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REVISION BDS > dental materials 93 > Flashcards

dental materials 93 Flashcards

1
Q

force in

A

mg

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2
Q

m =

A

mass (kg)

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3
Q

g =

A

gravitational acceleration (10ms-2)

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4
Q

compressive strength

A

resistance to breaking from a force acting to reduce its size

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5
Q

tensile strength

A

resistance to breaking from a force acting to elongate

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6
Q

shear strength

A

resistance of a material to moving along an axis which is parallel to the forces direction

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7
Q

strain

A

change in length / original length

(L1 - L0) /L0

given as a ration or %

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8
Q

Young’s Modulus =

A

Stress /strain

F/A or (L1-L0)/L0

given in MPa

assess how rigid a material is

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9
Q

opposite of rigid is

A

flexible

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10
Q

fracture

A

large force causes a catastrophic destruction of materials structure

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11
Q

hardness

A

ability of surface to resist indenetation (KHN)

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12
Q

abrasion

A

material surface removal due to grinding

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13
Q

abrasion resistance

A

ability to withstand layers being removed compromising surface integrity

grinding along opposing tooth surface

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14
Q

fatigue

A

repititive ‘small’ stresses cause material fracture

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15
Q

creep

A

gradual dimensional change due to repetitive small forces (amalgam when it creeps above margins - standing proud then fracture)

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16
Q

deformation

A

an applied force may cause a permanent change in materials dimensions (not fracture it)

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17
Q

elasticity

A

impression materials - strain and recoverery

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18
Q

de-bond

A

applied forces sufficient to break material tooth bond by shear forces (ortho appliances)

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19
Q

impact

A

large sudden forces causes fracture - curve of upper dentures to accomodate palate maean that they are liable to snap

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20
Q

bonding to enamel

A

hetergenous structure (5% organic, 95% inorganic)

‘dry’

acid etch technique - remove cores of enamel prism leaving just peripheral enamel (creates pores for resin)

bonding to enamel is simple

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21
Q

bonding to dentine

A

dentine composition - 20% organic (collagen), 70% inorganic (hydroxyapatite), 10% water

fluid from pulp flows up dentine base making the surface wet

dentine varies - aged dentine more mineralised, pulpal dentine has increased moisture content

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22
Q

requirements of dentine bonding agent DBA

A

flowability

intimate contact with dentine surface

low viscosity

adhesion to substrate - mechanical, chemical, van der waals

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23
Q

smear layer is

A

pulp, dentine, bacterial debris plug dentine holes

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24
Q

what to do with smear layer

A

has to be removed by acid conditioning to either dissolve or solubilise the plugs

expose the tubules to create pores for resin

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25
Q

critical surface energy

A

the surface tension of a liquid that will just spread on the surface of a solid

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26
Q

importance of critical surface energy and dentine

A

a liquid must have a lower surface energy than the surface it is being placed on to flow and then stick

lower SE liquid will flow onto a higher SE substrate = lower SE as a whole

Wet Dentine has a lower SE than Composite filling materials

  • Therefore this has to be reversed so that the Wet Dentine has a Higher SE than composite
  • DBAs increase surface energy of dentine surface to allow composite to flow and stick
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27
Q

dentine adhesion through molecular entanglement

A

Adhesive absorbed onto surface but also into interior of dentine due to good wetting/surface energies

  • absorbed component can polymerise
  • polymer meshes with substrate- molecular entanglement = high bond strength
  • phosphate-calcium bonds formed *can be hydrolysed by saliva/dentinal fluid = weakened bonds
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28
Q

3 components of total etch

A

dentine conditioner (acid e.g. phophoric 37%)

primer

bond

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29
Q

denitne conditioner in total etch

A

acid - phophoric 37%

  • removes smear layer
  • opens dentine tubules by removing smear plugs
  • decalcifies upper layer of dentine
  • etch washed off with water
  • collagen network in this top 10μm
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30
Q

primer in total etch

A

*!Adhesive part of agent!*

  • Hydrophilic end bond to dentine (think of philic and wetted dentine surface)
  • Hydrophobic Methycrylate end bond to composite

Molecule has to have a spacer group to allow it to be flexible in bonding for all sites

Has a solvent (acetone, ethanol or water) to dissolve primer agent

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31
Q

bond in total etch

A

Resin that penetrates into dentine surface attaching to primers hydrophobic surface

  • Mixture of resins (Usually Bis-GMA and HEMA)
  • Predominantly Hydrophobic
  • May contain filler and camphorquinone
  • forms micromechanical bond within tubules and exposed dentine collagen- Hybrid layer (collagen and resin)
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32
Q

problems with total etch

A
  • Overetching can cause collagen to collapse so no resin can penetrate
  • Overetching can mean the depth of etch is too much for the resin to penetrate fully leaving areas of unsupported collagen
  • Moisture dependence- too dry (dentine collapses) too wet (primer dilutes- reduced strength)
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33
Q

other option for DBA that isn’t total etch

A

Self Etching primer + Seperate adhesive

  • DO NOT remove the smear layer- instead incorporate into bonding matrix
  • not washed off!
  • not as technique sensitive in terms of moisture but bond itsef not as good

Usually found as a one bottle solution (Self etch and adhesive)

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34
Q

uses of composite resin

A

primary caries

abrasion

erosion

failed restorations (secondary caries)

trauma

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35
Q

mechanical properties for composite resin

A

smooth surface finsih/polishable

technique sensitive

low setting shrinkage (bonding agents and good technique to maintain this)

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36
Q

thermal properties of composite resin

A

thermal expansion coefficient pretty poor compared to amalgam and GI

under cold stimulus the composite can shrink away from cavosurface margins

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37
Q

biological properties of composite resin

A

biocompatible - generally ok (unreacted monomer can be issue)

anticariogenic - gennerally not but some release F

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38
Q

5 classes of components for composite resin

A

filler particles

resin

camphorquinone

low weight dimethycrylate

silane (coupling agent)

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39
Q

types of filler particles in composite resin

A

conventional

  • glass/quartz

microfilled

  • microfine silica

hybrid

  • combination of both
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40
Q

increased proportion of filler in composite resin =

A

decreased thermal expansion coefficient

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41
Q

role of filler in composite resin

A
  • improve mechanical properties of material
  • lower polymerisation contraction
  • some fillers are radiopaque
  • greater strength etc
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42
Q

resin in composite resin

A

BISGMA

  • Bisphenol-A
  • Glycidyle Methacrylate
    • difunctional molecule - free radicals in teh resin facilitate C=C cross linking (free radical additon polymerisation)
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43
Q

role of camphorquinone in composite resin

A

initiator

blue light activation -> releases free radicals

  • free radicals bond to BIS-GMA resin

confers increased molecular weight and so greater viscosity and strength

converts between 35-80% resin

  • toxic unreactd monomer left potential

reacts with blue light at 44nm - depth of cure 2mm approx

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44
Q

role of low weight dimethycrylate in composite resin

A

TEGOMA - triethylene glycol dimethycrylate

inc proportion of TEGOMA = dec. viscosity

  • almost like pain thinner
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45
Q

silane role in composite resin

A

coupling agent

acts as a wingman for glass to allow it to preferentially bond to resin and glass rather than water

  • water would normally adhere to glass particles preventing resin bonding to glass

silane methoxy groups do the following:

  • bind to absorbed water
  • bind to OH groups in filler
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46
Q

what can be inc in self cure composite

A

benzoyl peroxide and aromatic tertiary amine

2 pastes, react together to break C=C bonds and release free radicals

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47
Q

what are crowns usually made of

A

procelain fused alloys

  • porcelain on outside with a metal substructure
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48
Q

why use porcelain fused alloys

A

porcelain - good aeshtetic but microcracks form at the fitting surface = mechanincal failure

alloys - good mechanical properties

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49
Q

how can porcelain fused alloys restoration fail

A

fracture within porcelain itself

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50
Q

mechanical properities

compressive strength

A

stress needed to cause fracture

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51
Q

mechanical properities

elastic modulus (rigidity)

A

stress/strain ratio - i.e. stress needed to cause a change in shape

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52
Q

mechanical properities

brittleness/ductitility

A

dimensional change expereinced before fracture

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53
Q

mechanical properities

hardness

A

resistance of a surface to indent or abrasion

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54
Q

mechanical properties of porcelain

A

hard

strong

rigid

brittle (i.e. low tensile strength - can form defects, liable to fracture)

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55
Q

mechanical properties of alloy

A

hard

strong

rigid

ductile

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56
Q

porcelian metal resotrations properties

A

Metal Oxide sandwiched between porcelain and alloy

Metal Oxide also helps to eliminate cracks and defects on porcelain surface

Alloy acts as a support and limits the strain porcelain experiences

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57
Q

required properties for porcelian metal resotraions

A
  • thermal expansion coefficient
  • form good bond to porcelain
  • avoid discoloration of porcelain
  • mechnical
  • melting
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58
Q

thermal expansion coefficient for porcelain-metal resotrations

A

its important that the alloy has a similar thermal expansion coefficient to the porcelain

REDUCES STRAIN

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59
Q

metal good bond to porcelain for porcelain-metal resotrations

A

will allow the restoration to have longevity and maximises supporting property of alloy

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60
Q

metal avoid discolouration of porcelain

for porcelain-metal resotrations

A

porcelain chose for aesthetics

silive in AgPd can produce a green discoluration

copper not used with High Gold due to discolouration

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61
Q

metal mechanical properties

for porcelain-metal resotrations

A

bond strength

  • Gold (H/L), AgPd and CoCr all adequate (NiCr not)

hardness

  • all adequate (NiCr too hard)

elastic modulus

  • high (rigif) to support porcelain and prevent fracture
    • NiCr best
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62
Q

melting

for porcelain-metal resotrations

A

recrystallisation temp of alloy

must be harder than fustion temp of porcelain or creep will occur

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63
Q

High Gold alloys

properties for porcelain fused restoration

A
  • Match Thermal exp.
  • Increased melting pt
  • Forms oxide (Bonding)
  • Biocompatible v good
  • Cu presence can cause green discolourisation of porcelain
  • Melting range too low
  • Youngs modulus too low (Elastic)
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64
Q

low gold allous properties

for porcelain fused restoration

A
  • Increased melting temperature
  • Slightly better mech. props
  • Biocompatible good
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65
Q

silver palladium alloys (AgPd) properties for porcelain fused restoraitn

A

High melting point

Care needed in casting

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66
Q

nickel chromium alloys (NiCr) properties for porcelain fused restoration

A
  • High melting pt
  • High YM
  • Chromium forms oxide for bonding
  • High casting shrinkage
  • Not v biocompatible
  • Lowish bond strength
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67
Q

cobalt chromium alloys properties for porcelian fused restoration

A
  • High melting point
  • Minimal casting shrinkage
  • High YM
  • High tensile strength
  • High hardness
  • Lowish bond strength
  • Questionable Biocompatability
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68
Q

‘stressed skin’ effect

in porcelain fused alloys

A

Slight differences in thermal contraction coefficient

lead to compressive forces which aid bonding

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69
Q

chemical effects

in porcelain fused alloys

A

May be electron sharing in oxides

During firing porcelain flows and oxides in the metal oxide coating migrate

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70
Q

PMMA

A

polymethylmethacrylate

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71
Q

ideal properties of PMMA in general

A
  • replaces function of natural teeth
  • goes in pt mouth
  • seen by others - aesthetics
  • has to be cost effective
  • dimensionally accurate and stable in use - fit and be retained
  • high softening temp (Tg)
    • must not distorrt when eating or cleaning
  • unaffected by oral fluids over time
  • non-toxic/non-irritant
  • easy to repair
  • radiopaque
    • helps with detection of inhaled or ingested fragments if broken and swallowed
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72
Q

ideal properties of PMMA dimensionally

A
  • dimensionally accurate and stable in use - fit and retained
  • high softening temp (Tg) - must not distort when eating or cleaning
  • unaffected by oral fluids over time
  • high hardness and abrasion resistance
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73
Q

ideal mechanical properties of PMMA

A
  • high YM
  • high proportional limit - only large stresses will cause permanent deformation
  • high transverse strength - upper denture has 3pt loading (2 lateral and 1 middle downward force)
  • high fatigue strength - can withstand low stresses over a long time (design dependent)
  • high impact strength - withstand large stresses applied rapidly e.g. dropping onto hard surface - may form hairline fractures
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74
Q

ideal thermal properties of PMMA

A
  • artificial tooth - avoid internal stress on cool
  • high thermal conductivity - so don’t burn throat due to not being able to sense hot liquids
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75
Q

ideal density for PMMA

A

low

aids retention - simple gravity law

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76
Q

setting reaction of PMMA

A

free radical addition polymerisation - adding two molecules of either same or different form to make a bigger molecule without elimination of smaller molecule (i.e. breaking C=C bonds)

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77
Q

4 stages in setting reaction of PMMA

A

activation

initiation

propagation

termination

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78
Q

activation in PMMA reaction

A

heat to 72oC or more releases radical molecules from symmetrical benzoyle peroxide molecule

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79
Q

initiaion in PMMA reaction

A

free radicals break down C=C bond in methacrylate monomer and transfer free radical

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80
Q

propagation in PMMA reaction

A

growing polymer chain

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81
Q

termination in PMMA reaction

A

of polymerisation

chain stops growing

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82
Q

2 components in heat cure acrylic

A

powder

liquid

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83
Q

powder constituents for heat cure acrylic

A
  • initiator - Benzoyl Peroxide
  • PMMA particles - pre-polymerised beads
  • plasticiser - allows quicker dissolving in monomer liquid
  • co-polymers - improve mechanical properties
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84
Q

liquid constituents for heat cure acrylic

A
  • Methacrylate monomer - dissolves PMMA beads and polymerises
  • Inhibitor - hydroquinone - prolongs shelf life by reacting with free radicals
  • co-polymers - improve mechanical properites
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85
Q

why is it key PMMA has efficient polymerisation

A

increased molecular weight

=

better mechanical properties

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86
Q

undercured acrylic

A
  • free monomer - irritant
  • low molecular weight - poor mechanical properties
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87
Q

overcuring acrylic

A
  • gaseous porisity
    • voids in acrylic caused by monomer boiling
  • polymerisation shrinkage
    • monomer shrinks 20% due to poor packing, lack of excess material
  • contraction porosity
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88
Q

crazing acrylic

A

fine cracks forming in material

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89
Q

metal is

A

aggregate of atoms in crystalline structure

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90
Q

alloy is

A

combination of metal atoms in a crystalline structure (metals are the building blocks of these)

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91
Q

FS

A

fracture strength

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92
Q

EL

A

maximum strength without plastic deformation

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93
Q

UTS

A

ultimate tensile strength

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94
Q

ductility

A

amount of plastic deformation prior to fracture

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95
Q

extent to which a material can be shaped/manipulated calculated by

A

(UTS/EL)%

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96
Q

stages of metal in stress strain diagram

A

upward curve - molten metal

flat line - liquid>solid

lower descending curve - cooling

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97
Q
A

cubic

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98
Q
A

body centred cubic

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99
Q
A

face centred cubic

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100
Q

crystal growth

A

atoms at these sites act as nuclei of crystallisation

crystals grow to form dendrites (3D branched lattic network)

crystals (or GRAINS) grow until they impinge on other crystals

region where grains make contact is called a GRAIN BOUNDARY

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101
Q

types of metallic grains

3

A

equi-axed grains

radial grains

fibrous grains

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102
Q

equi-axed grains

A

if crystals growth of equal dimension in each direction

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103
Q

radial grains

A

molten metal cooled quickly in cylindrical mould

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104
Q

fibrous grains

A

wire pulled through die

cold worked metal/alloy

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105
Q

3 ways to alter crystals

A

fast cooling

more nuclei, small fine grains

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106
Q

slow cooling

A

few nuclei, large coarse grains

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107
Q

nucleating agents

A

impurities or additives act as foci for crystal growth

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108
Q

grains are

A

each grain is a single crystal (lattice)

with atoms orientated in given directions (dendrites)

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109
Q

grain boundary

A

change in orientation of the crystal planes

(impurities concentrate here)

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110
Q

small fine grains area advantageous because (2)

but (1)

A

high elastic limit

increased UTS and hardness

decreased ductility

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111
Q

dislocation of columns (SLIP)

A

forces applied and defect moves along (propagation)

  • when defect reaches grain boundary the lattic changes into new shape to free defect

dislocations - imperfections in crystal lattic

increases

  • elastic limit
  • UTS
  • hardness

decreases

  • ductility
  • impact resistance
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112
Q

3 factors impeding dislocation movement

A
  • grain boundaries (hence the fine grains)
  • different alloys have different atom sizes
  • when cold working builds up at grain boudaries
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113
Q

cold working

A
  • down at low temp - below recrystallisation temperature so some changes can be made
  • causes SLIP - dislocations collect at boundaries
  • results in stronger harder material
  • improves
    • elastic limit
    • UTS
    • hardness
  • decreases
    • ductility
    • corroision resistance
    • impact resistance

RESULTS IN INTERNAL STRESSES

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114
Q

residual stress

A

not in perfect position - causes instability in the lattic

  • results in distortion over time (undesirable!)

releived by annealing process

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115
Q

annealing

A

heating metal/alloy to cause thermal vibrations

  • vibration cause migration of atoms

re-arrangement of atoms within grains

  • doesn’t change mechanical properties or grain structure as a whole

care has to be taken as if temp too high causes grains to swell and poorer mechnical properties

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116
Q

recrystallisation

A

spoils cold work benefit but allows further cold working

continue bouts of cold work and recrystallisation until desired shape acheived

greater amount of cold work the lower recrystallisation temp

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117
Q

alloy

A

combination of 2 or more metals or metals with a metalloid (Si, C)

better mechanical properties than an individual metal

lower melting point than individual metal

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118
Q

definition of ‘phase’ used to define the metallic components of grains

A

physically distinct homogenous structure (can have more than one component)

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119
Q

defintion of ‘solution’ used to define the metallic components of grains

A

homogenous mixture at an atomic scale

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120
Q

one phase =

A

grains consising of metal A only

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121
Q

two phase =

A

individual grains of metal A+B in lattic network (distinct)

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122
Q

solution =

A

one phase but metal A+B in homogenous mixture (solid solution)

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123
Q

3 states upon crystallisation

A

be insoulble, no common lattic (2 phases)

intermetallic compound - with specific chemical formulation e.g. Ag3Sn

be soluble and form a solid solution (3 types)

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124
Q

3 stypes of solid solution

A

subsititution - atoms of one metal replace the other metal in the crystal lattic/grain

  • random substition
  • ordered substition

interstitial - atoms of markedly different in size - small atoms located in spaces in lattic/grain structure of a larger atom e.g. FeC

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125
Q

cooling curve - alloy

TL

A

crystallisation begins

126
Q

cooling curve - alloy

TS

A

crystallisation ends

127
Q

liquidus

A

temperature the alloy begins to crystallise

128
Q

solidus

A

temperature where it solidifies (completely crystallised)

129
Q

metals crystalise

A

at single temperature

130
Q

alloys crystalise

A

over a temperature range

131
Q

cooling and composition of alloys

A

must cool molten alloys slowly

allows metal atoms to diffuse through lattice

  • ensure grain composition is homogenous

downside is it has hard grains - so poor mechanical properties

132
Q

rapid cooling of alloys causes

133
Q

coring alloys

A

rapid cooling of alloys causes a concentration gradient to form

  • different proportions of metal with one starting in low conc and increasing and the other metal antithesis of this

coring helps improve mechanical properties by reducing dislocation movement and resulting in small grains

134
Q

homongenising anneal

A

gentle heat and vibrate atoms, helps atoms to diffuse (below recrystalistallisation temperature) to reduce the coring but not altering the grain structure allowing a more homogenous proportion of metal right the way through its depth

135
Q

dislocation in alloys

A

in metals the defect rolls smoothly over the lattic along the slip plane

in alloys the defect has to ‘fall’ into the space between the large and small atoms and climbs over the atoms alonge the grain boundary but requires a great degree of effort to do so

  • instead of flying over a crowd of people you have to climb over them because you cant get through them*
  • Requires more stress to move dislocations in a solid solution = inherently better mechanical properties
136
Q

eutetic alloys

A

alloy melts at a temperature higher than that of its individual metals (together = stronger)

137
Q

eutetic alloys properties

A

physically distinc grains

soluble in liquid state

insoluble in solid state

unusual in that it is an alloy which cools at a single temperature not over a range

138
Q

function of impression material

A

produce an accurate replica of the surface and shape of hard and soft oral tissues

139
Q

impression definition

A

negative reproduction of tissues

140
Q

mucostatic

A

e.g. ZOE, low viscosity alginates

fluid materials that displace the soft tissues slightly - i.e. give an impression of the undisplaced mucosa

141
Q

mucocompressive

A

e.g. impresion compound, high viscosity alginates/elastomers

viscous materials that record an impression of the mucosa under load i.e. give impression of displaced soft tissue

142
Q

viscoelastic behaviour

A

it is advantageous to wait for a time TF - TL after removing the L tray before you pour the cast - so as to minimise permanent strain (deformation)

remove the tray with a sharp pull to minimise permanent deformation

143
Q

3 elastic impression materials

(not perfectly elastic!)

A

hydrocolloid - alginate

elastomers

144
Q

hydrocollid

A

A colloid is a 2 phase system of fine particles
(1-200nm) of one phase dispersed in another phase (water is dispersing medium in hydrocolloid)

e. g. irreversible alginate
* reversible agar (no longer used as cross infection)*

145
Q

alginate reaction

A

irreversible hydrocolloid

2 NanAlg + nCaSO4 ——> nNa2SO4 + CanAlg

Cross linking with Calcium allows the alginate to set

  • intermediate reaction between sodium to calcium stage allows a delay in setting
  • Use perforated tray with adhesive for alginate!
  • remove tray with a sharp pull
  • large bulk reduces strain on material
146
Q

alginate composition

A

Sodium alginate (reacts w/ calcium ions)
Calcium Sulphate (provides calcium ions)
Trisodium sulphate (delays gel formation)
Filler (increases cohesion and strength)
Modifiers, flavourings, chemical indicators (surface, taste, pH)

147
Q

2 elastomers

A

polysulphides

polyethers (impregum)

silicone

148
Q

elastomers properties

A

monophase impression material

can break stock trays - must make special tray

made of prepolymer, catalyst and filler

undergoes addition polymerisation type reaction

149
Q

2 non elastic impression materials

A

impression compound

impression paste

150
Q

imp material accuracy needs

A

low viscosit

wetting ability

151
Q

impression material dimensional stability needs

A

immediate - on setting (contraction), on removal (deformation, viscoelasticity, rigidity)

long term

152
Q

impression material handling characteristics needs

A

strength

tear resistance

153
Q

needs for imp materials

A

accuracy

dimensional stability

handling characteristcs

cost

taste

colour

154
Q

decontamination policy for imps

A

wash under running water

soak in perform for min 10 mins

wash under running water again

bag in moist tissue

155
Q

perform is

A

potassium perozomonsulphate/sodium benzoate

156
Q

stock imp trays

A

need to be rigid

esp for single stage imp techniques involving putties

157
Q

custom imp trays

A

acrylic - cold cure or light cure

chairside silicone

can be modified with green stick

158
Q

perforations in imp trays

A

allows material to latch onto tray to prevent separation occuring (alginate)

non perforated trays used with compound

159
Q

cross section of imp trays

A

helps direct impression material

square - dentate pt

oval/rounded - edentulous

160
Q

components of stainless steel

A

72% iron

19% chromium

8% nickel

  1. 7% Titanium
  2. 3% carbon
161
Q

stainless steel % iron

162
Q

stainless steel % chromium

163
Q

stainless steel % nickel

164
Q

stainless steel % titanium

165
Q

stainless steel % carbon

166
Q

iron role in stainless steel

A

main constituent and when combined with carbon forms steel

167
Q

chromium role in stainless steel

A

lowers the temperature at which martensitic SS forms i.e. main component of hard SS used in ortho

168
Q

nickel role in stainless steel

A

lowers critical temp that the austentite breaks down on cooling

improve the corrosion resistance of the alloy

improves strength

169
Q

titanium role in stainless steel

A

prevents precipitation of chromium carbides at the grain boundaries

170
Q

iron

solid state phases

A

allotropic - undergoes 2 solid state phase changes

1) Temp >1400oC - Body centred cubic lattice with low carbon solubility (0.05%)
2) Temp > 900-1400oC - Face centred cubic lattice with higher carbon solubility (2%)
3) Temp > <900oC - Back to form in stage 1with temperature

171
Q

solid solutions iron forms with carbon

A

Austenite

  • Interstitial solid solution
  • face centred cubic which exists at >720oC

Ferrite

  • Very dilute solid solution, exists at low temp

Cementite

  • Fe3C, exists at low temp

Pearlite

  • Eutectoid (minimum transformation temp between solid solution and simple mixture)
  • mix of ferrite and cementite (i.e because they are both at low temp)
172
Q

aistentite quenched –>

A

martensite

173
Q

martenstie

A

formed because there is no time for diffusion of carbon through the lattice
has a distorted BCC lattice
is very brittle but this can be lessened by tempering

174
Q

tempering of iron

A

Heating at 450oC following quenching, helps to control poor mechanical properties

175
Q

how much chromium if stainless steel

A

>13% chromium

176
Q

chromium provides

A

corrosion resistance due to chromoium oxide layer in SS

but can be attacked by chlorides

Lowers the Austenite to Martensite temperature
Lowers the Austenite to Martensite transition rate
Decreases the % of carbon which forms a Eutectoid

177
Q

martensititc SS

A

12-13% Chromium + little carbon

Heat hardenable (tempering process)

dental instruments often mate of

178
Q

autenititic SS

A

contains sufficient Chromium and Nickel to suppress austenite to martensite transition i.e more than normal amounts of these metals

Used in

  • sterilisable instruments which don’t have a cutting edge
  • Ortho wire, due to them being readily cold worked and their corrosion resistance
  • Sheet form for denture bases

Corrosion resistance is more important than strength and hardness

179
Q

wrought alloys definition

A

manipulated by cold working

i. e. are drawn into wire shape
e. g. ortho wire and partial denture clasps

180
Q

requirement of wires

5

A
  • high springiness/ Elastic Modulus (YM)
  • stiffness (High Young’s Modulus)
  • high ductility
  • easily joined without impairing properties i.e. soldered or welded
  • corrosion resistant
181
Q

springiness

A

ability of a material to undergo large deflection (to form arc) without permanent deformation i.e. returns to its original shape

182
Q

weld decay

A

occurs at 500o-900oC

chromium carbides build up at grain boundaries

alloy becomes brittle

  • less chromium in central region of solid solution
  • periphery more susceptible to corrosion

solution -> low carbon steel, stabilised stainless stel with small amounts of titanium which forms carbides preferentially and not at grain boundaries

183
Q

dentura base made of stainless steel

advantages

A
  • much thinner than acrylic
  • lighter
  • fracture resistant
  • corrosion resistant
  • polishable
  • high thermal conductivity
  • high impact strength - won’t fracture when dropped
  • high abrasion resistance
184
Q

denture base made of stainless steel

negatives

A
  • dimensional inaccuracy during die process
  • elastic recovery of steel can lead to inaccuracy
  • damage of die under hydraulic pressure
  • loss of fine detail
  • difficult to ensure uniform thickness
185
Q

3 stages of dental ceramics setting reaction

A

vitreous phase

fusion

firing (sintering)

186
Q

virteous phase of dental ceramics setting reaction

A

formed by a flux i.e. Feldspar, this breaks the terahedral structure of silica

forms an amorphous 2D structure

feldspar lowers the fusion and softening temperature of the glass

during firing it forms a solid mass around the other components

187
Q

fusion phase of dental ceramics setting reaction

A

feldspar reacts with the outer layers of silica known as Kaolin - melding the particles together and forms leucite at 1150-1500oC

molten mass then is quenched and ground to a fine powder known as frit

this stage that you incorporate opacifiers, metal oxides and crystalline alumina for colouring and strengthening

188
Q

firing phase of dental ceramics setting reaction

(sintering)

A

heating leads to sintering

occurs just above the glass transistion temperature

glass phase will soften and the particles coalesce

causes contraction of the material by about 20%

189
Q

composition of dental ceramics

4 elements

A

feldspar

silica

kaolin

metallic oxides

190
Q

feldspar in dental ceramics

A

potassium aluminium silicate and sodium aluminium silicate

potash - PAS
soda - SAS

191
Q

silica in dental ceramics

A

formed by tetrahedra

when reacting it forms an amorphous high melting point glass which bonds to the feldspar

192
Q

Kaolin in dental ceramics

A

china clay

confers opacity on porcelain and contributes to the formation of the glass matrix

becomes sticky when mixed which allows the porcelain to be worked to shape a crown

193
Q

metallic oxides in dental ceramics

A

small amounts of metallic oxides provided

blended with unpigmented frit

194
Q

properties of dental ceramics

5

A

aesthetics

chemical stability

thermal properties

dimensional stability

mechanical properties

195
Q

aesthetics of dental ceramics

A

colour stable

retain surface very well without staining

smooth surface and have great optical properties

196
Q

chemical stablity of dental ceramics

A

very stable

unaffected by the pH changes and ranges in the mouth

doesn’t stain

good biocompatability

197
Q

thermal properties

A

similar to tooth subtance

similar thermal expansion coefficient to dentine

thermal diffusivity low too

198
Q

dimensional stablity of dental ceramics

A

20% shrinkage when fired but in the mouth literally no change

199
Q

mechanical properties of dental ceramic

A

very hard

can damage opposing teeth if not glazed

high compressive strength

very low tensile strength

200
Q

3 problems with dental ceramics

A

static fatigue

surface micro-cracks

slow crack growth

together mean feldspathic ceramics can only be used in anterior region

201
Q

static fatigue in dental ceramics

A

decrease in strength over time in the absence of any applied load, thought to be due to hydrolysis of Si-O groups within the material over time in an aqueous ennviroment

202
Q

surface micro-cracks in dental ceramics

A

can occur during manufacture, finsihing or due to occlusal wear

203
Q

slow crack growth in dental ceramics

A

cyclic fatigue under occlusal forces in a wet environment over time

204
Q

3 types of copings

A

metal copings (porcelain fused to metal alloy)

alumina core

zirconia core

205
Q

alumina core

A

core material in Porcelain Jacket Crowns

better flexure strength than feldspathic porcelain

alumina particles stock cracks propagating

however its opaque so can’t be used as a restorative material

206
Q

zirconia core

A

zirconium dioxide

  • zirconia powder doesn’t sinter unless heaated at over 1600oC
  • pure zirconia can crack on cooling
    • zirconia is a monoclinical structure
    • Ytrria stabilisation - allows zirconia to be used as a bridge
  • zirconia goes through manufacturing process and is veneered with feldspathic porcelin to produce the final restoration
207
Q

Yttria stablisation of zirconia

A

zirconia contains small amounts of Yttria

Yttria has a tetragonal structure (zirconia is monoclinical)

crack starts, tip of crack causes the yttria to reach a critical stress level and causes it to convert to the monoclinical structure like the rest of zirconia which will manifest as the crack is healing over

allows it be used for bridges

208
Q

pros of zirconia core

A

great aesthetics because of cores opacity

excellent fit

209
Q

cons of zirconia core

A

expensive equipment initial but material is cheap

porcelain can debond from core

inert fitting surface so cannot etch or bond

210
Q

cast/pressed ceramics process

A

lithium disilicate/reinforced glass is the ceramic used in this

wax up restoration

invested

cast from ceramic ingot

undergoes ceraming i.e. reheating it once cast to improve crystal structure and produce crack inhibiting crystals

211
Q

ceraming stages (2)

A

1 - crystals formed: max no. nuclei formed

2 - crystal growth: max physical properties

212
Q

luting crowns

A

any silica containing crowns can be etched with hydrofluric acid to produce a retentive surface

  • this surface can be bonded to using a silane coupling agent and then bonded to the tooth using an appropriate agent

Zirconia crowns are inert so cannot be ethced, howevere strong enough to be self supporting and can be bonded with a traditional cement

213
Q

milled Vs cast/pressed crowns

A

milled crowns have consistent physical properties meaning they are better overall

214
Q

key properties of luting agents

A
  • viscosity/film thickness
  • ease of use
  • radiopaque
  • marginal seal
  • biocompatibility
  • mechanical properties
215
Q

viscosity film thickness of luting agent

A

dependent on size of powder/filling particles

must be low to allow seating of restoration without interference

viscosity increases as the material sets - highlighting the importance of seasting it quickly and with pressure

25 micro-metres thickness ideally

216
Q

ease of use for luting agents

A

easy to mix

working time should allow seating of restoration

should have short setting time

217
Q

radiopaque quality of luting cement

A

some ceramic crowns are radiolucent

radiopacity allows marginal breakdown to be illicited on radiographs

218
Q

marginal seal of luting cements

A

ideally should bond chemically to the tooth and form an inpenetrable bond

219
Q

biocompatablity of luting agents

A

non toxic

low thermal conductivity

pulp friendly

220
Q

mechanical properties of luting agents

A

high compressive strength

high tensile strength

high hardness

YM similar to tooth

221
Q

powder components of zinc phosphate cement

A

zinc oxide - main reactive ingredient

magnium dioxide - gives white colour and compressive strength

other oxides (alumina, silica) - improve physical properties and alter shade of material

222
Q

liquid components of zinc phophate cement

A

phosphoric acid - aq solution

aluminum oxide - ensures consistency of set material

zinc oxide - retardant to reaction giving bettern working time

223
Q

setting reaction of zinc phosphate cement

A

acid/base

  • ZnO + 2H3PO4 -> Zn(H2​PO4) + H2O

hydration reaction

  • ZnO + Zn(H2​PO4) + 2H2​O -> Zn3(H2​PO4)2 . 4H2​O
    • makes Hopiete
224
Q

aluminium oxide role in setting reactin of zinc phosphate cement

A

prevents crystallisation leading to an amorphous glassy material

glassy matrix of acid salt surrounding unreacted ZnO powder

Matrix is almost insoluble but is porous and contains free water from the setting reaction

the cement matures and binds to the water leading to a stronger less porous material

225
Q

evaluation of ZnO

A

Low initial pH of 2 can cause pulpal irritation

Exothermic setting reaction

Not adhesive to tooth or restoration - acts almost like grout - just filling in spaces

Not cariostatic

final set takes 24 hrs

brittle

opaque

226
Q

evaluation of zinc polycarboxylate cement

A

Similar material to Zinc Phosphate cement but instead of Phosphoric acid it is replaced with Polyacrylic acid

Bonds to tooth surfaces a bit like GIC

Less exothermic

pH is low to begin with but returns to normal faster, long chain acids less damaging to the dentine

cheap

but

  • difficult to mix
  • difficult to manipulate
  • soluble in oral environment at low pH
  • opaque
  • lower modulus and compressive strength
227
Q

evaluation of glass ionomer cement

A
  • low shrinkage/stablity
  • relativly insoluble once fully set
  • aesthetically better than ZnPhos
  • self adhesive to tooth substance
  • F release
  • cheap
  • highly soluble
228
Q

composite luting cement

bonding to indirect composite

A
  • composite bonds to composite
  • micromechanical bonding occurs on the rought internal surface of the composite inlay
  • bond is also chemical to remaining unbroken C=C bonds on the inlay surface
  • using a dual curing cement as light penetration using conventional cement wouldn’t work
229
Q

composite luting cement

bonding to porcelain

A

is brittle and is required to be bonded to tooth to prevent fracture

untreated porcelain is smooth and non-retentive

can be treated with HF to etch the surface (v.toxic)

produces a rough retentive surface but is still not hydrophobic and compatible with composite resin luting agents

requires a surface wetting agent - silane coupling agent

thin porcelain - light cure composite can be used

thick porcelain - dual cure needed

230
Q

silane coupling agent when composite luting agent is bonding to porcelain

A

allows a strong bond to form between the silcon group in the porcelain and the base carbon as part of the composite monomer

231
Q

composite luting agent bonding to metal

A

like porcelain, composite doesn’t directly bond to metal

metal surface needs roughened - etching or sandblasting

electrolytic etching (beryllium alloys best)

need a dual cure luting agent

232
Q

composite luting agent bonding to non precious metals

A

use carboxylic/phosphoric acid derived materials

MDP and 4-META

both molecules have an acidic C=C end, this reacts with the metal oxide and renders the surface hydrophobic

233
Q

bonding to precious metals with composite luting agent

A

change to precious allow composition to allow oxide formation

increase copper content and head to 400oC

sulphur based bonding agent

234
Q

self etching composite resin luting agents

A

acid groups bind with calcium in HPA forming a stabilising attachement between tooth and resin

ions from dissolution of filler neutralise the remaining acidic groups forming a chelate reinforced methacrtylate network

limited removal of smear layer or significant infiltration into the tooth suface (only a couple of microns)

good bond strength to dentine

235
Q

self etching composite resin luting agents

bond to enamel vs dentine vs ceramics vs metals

A

enamel

  • lower than dentine
  • should be etched with acid prior to application

dentine

  • better than to enamel
  • no need to etch

ceramics

  • brand specific - RelyX unicem bonds quite weel to sandblasted Zirconia

metal

  • better to non-precious metal
  • not good enough for ortho brackets
236
Q

temporary cements

A

dont fully set and remain soft so can be removed easily

prep must be physical

base

  • ZnO
  • start
  • mineral oil

accelerator

  • resins
  • eugenol or ortho EBA
  • carnuaba wax

wax weakens the structure of the set cement and makes it easier to remove

material can be modified to make it weaker still by incorporating petroleum jelly into the mixture

eugenol not used when permenanet cement willl be resin as it inhibits set

237
Q

addition cured silicones

polyvinylsiloxanes

elastomer imp material

A

N.B Contain PDS but some of the methyl groups replaced by Hydrogen and Vinyl hence termed PS instead of PDS

Base paste

  • polydimethylsiloxane - some methyl (CH3) groups replaced by hydrogen
  • filler - variations change viscosity

Catalyst paste

  • polydimethylsiloxane - some methyl groups replaced by vinyl (CH2 =CH)
  • filler - variations change viscosity
  • platinum catalyst eg chloroplatinic acid

Base Paste ((PDS (Hydrogen)) + Catalyst ((PDS) Vinyl)) + Chlorplatinic acid ——> Cross linked Polymer formed

(NO BYPRODUCTS)

Hydrophilic Silicones

  • incorporate non-ionic surfactant
    • wets tooth surface
    • more easily wetted by water containing die materials
238
Q

condensation cured silicones

polydimethyl siloxane

elastomer imp material

A

TYPE 1 reaction

  • Silicone Polymer + Organhydrogen Siloxane (cross linking agent) ——> Cross linked Silicone Polymer + Hydrogen Evolved

TYPE 2 reaction

  • Silicone Polymer (Double OH) + Alkoxy orthosilicate (cross linking agent) + Silicone Polymer (Single OH) —-> Cross Linked polymer + Alcohol
    • within the monomer rather than the ends of chains
239
Q

polyether components

A

base paste

  • amine terminated prepolymer for cross linking
  • inert filler (viscosity and strength)

catalyst

  • ester derivative of aromatic sulphonic acid initiates polymerisation
  • inert oils and filler form paste

polyether + sulphonate ester -> cross linked material

240
Q

3 phases of polyehter and sulphonate ester setting reaction

A

1) Activation > Sulphonate Ester ionises and provides cations (+) to the reaction
2) Initiation > Cations then open up Epimime rings in the prepolymer which releases a further cation
3) Propagation > The chain reaction of cation release continues and the now ionic prepolymers join to form a larger chain as the ions are passed along

241
Q

3 key factors for analysing imp materials

A

viscosity

surface wetting

contact angle

242
Q

viscosity of imp material

A

determines a materials potential for making close contact with soft tissue surfaces = recording surface detail

243
Q

surface wetting for imp material

A

must make intimate contact with teeth/mucosa

244
Q

contact angle for imp material

A

determines how well the material envelopes hard/soft tissue surfaces

245
Q

accuracy considerations for imp materials

A

reproduction of surface detail - should reproduce at least 50μm of detail

viscoelasticity/elastic recovery- withdrawing impression quickly, less permanent deformation from strain

246
Q

dealing with removal from undercuts for imp materials

assessment

A

flow under pressure - shark fin test

tear tensile strength on removal

rigidity on removal

247
Q

dimensional stablity for imp materials assessment

A

should be low setting shrinkage

thermal expansion/contraction should be low - due to the disparity in temp from the oral cavity to the outside environment

storage - some materials undergo synersis or imbibition causing dimensional change

248
Q

stress concentration

A

abrupt changes in shape of a file that leads to a higher stress at that point aka a notch in the file

249
Q

shape memory

A

when elastic limit is significantly higher than that of conventional metals, will deform when heated returb to its original shape

250
Q

plastic deformnation

A

permanent bond displacement occuring when elastic limit is exceeded

251
Q

plastic limit

A

the point in which a plastically defomed file breaks

252
Q

cyclic fatigue

A

freely rotating in a curvature

generaton of tension and compression cycles

cyclic fatigue

failure

253
Q

torsional failure

A

when the load is suddently revered

i.e. turning the file in the opposite direction to which has been

254
Q

mechnical agitation

A

moving a substance by external vibration to allow said substance to fill a space

255
Q

preparation of the canal for obturation

A

smear layer formed during preparation

  • organic pulpal material and inorganic dentinal debris
  • superifical 1-5μm with packing into tubules
  • bacterial contamination, substrate and interferes with disinfection
  • also prevents sealer penetration

removal of smear layer

  • 17% EDTA
  • 10% citric acid
  • MTAD (mixture of a tetracylic isomeer, an acid and detergent)
  • sonic and ultrasonic irrigation
  • watch apical control
256
Q

EDTA and NaOCl in canal

A

should never be in the canal at the same time

because the EDTA neutralises the effectiveness of the NaOCl

also forms papachloanailine which is cytotoxic and carcinogenic

257
Q

ideal properties of obturation materials

A
  • easily manipulated with ample working time
  • seals the canal laterally and apically
  • non-irritant
  • impervious to moisture
  • unaffected by tissue fluids
  • inhibits bacterial growth
  • radiopaque
  • does not discolour tooth
  • sterile
  • easily removed if necessary
258
Q

gutta percha is

A

natural rubber and GP are polymers of the same monomer - transpolyisoprene

comes in 2 forms:

  • α - used in themoplastic manipulation techniques, the natural occuring form and when heated above 65o becomes amorphous
  • β - more commonly used in cold lateral compaction - rapidly cooled and recrystalised alpha, used in commercially produced GP

GP formulations are 60-75% Zinc Oxide

inc a variety of other agents to allow them to be radiopaque on radiographs

259
Q

ideal properties of sealing material

A
  • exhibit tackiness to provide good adhesion
  • establishes a hermitic seal
  • radiopacity
  • easily mixed
  • no shrinkage on setting
  • non-staining
  • bacteriostatic or does not encourafe growth
  • slow set
  • insoluble in tissue fluids
  • soluble on retreatment
260
Q

types of sealers

A
  • ZOE
  • GI sealers
  • resin sealers
  • calcium silicate sealers
  • medicated sealers
261
Q

ZOE sealer

A
  • effective antimicrobial
  • offers cytoprotection
  • resin acids are 90% of the colophony (Rosin) in the material and these are strongly antimicrobial and cytotoxic
  • altough toxic may be overall beneficial with longlasting antimicrobial effect and cytoprotection
  • formation of eugenolate consititutes hardening, this is accelerated by CaOH so this must be removed from the canals
  • remaining eugenol can act as an irritant
  • lose volume with time due to dissolution - resins can modify this
262
Q

GI sealers

A
  • advocated due to dentine bonding properties
  • removal upon retreatment is difficult
  • minimal antimicrobial activity
  • not enough data yet
263
Q

3 types of resin sealer

A

AH plus

Epiphany

EndoRez

264
Q

AH plus resin sealer

A
  • long history of use - development of AH26
  • epoxy resin
  • paste-paste mixing
  • slow setting - 8 hrs
  • good sealing ability
  • good flow
  • initial toxicity declining after 24hrs
265
Q

epiphany

resin sealer

A

dual cure dental resin composite sealer - used with Resilon

  • BisGMA
  • Ethoxylated BisGMA
  • Urethane-dimethacrylate UDMA
  • hydrophillic difucntional methacrylates
  • fillers of calcium hydroxide, barium sulphate, barium glass and silica

requires self-etch primer

266
Q

EndoRez

A

UDMA resin-based sealer

  • hydrophillic
  • good penetration into tubules
  • biocompatible
  • good radio-opacity
267
Q

Calcium silicate sealers

A
  • high pH (12.8) during intial 24hrs of the setting
  • hydrophillic
  • enhanced biocompatibility
  • does not shrink on setting
  • non-resobable
  • excellent sealing ability
  • quick set - 3-4hrs - requires moisture
  • easy to use
268
Q

medicated sealers

A

sealers containing paraformaldehyde not acceptable

lead and mercury components removed

severe adn permanent toxic effects on periradicular tissues

sargenti paste, endomethasone, SPAD

269
Q

how are investment materials used

A

wax pattern made (e.g. crown, inlay etc)

investment material poured around wax pattern and allowed to set (mould)

wax then eliminated (boiled out)

molten alloy is then forced into the cavity left by the wax via sprues prepared in the investment material

270
Q

components of all investment materials

A

refractory - silica (quartz/tridymite/crisotbalite)

binder - gypsum/phosphate/silicate

modifiers - change physical properties

271
Q

gypsum bonded material

A

Supplied as powders and mixed with water and silica and calcium sulphate hemihydrate with other components that control setting time

  • quartz withstands high temps and gives expansion

Thermal inversion is when the silica is heated so that it undergoes a phase transformation and expands (alpha > beta quartz)
Above 320oC there is contraction of the investment material which causes water loss, this can be reduced by modifiers like sodium chloride and boric acid

Heat Soaking

  • > 700​oC
  • CaSO4 + 4C ——> CaS + 4CO
  • (THEN) 3CaSO4 + CaS ——> 4CaO + 4SO2
  • Allow heat soaking to complete to allow gases to escape

Chemical stability

  • <1200​oC satisfies requirements
  • >1200​oC problems with sulphur trioxide production - causes corrosion and porosity in alloy castings
  • Therefore only allots with melting point of <1200 allowed
272
Q

phosphate bonded materials

A

Powder

  • Silica
  • Magnesium Oxide
  • Ammonium Phosphate

Liquid

  • Water
  • Colloidal Silica (increases strength, undergoes hygroscopic expansion)

Magnesium Oxide + Ammonium Phosphate + Water/Colloidal Silica —— > Magnesium Ammonium Sulphate

Type 1 PBMs

  • Inlays, crowns and other fixed restorations

Type 2 PBMs

  • RPDs and other cast removable restorations
  • Don’t require outer casting ring
273
Q

silica bonded materials

A
  • Powdered Quartz/Cristobalite bonded with silica gel
  • Silica gel becomes Silica and is a tightly packed mass of particles
  • Binder is usually Ethyl Silicate with Hydrochloric acid and Industrial spirit
  • the ethyl silicate is hydrolysed releasing alcohol and forming silica gel
  • the Hydrolysis and gelation can be accelerated by Piperidine but this causes alot of shrinkage
  • Ethyl silicate mediated materials dont dimensionally change on setting because they are Thermal expanders
  • their linear expansion = their linear thermal expansion unlike gypsum/phosphate
274
Q

GIC polyacid components

A
  • polyacrylic acid (ionic monomers)
    • copolymers of acrylic and itaconic acid
    • or Acrylic and maleic acid
  • tartaric acid
    • added to control the setting characteristics of material
275
Q

powder (base, metal) components of GIC

A
  • silica
  • alumina
  • calcium fluoride
  • aluminium fluoride
  • alumonium phosphate
  • sodium fluoride

adding Strontium + lithium salts increase radiopacity but dont take part in reaction

276
Q

setting reaction of GIC phases

A
  • dissoluion
  • gelation
  • hardening
277
Q

reaction for GIC

A

MO.SiO2 + H2A -> MA + SiO2 + H2O

M=metal A=polyacid

glass + acid -> salt + silica gel

278
Q

dissolution phase for GIC

A

Acid into Solution
H+ ions attack the glass surface
Ca, Al, Na and F ions are released
Leaves silica gel around the unreacted glass

279
Q

gelation phase for GIC

A

Inital set due to Ca2+ ions cross linkinh w/ polyacid by chelation with carboxyl groups

Bivalent so can bind to two carboxyl groups

Chelation can happen twice on same molecule of polyacid (gels toonmuch?)

At this point the material will appear hard in the mouth (after a fewminutes) caused by formation of calcium polyacrylate

280
Q

hardening phase for GIC

A

Trivalent Aluminium increases crosslinking

Formation of Aluminium Polyacrylate

Helps mechanical properties greatly

281
Q

imp points for GIC setting

A

Must be moisture and dessication free i.e not too wet or dry

This is most important during the hardening phase

if not achieved aluminium will leach out- less cross linking, water lost from matrix if dessicated, saliva contamination causes excess water absorption = A WEAK MATERIAL

282
Q

protection for conventional GIC

A
  • Varnishes
  • Copal ether
  • Acetate
  • Resins
  • Dentine/Enamel bonding agents
  • Unfilled Bis-GMA resins
  • Greases/Gels
  • Vaseline (not great as removed quickly by lips and tongue)
283
Q

properties of GIC

A

Can bond to enamel and Dentine w/o use of intermediate material

Bond strength pretty poor (5-20 MPa)

Poor Tensile strength

Lower compressive strength than composite (less than 50%)

Higher solubility than composite due to unprotected material during gelation phase

Usually seals well

Fluoride release (for short time)

Bonding**

  • Carboxyl (COO groups) in cement bond to Ca in the enamel!
  • In addition there is Hydrogen bonding and metallic ion bridging to the collagen in enamel too
  • IT IS CONDITIONED NOT ETCHED!! W/ POLYACRYLIC ACID!!
284
Q

powder components of RMGIC

A
  • Fluro-Alumino-Silicate glass
  • Barium glass (provides radiopacity)
  • Vacuum dried polyacrylic acid
  • Pottasium persulphate (redox catalyst - cures resin in the dark)
  • Ascorbic acid
  • Pigments (vary in shade for aesthetics)
285
Q

liquid components of RMGIC

A
  • HEMA (water miscible resin)
  • Polyacrylic acid with pendant methyacrylate (undergo acid/base reactions and polymerisation)
  • Tartaric Acid (speeds up setting reaction)
  • Water (allows reaction between polyacid and glass)
  • Photoinitiators - ALLOW LIGHT CURE
286
Q

two types of setting reaction for RMGIC

A

dual curing

tri curing

287
Q

dual curing RMGIC

A

On mixing proceeds like the normal GIC (dissolution)

Light activation causes free radical methycrylate reaction to occur = resin matrix formed

Acid Base reaction occurs for several hours afer

288
Q

tri curing for RMGIC

A

On mixing proceeds like the normal GIC (dissolution)

Redox reaction begins

Light activation- resin matrix formed

Redox reaction continues for 5 mins after initial mix

Acid base reaction occurs for several hours

Final hardening may take days

289
Q

RMGI vs GI

A

Better physical properties

Lower solubility

Fluoride release

Better translucency/aesthetics

Better handling

290
Q

role of cavity liners

A
  • prevents gaps
  • acts as a protective barrier

e.g in Amalgam

Cavity base lining placed in bulk to block undercuts for metal restorations

lining for exposed dentine- promotes pulpal health

291
Q

liner as a pulp protection

protects from

A
  1. chemical stimuli - unreacted chemicals in filling material/pH of filling material
  2. thermal stimuli - exothermic setting of composite#heat conduction amalgam/gold
  3. bacteria + endotoxins - microleakage (against oral fluids and bacteria and their toxins ingressing between material and cavosurface margins)
292
Q

liner therapeuatic and palliative role

A

therapeutic role in calming down pulpal inflammation and promote healing

palliative reduce symptoms in pts with irreversible pulpitis

293
Q

ease of use for liners

A

should be command set

workable

easy to mix

294
Q

thermal properties

conductivity

A

how well heat energy is transferred through a material

Heat flow through a cylinder of unit cross sectional area with a temp difference of 1oC between both ends

Cavity lining should have as low thermal conductivity as possible!!

295
Q

thermal properties

thermal expansion coefficient

A

change in length per unit length for a rise of 1*C (in ppm*C-1)

Liner should match thermal coefficient of tooth!

GIC has better TEC than RMGI

296
Q

thermal diffusivity

A

similar to conductivity

measured in cm2/sec

Liners have similar or lower thermal diffusivity than enamel

Amalgam much higher than tooth tissue- hence use of liner

297
Q

desired mechanical properties for cavity liners

A

High compressive strength

allows placement of filling without breakage

similar modulus to dentine

Radiopaque

Marginal seal

low solubulity

Cariostatic (fluoride release/ antibacterial)

Biocompatability (non toxic, pH neutral, not exothermic)

298
Q

4 types of cavity liners

A

calcium hydroxide liner (dycal, life)

zinc oxide cements

resin modified zoe

GIC/RMGIC

299
Q

calcium hydroxide liner

A

dycal, life

Components

  • Base- Calcium Hydroxide, Zinc Oxide Filler, Plasticiser
  • Catalyst- Butylene glycol Disalicylate (reactive element), filler, radiopaquer

Setting reaction

  • Butylene glycol disalicylate + Zinc Oxide filler = chelation and at pH 12

Mode of action

  • irritates odontoblast layer- forms reparative dentine, calcium from pulp helps to form a bridge between pulp and dentine, high alkaline content is bactericidal
300
Q

zinc oxide cements used as cavity base

A

Zinc Oxide Eugenol- used as a cavity liner, PD dressing and root canal sealer

Eugenol can have obtundant effect (reduce dentine sensation) and reduce pulpal pain

*inhibits set of composite resin material!! shouldn’t be used!

Zinc Oxide + Eugenol= Zinc Oxide Eugenolate + Water
(Base) + (Acid) = (Salt ) + (Water)

301
Q

resin modified ZOE as cavity base

A

adds resin to the ZnO/Eugenolate matrix to reduce solubility

302
Q

GIC/RMGIC as base or liner

A

can actually bond Amalgam to tooth as the bond goes Tooth—RMGI—Amalgam (it bond to rest.material)
Palliative cements- Base (seldom used)

303
Q

powder components of amalgam

A

50% by weight - mainly silver, tin and copper

  • Silver and Tin- intermetallic compound Ag3Sn
  • Copper- increases strength & hardness
  • Zn - scavenger during production - preferentially oxidises & slag formed / removed - some zinc free
  • *Hg in powder - (few materials)– “pre-amalgamated” alloys - react faster
304
Q

liquid amalgam components

A
  • Hg (50% by weight)

triple distilled (very pure) – reacts with other metals

305
Q

particle types for amalgam

A

lathe cut (coarse medium or fine) - formed by filing ingots

spherical/spheroidal - range of particle sizes, formed by sparying molten metal into inert atmosphere

306
Q

setting reaction for amalgam

A

Silver/Tin Powder reacts with Liquid Mercury

Some unreacted Silver/Tin Powder remains

Silver Mercury (y1) and Tin Mercury (y2) form amalgam matrix

Modern amalgam sets with a small amount of contraction (due to a solidsolution of mercury forming in with the Silver/Tin (only -0.2% contraction so very small)

307
Q

effect of Zn in amalgam clinically

A

Zn + H20 (saliva) -> ZnO + H2 (gas)

Hydrogen gas causes pressure expansion of amalgam

downward pressure = pulpal pain

upward pressure= sitting proud of surface - chipped off
∴ Zinc free materials!

308
Q

factors affecting Amalgam strength

A

Amalgam strength usually ok after 24 hrs however is decreased by the following

  • Undermixing
  • Too high Hg content after condensation
  • poor condensation pressure
  • slow rate of packing- increments dont bond
  • corrosion by oral fluid
309
Q

corrosion of amalgam

A

y2 most electronegative (tin and mercury)

this weakens amalgam particularly at the margins

orrosion products may actually hep with sealing margins

reduce corrosion by

  • copper enriched materials
  • polishing margins
  • avoiding galvanic cells (stops random redox reactions from occuring due to two different metals with different electronegavity contacting)
310
Q

Cu enriched amalgams

A
  • Gives higher early strength
  • Less creep
  • Higher corrosion resistance
  • Increased durability of margins

either

  • single composition formulations
  • dispersion modified setting reaction

REVISION BDS (43 decks)

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