Materials Science 4

Question 1

Advantages of Dental Amalgam as a Restorative Material

Answer 1

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

Question 2

Concerns about Dental Amalgam as a Restorative material

Answer 2

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
Wastewater pollution with mercury

Question 3

General Setting Reaction for Dental Amalgam

Answer 3

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 encapsulated products contain approximately 42 to 45% Hg by weight
Factors for setting process: composition, shape and size of alloy particles (based upon handling characteristics desired by manufacturer)

Question 4

Composition

Answer 4

Elements in alloy may include silver, tin, copper, zinc, indium, mercury, and/or noble metals gold, platinum and palladium. The total concentration of other elements cannot exceed 0.1 wt %, unless the manufacturer provides evidence of biocompatibility.

Question 5

Physical Properties

Answer 5

creep 1.0 % maximum, dimensional change between -0.15 and +0.20 %, and compressive strength minimum of 80 MPa after 1 hour and 300 MPa after 24 hours.

Question 6

Method of Making Particles

Answer 6

Filing or lathe-cut (machined from cast ingot)

Spherical (molten alloy blown through nozzle)

Question 7

Composition of Particles

Answer 7

All particles with same composition

Blend or admixture of particles with different compositions

Question 8

Varying Sizes of Alloy Particles

Answer 8

Spherical particles range from 50 μm diameter to over an order of magnitude smaller — three different sizes are typically observed
Wide range in sizes also observed for lathe-cut particles
Intentionally done by manufacturer for optimum condensation

Question 9

High-copper vs. low-copper

Answer 9

high-copper products contain >12 % Cu in alloy particles
High-copper products should be selected — benefit: greater clinical longevity of restorations — much lower creep values measured in laboratory

Question 10

Zinc-containing vs. zinc-free (< 0.01 wt % Zn)

Answer 10

not economically feasible for manufacturer to eliminate Zn
Zinc is considered to facilitate machining lathe-cut particles (makes ingot more brittle) and improves corrosion resistance of amalgam, but results in less plastic amalgam mix

Question 11

LCL, LCS

Answer 11

(low-copper alloy, lathe-cut or spherical particles)

Question 12

HCSS

Answer 12

(high-copper alloy, spherical particles of single composition)

Question 13

HCB

Answer 13

(high-copper alloy, blend of two different particles — shape and/or composition)

Question 14

Manufacturer Proprietary Heat Treatment Alloy for Dental Amalgam

Answer 14

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

Question 15

General Form of Setting Reaction Dental Amalgams

Answer 15

gamma (starting alloy particles) + Hg (liquid)  reaction phases (matrix) + unreacted alloy particles (core)
Incompletely consumed alloy particles exist in set dental amalgam microstructure
“Bricks” (alloy particles) and “mortar” (reaction phases) analogy for structure of set amalgam
No free mercury after setting reaction — Hg found in reaction phases

Question 16

Setting Reaction for Dispersalloy-Type Admix Dental Amalgams [Two Steps]

Answer 16

Alloy particles are admixture or blend of low-copper lathe-cut particles and spherical Ag-Cu particles of eutectic composition (72 wt% Ag, 28 wt % Cu)
First step of setting reaction is identical to low-copper dental amalgams — both 1 and 2 form initially
Second step of setting reaction is disappearance of 2 phase and formation of η phase
Slower setting reaction than for HCSS products because two-step process

Question 17

Dimensional Changes Setting of Dental Amalgams

Answer 17

Total dimensional change after 24 hr is limited by ADA Standard No. 1 and cannot be detected 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)

Question 18

Nature of Setting Dimensional Changes

Answer 18

Setting process is combination of solution and crystallization (precipitation)
Initial contraction from absorption of Hg (diffusion) by amalgam alloy particles
Can be subsequent expansion from formation and growth of gamma1 and gamma2 or Cu-Sn (η’) phases (matrix)
Final absorption of mercury by remaining amalgam alloy particles causes contraction
No free mercury in final set dental amalgam

Question 19

Characteristics of Microstructural Phases in Dental Amalgams

Answer 19

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 (η) is corroding phase that provides margin-sealing – because η is not interconnected, corrosion limited to marginal regions without bulk corrosion
Weaker interface between alloy particles and reaction phases

Question 20

Types of Corrosion in Dental Amalgams

Answer 20

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

Question 21

Corrosion of Dental Amalgam Restorations

Answer 21

Limited corrosion is beneficial because reduction in microleakage – 2 in low-copper amalgams and Cu6Sn5 (η) in high-copper amalgams
Tin-containing and copper-containing in vivo corrosion products have been identified
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

Question 22

Change in Mechanical Properties of Dental Amalgams with Time

Answer 22

ADA Standard No. 1 requires minimum values of compressive strength at 1 hr and 24 hr
Rate of strength increase is dependent on 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 to be after 1 week – nearly same strength after 1 day
Viscoelastic material (mechanical properties depend on rate of loading – creep occurs at constant load)

Question 23

Major Effects of Manipulative Variables on Properties of Dental Amalgams

Answer 23

Excessive mercury content (should not occur with current encapsulated products) – causes formation of greater amount of reaction phases, increases setting expansion, and decreases strength
Important to have adequate trituration time so that all alloy particles are coated with mercury and optimum mechanical properties are obtained
Sufficient condensation pressure is needed to adapt dental amalgam mix to prepared cavity and minimize porosity
Moisture contamination is problem with Zn-containing dental amalgam — causes delayed, excessive increase in setting expansion and decreased strength (H2 released from Zn reduction of water)

Question 24

Trituration and Condensation ― Clinical Considerations

Answer 24

Role of trituration ─ coat each alloy particle with mercury [Avoid undertrituration]
Overtrituration makes mixed material hot, reduces working time, and increases creep
Historically, small increases in trituration time have been considered to decrease setting expansion and increase strength
Optimum trituration time is highly important
Historic operative dentistry roles of condensation ― adapt restoration to cavity walls
minimize porosity in restoration
control final mercury content of restoration
Historically, small increases in condensation pressure have also been considered to decrease setting expansion and increase strength
Do not delay condensation after trituration


Question 25

Mercury and Mercury Toxicity

Answer 25

Mercury is liquid metal at temperatures above –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

Question 26

Mercury Hygiene Recommendations by ADA

Answer 26

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

Question 27

Mercury Hygiene Recommendations by ADA

Answer 27

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