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1
Impression materials
2
Index
1. Introduction2. Types if impressions3. Impression trays4. Ideal requirements5. Classification 6. Alginate7. Agar8. Polysulfide9. Condensation silicone10. Addition silicone11. Polyether
12. Viscoelastic property13. Manipulation14. Impression technique15. Removal of impression16. Preparation of stone
cast and die17. Dimensional stability18. Elasticity19. Tear strength20. Biocompatibility21. Common causes of
failures22. Comparison of
properties
3
introduction
Impression materials are used to make an accurate replica of the hard and soft oral tissues.
The area involved may vary from a single tooth to the whole dentition, or an impression may be made of an edentulous mouth.
The impression stage is the first of many stages involved in the production of dentures, crowns, bridges, orthodontic appliances etc.
The impression gives a negative reproduction of the tissues, and by filling the impression with dental stone or other model material, a positive cast is made.
4
Types of dental impressions
Click icon to add picturePreliminary impression
1. diagnostic models
2. custom trays
3. provisional coverage
4. orthodontic appliances
5. pretreatment and post‑treatment records
5
Final impression –
• most accurate reproduction of the teeth and surrounding tissues.
• Used to make indirect restorations, partial or full dentures, and implants.
6Bite registration –
• reproduction of the occlusal relationship between the maxillary and mandibular teeth.
• Provide an accurate registration of the patient’s centric relationship between the maxillary and mandibular arches.
7
Impression trays
Characteristics – Must be sufficiently rigid to:
1. Carry the impression material into the oral cavity.2. Hold the material in close proximity to the teeth.3. Avoid breaking during removal.4. Prevent warping of the completed impression.
8
Types of impression trays
Quadrant tray Covers one half of the arch.
Section tray Covers the anterior portion of the arch.
Full arch tray Covers the entire arch.
Perforated tray Holes in the tray create a mechanical lock to hold
the material in place.Smooth tray
Interior of the tray is painted or sprayed with an adhesive to hold the impression material.
9
Quadrant tray Section
tray
Full arch tray
10
11
Criteria for tray selection
Feel comfortable to the patient. Extends slightly beyond the facial surfaces of
the teeth. Extends approximately 2 to 3 mm beyond the
third molar, retromolar, or tuberosity area of the arch.
Sufficiently deep to allow 2 to 3 mm of material between the tray and incisal or occlusal edges of the teeth.
12
The use of custom trays for polyether and addition silicone impressions is not critical since these materials are stiffer and have less polymerization shrinkage than the polysulfide material.
When severe undercuts are present, the use of a custom tray should be avoided. Disposable stock trays are also used to support the putty when the putty-wash technique is used for making impressions.
13
Tray adhesives
VPS adhesives (blue) For polyvinyl siloxane and polyether impression
materials. butyl rubber or styrene acrylonitrile for polysulfide impression material
poly dimethyl siloxane, other silicone, Ethyl silicate
For condensation silicone impression material
14
Impression materials
15
16
Ideal requirements
1. A pleasant odor, taste, and esthetic color2. Absence of toxic or irritant constituents3. Adequate shelf life for requirements of storage and distribution4. Economically commensurate with the results obtained5. Easy to use with the minimum of equipment6. Setting characteristics that meet clinical requirements7. Satisfactory consistency and texture8. Readily wets oral tissues9. Elastic properties with freedom from permanent deformation after strain
17
Ideal requirements
10. Adequate strength so it will not break or tear on removal from the mouth11. Dimensional stability over temperature and humidity ranges normally found in clinical and laboratory procedures for a period long enough to permit the production of a cast or die12. Compatibility with cast and die materials13. Accuracy in clinical use14. Readily disinfected without loss of accuracy15. No release of gas during the setting of the impression or cast and die materials
18
Non-elastic
Elastic
Reversible hydrocolloids
Irreversible hydrocolloids
Condensation polymerized
Addition polymerized
Polysulfide
Condensation polymerized silicone
Addition polymerized silicone
Polyether
Non-aqueous
Aqueous
Thermal
Chemical
WaxThermoplastic compounds
PlasterZinc oxide eugenol paste
19
Alginate
The present alginate hydrocolloid, or irreversible impression material, was developed as a substitute for the agar impression material when its supply became scarce during World War II.
This material is based on a natural substance extracted from certain brown seaweed. The substance is called anhydro-P-d-mannuronic acid or alginic acid.
20
Potassium titanium fluoride
21
Gelation process
A reaction of soluble alginate with calcium sulfate and the formation of an insoluble calcium alginate gel.
Structurally, calcium ions replace the sodium or potassium ions of two adjacent molecules to produce a cross-linked complex or polymer network.
22
Role of retarder
23
Setting time
Fast setting alginate – 1.5 to 3 minutesNormal setting alginate – 3 to 4.5 minutesThis is regulated by the manufacturer by the
amount of retarder addedThe clinician can alter the setting time by the
temperature of water added. NOT BY THE AMOUNT OF ADDED WATER.
A 1 min reduction in setting time occurs for each 10 °C of temperature increase.
24
Temperature (°C)
25
Packaging and dispensing of alginate
26
Manipulation
The measured powder is sifted into premeasured water that has already been poured into a clean rubber bowl.
The powder is incorporated into the water by carefully mixing with a metal or plastic spatula that is sufficiently flexible to adapt well to the wall of the mixing bowl.
A vigorous figure-8 motion is best, with the mix being swiped or stropped against the sides of the rubber-mixing bowl with intermittent rotations (180°) of the spatula to press out air bubbles. All of the powder must be dissolved.
Mixing time - 45 sec to 1 min
27
MECHANICAL MIXER
28
The result should be a smooth, creamy mixture that does not readily drip off the spatula when it is raised from the bowl.
29
Making the impression
Perforated tray is generally used.Plastic tray or metal rim lock tray – alginate
tray adhesiveThickness – at least 3 mmImpression should not be removed from the
mouth at least for 3 minutes after gelation. (compressive strength doubles in the first 4 mins after gelation)
Not left in the mouth for longer than 6 – 7 mins.
30
Permanent deformation
The ANSI/ADA Specification requires that the recovery from deformation be more than 95% when the material is compressed 20% for 5 seconds at the time it would normally be removed from the mouth.
Lower permanent deformation (higher accuracy) occurs
(1) when the percent compression is lower (adequate bulk of the material)
(2) when the impression is under compression a shorter time (snap removal)
(3) when the recovery time is longer, up to about 8 minutes after the release of the load.
31
Flexibility
The ANSI/ADA Specification permits a range of 5% to 20% at a stress of 1000 g/cm2, and most alginates have a typical value of 14%.
32
Strength
Compressive strengths range from 5000 to 9000 g/cm2. (Acc. to ADA should be at least 3750 g/cm2)
Tear strengths vary from 380 to 700 g/cmThe tear strength is a measure of the
force/thickness ratio needed to initiate and continue tearing.
33
Causes of distortion and dimensional changes in alginate impression
syneresis
imbibition
34
Disinfection of the impression
IodophorBleachGluteraldehyde
After the impression is rinsed thoroughly, the disinfectant can be sprayed liberally on the exposed surface. The impression is then wrapped immediately in a disinfectant-soaked paper towel and placed in a sealed plastic bag for 10 min.
Alternate method is immersion in the disinfectant solution (not more than 10 mins)
35
Compatibility with gypsum
Water content of hydrocolloid inhibits setting of gypsum at the surface.
The gelation process of alginate impression produces not only insoluble calcium alginate but also sodium sulfate.
Sodium sulfate – low conc. – accelerator high conc. – retarderIn alginate impression, the amount is high
enough to act as retarder.
36
The pouring of a stone mixture to fill the impression should start from one end of the arch.
somewhat superior stone surfaces may be obtained if the impression is placed in a humidor while the stone hardens.
The filled impression should never be immersed in water while the stone sets.
The stone cast or die should be kept in contact with the impression for a minimum of 30 min, preferably for 60 min.
37
Shelf life
2 major factors affecting shelf life – 1. Storage temperature2. Moisture contaminationAvailable in individually sealed pouches or in
bulk form in cans.It is best not to stock more than 1 year's
supply in the dental office. The material should be stored in a cool, dry
environment.
38
Common causes of failures
• Improper mixing • Prolonged mixing• Excessive gelation• W/P ratio too low
Grainy material
• Inadequate bulk• Premature removal from the mouth• Moisture contamination• Prolonged mixing
Tearing
• Undue gelation preventing flow• Air incorporated during mixing
External bubbles
Irregularly shaped voids
• Moisture or debris on tissues
39
•Inadequate cleaning of impression•Excess water left in impression•Premature removal of the impression•Model left in impression too long•Improper manipulation of stone
Rough chalky stone
model
•impression not poured immediately•Movement of tray during gelation•Premature removal from mouth•Improper removal from mouth
distortion
40
Agar reversible hydrocolloids
The setting of a reversible hydrocolloid, often called gelation, is a solidification process that involves phase changes from sol to gel states.
This changes is brought about by temperature change.
The gel-to-sol and sol-to-gel transformations are dependent on time and temperature. The liquefaction and gelation temperatures are different (the latter being lower), and the effect is called hysteresis. A typical value of the gelation temperature is 43°C (109°F).
41
42
Composition
Thymol Bactericidal agentGlycerine PlasticizerColoring and flavoring agents
43
Tray material Packaged in plastic tubes. Used with water-cooled
tray.Syringe material
Packaged in plastic or glass cartridges that fit a syringe.
44
Manipulation
Liquefy the hydrocolloid gel in boiling water. The material must be held at this temperature for a minimum of 10 mins.
After it has been liquefied, it may be stored in the sol condition at 65°C until it is needed for injection into the cavity preparation or for filling a tray.
a gauze pad is placed over the top of the tray material, and the tray is placed in the water-filled tempering container (45° C) of the conditioning unit. The syringe material, on the other hand, is never tempered.
45
Conditioning unit
65 °C
46
47
Permanent deformation and Flexibility
The ANSI/ADA Specification requires that the recovery from deformation be greater than 96.5% after the material is compressed 20% for 1 second.(99%)
The ANSI/ADA Specification requirement for flexibility allows a range of 4% to 15%
48
Strength
The compressive strength of a typical agar hydrocolloid impression material is 8000 g/cm2.
The tear strength of agar hydrocolloid impression materials is about 800 to 900 g/cm, which is higher than the ANSI/ADA Specification requirement of 765 g/cm
49
Causes of failure of agar impression
• Inadequate boiling• Storage temperature too low• Storage time too long
Grainy material
• Water soaked tray material surface not removed
• Premature gelation of either material
Separation of tray and syringe material
• Inadequate bulk• Premature removal from mouth• Syringe material partially gelled
when tray was seatedTearing
• Gelation of syringe material; prevents flowExternal bubbles
50
• Material too cold
Irregularly shaped voids
• Inadequate cleaning of impression• Excess water left in impression• Premature removal of the
impression• Model left in impression too long• Improper manipulation of stone
Rough chalky stone
model
Distortion• impression not poured
immediately• Movement of tray during
gelation• Premature removal from mouth• Improper removal from mouth• Use of ice water during initial
stages of gelation
51
Laminate technique
Agar-Alginate combination techniqueSyringe material – agarTray material – alginate
Advantages • simplification of heating
equipment• the elimination of water-
cooled impression trays• Overall simplification of the
procedure• agar hydrocolloid is more
compatible with gypsum model materials than alginates
Disadvantages • the bond between the agar
and the alginate is not always sound
• the higher viscosity of the alginate material displaces the agar hydrocolloid during seating
• the dimensional inaccuracy of the alginate hydrocolloid limits its use to single units.
52
Non-aqueous elastomeric impression materials
A Group of flexible chemical polymers, which are either chemically or physically cross-linked. Generally they can be easily stretched and rapidly recover their original dimensions when applied stress are released.
1. Polysulfide 2. Condensation silicone3. Addition silicone4. polyether
Condensation polymerization
Addition polymerization
53
Polysulfide
Polysulfide rubbers are widely used for fixed partial denture application, due to their high accuracy and relatively low cost.
These materials are useful for multiple impressions when extra time is needed.
54
Composition
Base paste: mercaptan or polysulfide polymer (80 to 85%) lithopone and titanium dioxide (16 to 18%) dibutyl phthalate
Reactor paste: lead dioxide (reactor) (60-68%) lithophone or titanium dioxide (filler) Dibutyl phthlalate (plasticizer) (30-35%) Sulfur (3% ) (accelerator) oliec acid or stearic acid (retarder)
55
--SH HS---------------------SH HS--
S H
O
=
Pb
=
O
O
=
Pb
=
OO = Pb = O
HS
-S-S---------------S-S-
S
S
+ 3PbO + H2O
mercaptan + lead dioxide polysulfide rubber + lead oxide + water
Polysulfide Reaction
Linear polymer ,with 1 mol% of branches to provide pendant mercaptan groups
Condensation reaction
Chain lengthening
Cross linking
exothermic
H2O
Moisture and temperatureAlter the reaction
56
Manipulation
Available in light, medium, heavy viscosities.
Same matched tubes to be usedPseudoplasticitySetting time 5-8 min
• Temperature• humidity• Loss of Water • Altering base :accelerator ratio
57
Properties
Setting time is 6 – 10 mins; which is more than other elastomeric impression materials.
Very good tear resistance; can withstand about 700% tensile strain before tearing.
Least stiff material – most easily removed from undercut areas.
They are considered as viscoelastic and recover only slowly and not completely after being compressed or stretched.
Dimensional changes are mainly due to 2 reasons – Continued setting reaction By-product
58
Biocompatibility
The use of lead compounds in polysulfide materials has been questioned because of the known toxic effects of lead.
There have been some limited reports of allergic responses to these materials, from people with an allergy to latex products.
59
Advantages and disadvantages
AdvantagesLower cost
compared to silicones and polyethersLong working timeHigh tear strengthHigh flexibilityGood detail reproductionLess hydrophobicLong shelf life.
DisadvantagesPoor dimensional
stability water by-product pour within one hour single pour, second pour
less accurate.Custom traysMessy
bad odor (thiol group in base paste)
may stain clothing (lead dioxide in catalyst paste)
Long setting time.
60
Condensation silicone
Condensation silicone rubber impression materials are used mainly for fixed partial denture impressions.
They are ideal for single-unit inlays.
61
composition
Base pastepoly(dimethylsiloxane)
with terminal hydroxy groups (-OH),
orthoalkylsilicateFiller- 37%to
75%colloidal silica or microsized metal(added to form paste)or copper carbonate.
Reactor paste or liquid.Organo tin compound-
dibutyl tin dilaurate or stannous octoate
Oily diluent Organic dyes or
pigments
62
HO – Si – O – Si - O - H
CH3
CH3 CH3
CH3 n
HO – Si – O – Si - O - H
CH3
CH3 CH3
CH3 n
C2H5O OC2H5
Si
C2H5O OC2H5
HO – Si – O – Si - O -
CH3
CH3 CH3
CH3 n
HO – Si – O – Si - O -
CH3
CH3 CH3
CH3 n
OC2H5
Si
OC2H5
2C2H5OH
Condensation Silicone Reaction
stannous octoate
ETHANOL
exothermic
α-ω Hydroxyl terminated polydimethyl siloxane
Tetraethyl ortho silicate
+
63
Manipulation
one drop per inch of extruded base paste (for base paste/liquid catalyst system)Also available in putty consistencysetting time (6 to 8 minutes)
TemperatureAltering base catalyst ratio
64
Advantages and disadvantages
AdvantagesBetter elastic
propertiesClean, pleasantStock tray
putty-washGood working and
setting timeLess distortion on
removal.
Disadvantages HydrophobicPoor dimensional
stability high shrinkage
polymerization evaporation of ethanol
pour immediately within 30 minutes
poor wettability poor to adequate shelf life.Slightly more expensive.
65
Addition silicone
These impression materials are also called vinyl polysiloxane or polyvinyl siloxane impression materials.
They represent an advance in accuracy over condensation silicone.
Due to their high accuracy, these materials are suitable for fixed and removable partial denture impressions.
66
Composition
Base pastePolymethyl hydrogen siloxaneOther siloxane prepolymersFillersHybrid silicone
Catalyst pasteDivinyl polydimethyl siloxanePlatinum salt(catalyst)RetardersFillers.Platinum or palladium –to absorb the
hydrogen.
67
O
- Si – CH3
O
CH3 - Si -
O
---O – Si – CH2 - CH2
CH3
CH3
CH2 - CH2 – Si – O ---
CH3
CH3
Platinum salt
68
Secondary reaction between moisture and residual hydrides of the base polymer can lead to the development of hydrogen gas.
Can cause pin-point voids on
the cast
Prevention – • Platinum or palladium added
to the formulation acting as H2 scavengers
• Waiting for an hour or more before pouring a cast
69
Manipulation
Available in – light, medium, heavy and putty consistency.
2 paste systemMonophase systemPutty in 2 jarsPutty-wash
A silicone is most sensitive to temperature changes.Custom tray is required except for putty- wash
technique.
70
Major disadvantages
HYDROPHOBICCauses distortion or
loss of details at the margin due to undetected moisture.
Also prevents gypsum products from accurate reproduction of details.
Prevention – adding non-ionic surfactant to the paste renders the surface hydrophillic.
SULFUR CONTAMINATION
Natural latex glovesSome vinyl gloves
containing sulfur stabilizers.
Prevention – recording the impression without touching the oral structures with gloves.
71
Advantages Dimensionally stable pour up to one week
Stock or custom traysShorter setting timeLess distortion on removalAdequate tear strength.Multiple castsEasy to mixHighly accuratePleasant odor
DisadvantagesExpensiveSulfur inhibits set
latex gloves Ferric and Alsulfate
retraction solutionShort working timeLower tear strengthPossible hydrogen gas release
bubbles on die palladium added to absorb hydrophilic material also still needs to be carefully handled
for a very dry field.
72
Polyether
Polyether rubbers are used for accurate impressions of a few prepared teeth without severe undercuts.
Their high stiffness and short working time restricts their use to impressions of a few teeth.
73
composition
Base paste:Polyether polymerColloidal silica-fillerGlycolether or phthalate-plasticizer
Accelerator paste:Alkyl aromatic sulfonateFillerPlasticizer.Thinner-octyl phthalate
74
Manipulation
MonophaseAutomatic mixing devicesThinner addedLess sensitive to temperature changes.
75
Advantages and disadvantages
Advantages:Highly accurateGood dimensional stabilityStock or dual-arch traysGood elasticity.Good surface detailPour within one week
kept dryMultiple castsGood wettability
Disadvantages:ExpensiveShort working timeRigid
difficult to remove from undercutsBitter tastestiffnessAbsorbs water
76
General properties
77
Viscoelastic properties
Elastic solidSPRING
Viscous liquidOIL
DASHPOT
78
The amount of permanent deformation exhibited by an elastomeric impression material should be clinically negligible, provided that:
(1) the material has adequately gelled(2) negligible pressure is applied to the tray during polymerization(3) the impression has been removed rapidly along the path of tray insertion(4) the undercuts present in the cavity preparation are minimal
79
Manipulation
base paste and catalyst system
base paste and liquid catalyst
system
2 putty systemPutty and
liquid catalyst system
80
81
Automatic mixing devices
82
Multiple mix
technique
Monophase or single viscosity
technique
Putty-wash
technique
2 stage putty wash
technique or reline
technique
83
Multiple mix techniqueSingle viscosity technique
Simultaneous putty-wash technique 2 stage putty-wash technique
84
85
Removal of the impression
Should not be removed until it has adequately gelled
The borders of the tray should be pried loose parallel to the path of insertion until air leaks into the tray.
Then the tray can be removed rapidly with minimal rotation or twisting.
86
Preparation of stone casts and dies
Polyether and addition silicone – multiple casts can be poured because of their excellent dimensional stability
Polyvinyl siloxane – hydrophobic nature – epoxy resin
To pour casts in gypsum a surfactant needs to be applied to the surface of the impression. Soap slurry can be used.
Hydrophilic addition silicone exhibit a contact angle of 30 - 35° with water.
Pouring stone cast in polyether is easier than silicone impressions.
87
Dimensional stability
There are five major sources of dimensional change:
(1) polymerization shrinkage(2) loss of a by-product (water or alcohol) during the condensation reaction,(3) thermal contraction from oral temperature to room temperature(4) imbibition when exposed to water; disinfectant or a high humidity environment over a period of time(5) incomplete recovery of deformation because of viscoelastic behavior.
88
Elasticity
The relative amount of permanent deformation following strain in compression increases in the following order :
1. addition silicone2. condensation silicone3. polyether4. PolysulfideThe stiffness increases in following order:1. Polysulfide2. Condensation silicone3. addition silicone4. polyether
89
Tear strength
Tear strength increases in following order:1. Hydrocolloids (agar and alginate)2. silicones (addition and condensation)3. Polyether4. polysulfide.Tear strength is also influenced by the
consistency and the manner of removal of the materials.
90
Biocompatibility
Cell toxicity – polysulfide: lowest cell death count
polyether: highest cell death countMost likely biocompatibility problem – segment of impression material lodged in a patient’s gingival sulcus
91
92
Common causes of failure of elastomeric impression material
Rough or uneven surface on impression1. Incomplete polymerization caused by
premature removal from the mouth, Improper ratio or mixing of components, or presence of oil or other organic material on the teeth(for A silicone, agents that contaminate the material and inhibit polymerization)
2. Too rapid polymerization from high humidity or temperature
3. Excessively high accelerator/base ratio
93Bubbles1. Too rapid polymerization, preventing flow2. Air incorporated during mixingIrregularly shaped voids1. Moisture or debris on the surfaceRough chaulky stone cast 1. Inadequate cleaning of impression2. Excess water left on surface of the impression3. Excess wetting agent left on impression4. Premature removal of cast5. Improper manipulation of stone6. Failure to delay pour of addition silic0ne at least
20 mins
94
Causes of distortion
1. Resin tray not aged sufficiently2. Lack of adhesion of rubber to the tray caused by not
applying enough coats of adhesive, filling the tray with material too soon after applying adhesive or using wrong adhesive
3. lack of mechanical retention for those materials where adhesive is ineffective
4. Development of elastic properties in the material before tray is sealed
5. Excessive bulk of material6. Insufficient relief for the reline material (if such technique
is used)7. Continued pressure against impression material that has
developed elastic properties8. Movement of tray during polymerization9. Premature removal from mouth10. Improper removal from mouth11. Delayed pouring of the polysulfide 0r condensation silicone
impression
95
Comparison of Properties
Working time longest to shortest
polysulfide > silicones > polyether
Setting time shortest to longest
polyether < silicones < polysulfide
96
Comparison of Properties
Stiffness most to least
polyether > addition silicone > condensation silicone > polysulfides
Tear strength greatest to least
polysulfide > addition silicone > polyether = condensation silicone.
97
Comparison of Properties
Cost lowest to highest
polysulfide <condensation silicone < addition silicone < polyether
Dimensional stability best to worst
addition silicone 0.15%> polyether 0.2%> polysulfide 0.4%> condensation silicone0.6% Phillip’s 1996
98
References
Anusavice K. J. : Phillips’ science of dental materials, 11th edition.
Powers J. M., Sakaguchi R. L. : Craig’s restorative dental materials, 12th edition
O’Brien W. J. : Dental materials and their selection, 4th edition
McCabe J. F., Walls A.W.G. : Applied dental materials, 9th edition
99
Thank you