Why amalgam creep in the mouth

The amalgams used in dentistry have fusion temperatures only about 40 degrees C above mouth temperature, and they experience grain boundary sliding during creep deformation. Since grain boundary sliding, low-frequency cyclic stresses, and a temperature near the fusion temperature of the alloy are prerequisites for so-called "creep-fatigue fracture", this type of fracture may contribute to amalgam margin failure.

Amalgam made from seven different alloys was condensed into stainless steel dies. However, a combination of concerns by patients about its safety and its lack of esthetics has prompted many patients to request tooth-colored restorative materials.

An alloy is a mixture of two or more metals. The alloy used to produce dental amalgam is predominantly composed of silver but also contains copper and tin. A variety of other metals, such as palladium, indium, or zinc, may be added in much smaller quantities to produce specific properties in the alloy. When the silver-based alloy is mixed with mercury, a liquid metal, the reaction that occurs is called amalgamation and the material that is produced is called dental amalgam.

Silver-based amalgam alloys are classed according to the shape of the particles in the powder as irregular, admixed, or spherical Figure Each of these particle shapes contributes certain handling characteristics to the amalgam, and to some degree the amalgam type is selected by the dentist according to these characteristics. Irregularly shaped particles are formed by shaving fine particles off an ingot of the alloy with a lathe lathe-cut alloy.

The particles are sifted to separate them into fine and ultrafine particles. Spherical particles are produced by spraying molten alloy into an inert gas spherical alloy. Spherical particles are formed as the atomized droplets cool.

Admixed particles consist of a mixture of lathe-cut and spherical particles admixed alloy. Spherical alloys require less mercury to wet the particles and generally set faster than lathe-cut particles. Zinc may inhibit corrosion by reducing the oxidation of the other metals in the amalgam. In the low-copper amalgams, zinc was responsible for gradual expansion of the amalgam over time delayed expansion when moisture contamination was present during placement.

Contact of moisture with the zinc caused the formation of hydrogen gas within the amalgam, which caused it to expand. Delayed expansion could cause the restoration to expand beyond the cavity walls, causing cracking in the adjacent enamel. Because of this undesirable property, most high-copper alloys contain no zinc or very low levels of zinc and are called zinc-free alloys. When the alloy in powder form is mixed with the liquid mercury, a chemical reaction occurs.

The mixture has a putty-like consistency that can be packed into the cavity preparation. Over the next several minutes, it gradually becomes firmer. During the first part of this firming phase, the amalgam can be carved during the working time or time available to manipulate the amalgam to the anatomic shape of the tooth.

Once it reaches its initial set, it can no longer be carved and is firm but is not fully reacted. It is relatively brittle at this point, and the patient is advised not to bite on it for several hours.

Once fully set, they are hard, strong, durable restorations. The chemical reaction that occurs when the alloy and mercury are mixed has three phases. It is the strongest phase and has the least corrosion. It is strong and corrosion resistant, although not as resistant as the gamma phase. Gamma-2 is weak and corrodes readily. Tin is used to control the rate of set of the amalgam. Both silver and tin dissolve into the liquid mercury until the solution becomes saturated with them, and they also absorb mercury.

Newly formed particles begin to precipitate crystallize out of the mercury until there is no more mercury left to react. This process may take up to 24 hours to go to completion.

The low-copper amalgams had much more corrosion because of the chemical reaction of tin and mercury gamma-2 phase. Copper reacts with the tin to keep it from being available for the gamma-2 phase. High-copper amalgams do not have a gamma-2 phase and are superior in their clinical performance, displaying reduced corrosion, higher strength, and margins that hold up better.

As indicated in Figure 5, the creep percent of the amalgam decreased with increasing copper content until reaching the lowest value 0. For these reasons the resistance to creep is increases with increasing copper content of amalgam.

Also, its noted with all amalgam tested that there is a reduction in volume of the tested specimens. This reduction is resulted from the closer of the porosity that is formed during trituration and from the condensation processes. The mean value of dimensional change were obtained by three specimens of each amalgam are shown in Figure 6. ADA specification No. The result shown in Figure 6 indicated that all the amalgams within the allowable limit set by the ADA and the dimensional change of amalgam stabilized with increasing copper content.

The minimum dimensional change is 6. The microhardness test results were obtained by three specimens of each amalgam in two different aging time 1 day and 7 days from the end of the trituration are shown in Figure 7. Also the addition of copper increases the hardness of amalgams as compared with the low copper amalgam. Increasing copper content enhance the compression strength by Increasing copper content enhance the diametral tensile strength by Increasing copper content increases the creep resistance by The Vickers microhardness increases by Majed et al.

Aditya et al. Ferracane, Materials in Dentistry: Principles and Applications, 2nd ed. United States: Lippincott Williams and Wilkins, Bayindir and C. Sakaguchi and J. United States: Elsevier, McCabe and A.

Walls, Applied Dental Materials, 9th ed. UK: Blackwell Publishing Ltd, Fabianelli et al. Explain why these properties are important clinically.

List the goals of proper condensation of amalgam into a cavity preparation and why these goals are important clinically. List steps the dental team can take to limit the exposure of the patient and dental personnel to mercury and mercury vapor.

Understand the sources of mercury important to human exposure, and put the exposure to mercury from amalgam into context of total exposure.

Key Terms Admixed amalgam. In general, an amalgam is a mixture of a metallic element with the liquid element mercury. Dental amalgam is a mixture of a silver alloy with mercury. When the silver alloy , which is a powder composed mostly of silver, copper, and tin, is mixed with mercury, a chemical reaction ensues.

For 1 to 2 minutes after mixing, dental amalgam has a putty-like consistency, but it progresses to a carvable consistency for an additional 2 to 4 minutes. During this time, the amalgam is packed into a cavity preparation in a tooth and carved to the desired shape Figure In the following minutes to hours, the amalgam reaction proceeds, reaching maturity and full strength in about 24 hours. Dental amalgam is a mixture of an alloy of silver, copper, and tin with the liquid element mercury.

On a global scale, amalgam restorations still account for a significant portion of all dental restorations, although many modern dental practices today use amalgam sparingly. Prior to the development of tooth-colored restorations, amalgams were used to restore teeth throughout the mouth.

However, because of their gray color, today these restorations are limited to the posterior teeth where esthetics is less of a concern Figure More esthetic ceramic or direct resin composite materials are now used in situations where esthetics is important. However, these alternative materials have problems such as expense, shorter longevity, and technique sensitivity in placement.

Thus, amalgam remains a viable clinical choice where longevity, ease of placement, and clinical performance are paramount, especially when clinical conditions are challenging Table Concerns over mercury toxicity from amalgams remain despite no credible evidence that adverse health effects stem from the mercury in amalgam materials. Concerns about the mercury load on the environment from dental sources also remain. Other elements such as palladium and indium are sometimes included to reduce corrosion.

Particles of the silver alloy powder can be either irregularly shaped, spherical, or a mixture of the two Figure The shape of these particles will significantly influence the setting reaction and manipulation of the amalgam. If a mixture of particles is used, the resulting amalgam is referred to as an admixed amalgam. Amalgams containing irregular particles with or without spherical particles added require greater packing or condensation forces during placement than amalgams with spherical particles alone.

Most practitioners feel that amalgams containing irregular particles produce better proximal contacts and are easier to carve.