1. DENTAL AMALGAM Stephen C. Bayne Department of Operative Dentistry
School of Dentistry
University of North Carolina
Chapel Hill, NC
Dental amalgam has been the main restorative filling material for dentistry in the United States for almost 180 years (since about 1830). Due to better-and-better alternatives and increasing concerns for amalgam/mercury recycling challenges, the relative use of amalgam in first world countries is decreasing. A special caveat should be added. As dental care delivery increases in second and third world regions, dental amalgam is often the first tier strategy for treatment of dental caries. The actual amount of dental amalgam sold in the entire world actually continues to increase, despite the fact that its preference decreases.
[In the slide above, the alchemist’s symbols are shown for mercury (left) [CLICK] and amalgam (right). [CLICK] ]
 2006, Bayne and Thompson

2. Where is it? DENTAL AMALGAM
While they were used for virtually all operative sites about 30 years ago, there has been a strong shift away from anterior amalgams toward anterior esthetics. [CLICK] Amalgams are primarily used in posterior sites in the mouth. [CLICK] Amalgams continue to be used for Class I and II sites and for cores/foundations. However, if possible, tooth colored filling materials are generally preferred for most applications. If the Class I and II sites are relatively small, composites are often used as alternatives. For existing amalgam restorations, their margins may look a little harried, but that is generally not a reason for replacing them. In the image above there is actually an isthmus fracture as well. It may or may not be replaced for that reason.

3. AMALGAM TERMINOLOGY
AMALGAM = an alloy containing Hg as the major ingredient.
DENTAL AMALGAM = an alloy of Hg with Ag-Sn.
DENTAL AMALGAM ALLOY = a Ag-Sn alloy (to be mixed with Hg).
An “amalgam” means [CLICK] any material containing Hg as a major ingredient (or element) in the composition. [CLICK] A specific composition based on mixing Ag-Sn alloys with Hg is called a “dental amalgam.” [CLICK] The Ag-Sn alloys are referenced as the “dental amalgam alloy.”
The reaction is shown schematically above. [CLICK] Upon mixing the mercury begins to dissolve and react with the outer layers of the Ag-Sn particles (which are generally polycrystalline). About 15% of the Ag-Sn particles are consumed to produce complete reaction of the Hg and generate a matrix of solid reaction products. [The specific products will be addressed in a second.] The primary product is composed of Ag-Hg. The unreacted alloy particles remain embedded within the matrix and contribute mechanical reinforcement. In addition these dispersed particles have better corrosion resistance than the matrix.

4. ALLOY PRODUCTION Melting / Casting / Comminution  IRREGULAR Particles
“Cast ingots --> filed into powder”
Irregular particles = lathe cut = filings
Polycrystalline particles
Homogenized by HT to remove coring
Annealing HT to relieve cold work in filings
Melting / Spray Atomization  SPHERICAL PARTICLES
Alloy is produced predominantly as irregular-shaped particles, spherical particles, or mixture of the two types. [CLICK] Irregular particles are generally “lathe-cut,” meaning that a cast brick or ingot of alloy material is pulverized by filing it to pieces on a lathe. [CLICK] The pieces are polycrystalline and tend to have flat faces representing the sheared planes caused by fractures during filing. In most cases the ingots are cooled relatively quickly and require heat-treatment to remove coring. Because of the large amount of plastic deformation during filing, the particles require annealing to relieve the cold work. Both of these occur during the post-filing heat treatment.
[CLICK] Spherical particles are created by atomizing or spraying liquid dental amalgam alloy through a nozzle into a vertical column of cooled nitrogen gas. [CLICK] This solidifies the spheres and protects them from oxidation during the cooling steps. [CLICK] After the solid spheres are formed, they are generally acid washed and heat-treated to remove coring.
Hot alloy sprayed into cold air
Particles spherodize and solidify
Spheres are acid-washed
Generally spheres are HT

5. Hg / Alloy RATIOS
50:50
42:58
Originally, dental amalgams had relatively high Hg/alloy ratios. Since amalgam was generally mixed with an excess of mercury, it was crucial to remove as much as possible during actual manipulation and/or placement. During the 1800s, most of the excess mercury remained in the mix. It set slowly and was generally weaker due to that. During the early 1900s, the excess was removed by squeezing the mass with a squeeze cloth before placement. During the mid to late 1900s, the excess was removed by compressing the placed mass within the cavity preparation. [CLICK] This reduced the final Hg/alloy ratio well below 50:50 by weight. Compressing (condensing) extrudes excess matrix from the setting mass and improves both strength and corrosion resistance.
[CLICK] Most contemporary mixtures are now made with 45-50% initial mercury and end up with about 42% after the reaction and condensation steps are complete.

6. Powdered alloy + Hg  pre-capsulated  amalgamator
ALLOY MANIPULATION
Manual Trituration Procedures:
Alloy + Hg  mortar + pestle  manual mixing
Mechanical Trituration Procedures:
Powdered alloy + Hg  capsule + pestle  amalgamator
Pelleted alloy + Hg  capsule + pestle  amalgamator
A historical review of mixing dental amalgam is portrayed in the figure above. In early times, the powder particles of dental amalgam alloy and mercury were simply combined at the dentist’s discretion. However, by the late 1890s there was early scientific evidence that better amalgams resulted from proportioned materials. The small black bakelite balance shown in the upper left hand corner was in use in dentistry in the 1920s. The center of the rocker arm was adjusted to the desired Hg:alloy ratio. The arm contained a small groove allowing Hg to run from one end (left) to the other (right). Hg was proportioned on the left-hand rocker cup. Then alloy was added slowly to the right-hand cup. As soon as the proper ratio was achieved, it tipped the rocker arm and the Hg ran from the left to the right. This combination was then manually mixed in a mortar-and-pestle and carried to the cavity preparation for condensation.
While this process could be consistently accomplished, it was difficult for all operators to manually mix the mass in an equal manner. [CLICK] Therefore, in the late 1940s and early 1950s mixing (trituration) was accomplished using a mixing capsule with an internal steel pestle that was shaken back and forth in a figure-8 pattern to replace the action of the mortar and pestle. This was much more reproducible. To more easily load the capsule, the alloy powder was pressed into a pill under pressure. Then, it could easily be dispensed from a tube of pills. The Hg was proportioned as a spill (or droplet) that was calibrated to a compressed pill of alloy to preserve the Hg:alloy ratio recommended by the manufacturer.
In the late 1960s and early 1970s, [CLICK] pre-proportioned capsules containing alloy powder and Hg appeared. In the precapsulated package, the materials were separated by a thin foil barrier that was broken by twisting or pressing the ends of the capsule to allow them to come into contact just before trituration. The figure at the middle of the bottom of the frame above shows the large variety of designs available. While delivery of the materials in a disposable capsule was more expensive, it saved time, and tended to minimize the opportunity for Hg vapor to escape into the dental operatory.
Newer amalgamators have a cover over the jaws that grab the capsule and that produce the mixing motion. The cover insures that any spillage is prevented from become aerosolized in the region of the dentist and/or dental assistant. However, small amounts of Hg or mixture may still be spilled from the bottom of the mixing equipment.
Powdered alloy + Hg  pre-capsulated  amalgamator

7. CLASSIFICATION Application of system of nomenclature
Copper content = low copper, high copper
Particle size (and shape) = irregular, spherical
Number of particle types = 1 or 2
Zinc content = Zn-containing, Zn-free
Application of system of nomenclature
Alloy Copper Particle Number of Zinc
Name: Content: Shape: Particles: Content:
New True Dentalloy Low Irregular 1 Particle No
Velvalloy Low Irregular 1 Particle No
“Dispersalloy” High Both 2 Particles Yes
“Tytin” High Spherical 1 Particle No
Early dental amalgams ( ) contained less than 5% copper and historically then had limited corrosion resistance. In 1965, the first higher copper (high-copper, 12-30%) amalgams were commercialized, after it was discovered that they were much more corrosion resistant. A large part of the classification scheme for dental amalgams is related to the copper addition to the alloy powders.
[CLICK] The major classification is simply based on copper content, low copper versus high copper. Low copper dental amalgams were primarily made by filing particles from an ingot and were irregular-shaped. [CLICK] In the late 1950s and early 1960s, more and more spherical versions of the same alloys appeared on the market. Generally the texture of the setting amalgam was one of more crunchy feeling during condensation if irregular particles were involved. Spherical amalgams were lower viscosity. The very first high copper amalgams (Dispersalloy, Johnson & Johnson) actually added the copper as a Cu-Ag eutectic sphere to the Ag-Sn irregular particles. [CLICK] Other high-copper products (Tytin, Kerr) mixed the Ag-Sn-Cu in the same spherical particles. Regardless of how the elements are actually presented to the mixture, the same overall setting reaction takes place. Examples of two different low and high copper products are shown above. [CLICK] Finally, small amounts of Zn ( %) were traditionally added to the alloy to protect the mixture from oxidation during melting and casting of the ingots or spheres. It was discovered that if setting amalgams were prematurely contaminated by water from mixing or condensation, that it was possible for water to react with Zn producing ZnO and H2 gas. The hydrogen gas would increase expansion during setting and could contribute to tooth fracture. If the amalgam mixture was carefully manipulated this did not occur. In long term clinical trials, the presence of small amounts of Zn seems to contribute to ZnO production at cavity walls and margins and discourage corrosion, thereby increasing longevity.
[CLICK] Shown above are some examples of older and newer amalgam types as examples of the classification system.

8. Rx and MICROSTRUCTURE Ag Primary reactant
Sn Creates solubility, fluidity
Cu Reacts with Sn
Zn Alloy processing aid
Hg Reactant with Ag (and Sn)
Irregular
Alloy
Spherical
Individual compositions can produce quite complicated dental amalgam microstructures but basically the same important phases are present in all systems. [CLICK] As shown above on the left for cases of irregular and spherical alloys, the residual dental amalgam alloy powder particles are gray islands within the continuous matrix of reaction products that appear in white. The reaction products are about ~90% Ag-Hg and either ~10% Ag-Sn (low copper) or Cu-Sn (high-copper). The overall functions of each of the reactants is reported on the right-hand size of the slide. [CLICK] The SEM micrograph shows a high-resolution image of a spherical particle with a complex reaction zone surrounded by Ag-Hg grains in the matrix and some evidence of porosity from inadequate condensation.

9. SETTING REACTIONS Low-Copper Dental Amalgam:
I/S-Alloy + Hg  Residual Alloy + Matrix-1 + Matrix-2
Ag-Sn + Hg  Ag-Sn + Ag-Hg + Sn-Hg
Ag3Sn + Hg  Ag3Sn + Ag2Hg3 + Sn7-8Hg
g + Hg  g + g 1 + g 2
2-Particle High-Copper Dental Amalgam:
1-Particle High-Copper Dental Amalgam:
S-Alloy + Hg  Residual Alloy + Matrix-1 + Matrix-2
Ag3Sn/Cu+ Hg  Ag3Sn/Cu + Ag2Hg3 + CuSn + Cu3Sn
g + Hg  g + g 1 + e h
The specific reactions for both low-copper [CLICK] and [CLICK] high-copper amalgams are shown above in several ways. Examine the case for low-copper amalgam first. It can be represented [CLICK] (1) descriptively, [CLICK] (2) using the elements involved, [CLICK] (3) using the chemical formulas for the phases involved, and [CLICK] (4) using the Greek symbols for the phases.
In low-copper amalgams the alloy particles (Ag-Sn or Ag3Sn or ) react with Hg to form a matrix of Ag-Hg (Ag2Sn3 or 1) and Sn-Hg (Sn7-8Hg or 2) with substantial residual alloy. Learn these reactions. [CLICK] The Sn-Hg phase is the most corrosion prone phase and is the major culprit in amalgam deterioration.
Now examine an example of a high-copper amalgam. [CLICK] Note that the Sn-Hg phase is minimized or eliminated when excess copper preferentially reacts with the Sn forming Cu-Sn. Copper can be added to the amalgam by alloying it into the original phase or adding it as part of a second phase. The resulting copper reaction product phase can exist as Cu-Sn or Cu3Sn and may be abbreviated as  or , respectively. This new part of the matrix phase is much less corrosion prone but is still the weakest and most prone of all phases.
[CLICK] Yet, another method of accomplishing the same thing is to add copper by adding a Ag-Cu particle to a conventional dental amalgam composition. This is referred to as a 2-particle system. Overall, the same things occur.
I-Alloy + Hg  Residual Alloy + Matrix-1 + Matrix-2
g + Hg  g (Ag3Sn) + g 1 + (g 2)
S-Alloy + Hg  Residual Alloy + Matrix-1 + Matrix-2
Alloy + Hg  Alloy (Ag-Cu) + g 1 + e h

10. Penetrating Corrosion
Rx and MICROSTRUCTURE
+ Hg  + g g1 g2
As shown earlier, the reaction can be schematically described in terms of alloy particles mixed with mercury that produce a matrix of mercury reaction products surrounding residual dental amalgam alloy particles. Of special interest is the actual geometry of the phases that are involved. [CLICK] The Ag-Hg (1) crystals (grains) are relatively equiaxed as shown in the figure. [CLICK] Contrary to the geometry of these grains, Sn-Hg (2) crystals are elongated and look like tongue-depressor blades. [CLICK] This has a major consequence for corrosion. While the Sn-Hg crystals represent relatively little total volume of the mixtures, they tend to be touching (connected) throughout the entire dental amalgam (sort of like a pile of pick-up-sticks) allowing “penetrating corrosion” to take place. Shifting toward use of high-copper amalgam has both the advantage that Cu-Sn is less corrosion prone and the grains do not routinely touch each other, so that only “superficial corrosion” occurs.
Penetrating Corrosion

11. Rx and MICROSTRUCTURE + Hg  + g g1 g + spheres e + h 69 Ag 18 Sn
12 Cu
1 Zn
DISPERSALLOY
Let’s look at the reaction details now of a the first high-copper dental amalgam, Dispersalloy, that is still in common use today. Its overall chemical composition is shown to the left. [CLICK]
Dispersalloy is a typical 2-particle high-copper dental amalgam that is schematically summarized above. Ag-Sn-(Zn) irregular particles are mixed with Ag-Cu spheres to create the amalgam alloy powder. Mixed with Hg, Ag-Hg and Sn-Hg phases form. Almost as quickly as Sn-Hg forms, the phase is eliminated by the competitive reaction of Cu with Sn. As Cu3Sn or Cu-Sn forms, then the released Hg reacts with more Ag-Sn to produce predominantly Ag-Hg. Little or no Sn-Hg remains in the set amalgam as shown in the microstructural analysis above.
[CLICK] The microstructure is fairly complex because of the different phases. However, all high copper amalgams behave basically the same in the clinical environment, providing good corrosion resistance over long times.

12. PHASE DIAGRAMS Ag Hg g1 Ag3Sn + Hg >>> Ag2Hg3 = g1
g + b Ag Hg
Ag3Sn + Hg >>>
Ag2Hg3 = g1
(26Ag-74Hg)
The symbols for the amalgam reaction are derived from the phase abbreviations on phase diagrams.
Ag-Hg is a complex phase diagram. Most of the reaction product for the amalgam setting reaction is Ag-Hg around the stoichiometric composition for Ag2Hg3 which is 60 atomic percent Hg. [CLICK] By looking at the scale along the bottom of the phase diagram, one can see that 40Ag-60Hg in atomic percent corresponds to about 34Ag-66Hg in weight percent. [CLICK] At room temperature, on the phase diagram at 40Ag-60Hg occurs a narrow single phase region labeled gamma-1. Therefore, the matrix phase (which is almost exactly this composition) is often described as gamma-1 crystals. The actual composition of individual crystals may vary somewhat depending on the nucleation and growth conditions. g1

13. Overview of Manipulation
Placement and
Condensation
Carving
Burnishing
Polishing
TIME
Onset of
MIXING
WORKING
SETTING
End of
24 hours
The entire sequence for restoration with dental amalgam involves quite a few steps. [CLICK] You can monitor the process using a time line, [CLICK], examining the reaction progress [CLICK] and examining the manipulation steps.
[CLICK] Almost without exception, high copper dental amalgam is selected as the material of choice. Every product is available in a precapsulated form. [CLICK] After proper trituration in a mechanical amalgamator, [CLICK] a shiny pliable mass is removed from the capsule, [CLICK] temporarily placed in a small dish (dappen dish) to facilitate loading of an amalgam carrier to transport the material the cavity preparation. [CLICK] The plunger of the carrier pushes the fluid mass into the cavity. Quickly and efficiently the mass is compressed with an amalgam condenser (double-ended stainless steel hand instrument with a small and large nib on the ends). The large end is used initially (low stress = load / large area) followed by the smaller nib (high stress = load / small area). This compresses the particles of residual alloy together, expressing the unset Hg-rich matrix for removal from the restoration surface. This minimizes the final amount of matrix in the restoration and increases the strength and fatigue resistance. After slightly overfilling the cavity preparation, the surface is contoured with an amalgam carver. [CLICK] The blade of the carver is moved parallel to the margins to use the enamel surface as a guide and insure that the carved surface is confluent with the adjacent enamel. As this surface begins to harden (after just a few minutes), it is compressed with a ball burnisher to smoothen it. [CLICK] At this point, the amalgam is still relatively weak and unsuitable for final polishing. [CLICK] At the next recall or dental hygiene appointment, the amalgam is typically polished with a series of finer and finer abrasive polishing points and abrasives. This creates a very shiny surface, although this surface is no more effective in corrosion resistance that the burnished one. This has been demonstrated in numerous long-term clinical trials.
Selection / Proportioning / Amalgamation / Manipulation / Polishing

14. Amalgamators SPEED TIME ENERGY = Speed x Time
Now let’s consider some of the special features of the steps in amalgam mixing. Shown above are several mechanical amalgamators. On the left is an older amalgamator called the Caulk VariMix. On the right, is a rotational amalgamator from ESPE. Below on the right is a digital amalgamator from Caulk.
[CLICK] Each system includes a small vise to hold the precapsulated product. [CLICK] Each one also has a cover to protect against any leaked materials being aerosolized into the immediate breathing area of people.
Each equipment has options for setting the [CLICK] speed (rpm of the capsule throw, typically rpm) and [CLICK] time (seconds of mixing time, typically 8-10s). [CLICK] Increasing the speed or amount of time will increase the total mixing energy. Each amalgam product has a special combination of speed and time to produce good mixing without over-mixing. A well-mixed amalgam should be fluid, bright, and shiny. An over- or under-mixed mass will be dry in appearance and should be discarded.
ENERGY = Speed x Time

15. B A Setting amalgam Unset amalgam Excess Hg-rich matrix
Ag-Hg crystal
reaction products
Residual alloy
Excess Hg-rich matrix
eliminated by condensation.
The freshly mixed amalgam has some reaction products but is predominantly a mercury rich matrix. [CLICK] As described earlier, the condensation step forces the residual amalgam allow together, expressing the fluid unset Hg-rich matrix to the surface for removal. When setting does occur, the matrix will consist almost entirely of small gamma-1 (Ag-Hg, Ag2Hg3) crystals (as shown in the figure on the right). As setting starts to occur the texture of the mass changes to one of substantial resistance to condensation. [CLICK] You can follow the reaction by sensing the increase in pressure required to express any mercury rich matrix. As shown in the diagram, lathe-cut amalgams undergo much greater changes than spherical ones. Most clinicians prefer spherical dental amalgam alloys for this reason. Tytin (from Kerr) is probably the most widely used commercial spherical amalgam in dentistry.
Unset amalgam A

16. DIMENSIONAL CHANGES Dimensional changes on setting:
CONTRACTION during alloy dissolution
EXPANSION during impingement of reaction product crystals
(EXPANSION if side reactions due to H2O contamination)
EXP (+)
CONT (--)
TIME
ADA =
± 20 mm
The setting reaction involves a complex series of reactions occurring at different rates and producing different local volume changes in the mass. [CLICK] These can be followed as expansion/contraction versus time. [CLICK]
[CLICK] Dissolving the amalgam alloy into Hg and formation of Ag-Hg crystals leads to a slight decrease in volume (contraction). Most of this occurs during the mixing process. [CLICK] Growing crystals push on each other and create expansion. Most of this occurs during placement and condensation. [CLICK] Depending on the time of condensation and extent of reaction, the volume change may vary anywhere from -20 to +20 microns per cm over the setting reaction. Over long periods of time (weeks to a month) the reaction will slowly continue and can produce small amounts of solid state creep as well.
[CLICK] The actual dimensional changes associated with any amalgam are related to the particle size, Hg/alloy ratios, trituration times, and condensation pressures used.
Dimensional changes on depend on reaction variables:
Particle size, Hg/alloy ratio, trituration time, condensation, ...

17. AMALGAM PROPERTIES A. Introduction:
1. Specifications for Amalgam Properties
a. ADA / ANSI and ISO
b. Determination of “safety and efficacy”
2. Clinical Performance
a. Longevity = yrs ideally, 8-12 yrs practically
b. Modes of failure = caries, marginal fracture, bulk
UNITED
STATES
WORLD
ADA
FDI
ISO
ANSI
B. Properties:
1. Physical
2. Mechanical
3. Chemical
4. Biological
[CLICK] The properties of dental amalgam should be collected in the standard manner as physical, mechanical, chemical, and biological ones. Despite our general interest in all properties, there are a few which are particularly important -- because they help to establish the extent of reaction and minimum suitability of the material for clinical use. [CLICK] These select properties are used as “laboratory standards” for amalgam products. ADA/ANSI and ISO standards indicate specifications for acceptable products for “safety and efficacy.” On that basis, the ADA assigns is Seal for accepted products. [CLICK] Products can be sold without the ADA Seal. It is simply an acknowledgement that the materials meet the ADA standards. Many products have no ADA tests and could never acquire the ADA seal.
[CLICK] In the small figure in the lower right in the slide above, the general relationship of these special organizations is explained. The ADA is the national dental association for the United States. It, like all other organizations with standards, files its standards with the American National Standards Institute (sort of the registry for all US standards). The FDI (Federation Dentaire International) is the collection of all national dental associations. It files its standards with the ISO (International Standards Organization).
[CLICK] Standards do not guarantee the longevity of any restoration. Measured clinical performance is observed in controlled clinical trials and also in general dental practice. The latter results are often only about 40% as great (8-12y) as that for controlled clinical trials (20-25y). While secondary dental caries may be associated with amalgam restorations, primarily failure occurs by bulk fracture due to fatigue.
Now let’s consider the individual categories of properties of amalgam.

18. [Lustrous, shiny, white]
Physical Properties
1. Thermal conductivity =
2. Electrical conductivity =
3. Coefficient of thermal expansion =
4. Radiopacity =
5. Color =
[High]
[High]
25 ppm/ºC
[>2 mm Aluminum]
[Lustrous, shiny, white]
Generally amalgam restorations are considered esthetic restorations (although they are not tooth-colored). [CLICK] This is due to the shiny, lustrous surface that reflects the white color of neighboring teeth in the mirror-like finish.
[CLICK] All metallic restorations are greater in radiopacity (radio-density) than enamel or dentin. Since restorative materials are traditionally compared to the radiopacity of 2mm of aluminum (equivalent to enamel/dentin), the radiopacity is reported as >2mm Al.
[CLICK] Like all metals, dental amalgam is thermally and electrically conductive [CLICK] compared to the relatively insulating properties of enamel and dentin. As long as there is at least 2 mm of insulating restorative material and/or dentin below a restoration, there is very little chance of thermal insults to pulpal tissue during thermal transients in the mouth.
[CLICK] The most important problem for metallic restorations is that they have different coefficients of thermal expansion/contraction (amalgam = 25 ppm/C) compared to tooth structure (10 ppm/°C). During reductions in intraoral temperature, there is a strong tendency at the margins for amalgam restorations to contract away from tooth structure and allow marginal leakage of intraoral fluids (called percolation). Fluids are later expelled when the temperature returns to normal.

19. Mechanical Properties
Compressive Strength (psi) Tensile Strength (psi)
15-min 1-hr 24-hr 15-min 1-hr 24-hr
LOW COPPER:
Velvalloy 5,400 17,400 56, , ,000
Spheralloy 5,800 18,500 56, , ,800
HIGH COPPER:
Optalloy II 9,100 23,800 55, , , ,250
Dispersalloy 6,200 22,400 59, , ,990
Indiloy 4,600 26,300 64, , ,500
Sybraloy 23,800 50,000 72, , , ,600
Tytin 10,200 40,800 79, , ,300
Probably the most important characteristic of dental amalgam is its comparatively high strength compared to tooth structure. Dental enamel has a compressive strength of ~60,000 psi (i.e., ~400 MPa). Amalgam is as great or greater than that value. [CLICK] In the figure above, the compressive (CS) and tensile (TS) strengths of several low copper and high copper amalgams are reported. The TS values are much lower than the CS values indicating the low resistance to fracture.
[CLICK] During the setting reaction for amalgam, the strength does not increase quickly to near final levels. In many products, it takes several hours to get to 50-75% of the final strengths. Final strengths may not be realized for 30 days. Therefore, it is common to remind the patient to avoid creating any excessive stresses on the restoration for a day or so. [CLICK] Tytin, which is a spherical high copper dental amalgam, is famous for its high early strength and better than average ultimate tensile strength. For those reasons, it is used by 50-60% of all dentists.

20. Mechanical Properties
TYTIN (Kerr Dental Mfg) = “tie up the tin”
High-Copper, Spherical, 1 Particle, Zn-free
42% Hg mixed with alloy
Fast-setting
High early strength
Polished
Surface
Fracture
Shown above is the microstructure of TYTIN amalgam alloy powder. The name “Tytin” is derived from the companies play on words that the copper “ties up the tin.” The alloy particles are spherical and range in size from about 5 to 50 µm.
[CLICK] In the set amalgam, two faces are shown (polished and fractured). In the polished view, the residual alloy particles can be seen dispersed in a matrix of grey-colored Ag-Hg crystals that are weaker. In the fractured view, the fracture proceeds only through the matrix (going around all the alloy particles). This picture emphasizes the importance of condensing the setting amalgam as well as possible to minimize the matrix and make the path of any fracture as tortuous and slow as possible. Good condensation can almost double the fracture resistance of an amalgam.

21. Chemical Properties CHEMICAL CORROSION: AgS Sn-O-Cl Sn-O
ELECTROCHEMICAL CORROSION:
Galvanic corrosion
Local galvanic corrosion (structure selective)
Crevice corrosion (concentration cell)
Stress corrosion
Sn-O-Cl
Sn-O
Dental amalgam restorations undergo both chemical and electrochemical corrosion.
[CLICK] Chemical corrosion principally involves the reaction of Ag on the surfaces (Ag-Sn-Cu or Ag-Hg phases) with sulfur in the saliva from air pollution or food compounds. A very thin surface film develops that is very tight, adherent, and tough. While it is not very esthetic, it is not harmful either and does not cause long-term problems. Clinical trials of tarnished versus polished amalgams over 20 years indicate that there is no important clinical effect. Shown in the figure above is a tarnished Class V amalgam on the lingual surface of an incisor.
[CLICK] Electrochemical corrosion is much more destructive. Amalgams (with their multiple phases that are active compared to most of things) act as anodes -- which are susceptible to relative high corrosion rates. They may undergo galvanic corrosion (e.g., amalgam coupled to gold casting alloys), local galvanic corrosion (e.g., Ag-Hg versus Ag-Sn-Cu phase), crevice corrosion (e.g., amalgam margins or areas under plaque corrode), and stress corrosion (e.g., occlusal and proximal contact areas).

22. Biological Properties
Mercury Toxicity:
OSHA maximum TLV = 50 mg/m3 (vapor) per 40 hr work week.
Transient intraoral release (<35 mg/m3).
Mercury Hypersensitivity:
Low level allergic reaction.
Estimated to be < 1 / 100,000,000
Amalgam Tatoo:
Can occur during amalgam removal if no rubber dam.
Embedded amalgam particles corrode and locally discolor gum.
No known adverse reactions.
There are a number of possible biological reactions that may be caused by dental amalgam (as shown above).
First consider mercury toxicity and potential hypersensitivity. Hg is released in minute levels (by sublimation from the solid state) during chewing. However, the amounts are so low and for such short periods of time that the material represents no health hazard. What is released is assumed to be swallowed and passes harmlessly through the GI tract. The details of absoprtion will be discussed in a later presentation. A very small number of people (< 1 in 100,000,000 patients) may be allergic to Hg. The reactions are low level (not life threatening).
[CLICK] Amalgam tatoos are much less common today that in earlier times. During the removal of amalgam restorations, small particles are spewed away from the restored site. If there is inadequate isolation (e.g., rubber dam or cotton rolls), small pieces may become embedded into soft tissues and ultimately corrode. The light gray shadow that is generated is called an “amalgam tattoo.” This is very uncommon with today’s high-speed handpieces, good water cooling, and use of adequate isolation.

23. Clinical Performance Reasons for Failure:
Low-copper amalgam – marginal fracture and secondary caries.
High-copper amalgam – marginal fracture and bulk fracture.
Internal corrosion
Penetrating
Corrosion
Corrosion at margins
Superficial
Corrosion
Sn-O-Cl and Sn-O
Corrosion
Products
Jorgensen theory of mercuroscopic expansion (and marginal fracture)
The clinical performance of dental amalgam is closely aligned with its corrosion resistance. [CLICK] Low copper dental amalgams undergo continual “penetrating” electrochemical corrosion of the gamma-2 phase (Sn-Hg crystals that are long and needle-like) that undermine the entire microstructure of the restoration. This results in low strength and extensive marginal deterioration that usually allows secondary caries to occur. [CLICK] High copper dental amalgams only corrode “superficially.” Generally, they fail due to fatigue propagation of a crack that cause bulk fracture. [CLICK] Both low- and high-copper dental amalgams produce corrosion products of Sn-O (solid) and Sn-O-Cl crystals (soluble). Solid corrosion products help to seal the existing marginal crevices.
[CLICK] Over long terms there is a slow continuation of the amalgam reaction that produces expansion and causes creep. Since corrosion may be occurring at the same time, as Sn is consumed, local Hg becomes available that reacts with residual Ag-Sn particles and produce more Ag-Hg. This leads to further expansion (and creep) – and extrudes the amalgam margins above the normal anatomic contours allowing local stresses to fracture the margins. [CLICK] This creates an amalgam ditch. As corrosion occurs the general margin appearance may become more and more dramatic. This explanation is known as the Jorgensen theory of mercuroscopic expansion. [CLICK] If the expansion occurs in an area (e.g., a proximal or facial surface) where there is no natural wear or fracture occuring, then the amalgam appears to be pushing itself out of the preparation.

24. Clinical Evaluation ??? Hi-Cu Low-Cu Mahler scale: Hi-Cu, no Zn
Hi-Cu, +Zn
Low-Cu, +Zn
Low-Cu, no Zn
Many years ago, Dr. David Mahler developed a visual scale (1-11) using 5 photographs of progressively more deteriorated restoration margins (see the figure above from left-to-right) to rate low copper dental amalgams. For low-copper dental amalgams the restorations proceeded slowly from 1 to 5. [CLICK] In the range of a 3 to 5 score, the amalgam was often replaced, anticipating worsening margins and potential secondary caries. This was a great visual clue as to the clinical performance. [CLICK] Unfortunately, with high-copper dental amalgams, the margins rarely go beyond a rating of 3, since only superficial corrosion is involved. [CLICK] They should not be replaced. However, many clinicians still misinterpret the changes and pre-maturely replace them.
[CLICK] On average, high copper dental amalgam restorations last approximately 24 years. The percent survival data for 4 groups of amalgams (low and high copper dental amalgams with and without Zn) is shown in the figure. Amalgams with Zn in the composition appear to do a little bit better, presumably due to Zn forming ZnO and sealing the margins. As you can see, the high-copper group with zinc (e.g., Dispersalloy) is surviving at the 80% level at 14 years. The high-copper group without zinc (e.g., Tytin) is close behind.

25. THANK YOU
Dental amalgam was the first major restorative material and has survived as a part of dental practice for almost 180 years. However, concerns for the environment, esthetics, and bonding, have tilted the equation toward composite restorations. The relative use of amalgam has declined in the US and other first world countries. In the next lectures, we will look at those influences.
Thank you.