1
Q
function of dental composites (2)
A
- restorative (filling) materials
- special tray construction
2
Q
wide scope of dental materials/ how their use can differ from engineering applications
A
- light curing technology used in car repair fillers/paints as well as dental occlusins (ICI dental occlusion)
- fibre reinforced materials/kevlar (v strong) used in bullet proof vests/fluoroelastomer rubber, may be incorporated into dental treatments
- > unlike civil engineering, restoration must function within minutes of placement
3
Q
what triggers the polymerisation of light curing materials
A
camphorquinone (photo-initiator)
4
Q
dental composite definition
A
combination of materials in which indiv. components retain their identity and posses diff properties to the components acting alone
5
Q
components of dental resin composite (3)
A
- resin
- filler
- most importantly, a coupling agent
6
Q
what are fibre reinforced materials
A
type of composite consisting of:
- fibres of high strength
- a modulus
- > embedded/bonded to matrix
- > distinct interface/boundary between components
7
Q
legal requirements relating to the prescription of laboratory made dental appliances (5)
A
- patients name/confirmation made for them
- prescribers name/registered address
- technicians name/registered address
- description of appliance
- confirmation meets legal standards
8
Q
what are dental materials susceptible to within oral environment (5)
A
- loading up to 6kg
- cyclical forces causing fatigue
- chemicals
- temp fluctuations (-14 to 60 degrees)
- pH fluctuations (2-12)
9
Q
what is the shock absorber teeth are suspended within
A
periodontal ligament
10
Q
eg’s of dental materials (5)
A
- gypsum materials (plaster and stone)
- waxes
- alloys
- polymers
- ceramics
11
Q
what standards do dental materials have to make
A
- standards to protect the user and patient
- met by BSI (british standards institution) and ISO (international standards organisation)
- > lab tests are used to ensure materials meet these standards, inappropriate selection/application compromises durability/ risks safety of both patients/members of dental team
12
Q
strain calculation
A
=change in length divided by original length (has no units)
13
Q
stress equation
A
=force (newtons) divided by area (mm squared)
-units of stress = MPa
14
Q
mechanical properties of materials that are determined by lab tests (and how they are calculated) (7)
A
- tensile strength (elongation, force over area)
- compressive strength (not for brittle objects, force over area)
- diametral tensile strength (looks like compressive, to elongate spheres etc/brittle objects, placed between two supporting blocks, stress equation = 2F/3.14DL)
- flexural strength (3 point, stress=3FL/2b(d squared), where L is support separation)
- surface hardness (indented, dimensions measured and expressed as a number, high no’s = low hardness)
- shear bond strength (strength to separate bond between two structures, force div. by area)
- impact strength (in units of energy)
15
Q
how to measure stiffness on stress/strain curve
A
linear region of curve
16
Q
how to measure ductility on stress/strain curve
A
straight line from end point to…?
->potential for material to be drawn out into a wire
17
Q
what is given by measuring the area under stress/strain curve of the linear elastic region
A
resilience (energy absorbed by material in undergoing plastic deformation up to elastic limit)
18
Q
what is given by measuring the area under the entire stress/strain curve
A
- toughness (total amount of energy a material can absorb up until the point of fracture)
- > high value = tough
- > low value = brittle
19
Q
physical properties of materials determined by lab tests (4)
A
1-surface roughness (the higher the Ra the rougher the material)
2-thermal properties (conductivity/K - rate of heat transfer through material, energy required two heat up material/Q=mst, thermal diffusivity/D - rate temp will rise within material when heat is applied to its surface = K/Cp*p, -> Cp = spec. heat and p = material density)
3-rheological properties (material flow properties through syringe etc, viscosity is proportional to pressure/speed -> viscosity can increase (dilatant), decrease (pseudoplastic) or stay the same (newtonian) due to applied pressure)
4-thermal expansion and contraction (match as closely to tooth surf. being replaced)
20
Q
properties of materials that can be determined by lab tests (4)
A
- mechanical
- physical
- chemical
- biological
21
Q
chemical properties of materials that can be defined by lab tests
A
- corrosion (defines chemical reactivity of metals/alloys, should display good corrosion resistance)
- solubility
22
Q
ideal biological properties of materials used in dentistry (5)
A
- non toxic
- non irritant
- harmless to dental pulp
- no carcinogenic potential
- no irritancy
23
Q
ideal properties of model/die materials (5)
A
- dimensional accuracy
- fluid when poured
- good wetting
- once set, hard and also fracture resistant
- compatible with all materials it contacts
24
Q
classify gypsum products according to ISO standards (5)
A
1-impression plaster
2-dental plaster for models
3-dental stone for die/model
4-dental stone, die high strength, low expansion
5-dental stone, die high strength, high expansion
25
gypsum products used for models/dies (3)
- dental plaster (plaster of paris)
- dental stone
- densite
26
gypsum equation (manufacturing of gypsum products)
- gypsum -> gypsum product + water
- 2CaSO4*2H2O -> (CaSO4)2*H20 + 3H20
- calcium sulphate dihydrate -> calcium sulphate hemihydrate (alpha/beta)
27
chemical name of gypsum
calcium sulphate dihydrate
28
chemical name of gypsum products (2)
- beta calcium sulphate hemihydrate (dental plaster)
| - alpha calcium sulphate hemihydrate (dental stone)
29
conditions which favour formation of dental plaster from gypsum
120 degrees
30
conditions which favour formation of dental stone from gypsum
steam pressure in autoclave to 125 degrees
31
conditions which favour formation of densite from gypsum
boil in CaCl2
32
how do porosity and particle size differ between the 3 gypsum products
- in order of low to high:
| - dental plaster -> dental stone -> densite
33
reaction which occurs during setting of gypsum products to gypsum
(CaSO4)2*H20 + 3H20 -> 2CaSO4*2H20
34
how much water would you mix with 100g plaster for plaster and stone
-plaster = 50-60ml
-stone = 20-35ml
(uptake of water is accounted for by porosity)
35
why is it bad for air to be incorporated into mix whilst mixing (2)
leads to:
- >loss of detail
- >weakness
36
what are the 3 stages of setting of gypsum products
1-water becomes saturated with hemihydrate (0.8%solubility)
2-conversion of hemihydrate to dihydrate (0.2%solubility)
3-dihydrate crystallises out initiated by exposure to nuclei of crystallisation
37
what happens to the conditions of the gypsum product during setting (3)
- increase in temp (exothermic, up to 30 degrees)
- increase in viscosity (gilmore needles determine initial and intermediate set, not final)
- expansion (outward thrust of crystals, 0.15-30%)
38
what is used to control setting time of gypsum products (3)
- increasing concentration of nucleating agents can speed up
- accelerators (K2SO4) increase solubility of hemihydrate to speed up setting time
- retarders (borax) increase setting time by decreasing the rate of reaction
39
what is used to control expansion of gypsum products upon setting
accelerators and retarders
40
what can the operator/dentist do to control setting time of gypsum products
- temp (accelerates solution process and retards crystallisation)
- W/P ratio(an increase retards setting as decreases conc. of crystallisation nuclei)
- mixing time (an increase, increases set rate)
41
how are you able to tell the diff between gypsum products
- add dyes :
- > plaster = white
- > stone = yellow
- > die stone = blue/pink
42
properties of gypsum products once set
- stone is stronger than plaster as less porosity
- strength increases with time (progressive evaporation of water)
- brittle
- low flexural strength
- good dimensional stability
- good detail reproduction (type 3,4,5 are the best)
- soluble in water (surface can soften in lab processes causing issues)
43
advantages and disadvantages of gypsum products
```
advantages:
->inexpensive
->easy to use
disadvantages (both rare):
->may interact with borax in alginate
->hypochlorite may interact if not rinsed off impression before casting
```
44
requirements of impression material (8)
```
1-surface detail reproduction
2-dimensional accuracy and stability
3-no changes when exposed to disinfectants
4-no interaction with model/die materials
5-non toxic
6-easily manipulated
7-palatable taste
8-realistic working/setting time
```
45
what factors affect the surface detail reproduction of an impression material (2)
- >viscosity (high viscosity has less detail but can displace soft tissues whereas low viscosity allows more detail)
- >hydrophobicity (moist surfaces repel)
46
what factors affect the dimensional accuracy and stability of an impression material (3)
- >type of tray (flexible tray distorts, must have effective bonding adhesive)
- >flexibility and recovery (removal from undercuts)
- >storage prior to casting (no dimensional changes)
47
what are the 2 classes of impression materials
- non elastic
- elastic
- synthetic elastomers
48
eg's of non elastic impression materials (3)
- impression plaster
- impression compound
- zinc oxide and eugenol pastes
49
types of elastic impression materials (2) including examples
- hydrocolloids:
- >reversible (agar)
- >irreversible (alginates)
- synthetic elastomers:
- >polysulphides
- >silicone rubbers (condensation curing or addition curing)
- >polyether's
50
what are hydrocolloids
colloidal suspensions of polysaccharides in water
51
what states can hydrocolloids exist in (2)
- sol (fluid, low viscosity, random chain arrangement)
- gel (increased viscosity, parallel chain arrangement, elastic properties)
- temp/ chemical convert sol -> gel
- temp converts gel -> sol
52
what is agar
- a reversible hydrocolloid (elastic impression material)
| - complex polysaccharide from seaweed
53
what is agar supplied in a gel along with (2)
- borax (to give more body)
| - potassium sulphate (to counteract the retardant effect of borax upon dental plaster/gypsum)
54
state of agar in different temperatures (3)
- 100 degrees favours gel -> sol
- 65 degrees -> holding temperature
- 45 degrees -> clinical usage temperature (clinical use requires water cooled trays)
55
advantages and disadvantages of agar (reversible hydrocolloid elastic impression material)
- advantages:
- >takes up moisture
- >re-use
- disadvantages:
- >special equipment
- >cost
- >cross infection concerns of re-use
- >low tear strength
- >dimensionally unstable due to water content (syrnerisis = loss of water from squeezed chains and imbibition = uptake of water causing swelling due to chains separation)
56
functions of agar (2)
- clinical use (partial dentures, crowns, bridges)
| - lab use (duplicating material)
57
how are alginates (irreversible hydrocolloids) supplied
as a powder that is mixed with water
58
composition/components of alginate and functions of each component (5)
- Na or K salt of alginic acid (forms sol with water and in set becomes cross linked gel)
- CaSO4 2H20 (source of calcium for cross linking)
- Na3PO4 (controls working time)
- inert filler (provides body for manipulation)
- reaction indicator in some to signify set
59
mechanism of rate retardant (slowing) for alginate setting
- sodium phosphate limits availability of calcium ions
| - 2Na3PO4 + 3Ca -> 6Na + Ca3(PO4)2
60
properties of alginate (irreversible hydrocolloid)
- low viscosity (mucostatic)
- adequate working time (operator may control with water temperature)
- rapid set (sets first by tissue, avoid movie tray)
- elastic
- must use no less than 3-5mm thick to prevent distortion on removal
- susceptible to syneresis (loss of water) and imbibition (uptake of water) therefore you must pour soon
61
how are alginates modified
-incorporate silicone polymers (supplied as two pastes, this offers marginally better detail reproduction/tear resistance however has poor dimensional stability)
62
how do agar/alginates differ in properties (4)
- alginate has higher compressive strength
- alginate has a quicker deformation recovery
- alginate has greater detail reproduction
- agar has greater tear resistance
63
why are impression materials disinfected
- to prevent transmission of blood borne viruses to patients, postal workers and members of the dental team
- BDA advice sheet A12 states that this responsibility lies solely with the dentist and that it is good practice to agree this process with the lab to prevent repeated disinfection occurring
- >rinsing/brushing away debris is essential to maximise contact with disinfection agent
64
steps involved in disinfecting impression material
- rinse
| - timed immersion in fresh solution (for 2 mins)
65
alternatives to dental impressions (2)
- CAD CAM
| - intraoral scanner
66
advantages of elastomeric impression materials
- offer potential solutions to main hydrocolloid problems (poor tear resistance and poor dimensional stability)
67
4 types of elastomeric impression materials
- polysulphides
- silicone rubbers (condensation curing)
- silicone rubbers (addition curing)
- polyether
68
what does the ISO 4823 classify elastomeric impression materials according to (2)
- chemical nature
| - consistency (dictates clinical application)
69
types of elastomeric impression materials, their consistency and nature (4)
- type 0 = very high consistency, putty like
- type 1 = high consistency, heavy bodied
- type 2 = medium consistency, medium bodied
- type 3 = low consistency, light bodied
70
properties of elastomeric impression materials of diff consistencies (high and low)
- putty (high consistency):
- >high dimensional stability
- >tissue displacing
- >poor fine detail capture
- low bodied (low consistency):
- >low dimensional stability
- >limited tissue displacement
- >fine detail capture
->clinical techniques combine viscosities to maximise applications
71
application of polysulphides (elastomeric impression material)
- base paste (white)
| - catalyst paste (brown)
72
components of the base paste of polysulphides (3)
- polysulphide prepolymer (with SH terminal groups)
- plasticiser (di-n-butyl phthalate)
- inert filler (chalk/TiO2)
73
components of polysulphide catalyst paste (3)
- PbO2
- sulphur
- inert oil (paraffinic oil/di-n-butyly phthalate)
74
what reaction occurs during setting of polysulphide (elastomeric impression material)
condensation polymerisation
75
what happens to the properties of polysulphides as cross linking occurs (2)
- increased viscosity (gradual)
| - increased elasticity (slow)
76
properties of polysulphides (elastomeric impression material) (7)
- viscosity & elasticity increase following mixing
- high tear resistance but slow incomplete elastic recovery (sharp pull removal required)
- heavy bodied best dimensional stability
- reaction continues with water loss
- concerns of the use of lead
- bad odour and taste
- impression easily read
77
what are the 2 compositions of condensation cured silicone rubbers that can be used (elastomeric impression material)
- either 2 pastes
- or
- >light, regular, heavy bodied and puty consistencies are available
- paste 1 = hydroxyl terminated polydimethylsiloxane chains/liquid silicone prepolymer and inert fillers such as silica
- paste 2/liquid = silicate, e.g. tetraethyl silicate and tin compound such as dibutyl tin dilaurate
78
what occurs during the setting reaction of condensation cured silicone rubbers (elastomeric impression material) (inc. cross linking agent, catalyst and by product)
- cross linking of hydroxyl terminated polydimethylsiloxane chains (cross linking increases viscosity and elastic properties)
- >alkyl silicate is the cross linking agent
- >tin compound is the catalyst
- >by product = C2H5OH
79
properties of condensation cured silicone rubbers (8)
- set time less than polysulphides
- elasticity develops more rapidly than polysulphides
- hydrophobic
- adequate tear resistance
- good elastic recovery
- post setting dimensional changes (therefore must pour soon)
- handle liquid components with respect
- considered non toxic in intraoral application
80
available forms/compositions of addition curing silicone rubbers (4)
- paste 1:
- >filler
- >white liquid silicone prepolymer and in some methyl groups replaced with vinyl groups
- >catalyst eg. chlorplatinic acid
- paste 2:
- >filler
- >liquid silicone prepolymer
- >polydimethylsiloxane prepolymer with some methyl groups replaced by hydrogen
- available also in auto mix format (cartridge/pentamix)
- viscosities = putty, regular, light
81
addition cured silicone rubbers setting reaction
- platinum catalysed addition reaction
- >no byproducts
- >potential hydrogen gas evolved therefore delay casting (poss. due to reaction of platinum catalyst with moisture)
82
properties of addition cured silicone rubbers
- same as condensation silicones:
- set time less than polysulphides
- elasticity develops more rapidly than polysulphides
- hydrophobic
- adequate tear resistance
- good elastic recovery
- post setting dimensional changes (therefore must pour soon)
- handle liquid components with respect
- considered non toxic in intraoral application
- >however no by-products
- >hydrophobic though newer formulations incorporate surface active agents to render more hydrophilic
83
available forms/compositions of polyethers (3)
-two pastes:
1-baste paste (in large tube, components=imine terminated polymer and inert filler)
2-catalyst (smaller tube, components = reaction initiator in form of aromatic sulphonic acid ester and paste forming oils and fillers)
->also available in auto mix units
84
setting reaction of polyethers
- by cationic ring opening addition polymerisation
- no by-products
- as setting occurs, viscosity increases resulting in a rigid cross linked rubber
- steps involved:
- >activation
- >initiation
- >propagation
85
properties of polyethers (6)
- similar tear resistance and elastic properties to silicones
- relatively rigid (force to remove, difficult to remove die without, excellent for implant work)
- lack of heavy body/putty limits use
- hydrophilic (issue in humid environments)
- allergy
- must limit immersion to 10 mins in hypochlorite
86
comparison/overview of the 4 elastomeric impression materials:
- viscosity
- tear resistance
- elasticity
- accuracy
- dimensional stability
- viscosity:
- >polysulphides = 3 (no putty)
- >silicone condensation = 4
- >silicone addition = 4
- >polyethers (single regular, however diluent available and putty)
-tear resistance = average for all 4
- elasticity:
- >polysulphides = viscoelastic
- >silicone condensation = very good
- >silicone addition = very good
- >polyethers = average
- accuracy:
- >polysulphides = good with special tray
- >silicone condensation = average with stock
- >silicone addition= average with stock
- >polyethers = good with special tray
- dimensional stability:
- >polysulphides = average but no pour delay
- >silicone condensation = average if poured ASAP
- >silicone addition = very good
- >polyethers = very good if low humidity
87
if the stress applied to a ductile material results in a permanent change in shape, what type of deformation occurs in the material
plastic
88
what is the name given to the stress (on a stress strain curve) at the point where plastic deformation starts
yield point/region
89
which point on a stress/strain curve has been reached when a material fractures through application of stress
maximal tensile stress
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