental effects, Morris said.
“Technicians tend to be concerned with miscasts if the metal cools and solidifies too quickly so they try to make sure enough heat is applied,” he said. “But overheating can cause problems such as porosity and rough surfaces.”
Excessive temperatures will cause volatile elements in the alloy to vaporize and/or oxidize, changing its composition and properties. Palladium based alloys absorb hydrogen from propane or natural gas, so prolonged torch flame exposure can increase their hydrogen content, making it brittle and susceptible to cracking, Morris explained. To counter these effects, it’s always a good idea to add some new alloy to previously melted buttons and castings.
“You need to focus on the appearance of the alloy in the crucible. Leaving the flame of the torch on the alloy even a few seconds too long can cause it to overheat by 100º F or more,” he said. “Visually determining casting temperature requires some practice. If a gentle tap on the side of the casting machine arm causes ripples on the surface of the molten alloy, it is ready to cast.”
Cook prefers alloys that melt and are ready for casting in 45 seconds or less because the loss of volatile elements such as silver, indium, tin and gallium is minimized, assuring the physical and mechanical properties of the casting are essentially those reported by the alloy manufacturer.”
There are decades of experience behind producing today’s alloys and a variety of resources available for support.
“Always seek professional guidance before switching unilaterally between alloys,” Cascone recommended. “As we all know, the lab process is very intricate.
At the end of the day, you need to feel comfortable with the technical support offered by your alloy provider and confident that the support will be there when needed.”
Trust the manufacturers. They’ve been at it for quite a long time.
“They’ve been manufacturing ceramic alloys for over 50 years now,” Morris said. “If a technician has a problem with an alloy, it has been engineered to the ‘nth’ degree, so it’s highly improbable that anything is wrong with the alloy.
“Alloy manufacturers adhere to strict control standards that are governed by ISO13485 regulations. Slight but inadvertent variations in technique within a laboratory can result in a problem, particularly with technique sensitive materials. The manufacturer’s technical support staff can generally resolve any questions or issues you may encounter.”