DMS Flashcards
1
Q
Components of a preformed metal crown
A
Porcelain surface
Metal alloy substructure
2
Q
Porcelain as a crown material
A
Good aesthetics but microcracks tend to for at the fitting surface, making it prone to mechanical failure
3
Q
Metal alloys as crown material
A
Good mechanical properties
Poor aesthetics
4
Q
Compressive strength
A
Stress required to cause fracture
5
Q
Elastic modulus
A
Rigidity
Stress:strain ratio
Stress required to cause change in shape
6
Q
Brittleness/ductility
A
Dimensional change experienced before fracture
7
Q
Hardness
A
Resistance of surface to indentation or abrasion
8
Q
Which mechanical properties does a stress strain curve give information on?
A
Rigidity/elastic modulus
Brittleness/ductility
Compressive/tensile strength
9
Q
What does the difference between proportional limit and fracture stress on a stress-strain curve indicate?
A
The brittleness or ductility of a material
Big difference - ductile
Small difference - brittle
10
Q
Describe hardness, strength, rigidity and ductility of metal alloy and porcelain
A
Porcelain - quite hard, strong and rigid, very brittle (not ductile at all)
Metal alloy - very hard, strong, rigid and quite ductile
11
Q
Characteristics of porcelain
A
Rigid - large stress required to cause strain
Hard - surface withstands abrasion/indentation well
Strong - high compressive strength but low tensile strength
Tendency to form surface defects, leads to fracture at low stress
Brittle - low fracture toughness
12
Q
What bonds porcelain to alloy in preformed metal crowns?
A
Metal oxide on the surface of the alloy
13
Q
What is the role of an alloy in a preformed metal crown?
A
Support and limit the strain that porcelain experiences
With the two materials bonded together, the stress applied causes a small strain to be experienced, small enough for porcelain to withstand
14
Q
Porcelain fused to metal alloys
A
High gold alloy
Low gold alloy
Silver palladium
Nickel chromium
Cobalt chromium
15
Q
Required properties of alloy to be used in porcelain alloy crown
A
Form good bond to porcelain - i.e. good wetting, porcelain forms bond with metal oxides on the surface
Thermal expansion coefficient similar to porcelain, to avoid setting up stresses during fusing of porcelain on to alloy
Avoid discolouration of porcelain
Mechanical - bond strength, hardness and elastic modulus
Melting, recrystallisation temperature must be higher than fusion temperature of porcelain
16
Q
Creep
A
Gradual increase in strain (permenant) experienced under prolonged application of stress (<EL)
17
Q
Which alloys are more difficult to bond to porcelain?
A
Nickel chromium
18
Q
Which alloys can cause discolouration of porcelain?
A
Silver in silver palladium
19
Q
Which alloys have the highest elastic modulus?
A
Nickel chromium
20
Q
Why is no copper usually in alloys to be bonded to porcelain for crowns?
A
Gives green hue to porcelain
21
Q
High gold alloys constituents and their effects
A
80% gold
14% Pt/Pd - helps match thermal expansion to porcelain, increases melting point
Ag 1%
Small amount of indium or tin - form metal oxide layer
22
Q
What is the issue with metal alloys having a lower melting/recrystallisation temp than fusion temperature of porcelain?
A
Can cause creep to occur
23
Q
Problems with high gold alloys
A
Melting range may be too low
Young’s Modulus - too low
24
Q
What alternative to high gold alloys can be used to improve its limitations?
A
Low gold alloys - Au 50%, Pd 30%, Ag 10%, Indium/Tin - 10%
Increased melting temperature
Slightly better mechanical properties
25
Constituents of silver palladium alloys and their characteristics
Pd 60%
Ag 30%
Indium/tin - 10%
High melting point
Care needed in casting
26
Constituents of Nickel chromium alloys and their characteristics
Ni 70-80%
Cr 10-25% (oxide bond)
High melting point
High young's modulus
High casting shrinkage
Low-ish bond strength
27
Characteristics of cobalt chromium alloys
High melting point (1300-1400C)
Casting shrinkage 2.3%
Low ish bond strength 50MPa
High Young's modulus 220GPa
High tensile strength 850MPa
High hardness 360-430 VHN
28
Bonding type between porcelain and metal in crowns
Mechanical - probably least important, due to irregularities on surfaces
Stressed skin effect - slight difference in thermal contraction coefficients - lead to compressive forces which aid bonding
Chemical - may be electron sharing in oxides, during firing porcelain flows and oxides in the metal-oxide coating migrate
29
Ideal thermal expansion coefficient of a metal alloy to be bonded to porcelain
0.5ppm/C higher than porcelain (14ppm/C)
30
Uses of stainless steel in dentistry
Denture bases
Orthodontic wire
31
Wrought alloy
An alloy which can be manipulated or shaped by cold working
e.g. drawn into a wire for use in ortho appliance or as denture clasps
32
Steel composition
Steel is an alloy
>98% iron
<2% carbon
0.5%-1% chromium to improve tarnish resistance
Manganese, molybdenum, nickel, cobalt, silicon
Any more than 2% carbon and it is considered as CAST IRON
33
Allotropic
Undergoes two solid state phase changes with temperature
34
Iron characteristics
Allotropic
Temp >1400C - Body Centred Cubic crystalline lattice structure, low carbon solubility (0.05%)
Between 900-1400C - face centred cubic lattice structure, higher carbon solubility (2%)
<900 - BCC structure, low carbon solubility again
35
Austenite
Interstitial solid solution of iron and carbon, FCC, exists at high temp >720C
36
Ferrite
Very dilute solid solution
Exists at low temp
37
Cementite
Fe3C exists at low temperature
38
Pearlite
Eutecoid mix of ferrite and cementite
39
Solid solution
Two metals that are soluble in one another form a common lattice structure
40
Substitutional solid solution
Two types
1. Random - both types of atom in the lattice structure are arranged at random
2. Ordered - types of atoms located in predictable layers
41
Interstitial solid solution
Two atoms of markedly different size
Larger atom forms lattice
Smaller fits into spaces in random fashion
42
Why is austenite more desirable than ferrite or cementite?
They have large grains with poor mechanical properties
43
What does quenching of austenite produce?
Martensite
44
Martensite properties
Distorted lattice - no time for diffusion of carbon
Hard
Brittle
45
What does slow cooling of austenite give us?
Pearlite
Ferrite
Cementite
46
What is tempering of martensite and why is this done?
Heating 450C followed by quenching
Temp and duration affect conversion into ferrite (soft, ductile) and cementite (hard, brittle)
Non dental uses for these products
47
Stainless steel composition
Iron
Carbon
Chromium
Nickel
Only stainless steel if >12% Cr
48
What is the role of chromium in stainless steel?
To lower the temperature of austenite to martensite conversion and to lower the rate of this conversion
49
What gives stainless steel corrosion resistance?
Chromium oxide layer
50
What is the role of Nickel in stainless steel?
Improves strength and corrosion resistance
Lowers austenite to martensite transition temp
51
Type of stainless steel
Martensitic
Austenitic
52
Martensitic stainless steel properties
12-13% chromium, little carbon
Heat hardenable
Used to make dental instruments
53
Austenitic stainless steel properties
Contains sufficient chromium and nickel to suppress austenite to martensite transition
E.g. 18% Cr 8% Ni ratio
or
12% Cr 12% Ni ratio
54
Austenitic stainless steel uses
Dental equipment and instruments - can withstand WD temps
Wires - ortho, readily cold worked, resists corrosion
Sheet forms for denture bases - swaged
55
18-8 stainless steel composition
18% chromium
8% Nickel
0.1% carbon
74% iron
56
Properties of 18-8 stainless steel
Does not heat harden
Soft and malleable when cast but work hardens rapidly
57
Cold working
Work done on metal alloys at low temperature - below recrystallisation temp.
Bending, rolling or swaging, cause SLIP - dislocations collect at grain boundaries
Hence, stronger, harder material
58
18-8 stainless steel wire uses
Ortho springs, clasps
Partial denture clasp arms, wrought rests
59
Grades of 18-8 stainless steel wire
Soft
Half hard
Hard
Spring temper
60
Wire alloys
Stainless steel
Gold
Cobalt chromium
Titanium - various types
61
Cobalt chromium wire constituents
Co 40%
Cr 20%
Ni 15%
Fe 16%
62
Gold wire constituents
Au 60%
Ag 15%
Cu 15%
Pt/Pd 10%
63
Nickel titanium wire constiuents
Ni 55%
Ti 45%
+ some cobalt
64
Springback ability
Ability of material to undergo large deflections (to form an arc) without permanent deformation
EL/YM
65
Requirements of wires
High springback ability
Stiffness (YM)
High ductility
Easily joined
Corrosion resistant
66
Stainless steel wire soldering
Can be done with gold or silver solder, care must be taken to avoid recrystallisation, which negatively impacts mechanical properties
Quench rapidly to maintain
67
Weld decay
Occurs when stainless steel is heated to between 500-900C
Chromium carbides precipitate at grain boundaries
Alloy becomes brittle
Less chromium in central region of solid solution - more susceptible to corrosion
68
How to minimise weld decay
Low carbon steels - expensive
Stabilised stainless steel - contain small quantities of titanium or niobium, forms carbides preferentially - not at grain boundaries
69
Stress relief anneal
Done to ensure the configuration of metal atoms into alloy grains settle into equilibrium
Stainless steel held at 450C for 1-2min
CAREFUL - Above 650C grain structure is affected and above 500C precipitation of carbides at grain boundaries occurs
70
Properties of stainless steel as a denture base
Thin 0.11mm (acrylic 1.52mm)
Light
Fracture resistant
Corrosion resistant
High polish obtainable
High thermal conductivity
High impact strength
Abrasion resistant
Disadvantages
Possible dimensional inaccuracy
Elastic recovery of steel - inaccuracy
Damage of die under hydraulic pressure during swaging
Loss of fine detail during the many stages
Difficult to ensure uniform thickness
Uneven pressure on die and counter die
71
Ideal properties of a denture
Replace function of natural teeth
Fit properly
Appropriate aesthetics
72
Ideal properties for denture base
Dimensionally accurate and stable in use
High softening temp
Unaffected by oral fluids
Thermal expansion
Low density
High thermal conductivity
Radiopaque
Non toxic
Colour
Easy and inexpensive to manufacture
Easy to repair
73
Mechanical properties required for a denture base material
High Young's modulus
High proportional limit
High transverse strength
High fatigue strength
High impact strength
High hardness/abrasion resistance
74
What type of polymerisation reaction does acrylic resin undergo?
Free radical addition polymerisation - chemical union of two molecules either the same or different to form a larger molecule, without the elimination of a smaller molecule
Involves molecules with C=C
75
Which three groups are joined to the central carbon atom in methacrylate monomer?
-CH3
=CH2
-COOCH3
76
Stages of polymerisation reaction
Activation - of initiator to provide free radicals
Initiation - free radicals break C=C bond in monomer and transfer free radical
Propagation - growing polymer chain
Termination - of polymerisation
77
What is the initiator in heat cured acrylic powder?
Benzoyl peroxide 0.2%-0.5%
78
Components of heat cured acrylic powder
Initiator - benzoyl peroxide
PMMA particles (pre polymerised beads)
Plasticiser - allows quicker dissolving in monomer liquid e.g. dibutyl phthalate
Pigments to give natural colour
Co-polymers to improve mechanical properties e.g. ethylene glycol dimethacrylate
79
Components of heat cured acrylic liquid
Methacrylate monomer (dissolves PMMA particles, polymerises)
Inhibitor - Hydoquinone 0.006% prolongs shelf life - reacts with any free radicals produced by heat or UV light
Co-polymers to improve mechanical properties, particularly crosslinking of polymers
80
What is efficient polymerisation?
Low number of crosslinks in polymer, giving high molecular weight and good mechanical properties
81
Properties of acrylic resin
Non toxic/irritant
Unaffected by oral fluids
Mechanical properties - poor
Fairly resistant fatigue strength, but can fail
High hardness/abrasion resistance, some wear over time
Thermal expansion - ok if acrylic teeth used, far higher than porcelain teeth
Low thermal conductivity - bad
Low density - good
Ok softening temperature - must not clean in boiling water
Ok dimensional accuracy and stability in use
82
What is the difference in composition between self cured and heat cured acrylic?
Tertiary amine in liquid activates benzoyl peroxide initiator, instead of heat
83
Advantages of self cure acrylics
No heating stage - less thermal contraction and better fit
84
Disadvantages of self cure acrylic
Chemical activation leads to less efficient polymerisation - lower molecular weight and poorer mechanical properties and more unreacted monomer - acts as plasticiser softening the base and more likely to cause irritation
Poorer colour stability
Fits cast better BUT water absorption in mouth makes oversized
85
How much unreacted monomer can be expected in self cure vs heat cure acrylic?
Self - 3-5%
Heat - 0.2-0.5%
86
What can the issues with sizing of heat cured acrylic denture bases be?
Slightly undersized due to thermal contraction
BUT water absorption gives expansion, cancelling some of this out
87
Advantages of heat cured acrylic resin
Higher molecular weight - stronger
Better fit (likely slightly undersized rather than slightly oversized)
Less uncured monomer
88
Disadvantages of heat cured acrylic resin
Curing process may cause porosity
89
Improved versions of acrylic resin available
High impact resistant materials - incorporate rubber toughening agent to stop crack propagation - long term fatigue problems
and fibres (carbon, ultra high molecular weight polyethylene, glass) - difficult processing
90
Heat cure denture base product Ultra Hi
A high impact heat cure acrylic resin formulated with exceptional flexural strength and superior fracture toughness (ductility)
These two key features together gives ultra hi
A slight bending aspect which keeps material from being brittle to reduce cracking
91
Pour n Cure resins
Similar to self cure
Smaller powder particles creates fluid mix rather than dough like substance to be poured into mould
Good fitting but poor mechanical properties
92
Light Activated Denture Resins
Urethane dimethacrylate matrix plus acrylic copolymers, microfine silica fillers and photoinitiator system
Adapted to cast
Cured in light chamber - limits thickness
Used mostly as customised impression tray material and for repair of fractured dentures
93
Radiopaque polymers
Some additions are made to denture base materials to achieve the desired radiopaque quality
Metal inserts - weaken, poor aesthetics
Inorganic salts (high conc required for radiopaqueness) - weak base
Comonomers containing heavy metals - poor mechanical properties
94
What can be used for patients with allergies to acrylic resin, and what are their main drawbacks?
Nylons - water absorption, swelling
Vinyl polymers - low softening temp (60%)
Polycarbonates - require expensive injection moulding technique, become distorted
95
Most commonly used denture base material
Heat cured acrylic resin
96
What are investment materials used for?
To produce metal/alloy inlays, onlays, crowns and bridges by casting the molten alloy into a mould cavity of the required shape
The mould cavity is made of an investment material
97
Stages in casting an alloy by lost wax technique
Wax pattern of required prosthesis is made
Investment material poured around wax pattern and allowed to set to create a mould
Wax is then eliminated (with boiling water)
Molten alloy is forced into the cavity through sprues (hollow tubes prepared in the investment material)
98
Conditions for casting of molten alloy
1000C + and high pressure
99
Investment materials and their use
Dental stone or plaster - acrylic dentures
Gypsum bonded materials - gold casting alloys
Phosphate bonded materials - base metals/cast ceramics
Silica bonded materials - base metal alloys
100
Requirements of investment material of metal alloys
Must expand to compensate for cooling shrinkage of alloy
Porous to allow escape of trapped gases on casting to prevent back pressure effect
Strong - room temp ease of handing, casting temp withstand casting forces
Smooth surface - easy finishing
Chemically stable
Easy removal from cast
Handling not complicaed
Relatively inexpensive
101
Back pressure effect
Voids and defects in cast alloys due to gas trapped during the casting process
102
Typical contractions (by volume) from alloy melting point to room temp of alloys
Gold alloys - 1.4%
Ni/Cr alloys 2.0%
Co/Cr alloys 2.3%
103
Components of investment materials and their roles
Binder - to form coherent solid mass, gypsum, phosphate and silica
Refractory - withstand high temperature and undergoes expansion, silica (quarts or cristobalite)
104
Composition of gypsum bonded investment
Powder (mixed with water) Silica 60-65%, calcium sulphate hemihydrate 30-35%, reducing agent for oxides, chemicals to inhibit heating shrinkage and control setting time - boric acid, NaCl
105
Setting reaction of gypsum bonded investment material
(CaSO4)2.H2O +3H2O -> 2CaSO4.2H2O
Hemihydrate + water -> dihydrate
106
Dimensional changes of gypsum bonded investement
Silica undergoes thermal expansion and inversion expansion
Gypsum undergoes expansion during setting - hygroscopic expansion
and contraction above 320C
107
Hygroscopic expansion
Not fully understood
Varies, causing up to 5 fold change in volume within gypsum bonded investment
Thought to be water molecules attracted to gaps between crystals by capillary forces, forcing crystals apart
108
Factors increasing hygroscopic expansion
Lower powder/water ratio
Increased silica content
Higher water temp
Longer immersion time
109
What causes the contraction of gypsum above 320C?
Water loss
Presence of NaCl and boric acid
110
Properties of gypsum bonded investment material
Expansion 1.4% - sufficient for gold alloys
Smooth surface due to fine particles
Manipulation is easy and setting time is controlled
Porous
Strength is adequate if correct powder/liquid ratio and correct manipulation
111
What is heat soaking
About 700C, if there is any wax residue remaining, a reaction between CaSO4 and C occurs, releasing carbon monoxide
CaSO4 +4C -> CaS + +4CO
Then the CaS reacts with CaSO4 to give SO2 gas
3CaSO4 +CaS -> 4CaO +4SO2
Crucial these gases escape, to ensure this happens, heat soaking is done
Held at this temperature for some time to allow gases to escape
112
When is gypsum bonded investment suitable?
If the metal to be cast melts below 1200C
Above 1200, the CaSO4 in gypsum reacts with SiO2 to create SO3 which will cause voids in the cast
113
Composition of phosphate bonded investment
Powder - silica, magnesium oxide, ammonium phosphate
Liquid - water or colloidal silica
114
What is the purpose of mixing phosphate bonded investment with colloidal silica?
Increases strength
Gives hygroscopic expansion (2%) to compensate for alloy shrinkage on cooling to room temp
115
Setting reaction for phosphate bonded investment material
NH4H2PO4 +MgO +5H2O --> MgNH4PO4+6H2O
Ammonium phosphate reacts with magnesium oxide and water to give
MAGNESIUM AMMONIUM PHOSPHATE and water
116
What are the effects of heating phosphate bonded investment to around 1000-1100C?
At 330C water and ammonia are liberated
At higher temps complex reactions with silico-phosphates take place, leading to increased strength of the material
117
Properties of phosphate bonded investment
High strength
Sufficiently porous
Chemically stable
Easy to use
High green strength
118
What is green strength?
Strength for handling at room temperature
119
Properties of silica bonded investment
Sufficient strength
Not porous - would create weak alloy therefore needs vents
Complicated manipulation
120
Types of elastomer used
Polyether
Addition silicone
121
Factors in selecting an impression material
Material characteristics
Clinical performance - ease of use and patient acceptance
122
How are elastomers formed?
Polymerisation and crosslinking of polymer chains
Crosslinking generates elastic properties, causing fluid -> solid transition
Polymerisation also may produce byproducts such as water, hydrogen and alcohol which affect dimensional stability and cast compatibility
123
Types of silicone elastomers
Addition curing
Condensation curing
124
How are addition cured silicones and polyethers usually sold?
Large twin cartridges of base paste and catalyst paste
125
Important material properties of elastomers
Surface detail
Flow/viscosity
Contact angle/wettability
(first 3 determine quality of impression small details recorded)
Elastic recovery
Stiffness
Tear strength
(Determine accuracy of dimensions and shape of impression)
Mixing time
Working time
(Practical considerations)
Shore A hardness
Shark fin (flow under pressure, essential for undercuts)
Setting shrinkage
Dimensional stability
Thermal expansion coefficient
Biocompatibility
126
What type of material is Virtual?
Addition silicone elastomer
Polyvinylsiloxane
127
Factors affecting the quality of surface interaction between material and tooth/soft tissue surfaces
Viscosity - must be able to flow readily
Surface wetting - must make intimate contact with teeth/mucosa
Contact angle - determines how well material envelopes the hard/soft tissue surface
128
Factors affecting the accuracy of an impression material
Surface reproduction
Visco-elasticity/elastic recovery
129
Factors affecting an impression materials ability to cope with removal stage and undercuts
Flow under pressure (Shark fin test)
Tear/tensile strength
Rigidity
130
Factors affecting impression materials dimensional stability
Setting shrinkage
Thermal expansion/contraction
Storage
131
Viscosity of impression materials
A measure of materials ability to flow
Must flow readily in order to make close contact with hard/soft tissue surfaces
This therefore affects how well it records surface detail
132
Wettability - contact angle of impression material
How readily the material wets the surface of tooth and how closely it envelopes the tooth surface
Small contact angle = large % of volume will make contact with target surface - ideal
133
What is the effect of a high contact angle of impression material on the final impression?
Results in spaces between globules of material when applied to teeth, so some of the tooth surface will not be replicated
134
Hydrophilic silicones benefits
Incorporate a non-ionic surfactant to wet tooth surface
Makes better contacts with tooth surface
135
ISO 4823:2000
International standard for measuring how well surface is represented by impression material
Measured by spreading material over surface with grooves of A 20micrometres B 50 and C 75
136
Ideal elastic behaviour of impression material
100% elastic recovery following removal of impression tray from mouth - no permanent strain
137
How can clinical technique influence elastic recovery of an impression?
If load time is less (tray is removed quickly) there is less overall permanent strain, better elastic recovery
138
Why should impression trays be removed from the mouth quickly?
To reduce the permanent strain on the impression and achieve better elastic recovery, by reducing the load time while the impression is stretched over the bulbous part of the tooth
139
Viscoelastic behaviour
Occurs when after being stretched or compressed, a material fails to return to its original dimensions/shape and there is permanent deformation
140
What happens to elastomers ability to flow following mixing?
Gradually decreases while the material sets
141
What happens to an elastomers elasticity following mixing of the material?
Starts to increase gradually
142
How is the shark fin test carried out?
Impression material is applied to a cylindrical chamber with slot of specified depth
Slot is shark fin shape
Pressure is applied from above
Longer fin length = high flow, better for recording undercuts
143
Tear strength
Stress material will withstand before fracturing
144
Rigidity
Stress/strain rate
Low rigidity - flexible - ideal for impression materials as removal can cause strain and tears, especially from undercut regions
145
Dimensional stability of impression materials
Setting shrinkage should be low to maintain shape of the dentition
Thermal expansion coefficient should be low to reduce the change in shape when the impression material is taken through the large temperature shift from the mouth to room temp
Storage - some materials absorb/release moisture, causing a change in dimensions
146
Which type of elastomers tend to have higher setting time and working time?
Addition silicones
147
How to choose the appropriate material checklist
Know KEY material properties
Review product specification data
Know typical values expected for specific properties
Identify properties NOT mentioned
REJECT claims not supported with evidence
148
Temporary materials
PMMA
PEMA
Bis-acryl composite
UDMA
149
Methacrylate monomer
CH3
I
C = CH2
I
COOCH3
150
Polymethylmethacrylate - JET temporary material
Powder/liquid formation
Self curing
Good marginal fit
Good transverse fit
Polishable
BUT - poor abrasion resistant, high shrinkage, high thermal release and FREE MONOMER TOXIC
151
What type of monomer does PMMA have?
Mono-functional - has only one C=C bond, forms linear chain like polymer
152
Brand names for PEMA temporary materials
Trim II
Snap
153
Bis GMA monomer type
Bifunction - 2 C=C bonds - forms 3D polymer network
154
Brand names of Bisacrylate composite temporary materials
Protemp4
Quicktemp
155
Brand name of urethane dimethacrylate temporary material
Provipont DC
156
How are temporary restoration materials sold?
Powder and liquid
OR
Two pastes in cartridge system
157
Why is product literature not always the best source of information when choosing a material?
Can be biased
Not always complete or accurate data
158
What is a reliable source of information when choosing a dental material?
Journals
159
Examples of product literature
Brochures
Web sites
Product profile
160
Findings of Zach and Cohen study of temperature increase impact on the pulp in 1965
2C increase - no effect on pulp histology
5.5C increase - significant tissue changes over first few days, two months for recovery of MOST pulps, some smaller teeth were necrotic
11C increase - most suffer irreparable necrosis
161
Two factors of heat stimulus effects on dental pulp
Duration of stimulus
Temp increase
162
What did the Baldissara study from 1997 find, in comparison to the earlier Zach and Cohen study?
It contradicts what Zach and Cohen found that temp increase of 11.5C will cause pulp tissue damage
163
Baldissara experiment
Pair of teeth from same individual
One tooth exposed to thermal stimulus believed to be harmful, monitored for symptoms for 2-3months then extracted and examined histologically for damage to pulp
Other tooth extracted and exposed to same stimulus in order to assess the temp increase induced in tooth
No damage <11C increase
164
Temperature increase shown by a range of temporary materials
3.5-10.5C, 1.5-2 mins taken
All safe
165
What was the worst case involved in the Baldissara experiment?
Change of more than 7C, held for 170 sec, with a peak of over 14C - pulp was unaffected
166
Three important characteristics of temporary materials
Temperature - exothermic reaction, safety of the pulp important
Colour stability - aesthetics
Polymerisation shrinkage - clinical fit
167
Biting forces can reach what force?
700N
168
Luting agents include
Dental cements
Composite resins
Self adhesive composite resins
Surface modifying chemicals
169
Properties of a luting agent
Viscosity and film thickness
Ease of use
Radiopaque
Marginal seal
Aesthetics
Solubility
Cariostatic
Biocompatible
Suitable mechanical properties
170
Viscosity and film thickness of luting agents
Dependent on the size of powder or filler particles in the material
Must be low to allow seating of the restoration without interference
Viscosity increases as material sets - must seat rest. quickly and maintain pressure
Film thickness should be as thin as possible ideally 25 micrometers or less
171
Ease of use of luting agents
Easy to mix - many products encapsulated, clicker system
Working time should be long to allow for seating of restoration
Setting time to be short
172
Radiopaque - luting agents
Some ceramic crowns are radiolucent - makes it easier to see marginal breakdown
173
Marginal seal of luting agents
Ideally should bond chemically to the tooth and the indirect restoration with a permanent impenetrable bond
174
Aesthetics of luting agents
Ideally tooth coloured - variation in shade and translucency
Non staining
175
Solubility of luting agents
LOW
176
Cariostatic - luting agents
Fluoride releasing
Antibacterial
Important in prevent secondary caries around crown margins
177
Biocompatibility of luting agents
Non toxic
Not damaging to pulp - pH or heat on setting
Low thermal conductivity
178
Ideal mechanical properties of luting agents
High compressive strength (dentine around 275 MPa)
High tensile strength (Dentine around 50MPa)
High hardness (dentine around 70K enamel around 400K)
Young's modulus similar to tooth (dentine around 15GPa)
Currently no luting agent gets close to tooth values for more than one or two properties
179
Types of material used as luting agents
Dental cements - zinc phosphate, zinc polycarboxylate
GIC - conventional and RM
Composite resin - total etch for use with DBA, self etch, requires etch but has own bonding agent incorporated
180
Zinc phosphate as luting agent
Acid base reaction
Powder (ZO90% MgO2 10% to change colour) and liquid (phosphoric acid 50%, oxides to buffer and slow down setting)
Excellent clinical service
Easy to use
Cheap
ZnO + 2H3PO4 -> Zn(H2PO4)2 +H2O followed by hydration reaction resulting in formation of crystalised phosphate matrix Zn3(H2PO4)2.4H2O
181
Problems with zinc phosphate cement
Initial pH 2
Exothermic setting reaction
Not adhesive to tooth or restoration - just fills gap
Not cariostatic
Final set takes up to 24h
Brittle
Opaque
182
Zinc polycarboxylate cement as luting agent
Similar but polyacrylic acid instead of phosphoric acid
This material has advantage of bonding to tooth
Less heat of reaction
pH low to begin with but returns to neutral more quickly AND longer chain acids which do not penetrate dentine as easily
Cheap
183
Disadvantages of zinc polycarboxylate luting agent
Difficult to mix and manipulate
Soluble in oral environment at lower pH
Opaque
Lower modulus and compressive strength than zinc phosphate
184
In what circumstances are dental cements used today?
Rarely
If trying to cement a temporary indirect such as onlay, without many retentive surfaces, use zinc phosphate as it will stay much better than with temporary cement, and it does not stick to tooth like other permanent cements
185
What type of luting agents are used most?
Glass ionomer cement
186
What is the main difference between GI for restorations and GI as a luting agent?
Much smaller glass particle size in the luting agent which is less than 20 micrometres to allow for suitable film thickness
187
Reaction for glass ionomer luting agent
Acid base between glass (SiO2,Al2O3, CaF2) and acid (poly acid mix of acrylic, maleic and itaconic acid)
Reaction goes through dissolution, gelation and hardening stages
188
Glass ionomer cement bonds
Ion exchange with calcium
Hydrogen bonding with collagen
Strong durable and possibly dynamic bond to tooth
No chemical bond to restoration surface - surface of the restoration should be sandblasted to allow mechanical adhesion
189
Benefits of GIC as luting agents
Low shrinkage
Easy to use
Long term stability
Relatively insoluble once fully set
Self adhesive to tooth surface
Fluoride release
Cheap
190
RMGI cements
Chemistry same as RMGI filling material however glass particles smaller to allow acceptable film thickness
In addition to conventional GIC powder and liquid, the liquid contains hydrophilic monomer - HEMA
Same acid base reaction occurs
Light activation causes polymerisation of HEMA - rapid initial set, dark cure by REDOX reaction
Acid base reaction continues for some time
191
Benefits of resin modified GIC luting agents
Shorter setting time
Longer working time
Higher compressive and tensile strengths
Higher bond strength to tooth
Decreased solubility
192
Problems with RMGIC that do not affect conventional GIC luting agents
HEMA - cytotoxic
HEMA swells/expands in wet environment
No bond to indirect restoration
193
Where can resin modified glass ionomer cements not be used, and why?
Cement posts in as may split the root
To cement porcelain crowns as they may crack
both due to swelling when HEMA expands in wet environment
194
Composite luting agents
Variants on composite filling materials with suitable viscosity and filler particle size
Must be used in conjunction with dental bonding agents
Can be light cured or dual cured
Have better physical properties, lower solubility and better aesthetics
Technique sensitive
Moisture control important
Physical properties reduced 25% if not light cured
195
Bonding to indirect composite
Composite bonds to composite
Bond strength is lower to inlay fitting surface than to new composite
Bond is micromechanical to rough internal surface of inlay
Bond is also chemical to remaining C=C bond on the fitting surface of the inlay
Use a dual curing cement as light penetration through the inlay will be poor
196
What is the role of DBA between tooth and composite luting resin?
Hydrophilic end bonds to tooth and hydrophobic end bonds to composite luting resin
197
Bonding to porcelain
Porcelain is brittle and requires to be bonded to tooth to prevent fracture
Untreated porcelain is smooth and non retentive
It can be treated with HF to etch surface but toxic
Produces rough retentive surface but it is still not hydrophobic and compatible with composite resin luting agents there for a surface wetting agent is required
198
Silane coupling agent
Hydrophobic and hydrophilic ends
Applied to etched porcelain - very strong bond between oxide groups
Other end has C=C bonds which react with composite resin luting agent
Works in the same manner as dentine bonding agent
Strong durable bond
199
Why do composite resin luting agents not bond directly to tooth?
Tooth - wet/hydrophilic
Composite - oily/hydrophobic
200
Bonding to metal
Like porcelain, composite materials do not bond directly to metal
Metal surface needs roughened - done by etching or more often sandblasting
Chemical bonding required to strengthen the bond
MDP and 4META used, acidic end bonds to metal oxides on surface, C=C bond to composite
201
Why must metal bonding agents be dual cure?
Light will not penetrate metal
202
Limitations of metal bonding agents
Technique sensitive
Moisture control essential
Will not bond to precious metals
203
Bonding to precious metal
Very complex
Must change precious alloy composition to allow oxide formation
204
Self adhesive composite resin
Simplifies bonding process
Contains MDP for bonding to metal surfaces
Anaerobic self cured material
Consistent results over many years
Good film thickness
Opaque
Moisture sensitive
Expensive
Mostly used to retain bridgework
205
Self etching composite resin cements
New material
Composite cement with self etching dentine bonding agent built in
Requires good moisture control
Acidic groups bind with calcium in hydroxyapatite - stabilising attachment
Ions from dissolution of filler neutralise the remaining acidic groups - chelate reinforced methacrylate network
Limited removal of smear layer
Good bond strength to dentine
Good mechanical properties
206
Self etching composite resin cements bonds to different materials
Enamel - lower than to dentine, should be acid etched first
Dentine - should not be etched first
Ceramics - brand specific
Metal - better to non precious, not good enough to cement ortho brackets
207
Uses for GIC cement
MCC
Metal post
Zirconia crown
Gold restoration
(NOT fibre post, veneer, adhesive bridge, composite inlay, porcelain inlay)
208
Uses for RMGIC luting agent
MCC
Zirconia crown
Gold rest
209
Uses for Light cure composite + DBA as luting agent
Veneer
210
Uses for dual cure composite + DBA luting agent
Fibre post
Zirconia crown
Composite inlay
Porcelain inlay
211
Uses for anaerobic cure composite luting agent
Adhesive bridge
Zirconia crown
212
Uses for self adhesive composite luting agent
Fibre post
Zirconia crown
Composite inlay
Porcelain inlay
213
Suitable luting agents for MCC
GIC
RMGIC
214
Suitable luting agents for metal post
GIC
215
Suitable luting agents for fibre post
Dual cure comp + DBA
Self adhesive comp
216
Suitable luting agents for veneers
Light cure comp + DBA
217
Suitable luting agents for adhesive bridge
Anaerobic cure comp
218
Suitable luting agents for zirconia crown
GIC
RMGIC
Dual cure comp +DBA
Anaerobic cure comp
Self adhesive comp
219
Suitable luting agents for composite or porcelain inlay
Dual cure comp + DBA
Self adhesive comp
220
Suitable luting agents for gold restoration
GIC
RMGIC
221
Temporary cements
Made to cement temporary rests in place while permanent is fabricated
Soft for easy removal - some don't set at all
Prep must be physically retentive or they don't work
Can be used for trial lute of permanent rest to allow assessment by pt or clinician
Two pastes base and catalyst/accelerator
Base - ZnO, starch and mineral oil
Accelerator - resins, eugenol or ortho EBA and carnauba wax
Wax weakens structure of set cement making it easier to remove
Incorporate petroleum jelly to weaken further
222
When can eugenol containing temporary cements not be used and why?
When the definitive is to be cemented with a resin cement
Any residual eugenol may interfere with setting
223
Endo material categories
Instruments
Irrigants
Intra-canal medicaments
Obturation materials
Sealers
Pulp capping materials
Root end filling materials
224
Purpose of endodontic instruments
Used in mechanical phase of chemomechanical disinfection
Help create space for introduction of irrigants, medicaments and for obturation
Metal files are used to remove soft and hard tissues, as well as micro-organisms
Provides appropriate shape for obturation
225
Strain formula
Change in length/length
226
Stress
Deforming force measured across a given area F/A
227
Stress concentration point - endo files
Abrupt changes in the geometric shape of a fil that leads to a higher stress at that point
228
Strain
Response of a material to stress, amount of deformation undergone
229
Elastic limit - endo files
Set value representing the max strain that when applied to a file, allows the file to return to original dimensions
230
Elastic deformation
Reversible deformation that does not exceed the elastic limit
231
Plastic deformation
Permanent bond displacement when elastic limit exceeded
232
Prevention of endodontic instrument failure
Manual glide path
Straight line access
NiTi files should be used in constant motion with gentle pressure
Avoid use of rotary files in abruptly curved or dilacerated canals
233
Ideal root filling (materials)
Various endo materials used
Generally, core material and a sealer
Always use sealer to create a fluid-tight seal
Biologically acceptable chemical and mechanical treatment of the root canal to promote healing and repair of periradicular tissues
234
Modern alternative to bioceramic cement
Biodentine - similar material with modifications allowing much more rapid setting, good for pulp capping
Reduced discolouration
235
Classifications of endo instruments
Manually operated
Low speed
Engine driven NiTi rotary instruments
Engine driven instruments that adapt to canal shape
Engine driven reciprocating instruments
Ultrasonic instruments
236
Stainless steel hand instruments - endo
Alloy or iron, carbon, chromium and nickel
13-26% chromium - anti rust
Machined stainless steel wire cut into square or triangular cross sections, twisted together creating work hardening, modifying properties
Fluted pattern
237
Work hardening
Strengthening of a material by plastic deformation
Crystal structure dislocated, becomes less regular, leading to less easily dislocated structure as no easy sliding of lattice over itself
238
Shape memory
Alloys can be deformed at one temp but when heated or cooled, return to their original shape
239
Nitinol
Equiatomic alloy of nickel and titanium
Does not display normal metal characteristics
Super elasticity
Crystal structure in either austenite or martensite structure depending on temp
Proportions determine the mechanical properties
240
Austenite vs martensite
Martensite - soft, ductile, easily deformed
Austenite - strong, hard
241
Components of endo rotary instrument
Taper - diameter change along working surface
Flute - groove to collect dentine and soft tissue
Leading/cutting edge - forms and deflects dentine chips
Land - surface between flutes
Relief - reduction in surface of land
Helix angle - angle cutting axis forms with long axis of file
242
Purpose of lands and reliefs on endo rotary instrument
Modify the friction created between instrument and dentine, also affecting the force applied to instruments
243
What are irrigants used for in endo treatment?
To help remove debris, lubricate and act as anti-microbials by dissolving organic tissue in the root canal
Also penetrate canal periphery and kill bacteria/yeast/viruses
Cause biofilm disruption creating biological compatibility in the canal, without weakening tooth
244
Primary irrigant used
NaOCl sodium hypochlorite
245
NaOCl as an irrigant
Ionises in water into Na+ and the hypochlorite ion OCl- Establishes an equilibrium with hypochlorous acid (HOCl)
In acid or neutral HOCl predominates
pH 9 + OCl- predominates
HOCl responsible for antibacterial activity
NaOCl effects organic material but is not able to remove smear layer by itself
Potentially has effects on the dentine by reacting with organic component
246
Concentration of NaOCl as irrigant
3%
247
What must be removed from root canals before obturation?
Smear layer - would cause bacterial contamination and interfere with disinfection, as well as preventing penetration with sealer
248
Smear layer
Organic pulpal material and inorganic dentinal debris layer of 1-5 micrometers, with packing into tubules
249
What is used to remove the smear layer from root canals?
17% EDTA (or 10% citric acid) with sonic or ultrasonic irrigation - careful with apical control
Do not mix irrigants, make sure NaOCl is fully removed with sterile water rinse before
250
Obturation materials ideal properties
Easily manipulated with ample working time
Dimensionally by tissue fluids stable
Seals canals 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
251
Most commmon obturation material
Gutta percha
252
Monomer in GP
Isoprene
CH3
I
C = CH2
I
C = CH2
I
H
253
Two crystalline forms of GP
Alpha - naturally occurring, heated above 65C it melts into amorphous phase and cooled slowly returns to alpha, cooled rapidly it recrystallises as beta phase
Beta phase is used in commercially produced GP
254
GP cones constituents
20% GP
65% ZO
10% radiopacifiers
5% plasticisers
255
Sealers - endo
Fill the void between dentinal wall and core material
Also fills irregularities in the canal, lateral canals and between GP points used in lateral condensation
Lubricates during obturation
256
Ideal sealer properties - endo
Tackiness for good adhesion
Hermetic seal
Radiopacity
Easily mixed
No shrinkage on setting
Non staining
Bacteriostatic or doesn't encourage growth
Slow set
Insoluble in tissue fluids
Tissue tolerant
Soluble on retreatment
257
Most common endo sealer
Resin sealers
258
Radiopacity of ZOE compared to GP
ZOE less radiopaque
259
What is added to ZOE to improve dentine adhesion?
Rosin or Canada balsam
260
Setting process of ZOE sealer
Chemical process combined with physical embedding of ZO in eugenolate matrix
Eugenolate formation constitutes hardening
CaOH accelerates this so must be fully removed
261
Advantage of GIC sealers in endo treatment
Bond to dentine
262
Disadvantages of GIC sealers in endo treatment
Removal upon retreatment difficult, minimal antimicrobial activity
263
Main advantage of ZO sealers
Effective antimicrobial
264
How are resin sealers sold?
As paste-paste mix
265
Advantages of resin sealers
Good seal
Good flow
Slow set
266
Disadvantages of resin sealers
Initial toxicity - declining after 24 hours
267
Why is it good to have a hydrophilic material as endo sealer?
Good penetration into dentinal tubules
268
Calcium silicate sealers
High 12.8 pH during initial 24 hours
Hydophilic, enhanced biocompatible, no shrink on setting, non resorbable, easy to use, excellent seal, quick set
269
Options for endo sealers
Resin sealers - mainly used
ZO sealers - long lasting antimicrobial, but toxic
GI sealers - dentine bonding good but other undesirable traits
Medicated sealers - not favoured
Calcium silicate sealers - many advantages
270
Uses for pulp capping materials
Pulp exposure due to trauma or removal of deep caries
Pulpotomy and pulp regeneration
Root resorption repair
Apexification
Lateral perforation repair
Apicoectomy
271
Characteristics of white vs grey MTA
Grey - Better setting characteristics but cause tooth discolouration
White - smaller particle size and reduced colouration
272
Constituents of grey MTAs
Tricalcium silicate (both)
Dicalcium silicate (both)
Tricalcium aluminate
Tetracalcium aluminoferrite
Bismuth oxide (both)
273
Constituents of white MTAs
Tricalcium silicate (both)
Dicalcium silicate (both)
Calcium aluminate
Bismuth oxide (both)
Calcium sulphate dehydrated
274
Setting reaction of MTAs
Hydraulic cements composed of several phases
When mixed with water, chemical hydration reaction takes place
Different setting reactions for white and grey MTA
275
Porcelain vs ceramic
Porcelain is a type of ceramic
All porcelain is ceramic but all ceramic is not porcelain
276
What must be removed from ceramics for dental uses?
Kaolin - it is opaque and dental ceramics must be translucent
277
What component in dental ceramics is not in decorative ceramics?
Glass (up to 15%)
278
Constituents of dental ceramics
Kaolin <5%
Quartz (silica) 12-25%
Feldspar 70-80%
Metal oxides 1%
Glass up to 15%
279
Feldspar
Potash feldspar - potassium alumina silicate
Soda feldspar - sodium alumina silicate
Acts as a flux - lowers the fusion and softening temp of the glass
It is the lowest fusing component and flows during firing, forming a solid mass around the other components
280
Metal oxide colours conveyed to ceramic
Chromium - green
Cobalt - blue
Copper - green
Iron - brown
Manganese - lavender
Nickel - brown
281
How are conventional dental ceramics supplied?
As a powder
Powder is made by heating the constituents to >1000C, then cooling rapidly (fritting) in water creating cracks
Mill the frit to fine powder
Add binder - often starch
Powder is mixed with distilled water and built up into restoration
282
What do feldspathic ceramics for when heated to 1150-1500 in crown fabrication?
Leucite - potassium aluminium silicate around the glass phase of the ceramic
Gives a powder
Powder melts together to form the crown
283
Properties of dental ceramics
Good aesthetics
Chemical stability
Biocompatibility
Thermal properties similar to tooth substance and low thermal diffusivity
Dimensional stability - shrinkage is an issue during firing but once fired very stable
Mechanical properties - high compressive strength and hardness but low tensile, flexural and fracture strength leading to fracture during loading
284
Which dental material has best aesthetics?
Ceramics - stable colour and very smooth surface, less staining in the long term
285
Rule of thumb for choosing a luting agent
If it won't break when thrown at a wall use GIC
286
What is the effect of mechanical properties on use of ceramics?
Can only be used in low stress areas
287
Alumina core
Used in ceramic crowns to improve mechanical properties - not strong enough for ruse in posterior crowns - lack of flexural strength
288
Most popular material for a ceramic crown core
Zirconia
289
What type of zirconia is used in dentistry?
Ytria stabilised (3-5% Ytria)
290
Problems with zirconia cored crowns
Expensive equipment required
Potential for veneered porcelain to debond
Zirconia core is opaque - doesn't improve aesthetics much from MCC
Inert fitting surface - cannot etch or bond
291
Fabrication of Milled Crown
CAD/CAM technology used
Scanned image of cast
Lower cast scanned and articulated
Select and adjust crown margin
Select crown type and place on virtual model
Adjust shape and size
Save file and send to milling machine 30-40 to produce crown
Still requires final finishing
292
Cast and pressed ceramics
Restoration is waxed up
Invested
Cast from a heated block of ceramic 1100C
Devested
Heated to form crack inhibiting crystals - ceraming
293
Is the sintered or milled version of the same material stronger?
Milled
294
What crown should be used for posterior teeth?
Monolithic zirconia
295
Which crown for anterior use?
LiDiSi (Lithium disilicate)
296
What should be used for longer bridgework or heavily occluding crowns?
Zirconia cored with zirconia where occlusal contacts meet
297
What should be used to lute crowns?
Zirconia or LiDiSi - conventional or resin cements
298
Sintering
Occurs on heating of ceramic crowns - ceramic particles begin to fuse into a single mass, glass phase softens and comes together
Over time there is controlled diffusion and a solid ceramic mass is formed
During sintering material contracts about 20%
