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Dent 633 Dental 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: [email protected] Textbook References.

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Dent 633 Dental Amalgam

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Dent 633 dental amalgam l.jpg

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: [email protected]

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Textbook References

O’Brien WJ (editor). Dental Materials and Their Selection (3rd ed). Chicago: Quintessence, 2002. Chapter 12.

Powers JM, Sakaguchi RL (editors).

Craig’s Restorative Dental Materials (12th ed).

Mosby, 2006. Chapter 11.

Anusavice, KJ (editor). Phillip’s 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 – γ (Ag3Sn), β (Ag-Sn) and ε (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 mercury:alloy 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 >12 % 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 (< 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

γ (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 – γ1 (Ag2Hg3) and γ2 (Sn8Hg)

High-copper dental amalgams –γ1 and η (Cu6Sn5)

Note that high-copper dental amalgams are γ2-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 γ2 phase and formation of η 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 < 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: LCLFromAnusavice (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 γ1, γ2 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 AmalgamOriginal Magnification 1000From Anusavice (11th ed), Fig. 17-5

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Microstructure of HCB Dental AmalgamOriginal Magnification 1000Mercury-rich droplets from polishing specimenFrom Anusavice (11th ed), Fig. 17-7

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Microstructure of HCSS Dental AmalgamRelief polish with original magnification 560From Anusavice (11th ed), Fig. 17-8

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Characteristics of Microstructural Phases in Dental Amalgams

Strongest phase – incompletely consumed starting alloy particles (γ)

Weakest phase – γ2 in low-copper amalgams (most corrosion prone)

Completely interconnected nature of γ2 can result in bulk corrosion of low-copper dental amalgam

High-copper amalgams – Cu6Sn5 (η) is corroding phase that provide margin-sealing – because η 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 – γ2 in low-copper amalgams and Cu6Sn5 (η) 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 > about 350 MPa

Tensile strength < 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 (<1%)

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 γ1 phase (blocked by η in high-copper amalgams)

Values of strength and creep representative dental amalgams anusavice 11th ed table 17 2 l.jpg

Values of Strength and CreepRepresentative Dental AmalgamsAnusavice (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

Delayed expansion of moisture contaminated zinc containing amalgam from anusavice 11th ed fig 17 11 l.jpg

Delayed Expansion of Moisture-Contaminated Zinc-Containing AmalgamFrom 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

Effect of mercury alloy ratio strength of dental amalgams from anusavice 11th ed fig 17 11 l.jpg

Effect of Mercury/Alloy RatioStrength of Dental AmalgamsFrom 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 –39°C) 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 1983;49:652-656) 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 1987;66:1775-1780) 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 1990;69:1646-1651), 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 1989;68:780-785) 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 1994;10:298-303) 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 2000;35:819-820. Comment on J Am Coll Cardiol 1999;33:1578-1583.

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 1997;778:222-232.

Vimy MJ, Lorscheider FL. Renal function and amalgam mercury. Am J Physiol 1997;273 (3 Pt 2):R1199-1200. Comment on Am J Physiol 1996;271 (4 Pt 2):R941-945.

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Profile in New England Children's Amalgam Trial

Bellinger DC et al. JAMA 2006;295:1775-1783.

[Source of following slides]

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