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Flashcards in Exam II Deck (69):

define the words cast and die

-Cast: replica of the teeth and/or associated supporting bony tissues of one jaw (prepared from an impression)
-Die: model of a single tooth prepared from an impression


what are the desirable properties of cast and die materials

-mechanical: bulk properties (strength) and surface properties (hardness; resistance to abrasion)
-detail reproduction
-dimensional accuracy and stability
-compatibility with impression materials
-color contrast


what is the dihydrate of calcium sulfate?



how are hydrates of calcium sulfate dihydrate (gypsum) made?

when gypsum is heated, it loses water of crystallization creating a hemihydrate


how do you reverse the creation of a hemihydrate from a dihydrate (gypsum)

mix with water
reforms the dihydrate form causing setting


classifications of dental gypsum products

- Type I: impression plaster
- Type II: model plaster
- Type III: dental stone
- Type IV: high strength dental stone
- Type V: high strength, high expansion dental stone


describe the chemical setting reaction of calcium sulfate hemihydrate with water

- dissolution of some of the hemihydrate --> dihydrate formed in solution/diffusion of Ca2+ and SO42- ions
- crystal growth of dihydrate from crystal nuclei
- more hemihydrate dissolves
- interlocking crystals give rigidity and strength
- porosity because of excess water
- set material is almost entirely calcium sulfate dihydrate


why is water required for setting reactions

- hydration
- give smooth workable mix


describe the setting process of gypsum product

- initial fluid mix
- then becomes rigid, not hard (initial set)
- exothermic heat
- dimensional changes - expansion
- hygroscopic expansion
- hard set material (final set)
- porosity (due to excess water that doesnt react and finally evaporates)


describe the mechanism of setting expansion of gypsum products

- crystals growing from nuclei
- volumetric expansion caused by thrust of growing crystals


what are the two main forms of calcium sulfate hemihydrate

- Calcined (beta) -->plaster
- Autoclaved (alpha) -->stone
*the initial form of hemihydrate influences the application of set gypsum


differences in manufacturing and particle size/shape in beta in alpha hemihydrates

- manufacturing process influences size and shape of supplied hemihydrate crystals
- Calcined (beta): irregular crystals
- Autoclaved (alpha): smaller prismatic crystals


water-powder ratio and set material denstiy in beta and alpha hemihydrates

- beta is mixed with more water than alpha
- result of the difference in water excess = difference in porosity of set gypsum
- dental plaster is more porous than set dental stone


difference in set material mechanical properties and applications - dental plaster vs stone

- plaster is more brittle and weaker than stone
- set dental plaster used for mounting study casts (type II gypsum)
- set dental stone used for working casts (type III) and dyes (type IV and V)


what are the effects of additives to gypsum

- effect on setting time
> accelerators - gypsum, potassium sulfate
> retarders - borax, potassium citrate
- effect of setting expansion
> increase - calcium acetate
> decrease - potassium sulfate
- effect of mechanical properties
> usually weakening


correct manipulation of setting materials to obtain optimum properties

- select correct material
- use correct water/powder ratio
- smooth mix
- eliminate air bubbles (vacuum mixing/vibration on pouring)
- dry set material


what is setting time dependent on

- not greatly affected by temp
- depends on: type of material, water/powder ratio, and mixing time


what are critiques of gypsum products

- mechanical: brittleness; abrasion resistance
- fine detail, sharp margins: good reproduction
- accuracy and stability: good
- compatibility with impression materials
- color contrast
- cost - inexpensive


what is working time

measurement of working begins with start of mixing and ends just before impression material has developed elastic properties


what is working time dependent on



what is setting time

time measured from when mixing the impression material begins until complete reaction occurs


what is viscosity of a fluid

measure of its resistance to gradual deformation by shear stress or tensile stress
(informal concept of "thickness" of a liquid)


viscosity depends on:

- composition: (molecular weight, filler content,...)
- rate of deformation (shearing): (Newtonian and non-newtonian liquids)
- temperature: (viscosity decreases with temp)


distinguish between direct and indirect restorations

- Direct: where material is placed directly into the mouth, where it sets (amalgam, resin composite)
- Indirect: where a restoration or prosthesis is prepared in a lab (dentures, cast gold, ceramics)


what are the applications of dental impression materials

to produce cast/models for:
- pros and ortho treatment planning
- occlusal analysis
- fabrication of provisional restoration
- fabrication of definitive indirect restoration


what are the requirements for impression materials

General: BICMEP
- B: non-toxic, non-irritant
- I: good wetting
- C: long shelf life
- M: good resistance to tearing
- E: acceptable odor and taste
- P: appropriate working and setting time

C, M, P: dimensional accuracy and stability


what are the 7 stages of impression taking that needs to be considered (for most accurate dimensions)

1. insertion
2. setting
3. removal
4. cooling to room temp
5. disinfection
6. storage
7. compatibility with gypsum (cast/die materials)


what are the classifications of dental impression materials

- Non-elastic materials
>impression plaster/paste/compound
- Elastic Materials
+reversible (agar)
+irreversible (alginate)
+polysulfides, condensation silicones, polyethers, addition silicones


non-elastic materials as dental impression material

- not used in dentistry anymore
- rigid when set, will deform/fracture when displaced from tissue


distinguish between hydrocolloids and non-aqueous elastomers

- both are elastic impression materials
- Hydrocolloids: aqueous materials with poor strength and stability
- Elastomers: non-aueous synthetic rubbers which are more stable and stronger than hydrocolloids


define colloid and hydrocolloid

- Colloid: a suspension of finely divided particles as a continuous medium in a dispersion medium from which the particles do not settle out rapidly and cannot readily be filtered
- Hydrocolloid: dispersion medium is water


difference between a sol and a gel

- sol: fluid
- gel: elastic


dental application of sols and gels

- convert sol to gel (setting mechanism)
- two ways to do this:
> reversibly: by cooling material (agar)
> irreversibly: by chemical reaction (alginate)


physical structure of a irreversible hydrocolloid gel and the consequences of this structure

- Set material of irreversible hydrocolloid gel consists of bonded chains of the particles entrapping a significant amount of water

- set materials are elastic (good)
- poor strength properties --> may tear easily when withdrawn from undercuts (water acts as plasticizer)
- water may be lost or taken up by material at different steps of the process --> Compromises stability of the dimensions and mechanical properties of the impression (impression can expand and shrink)


define syneresis and imbibition - then explain the significance of these factors in relation to dimensional stability

- syneresis: loss of fluids by the material
- imbibition: uptake of fluids by the material

- syneresis results in the colloid molecules being drawn closer together--> substantial shrinkage
- inbibition results in expansion

*syneresis and imbibition must be avoided


what are the main constituents of alginates

- soluble salt of alginic acid (polysaccharide)
> sodium alginate
- calcium salt
- trisodium phosphate
> setting retarder
- about 70% filler to increase gel strength



what are the two sequential reactions of alginates

1. formation of calcium phosphate (precipitate)
>reaction 2 cannot take place until reaction 1 is substantially complete
2. formation of calcium alginate (gel)


list the factors that are important in correct manipulation of alginate material

- shake powder container
- correct powder to water ratio
- ensure retention to tray
- vigorous mixing
- don't move material during impression taking
- recognize when material has fully gelled
- displace sharply from the tissues
- wash, remove saliva, disinfect (disinfection by immersion is not recommended, spray preferred)
- keep material moist and prepare cast as soon as possible


critiques of alginates

- well-defined working time
- elastic
- records fine detail
- comparatively inexpensive

- dimensionally unstable
- tear easily


dental applications of alginates

- used for cast models: pros and ortho impressions
- generally considered to be insufficiently accurate and stable for inlay, crown and bridge impressions


define amalgam and amalgamation

- Amalgam: alloy of mercury with another metal
- Amalgamation: reaction of mercury with other metallic materials - this can occur at room temperature


list the practical stages in the preparation of an amalgam restoration

- mix powdered alloy with liquid mercury
- metallic paste is formed
- paste is packed (condensed) into tooth cavity
- material is carved to correct anatomy
- amalgamation reaction occurs
- material sets and hardens
- restoration is polished


Ag3Sn (gamma-phase)

Alloy powder is based in the intermetallic alloy Ag3Sn (a silver tin alloy) this is the metallic alloy that will react with mercury during amalgamation --> this intermetallic compound is called gamma-phase


what is the structure of set material resulting from the reaction of Ag3Sn (gamma phase) with mercury

- silver-tin reacts with mercury to form:
>silver-mercury compound (gamma1)
>tin-mercury compound (gamma2)
-also unreacted Ag3Sn present

-core of unreacted powder surrounded by reaction products from powder-liquid reaction (like a brick wall)


traditional low copper amalgams

- older material
- less than 6% Cu
- added Cu to increase strength and hardness and to reduce excess contraction (Sn) and expansion (Ag) on setting
- low copper amalgams have limited durability because they corrode
- now make amalgams with higher copper concentrations


limitations of low copper amalgam

- gamma 2 phase is weakest and softest phase and is most liable to electrolytic corrosion
- gamma 2 phase acts as anode and corrodes
- products of gamma 2 corrosion:
> tin salts: weak amalgam
> amalgamation from free mercury released: expansion (mercuroscopic expansion)


why is gamma2 phase not formed in high copper amalgams

1. Powder in high Cu amalgams contains silver-copper particles in addition to silver-tin particles
2. Gamma2 phase initially forms, as in low Cu amalgams
3. The initially formed gamma2 phase further reacts with added silver-copper particles and form new phase made of copper-tin, n-phase (eta phase) that doesn't corrode
4. All the initially formed gamma2 phase reacts to form eta phase = final amalgam is free of gamma2
Eta phase located around silver-copper particles forming a halo in the final set


classify contemporary high copper amalgams

1. Blended Alloys (dispersalloy or admixed alloys)
- 2 parts irregular shaped lathe cut silver-tin particles
- 1 part spherical silver-copper
2. Single Composition
- Contains only one type of spherical silver-tin-copper particles


what does excess Hg in the final set amalgam result in

weaker restoration


what happens with overtrituration and overtrituration of amalgam

- may give unworkable mix
- dry and brittle: grainy/crumbly mix

- shiny, too soft, sticks to capsule
- decreases working time
- may result in excessive contraction during setting


toxicity of mercury depends on:

- form in which mercury is present (vapor is most toxic)
- quantity of exposure
- frequency of exposure


what are the forms of mercury

- liquid
- vapor
- intermetallic compound
- organometallic compound

*must consider each separately


mercury as a liquid

- high vapor pressure at room temp
- if swallowed, poor absorption by gut --> rapidly and easily excreted
- two main problems: skin contact may result in absorption and a few people have allergy to Hg


mercury as an intermetallic compound

- formed in set amalgam
- comparatively insoluble and harmless
- if in dissolved salts, much more toxic


mercury as a vapor

- very toxic
- Hg can rapidly cross alveolar membranes and directly into bloodstream
- small quantities in expired air from patients (1-2 micrograms/day) less than environmental level (10-20 micrograms/day)
- acute toxicity is rare


mercury as an organo-metallic compound

- methyl mercury is very toxic
- sources: in food chain (fish) and other environmental sources


mechanical strength of dental amalgam

strength increases with time


weak amalgam restoration results from:

- undertrituration
- too much mercury in set material
- too low condensation pressure
- slow rate of packing
- corrosion
- contamination with blood or saliva during placement


dental amalgam creep

- amalgam shows phenomenon of dynamic creep (material slowly deforms during mastication and bruxing)
- modern high Cu alloys show much less creep than older materials


thermal conduction of dental amalgam

- enamel and dentin are good thermal insulators
- amalgam conducts heat
- consideration must be given to pulpal protection if large amalgam restoration placed


what are the types of demineralization

1. surface demineralization (surface softening)
-no white patches, but surface is rough and looks matte (not opaque)
-beverages, stomach acid, various acids
- dental erosion
2. sub-surface demineralization (lesions)
- white, opaque lesions
- surface less mineralized
- main demineralization is sub surface
3. etched enamel demineralization
- similar to #1 but part of surface is removed...can be more advanced stage of #1

*remineralization looking at mostly for subsurface, surface one still being researched


what leads to surface opacity in enamel demineralization

Ca and PO4 redeposit during remineralizaiton leads to opaque color (pH there is higher...further down pH is lower and diffusing)


what is needed for enamel remineralization

- need bioavailable calcium and phosphate
- need partically demineralized crystals
- remineralization no possible after mineral phase if its nucleation sites are lost completely


definition of remineralization

- process of ion deposition, sourced from the surrounding environment into voids in demineralized enamel to produce net mineral gaini
- void: any intercrystal and interrod spaces caused by dissolution
- includes crystal repair
- does not include precipitation of solid phases onto enamel surfaces
- needs bioavailable calcium phosphate


what is needed for fluoride to be effective

Ca and PO4 from saliva
low salivary flow, fluoride does not help


what does fluoride do in oral cavity

- fluoride increases deposition rate by factor of 2-3
- fluoride is incorporated into mineral as fluorohydroxyapatite
- results in: reduced acid solubility of remineralized enamel, and development of fluoride gradients if repeated remineralization occurs


where on the tooth is fluoride most effective

- highly effective on smooth surface caries
- less so on pit and fissure caries


what are ideal properties of remineralizing materials

- diffuses into the subsurface and delivers calcium and phosphate onto body of lesion
- does not deliver an excess of calcium
- works at acidic, neutral, and basic pHs (fluoride better at slightly acidic)
- works in xerostomic patients
- enhances or maintains the remineralizing effect of saliva
- for novel materials, shows a benefit over fluoride, or establishes a synergistic effect with fluoride to promote remineralization


what are 5 therapeutic options for remineralization

1. fluoride
2. compounds that increase mineral saturation
3. biofilm modifiers
4. self-assembling peptides
5. diffusion barriers (like sealants)