impression materials
TRANSCRIPT
2
WHAT IS AN IMPRESSION???
• negative replica of the hard and soft oral tissues
• Register or reproduce the form and relationship of the teeth and oral tissues
3
Purpose of Impression Materials
• Impression –negative reproduction of tissues• Positive cast made by filling the impression
with dental stone or other model material
4
Desirable qualities of Impression Materials
A pleasant odour, taste and acceptable colorAbsence of toxic or irritant constituentsAdequate shelf life for requirements of storage and distribution
5
Desirable qualities contd…
Easy to use with minimum of equipmentSetting characteristics which meet clinical requirementsLow enough viscosity to adapt to the oral tissues,yet be viscous enough to be contained in the impression tray
6
Desirable qualities contd…
Should have adequate wettability of the oral tissues Set impression should show adequate elastic recovery with no permanent deformation upon removal from mouth
7
Desirable qualities contd….
Dimensionally stable after setting over temperature and humidity ranges normally found in clinical and lab procedures until the pouring of the castCompatible with cast and die materials
8
Desirable qualities contd….
Biocompatible ,non-toxic Must be of color and opacity that allows the dentist to evaluate the impressionReadily disinfected without significant loss of accuracy or loss of mechanical properties
9
Desirable qualities contd….
Materials ,associated processing time and equipment –cost effectiveNo release of gas or other by-products during setting of the impression materials
11
CLASSIFICATON:
Classified on the basis of setting and elasticity.
Mode of setting rigid Elastic
Set by chemical reaction(irreversible)
Impression plaster, zinc oxide eugenol.
Alginate, polysulfide, polyether, silicone,
Set by temperature change(reversible)
Compound, waxes Agar hydrocolloid.
13
Impr
essio
n M
ater
ials
Non-elastic
Elastic
Aqueous Hydrocolloids
Non-aqueous Elastomers
Polysulfide
Silicones
Polyether
Condensation
Addition
Agar (reversible)
Alginate (irreversible)
Plaster
Compound
ZnO - Eugenol
Waxes
O’Brien, Dental Materials & their Selection 1997
14
NON-ELASTIC IMPRESSION MATERIALS
NON-ELASTIC IMPRESSION MATERIALS
Impression plaster
Zinc-oxide eugenol
impression paste
Impression compound
15
Impression Plaster• used as mucostatic impression material for
making final impressions for edentulous patients
• Doesn’t compress and displace tissues during seating of tray due to its fluidity
16
Impression plaster contd…
• Applicable to patients with displaceable soft tissues that should be recorded in a passive state
17
Composition
• ß-calcium sulphate hemihydrate • Reacts with water to form calcium sulphate
dihydrate• W/P ratio– 0.5-0.6• Expansion and setting times controlled by
incorporating compounds designed to mediate handling properties
18
• Potassium sulphate added as an anti-setting expansion agent
• Borax(retarder)- added to the powder to balance the setting acceleration caused by Pot. Sulphate and to bring the setting time under control
20
• Custom tray constructed using 1-1.5mm spacer with acrylic resin or shellac
• Impression plaster can be used as wash material
• Techinque- “Puddling” the impression into place
21
• With remaining plaster in tray, the tray is seated in single movement
• Then tray is gently moved from side-to side and antero-posteriorly to take advantage of fluidity of material
22
• Hemihydrate particles absorb moisture from the surface of the oral tissues allowing intimate contact between impression material and the tissues
23
• Plaster impression material –very brittle and fractures easily
• When undercut is involved, fracture the impression to facilitate removal from mouth
• Fragments reconstructed to form completed impression
24
• Beading of the impression done• Coated with separating medium and cast in
fresh plaster• Disinfection- achieved with a 10 min soak in
sod hypochlorite solution
25
• Not used regularly due to mechanical limitations
• Used frequently as occlusal registration material
26
Zinc-oxide Eugenol Impression Paste
• Composition-2 separate pastes dispensed in tubes
• One tube contains zinc oxide and vegetable or mineral oil
• Other tube contains eugenol and rosin
27
Composition
Components Percentage
Tube no 1 (base)
Zinc-oxide 87
Fixed vegetable or mineral oil 13
Tube no 2 (accelerator)
Oil of cloves or eugenol 12
Gum or polymerised rosin 50
Filler(silica type) 20
Lanolin 3
Resinous balsam 10
Accelerator solution(CaCl2) and color 5
28
Setting reaction of ZOE
• Ionic in nature• Requires ionic medium in which it can proceed
at any desired rate• 1st reaction-hydrolysis of zinc oxide to its
hydroxide form
29
Zinc Oxide Eugenol
• When the 2 pastes are mixed,the phenol –OH of the eugenol acts as a weak acid and undergoes an acid-base reaction with zinc hydroxide
• Forms a salt- zinc eugenolate
30
ZOE contd…
• Two further coordinate bonds are formed by donation of pairs of electrons from methoxy oxygen to zinc
31
ZOE contd…
• Disadvantage-• Stinging or burning sensation caused by
eugenol • Orthoethoxybenzoic acid -substitute
32
Manipulation
• Mixed on oil impervious paper or glass mixing slab
• Proper proportion of two pastes obtained by squeezing 2 strips of paste of the same length,one from each tube ,onto the mixing slab
33
Manipulation of ZOE contd…
• Flexible stainless steel spatula used for mixing• 2 strips of contrasting colors combined with
the first stroke of the spatula ,mixing is continued for approx 1 min, until a uniform color achieved
34
Types of ZOE
• Classified as Hard paste(type I) soft paste(type II)• Final set for type I paste-10 min type II paste-15min Actual time shorter when setting occurs in
mouth
35
• Shorten the setting time – by adding small amount of Zinc acetate or additional accelerator or a drop of water in the paste before mixing or by extending mixing time
• Prolonging the setting time- cool spatula and mixing slab
36
• Paste of thick consistency –compresses the tissues
• Thin,fluid material results in little or no compression
Advantage of heavier consistency – increased strength
37
Dimensional stability
• Negligible shrinkage(less than 0.1%) may occur during hardening
• No significant dimensional change• Impression can be preserved indefinitely
without change in shape provided the tray material is dimensionally stable
38
Disinfection
• 2 % alkaline glutaraldehyde solution• Immersed in solution for required time,rinsed
and poured immediately
39
Applications of ZOE
• Final impression of edentulous ridges• As a wash impression with other impression• As an interocclusal registration material• As a temporary liner material for dentures• As a surgical dressing
40
Impression compound• Also called “modelling plastic”• Thermoplastic material• Supplied in the form of cakes(red) and sticks
(green, gray or red)
41
Composition
• Mixture of –waxes(principal ingredient) thermoplastic resins filler(increase viscosity and
rigidity) coloring agent• Shellac, stearic acid and gutta-percha added to
improve plasticity and workability
43
Type I (Lower fusing material)
• Cakes- as an impression material for completely edentulous patients, the material is softened by heat, inserted into the tray and placed against the tissues before it cools to a rigid mass
44
• Sticks- as a border molding material for the custom tray ,the material is used before making the final impression
45
Type II( Higher fusing material)
• Used as an adaptation material which requires more viscous properties
• Used for making primary impression of the soft tissues and then used a tray to support a thin layer of a second impression material such as ZnOE paste, hydrocolloids or nonaqueous elastomers
46
Manipulation
• Setting mechanism– reversible physical process
• Softening by heat – prerequisite • Preheated and used warm (~450C)• Then cooled to the intraoral temperature(370C)
at which it is fairly rigid
47
• Once the impression tray is seated , it should be held gently(passively) in position until the impression cools below the fusion temperature
48
• Softened by heat over flame(green stick) or in a temperature controlled water bath
• When direct flame is used, the material should be moved over the flame in such a manner that it will not be allowed to boil or ignite so that the constituents are volatilized
49
• Prolonged immersion or overheating in water bath makes the compound brittle or grainy due to leaching of low molecular weight ingredients
50
• Dimensional stability- allow thorough cooling of the impression before removal from the mouth and to construct the cast or die as soon as possible after the impression has been obtained(at least within the hour)
• Disinfection – 2% alkaline glutaraldehyde solution
51
Thermal Properties
a) Thermal conductivity-low thermal conductivitySignificance• During softening of the material, the outside will
soften first and inside last, so to ensure uniform softening, the material should be kept immersed for a long time in water bath
• The layer adjacent to tissues will remain soft . Thus it is important to cool the compound thoroughly before removing the impression
52
b)Coefficient of Thermal Expansion- high COTE (0.3% acceptable)
c)Glass Transition Temperature• The temperature at which the material loses
its hardness or brittleness on heating or forms a rigid mass upon cooling
• Approx 39°C
53
d)Fusion temperature– corresponds to a definite reduction in plasticity of the material during cooling
• Above this temperature, the material remains plastic while the impression is being made
• Approx 43.5°C
54
Significance of Fusion temp and Glass transition temp
• Above Fusion temp,the fatty acids are liquid and lubricate the softened material to form a smooth plastic mass while the impression is being obtained.
• Thus all impressions with compound should be made above this temperature
55
• Once the impression tray is seated,it should be held firmly in position until first fusion temperature and later the glass transition temperature is reached
• Thus, impression is made above the fusion temperature and removed after it cools down to its glass transition temperature
56
Important considerations for proper use of impression compound
• Low thermal conductivity- adequate time needed to attain thorough heating and cooling
• Incorporation of water(wet kneading)- excessive flow of the material at mouth temp. producing distortion as the impression is removed from mouth
57
• Tray used for impression must be strong and rigid enough to support the material and to avoid distortion of the impression
• Relatively high viscosity limits its ability to record fine details
• Cast should be poured as soon as possible to minimise distortion due to relaxation of the compound
59
Elastomeric Impression Materials
ELASTOMERIC IMPRESSION MATERIALS
Aqueous hydrocolloids
Agar-agar Alginate
Non-aqueous elastomers
Polysulfides
Silicones Polyethers
61
Agar • Reversible hydrocollloid• Physical change of agar from sol to gel induced
by lowering temperature• Gel liquefies to sol when heated to a
temperature known as liquefaction temperature(700C-1000C)
62
• When sol is cooled, it becomes gel at a point known as the gelation temperature( btw 370C and 500 C)
• Thus called reversible hydrocolloid
63
• Gelation temp- critical for impression making• If too high,heat from the sol may injure the
oral tissues• If too low,below oral temperature,impossible
to make impression because the sol will not convert to a gel
64
• Polysaccharide- extracted from certain types of seaweed
• Water major constituent• Supplied as gel• Available in tray and syringe consistencies• Tubes used to fill water cooled trays and
cartridges used with syringes
65
Composition Agar Gelling agent
Borax Improves strength
Potassium sulphate Gypsum hardener
Alkyl benzoates Preservatives
Water Reaction medium(>80%)
Coloring agents
Flavouring agents
66
• Fillers such as diatomaceous earth,wax, clay,silica,rubber and similar inert powders– used to control strength , viscosity and rigidity
• Thymol and glycerine added as bactericidal agent and plasticiser
67
Making the Agar impression
• Process requires a 3 compartment conditioning unit for the agar tray material
• Allows liquefaction,storage and tempering• Syringe material used only in liquefaction and
storage compartments
68
Liquefy the hydrocolloid gel in the tube in boiling water at 1000C for minimum 10 min
tube then placed in a storage bath at 65°C to retain the sol condition until needed
impression tray filled with hydrocolloid sol from the tube taken from storage bath , gauze pad placed over the top of the tray material
Tray placed in water filled tempering compartment(at abt 45°C)
70
Just before tempering completed,syringe material taken directly from storage compartment and applied to the prepared teeth
• Note--- tempering time-3-10 min• if >10 min,partial gelation occurs• syringe material doesn’t require
tempering bcoz maintained in fluid state to enhance adaptation to tissues
71
• Syringe material first applied to the base of the preparation,then remainder of the prepared tooth is covered
• Tip of the syringe is held close to the tooth and it remains embedded below the surface of the syringe material to prevent entrapment of air bubbles
72
• Water soaked outer layer of hydrocollloid loaded tray and the gauze covering the tray impression material are removed to ensure firm bonding to the syringe hydrocolloid
• Tray immediately brought into position,seated with light pressure and held with a very light force
73
• Gelation accelerated by circulating cool (18-21°C)through tray for 3-5 min
• During gelation process, tray must be held in mouth until gelation has proceeded to a point at which gel strength is sufficient to resist deformation or fracture
• Tray removed with a snap
74
Distortion during gelation
• Some contraction occurs due to physical change (sol gel)
• If held rigidly in the tray,shrink towards the center of its mass,thus creating larger dies
• Rapid cooling may cause stress concentration near the tray
77
Compatibility with Gypsum
• Contains borax- retarder for setting of gypsum products
• Deficiency of gypsum setting can be overcome by--Immersing agar impression in a solution containing a gypsum accelerator(2% pot sulfate solution) prior to pouring of the impression
• By incorporating gypsum surface hardener in the material such as sulfate
79
Alginate • Irreversible hydrocolloid• Most widely used material in dentistry• Developed as a substitute for agar
80
Advantages
• Ease of manipulation• No need of expensive
equipments• Relatively low cost• Comfort to patients
• High viscosity• Ability to displace
tissues
Disadvantages
81
Potassium or sodium alginate dissolves in water and reacts with calcium ions
Calcium sulphate dihydrate A reactor ,reacts with potassium alginate to form a dihydrate insoluble alginate gel
Zinc oxide Filler particles, affects properties and setting time
Potassium titanium fluoride Accelerator ,counteracts the inhibiting effect of the hydrocolloid on the setting of stone,ensures good quality surface of the cast
Diatomaceous earth Filler particles, controls the consistency of the mix and the flexibility of the set alginate
Trisodium phosphate Retarder,controls the settting time to produce either regular or fast set alginates
Coloring agents
Flavoring agents
Composition
82
Modified alginates
• Dustless alginates• Include polyethylene glycol or polypropylene
glycol on the alginate powder to agglomerate the particles
• Color indicators added to reveal the stage of setting reaction
83
Modified alginates
• Two paste alginate materials• One paste contains sol of
alginate,fillers,retarders and other ingredients like glycols and dextrose
• Other paste contains gypsum dihydrate, fillers, retarder , glycerol or glycol, gypsum surface modifier and some silicone oil
84
Gelation process
• Typical sol-gel reaction• Soluble alginate reacts with calcium ions from
calcium sulphate and forms insoluble calcium alginate
• Production of calcium alginate- rapid,doesn’t allow sufficient working time
• Retarder trisodium phosphate added to extend working time
85
Setting reaction of alginate
(a) K2n-Alginate+ n CaSO4 nK2SO4 + Can-alginate
(b)2Na3PO4 + 3CaSO4 Ca3(PO4)2 + 3Na2SO4
87
Controlling Setting Time
• Ideal W/P ratio- 20 ml water/8gms of powder 40 ml water/16gms of powder• Powder should be weighed not measured• Approx 2.5:1• Slight modification in W/P ratio affects 2
important properties--- tear strength elasticity
88
• Thus setting time best regulated by amount of retarder added during manufacturing
• Fast-set alginate- 1.5-3min• Regular set alginate- 3-4.5 min• Can also be influenced by altering the
temperature of water
89
• Cool water in hot weather• Precool mixing bowl and spatula• Tap water-contains certain levels of metallic
ions(Ca,Mg)• Tap water with a high hardness may accelerate
setting time
90
Preparation of Alginate Impression Materials
• Measured powder added slowly to premeasured water already poured into clean rubber bowl
• Powder incorporated into water by carefully mixing with a metallic spatula flexible enough to adapt well to the wall of the mixing bowl
91
• Avoid incorporating excessive air into the mix• Vigorous figure of 8 stropping motion • Mixing time- 45sec to 1 min• Result should be a smooth creamy mixture
that doesn’t drip off the spatula when raised from bowl
92
• Mechanical mixing devices• Include rotating mixing bowl , mechanical
mixer with time-control unit, a vaccum mixer for water/powder mixing
• Advantages- convenience, speed and reduction of human error
93
Making the alginate impression
• Perforated metal tray preferred• Thickness of alginate impression between the
tray and the tissues should be at least 3mm• Compressive strength doubles during first 4 min
after gelation,but doesn’t increase appreciably thereafter
• Improve elasticity over time which minimizes distortion of the material during impression removal(undercut areas)
94
Compressive strength of an Alginate gel as a function of Gelation time
Time from Gelation(min) Compressive strength(KPa)
0 330
4 770
8 810
12 710
16 740
95
• Alginate impression should not be removed from the mouth for at least 3 min after gelation has occurred
• Tear strength increased when the impression is removed along a vertical path with a snap
• Speed of removal- between rapid movement and a slower rate
96
STRENGTH• Manufacturer’s directions should be followed• Any deviation from instructions can have
adverse effects on the gel strength
SHELF LIFE• 2 factors affecting shelf life- storage
temperature and moisture contamination
97
Dimensional stability
• Syneresis- loss of water when exposed to air at room temperature associated with shrinkage
• Imbibition- swelling of the impression if immersed in water
98
How can distortion be minimised??
• Poured immediately after making impression• If pouring delayed, then rinsed in tap
water,disinfected wrapped in a surgical paper towel saturated with water and placed in a sealed plastic bag or humidor
99
Compatibility with Gypsum
• Poatssium titanium fluoride-surface hardener or gypsum hardener
• Solubility-1.3gm per 100 ml of water at 20°C• Fluoride on surface of alginate will form fine
calcium fluoride precipitates with the calcium from the gypsum
• Fine calcium fluoride particles become nuclei that accelerate the setting of gypsum
100
• Rough stone surface will result if excess rinsing water collected on the surface of the impression at the time of pouring the stone mixture
• A dried gel results in its adherence to the surface of the cast which results in tearing upon removal
101
• Surface of impression should be shiny but with no visible water film or droplets at the time of pouring
• Stone cast or die should be kept in contact with the impression for a minimum of 30 min, preferably for 60 min before the impression is separated from the cast
102
Disinfection
• Household bleach(1-10 dilution) iodophors or synthetic phenols
• After rinsing,disinfectants sprayed on exposed surface
• Impression immediately wrapped in a disinfectant-soaked paper towel and placed in a sealed plastic bag for 10 min
103
• Wrapped impression removed from the bag, unwrapped, rinsed and shaken to remove excess water
• Then poured with stone of choice AccuracyNot capable of reproducing the finer details
compared with other impression materials
105
Laminate technique(Alginate -Agar method)
• Modification of agar procedure• Agar in tray replaced with a mix of chilled
alginate that bonds to the agar expressed from a syringe
• Alginate gels by chemical reaction while agar gels by means of contact with the cool alginate rather than water circulating through the tray
106
• Agar contacts the prepared teeth ,maximum detail reproduced
• Equipment cost low, less preparation time needed
• Main disadvantages- bond btw agar and alginate not always
sound higher viscosity of alginate displaces agar
during seating dimensional inaccuracy of alginate limits its
use to single units
108
Elastomeric impression materials
• Comprise a group of synthetic polymer-based impression materials that are chemically cross-linked when set and can be stretched
• Rapidly recover to their original dimensions
110
Elastomers
• Supplied in 2 components: base paste catalyst paste• Formulated in several consistencies in increasing
content of filler Extra light body Light body Medium or regular body Heavy body Putty(extra heavy)
111
• Extra low and putty available only for condensation and addition silicones
• Polysulfide provided only in light body and heavy body
• No heavy body for condensation silicone
112
• Different colored pastes dispensed either through a spiral mixing tip or in equal lengths on a mixing pad
• Setting occurs through a combination of chain-lengthening polymerization and chemical cross-linking by either a condensation or addition reaction
113
• 3 types of systems available to mix the catalyst and base : hand mixing, static automixing, dynamic mechanical mixing
• To record soft tissues for edentulous patients under minimum compression: polysulfide or addition silicones
(free flowing, minimum viscosity)
114
• If moderate compression required: medium-viscosity polysulfides, addition silicones, polyethers
• Undergo shrinkage upon polymerisation• Condensation type silicones undergo
additional contraction
115
• Polysulfides and condensation silicones : highest dimensional changes during setting
• Addition silicones and polyethers: lesser dimensional changes
116
Polysulfides • First synthetic elastomeric impression
material introduced in 1950• 2 paste system• Available in low, medium and high
consistencies• Made up of a base and accelerator/reactor• Brands- COE-FLEX,PERMALASTIC,NEOPLEX etc
117
Composition The BasePolysulfide polymer(-SH,mercaptan gp) Principal ingredient
Titanium oxide and Zinc Fillers
Sulphate ,copper carbonate or silica Strengthener
Dibutyl phthalate Plasticizer (confers viscosity to base)
The AcceleratorLead dioxide, hydrated copper oxide or organic peroxide
Reactor
Sulfur Promoter, accelerates the reaction
Oleic acid or Stearic acid Retarder, controls setting reaction
118
• Working time: 5-7 min( longest among elastomers)
• Setting time: 8-12 min• Pouring the cast: impression must be poured
within 30 min to 1 hr
119
• Each paste supplied in a dispensing tube with approx sized bore diameters at the tip
• Equal lengths of paste extruded from each tube to provide the correct ratio of polymer to cross-linking agent
120
• Reaction starts at the beginning of mixing and reaches its maximum rate soon after spatulation is complete
• Resilient network started to form• During final set, a material of adequate
elasticity and strength is formed that can be removed past undercuts
121
• Polymerisation results in chain lengthening and cross –linking with an increase in molecular weight
• Setting indicated by change of the color of the paste to dark- brown or gray- brown
• Color- presence of lead oxide
122
Polymerisation of ploysulfide impression material SH groups interact with oxygen released from lead dioxideCompletion of the condensation reaction results in water as a by-productPendant –SH for cross-linking and terminal for chain lengthening
123
• Hot and humid conditions accelerate the setting of polysulfide impression material
• Reaction slightly exothermic and yields water as a by-product
124
• Good flexibility• High tear strength• Hydrophobic• Messy, stains clothes and has an offensive
odor• Uses—impression for crown and bridge edentulous impressions
125
Condensation silicone• Followed in 1955• Supplied as two-paste system or base-paste and a
low viscosity liquid catalyst or a two-putty system• Putty used as tray material in conjunction with a
low-viscosity silicone • Referred to as the Putty-wash technique• Brands- Speedex
126
Composition The Base paste
- -hydroxyl-terminated polydimethyl siloxane
High molecular weight polymer
Silica or calcium carbonate Fillers
The Liquid Accelerator
Tin octoate Metal organic ester
Orthoalkyl silicate
Oil-based diluents
Thickening agents Increase viscosity
127
Condensation silicone
• Working time: 3 min• Setting time: 6-8 min• Impression must be poured as soon as
possible within first 30 min
128
• Curing involves a reaction of tri- and tetra-functional alkyl silicates in the presence of stannous octoate as a catalyst
• Sets by cross-linking between terminal groups of the silicone polymers and the alkyl silicate to form a 3-D network
129
Condensation polymerisation of alpha-omega hydroxy –terminated poly(dimethyl siloxane) with tetraethyl orthosilicate in the presence of stannous octoate(catalyst)This reaction results in the release of ethanol molecules
130
• By-product: Ethyl alcohol• Subsequent evaporation accounts for much of
the contraction that takes place in the setting impression
• Extra-heavy or putty consistency developed to counteract the large polymerisation shrinkage
131
Addition silicone• Commonly referred to as Polyvinyl siloxanes(PVS)• Supplied as low, medium, high and very high
consistencies• Based on silicone prepolymers that carry vinyl
and hydrogen side groups which can polymerize by addition polymerization
• Brand name- Aquasil
132
Composition The Base
Polymethylhydrosiloxane Low molecular weight polymer
Fillers
The Accelerator
Divinyl polymethyl siloxaneOther siloxane pre-polymers
Platinum salt Catalyst
Reatrder Controls working and setting times
133
• Working time: 2-4.5 min• Setting time: 3-7 min• Cast can be poured upto 1 week after making
the impression
134
• Reaction activated by a platinum salt catalyst (chloroplatinic acid) without the release of by-
products• In presence of impurites or moisture,
secondary reaction takes place between the residual hydrides and moisture leading to evolution of hydrogen gas
135
Hydrogen atoms along the backbone str of PVS chain move to the vinyl group during addition polymerisation(top)Final str after platinum salt has initiated the addition polymerisation reaction(bottom)The zigzag line at the other end of divinylpolysiloxane represents repeating units of dimethylsiloxane with a vinyl terminal
136
• This can cause minute gaseous voids in the gypsum casts and reduce the effectiveness of cross-linking polymer structure
• Automatic mixing systems simplified their manipulation, reduced voids in impressions, reduced the amount of material wasted and reduced the sensitivity of their mixing technique
138
Advantages
• Most elastic of currently available materials• Virtually negligible distortion upon removal
from undercuts• Exceptional accuracy in reproducing anatomic
details • Dimensional stability allows pouring long after
impression making• Excellent occlusal record registration material
139
Disadvantages
• Inherent hydrophobic natureNon-ionic surfactant wetting agent added to
silicone paste rendering the surface of the impression more hydrophillic and called hydrophilized addition silicone
140
• Sulfur contamination from natural latex gloves inhibits the setting of addition silicone
• Touching the tooth with latex gloves before seating the impression can inhibit the setting of critical surface next to tooth
141
Polyether • Supplied as two-paste system in low, medium
and high consistencies• 2 types: 1. based on ring-opening polymerization
of aziridine rings which are at the end of branched polyether molecules
2. based on an acid-catalyzed condensation polymerization of polyether prepolymer with alkoxysilane terminal groups
• Brands- IMPREGUM
142
1st type
• Main chain probably a copolymer of ethylene oxide and tetrahydrofuran
• Cross-linking and setting promoted by an initiator and an aromatic sulfonate ester
• R alkyl group• Produces cross-linking by cationic
polymerisation via the imine end groups • Supplied as 2 pastes: base and accelerator
143
Composition
The Base
Polyether polymer
Colloidal silica Filler
Glycol ether or pthalate Plasticizer
The Accelerator
Alkyl aromatic sulfonate Initiator
Filler
Plasticizer
145
Initiator ,aromatic sulfonate ester dissociates and forms alkyl cations that bind the nitrogenatoms of the azridine ring terminals of the prepolymer(top,left)The arrows indicate binding between the cations (R) with nitrogen atomsThis action opens up the ring,and the reacted pre-polymer (center) now has 2 ethylene imine terminals(-NR-CH 2-C+H2) which can react with nitrogen atoms of adjacent unreacted prepolymers.(R2 Aziridine ringChain propagation polymerization reaction yields a larger molecule(right) which continues growing by binding with aziridine rings of additional unreacted prepolymersPolymerisation reaction terminates when the growing chain combines with a counter ion
146
2nd type
• Based on an acid-catalyzed condensation polymerization of polyether prepolymer with alkoxysilane terminal groups
• Mechanism similar to condensation silicones• Material often called hybrid• Behave very much like the 1st type due to
ether linkages
147
• High degree of wettability• Inherent hydrophillic
nature• Relative stiffnessExcellent material for good
duplication of fine details and rigid support for pick-up copings
148
Making impressions with elastomeric materials
• Fabrication of gypsum models ,casts and dies involves 6 major steps:
1. Preparing a tray2. Managing tissue3. Preparing the material4. Making the impression5. Removing the impression6. Preparing stone casts and dies
149
1.Impression trays
• Custom tray recommended to reduce the quantity of material required
• In case of severe undercuts, custom tray avoided
• Prior to impression making, uniform thickness of tray adhesive applied
150
2.Tissue management
• Displace the gingival tissues, control gingival haemorrhage and control sulcular fluids to ensure access for the tooth preparation and making impression
• Gingival retraction cord- most commonly used
151
3.Manipulation of impression materials
• Supplied for 3 modes: hand mixing, static mixing and dynamic mechanical mixing
152
Hand mixing
• Dispense the same length of materials onto a mixing pad or glass slab
• Catalyst paste first collected on stainless steel spatula and then spread over base paste
• Mixture is then spread over the mixing pad• Mass is then scraped up with the spatula
blade and spread uniformly back and forth on the mixing pad
153
• Process continued until the mixed paste is uniform in color with no streaks of the base or catalyst appearing in the mixture
• 2 putty systems(condensation and addition silicone) dispensed by volume using equal number of scoops of each material
• Knead the material with fingers until a uniform color is obtained
154
Static mixing• Transforms 2 fluid(or paste-like)
materials into a homogenous mixture without mechanical mixing
• Device used- gun for compressing materials into a 2-cylinder cartridge, which contains the base and catalyst separately, as well as mixing tip
155
• Mixing tip is made of helical mixer elements in a cylindrical housing
• Mixer elements are series of alternating right and left –turn 180°helixes positioned so that leading edge of one element is perpendicular to the trailing edge of the next
• Length of each material is the same as the inner diameter of the cylinderical housing
156
Dynamic mechanical mixing
• Device uses motor to drive parallel plungers,forcing the materials into a mixing tip and out into an impression tray or syringe
• Motor driven impeller mixes the materials as they are extruded through the tip
• Materials supplied in collapsible plastic bags housed in cartridge
• Polyether and addition silicone
157
4.Making an impression
• 3 techniques:• Multiple-mix technique• Monophase technique• Putty wash technique
158
Multiple-mix technique
• Syringe material(light body)and tray material(heavy body)
• Lighter material injected within or around the tooth preparation
• Filled tray then inserted in the mouth and seated over the syringe material
• Tray material force the syringe material to adapt to the prepared tissues
159
Monophase technique
• Medium body polyether and addition silicone • Only one mixture is made and a part of the
material is placed in the tray and another portion in syringe for injection in the prepared tissues
• Success depends on pseudoplastic (shear thinning) property of material
160
Putty –wash technique
• Originally developed for condensation silicone to minimize the effect of associated dimensional changes
• Thick putty material placed in stock tray and a primary impression made
• Space for light-body “wash” material provided • Mixture of thin consistency wash material
placed into putty impression and preparation
161
5.Removal of the impression
• Shouldn’t be removed until curing progressed sufficiently to provide adequate elasticity ,so distortion doesn’t occur
• Typically impression should be ready for removal within at least 10 min from time of mixing,allowing 6-8 min for impression to remain in mouth
162
• Mechanics of removing impression– separation at the impression/tissue interface and stretching of the impression
• 1st step to break the physical adhesion between the tissue and the impression
• Polyether requires extra effort• 2nd step stretches the impression enough to pass
under the height of contour of hard tissue to remove impression
163
6.Preparation of stone casts and dies
• Silicones– hydrophobic• Surfactant sprays- debubblizers improve
surface wettability of silicone impression material for stone slurry
• Dilute solution of soap also acts as surfactant• Polyether n polysulfide- don’t require
surfactant
164
Properties of elastomeric impression materials
1. Working and setting time2. Reproduction of oral structure detail3. Rheological properties4. Elasticity and visco-elasticity5. Tear strength6. Dimensional stability7. Disinfection8. Wettability and hydrophillization9. Biocompatibility
165
1. Working and setting timeMean working time(min) Mean setting time(min)
Impression material 23°C 37°C 23°C 37°C
POLYSULFIDE 6.0 4.3 16.0 12.5
CONDENSATION SILICONE 3.3 2.5 11.9 8.9
ADDITION SILICONE 3.1 1.8 8.9 5.9
POLYETHER 3.3 2.3 9.0 8.3
166
2.Reproduction of detail
• Record detail to the finest degree• When stone poured on the surface, finest
details not always reproduced
167
3.Rheological properties
• Viscosity and flow behaviour depends upon - ease of mixing -air entrapment during mixing -tendency of trapped air to escape before the
impression is made• All elastomers exhibit shear-thinning
characteristics before setting
168
• 2 categories of shear thinning phenomena—• Pseudoplasticity • Thixotropy • Pseudoplastic material- displays decreasing
viscosity with increasing shear stress and recovers its viscosity immediately upon a decrease in shear stress
169
• Thixotropic material- doesn’t flow until sufficient surface energy in the form of an impact force or vibration force is applied to overcome the yield stress of material
• Extreme shear thinning- material retains an immobile state at rest but flows freely under stress
170
Significance of shear thinning
• Exhibited by addition silicone and polyether impression materials
• Enable the clinician to use a monophase impression making technique to capture details needed for fixed prostheses
171
4. Elasticity and viscoselasticity
• Explained well by Maxwell- Voigt model• Relative amount of permanent deformation• Addition silicone<polyether<polysulfide• Recovery of elastic deformation following
strain is less rapid for the polysulfide material than for other three types of impression materials
172
MAXWELL-VOIGT MODEL
A-Maxwell-Voigt model in a stress-free stateB-during loading,only S1 spring contracts in response to loadC-when loading continues,the pistons in dashpot D1 and D2 move proportionally to the duration of loading . S2 spring contracts alongwith dashpot D2. No change in S1D-the moment the load is released,S1 spring recovers instantly, whereas rest of the elements remain unchanged. S2 should also recover instantly but retarded by the sluggishness of dashpot D2E- as time passes, S2 spring recovers and extends dashpot D2 slowly near to its original position. Dashpot D1 remains unchanged
173
• Polyvinyl siloxane- exhibit most elastic recovery
• Distortion on removal from undercuts is virtually non-existent
• If material is in advanced stage of elasticity, and compressed excessively during seating of impression,distortion can occur when the material elastically rebounds
175
5.Tear strength
• Low viscosity materials used in interproximal and subgingival areas
• Measures the resistance of an elastomeric material to fracture when subjected to a tensile force acting perpendicular to a surface flaw
• Tear strength in increasing order• Silicones<polyethers<polysulfides
177
6. Dimensional stability• Dimensional accuracy v/s dimensional stability• 6 major sources of dimensional change1. Polymerisation shrinkage2. Loss of condensation reaction by-product3. Thermal contraction from oral temp to room temp4. Absorption of water or disinfectant over a period of
time5. Incomplete recovery of deformation because of plastic
deformation6. Incomplete recovery of deformation because of
viscoelastic behaviour
178
• Absorption of water or fluids- negatively affects polyether impression
• Simultaneous leaching of water soluble plasticizer
• Stored in dry(relative humidity<50%),cool environment to maintain its accuracy
• Should never be left for protracted periods in disinfecting solutions
179
7.Disinfection • Polysulfides and silicones--Glutaraldehydes,
chlorine compounds ,iodophors,phenolics• Polyether- chlorine compounds or iodophors
180
8.Wettability and hydrophilization
• Silicones-most hydrophobic• Polyethers-hydrophillic• Spray surfactant on hydrophobic impressions
prior to pouring with gypsum• Non-ionic surfactant added during
manufacturing
181
• Hydrophyllized PVS- ether group –hydrophillic and oriented towards the surface when the surfactant migrates by diffusion to the surface region
• Depends on the contact angle made by water droplets with the impression surface and surface energy
182
• As soon as hydrophillized PVS encounters oral fluids during impression making, begins to release surfactant to its surroundings
• Adequate amount of surfactant trapped on the surface of set PVS impressions ,thus providing hydrophilicity for pouring of gypsum dies
183
9.Biocompatibility
• Tests covered in ISO 10993-5,Biological Evaluation of Medical Devices:Tests for in-vitro cytotoxicity
• Polysulfide – lowest cell death count• Polyether –highest cell toxicity scores
184
• Elastomer induced biocomaptibility problem– fragment of impression material trapped in patient’s gingival sulcus
• Causes severe gingival imflammation• Can also occur in 2nd stage implant surgery• Contact dermatitis from Polyether in dentists
or dental technicians
185
10. Shelf life
• Don’t deteriorate appreciably in tube or container before the expiration date when stored in dry , cool environment
• Clear liquid expressed along material-plasticizer segregation
• Indicates manufacturing error or excessive temperature extremes during storage
187
Common failures occurring with use of elastomeric impression materials
Type of failure Causes
Rough or uneven impression surface Incomplete polymerization,improper ratio or mixing of components,presence of oil or plaque on teeth
Surface agents (latex for PVS) inhibit polymerization
Too rapid polymerization from high humidity or temp
Excessively high accelerator /base ratio(cond silicone)
Bubbles Air incorporated during mixing
Irregularly shaped voids Moisture debris on teeth surfaces
Rough or chalky stone casts Inadequate cleaning of impression
Excess water not blown off impression
Excess wetting agent left on impression
Premature removal of cast,improper w/p ratio of stoneFailure to delay pour(PVS) that doesn’t contain Palladium salt for 20 min
188
Distortion Resin tray not aged sufficiently , still undergoing polymerisation shrinkage
Lack of adhesion of elastomer to tray
Excessive bulk of material
Lack of mechanical retention to the impression tray
Excessive bulk of material
Insufficient relief for the reline material
Development of elastic properties in material before tray is fully seated
Continued pressure against impression material that developed elastic properties
Movement of tray during polymerization
Premature/improper removal from mouth
Delayed pouring of polysulfide or condensation silicone impression
189
Comparative properties of elastomeric impression materials
Property Polysulfide Condensation silicone
Addition silicone
Polyether
WT(min) 4-7 2.5-4 2-4 3ST(min) 7-10 6-8 4-6.5 6
Tear strength(N/m) 2500-7000 2300- 2600 1500-4300 1800-4800
Percent contraction(at 24h) 0.40-0.45 0.38-0.60 0.14-0.17 0.19-0.24
Contact angle(°) 82 98 98/53 49
Hydrogen gas evolution N N Y N
Automatic mixing N N Y Y
Custom tray Y N N N
Unpleasant odour Y N N N
Multiple casts N N Y Y
Stiffness 3 2 2 1
Distortion on removal 1 2 4 3