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Dental amalgam

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When that solubility is exceeded, crystal of two binary metallic compounds precipitate in to the mercury. ... amalgam can tarnish in the presence of sulpher, ... – PowerPoint PPT presentation

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Title: Dental amalgam


1
Dental amalgam
  • Dr. Waseem Bahjat Mushtaha
  • Specialized in prosthodontics

2
Terminology
  • Amalgam an alloy of mercury.
  • Amalgamation the process of mixing liquid
    mercury with one or more metals or alloys to form
    an amalgam.
  • Creep the time-dependent strain or deformation
    that is produced by a stress. The creep process
    can cause an amalgam restoration to extend out of
    the cavity preparation, thereby increasing its
    susceptibility to marginal breakdown.

3
  • Delayed expansion the gradual expansion of a
    zinc-containing amalgam over a period of weeks to
    months that is associated with hydrogen gas
    development caused by contamination of the
    plastic mass with moisture during its
    manipulation in a cavity preparation.
  • Dental amalgam an alloys of mercury, silver,
    copper, tin, which may also contain palladium,
    zinc, and other elements to improve handling
    characteristics and clinical performance.

4
  • Dental amalgam alloy an alloy of silver,
    copper, tin and other elements that is formulated
    and processed in the form of powder particles or
    as a compressed pellet.
  • Trituration the process of grinding powder,
    especially within a liquid. In dentistry, the
    term is used to describe the process of mixing
    the amalgam alloy particles with mercury in an
    amalgamator.

5
Trituration
6
Alloys composition
  • American dental associated (ADA) specification
    No. 1 requires that amalgam alloys be
    predominantly silver and tin. Unspecified amount
    of other elements, such as copper, zinc, gold,
    and mercury, are allowed in concentrations less
    than the silver or tin content. Alloys containing
    zinc in excess of 0.01 or less of zinc are
    designated as nonzinc.

7
Metallurgic phase in dental amalgam
  • The setting reactions of alloys for dental
    amalgam with mercury are usually described by the
    metallurgic phases that are involved. These phase
    are named with Greek letters that correspond with
    the symbols found in phase diagram for each alloy
    system

8
Symbols and stoichiometry of phases that are
involved in the setting of dental amalgam
  • Phases in amalgam stoichiometric formula
  • Alloys and set dental
  • Amalgam.
  • ? Ag3Sn
  • ?1 Ag2Hg3
  • ?2 Sn7-8Hg
  • e Cu3Sn
  • ? Cu6Sn5
  • Silver-copper eutectic Ag-Cu
  • The Greek letter named as follows
    ?(gamma)e(epsilon)? (eta).

9
Manufacture of alloys powder
  • Lathe-cut powder to make lathe-cut powder, an
    annealed ingot is placed in a milling machine or
    in lathe and is fed in to cutting tool or bit.
    The chips removed are often needlelike.
    (irregular particles).

10
  • Homogenizing Anneal because of the rapid cooling
    conditions form the as-cast state, an ingot of an
    Ag-Sn alloy has a cord structure and contains
    nonhomogeneous grains of vary composition. A
    homogenizing heat treatment is performed to
    re-establish the equilibrium phase relationship.
    The ingot is placed in an oven and heated at a
    temperature below the solidus for sufficient time
    to allow diffusion of the atoms to occur and the
    phases to reach equilibrium.

11
  • Particle treatments once the alloy ingot has
    been reduced to cuttings, many manufactures
    performed some type of surface treatment of the
    particles. Treatment of the alloys particles with
    acid has been a manufacturing practice for many
    years. The exact function of this treatment is
    not entirely understood, but it is probably
    related to the preferential dissolution of
    specific components from the alloy. Amalgams made
    from acid-washed powder tend to be more reactive
    than those made from unwashed powder.

12
  • Atomized powder atomized powder is made by
    melting together the desired elements. The liquid
    metal is atomized into fine spherical droplets of
    metal. (spherical powder).
  • Particle size maximum particle size and the
    distribution of sizes within an alloy powder are
    controlled by the manufacturer. The average
    particle sizes of modern powders range between 15
    and 35 µm. The most significant influence on
    amalgam properties is the distribution of sizes
    around the average. For example very small
    particles (lt 3 µm) greatly increase surface area
    per unit volume of the powder. A powder
    containing tiny particles requires a greater
    amount of mercury to form an acceptable amalgam.

13
  • Lathe-cut compared with atomized alloys
  • Amalgams made from lathe-cut powders, or admix
    powder of blend of lathe-cut and spherical
    powders, tend to resist condensation better than
    amalgams made entirely from spherical powder.
  • Spherical alloys require less mercury than
    typical lathe-cut alloys because spherical alloys
    have a smaller surface area per volume than do
    the lathe-cut alloys. Amalgam with a low mercury
    content generally have better properties.

14
Amalgamation and resulting structure
  • Low-copper alloys
  • Amalgamation occurs when the mercury comes into
    contact with the surface of the Ag-Sn alloy
    particles. When the powder is triturated, the
    silver and tin in the outer portion of the
    particles dissolve in to mercury. At the same
    time, mercury diffuse into alloy particles. The
    mercury has a limited solubility for silver
    (0.035 wt) and tin (0.6 wt).

15
  • When that solubility is exceeded, crystal of two
    binary metallic compounds precipitate in to the
    mercury. These are the body-centered cubic Ag2Hg3
    compound (the ? phase) and the hexagonal closed
    packed Sn7-8Hg compound (the ?2 phase). Because
    the solubility of silver in mercury is much lower
    than of tin, the ?1 phase precipitates first, and
    the ?2 phase precipitates later. Immediately
    after trituration, the alloy powder coexists with
    the liquid mercury, giving the mix a plastic
    consistency. As the remaining mercury disappears,
    the amalgam hardens.

16
  • As the particles become covered with newly formed
    crystals, mostly ?1, the reaction rate decreases.
    The alloy is usually mixed with mercury in
    approximately a 11 ratio. This is insufficient
    mercury to completely consume original alloy
    particles consequently, unconsumed particles are
    present in the set amalgam. Alloys particles (
    smaller now, because their surfaces have
    dissolved in mercury) are surrounded and bound
    together by solid ?1 and ?2 crystals.

17
  • Thus, atypical low-copper amalgam is a composite
    in which the unconsumed particles are embedded in
    ?1 and ?2 phases. The reaction can be
    conveniently expressed in terms of the phases
    that form during amalgamation
  • Alloy particles (ß ?) Hg ?1 ?2
    unconsumed alloy particles (ß ?) .
  • The physical properties of the hardened amalgam
    depend on the relative percentages of each of the
    microstructural phases. The unconsumed Ag- Sn
    particles have a strong effect. The more of this
    phase that is retained in the final structure,
    the stronger is the amalgam. The weakest
    component is the ? 2 phase. The hardness of ?2 is
    approximately 10 of the hardness of ?1.
  • ?2 phase is the least stable in a corrosive
    environment and experience corrosion attack,
    especially in cervices of the restorations.
    Pure ?1 phases are stable in an oral environment.
    However, ?1 in amalgam does contain small amounts
    of tin, which can be lost in a corrosive
    environment.

18
  • High copper alloys
  • High copper alloys have become the material the
    materials of choice because of their improved
    mechanical properties, corrosion characteristics,
    and better marginal integrity and performance in
    clinical trials, as compared with traditional
    low-copper alloys. Two different types of
    high-copper alloy powders are available
  • 1) Admix alloy powder.
  • 2) Single composition alloy powder.
  • Both types contain more than 6 wt copper.

19
1) Admix alloy powder
  • In 1963, Innes and Youdelis added apherical
    silver-copper (Ag-Cu) eutectic alloy (71.9 wt
    silver and 28.1 wt copper) particles to
    lathe-cut low-copper amalgam alloy particles.
    This was the first major change in the
    composition of alloy for dental amalgam since
    Blacks work. Theses alloys are often termed
    admix alloys because the final powder is a
    mixture of at least two kinds of particles. An
    admix powder, showing lathe-cut low-copper alloy
    particles and spherical Ag-Cu alloy particles.

20
  • Amalgam made from these powder is stronger than
    amalgam made from lathe-cut low-copper (composite
    materials materials that consist of a matrix and
    filler can be strengthened by the addition of
    strong fillers) and the Ag-Cu particles probably
    act as strong fillers, strengthening the amalgam
    matrix.
  • Several classic studies have shown that
    restorations made with this prototype admixed
    amalgam were clinically superior to low-copper
    amalgam restorations when they were evaluated for
    resistance to marginal improved clinical
    performance.

21
  • Admix alloy powders usually contain 30 wt to 55
    wt spherical high-copper powder. The total
    copper content in admixed alloys ranges from
    approximately 9 wt to 20 wt. The phases present
    in the copper-containing particles depend on
    their composition. The Ag-Cu alloy consists of
    mixtures of two phases-silver rich and copper
    rich-with the crystal structures of pure silver
    and pure copper, respectively. Each phase
    contains a small amount of the other element. In
    the atomized powder (which is fast cooled), the
    eutectic two-phase mixture from very fine
    lamellae. Compositions on either side of the
    eutectic from relatively large drains of
    copper-rich phase or silver-rich phase amide the
    eutectic mixture.

22
  • When the mercury reacts with an admixed powder,
    silver dissolves into the mercury from Ag-Cu
    alloy particles, and both silver and tin dissolve
    into the mercury from Ag- Sn alloy particles. The
    tin in solution diffuses to the surfaces of the
    Ag-Cu alloy particles and reacts with the copper
    phase to form the ? phase (Cu6Sn5). A layer of ?
    crystals form around unconsumed Ag-Cu alloy
    particles. The ? layer on Ag-Cu alloy particles
    also contains some ?1 crystals. the ?1 phase form
    simultaneously with the ? phase and surrounds
    both the ? covered Ag-Cu alloy particles and the
    Ag- Sn alloy particles. As in the low-copper
    amalgams, ?1 is the matrix phase, that is, the
    phase that binds the unconsumed alloy particles
    together

23
  • The reaction of the admixed alloy powder with
    mercury can be summarized as follows
  • Alloy particles (ß ?) Ag-Cu eutectic Hg
  • ?1 ? unconsumed alloy of both types of
    particles.
  • N.B ?2 has been eliminated in this reaction.

24
Single composition alloys
  • Unlike admixed alloy powders, each particle of
    these alloy powders has the same chemical
    composition. Therefore, they are called
    single-composition alloys. The major components
    of the particles are usually silver, copper, and
    tin. The first alloy of this type contained 60
    wt silver, 27 wt tin, and 13 wt copper. The
    copper content in various single composition
    alloys rang from 13 wt to 30 wt. In addition,
    small amounts of indium or palladium are also
    found in some of the currently marked
    single-composition alloys.

25
  • A number of phases are found in each
    single-composition alloy particle, including ß
    (Ag- Sn) ?(Ag3Sn), and e (Cu3Sn). Some of the
    alloys may also contain some ? phase (Cu6Sn5).
  • When triturated with mercury, silver and tin from
    the Ag-Sn phases dissolve in mercury, little
    copper dissolves in mercury. The ?1 crystals
    grow, forming a matrix that bind together the
    partially dissolved alloy particles. the ?
    crystals are found as meshes of rod crystals at
    the surface of alloy particles, as well as,
    dispersed in the matrix. Theses are much larger
    than the ? crystals found in the reaction layers
    surrounding Ag-Cu particles in admix amalgams.

26
  • To summarized the reaction of the
    single-composition alloy powder with mercury is
    as follows
  • Ag-Sn-Cu alloy particles Hg
  • ?1 ? unconsumed alloy particles.

27
Manipulation
  • 1) proportioning
  • 2) Trituration
  • 3) Condensation
  • 4) Trimming and carving
  • 5) Polishing
  • 6) Some precautions

28
Proportioning
  • a) Mercury the required quantity can be
    obtained by weighing or by using a volume
    dispenser. Clearly the latter method is quicker.
    It is important to use pure clean mercury.

29
  • b) Alloy this can be proportional by
  • 1) Weighing
  • 2) Using tables of alloy, particularly with
    mechanical mixing.
  • 3) Having envelopes with pr-weighed quantities.
  • 4) Using a volume dispenser.
  • Two disadvantages of a volume dispenser are
  • 1) It is difficult to measure any powder
    accurately by volume, as the weight of material
    per volume depends on the efficiency with which
    the particles are packed together.
  • 2) Alloy can cling to the walls of the dispenser.

30
  • c) Alloy/mercury ratio. In the final set amalgam
    it is desirable to have less than 50 mercury .
  • Two techniques have been recommended
  • 1) The use of an alloy/mercury ratio of 5/7 or
    5/8. the excess mercury makes the trituration
    easier, giving a smooth plastic mix of material.
    Before insertion into the cavity, excess mercury
    is removed from the mix by squeezing it in a
    dental napkin.
  • 2) Minimal mercury techniques, where about equal
    weights of alloy and mercury are used and no
    mercury is squeezed out of the mix before
    condensation .
  • d) Many materials are supplied in capsules with
    per-proportioned alloy and mercury.

31
Trituration
  • a) Hand mixing by mortar and pestle. A glass
    mortar and pestle are used. The mortar has its
    inner surface roughened to increase the friction
    between the amalgam and the surface. A rough
    surface can be maintained by occasionally
    grinding with carborandium paste. The pestle is a
    glass rod with a rounded end.

32
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33
  • b) Mechanical mixing the proportioned alloy and
    mercury can be mixed mechanically in a capsule,
    either with or without a stainless steel or
    plastic pestle. A pestle, which should be of
    considerably smaller diameter than the capsule,
    should be used with tablet alloys, to help break
    up the material. The mechanical amalgamators have
    time switches to ensure a correct mixing time. A
    number of these materials are available in an
    encapsulated form, each capsule containing a
    controlled weight of alloy, and having the right
    quantity of mercury sealed in its lid. The choice
    of trituration time is important, and will depend
    both on the type of alloy and the speed of mixer.
    In particular rich copper alloys require precise
    control of trituration conditions. Some products
    require high energy mixing to break up the oxide
    coating which forms on copper rich particles.

34
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35
Condensation
  • 1) Each portion is properly adapted by a
    condenser of suitable size.
  • 2) A load of up to 4-5 kg is applied to each
    increment.
  • 3) As the mix is condensed, some mercury- rich
    material rises to the surface. Some of this can
    be removed, to reduce the final mercury content,
    and improve the mechanical properties. The
    remainder will assist bonding with the next
    increment, to avoid the production of a weak
    laminated restoration

36
  • A material should be condensed as soon as
    possible after mixing. If it is left too long,
    and has begun to set
  • 1) Proper adaptation to the cavity will be
    impossible
  • 2) Elimination of excess mercury will be
    difficult
  • 3) Bonding between increments will be poor
  • 4) Lower strength values will result.

37
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38
Trimming and curving
  • When the cavity is overfilled, the top
    mercury-rich layer can be trimmed away and the
    filling carved to the correct contours. The
    amalgam prepared from a coarser grain alloy may
    be more difficult to carve, as the instrument may
    pull out large pieces of alloy from the surface.
    Spherical alloys are used where ease of carving
    is desired.

39
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40
Polishing
  • Conventional amalgams are polished not less than
    24 hours after insertion that is, when the
    material has gianed considerable strength. Since
    amalgams from rich copper alloys gain strength
    rapidly .

41
Some precautions
  • a) Mercury is toxic, so free mercury should not
    be allowed to enter the atmosphere. This hazard
    can arise during trituration, and condensation
    and finishing of restorations, and also during
    the removal of old restorations at high speed.
  • b) Skin contact with mercury should be avoided,
    as it can be absorbed by the skin.
  • c) Any excess mercury should not be allowed to
    get into sinks, as it can react with some of the
    alloys used in plumbing.
  • d) Contamination of the amalgam by moisture must
    be avoided.

42
Properties
  • 1) Toxicity
  • 2) Corrosion reactions
  • 3) Marginal leakage
  • 4) Strength
  • 5) Marginal failure
  • 6) Thermal diffusivity
  • 7) Dimensional changes

43
Toxicity
  • a) The wisdom of using a restorative material
    containing mercury has often been questioned.
  • b) The potential danger of any form of mercury is
    related to
  • 1- The form in witch the mercury is present
  • 2- The quantity and frequency of exposure
  • c) There is no evidence of harmful effect of the
    amalgam.

44
Corrosion reaction
  • a) Tarnish amalgam can tarnish in the presence
    of sulpher, to give layer of sulphides on the
    surface of restoration.
  • b) Corrosion of conventional amalgams the set
    material is heterogeneous stimulate corrosion. Of
    the three phases present, the ?2 is the most
    active electrochemically, being anodic in
    relation to both the ? and ?1 phases.

45
  • As ?2 corrodes, essentially two products result
  • 1) Ionic tin produced in the presence of saliva,
    corrosion products such as SnO2 and Sn(OH)6Cl are
    found.
  • 2) Hg is produced, which can react with some of
    the remaining hitherto unreacted ? phase.
  • c) Corrosion of rich copper amalgam
  • 1) No ?2 is present.
  • 2) However, the corrosion currents associated
    with those with conventional amalgam.

46
  • 3) The volume of corrosion products is less than
    with conventional amalgam.
  • 4) No mercury is produced as a result of the
    corrosion.
  • d) Practical considerations
  • 1) Corrosion resistance is greatly improved if
    the amalgam is polished. This process removes
    pits and voids on the surface, which aid
    concentration cell corrosion

47
  • 2) If amalgam comes in contact with a gold
    restoration, an electrolytic cell may be set up
    leading to corrosion of the amalgam and
    incorporation of mercury on the gold restoration.
  • 3) Corrosion of conventional amalgam can have a
    significant effect on long-term mechanical
    properties. It has been shown, fore example, the
    tensile strength is reduced by 30 when the
    network of ?2 has corroded.

48
Marginal leakage
  • The initial marginal leakage of an amalgam
    restoration, reduces with time, because of
    sealing of the micro-fissures by products of
    corrosion breakdown.

49
Strength
  • The following factors can lead to the production
    of a weak restoration
  • 1) undertrituration
  • 2) Too high a mercury content
  • 3) Too low condensation pressure
  • 4) Slow rate of packing
  • 5) Corrosion

50
  • The rate of development of strength of an amalgam
    is of importance. With amalgams which develop
    strength slowly, there is danger of early
    fracture of such a restoration. Generally,
    spherical and rich copper amalgams have high
    early strengths. Of the phases present in
    conventional amalgams, the ?2 is the weakest and
    softest.

51
Marginal failure
  • Ditching of the margins of amalgam is a common
    occurrence. Clinical trials have shown that
    higher copper alloy formulations show much less
    marginal breakdown than conventional materials.
    The following observations are relevant
  • a) Poor technique can cause breakdown, e.g. an
    unsupported ledge of amalgam extending over the
    enamel may fracture during mastication

52
  • b) A theory has been propounded which links
    marginal breakdown with corrosion
    characteristics. It has been suggested that the
    mercury corrosion product (from conventional
    materials) reacts to form more ?1 and ?2
    material, with associated expansion, termed
    mercuroscopic expansion. The expanded material,
    weakened by corrosion, protrudes away from the
    supporting tooth structure, and fractures.

53
Thermal diffusivity
  • Dental amalgam is a conductor of heat, whereas
    the enamel and dentin it replaces is a thermal
    insulator. Consequently, large amalgam
    restorations are usually lined with a thermal
    insulating cement to product the pulp from
    temperature changes in the mouth caused by hot
    and cold foods and liquids.

54
Dimensional changes
  • Ideally there should be little or no contraction
    on setting of a dental amalgam, otherwise a gap
    between filling and cavity walls may result,
    enhancing the possibility of further decay. Too
    great an expansion should also be avoided, as
    this will cause the filling to protrude from the
    cavity.

55
  • In laboratory experiments, where free expansion
    is measured, it has been shown that a greater
    expansion on setting will result if
  • 1) A higher alloy/mercury ratio is used.
  • 2)There is a shorter trituration time
  • 3) Lower pressure during condensation is used
  • 4)The alloy has a larger particle size
  • 5) There is contamination by water before setting
    in zinc-containing materials.

56
  • An electrolytic reaction between zinc (the anode)
    and the other metals which are cathodic and the
    water as an electrolyte.
  • Hydrogen is evolved as a result of this reaction.
  • The pressure of the evolved hydrogen may cause
    the amalgam to flow.
  • This causes an expansion, which may not appear
    within the first 24 hours, but may become
    evidence some days after insertion of the
    restoration.
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