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B4 Flashcards

1
Q

State the classification of dental ceramics

A

A) Uses or indications ( anterior & posterior crown , veneer , post and core fixed dental prosthesis , ceramic stain , glaze )

B) Composition

C) Principal crystal phase and /or matrix phase ( silica glass , alumina / zirconia and zirconia )

D) Processing method ( casting , sintering , partial sintering & glass infiltration )

E) Firing temperature ( ultra low fusing , low fusing , medium fusing and high fusing )

F) microstructure ( amorphous glass , crystalline , crystalline particle in a glass matrix )

G) Translucency ( opaque , translucent , transparent )

H) Fracture resistance (low , medium , high )

I) Abrasiveness ( Comparison relative tooth enamel against tooth enamel )

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2
Q

Describe about the composition of dental ceramics

A
  • Nonmetallic , inorganic structures , primarily containing compounds of O2 with one or more metallic or semi metallic elements ( aluminium , boron , calcium , cerium , lithium , magnesium , phosphorus , potassium , silicon , sodium )
  • contain a crystal phase and silicate phase. These structures characterised by chains of (SiO4)4 - tetrahedra in which Si4+ cations are positioned at the centre of each tetrahedron with O - anions at each of the four corners.
  • SiO4 tetrahedra are linked by sharing their corners. They are arranged as linked chains of tetrahedra , each of which contains two O2 atoms for every silicon atom.
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3
Q

Molecules with one oxygen atom ( such as Na2O , K2O or CaO ) useful in ( …………) as fluxes , may acts as (……)

A

Molecules with one oxygen atom ( such as Na2O , K2O or CaO ) useful in ( dental porcelain ) as fluxes , may acts as ( opacifiers )

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4
Q

Molecules that contains 3 oxygen atoms for every 2 other atom ( such as Al2O3) are used as (……..) They also added as crack blockers or toughening crystals.

A

Molecules that contains 3 oxygen atoms for every 2 other atom ( such as Al2O3) are used as ( stabilisers) They also added as crack blockers or toughening crystals

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5
Q

Explain about the Zirconia Ceramics

A
  • Major dental importance because of its high fracture toughness.
  • Pure ZrO2 is not useful because cracks occur during sintering as a result of transformation from the tetragonal to monoclinic phase.
  • Transformation can be fully or partially suppressed by the addition of certain oxides such as MgO, Y2O3 , CaO or CeO
  • Zirconia ceramic materials for dental prostheses are based on tetragonal zirconia particles ( TZP) that are fully stabilised with Yttria ( Y2O3)
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6
Q

(……….) 2370C

Monoclinic —————> Tetragonal ————-> (……….)

A

(1170C) 2370C

Monoclinic —————> Tetragonal ————-> (Cubic)

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7
Q

Explain the method for strengthening ceramic ( minimising the effect of stress concentration )

A
  • Stress concentration geometry at the tip of each surface flaw can increase the localised stress to extremely high levels even though a relatively low average stress exits throughout the bulk of structure.
  • Induced tensile stress > nominal strength of material structure the bonds at the notch tip rupture —-> crack

Condition producing stress concentration
1) folds of the platinum or gold foil substances that become embedded in the porcelain leave notches that acts as stress raiser

2) Sharp line angles in the preparations also create areas of stress concentration in the restoration
3) poor framework design of metal ceramic Pontic
4) Large changes in porcelain thickness , a factor that is also determined by tooth preparation
5) Occlusion is not adjusted properly on a porcelain surface , contact points rather than contact areas will greatly increase the localise stresses in the porcelain surface as well as within the internal surface of the crown

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8
Q

Explain the method for strengthening ceramics ( development of residual compressive stress )

A
  • process of cooling to room temperature offers the opportunity to take advantages of mismatches in coefficients of thermal contraction of adjacent materials in ceramic structure.
  • self glazed surface layer can also set up compressive stress on cooling
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9
Q

Explain the method for strengthening ceramics ( minimising the number of firing cycles )

A
  • porcelain firing procedure is to densely sinter the particles of powder together produce relatively smooth , glassy layer ( glaze ) on the surface.
  • This increases the concentration of crystalline leucite in the porcelains designed for fabrication of metal ceramic restoration.
  • Changes in leucite content caused by multiple firing can alter the coefficient of thermal concentration of some porcelain products
  • This increase their coefficient of thermal expansion.
  • If the expansion coefficient increases above that , expansion mismatch between the porcelain and the metal can produce stresses during cooling sufficient to cause immediate or delayed crack formation in the porcelain.
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10
Q

Explain the method for strengthening ceramics ( thermal tempering )

A
  • most common for strengthening glasses
  • creates residual surface compressive stresses by rapidly cooling the surface of the object while it is hot and in the softened ( molten ) state .
  • rapid cooling produces skin of rigid glass surrounding a soft core . As the soft core solidifies , it tend to shrink but the outer skin remain rigid . The pull of solidifying molten core , as it shrinks , creates residual tensile stress in the core and residual compressive stresses within the outer surface.
  • Thermal tempering produces a protective region of compressive stress within the surface
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11
Q

Explain the advantages and disadvantages of metal ceramic restoration

A

Advantages :
A) Resistance to fracture , especially with metal occlusal surfaces , the fractures rate in posterior sites could be reduced further.
B) Less tooth structure need to be removed to provide the proper bulk for the crown.
C) Less wear of antagonist enamel than occurs when enamel is opposed by ceramic surfaces

Disadvantages
A) Potential for metal allergy
B) Not the best esthetic choice for restoring .

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12
Q

Explain the requirement of alloy for metal ceramic restoration

A

1) They have potential to bond to dental porcelain
2) They exhibit coefficients of thermal contraction compatible with those of dental porcelain.
3) Their solidus temperature is sufficiently high to resists softening during sintering of porcelain.

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13
Q

State the types of metal alloy for metal ceramic restoration

A

A) High noble and noble alloy

B) Base metal alloy

C) Titanium based alloy

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14
Q

Explain the advantages for high noble and noble alloy in metal restoration

A
  • alloys contain palladium
  • Has high melting point
  • Improves sag resistance during firing and thermal contraction coefficient is lower than silver , gold and platinum
  • helpful in developing lightweight metal ceramic alloys that are compatible with currently used dental ceramic

Ex : Gold- Platinum - Palladium Alloy
: Gold - Palladium Alloy
: Palladium - Gold - Silver Alloys

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15
Q

Give some examples for base metal alloys

A

A) Ni- Cr

B) Cr- Co

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16
Q

Explain the characteristic of base metal alloys

A

A)Density of base metal alloy is about half that of gold based alloy , lighter weight

B)High hardness and high strength of these base metal alloys contribute difficulty in sandblasting , difficulty for occlusal adjustment

C) Thickness of oxide layer and wetting of the oxide layer by porcelain similar to noble alloys

D) Thermal contraction differential between base metal alloys and dental porcelains may under certain condition , contribute to high levels of residual tensile stress in porcelain and induce transient cracking of porcelain or delayed failure

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17
Q

Explain the characteristics of titanium base alloys

A
  • pure titanium undergoes phase transformation at 882 C
  • For CP Ti metal and alpha-beta titanium alloy , the firing temperature for porcelain sintering must be performed well below that temperature to prevent any phase transformation that may alter metal properties upon cooling
  • so low fusing fusing ceramics should be used
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18
Q

Explain the composition of ceramics

A
  • Silica ( SiO2) network , potash feldspar , soda feldspar or both .
  • Pigments , opacifiers & glasses are added to control fusion temperature , sintering temperature , coefficient of thermal contraction and solubility
  • Pigments are added to produce the hues of natural teeth or the colour appearance of tooth coloured restorative materials that may exist in adjacent teeth
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19
Q

Explain the factor affecting metal ceramic bonding

A

1) Mechanical interlocking or interatomic bonding at the interface between porcelain and the metal oxide
3) Type and magnitude of residual stress in veneering ceramics

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20
Q

Explain the types of metal ceramics bond failure

A

1) Cohesive failure : Porcelain - porcelain , metal-metal , oxide - oxide
2) Adhesive failure : Porcelain - oxide , metal -oxide , metal - porcelain
3) Mixed failure : combination of types of failure

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21
Q

List the 4 types of veneering ceramics

A

1) Ultralow and low fusing ceramics ( feldspar based porcelain , nephelometer syenite based porcelain and apatite based porcelain)
2) Low fusing specialty ceramics ( shoulder porcelains and wash coat ceramics )
3) Ceramic stains
4) Ceramic glazes ( auto glaze and overglaze)

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22
Q

(………….) is the main glass forming structure used in all dental veneering ceramics

A

( Silica [SiO2] ) is the main glass forming structure used in all dental veneering ceramics

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23
Q

Describe the procedure involved in fabrication of metal-ceramic restoration

A

1) Porcelain condensation
- may be accomplished by 3 methods : vibration , spatulation or brush technique
- usually the porcelain is layered to be larger than the desired prosthesis to accommodate for shrinkage during sintering

2) Sintering procedure
- to sinter the powder particles together
- leads to changes in leucite content of the porcelains which affects the COTE of the porcelain
- condensed porcelain mass is placed into furnace & firing cycle is initiated
- as porcelain particles bond together , the structure shrinks and densifies

3) Cooling

4) Glazing
- to obtain a smooth surface that stimulate a natural tooth surface
- 2 methods : auto glazing or add-on glazing
- add on glazing : a layer of glaze is added & fired
- auto glazing : rapid heating of the porcelain up to fusion temperature for 1-2 min to melt the surface particles ( preferred method )

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24
Q

Describe the methods of condensing porcelain

A

1) Vibration technique
- mild vibration is used to pack the wet powder onto the metal coping
- excess water is bottled away w/ tissue & condensation occurs towards the bottled area

2) Spatulation technique
- small spatula is used to apply & smooth the wet porcelain
- smoothing actions brings excess water to the surface where it is removed

3) Brush technique
- Dry powder is placed by a brush to the side opposite from an increment of wet porcelain
- as the water is drawn towards & absorbed by the dry porcelain , the wet particles get pulled tgt

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25
Q

Welding is the process (…………….) through application of heat pressure or both ( ………. ) to produce a localised union across an interface between the parts.

A

Welding is the process (fusing 2 or more metal parts )through application of heat pressure or both ( without filler metal ) to produce a localised union across an interface between the parts.

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26
Q

Explain the technical consideration for welding

A
  • welding is done by passing electric current through the pieces to be joined
  • these pieces are simultaneously pressed together. The resistance of the metal to flow of current causes intense localised heating and fusion of the metal.
  • The combination heat and pressure fuses the metals into a single piece.
  • These called electric spot welding apparatus hence called as spot welding
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27
Q

Explain the types of welding

A

1) Cold welding
- process of plastically deforming a metal ( room temp )
: Gold foil filling

2) Hot welding
: use heat of sufficient intensity to melt the metals being joined

3) Spot welding
: involves both heat and pressure
: joining orthodontics component

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28
Q

Soldering is a process of joining metal by use a ( ……. ) which has low fusion temperature ( …….) than that of the metal being joined

: Brazing means the fusion temperature of filler metal exceed ( ……..)

A

Soldering is a process of joining metal by use a ( filler metal ) which has low fusion temperature ( <450C) than that of the metal being joined

: Brazing means the fusion temperature of filler metal exceed (450C)

29
Q

Compare about soft and hard solders

A

Soft solders

  • low melting range ( 260C)
  • Lack corrosion resistance
  • Not suitable for dental use

Hard solders

  • Higher melting temperature
  • Have greater strength and hardness
  • Commonly used on dentistry : Gold and silver solder
30
Q

Explain the properties of dental solder

A

: Need have food flow and wetting of parent metal

Factor affect flow :
A) Short Melting range -> good flow
B) Gold and silver based alloy better flow than nickel based alloy
C) Presence of oxides on parent metal reduce the flow
D)Gold content increases ; Tarnish and corrosion resistance increase
E) Gold and silver have adequate strength and hardness compare to dental casts gold alloy

31
Q

Explain the function of flux

A
  • To remove any oxide coating on the parent metal

- To protect the metal surface from oxidation during soldering

32
Q

Describe the steps in soldering

A

1) Selection of solder
2) Cleaning and polishing of components
3) Assembly of the prosthesis in soldering investment
4) Application of flux
5) Preheating the bridge assembly
6) Placement of solder
7) Application of hot gas flame to joint and solder
8) Cooling of assembly followed by quenching in water

33
Q

Explain the technical considerations for successful soldering

A

A) Selection of solder : proper fusion temperature, colour and flow
B)Cleanliness : free of oxides and dirts
C) Gap between parts : ranging from 0.13 to 0.30
D) Flux : much of flux weaker the joint
E) Flame:Neutral or reducing in nature
F)Temperature: Optimum temperature
G)Time : flame is held until the filler metal has flowed completely into the gap .

34
Q

All ceramic restoration come in 2 system :

A

1) Bilayered ( veneered )
- composed of high strength core ( zirconia ) , covered w/ highly esthetic ceramic veneer.
- ex : porcelain-fused-to-zirconia (PFZ)

2) Monolithic ceramic systems
- composed of one layer , no core or veneer
- ex : monolithic zirconia

35
Q

State the method of production all ceramic system :

A

1) Refractory die method
2) Heat-pressed ceramic systems
3) CAD-CAM processing

36
Q

In all- ceramic restoration,

Leucite-based glass ceramics ( flexural strength : (……….) ) are comparatively (………) than lithia disilicate-based glass ceramics ( flexural strength (…….)

A

In all- ceramic restoration,

Leucite-based glass ceramics ( flexural strength : (112MPa) ) are comparatively (weaker ) than lithia disilicate-based glass ceramics ( flexural strength (350MPa)

37
Q

Lithia dislocate-based glass ceramics is pressed at ( ……..) and layered with a glass containing some dispersed apatite crystals during the heat-pressing technique for fabrication of all - ceramics restoration

A

Lithia dislocate-based glass ceramics is pressed at ( 920C)and layered with a glass containing some dispersed apatite crystals during the heat-pressing technique for fabrication of all - ceramics restoration

38
Q

Describe the fabrication of heat pressed ceramic

A
  • ceramics ingots are vacuum pressed at a high temp (1165 C) using an alumina plunger into a refractory mold made by lost wax technique.
  • Solidified sprue and ceramic restoration is divested
  • sprue is removed
  • restoration is then seated on the die
  • Then it is finished 2 techniques ( characterised techniques) using a conventional furnace
    I) Surface stain — supplies stain paste is mixed 2/ glaze , applied to the framework and fired
    II) A layering technique involving applications of a veneering porcelain
39
Q

Compare btw the all ceramic and PFM crown

A

1) PFM indicated in bruxism , excessive occlusal force ; All ceramic not indicated
2) All ceramic better in aesthetics ; PFM worse , due to metal at the gingival margin esp seem in pt w/ gingivitis
3) All ceramic have less tooth preparation required while PFM have more tooth preparation required
4) All ceramic less suitable for multiple unit prosthesis ( long span bridges ) compared to PFM
5) PFM have higher strength than all ceramics
6) PFM cause metal allergy n ceramic more compatible

40
Q

Describe the stages of firing during the sintering procedure of PFM fabrication

A

1) Low bisque stage
- particles lack complete adhesion
- Very porous
- Low amount of shrinkage seen

2) Medium bisque stage
- better cohesion b/w powder particles
- some porosity seen

3) High bisque stage
- particles fused to form a continuous mass
- complete cohesion
- no shrinkage seen

41
Q

Compare the porcelain teeth and acrylic teeth

A

Retention :
P - Retains to denture base via mechanical retention ( holes and pins )
A - retains to denture base via chemical bonds

Hardness and abrasiveness
P- Greater ( 500MPa ) -> leads to attrition of opposing enamel . Also makes it harder to adjust
A- Lesser ( 20MPa) , less attrition of opposing enamel due to bruxism. Easier to make occlusal adjustments

Strength :
P - Greater
A-Lesser

Density
P - More , hence heavier
A- Lesser , hence lighter

Aesthetics
P- better
A- worse

Biocompatibility
P- More
A- less

42
Q

Feldsparhic porcelains are used for making ( ………. ) and (………….)

Lithia dislocate porcelains are used for making ( ………. ) as well as ( ……… )

Zirconia porcelains are used for making ( ………. )

A

Feldsparhic porcelains are used for making ( anterior laminate veneers ) and ( metal-ceramic veneers )

Lithia dislocate porcelains are used for making ( crowns for anterior and premolar ) as well as ( anterior three unit bridges)

Zirconia porcelains are used for making ( posterior crown & bridges )

43
Q

Describe the types of dental implants

A

1) Subperiosteal
- A framework that rests upon the bony ridges but does not penetrate it
- implant lies over the cortical bone but beneath the periosteum

2) Transosteal
- An implant that penetrate completely through both cortical plates and the thickness of the alveolar bone
- combination of subperiosteal & endosteal
- very painful , extensive surgery required

3) Endosteal
- partially submerged & anchored within the bone — placed into alveolar bone and/or basal bone of the mandible or maxilla that transects only one cortical plate
- used currently

4) Intramucosal
- only into the soft tissue
- associated w/ very painful healing

44
Q

Classify the types of materials used for dental implants

A

1) Metals
- Titanium , titanium alloys , surface coated titanium
- stainless steel
- Co-Cr-Mo based ( vitallium )

2) Ceramics
- Hydroxyapatite
- bioglass
- aluminium oxide
- zirconium

3) Polymers& composite
- PMMA
- polytetrafluoroethylene
- polyurethane
- polyamide
- polyethene

4) Others
- carbon , carbon-silicone
- gold
- tantalum

45
Q

Describe the forms of titanium-based implants

A

Available in few forms :
I) Commercially pure Ti ( cp Ti ) , available in grades I-IV
II) Ti alloy s : Ti-4Al-6V
III) surface - coated titanium ( coated w/ tricalcium phosphate, HA ) -> bioactive materials which stimulate bonding to bone & bone growth

46
Q

Describe the properties of titanium-based implants which makes it the first choice for dental implants

A

1) Ability to osseointegrate
- bone is deposited on the dental implant without intervening fibrous tissue

2) High corrosion & tarnish resistance
- due to passivation —> formation of TiO2

3) High strength
4) Low density ( 4.5 gm/cm2 )
5) Modulus of elasticity similar to bone
6) Good stiffness
7) High heat resistance

47
Q

Describe the zirconia-based dental implants

A
  • alternative to Ti implant in case of Ni allergy
  • better aesthetics due to its white colour , suitable for anterior regions
  • bioinert material ( bone is deposited onto the implant )
48
Q

State the steps of compression moulding of heat-cured denture base

A

1) Construction of denture wax pattern

2) Flasking
- seperating media is coated onto the master cast and temporary denture base to prevent adherence of dental plaster.
- lower portion of denture flask is filled w/ plaster & master cast is placed onto the mixture
- dental plaster is contoured , exposing the land area
- stone is also coated with separating media
- upper portion of denture flask in then put over the lower flask & filled w/ plaster
- lid of the flask is seated over the flask and allowed to harden

3) Dewaxing
- flask is immersed in boiling water for 15 min to soften wax
- flask is opened & wax is removed
- teeth remain adhered to dental plaster

4) Application of separating media
- applied all over the mold surface and cast but not over the denture teeth

5) Proportioning & mixing of heat cure acrylic
- polymer : monomer ratio of 3:1 by volume

6) Packing of heat cure acrylic
- occurs during the dough stage
- the flask assembly is then placed into a press & pressure is applied incrementally , causing the acrylic to flow into the mold spaces
- excess acrylic ( flash) is removed
- continue until no more flash

7 ) Curing

  • to convert monomer into polymer
  • 70C for 7 hours followed by 100C for 3 hours
  • 70 C for 7 hours followed by 100 C for 1 hour

8) Retrieval of prosthesis , finishing & polishing
- pumice is used to polish the denture

49
Q

State the objective of relining and rebasing

A

1) Restore retention and stability to the denture
2) Restore lost occlusal and maxillae-mandibular relationship
3) Re-establish the correct relation of the denture to the basal tissue.

50
Q

Describe the addition polymerization

A
  • self addition of monomer molecules to each other , very rapidly through a chain reaction without simultaneous production of byproducts
  • 3 major steps are involved : initiation , propagation & termination.
51
Q

Describe condensation polymerization

A
  • brought about by monomer containing 2 or more reactive functional groups such as -COOH , -OH and -NH2
  • byproduct formed : water
  • ex: polysulfide
  • basically functional groups of monomer react together to form the polymer
52
Q

Describe the stages of free radical addition polymerisation

A

1) Initiation
- activators ( heat , N-N-dimethyl P-toludine, microwave , visible light ) react with initiators (benzoyl peroxide , benzoin methyl ether , camphoroquinone ) to form free radicals
- free radicals reacts with monomer molecule to form free radical monomer complex
- R’ + Monomer =R-M’

2) Propagation
- free radical monomer complex acts as a new free radical centre —> reacts w/ another molecule to form dimer ( which also become a free radical )
- R-M’+M—> R-M-M’
- continues until the chain growth is stopped by the free radicals being killed by some impurities or all the monomer has been reacted

3) Termination
- any further addition of the monomer units to the growing units is stopped and growth of the polymer chain is arrested
- may be terminated by direct coupling of 2 free radicals ends or by the exchange of 1 H2 atom from one growing chain to the other to form double bond ( disproportionation)

4) Chain transfer
- in this process the free radical molecule of a growing chain is transferred onto another molecule ( eg : inactive monomer or already terminated polymer chain )
- cause termination of the original chain & activation of the chain in which the free radical is tranferred onto.

** although chain transfer can result in termination, addition polymerisation results are usually terminated via direct coupling or exchange of H atom from one chain to the other.

53
Q

Describe the copolymers and its types

A
  • polymer which is composed of more than one types of repeating units
  • monomers from which a copolymer is made are known as comonomers
TYPES : 
I) alternating : nA+ nB -> A-B-A-B-A-B
II) random : nA+nB -> A-B-B-A-A-B 
III) block : nA+nB -> A-A-A-A-B-B-B-B
IV) graft copolymer : A forms the backbone , B forms branches
54
Q

Describe the cross linking

A
  • it is formation of chemical bonds between linear polymer molecules
  • forms a non-linear , branched or crosslinked polymer
  • cross-linking agents : ethylene glycol dimethacrylate
55
Q

Describe the plasticizers and its types

A
  • Substances added to resins to decrease their softening or melting / fusion temperature -> less heat required for the material to become soft / plastic like
  • It increase the solubility of the polymer in monomer
  • it decrease the brittleness ( harder to crack , more flexible) , but it decrease the strength & hardness
  • used to form soft , cushioning liners for denture
  • Types :
    i) external plasticizers
  • it penetrates macromolecule and partially neutralises the intermolecular attractions which normally prevent the resin chains from slipping past one another ( example : undergoing plastic flow ) when the material is stressed —> hence it helps make the resin flow more easily when stressed as bonds are neutralised
  • insoluble , high boiling compound
  • not widely used as it may evaporate or leach out during normal use of resin —> loss of plasticity

II) internal plasticizer
: accomplished by copolymerization with a suitable commoner
- plasticizer become part of the polymer
- ex : butyl methylacrylate is added to methylacrylate

56
Q

State the classification of denture base resin & its ADA’s specification number.

A

ADA specification number : 12

Type 1 : Heat polymerisable polymer
I) Class 1 - in powder or liquid form
II) Class 2 - in cake form

Type 2 : Autopolymerizable polymers / cold cured

Type 3 : Thermoplastic blank or powder

Type 4 : Light activated materials

Type 5 : Microwave-cured materials

57
Q

List some requirements of denture base resins

A

1) Dimentionally stable
2) Easily manipulated
3) High strength , stiffness , hardness & toughness
4) Ease of repair
5) natural appearance
6) resistance of absorption of oral fluids
7) absence of odor , taste , toxic products

58
Q

Describe the 5 stages of polymer-monomer interaction.

A

Polymer-to-monomer ratio —> 3:1 by volume — limits volumetric shrinkage to approx 7%

1) Sandy stage
- Little or no interaction occurs on molecular level
- polymer beads remain unaltered
- consistency of the mix —> coarse / grainy

2 ) Stringy stage

  • Monomer attackers the surface of individual polymer beads & is absorbed into the beads
  • polymer chains uncoil , increasing the viscosity of the mix
  • characterised by stringiness when the materials is touched

3) Dough stage
- an increased number of polymer chains enter the solution — monomer & dissolved polymer are formed
- mass behaves as pliable dough , no longer tacky and does not adhere to the surface of the mixing vessel or spatula.
- ideal for compression moulding

4) Elastic / rubbery stage
- monomer is dissipated by evaporation & further penetration into remaining polymer beads
- mass rebounds when compressed or stretched
- no longer flows freely & cannot be modded by conventional compression technique

5) Stiff stage
- due to continued evaporation of unreacted monomer
- mixture is very dry & resistant to mechanical deformation

59
Q

Describe the composition of heat cured & autopolymerizing denture resins

A

Powder component
*I) prepolymerized spheres of polymethyl methacrylate
*II) Benzoyl peroxide ( initiator — forms free radicals
III) pigments ( salts of cadmium , mercury , iron or organic dyes )
IV) radioopacifiers ( titanium / zinc dioxides )
V) dyed organic fibers for natural appearance of tissue ( nylon , acrylic )
VI) dibutyl phtalate ( plasticisers)

Liquid component
I) methyl methacrylate monomer
II) hydroquinone ( inhibitor )
III) glycol dimethyacrylate at 1-2% by volume ( cross linking agent )
IV) ** N N’ - dimethyl-p-toludine ( activator , only in autopolymerizing

-Glycol dimethyacrylate —> 2 C=C bonds per molecule while MMA —>1 C=C bond per molecule , hence 1 molecule of glycol dimethyacrylate can participate in the polymerisation of 2 separate polymer chains that unites 2 polymer chains , if sufficient glycol dimethyacrylate —> form extensive interconnection —> increase resistance to deformities

60
Q

Describe the light activated / light cured denture base resins ( composition , activator and initiator )

A

Composition
I) Urethane dimethacrylate ( UDMA )
II) Microfine silica
III) High-molecular-weight acrylic resin monomers —> acts as organic fillers

Activators
- high intensity visible blue light

Initiator
- camphorquinone ( photosensitizing agents )

  • supplied in sheet & rope forms in lightproof package ( prevent accidental polymerisation ) —> denture base mold Ed on cast & teeth positioned —> exposed to blue light —> polymerisation —> denture is finished & polished
61
Q

Compare heat cured acrylic resins with autopolymerized ( cold cured ) acrylic resins ( classification , activator , degree of activation , dimensional stability , colour stability )

A

Heat cure denture base resin

  • Type 1
  • Thermal heat >60 ( water bath , microwave oven)
  • Benzoyl peroxide
  • More complete ( Hence stronger impact strength , hardness , lower creep )
  • less dimensional stability
  • more stable in colour stability & aesthetics

Auto polymerized denture base resin

  • Type 2
  • N-N’ dimethy-P-toluidine
  • Benzoyl peroxide
  • Less complete ( greater amt of unreacted monomer present —> acts as a plasticizer & decrease transverse strength of denture base resin , also act as potential tissue irritant
  • Fractures & abrades more easily
  • More ( Less shrinkage displayed )
  • Less stable ( Due to N-N’ dimethyl-P- toluidine —> susceptible to oxidation —> Color change ) (bleaching)
62
Q

State the reasons why porosities may appear in denture base resin

A

1) Inadequate mixing of liquid & powder components
2) Inhomogeneity of resin mass
3) Inadequate pressure or insufficient material
4) Air inclusion incorporated during mixing & pouring procedure
5) Excess monomer used
6) Too high curing temperature leading to gaseous porosities

63
Q

Curing cycle:
Popular method is to heat the flask containing the dough for (……….) hours at (……)C ; followed by (…….)hours at (……..) C. Most of the conversion of (……….) occurs during the seven hours 70C .The final 3 hours at 100C ensure (…………….) in those thinner areas of the denture base where the effect of the exothermic heat is less pronounced.

A

Curing cycle:
Popular method is to heat the flask containing the dough for ( 7) hours at (70) C ; followed by (3) hours at (100) C. Most of the conversion of (monomer to polymers) occurs during the seven hours 70C .The final 3 hours at 100C ensure (complete conversion of polymer ) in those thinner areas of the denture base where the effect of the exothermic heat is less pronounced.

64
Q

Define relining

A

Process of replacing the tissue-contacting surface of an existing denture with a new material.

65
Q

Define rebasing

A

Process of replacing the entire denture base of an existing complete or partial denture

Only the original teeth & their arrangement remain

66
Q

Distinguish b/w hard liners , soft liners and tissue conditioner

A

Hard liners

  • a layer of plastic is removed from denture interior surface & then filled with putty like material
  • done every 2 years

Soft liners

  • a layer of soft, pliable material that is fitted onto the impression surface of the denture
  • it absorb shock b/w the hard base of denture and gums

Tissue conditioner

  • A short term soft liner ( up to 14D)
  • applied to tissue surface of the denture
  • mainly elastomer used to treat irritated tissues , once it loses their elasticity it should be replaced

Example : PMMA ( heat and cold cured ) , butyl methacrylate , silicone materials

67
Q

State the indications for relining and rebasing

A

1) Alveolar ridge resorption
2) Loss of retention & stability due to loss of correct relationship to supporting tissue
3) Decreased occlusal vertical dimension

4) Socioeconomic constraints
- pt. does not have money to replace .

68
Q

State the contraindications of relining & rebasing

A

1) Unresolved TMJ dysfunctional & myofacial pain
2) Abused, inflammed and pathologic basal seat mucosa
3) malpositioning of articulation teeth
4) Multiple fracture & severely worn artificial teeth

DM (4 decks)

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