Dental Materials Flashcards
(332 cards)
1
Q
Name 5 applications for fibre reinforcement
A
1) improve fatigue resistance and impact strength of acrylic dentures
2) approximate dentine characteristics for post construction
3) temporary bridges
4) periodontal splinting
5) elimination of metal in porcelain fused to metal restorations (better aesthetics and biological tolerance)
2
Q
What is the concept behind fibre reinforcement?
A
high strength and modulus fibres embedded/bonded to a matrix.
Fibres and matrix retain both physically and chemically
3
Q
What are the dental components of fibre reinforcement?
A
fibres embedded in matrix which is either acrylic or resin with an effective coupling agent between the glass fibres
4
Q
What is the matrix made of in fibre reinforcement?
A
either acrylic or resin
5
Q
What coupling agents are used in fibre reinforcement?
A
resin bonding agents
silanes
6
Q
Name the four fibre types and the two most commonly used
A
1) carbon
2) kevlar
3) polyethylene
4) glass
glass and polyethylene most commonly used as shade matches tooth
7
Q
What are the characteristics of carbon fibres?
A
-polyacrylonitrile precursors drawn into long strands
-heated at very high temp in absence of oxygen to avoid combustion
-tightly interlocked chains of C atoms
-twisted to form yarn
-fibre diameter 5-10 microns
-black
8
Q
What are the characteristics of kevlar fibres?
A
- aromatic polyamide
- pleated structure - atoms radially formed in sheets, weak flexural, compression and low abrasion resistance
- yellow
- high thermal and mechanical stability
- resistant to chemicals
9
Q
What are the characteristics of polyethylene?
A
- natural crystalline polymers
- drawn at lower temp than MP into filamentous fibres
- in axial directions have improved modulus
- ductile
- low density
- translucent so used in dentistry
10
Q
What are the characteristics of glass fibres?
A
- melts of mixtures of oxides - cool without crystallisation
- translucency and ability to match tooth
- high modulus of elasticity - little deformation in function
11
Q
What is critical for success fibre reinforcement?
A
correct fibre orientation
12
Q
What are the three fibre orientation options?
A
1) unidirectional
2) bidirectional
3) random
13
Q
What does anisotropic mean?
A
exhibiting properties with different values when measured in different directions (orientation)
14
Q
What does unidirectional mean?
A
Longitudinal OR transverse (i.e. one or the other) - reduced strength and modulus
15
Q
What does bidirectional mean?
A
enables great strength variation by the variation of:
- amounts/types of fibres
- longitudinal/transverse
16
Q
What do random fibre orientations mean?
A
some properties in all loading directions (isotropic)
This means that when a specific load is applied at any point, the material will exhibit the same strength, stress, strain, young’s modulus and hardness
17
Q
What is Young’s modulus?
A
property of the material that tells us how easily it can stretch and deform and is defined as the “ratio of tensile stress”
18
Q
What are six factors which generally affect the physical properties of fibre reinforcement?
A
1) type of matrix
2) type of fibre
3) quality of fibres
4) length of fibres - must exceed critical length
5) quality of bond fibre/matrix - preimpregnated better than point of use
6) orientation - perpendicular to point of load is best
19
Q
What fibre orientation is best?
A
perpendicular to point of load
20
Q
Fibre reinforced composites are superior to metallic materials in terms of what ratios?
A
weight : strength
modulus : weight
21
Q
Is fibre reinforced composite biocompatible?
A
Yes
22
Q
What are two production advantages of fibre reinforced composite?
A
easily produced
cost effective
23
Q
Comment on the flexural strength of fibre reinforced composite:
A
improved for both PMMA dentures and resin composite bridges. The improvement however, is short-lived as surrounding resin absorbs water and degrades the bond between the fibres and the resin matrix with time
24
Q
What is water sorption?
A
the weight gained when a dental material is immersed in water
25
How is water sorption calculated?
weight increase / surface area
26
What effect does water sorption have on fibre reinforced composite?
water absorbed into material by diffusion
enters voids, produces hygroscopic expansion, material dimensional instability (can be beneficial as counteracts polymerisation contraction)
decreased flexural properties
27
What is wear resistance?
the process by which material is displaced/removed by interfacial forces generated as two surfaces rub together
28
Why is there a limit to how much fibre you should put in a fibre reinforced composite?
must have sufficient for effect but not too much as if so, abrasive and adhesive wear plucking occurs.
29
At what percentages of fibre weight content is the relative wear resistance of fibre reinforced composite low and high?
2% - Low
5% High
7% Low
30
Fibre reinforced composites increase what characteristics?
toughness
impact strength
31
What cycle can reduce all properties over time of a fibre reinforced composite?
thermal cycling
32
What are the clinical uses of fibre reinforced materials?
1) improve fatigue resistance and impact strength of acrylic dentures
2) approximate dentine characteristics for post construction
3) temporary bridges
4) periodontal splinting
5) elimination of metal in PFM restorations
33
Alloys of noble metals contain what percentage of noble metal content?
at least 25% but less than 75%
34
What welding process does pure gold utilise?
cold welding - pressure applied, metallic bonds at point of contact, prior to placement heated to 250 degrees to drive off grease, gold foil increments of 1 micro m
35
What is cohesive gold and how is it placed dentally?
tiny pieces of gold, placed in cavity incrementally, filled over hours with gold foil
36
What are the disadvantages of utilising pure gold?
- time consuming
- no cement lute
- rigidity and elasticity insufficient in high stress situations
- overworking at placement can work harden
37
What are the four types of traditional casting gold alloys?
1) Type 1 low strength
2) Type 2 medium strength
3) Type 3 high strength
4) Type 4 extra high strength
38
What is the main characteristic of type 1 casting gold alloys and what are they used for?
type 1 Low strength - castings subject to slight stress e.g. inlays
39
What is the main characteristic of type 2 casting gold alloys and what are they used for?
type 2 medium strength - castings subject to moderate stress e.g. inlays/onlays
40
What is the main characteristic of type 3 casting gold alloys and what are they used for?
type 3 high strength - high stress situations e.g. onlays, thin cast backings, pontics, full crowns
41
What is the main characteristic of type 4 casting gold alloys and what are they used for?
type 4 extra high strength - for casting thin in cross section e.g. saddles, bars, clasps, crowns, bridges and partial denture frameworks
42
How is gold content expressed?
Carat - parts by weight of gold in 24 parts of alloy
Fineness - parts by weight of gold in 1000 parts of alloy
43
An alloy with 75% gold content has what carat and fineness?
18 carat
750 fine
44
What 5/6 elements generally make up a traditional casting gold alloy?
gold Au
silver Ag
copper Cu
platinum/palladium Pt/Pd
zinc Zn
45
As we move from Type 1 to type 4 traditional casting gold alloys, what properties increase?
hardness
proportional limit
strength
46
As we move from type 1 to type 4 traditional casting gold alloys, what properties decrease?
ductility
corrosion resistance
47
Why, as gold content decreases, does hardness increase?
solution hardening - formation of solid solutions with gold
48
As we move from type 1 to type 4 traditional casting gold alloys, what happens to the gold content?
it decreases
85%, 75%, 70%, 60%
49
What properties does Ag provide in a traditional casting gold alloy?
slight strengthening effect and counteracts reddish copper tint
50
What properties does Cu provide in a traditional casting gold alloy?
increases strength and lowers MP.
If content >16% Cu will oxidise and will tarnish
51
What properties does Pt/Pd provide in a traditional casting gold alloy?
increase both strength and MP
52
What properties does Zn provide in a traditional casting gold alloy? And what happens if it runs out?
acts as a scavenger. When used up, this function is then taken over by Cu. This is undesirable as impairs physical properties of the casting if Cu is used up in this way.
53
What happens when the Zn in a traditional casting gold alloy is used up?
The function of scavenger is then taken over by Cu, which is undesirable as it impairs the physical properties of the alloy
54
What traditional casting gold alloys can be heat treated?
types 3 and 4
NOT type 1 and 2
55
What does the heat treatment of type 3 and 4 traditional casting gold metal alloy do?
results in further hardening by precipitation hardening of Ag/Cu and Au/Cu systems
56
Why can type 1 and 2 traditional casting gold metal alloys not be heat treated?
due to lack of silver and copper that are required for the necessary atomic diffusions
57
How are heat treatments achieved in a traditional casting gold metal alloy?
melting alloy and cooling it - new crystal structure
58
What influences the new structure formed in heat treatments?
length of time of treatment
slowly to room temp
59
How is premature hardening avoided during heat treatments?
- cool casting rapidly from excess of 600 degrees
- red colour of sprue region of casting indicates above 600
- once red colour lost through cooling - cold water plunge, disintegrates investment and results in fine grain structure
60
Cold water cooling an alloy containing Pt/Pd risks what?
coring
61
How is an alloy containing Pt/Pd heat treated to avoid the risk of coring?
- heat to 700 degrees for 10 mins
- quenching
- known as homogenisation heat treatment
62
What is the name of the heat treatment used to eliminate coring (while increasing corrosion resistance) in Pt/Pd containing alloys?
Homogenisation heat treatment
63
What gold content is generally found in low gold alloys?
normally Au 45-50% but could be down to 10%
64
What element has a high content in low gold alloys?
High palladium
65
What colour are low gold alloys?
"white" in colour
66
What constituents are contained in a silver palladium alloy?
primarily Ag and Pd
little to no Au
67
What are the advantages of low gold alloys?
Low cost (relative)
good clinical performance
68
How are low gold alloys classified?
As for conventional gold casting alloys (4 types)
69
What are the possible disadvantages of silver palladium alloys?
- lower density compared to gold alloys (affects castability)
- may dissolve oxygen - porosity
- lower ductility than conventional gold alloys
70
How are dental porcelains classified by manufacturers?
according to their firing temperatures
e.g. low fusing 850-1100 degrees
high fusing 1300-1400 degrees
71
What are high and medium fusing porcelains utilised for?
denture teeth
72
What are low fusing porcelains used for?
metal ceramic retainer restorations (crowns and bridges)
73
What kind of effect does the addition of leucite to ceramic have?
dispersion strengthening effect upon the ceramic (if at 35-50 mass %)
74
Leucite inclusions by manufacturers of ceramics aim to do what?
customisation of ceramics expansion/contraction to match range of dental alloys.
Aim to have slightly higher expansion/contraction than underlying alloy to put porcelain in slight tangential compression (prevent crack propagation)
75
What is a tangential stress?
a force acting in a generally horizontal direction. simply it is acting in parallel to surface
76
What is the more common word for kaolin?
clay
77
What is kaolin?
china clay
Hydrated aluminosilicate
Al2O3 2(SiO2) 2(H2O)
78
What is feldspar?
mixture of aluminosilicates
naturally occurring alumino-silicate minerals containing varying amounts of potassium, sodium, calcium, and/or lithium
79
What are the properties of kaolin at room temperature?
forms colloidal suspension in water
by surface tension effects provides coherence and plasticity to porcelain powder by permitting manipulation
80
What does kaolin turn to at around 450 degrees?
unstable metakaolinite
81
At what temperature does feldspar melt and decompose?
1150 degrees
82
What happens when feldspar melts and at what temperature does this occur?
1150 degrees
flows and consolidates powder particles together
83
What forms when the molten feldspar cools?
on cooling glassy silicate matrix results
84
The production of glassy silicate matrix from feldspar depends on calibration of what functions?
Time
temperature
85
What is added to the making of glassy silicate matrixes to promote scattering of light and colouration (opalescence)?
particles of metal oxides
86
The addition of metal oxide indium or praseodymium would result in what colouration of glass silicate matrix?
ivory
87
The addition of what metal oxide would cause a brown colouration of the glass silicate matrix?
iron or nickel
88
The addition of what metal oxide would cause a blue colouration of the glass silicate matrix?
cobalt
89
The addition of what metal oxide would cause a green colouration of the glass silicate matrix?
copper or chromium
90
The addition of what metal oxide would cause a lavender colouration of the glass silicate matrix?
manganese
91
The addition of what metal oxide would cause a brown/yellow/green colouration of the glass silicate matrix?
vanadium
92
The addition of what metal oxide would cause a yellow/pale green colouration of the glass silicate matrix?
cerium
93
The addition of what metal oxide would cause a yellow/brown colouration of the glass silicate matrix?
titanium
94
The addition of what metal oxide would cause an opaque white colouration of the glass silicate matrix?
tin, zirconium, zinc
95
What does the packing of porcelain do?
reduces firing shrinkage
96
How are porcelains fired?
porcelain furnace
electrically heated muffler (surrounds porcelain)
pyrometer to indicate muffle temperature
97
Why is firing a porcelain in a vacuum essential?
reduces porosity from 4.6% to 0.5%
98
Why is controlled cooling of a porcelain following firing important?
to avoid cracking
99
Why does re-firing of a porcelain require incremental heat build up?
so thermal stresses are not generated
100
What are porcelains glazed with?
low fusing transparent glass
101
What forces stop crack propagation?
compressive forces
102
In what direction do cracks propagate?
from within outwards
103
How can we limit crack propagation?
1) aluminium core (pure)
2) addition of alumina powder to porcelain
3) sintered alumina core
4) metal core - porcelain fused to metal
104
What is PFM?
Porcelain fused to metal
alloy substructure, bonded porcelain veneer
105
Porcelain has an increased coefficient thermal expansion by the addition of what?
leucite
106
What is the coefficient of thermal expansion?
extent to which a material expands upon heating
107
What are the desirable properties of a metal to be used for PFM?
1) not melt when firing porcelain
2) rigid to support thin porcelain veneer
3) bond to porcelain
4) similar coefficient of expansion to porcelain
108
What are the main four PFM alloy options?
1) High gold alloys
2) Low-gold alloys
3) silver-palladium alloys
4) nickel-chromium alloys
109
What are the characteristics of high gold alloys when used for PFM?
-Sn and indium become oxidised and chemically bond to porcelain
-need thick coping to prevent flexure
110
What are the characteristics of low gold alloys when used for PFM?
50% Au
30% Pd - increased melting temp, decreased coefficient of thermal expansion
10% Ag
10% Indium - for bonding
similar properties to high gold alloys
111
What are the characteristics of silver palladium alloys when used for PFM?
advantageous higher modulus value and melting range compared to high gold
Ag may lead to greening
cost saving
112
What are the characteristics of nickel-chromium alloys when used for PFM?
high modulus and melting temperature are advantageous
prone to high casting shrinkage with voids
poorer bond strength to porcelain than other alloys
poor biocompatibility due to Be (animal carcinogen) and Ni (risks contact dermatitis)
113
What is an alternative to PFM alloys?
Captek capillary technology
114
What is CapTek capillary technology?
- wax strip (powdered Pd metal rich) is adapted onto the refractory die.
- It is then fixed - sinters metal and burns off wax
- to the capillary network 2nd wax strip (loaded with pure gold) is applied
- fired
- gold infiltrates the network and forms substructure for porcelain build up
115
x
x
116
What kind of material are composites?
ceramics
117
What are the three classes of composite materials?
1) predominantly glassy materials
2) particle filled glasses
3) polycrystalline ceramics
118
What kind of composite materials are highly aesthetic?
glassy composite materials
119
What kind of composite materials are examples of high strength ceramics?
crystalline ceramics
120
What is a 2nd example of an alternative to PFM alloys?
CAD-CAM
- substrate optimised block of factory produced ceramic
- impression to make die or optical (scanned)
- preparation coated with optically reflective powder
- digital model - edited
- computed controlled milling of final restoration/coping
- porcelain build up on coping thereafter
121
What are the characteristics of a glassy ceramic (composite material)?
- amorphous form
- 3D network of atoms of no regular pattern
- feldspar based
- resistant on firing to crystallisation and slumping
- alterations of Na and K content have affects on firing temperature and thermal expansion/contraction
122
Alterations of Na and K content of glassy ceramics have affects on what?
firing temperature
thermal expansion/contraction
123
What are particle filled glasses and why are they utilised?
base glass compositions with filler particles added.
improve mechanical properties, such as strength
and thermal expansion and contraction behaviour.
124
What structure are most particle filled glasses fillers?
crystalline
125
Name two crystalline filler particles added to particle filled glasses
1) leucite
2) Al2O3
126
What does the addition of filler particle leucite do?
dispersion strengthening
127
What does the addition of filler particle Al203 do?
fosters differential etching (etches at a greater rate) and hence micromechanical attachment
128
How can filler particles be added to form a particle filled glass?
1) mixing in the factory
2) grown in the pressed pellet for pressing by heat treatments
129
What are the characteristics of a polycrystalline ceramic?
- have no glassy component
- atoms packed in dense array
- relatively opaque (but unlike metals, transmit some light)
130
How are polycrystalline ceramics manufactured?
either pressed into over sized die or machined from block and sintered
131
What characteristics are added by the atoms of polycrystalline ceramics being packed in a dense array?
inhibit crack propagation
tougher and stronger than glass ceramics
132
What is an allotrope?
each of two or more different physical forms in which an element can exist. Graphite, charcoal, and diamond are all allotropes of carbon.
133
The allotropes of zirconia exist in what formations?
cubic, tetragonal and monoclinic
small Zr4+ and large 02-
134
What are the disadvantages of ceramics?
prone to crack propagation
brittle - time dependent strength decrease (Static fatigue - longer time is, lower strength becomes)
135
How do transformation toughened zirconium oxides prevent crack propagation?
surface stresses encountered during wear induce the toughening particles to transform, putting the surface into compression
136
What form of zirconia exists at room temperature?
monoclinic
137
What form of zirconia exists at firing temperature (2370-1170 degrees)?
tetragonal
138
How does transformation toughened zirconium oxide prevent crack propagation?
- high strength of the surrounding cubic Zirconia prevents expansion, so the Zirconia retains its tetragonal form down to room temperature following firing.
- each tetragonal Zirconia precipitate is under stress and full of energy that wants to be released
- tetragonal precipitates next to the crack are now able to expand and transform back to their stable monoclinic form. This expansion adjacent to the crack presses against the crack and stops it.
139
What is stress corrosion cracking?
Stress corrosion cracking is the growth of crack formation in a corrosive environment
140
What are the main properties of porcelain?
- hard - risks wear of opposing teeth
- relatively resistant to chemical attack
- good thermal insulator
141
What is known as the "Achilles heel" of porcelain?
crack propagation
142
What stops crack propagation?
compressive forces
143
What can help to prevent crack propagation by tensioning the surface of the material?
a glaze of low MP glass
144
How can you maximise clinical durability of ceramics?
- use max. occlusal thickness (strength increases with square of thickness)
- use highest elastic modulus substrate possible for core
- bond the restoration - ceramic = cement, cement - tooth or substructure
-develop broad NOT pinpoint occlusal contacts
145
How do broad rather than pinpoint occlusal contacts help to increase clinical durability?
spread the load
146
What increases with square of thickness of ceramic?
strength
147
What are two challenges regarding the cementation of ceramic prostheses?
aesthetics - often translucent restorations
brittle nature of ceramic
148
Why is the use of trial pastes when cementing a ceramic prostheses essential?
to establish which shade
149
What is shear thinning?
the viscosity of the fluid reduces as the shear increases
- as you seat it it becomes runnier then once seated will become viscous again
150
What does silanisation do?
promotes adhesion between resin composites and silica based or silica coated indirect restorations
151
What must be done prior to cementation if using resin cementation?
silanisation just prior to cementation because silane will degrade in presence of oxygen. Silane bonds to the Si-O of the ceramic system and effectively, the other end becomes incorporated into the polymerised resin luting cement
152
Generally, what cements are avoided and why?
conventional acid base cements must not be used EXCEPT in the case of zirconium oxide based ceramics where this is an option. Risk that etchant effect can exacerbate flaws
153
What are the advantages of dual cure resin composite?
- some operator control
- ensures deep curing of all shades
154
Feldspathic based ceramics require what type of etching?
etched in lab with HF - air abrasion is a suitable alternative to generate micromechanical undercut surfaces for attachment
155
Name two examples of polycrystalline oxide ceramics
1) aluminium oxide ceramics
2) zirconium dioxide ceramics
156
Name three examples of glass infiltrated oxide ceramics
1) glass infiltrated alumina
2) glass infiltrated zirconia
3) glass infiltrated spinell
157
Name three examples of glass ceramics
1) feldspathic porcelains
2) leucite-reinforced glass ceramics
3) lithium disilicate glass ceramics
158
How are ceramic failures that are repairable fixed?
- roughen surface (mechanical/chemical)
- prime surface for bonding (silane)
- bond on repair (composite/porcelain fragments)
159
What is a PJC, what are they made of and how are they prepared?
-porcelain jacket crown
-no longer made, feldspathic porcelain with added alumina
-need at least 1-2mm tooth reduction
-shoulder all round margin
-prone to fracture
160
What is the disadvantage of PJCs?
need at least 1-2mm tooth reduction
prone to fracture
161
What kind of margins are required on a PJC?
Shoulder margins all round
162
What are PJCs made of?
feldspathic porcelain with added alumina
163
How are veneers made and placed?
- pour up impression - refractory die or stone die
- foil over stone die
- build up veneer and fire
- try-in
- etch fit surface
- bond
164
What are dentine bonded crowns made of and how are they placed?
- feldspathic porcelain on refractory die
- trial for fit (NOT occlusion, too fragile to check)
- etch fit surface
- bond
- strength comes once bonded in situ
165
What is a dentine bonded crown essentially?
an "all over veneer"
166
How are hot pressed/injection moulded ceramics made and what are they used for?
- lost wax technique
- ceramic pressed into space
use: leucite reinforced glass ceramic, lithium di-silicate glass ceramic
167
What is a coping?
A thin metal, resin, or ceramic cap covering a prepared tooth. like a thimble
168
What are copings made of by VITA?
-alumina (In-ceram alumina
alumina with magnesium (In-ceram Spinell)
alumina with zirconia (In-ceram zirconia)
169
What is the function of a shrinking refractory die when making ceramic crowns?
allows room for removal of crown from die
170
What material can be used with CAD-CAM for greater strength?
Milled cores of yttrium-stabilised zirconia
171
How are CAD-CAM crowns produced?
CAD-CAM to produce a coping which is subsequently veneered with feldspathic porcelain by a technician
172
How are CAD-CAM copings produced?
- copings milled from 'green blocks' i.e. when not so hard
- sintered/hardened in furnace which causes around 20% shrinkage, computer calculates how much bigger to make it originally to compensate
- coping covered with feldspathic porcelain
173
Does porcelain bend and what impact can this have?
no
parafunctional habits - abrasive?
174
What types of porcelain and polycrystalline ceramic are used at DDH?
- high alumina porcelain (DBC)
- feldspathic porcelain (DBC and MCC)
- lithium di-silicate glass ceramic (E-max)
- zirconia (Lava)
175
What are high alumina porcelains used for at DDH?
dentine bonded crowns
176
What are feldspathic porcelains used for at DDH?
DBC and MCC
177
What can high alumina porcelain be used for?
- DBC
- occasionally used to support and strengthen feldspathic porcelain for DBCs
- can be etched and be used to opaque metal substructure of MCCs prior to placement of (aesthetic) feldspathic porcelain
178
What can feldspathic porcelains be used for?
- aesthetic and etchable porcelains which lack strength used for:
- covering high alumina porcelain in DBCs and opaque ceramic layer of MCCs
- veneers made on either foil or refractory dies
- covering zirconia frameworks
179
What can lithium di-silicate glass ceramics (E-max) be used for?
aesthetic, high strength and etchable glass ceramic made using CAD-CAM used for:
- veneers, onlays, inlays, crowns and covering zirconia frameworks
180
What are the characteristics of feldspathic porcelains?
aesthetic and etch-able but lack strength
181
What are the characteristics of lithium di-silicate glass ceramics (E-max)?
aesthetic, high strength and etch-able glass ceramic made using CAD-CAM
182
How are lithium di-silicate glass ceramics (E-max) made?
CAD-CAM
183
What is zirconia used for?
Yttrium-stabilised zirconia (Lava) using CAD-CAM which is NOT etch-able is used for:
- high strength frameworks for C+B to be later veneered with feldspathic porcelain or lithium di-silicate glass
- high-strength copings to replace use of gold crowns (retained by retention and resistance form)
184
What are the characteristics of Yttrium-stabilised zirconia (Lava)?
made using CAD-CAM and is NOT etch-able
high strength
185
Can zirconia be bonded?
No.
placed using luting cement
186
What is zirconia placed with?
luting cement
187
What can be done to zirconia to make a crown etch-able?
fired with feldspathic glaze to fit surface and then etch
188
What are ceromeric restorations? (ceromers)
porcelain filled composite
high quality long term provisional crowns
189
What are milled composites used for and what are their key characteristics?
inlay-onlay restorations only
aesthetic
cheap
can be polished in the mouth
CAD-CAM to mill from a block of composite
190
What materials can be utilised to produce tooth coloured crowns?
- feldspathic porcelain (strengthened core overlayed with feldspathic porcelain)
- lithium di-silicate glass ceramics
- all polycrystalline ceramic crowns
- glass (leucite reinforced)
- ceromeric crowns
- composite crowns
191
How are tooth coloured crowns manufactured?
- addition of porcelain slurry
- injection moulded/pressed/cast
- CAD-CAM
192
What are 5 trade names of tooth coloured crown materials?
- Procera
- Empress
- Lava
- In-ceram
- E-max
193
What is "regular dental porcelain"?
feldspathic porcelain
194
What are the advantages of feldspathic porcelain?
- can bond to metal
- great range of shades
- can etch fit surface and silanate (to bond composite)
195
How is feldspathic porcelain mixed?
powder mixed with water as a slurry
196
Is feldspathic porcelain strong?
weak until cemented with composite (=monoblock)
197
What does adding aluminium to porcelain to create a high alumina porcelain do?
increases strength but makes it 'very white' although still etch-able
198
How can the very 'white' appearance of high alumina porcelain be disguised?
by overlaying with regular feldspathic porcelain
199
What is an example of a high alumina porcelain?
Procera
200
Is there a clinical advantage of yttrium-stabilised zirconia over regular zirconia?
No obvious clinical advantage
201
What is an example of a regular zirconia product?
3M ESPE Lava
202
What is an example of a yttrium-stabilised product?
Vita In-ceram
203
How is the strength of zirconia achieved?
sintering (around 1500 degrees)
204
What amount of shrinkage occurs in zirconia due to sintering?
around 20%
205
Can you mill zirconia once it is sintered?
No, too hard
hence why made 20% larger to compensate for shrinkage prior to sintering
206
Can you etch zirconia?
No, can sandblast
207
Can you achieve many shades with zirconia?
Yes, innovative colouring of zirconia, different shades
208
What are the options for milling zirconia to be sintered?
- mill as a single block
- mill coping and cover with porcelain fired onto surface
209
How do you bond sintered zirconia?
- can't etch
- can't silanate
- may sandblast - try sandblasting with silica = tribochemical silica coating, then silanate
- can fire porcelain glaze onto fit surface, etch, silanate and bond
210
What can zirconia be used for?
- any type of crown
- bridgework as big as can be cut from the block
regular blocks = 4-5 units, max 4 pontics between abutments
211
How is a tooth prepared for a zirconia crown? (measurements)
axial - 1.0mm
incisal/occlusal - 1.5-2.0mm
chamfer margin
212
What does the incorporation of lithium di-silicate glass ceramic do?
results in significantly increased strength with excellent aesthetics
213
What restorations are lithium di-silicate glass ceramics incorporated in?
veneers, inlays/onlays, single crowns and 2 unit cantilever bridgework
Not strong enough for fixed-fixed
214
What thickness can lithium di-silicate glass ceramic work in?
works in thin section (0.8mm)
215
What is an example of a lithium di-silicate glass ceramic?
E-max
216
What is a ceromer?
porcelain filled composite
"posh composite crown"
difficult to bond
217
What is a direct restoration?
material placed in a cavity or on a tooth which then hardens into a solid either chemically or by light curing e.g. amalgam, composite, GI restorations
218
What is an indirect restoration?
a solid object made outside of the mouth which is placed in or on a prepared tooth e.g. crown, bridge, veneer, inlays, onlays
219
What is a luting cement?
material used for retention of indirect restorations and to seal the space between the restoration and tooth
220
What does the choice of luting cement depend on?
the type of restoration being cemented
221
What are the two types of luting cement?
- active luting material
- passive luting material
222
How does an active luting material work?
- bonds to tooth and restoration
- role in retention
- marginal seal
223
How does a passive luting material work?
-fills gap between tooth and restoration
- no bond between tooth and restoration
- retention due to tooth preparation - taper, preparation height, surface roughness and mechanical interlocking etc
224
What factors on tooth preps can make them retentive?
taper, preparation height, surface roughness and mechanical interlocking
225
What level of solubility in a luting cement helps to create a good marginal seal?
low solubility - active luting advantageous
causes reduced hypersensitivity if seal remains intact
226
Why is low film thickness of luting cement advantageous?
- allows full seating of restoration
- good marginal adaptation
227
What does pseudoplastic mean?
non-Newtonian behaviour of fluids whose viscosity decreases under shear strain
228
Why is being pseudoplastic a desirable property of a luting cement?
coats fit surface of restoration without slumping, but flows readily under pressure on fitting
229
Name four types of passive luting cements
1) zinc phosphate
2) zinc polycarboxylate
3) glass ionomer luting materials
4) resin modified glass ionomer luting cements
230
How does zinc phosphate luting cement present before mixing?
1) powder - zinc oxide, up to10% magnesium oxide (improves compressive strength, adds colour)
2) liquid - aqueous phosphoric acid (45-64%)
231
How is zinc phosphate luting cement mixed?
- working time 3-6mins
- gradual incorporation of powder over large area of mixing slab
- slaking the fluid - small amount of powder added to fluid about 1 min before - extend setting time
- use chilled glass slab
232
Why should you use a chilled glass slab when mixing zinc phosphate luting cement?
- increases working time
- increases amount of powder incorporated
- increases strength
- decreases solubility
233
What is the setting reaction involved in zinc phosphate luting cement?
- acid base reaction
- slightly exothermic
- viscosity increases rapidly
- 50% strength in 10mins, 100% in 24hrs
- shrinks slightly on setting
234
Is zinc phosphate luting cement antibacterial?
No
235
How quickly does zinc phosphate luting cement reach full strength?
50% of strength in 10 mins
100% in 24 hours
236
What kind of pH does unset zinc phosphate luting cement have?
1.6-3.6 depending on thickness of mix - relevant to vital pulp
237
What are three main characteristics of zinc phosphate luting cement?
- good compressive strength
- low tensile strength (very brittle)
- high solubility
238
How does zinc polycarboxylate luting cement present before mixing?
1) powder - zinc oxide (up to 10% magnesium oxide)
2) liquid - aqueous copolymer of polyacrylic acid (30-40%) (high viscosity)
239
What is a possible alternative presentation to the regular presentation of zinc polycarboxylate before mixing?
liquid - acid freeze dried and added to powder
liquid - distilled water
240
What does the addition of magnesium oxide to luting cement powders do?
improves compressive strength and adds colour
241
What setting reaction occurs in the mixing of zin polycarboxylate luting cement?
- acid dissolves zinc oxide
- zinc ions form cross links between carboxyl groups on polyacrylic acid polymer chains
- unreacted powder bound in matrix of zinc polyacrylate
242
What is the working time of zinc polycarboxylate luting cement?
short working time - 30-40 seconds
243
How can the working time of zinc polycarboxylate luting cement be extended?
addition of tartaric acid, mix on cold glass slab
244
What can cause a shortened working time when mixing zinc polycarboxylate luting cement?
higher powder:liquid ratio
245
What is the initial pH of zinc polycarboxylate luting cement?
pH 3-4 - less injurious to pulp
246
Does zinc polycarboxylate luting cement have antibacterial properties?
Yes
247
Is zinc polycarboxylate luting cement adhesive and to what?
adhesive to enamel and dentine and some metals (base metal alloys via oxide layer)
248
Compared to zinc phosphate luting cement, does zinc polycarboxylate luting cement have higher or lower strength?
lower compressive strength
higher tensile strength
249
How long does it take for zinc polycarboxylate luting cement to reach full strength?
80% full strength in 1 hour
250
Zinc polycarboxylate luting cement is soluble in what?
acid
251
Does zinc phosphate luting cement shrink upon setting?
Yes, slightly
252
How does glass ionomer luting cement present before mixing?
powder - fluoro-alumino-silicate glass
liquid - aqueous poly alkenoic acid
253
What is an alternative presentation for glass ionomer luting cement before mixing?
acid freeze dried and added to powder
liquid - distilled water
254
What is the setting reaction involved in the mixing of glass ionomer luting cement?
chemical set
dissolution - Ca2+ ions released first, then Al3+
gelation
hardening - Al3+ - trivalent, slower to be released from glass, Al3+ ensures strong cross linking of polymer chains
255
How quickly does glass ionomer luting cement set?
slow
dissolution and initial set 3-6mins (viscosity increases quickly, seat within 2-2.5mins)
hardening can take 7 days
needs protecting to prevent dissolution or contamination
256
What are the key properties of glass ionomer luting cement?
- fluoride release - anti-caries?
- bond to enamel and dentine - seal, retention
- initial acidity - may cause pulpal inflammation
257
How does glass ionomer luting cement compare to zinc oxide cements?
better compressive strength
low tensile strength and fracture toughness
less soluble
258
What are the constituents of resin modified glass ionomer luting cements?
components of glass ionomer plus monomer (e.g. HEMA, BisGMA)
no photo initiator - chemical cure only (acid base reaction of GI)
259
What are the advantages of resin modified glass ionomer luting cements over conventional GI?
- low solubility
- improved biocompatibility
- improved fluoride release
- improved physical properties (tensile strength)
- improved adhesion to tooth structure
260
What are the disadvantages of resin modified glass ionomer luting cements compared to conventional GI?
- can undergo hygroscopic expansion
- suitable under zirconia core/CAD CAM crowns
261
When should resin modified glass ionomer luting cements not be used?
avoid under conventional all ceramic crowns
262
When are resin modified glass ionomer luting cements suitable to use?
under zirconia core/CAD CAM crowns
263
What are the constituents of resin-based luting cements?
Silanated filler
resin e.g. BisGMA
- lower filler content and hence lower viscosity
- most dual cured - chemical and light
264
How are most resin-based luting cements cured?
chemical and light
265
What are conventional resin luting cements used for?
for use with all ceramic restorations, indirect composite or quartz fibre posts e.g. veneers, DBC
266
What are chemically adhesive resin luting cements used for?
for adhesively bonding to metals e.g. resin retained bridges, metal veneers, poorly retained indirect restorations
267
What do conventional resin luting cements bond to?
bonding to ceramic
268
How can you bond to ceramic?
1) conventional way - acid etch, rinse, dry, dentine bonding agent (self etching versions available)
2) bond to fit surface of ceramic - etched with hydrofluoric acid and just before cementing, silane coupling agent applied and air dried
= micromechanical and chemical bonding
269
How can you bond the fit surface of ceramic?
etch with hydrofluoric acid and just before cementing, silane coupling agent applied and air dried
= micromechanical and chemical bonding
270
Why is hydrofluoric acid only used in the laboratory, not on clinic?
very toxic
271
Why must hydrofluoric acid be neutralised when used to etch ceramic?
if not may leach out causing tissue damage later
272
From the outer surface of a veneer, in to the dentine, name the layers of materials present to bond a ceramic veneer
1) veneer
2) hydrofluoric acid etch fit surface and silane coupling agent
3) resin luting cement
4) etched tooth surface and dentine bonding agent
5) enamel
6) dentine
273
How can you bond a base metal alloy (Ni-Cr)?
- roughen fit surface with 50micro m alumina grit (micromechanical retention)
- chemically adhesive resin luting cement - modified BisGMA resin
274
How can you bond a precious metal alloy?
- chemically adhesive resin luting cement -low affinity
- modify fit surface - tin plate (irregular surface, attracted to tin oxide on alloy surface), silica coating to metal (then silane coupling agent), metal primers (bifunctional monomers)
275
What kind of consent do you require to retrospectively go through notes?
Caldicott Guardian consent
276
What is a popular method of failure analysis?
Survival analysis
277
What is the aim of survival analysis?
to plot a survival curve
278
What does a survival curve plot?
plots time until failure
permits comparison of survival of two or more groups
279
What is termed censored data?
data where you don't know or can't judge survival of a restoration. e.g. pt has not been seen since placement
280
How does a Kaplan-Meier survival analysis plot failure?
-gather data (date of placement, last date of follow up, outcome, survival/failure)
-state if an event (1) or censored data (0)
-place into statistical programme
-survival curve generated
281
What are on each of the axes in a Kaplan-Meier survival analysis curve?
x-axis = time in days
y-axis = probability of surviving or the proportion of people surviving
282
How is durability and load limit predicted in the laboratory?
- fatigue testing
- restorations subject to on/off loading
- define in advance your survival number of cycles and cross head speed
- apply cyclical force 2 newtons - to test upper load limit
- initial test upper load limit is 60% of applied equivalent for mean flexural strength
- record no. of cycles to failure or survival
- if survives, increase upper load limit by set increment
- if fails, decrease upper applied load limit
- typically test 15 specimens
283
x
x
284
What is considered the Achilles heel of brittle dental materials?
porosity
285
What does Weibull statistical distribution assess?
recognised method of assessing dependability worldwide
286
What does a low Weibull modulus indicate?
wide distribution of fracture stress values and a long tail at low values of stress - possible vulnerability
287
What does a high Weibull modulus indicate?
indicative of a close grouping of fracture stress values - Better dependability
288
x
x
289
In a strength-probability-time diagram, what does fracture strength reflect?
size of the flaw initiating failure
290
How will a prolonged application of a stress insufficient to induce short term failure cause failure?
it will ultimately fail in time (t) due to the subcritical growth existing defects of flaws (static fatigue)
291
What is CAD-CAM?
Computer aided design, computer aided manufacture
292
What is involved in the use of CAD-CAM?
record a digital impression - number of possibilities
intraoral scanner, digitise a lab conventional impression, digitise in lab model cast from impression
293
What are the general principles of non-contacting scanners?
-emit some form of radiation
- detect the reflection from the surface of the radiation
- time the round trip (scanner-surface-scanner)
- determines elevation of only one point
- add in rotatable mirrors to move around surface and measure more points
294
The camera in a non-contacting scanner is usually what?
a charge coupled device (CCD)
295
What are the six degrees of freedom of a non-contacting scanner?
1) forward/backward (surge)
2) up/down (heave)
3) left/right (sway)
translation in 3 perpendicular axes :
4) Yaw (normal axis)
5) Pitch (transverse axis)
6) roll (longitudinal axis)
296
What does the parallel confocal imaging technique (non-contacting scanner) do?
utilises red light laser beams
- pass through focussing optics
- leave scanner and hit object surface
- intensity of reflected light measured
- for each spot specific position software adjusts optics to maximise intensity of reflection
- info yields elevation for each point
297
What is the Moire effect?
mechanical interference of light by superimposed networks of lines
the resulting moire pattern can be analysed to yield depth info
298
What is active wavefront sampling?
-scanner has an aperture that rapidly moves (by rotation) permitting image capture at several positions
-in effect, the "camera" CCD records images from different positions from which depth can be determined using photogrammetric principles
299
What patient factors can affect the accuracy of hand held scanners?
- limited patient mouth opening
- patient movement
- saliva flow
- intraoral humidity
300
What anatomical factors can affect the accuracy of hand held scanners?
tooth form
tooth position
301
How is the data from a hand-held scanner processed?
- raw data in x, y, z format
- converted to STL file format (standard triangle language) - only describes surface geometry, not colour or texture
302
What is XML and why is it used for processing hand-held scanner data?
eXtensible markup language - software and hardware independent of tool for storing and transporting data.
Simplifies - data sharing and transport, platform changes, data availability, expansion/upgrade to new operating systems without losing data, addition/removal of data as the XML application will still work
303
What are the two main current CAM methods?
Reductive
Additive
304
What is the "reductive" CAM method?
- cut from block of optimised properties OR green stage block (softer and readily cut) with post machining treatment to optimise properties
- CAD model gives template
- CAM informed by CAD specifies sequence, tools and tool motion, magnitude and direction
- 3 axis milling machine (x, y, z) most common
305
What is the "additive" CAM method?
-3D printing - build up in successive layers to desired shape. Pattern produced in burnout plastic
- rapid prototyping techniques
306
What are three current rapid prototyping methods in CAM?
1) stereolithography (SLA)
2) selective laser melt (SLM)
3) selective laser sintering
307
What is stereolithography (SLA)?
- rapid prototyping method
- UV laser beams solidify photosensitive resin laters into a 3D polymer
- structure needs supported and rinsed of solvent
- cured in UV oven
- support removed
308
What is selective laser melt (SLM)?
metal 3d printing
309
What is selective laser sintering (SLS)?
- rapid prototyping method
- applied to plastics, ceramics, glass and alloys
- a CO2 laser is directed to incrementally fuse powdered material into a 3D model
- no support required as first sintered layer supports this
310
What are some common machining and performance concerns?
1) machining damage leaves history, in form of residual stresses, that contribute to early clinical failure by future crack propagation
2) minimal machined thickness
3) as thickness decreases % failure increases
4) as speed/aggression of milling increases, % failure increases
311
How are zirconia and alumina ceramics made with CAM?
- supplied as green/partially sintered for machining
- on sintering shrinkage of 20-35%
- thus for accuracy, need to measure and convey blank density to milling unit
- allows for correct oversizing and shrinkage for good fit
- quality assurance of product important for if block supplied is not homogenous, differential shrinkage and warping occur
312
How are glass ceramics made with CAM?
- correct machining essential
- post machining crystallisation may produce distortions
- machining when crystallised risks machining damage adversely affecting materials properties
313
What are the three types of ceramic blanks for CAM?
1) green - powder pressed/cast to form by manufacturer
2) partially sintered - blanks subjected to heating by manufacturer
3) fully dense - fully sintered and fully dense
314
Name the three types of ceramic blanks used for CAM in order of increasing strength and decreasing porosity:
1) green - least strong, highest porosity
2) partially sintered
3) fully dense - strongest, least porosity
315
What is the digital workflow?
combination of technologies employed by the dental team to primarily provide patients with restorative care
316
What are the main four components of the digital workflow?
1) data acquisition
2) design stage (CAD)
3) manufacturing phase (CAM)
4) final processing and treatment delivery
317
What are the two main methods of data acquisition, clinical and laboratory for the digital workflow?
1) intra-oral scanning
2) laboratory desktop scanning
318
What does the IOS output provide in the digital workflow?
the output screen provides the user with a real time view of the scanning process during the procedure, allows for 360 degree manipulation of scan
319
How is laboratory data acquisition acquired?
- desktop optical scanner
- surface scan
- dental model scan
- conventional impression scan
- STL output
320
What is structured light scanning and what does it achieve?
series of structured light patterns projected onto the surface of the dental model and then recoded by a sensor
it provides higher levels of accuracy which makes it capable of producing identical copies of dental models or impressions
321
How do benchtop scanners work?
high resolution camera technology is incorporated to allow texture scanning, where cameras can capture 2D images of the gypsum model/impression surface and precisely overlays these onto 3D models. This enhances visualisation and surface details
322
What materials are used in digital fixed restorative prostheses?
- all ceramic
- digital precious metal (DPM) crown, digital non-precious (DNP) post and core
- digital temporary , burnout wax
323
What output materials are used for digitally designed removable partial dentures?
- sintered cobalt chrome
- flexible denture
- acetal
324
What is a ScanBody?
scannable object used to accurately translate the position of an implant in the oral cavity into a digital file which can be utilised within the implant design software. It is screwed into the implant intraorally or alternatively into the dental cast. From there, angle and position is calculated and saved to allow design to commence.
325
What is the leading material used in digital implant manufacture and why?
titanium
due to biocompatibility and strength:weight ratio
326
What are the three familites of ceramics?
1) glass-matrix ceramics
2) polycrystalline ceramics
3) resin-matrix ceramics
327
What type of ceramic is an all-ceramic material with a glass-matrix present?
glass matrix ceramix
328
What type of all-ceramic material has no glass-matrix present?
polycrystalline ceramics
329
What kind of all-ceramic material contains an organic matrix highly filled with ceramic particles?
resin-matrix ceramics
330
What is modulus of elasticity measured in?
has no units
331
What does an high modulus of elasticity indicate?
rigid
332
What does a low modulus of elasticity indicate?
flexibility
