Flashcards in Biomaterials Exam - Amalgam as a Dental Material Deck (38)
What are the 3 routes of mercury exposure?
-inhalation of vapor
At how many nanograms of mercury per mL blood are symptoms of mercury poisoning observed?
100 ng/mL blood
How should mercury be disposed of in a dental practice?
-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
What does the ADA recommend for operators using mercury in their practices?
-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
What type of reaction may some patients experience with exposure to dental amalgams?
allergic skin reaction; other options other than amalgam are available for restorations
Is it better to have high-copper or low-copper materials? Why?
-greater clinical longevity of restorations
-much lower creep values measured in laboratory
What are the consequences of placing zinc in a material?
-facilitates machining lathe-cut particles (makes it more brittle)
-improves corrosion resistance
-less plastic amalgam mix
What are the consequences of having a zinc-free alloy?
no concerns about moisture contamination during trituration or condensation
If there is a higher mercury-alloy ratio, would the setting time increase or decrease?
setting time would increase (slower setting time)
What is the composition of gamma 1?
What is the composition of gamma 2?
In low-copper dental amalgams, what is the setting reaction equation?
alloy (gamma) + Hg --> (gamma)1 + (gamma)2 + unreacted alloy particles
In high-copper dental amalgams, what is the setting reaction equation?
alloy (gamma) + Hg --> (gamma)1 + n' + unreacted alloy particles
*NOTE: (gamma)2 does not form
What are the three metal elements that are reacted with mercury in high- and low-copper amalgams?
silver (Ag), tin (Sn), and copper (Cu)
What is the composition of n' that is produced in the high-copper amalgam setting reaction?
For dispersalloy-type dental amalgams (low-Cu lathe-cut and spherical Ag-Cu particles of eutectic composition), what is the setting reaction equation?
1. (gamma)1 + (gamma)2 form
2. (gamma)2 disappears; n' phase appears
Which sets faster: high-copper spherical particles of single composition (HCSS) or dispersalloy-type dental amalgams?
HCSS is faster because it is only 1 step as opposed to 2
When amalgams set, do they expand or contract?
slightly contract; clinical problems would occur with excessive setting expansion (loss of anatomy and postoperative pain) or excessive setting contraction (microleakage)
Describe the steps of the setting process of amalgams.
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!*
What is the strongest phase of dental amalgams?
gamma phase (incompletely consumed starting alloy particles)
What is the weakest phase of dental amalgams?
(gamma)2 phase (in low-copper amalgams
What are the 6 types of corrosion in dental amalgams?
- 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
Limited corrosion is beneficial. Why?
because it reduces microleakage ((gamma)2 in low-copper amalgams and n' in high-copper amalgams)
How can corrosion be minimized?
by polishing amalgam restorations (scratches and pits trap debris, enhancing corrosion because lower oxygen concentration under deposit)
What metal may reduce corrosion in amalgam?
Does amalgam have low or high tensile strength?
low tensile strength; allows fracture of edges easily
Does amalgam have low or high compressive strength?
high compressive strength; strength increases as the amalgam sets until it reaches maximum strength at approximately 1 week
What is creep? How does it occur?
- 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
Do high-copper amalgams have high or low creep?
low creep; the n' phase blocks sliding of the (gamma)1 phase in high-copper amalgams