Exam II Flashcards
1
Q
define the words cast and die
A
- 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
2
Q
what are the desirable properties of cast and die materials
A
- mechanical: bulk properties (strength) and surface properties (hardness; resistance to abrasion)
- detail reproduction
- dimensional accuracy and stability
- compatibility with impression materials
- color contrast
- economical
3
Q
what is the dihydrate of calcium sulfate?
A
Gypsum
4
Q
how are hydrates of calcium sulfate dihydrate (gypsum) made?
A
heating
when gypsum is heated, it loses water of crystallization creating a hemihydrate
5
Q
how do you reverse the creation of a hemihydrate from a dihydrate (gypsum)
A
mix with water
reforms the dihydrate form causing setting
6
Q
classifications of dental gypsum products
A
- 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
7
Q
describe the chemical setting reaction of calcium sulfate hemihydrate with water
A
- 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
8
Q
why is water required for setting reactions
A
- hydration
- give smooth workable mix
9
Q
describe the setting process of gypsum product
A
- 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)
10
Q
describe the mechanism of setting expansion of gypsum products
A
- crystals growing from nuclei
- volumetric expansion caused by thrust of growing crystals
11
Q
what are the two main forms of calcium sulfate hemihydrate
A
- Calcined (beta) –>plaster
- Autoclaved (alpha) –>stone
- the initial form of hemihydrate influences the application of set gypsum
12
Q
differences in manufacturing and particle size/shape in beta in alpha hemihydrates
A
- manufacturing process influences size and shape of supplied hemihydrate crystals
- Calcined (beta): irregular crystals
- Autoclaved (alpha): smaller prismatic crystals
13
Q
water-powder ratio and set material denstiy in beta and alpha hemihydrates
A
- 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
14
Q
difference in set material mechanical properties and applications - dental plaster vs stone
A
- 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)
15
Q
what are the effects of additives to gypsum
A
- 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
16
Q
correct manipulation of setting materials to obtain optimum properties
A
- select correct material
- use correct water/powder ratio
- smooth mix
- eliminate air bubbles (vacuum mixing/vibration on pouring)
- dry set material
17
Q
what is setting time dependent on
A
- not greatly affected by temp
- depends on: type of material, water/powder ratio, and mixing time
18
Q
what are critiques of gypsum products
A
- mechanical: brittleness; abrasion resistance
- fine detail, sharp margins: good reproduction
- accuracy and stability: good
- compatibility with impression materials
- color contrast
- cost - inexpensive
19
Q
what is working time
A
measurement of working begins with start of mixing and ends just before impression material has developed elastic properties
20
Q
what is working time dependent on
A
temperature
21
Q
what is setting time
A
time measured from when mixing the impression material begins until complete reaction occurs
22
Q
what is viscosity of a fluid
A
measure of its resistance to gradual deformation by shear stress or tensile stress
(informal concept of “thickness” of a liquid)
23
Q
viscosity depends on:
A
- composition: (molecular weight, filler content,…)
- rate of deformation (shearing): (Newtonian and non-newtonian liquids)
- temperature: (viscosity decreases with temp)
24
Q
distinguish between direct and indirect restorations
A
- 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)
25
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
26
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
Specific:
C, M, P: dimensional accuracy and stability
27
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)
28
what are the classifications of dental impression materials
```
- Non-elastic materials
>impression plaster/paste/compound
- Elastic Materials
>Hydrocolloids
+reversible (agar)
+irreversible (alginate)
>Elastomers
+polysulfides, condensation silicones, polyethers, addition silicones
```
29
non-elastic materials as dental impression material
- not used in dentistry anymore
| - rigid when set, will deform/fracture when displaced from tissue
30
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
31
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
32
difference between a sol and a gel
- sol: fluid
| - gel: elastic
33
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)
34
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
Consequences:
- 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)
35
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
CONSEQUENCE:
- syneresis results in the colloid molecules being drawn closer together--> substantial shrinkage
- inbibition results in expansion
*syneresis and imbibition must be avoided
36
what are the main constituents of alginates
POWDER:
- soluble salt of alginic acid (polysaccharide)
> sodium alginate
- calcium salt
- trisodium phosphate
> setting retarder
- about 70% filler to increase gel strength
WATER
37
what are the two sequential reactions of alginates
1. formation of calcium phosphate (precipitate)
>retarder
>reaction 2 cannot take place until reaction 1 is substantially complete
2. formation of calcium alginate (gel)
38
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
39
critiques of alginates
POSITIVE:
- well-defined working time
- elastic
- records fine detail
- comparatively inexpensive
NEGATIVE:
- dimensionally unstable
- tear easily
40
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
41
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
42
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
43
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
44
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
STRUCTURE:
-core of unreacted powder surrounded by reaction products from powder-liquid reaction (like a brick wall)
45
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
46
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)
47
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
48
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
49
what does excess Hg in the final set amalgam result in
weaker restoration
50
what happens with overtrituration and overtrituration of amalgam
Overtrituration:
- may give unworkable mix
- dry and brittle: grainy/crumbly mix
Undertrituration:
- shiny, too soft, sticks to capsule
- decreases working time
- may result in excessive contraction during setting
51
toxicity of mercury depends on:
- form in which mercury is present (vapor is most toxic)
- quantity of exposure
- frequency of exposure
52
what are the forms of mercury
- liquid
- vapor
- intermetallic compound
- organometallic compound
*must consider each separately
53
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
54
mercury as an intermetallic compound
- formed in set amalgam
- comparatively insoluble and harmless
- if in dissolved salts, much more toxic
55
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
56
mercury as an organo-metallic compound
- methyl mercury is very toxic
| - sources: in food chain (fish) and other environmental sources
57
mechanical strength of dental amalgam
strength increases with time
58
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
59
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
60
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
61
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
62
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)
63
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
64
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
65
what is needed for fluoride to be effective
Ca and PO4 from saliva
| low salivary flow, fluoride does not help
66
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
67
where on the tooth is fluoride most effective
- highly effective on smooth surface caries
| - less so on pit and fissure caries
68
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
69
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)
