Dent 633 Dental Amalgam

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Dent 633Dental Amalgam

• William A. Brantley (PhD)
• Professor and Director
• Graduate Program in Dental Materials Science
• Section of Restorative and Prosthetic Dentistry
• College of Dentistry, The Ohio State University
• Postle Hall Room 3005-L
• E-mail brantley.1_at_osu.edu

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Textbook References

• OBrien WJ (editor). Dental Materials and Their
  Selection (3rd ed). Chicago Quintessence, 2002.
  Chapter 12.
• Powers JM, Sakaguchi RL (editors).
• Craigs Restorative Dental Materials (12th ed).
• Mosby, 2006. Chapter 11.
• Anusavice, KJ (editor). Phillips Science of
  Dental Materials (11th ed). Saunders/Elsevier,
  2003. Chapter 17.

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Advantages of Dental Amalgam as Restorative
Material

• Relatively inexpensive compared to gold alloy
• Easily prepared direct restorative material
• Margin-sealing capability (decreased marginal
  microleakage with time) corrosion products
• Over 100 years of successful clinical history
  (dating from GV Black dental amalgam)

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Concerns about Dental Amalgam as Restorative
Material

• Poor esthetics compared to resin composites
• Weakening of tooth from removal of tooth
  structure
• Recurrent caries
• No adhesive bonding unless bonded restoration
• Sensitivity of properties to manipulation
• Brittle nature of material
• Biocompatibility not generally considered
  problem for patients (supported by recent JAMA
  article)
• Wastewater pollution with mercury

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General Setting Reaction for Dental Amalgam

• Alloy (for dental amalgam) Hg ? Dental amalgam
  Components in two compartments of capsule
• Mercury/alloy ratio - approximately 0.5 and
  depends upon particular commercial product
• Modern precapsulated products contain
  approximately 42 to 45 Hg by weight
• Factors for setting process composition, shape
  and size of alloy particles (based on handling
  characteristics desired by manufacturer)

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Alloy for Dental Amalgam(Particles Mixed with
Mercury)

• ANSI/ADA specification no. 1 does not require
  specific percentages of elements
• Major element is Ag, Sn has second-largest
  amount, Cu about 2 to nearly 30, Zn from 0 to
  about 1
• Other elements if manufacturer submits results of
  clinical and biological testing (e.g., In and Pd)
• Particles have complex structure with three
  phases ? (Ag3Sn), ß (Ag-Sn) and e (Cu3Sn)

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Classification of Products by Particle Shape and
Composition

• Filing or lathe-cut (machined from cast ingot)
• Spherical (molten alloy blown through nozzle)
• All particles with same composition
• Blend or admixture of particles with different
  compositions
• Spherical particles range from 50 µm diameter to
  over order of magnitude smaller also wide range
  in sizes of lathe-cut particles
• Intentionally done for optimum condensation

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Importance of Particle ShapeSpherical vs.
Lathe-Cut Products

• Spherical particles are wetted with lower
  mercuryalloy ratio than lathe-cut particles
• Spherical particles resist forces of condensation
  less than lathe-cut particles

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Microstructures of Lathe-Cut (?100), Spherical
(?300), and Admixed (?500) Alloys for Dental
AmalgamFigures from Anusavice (11th ed), Chapter
17
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Classification of Products by Alloy Composition

• High-copper vs. low-copper high-copper products
  contain gt12 Cu in alloy particles
• High-copper products should be selected greater
  clinical longevity of restorations and much lower
  creep values measured in laboratory
• Zinc-containing vs. zinc-free (lt 0.01 wt Zn)
  not economically feasible to eliminate Zn
• Zinc considered to facilitate machining lathe-cut
  particles (more brittle) and improves corrosion
  resistance of amalgam, but less plastic mix
• No concern with Zn-free alloys about moisture
  contamination during trituration or condensation

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Frequent Letter Codes for Dental Amalgam Products
in Books and Articles

• LCL, LCS (low-copper alloy, lathe-cut or
  spherical particles)
• HCSS (high-copper alloy, spherical particles of
  single composition)
• HCB (high-copper alloy, blend of two different
  types of particles shape and/or composition)

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Composition Details for Two ProductsValiant PhD
() formerly used in CollegePermite C ()
currently used in College
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Heat Treatment of Alloy for Dental Amalgam by
Manufacturer

• Eliminates compositional nonuniformity that
  exists in ingot before lathe-cutting (machining)
  or in spherical alloy particles due to rapid
  freezing
• Relieves stresses in alloy particles (both
  lathe-cut and spherical)
• Provide manufacturer control of setting time
  great clinical importance

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General Form of Setting Reaction

• ? (starting alloy particles) Hg (liquid) ?
  reaction phases (matrix) unreacted alloy
  particles (core)
• Incompletely consumed alloy particles in set
  dental amalgam microstructure
• Bricks (alloy particles) and mortar (reaction
  phases) analogy for structure and strength of set
  amalgam
• No free mercury after setting reaction Hg found
  in reaction phases
• Microstructure will contain some porosity from
  incomplete condensation

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Setting Reaction ProductsThree Types of Dental
Amalgams

• Low-copper dental amalgams ?1 (Ag2Hg3) and ?2
  (Sn8Hg)
• High-copper dental amalgams ?1 and ?? (Cu6Sn5)
• Note that high-copper dental amalgams are ?2-free

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Setting Reaction for Dispersalloy-Type Dental
Amalgams

• Alloy particles are admixture or blend of
  low-copper lathe-cut particles and spherical
  Ag-Cu particles (72 wt Ag, 28 wt Cu)
• First step of setting reaction identical to
  low-copper dental amalgams
• Second step of setting reaction is disappearance
  of ?2 phase and formation of ?? phase
• Slower setting reaction than for HCSS products

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Dimensional Changes During Setting of Dental
Amalgams

• Total dimensional change after 24 hr lt 20 µm/cm
  (0.20) is ANSI/ADA Specification no. 1 limit
  and cannot detect by unaided eye or explorer
• Most modern dental amalgam products undergo an
  overall contraction of the setting mass
• Clinical problems would occur with excessive
  setting expansion (loss of anatomy and
  postoperative pain) or excessive setting
  contraction (microleakage)

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Setting Dimensional ChangesA HCB, B HCSS and
C LCL From Anusavice (11th ed), Fig. 17-10
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Nature of Setting Dimensional Changes

• Setting process is combination of solution and
  crystallization (precipitation)
• Initial contraction from absorption of Hg
  (diffusion) by amalgam alloy particles
• Subsequent formation and growth of ?1, ?2 and
  Cu-Sn phases (matrix)
• Final absorption of mercury by remaining amalgam
  alloy particles
• No free mercury in final set dental amalgam

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Microstructure of LCL Dental Amalgam Original
Magnification ?1000From Anusavice (11th ed),
Fig. 17-5
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Microstructure of HCB Dental Amalgam Original
Magnification ?1000Mercury-rich droplets from
polishing specimen From Anusavice (11th ed),
Fig. 17-7
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Microstructure of HCSS Dental Amalgam Relief
polish with original magnification ?560 From
Anusavice (11th ed), Fig. 17-8
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Characteristics of Microstructural Phases in
Dental Amalgams

• Strongest phase incompletely consumed starting
  alloy particles (?)
• Weakest phase ?2 in low-copper amalgams (most
  corrosion prone)
• Completely interconnected nature of ?2 can result
  in bulk corrosion of low-copper dental amalgam
• High-copper amalgams Cu6Sn5 (??) is corroding
  phase that provide margin-sealing because ??
  is not interconnected, corrosion limited to
  marginal regions without bulk corrosion

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Types of Corrosion in Dental Amalgams

• Galvanic corrosion at interproximal contacts with
  gold alloys
• Electrochemical corrosion because multiple phases
• Crevice corrosion at margins
• At unpolished scratches or secondary anatomy
  lower pH and oxygen concentration of saliva
• Corrosion under retained plaque because of lower
  oxygen concentration
• Chemical corrosion from reaction with sulfide
  ions at occlusal surface

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Corrosion of Dental Amalgam Restorations

• Limited corrosion is beneficial because reduction
  in microleakage ?2 in low-copper amalgams and
  Cu6Sn5 (??) in high-copper amalgams
• Tin-containing and copper-containing phases have
  been identified as corrosion products
• Corrosion minimized by polishing amalgam
  restoration scratches and pits trap debris,
  enhancing corrosion because lower oxygen
  concentration under deposit
• Clinical trials suggest that Zn-containing
  amalgam restorations have superior marginal
  integrity and longevity preferential Zn
  corrosion may occur

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Mechanical Properties of Dental Amalgams

• Brittle material for normal rates of loading (CS
  TS) and need dentinal support to resist forces
  of mastication
• Poor edge strength fracture of ledge on poorly
  finished restoration readily occurs (low tensile
  strength leads to fracture in bending)
• Insufficient strength of set dental amalgam would
  also increase amount of marginal breakdown

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Change in Mechanical Properties of Dental
Amalgams with Time

• ANSI/ADA specification no. 1 requires specific
  compressive strength after 1 hr has practical
  significance
• Rate of strength increase is dependent upon
  particular product ? HCSS has most rapid setting
  reaction
• Much greater difference in strength for wide
  range of products after 1 hr compared to 1 day
• Final strength considered after 1 week nearly
  same strength after 1 day
• Mechanical properties measured in laboratory are
  dependent upon rate of loading creep at
  constant load)

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Typical Strength for Dental Amalgams (24 hr)

• Compressive strength gt about 350 MPa
• Tensile strength lt about 70 MPa
• High ratio for CS divided by TS ? brittle material

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Laboratory Creep Test forDental Amalgams

• Cylindrical specimen stored 1 wk (37º C),
  compress at 36 MPa (37º C), measure length change
  for 1 - 4 hr
• Maximum creep limit in ANSI/ADA specification no.
  1
• High-copper amalgams generally have low creep
  (lt1)
• Creep is only mechanical property correlated with
  clinical marginal fracture of low-copper amalgam
  restorations (not high-copper which have low
  creep)
• Creep mechanism is grain boundary sliding of ?1
  phase (blocked by ?? in high-copper amalgams)

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Values of Strength and CreepRepresentative
Dental Amalgams Anusavice (11th ed), Table 17-2
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Effects of Manipulative VariablesSetting
Expansion of Dental Amalgams(Increased with more
setting reaction phases)

• Excessive mercury content increases SE
• Increased tritutation time decreases SE
• Increased condensation pressure decreases SE
• Moisture contamination of Zn-containing amalgam
  causes delayed, excessive increase in SE ? reason
  why Zn-free products often selected

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Delayed Expansion of Moisture-Contaminated
Zinc-Containing Amalgam From Anusavice (11th
ed), Fig. 17-11
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Effects of Manipulative VariablesStrength of
Dental Amalgams(Increased with less setting
reaction phases)

• Excessive mercury content decreases strength
• Increased tritutation time increases strength
• Increased condensation pressure increases
  strength
• Moisture contamination of Zn-containing dental
  amalgam large decrease in strength
• Zinc reduction of H2O releases H2 gas, causing
  excessive delayed expansion and possible
  postoperative pain from pulpal pressure

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Effect of Mercury/Alloy RatioStrength of Dental
Amalgams From Anusavice (11th ed), Fig. 17-11
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Some Clinical Considerations for Trituration

• Role of trituration - coat each alloy particle
  with mercury
• Overtrituration makes mixed material hot, reduces
  working time, and increases creep
• Optimum trituration time is highly important
• Also important to avoid undertrituration

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Roles of Condensation

• Adapt restoration to cavity walls
• Minimize porosity in restoration
• Control final mercury content of restoration
• Do not delay condensation after trituration

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Mercury and Mercury Toxicity

• Mercury is liquid metal (temperatures greater
  than 39C) with high density (13.6 gm/cm3) and
  high vapor pressure that rapidly increases with
  temperature
• Because of mercury toxicity, US government has
  set threshold limit value (TLV) for sustained (40
  hr/wk) exposure at 0.05 mg Hg/m3
• Routes for mercury exposure - skin contact,
  inhalation of vapor, airborne droplets
• At level of 100 ng Hg per mL blood, symptoms of
  mercury poisoning are typically observed
• Some patients may exhibit an allergic skin
  reaction to dental amalgams

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Mercury Hygiene Recommendations by ADA

• Use single-use capsules when preparing dental
  amalgams
• Use a no-touch technique and clean up any spilled
  mercury
• Discard any old or damaged mixing capsules which
  might be prone to leakage
• Store dental amalgam scrap in cool space in
  capped, unbreakable jar holding water with finely
  divided sulfur
• Avoid baseboard heating in operatories where
  dental amalgam is used

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Mercury Hygiene Recommendations by ADA

• Use face mask and water spray with high vacuum
  evacuation when finishing new dental amalgam
  restorations or removing old restorations
• Do not use ultrasonic condensers for dental
  amalgam restorations
• Mercury vapor levels in offices and operatories
  where dental amalgam restorations are prepared
  and placed should be regularly checked
• Office personnel involved with dental amalgam
  restorations should have their mercury levels
  periodically monitored by urinalysis

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Biocompatibility of Dental Amalgams

• Concern about mercury poisoning arises from high
  vapor pressure of liquid Hg (1.20 x 10-3 Torr at
  20ºC), which rapidly increases with temperature
• Modern analytical equipment can detect mercury
  vapor levels as low as 1 ?g/m3 in air or 0.2
  ng/mL in solution
• Possible toxicity effects from minute amounts of
  mercury released by dental amalgams can now be
  more readily investigated than previously

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Biocompatibility of Dental Amalgams

• A clinical study (Reinhardt et al, J Prosthet
  Dent 198349652-656) found that the amount of
  mercury exhaled by a patient following removal or
  placement of a single amalgam restoration is very
  small, occurs for a relatively brief period of
  time, and is largely avoidable by proper clinical
  procedures (e.g., use of rubber dam, handpiece
  water spray and high-volume evacuation coolant)

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Biocompatibility of Dental Amalgams

• Theoretical calculations (Mackert, J Dent Res
  1987661775-1780) have shown that, even for an
  individual with twelve or more occlusal surfaces
  containing amalgam restorations and not
  occupationally exposed to mercury, the daily dose
  of mercury from these restorations would be only
  10 of the normal daily intake from food, air and
  water

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Biocompatibility of Dental Amalgams

• In a clinical study on patients with controlled
  diets (Berglund, J Dent Res 1990691646-1651),
  the estimated average daily dose of mercury vapor
  inhaled from dental amalgam restorations was 1.7
  ?g, with a range of 0.4 - 4.4 ?g
• This is about 1 of the dose that would be
  obtained from the threshold limit value (TLV) of
  50 ?g/m3 of airborne mercury set by the US
  government for an individual exposed eight hours
  per day for a five-day work week

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Biocompatibility of Dental Amalgams

• Snapp et al (J Dent Res 198968780-785) examined
  ten adults with average of fourteen restoration
  surfaces
• Mean baseline total blood Hg of 2.18 ng/mL was
  significantly correlated with number of occlusal
  surfaces
• After removal of amalgam restorations, nine
  subjects had significant decrease in blood
  mercury (mean 1.13 ng/mL)
• Removal of restorations caused exposure 1.46 ng
  Hg/mL, which disappeared within three days
• Mercury toxicity in most sensitive adults
  occurred at blood concentrations of 30 ng/mL,
  indicating that dental amalgam restorations do
  not appear to be health hazard

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Biocompatibility of Dental Amalgams

• Wataha et al (Dent Mater 199410298-303) placed
  specimens of two dental amalgams in contact with
  Balb/c mouse fibroblasts for 24 hr and
  investigated succinic dehydrogenase activity of
  cells as monitor of cytotoxicity
• HCSS dental amalgam Tytin exhibited no
  cytotoxicity compared to teflon controls
• HCB dental amalgam Dispersalloy was severely
  cytotoxic initially when release of Zn ions was
  greatest, but less toxic between 48 and 72 hr as
  Zn release decreased

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Contrary ViewpointBiocompatibility of Dental
Amalgams

• Lorscheider F, Vimy M. Mercury and idiopathic
  dilated cardiomyopathy. J Am Coll Cardiol
  200035819-820. Comment on J Am Coll Cardiol
  1999331578-1583.
• Aschner M, Lorscheider FL, Cowan KS, Conklin DR,
  Vimy MJ, Lash LH. Metallothionein induction in
  fetal rat brain and neonatal primary astrocyte
  cultures by in utero exposure to elemental
  mercury vapor (Hg0). Brain Res 1997778222-232.
• Vimy MJ, Lorscheider FL. Renal function and
  amalgam mercury. Am J Physiol 1997273 (3 Pt
  2)R1199-1200. Comment on Am J Physiol 1996271
  (4 Pt 2)R941-945.

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Profile in New England Children's Amalgam Trial
Bellinger DC et al. JAMA 20062951775-1783. Sour
ce of following slides
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