Biomaterials Exam - Amalgam as a Dental Material

Question 1

What are the 3 routes of mercury exposure?

Answer 1

• skin contact
• inhalation of vapor
• airborne droplets

Question 2

At how many nanograms of mercury per mL blood are symptoms of mercury poisoning observed?

Answer 2

100 ng/mL blood

Question 3

How should mercury be disposed of in a dental practice?

Answer 3

• use single-use capsules
• use a no-touch technique and clean up any spilled mercury
• store/discard dental amalgam scrap in cool space in capped, unbreakable jar holding water with finely divided sulfur

Question 4

What does the ADA recommend for operators using mercury in their practices?

Answer 4

• single-use capsules
• no-touch technique to clean any spills
• discard old or damaged mixing capsules
• store any scraps in a cool space in a capped, unbreakable jar holding water with finely divided sulfur
• avoid baseboard heating in operatories where dental amalgam is used
• 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
• check mercury vapor levels frequently
• office personal should have mercury levels monitored periodically through urinalysis

Question 5

What type of reaction may some patients experience with exposure to dental amalgams?

Answer 5

allergic skin reaction; other options other than amalgam are available for restorations

Question 6

Is it better to have high-copper or low-copper materials? Why?

Answer 6

high-copper

• greater clinical longevity of restorations
• much lower creep values measured in laboratory

Question 7

What are the consequences of placing zinc in a material?

Answer 7

• facilitates machining lathe-cut particles (makes it more brittle)
• improves corrosion resistance
• less plastic amalgam mix

Question 8

What are the consequences of having a zinc-free alloy?

Answer 8

no concerns about moisture contamination during trituration or condensation

Question 9

If there is a higher mercury-alloy ratio, would the setting time increase or decrease?

Answer 9

setting time would increase (slower setting time)

Question 10

What is the composition of gamma 1?

Answer 10

Ag2Hg3

Question 11

What is the composition of gamma 2?

Answer 11

Sn8Hg

Question 12

In low-copper dental amalgams, what is the setting reaction equation?

Answer 12

alloy (gamma) + Hg –> (gamma)1 + (gamma)2 + unreacted alloy particles

Question 13

In high-copper dental amalgams, what is the setting reaction equation?

Answer 13

alloy (gamma) + Hg –> (gamma)1 + n’ + unreacted alloy particles
*NOTE: (gamma)2 does not form

Question 14

What are the three metal elements that are reacted with mercury in high- and low-copper amalgams?

Answer 14

silver (Ag), tin (Sn), and copper (Cu)

Question 15

What is the composition of n’ that is produced in the high-copper amalgam setting reaction?

Answer 15

Cu6Sn5

Question 16

For dispersalloy-type dental amalgams (low-Cu lathe-cut and spherical Ag-Cu particles of eutectic composition), what is the setting reaction equation?

Answer 16

Two steps:

1. (gamma)1 + (gamma)2 form
2. (gamma)2 disappears; n’ phase appears

Question 17

Which sets faster: high-copper spherical particles of single composition (HCSS) or dispersalloy-type dental amalgams?

Answer 17

HCSS is faster because it is only 1 step as opposed to 2

Question 18

When amalgams set, do they expand or contract?

Answer 18

slightly contract; clinical problems would occur with excessive setting expansion (loss of anatomy and postoperative pain) or excessive setting contraction (microleakage)

Question 19

Describe the steps of the setting process of amalgams.

Answer 19

Setting process is combination of solution and crystallization (precipitation).

1. initial contraction from absorption of mercury (diffusion) by amalgam alloy particles.
2. can be subsequent expansion from formation and growth of (gamma)1 and (gamma)2 or Cu-Sn (n’) phases (matrix)
3. final absorption of mercury by remaining amalgam alloy particles causes contraction
   * no free mercury in final set dental amalgam!*

Question 20

What is the strongest phase of dental amalgams?

Answer 20

gamma phase (incompletely consumed starting alloy particles)

Question 21

What is the weakest phase of dental amalgams?

Answer 21

(gamma)2 phase (in low-copper amalgams

Question 22

What are the 6 types of corrosion in dental amalgams?

Answer 22

• galvanic corrosion - at interproximal contacts with gold alloys
• electrochemical corrosion - because multiple phases
• crevice corrosion - at margins
• corrosion 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 23

Limited corrosion is beneficial. Why?

Answer 23

because it reduces microleakage ((gamma)2 in low-copper amalgams and n’ in high-copper amalgams)

Question 24

How can corrosion be minimized?

Answer 24

by polishing amalgam restorations (scratches and pits trap debris, enhancing corrosion because lower oxygen concentration under deposit)

Question 25

What metal may reduce corrosion in amalgam?

Answer 25

zinc

Question 26

Does amalgam have low or high tensile strength?

Answer 26

low tensile strength; allows fracture of edges easily

Question 27

Does amalgam have low or high compressive strength?

Answer 27

high compressive strength; strength increases as the amalgam sets until it reaches maximum strength at approximately 1 week

Question 28

What is creep? How does it occur?

Answer 28

- metals may become elongated or deformed as a result of a load being applied for a long period of time - grain boundary sliding of (gamma)1 phase

Question 29

Do high-copper amalgams have high or low creep?

Answer 29

low creep; the n' phase blocks sliding of the (gamma)1 phase in high-copper amalgams

Question 30

Do low-copper amalgams have a high or low creep?

Answer 30

high creep; creep is the only mechanical property correlated with clinical marginal fracture of low-copper amalgam restorations

Question 31

Why is it important to have adequate trituration time?

Answer 31

so that all alloy particles are coated with mercury and optimum mechanical properties are obtained

Question 32

With what type of amalgam is moisture contamination a problem? What will moisture contamination cause?

Answer 32

- zinc-containing dental amalgam - causes delayed setting, excessive increase in setting expansion and decreased strength (H2 is released from Zn reduction of water)

Question 33

What will happen if amalgam is overtriturated?

Answer 33

makes the material hot, reduces working time, and increases creep

Question 34

What 3 things were considered historically when placing amalgam?

Answer 34

- adapt amalgam to cavity walls - minimize porosity in restoration - control final mercury content of resotration

Question 35

What elements make up the alloy in amalgam?

Answer 35

silver, tin, copper, zinc, indium, mercury, and/or noble metals, gold, platinum, and palladium

Question 36

What are the two methods of making amalgam particles?

Answer 36

- filing or lathe-cut (machined from cast ingot) | - spherical (molten alloy blown through nozzle)

Question 37

Which type of amalgam particles (lathe-cut or spherical) are wetted with a lower mercury:alloy ratio?

Answer 37

spherical particles

Question 38

Which type of amalgam particles (lathe-cut or spherical) are able to resist forces of condensation more?

Answer 38

lathe-cut particles