Dental Amalgam: Part 1

DLA 1201, exam 3

1. Dental Amalgam Part One

2. Dental Amalgam Combination of metals Easy to manipulate, has good clinical durability, and low cost In the United States, dental amalgam has been used successfully for the restoration of teeth for more than 175 years. Millions of amalgam restorations are placed each year. Amalgam is a direct placement material that is used in the posterior region of the mouth. 2

3. Dental Amalgam No other direct restorative material has the durability, ease of handling, and good physical characteristics of amalgam Wear resistance and compressive strength are superior to composite resin and glass ionomer cements 3

4. Dental Amalgam Insurance carriers have seen a reduction in the use of dental amalgam for posterior restorations by approximately 45% in the past 20 years. Probably because of concerns by patient about its safety and its lack of esthetics have prompted patients to request tooth-colored restorations. A study conducted by the National Institute of Health (NIH) in 1991-1993 concluded that amalgam is safe for human use. 4

5. Alloy Used in Dental Amalgam Alloy: Mixture of two or more metals 5

6. Alloy Used in Dental Amalgam The alloy used to produce dental amalgam is composed predominantly of silver but also contains copper and tin. A variety of other metals, such as palladium, iridium, or zinc, may be added in smaller quantities. When the silver-based alloy is mixed with mercury, the reaction that occurs is called amalgamation and the material that is produced is called dental amalgam. 6

7. Silver-Based Amalgam Alloy Particles Classified according to the shape of the particles in the powder Irregular Lathe-cut alloy Admixed Mixture of lathe-cut and spherical Spherical Spherical alloy 7 Courtesy of Grayson W. Marshall, University of California School of Dentistry (San Francisco, CA).

8. Silver-Based Amalgam Alloy Particles Each of these particle shapes contributes certain handling characteristics to the amalgam. Irregularly shaped particles are formed by shaving fine particles of 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 (atomizing) a mist of molten alloy into an inert gas (spherical alloy). Spherical particles are formed as the atomized droplets cool . Require less mercury to wet the particles and generally set more quickly than lathe-cut particles Admixed particles consist of a mixture of lathe-cut and spherical particles (admixed alloy). 8

9. Spherical vs Admixed When spherical high-copper amalgam alloys are contrasted with admixed dental amalgam alloys, the spherical alloy has very little resistance to condensation into the cavity preparation and feels soft compared with an admixed amalgam. The spherical alloy needs approximately 10% less mercury than the admixed alloy for the amalgamation process. Spherical alloys do not displace a matrix band and keep it in contact with the adjacent tooth in class II preparations as well as admixed amalgams do. Spherical amalgams have higher 1-hour and 24-hour compressive strengths than admixed amalgams do. 9

10. Composition Silver Tin Copper Indium Palladium Zinc 10

11. Dental Alloys- Composition Dental alloys for amalgam are composed mainly silver and tin . Copper is added to replace some of the silver to lessen the brittleness . Modern dental alloys are considered to be high in copper content (13% to 30%) compared with their predecessors, which had 4% to 6% copper by weight. They generally contain 40% to 70% silver and 12% to 30% tin . 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, they also absorb mercury. They are mixed with mercury 42% to 52% by weight . 11

12. Dental Alloys- Composition Manufacturers may also add indium, palladium, and zinc. Zinc may inhibit corrosion by reducing the oxidation of the other metals in the amalgam. In low-copper amalgams, zinc is responsible for gradual expansion for the amalgam over time when moisture contamination is present during placement . Contact of the moisture with 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 and could cause cracking in the adjacent enamel. 12

13. Amalgamation When the alloy in powder form is mixed with the mercury, a chemical reaction occurs. The setting transformation is also known as amalgamation . The mixture has a putty-like consistency that can be packed into the cavity preparation. Over the next several minutes the mix becomes firmer. During the firming phase , the amalgam can be carved to the anatomic shape of the tooth. Once it reaches its initial set , it no longer can be carved. It is firm but is not fully reacted. It is relatively brittle at this point. It takes up to 24 hours for most amalgams to gain their maximal strength. 13

14. Dental Amalgam Many of the high-copper spherical amalgams gain approximately 50% of their compressive strength in the first hour Some gain approximately 80% of their strength in the first 8 hours High and low-copper amalgams are comparable in compressive strength once they have completely set at about 24 hours. Once fully set, they are hard, strong and durable restorations. 14

15. Setting reaction phases The first phase, called the gamma phase, is the silver alloy phase . It is the strongest phase and involves the least corrosion. The second phase is the gamma-1 phase, which consists of mercury reacting with silver. It is strong and corrosion resistant . Not as resistant as the gamma phase The third phase is the gamma-2 phase, which consists of the reaction of mercury with tin. Gamma-2 is weak and corrodes easily . High-copper amalgams DO NOT have this phase, and are superior in their clinical performance to low-copper amalgams. 15

16. High-Copper and Low-Copper Amalgams High-copper amalgams are more durable than low-copper amalgams The higher copper content of newer alloys eliminates the formation of the gamma-2 reaction product that caused weakening of the amalgam. High-copper alloys have virtually replaced low-copper alloys because high-copper have: Less deterioration at the margins Less corrosion Greater strength 16

17. Tarnish Oxidation that attacks the surface and slightly below the surface making it discolor Results from contact with: Oxygen Chlorides Sulfides 17 Courtesy Dr. Steve Eakle.

18. Tarnish Tarnish causes a dark, dull appearance that is not destructive to the amalgam. The rougher the surface, the more it tends to tarnish. Polishing of amalgams is best done after the restoration has set for a period of 24 hours or longer. Why?? 18

19. Corrosion Chemical reaction between amalgam and substances in saliva or food, resulting in oxidation of the amalgam Galvanism 19

20. Corrosion Corrosion can occur when two dissimilar metals interact in a solution that contains electrolytes (saliva). An electrical current is generated between the metals (much like a battery) in a process called galvanism , resulting in corrosion. It weakens the amalgam over time, can stain surrounding tooth structure as corrosion products enter the dentinal tubules, and can lead to deterioration of the margins. 20

21. Creep Gradual change in shape of the restoration Creep in dental amalgams refers to the gradual change in shape of the restoration from compression by the opposing dentition during chewing or by pressure from adjacent teeth. This phenomenon is associated with the gamma-2 phase seen with low-copper alloys and results in deterioration of the margins. High-copper alloys exhibit far less creep and have superior marginal integrity 21

22. Dimensional Change Some expansion and contraction occurs during the setting reaction of the amalgam. If it contracts excessively, it will open gaps at the margins and contribute to microleakage of fluids and bacteria, and cause sensitivity If the material expands excessively, it puts pressure on the cusps and cause pain with biting pressure, or it may cause fracture of the cusps. Low-copper amalgams containing zinc are prone to expansion over time if they are exposed to moisture during placement. The gradual expansion after placement is called delayed expansion. 22

23. Dimensional Change Ideally, the dimensions of a newly placed amalgam should not change. Some expansion and contraction occur during the setting reaction of the amalgam. It is the net effect of these two processes that is important. 23 Courtesy Dr. Steve Eakle.

24. Strength Amalgam: Strongest of the directly placed restorative materials Amalgams are stronger in compression than composite or glass ionomer. They are relatively weak in tension and shear Therefore require adequate bulk to resist breaking. If amalgams are too thin in areas, they may be broken by the biting forces. 24

25. Thermal Conductivity Amalgam: Good conductor of heat and cold In shallow cavity preparations, the thickness of the dentin remaining over the pulp is usually adequate to dissipate the heat or cold. In deeper cavity preparations or in teeth that were sensitive prior to the placement of the amalgam restorations, a base or liner should be used for the comfort of the patient. 25