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B3 Flashcards

1
Q

Classify dental waxes

A

According to the Origin
A) Minerals - Paraffin Wax , Microcrystalline wax
B) Animal - Bees wax
C) Vegetable - Carnauba wax , Candelilla wax

According to Use
A) Pattern Wax - Modelling wax , Inlay wax , Casting wax , Base plate wax
B) Processing wax - Boxing wax , Utility wax , Sticky wax
C) Impression wax - Corrective wax

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2
Q

State the uses of modelling wax

A
  • make occlusal rim for recording jaw relationship
  • make wax pattern for complete & partial denture
  • wax pattern for orthodontic appliances
  • type 1 can be used to build veneers in laboratory
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3
Q

State the general composition of dental waxes

A

1) Waxes : beeswax , carnauba wax , paraffin wax , Ceresin wax
2) Resins
3) Gums - gum dammar
4) Fats & oil
5) Pigment - provide Color

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4
Q

State the use of inlay wax

A

To make wax pattern for inlay , crown & bridges

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5
Q

State the use of baseplate wax

A
  • to make temporarily denture base

- to make special trays

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6
Q

State the use of casting wax

A
  • To make wax pattern of the metallic framework of cast partial denture
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7
Q

State the use of boxing wax

A

To do boxing & beading ( building up vertical walls) of an impression in order to preserve the height & width of limiting areas

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8
Q

State the use of utility wax

A

To change the contour of stock tray for hydrocolloid example to raise flange height , to extend the tray posterior lay & raise palatial portion in cases of deep palate

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9
Q

State the use of sticky wax

A
  • to solder join broken metal pieces before soldering ( example in cast partial denture fracture )
  • to join acrylic pieces ( broken dentures) together before repair.
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10
Q

State the use of corrective impression wax

A
  • to make functional impression of free end saddles ( class 1 and 2 partial denture)
  • to record posterior palatial seal
  • as a wax veneer on original impression to contact & register detail of soft tissue
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11
Q

State the use of bite registration wax

A
  • to record the relationship between upper and lower teeth
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12
Q

What dental amalgam contains ?

A

Silver , tin ,copper , zinc and mercury

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13
Q

What the function of copper in dental amalgam

A

To increase strength

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14
Q

What the function of zinc in dental amalgam?

A

Act as deoxidiser , which is an oxygen scavenger that minimises the formation of oxides of other elements in the amalgam alloys during melting

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15
Q

What is the weakest metallurgic phase of dental amalgam?

A

Tin-mercury phase

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16
Q

Classify dental amalgam according to the particle shape

A

A) Spherical - produced by atomising the liquid alloy in a chamber filled with inert gas.

B) Irregularly- shaped particles produced by milling or lathe p-cutting a cast of the amalgam alloys

C)As a mixture of both lathe-cut and spherical particles

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17
Q

If amalgam is undertriturated , the mix is ( …..) —forming a restoration that is ( …..) and has a ( …….. )

Overtrituration results in a ( …… ) that is also (…. )

A

If amalgam is undertriturated , the mix is ( grainy ) —forming a restoration that is ( weak ) and has a ( rough surface which is susceptible to tarnish )

Overtrituration results in a (softer , shinny and flat mixture) that is also ( weak )

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18
Q

Delayed expansion of amalgam occurs if amalgam is contaminated with ( ….) during (…..) or (….) phase.

It usually starts (….) after the placement and may continue for months , reaching values > 400micrometer/ cm

It is caused by the electrolytic action involving (…..) and ( …. ) which produces ( ….)

Accumulation of (…) increases the internal pressure , causing the amalgam to creep & produces expansion and leads to leaky margins

A

Delayed expansion of amalgam occurs if amalgam is contaminated with ( moisture [blood / saliva] during (trituration) or ( condensation ) phase.

It usually starts ( 3-5 days )after the placement and may continue for months , reaching values > 400micrometer/ cm

It is caused by the electrolytic action involving (zinc ) and ( water ) which produces ( hydrogen )

Accumulation of (hydrogen gas )increases the internal pressure , causing the amalgam to creep & produces expansion and leads to leaky margins

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19
Q

Tarnishing of dental amalgam is result of ( …. ) forming on the surface . It does not usually affect the mechanical properties of the restoration.

Corrosion refers the formation of ( … ) usually at the tooth interface and within the bulk of amalgam restoration. They have negative effects on the mechanical properties of the dental amalgam.

A

Tarnishing of dental amalgam is result of ( silver sufide ) forming on the surface . It does not usually affect the mechanical properties of the restoration.

Corrosion refers the formation of ( tin oxides and chlorides) usually at the tooth interface and within the bulk of amalgam restoration. They have negative effects on the mechanical properties of the dental amalgam.

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20
Q

High copper amalgam is advantageous over low copper amalgam as it prevents the ( …..) which has low strength and low corrosion resistance.

A

High copper amalgam is advantageous over low copper amalgam as it prevents the ( y2 phase [ tin-mercury phase ] which has low strength and low corrosion resistance.

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21
Q

State the ideal requirement of investment material

A

A) Chemically stable at high temperature
B) Strong enough to resist the force produced during casting procedure
C) Produce smooth casting without nodules
D) Sufficient porosity to allow gas escape (during burnout of wax)
E) Easy recovery of casting
F) Controllable expansion to compensate for shrinkage

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22
Q

List the types of investment materials and their uses

A

A)Gypsum-bonded investment materials
- to make gold inlay , onlay , crown and other larger fixed dental procedure.

B) Phosphate-bonded investment materials

  • to make restoration made out of base metal alloys ( Co-Cr) , palladium, porcelain fused to metal ( PFM ) restore, partial dentures
  • due to having higher melting temperature than gold, only PBI can handle the high melting temperature of these alloys

iii) ethyl silicate bonded investment materials
- used for high fusing base metal alloys to cast partial denture framework

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23
Q

Describe the composition of gypsum-based investment materials

A

A) Refractory material

  • silica ( cristoballite , quartz )
  • to provide refractory component ( thermal-shock resistance and regulate thermal expansion ( facilitate expansion of the investment material to compensate for the casting shrinkage of the solidifying alloy )

B) Binder

  • Alpha-hemihydrates of gypsum
  • Increase the strength of the investment

C) Additives / modifiers

i) reducing agents
- reduce any metal oxides formed on the metal
- example : copper

II)modifying chemicals

  • regulate setting expansion & thermal expansion & prevent shrinkage of gypsum > 300 C
  • Example : boric acid, soluble salts of alkali , alkaline earth metals

iii) colouring pigments

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24
Q

Describe the composition of phosphate-bonded investment materials

A

A) Refractory fillers

  • Silica ( quartz , cristoballite )
  • provide high thermal expansion , provide thermal shock resistance

B) Binder

  • magnesium oxide
  • monoammonium phosphate

C) Modifiers

  • carbon : facilitates clean casting & easy divesting, but only used when alloy is gold.
  • if silver- palladium or base metal alloys are used —> brittleness of casting
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25
Q

Type 1 phosphate bonded investment materials is used for casting of ( …) esp for alloys like gold, platinum , cobalt-chromium and nickel-chromium

Type 2 phosphate bonded investment materials is used for casting of ( …)

A

Type 1 phosphate bonded investment materials is used for casting of ( inlay , crowns & other restoration ) esp for alloys like gold, platinum , cobalt-chromium and nickel-chromium

Type 2 phosphate bonded investment materials is used for casting of ( removable partial denture [ RPD] )

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26
Q

Describe the composition of ethyl silicate bonded investments.

A

Refractory materials
-silica ( cristoballite , quartz )

Binder
-silica gel or ethyl silicate

Chemical modifier

  • MgO : strengthens the silica gel
  • Ammonium chloride : accelerator
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27
Q

State the setting time of investment materials

A

No shorter than 5 mins , no longer than 25 min

Avg : 9-18 min

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28
Q

State the steps involved in casting procedure

A

1) Construction of wax pattern
- direct or indirect technique

2) Attachment of sprue former to wax pattern
- sprue former attached to the thickest cusp at 45 degree angulation

3) Attachment of sprue former and wax pattern to crucible former
4) Investing of wax pattern in the casting ring
5) Burnout of wax
6) Casting
7) Recovery

8) Finishing & polishing
- pickling , sandblasting

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29
Q

For molars and metal - ceramic crowns, a sprue with a diameter of ( ……) is recommended.

For premolars and partial coverage restoration ( …) is recommended

A

For molars and metal - ceramic crowns, a sprue with a diameter of ( 2.5mm( 10 gauge)) is recommended.

For premolars and partial coverage restoration (2mm (12 gauge )is recommended

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30
Q

Describe how the sprue former should be attached to the wax pattern

A

Sprue former attached at 45 angulation to the point of greatest bulk ( usually thickens cusp ) prevent air entrapment during investment and suck back porosities after casting

Attachment should also be flared to prevent turbulence during flow of molten metal

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31
Q

When selecting recasting ring,
The height should be about ( …. ) above the wax pattern

The internal diameter should be (… ) greater than the widest measurement of the pattern

When casting single crowns / inlays , a small ring with diameter of (…..) is used

When casting FPDs , a ( …..) round / oval shaped casting ring is used

A

When selecting recasting ring,
The height should be about ( 6mm )above the wax pattern

The internal diameter should be ( 5- 10 mm)) greater than the widest measurement of the pattern

When casting single crowns / inlays , a small ring with diameter of ( 32mm ) is used

When casting FPDs , a ( 63mm ) round / oval shaped casting ring is used

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32
Q

State the types of casting ring liners

A

A) Asbestos
B) Cellulose paper
C) Fibrous ceramic aluminous silicate
D) Ceramic - cellulose combination

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33
Q

State the function of casting ring liners

A

A) Allows for mold expansion

B) Acts as thermal insulator , preventing heat loss while the casting ring is transferred from the furnace to the casting machine.

C) Allows for easy divesting of the investment

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34
Q

Explain the alloy classification by noble metal content ( ADA)

A

A) High noble - Must contain >= 40% Au and >=60% by weight of noble metal elements .

B) Noble : must contain >= 25% by weight of noble metal elements

C) Predominantly base metal (PB) : contains <25% by weight of noble metal elements

** Noble metals comprise of 7 metals that are resistant to corrosion and tarnish in the mouth.
( Gold , palladium , platinum , rhodium , ruthenium ,iridium and osmium )

**Silver called precious metal and is reactive in oral cavity and not considered a noble metal

35
Q

Explain the alloy classification by mechanical properties

A

(Type) (Descriptor) ( Yield strength ) ( Elongation ) ( Example of Application)

1 : Low : 80 : 18 : Inlays
2 : Medium : 180 : 10 : Inlays and onlays
3 : Hard : 270 : 5 : Onlays , thin cast backings , Pontiacs , full crowns , saddles
4 : Extra hard : 360 : 3 : Saddles , bars , claps , crowns, bridges and partial denture frameworks

36
Q

Explain the alloy classification by principal elements

A

(Metal type ) ( All Metal Prostheses ) (Metal - Ceramic Protheses ) ( Partial denture Frameworks)

High Noble : Au-Ag Pd : Au-Pt-Pd : Au-Ag-Cu-Pd

Noble : Ag-Pd-Au-Cu : Pd-Au

Predominantly base metal : Ti- Al -V : Cp Ti : Cp Ti

37
Q

Explain the alloy classification by dental applications

A

A) All metal fixed prostheses
B) Metal ceramic prostheses
C) Removable partial denture

High noble and noble alloys for metal ceramic prostheses can be used for all metal restoration ;
Whereas those all metal restoration are rarely used for metals ceramic restoration
Reason:
1) Alloys may not form thin , stable oxides required for porcelain bonding.
2) Their melting range may be too low to resist formation or melting range may be too low to resists deformation or melting at porcelain-firing temperature.
3) Their thermal contraction coefficient may not be close enough to those of commercial porcelains.

38
Q

State some properties of dental casting alloys

A

A)Biocompatibility

  • Alloys must tolerate oral fluids and not release any harmful products into oral environment.
  • When components of the alloy are released in the oral environment , they can cause a toxic or allergic reaction.

B) Tarnish and corrosion resistance
- Corrosion resistance is derived from the use of noble metals that do not react in the oral environment (gold , palladium)or by the ability of one or more of the metallic elements to form an adherent passiveting surface film , which inhibits any subsurface reactions ( chromium and titanium )

C) Porcelain bonding :
To achieve sound chemical bond to ceramic veneering materials , the alloys must be able to form a thin adherent oxide , preferably one that is light in colour so that it does not interfere with esthetic potential of ceramic

D) Ductility
- The mechanical property represent they amount of plastic deformation under tensile stre that an alloy can undergoes before it fracture

E) Hardness
- Hardness of the metal should be high enough to resist scratching and abrasion and also to maintain the smoothness of the prosthesis in the oral environment.

F) Fatigue resistance
- This phenomenon occurs when a materials is subjected to repeated loading and unloading below its elastic limit

39
Q

Explain about the stress-strain curve

A

Resilience : The resistance of a material to permanent deformation

A : Proportional limit , Elastic limit
Proportional limit : The greatest stress that a material will sustain without a deviation from the proportionality of stress to strain, below which no permanent deformation happens.

Elastic limit : Maximum stress that a material will withstand without permanent deformation.

B : Yield strength

  • The stress at which a material exhibits a specified limiting deviation from proportionality of stress to strain.
  • YS indicates a degree of permanent deformation ( usually 0.2%)

C)Ultimate Strength
: Tensile or Compressive
: The maximum that a material can withstand before failure ( tension or compressive )

D) Fracture Strength
: The stress at which the material fractures

40
Q

Define the Dislocation

A
  • Dislocation of the atoms from their specified place to some other place is called the dislocation in the crystal.
  • type of defect found in the crystal structure. When the load is being applied on the body it gets deformed and the dislocation is generated in the crystal structure.
  • This dislocation leads to slip in the crystal and the plastic deformation to occur.
41
Q

Define the slip

A
  • important mechanism of the plastic deformation in the material basically metal.
  • Whn the one plane is slide over the other plane along a definite crystallographic plane.
  • When the shear force is applied in the metal and this applied force exceeds their limit then slip occurred.
42
Q

Define the twinning

A
  • Interconnection of crystal
  • occurs in the crystal due to the crystal growth of the separate crystal.
  • They shares some of the same crystal lattices in a symmetrical manner
  • When the two crystal get together and share their crystal lattice , a twin boundary surface separate the 2 crystals.
43
Q

Explain the 3 Successive stages of annealing

A

A) Recovery

  • Cold worked properties begin to disappear.
  • Slight decrease in tensile strength.
  • No change in ductility
  • No changes in microscopic structure

B) Recrystallisation

  • The old grains are replaced by new set of grains
  • The material attains it’s original soft and ductile condition.
  • The fibrous structure is transformed to small grains.

C) Grain growth

  • Grain size range from fine to coarse
  • Fine grain structure if annealed further , grains begin to grow.
  • Large grains consume smaller grains
  • Grain growth process does not progress indefinitely to form single crystal.
  • Rather, an ultimate coarse grain structure is formed
44
Q

Explain the classification of casting defects

A

A) According of Anusavice

  • Distortion
  • Surface roughness and irregularities
  • Porosity
  • Incomplete or missing details

B) According to Rosensteil

  • Roughness
  • Nodules
  • Fins
  • Incompleteness
  • Voids or porosity
  • Marginal discrepancy
  • Dimensional inaccuracies

C) Based on location

  • internal
  • External
45
Q

Explain the distortion

A

Distortion of wax pattern occurs due to thermal changes relaxation of stresses that are caused by contraction on cooling , occluded air , moulding , arcing , removal and the time and temperature of storage

46
Q

State the remedy of distortion

A
  • Don’t over heat wax ,
  • Place in increments
  • Never cool the pattern suddenly
  • Carve with sharp instrument
47
Q

Describe how nodules are formed during the casting procedure and how to remediate them

A
  • Caused by air bubbles which attach to the pattern during or subsequent to the investing procedure.
- Caused by : 
I) air entrapment during investing 
II) inadequate vacuum during investing 
III) Improper brush technique 
IV) lack of surfactant —> poor wettability  of investment 
V) excessive & prolonged vibrations

Remedy

  • Use surfactant
  • Use proper vacuum investing technique.
48
Q

Describe how ridges / veins / water films form during the casting procedure and how to remediate them

A
  • occurs when a water film forms over the hydrophobic wax pattern

-caused by :
I) movement of the wax pattern after investing
II) lack of intimate contact b/w wax pattern & investment
III) too high L:P ratio

-remedy :
I) use surfactant

49
Q

Describe how fins form during the casting procedure and how to remediate them

A
  • Cracks in the investment material form allowing molten metal to fill them.
  • caused by:
    I) too high W:P ratio
    II) Improper positioning of the pattern in investment / wax patterns positioned too closely together
    -wax expands more than the investments —: formation of cracks
    III) too rapid heating —> steam formation
    IV ) excessive casting force
    V) rough handling of ring after investment.

Remedy :

  • Appropriate W:P ratio
  • Careful handling of investment
50
Q

Describe how spines are formed during casting procedures & how to remediate them

A
  • Due to rapid heating of investment

- Remedy : use at least 1 hour to heat the investment from room temperature to 700C , slow heating

51
Q

Describe the types of inclusions that can happen during casting

A

A) Carbon inclusions

  • Improperly adjusted torch
  • From crucible
  • From carbon- containing investment

B) Foreign body inclusions

  • Flakes of investment which may be carried into the mold during casting
  • Bits of carbon from flux
52
Q

State the reasons why incomplete casting may occur

A 
I) Insufficient metal alloy 
II) Incomplete melting of alloy 
III) blockage of sprue due to solidification, presence of loose investment particles 
IV) incomplete dewaxing 
V) Too low casting forces
53
Q

The average particle size of modern amalgam powders range from ( …..)

The (……) the particle size , the greater the surface area per unit volume.
Hence ( …..) amounts of mercury are required to react w/t the powder to form an acceptable amalgam .

Smaller average particle sizes tends to lead to more (……) rapid hardening of the amalgam which leads to ( ….)early strength

A

The average particle size of modern amalgam powders range from ( 15 - 35 micrometer )

The (smaller )the particle size , the greater the surface area per unit volume.
Hence ( larger )amounts of mercury are required to react w/t the powder to form an acceptable amalgam .

Smaller average particle sizes tends to lead to more (rapid ) hardening of the amalgam which leads to ( greater ) early strength

54
Q

State the objective of trituration of dental amalgam

A

On the alloy particles there is an oxides layer which hinders diffusion of mercury into the alloy
Trituration removes the oxide layer via abrasion , allowing intimate contact b/w mercury and alloy

55
Q

State ways to reduce microleakage in an amalgam filling

A
  • Use of cavity varnish
  • Use of dentin bonding agents
  • Self - sealing property of amalgam : corrosion product form b/w tooth and filling —> prevent leakage
56
Q

What is the compressive strength of a satisfactory amalgam should be ?

A

310MPa

57
Q

High copper dental amalgam alloys are available in 2 forms :
(……) & (….. )

Both contain ( ….) of Cu by weight

A

High copper dental amalgam alloys are available in 2 forms :
( Two phase admixed powder & Single composition single phase powder )

Both contain (>6%) of Cu by weight

58
Q

In wax burnout of casting procedures , the wax should be initially melted at (….. )for (….)min then the final wax burnout should be carried out (…..) for (…..) min

A

In wax burnout of casting procedures , the wax should be initially melted at ( 200C ) for ( 3) min then the final wax burnout should be carried out ( 650C ) for( 45 )min

59
Q

State the uses of wrought alloys in dentistry

A

1) Orthodontic wires
2) Endodontic files
3) Partial denture clasps
4) Prefabricated crowns
5) Surgical instruments

60
Q

Describe the types of point defects

A

1) Vacancy : a vacant lattice site
2) Di / trivacancy : 2 or more missing atoms
3) Interstitial :extra atom present in lattice space

61
Q

Describe passivation , sensitisation and stabilisation in stainless steel

A

Passivation : property of stainless steel to resists corrosion & tarnish due to formation of a layer of chromium oxide

Sensitisation
: formation of chromium carbides at grain boundaries due to heating processes in temp angle ( 400- 900c
Leads to the loss of corrosion resistances ad chromium is lost.
- Method to reduce involve : reducing carbon content and limiting time of exposure to the critical temperature range.

Stabilisation

  • addition of titanium and tantalum to form titanium / tantalum carbides
  • preserve the chromium
  • prevents corrosion& tarnish
62
Q

Describe the amalgamation reaction of low copper alloys

A
  • Final set product is a composite in which unconsumed particles of Ag3Sn is embedded within y1(Ag2Hg3 ) and y2 ( Sn7-8Hg) particles
63
Q

State how porosities can occur in dental amalgam

A

Compressive strength is reduced

  • Insufficient condensation pressure
    • Late condensation —> harden , difficult for intimate contact to form
  • Undertrituration - forms grainy mix which is not sufficiently plastic
64
Q

State some methods to maintain mercury hygiene

A

1) Work in a wells ventilate area
2) Properly store & dispose all amalgam waste through reputable dental vendors
3) Install amalgam particle separators in dental wastewater stream

4) Wear gloves while handling amalgam
- if comes in contact w bare skin

5) Eye, mask & gloves

65
Q

State the consequences of strain hardening

A

1) Increased surface hardness
2) Increase yield strength : more force required to produce further slip
3) Greater ultimate tensile strength
4) Decreased ductility : Internal stresses increase —> easier for brittle fracture to occur
5) Decrease corrosion resistance

66
Q

State the types of crystal structure seen in stainless steel & their applications in dentistry.

A

1) Ferritic : no applications in dentistry
2) Austenitic : Orthodontics wires , endodontic instruments , paediatric crowns
3) Martensitic : Surgical instruments due to high hardness

67
Q

Describe the martensitic stainless steel

A
  • Body centered tetragonal ( BCT ) crystal structure
  • produced by quenching of Austenite to undergo spontaneous , diffusion less transformation
  • very strong , brittle and hard alloy ; least corrosion resistance
    -uses : surgical & cutting instruments
  • composition :
    I) Chromium - 11.5% - 17%
    II) Nickel - 0-2.5%
    III) Carbon - 0.15 - 1.2%
68
Q

Describe the austenitic stainless steel

A
  • face centered cubic ( FCC) crystal structure
  • 18-8SS ( 18 chromium , 8% nickel j
  • most corrosion resistance
  • use for orthodontic wires , endodontic instruments & paediatric crowns
69
Q

Describe shrink spot porosities

A
  • Caused by premature termination of molten metal flow during solidification
  • Occurs of the sprue solidifies before casting
Remedy : 
A) Flare sprue at attachment 
B) Attach sprue at thickest cusp 
C) Use sprue of correct thickness 
D) Place reservoir close of wax pattern

Molten metal shrinks as it solidifies — if sprue freezes first there will be no molten metal to compensate the shrinkage - lead to localised shrinkage void

70
Q

Describe suck back porosities

A
  • entering molten metal impinges onto the mold surface at the occlusoaxial / incisoaxial ine angle , creating a high localised mold temperature at a region ( hot spot )
  • A hot spot may retain a localised pool of metal after other areas of the casting have solidified , creating a shrinkage void or suck back porosity.

Remedy - Flare sprue @ point of attachment
- low casting temperature @ 30C

71
Q

Describe the back pressure porosities

A
  • due to inability of air to escape through the investment or by pressure gradient that displaces air towards end of the investment
Remedy : 
I) proper W:P ratio 
II) Clearance should not exceed 6 mm 
III) sufficiency high casting pressure 
IV ) Adequate mold & casting temperature
72
Q

Co-Cr- Ni also called ( ….)

A

Co - Cr - Ni also called ( elgiloy )

73
Q

Describe the shape memory and superelastic properties of nickel titanium alloys

A

Both phenomena occurs due to twinning

Shape memory

  • Nitinol is made into certain shape at a high temperature (482C) (austenitic)
  • Then ,it is allowed to cool and formed into second shape (martensitic)
  • Upon reheating , the nitro reverts back to its original shape ( revert to original austenitic crystal structure )

Superelasticity
- ability of nitinol to undergo large deformations & immediately return to its unreformed shaped upon load removal ( austenitic —> martensitic transition is induced purely by stress )

74
Q

State the purpose of cobalt in nitinol

A

To control the transition temperature ( martensitic revert to austenitic ) at 37 C ( oral temperature)

75
Q

State the indication and contraindications of direct filling gold

A

Indications : Ideal for non esthetic non stress bearing areas
A) Class I small carious lesions - in pits on posterior teeth and lingual surface of anterior teeth.
B)Cervical 1/3rd of teeth
C) Class VI restoration on incisal edges or cusp tips provided there is no heavy occlusal stress

D) Repair of cast gold crown

Contraindications
A) Young patients or teeth with very large pulp chamber - hammering forces during compaction - not tolerated by pulp
B)Severely periodontally weakened teeth
C) Handicapped patient -unable to,sit for the long dental appointments
D)Root canal treated teeth -Teeth are brittle
E) Cases where cost is a limiting factor
F) Moderate to large carious defects

76
Q

State the advantages and disadvantages of direct filling gold

A

Advantages :

  • Biocompatible ( least pulpal & gingival irritation )
  • Insoluble in oral fluids
  • Excellent tarnish & corrosion resistance
  • Retains high polish - plaque accumulation is minimal
  • Ductile & malleable
  • Good adaptation to cavity walls
  • COTE similar to dentin

Disadvantages

  • Yellow color - not esthetic
  • Placement is time consuming & require technical skill
  • Expensive
  • High thermal conductivity (pulp protection needed in deep cavities
  • Cannot withstand high masticatory forces
77
Q

State the types of DFG

A 
1) Foil ( fibrous gold ) 
A) Sheet :Cohesive and non cohesive 
B) Cylinders, pellets 
C) Ropes 
D) Laminated
E) Platinized

2) Electrolytic Precipitate ( crystalline gold )
A) Mat gold
B) Mat foil
C) Gold- Ca alloy

3) Powdered Gold ( granular gold )

78
Q

Cohesive gold foil is gold foil which is (………..)

Non cohesive gold foil is gold foil which is ( ……..)

A

Cohesive gold foil is gold foil which is ( free of surface contaminants & inherently cohesive )

Non cohesive gold foil is gold foil which is ( Coated with a layer of ammonia to prevent adsorption of gases which prevent cold welding of gold )

79
Q

Describe the advantages of mat foil over mat gold

A

Mat foil is a combination of gold foil & mat gold

Mat gold is easy to compact & offers good adaptation to the cavity wall , but it is spongy in nature — can lead to pitting on the external surface

To solve this problem, the bulk of the cavity is filled with Mat gold and the surface is veneered with gold foil.

Mat foil prevents the need for surface veneering with gold foil

80
Q

Describe the manipulation of DFG

A

1) Degassing / Desorbing
- any DFG has to be first heated to remove any wax ( example ( powdered gold ) ,ammonia coating ( non- cohesive gold ) or adsorbed gases . this produces a surface free of impurities which is needed for cold welding

2) Compaction
- to adapt gold into preparation & prevents any voids between increments of gold
- Incremental pieces of gold attach together by colds welding.
- may be manual or mechanical compaction

3) Finishing & polishing
- surface is probed with explorer — if surface penetrates easily — condensation is continued with more force
- the gold is burnished to obtain better marginal adaptation & surface hardening
- for polishing , tin oxide paste is used

81
Q

Describe the methods that can be used in degassing of DFG

A

1) Bulk method ( B,C)
- Using a mica tray or electric annealer
- Used for all forms of DFG except powdered gold
- Gold is heated until a dull red glow appears
- may results in non uniform heating causing the pellets to stick together

2) Piece by piece method (A)
- Used for all forms of DFG
- Gold is heated over the inner blue region of a flame until a dull red glow appears

82
Q

(…..) is the only form of DFG which can be adequately condensed by hand pressure — the rest must be hand condensed together with a mallet or by mechanical ( electromallet ) condensation

A

(Powdered gold) is the only form of DFG which can be adequately condensed by hand pressure — the rest must be hand condensed together with a mallet or by mechanical ( electromallet ) condensation

83
Q

Describe the principle of compaction of DFG

A

1) Condenser
- tips of condenser is serrated to prevent slipping on gold
- tip size is between 0.5 - 1.0mm if too small the tip may punch through the gold restoration.

2) Angle of compaction forces
- Initially , the forces should be 45D to the cavity all and floor
- Then , the forces of condensation should be 90D to the previously condense gold

DM (4 decks)

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