Impression Materials Flashcards
What are impression materials and how are they used?
Impression materials are materials designed to take the “negative” shape of teeth materials and surrounding structures when the impression material is in a more plastic unset condition and then it hardens and is used as a mold for a positive replication of tooth structure in die or another suitable model.
What are the key properties of impression materials?
Flow = ability to replicate intraoral surface details
Dimensional stability = Ability to retain absolute dimensional size over time
Tear resistance = Ability to resist tearing in thin sections (such as through the feather edged material within the gingival sulcus)
What are the desirable properties for impression materials?
Adequate shelf life
Acceptable odor and taste
Biologically acceptable: No toxic or irritant constituents
Easily manipulated: Easy to use with the minimum of equipment
Setting characteristics suitable for the purpose
Good set material properties
Can handle addition and correction
Satisfactory consistency and texture
Readily disinfected without loss of accuracy
Compatiblity with cast and die materials
Economically commensurate with the results obtained
What are the setting characteristics that meet clinical requirements?
Suitable working time
Suitable setting time
Readily wets oral tissues
Not affected by oral fluids
Does no release gas during setting
What are the characteristics of set material that make them ideal for clinic?
Sufficient mechanical strength to handle resistance to tear on removal from the mouth
Elastic properties with no permanent deformation after strain
Good dimensional accuracy
Dimensional stability over temperature and humidity ranges met in clinic and lab
Can any impression material fulfill all the desirable characteristics?
No, selection of material should be best suited for a particular clinical situation. The technique depends on the dentist.
What factors affect the accuracy of impression materials?
Must be in fluid or plastic state on insertion
Dimensional changes on setting are negligible
Must be elastic on removal from the mouth
Must adhere to the tray
Must have dimensional stability with storage
Must be compatible with model and dies
How are impression materials classified by method of setting?
Hardening by chemical reaction (eg alginates)
Softening and hardening by heat (eg impression compounds)
How are impression materials classified by behaviour after setting?
Non-elastic impression materials: Used with no undercuts exist (eg plaster of paris)
Elastic impression materials: Can be used when undercuts exist. (eg hydrocolloids)
How are impression materials classified?
By method of setting
Behavior after setting
According to viscosity
How are impression materials classified by viscosity?
Mucodisplasive impression materials (eg high viscosity alginates, impression compound)
Mucostatic impression materials (eg Plaster)
What are the types of non-elastic impression materials?
Wax
Impression plaster (Plaster of Paris)
Impression compound
Zinc oxide-Eugenol impression material
What limits the use of wax for occlusal records?
It is distorted upon removal
Release of internal stresses
High flow properties
Large dimensional changes upon cooling
What is the application for plaster of paris?
Application: Used to make impression for edentulous patients
How is Plaster of Paris mixed?
Powder and water are mixed
What is the composition of plaster of paris?
CaSO4.1/2 H2O
Modifiers: Chemicals used to control setting time and setting expansion.
4% K2SO4
1% borax
Natural gums
Flavoring agents
Colouring agents
Potato starch
What are the types of elastic impression materials?
Hydrocolloid: Alginate, agar, and agar/alginate
Rubber base: Polysulfide, polyether, and silicone (Condensation polyer and addition polymer)
How is gypsum produced?
By heating the plaster to 110 - 130 degrees
What is formed from plaster if it is heated?
Gypsum = 110 - 130 degrees celsius
Platic or stone = 110 - 130 degrees celsius
Hexagonal anhydrite = 130 - 200 degrees celsius
Orthorhombic anhydrite = 200 - 1000 degrees celsius
What is the purpose of using chemical modifiers in plaster of paris?
They control the setting time and setting expansion
What are the types of modifiers used in plaster of paris?
K2SO4 (4%)
Borax (1%)
Natural gums
Flavouring agents
Colouring agents
Potato starch
What is the setting reaction for plaster of paris?
CaSO4.1/2H2O + 1.5 H2O -> CaSO4.2H2O + heat
What are the advantages to using plaster of paris?
Accuracy:
High flowy
Minimum linear setting expansion (+0.06%)
It fractures into clean clear cut fragments so it can be reassembled with great accuracy
Dimensional stability
It is easily manipulated, cheap and has a good shelf life
What are the disadvantages to using plaster of paris impression material?
Its taste and consistency are usually unpleasant to the patient and dehydrates the soft tissues
Does not accept additions or corrections
Not elastic
A separating medium should be used
Cannot be electroplated
What is impression compound?
It is one of the oldest impression materials in dentistry
It is a thermoplastic material
What is the application of impression compound?
Impression for edentulous patients
As an impression tray
Peripheral seal material
Single impression with copper band
How is impression compound presentation?
In the form of:
Sheets
Sticks
Cones
What is the composition of impression compound?
40% resins
7% waxes
Plasticizers (3% organic acids)
50% fillers: Talc powder, chalk or iron oxide
Colouring agents
What are the classifications of impression compounds?
Type 1 “lower fusing materials”
Type 2 “Higher fusing materials”
How is impression compound manipulated and how does it set?
Impression compounds are thermoplastic materials
They are hard at 37 degrees and are soft at 45 degrees. Cooling must continue until the entire mass is rigid to reduce plastic flow on removal from the mouth.
What is the accuracy like for impression compound?
Dimensional accuracy: They are not sufficiently fluid to record all the fine details.
Dimensional stability: They have a high coefficient of thermal expansion. Cooling shrinkage is 0.3 - 0.4%
How can the poor dimensional stability be overcome?
By making a secondary impression or spray cold water on the metal tray.
What are the disadvantages of using impression compound?
Not very fluid
High coefficient of thermal expansion meaning they experience cooling shrinkage
The material is non-elastic, most serious drawback of this material
Internal stresses are relieved leading to warpage or distortion. (can also be caused by premature removal before it hardens and cools)
Poor thermal conductivity
What are the advantages to using impression compound?
Accepts addition and correction
Does not need a separating medium
Non-toxic and non-irritant
Hardens in a reasonable time in the mouth
Has adequate shelf life
Can be electroplated with copper
How is zinc oxide-eugenol impression material applied?
Full mouth edentulous impressions
Largely replaced by light bodied rubber impression materials
How is zinc oxide-eugenol impression material presented?
Soft or hard set types usually in the form of 2 pastes in collapsible tubes
What is the composition of zinc oxide-eugenol impression material?
One paste is called the base paste contains:
Zinc oxide 80% (powder)
Inert oil 15% (forms a paste)
Hydrogenated resin
The other paste is called the accelerator and contains:
Eugenol or oil of cloves (12 - 15%)
Filler talc powder
Gum rosin and oils (65%)
MgCl2 or zinc acetate
Inert oils
Moisture
What is the setting mechanism for zinc oxide-eugenol?
Chelation reaction:
ZnO + eugenol undergoes a chelation reaction to form zinc eugenolate + ZnO
How is the setting time of zinc oxide-eugenol controlled?
Setting time can be accelerated by:
Heat
Ethyl alcohol
High humidity
Setting time can be retarded by glycerin
What are the disadvantages of using zinc-oxide eugenol?
Must have adequate adhesion to the tray
Eugenol can be irritant
Eugenol can adhere to tissues
It cannot be electroplated
What are the advantages of zinc oxide-eugenol?
Has satisfactory working and setting time
Has satisfactory shelf life
What are the types of elastic impression materials?
Hydrocolloids (Such as agar and alginate)
How do hydrocolloids set? (setting reaction)
Via a sol - gel transformation:
Sol -> Agglomeration of molecules of dispersed phase -> Fibrils (micelles) -> Network pattern enclosing dispersion phase particles by adhesion/capillary attraction
What is the difference between alginate and agar impression material?
Reduction in temperature -> Agar impression material
Chemical reaction -> Alginate impression material
What affects dimensional stability of hydrocolloids?
Syneresis (the contraction of a gel accompanied by the separating out of liquid) -> Loss of water -> Shrinkage
Imbibition -> Absorption of water -> Expansion
What are the advantages of using alginate?
Ease of mixing and manipulation
Minimum equipment
Flexible
Accuracy
Low cost
What are the disadvantages of alginate impression material?
Restrict the model and die material to only gypsum
Not accurate in reproducing fine details
What is alginate used for?
Preparing study cast
Preparing gypsum models
Removable partial denture
Athletic mouth protector
NOT RECOMMENDED FOR IMPRESSION OF CAVITY PREPARATIONS
What is the composition of alginate impression material?
Sodium potassium alginate
Calcium sulfate
Sodium sulfate
Diatomaceous earth
Potassium sulfate or fluoride
Glycol (in dustless alginates)
What is the setting reaction for sodium alginate?
Water + powder mixing plastic mass
Retarding Reaction (gives use working time): Na2PO4 + CaSO4 -> Na2SO4 + CaPO4
Setting reaction: Sodium alginate + CaSO4 -> Ca++alginate + Na2SO4
THIS REACTION IS IRREVERSIBLE
What are the properties of alginate impression material?
Permanent deformation <3% (ADA specification)
Strength: Compressive and tear strength are affected by W/P ratio, proper spatulation, rate of deformation, and thickness of impression material.
Storage: Should be stored in 100% relative humidity to avoid syneresis/imbibition and thus avoid shrinkage and expansion.
What equipment is required for alginate material manipulation?
Equipment needed includes:
Powder-dispensing cup
Water-dispensing cup
Rubber mixing bowl
A spatula
What are the steps in alginate material manipulation?
Dispensing
Mixing
Loading the tray
Making the impression
Removal of the impression
Pouring the impression
Storage
What are the advantages to using agar?
It is highly accurate
Elastic
What are the disadvantages to using agar?
Difficult to manipulate
No method for obtaining model die
Dimensionally unstable
Patient discomfort of thermal shock
What is the composition of agar impression materials?
Agar 12.5%
Potassium sulfate: 1.7%
Borax 0.2%
Alkyl benzoate 0.1%
Water 85.5%
Colour and flavours (trace)
What are the types of agar material and how are they different?
Tray type which is stiffer
Syringe type which has less agar
What is the setting reaction for agar?
Sol is cooled to form the gel and heated to form sol. (37 degrees = setting temp; 45 degrees = sol)
What is the elastic recovery requirement for agar impression material?
ANSI/ADA specification requires elasstic recovery greater than 96.5%. it depends on time under compression and rate of loading.
How flexibly is agar impression material?
Flexibility requirement by ANSI/ADA is 4 - 5%
What is the strength of agar impression material?
- 8MPa for compressive strength
0. 8 - 0.9 N/mm tear strength
How is dimensional stability maintained in agar?
Storage should be in 100% relative humidity to avoid synersis / imbibition which in turn avoid shrinkage / expansion. Immediate pouring the cast is recommended
How is agar manipulated in the tray?
Material liquefaction takes place in boiling water for 8 to 12 minutes
Tube is then immersed in water at 43 degrees celsius to 49 degrees celsius and manipulated to ensure even cooling
The tray is filled and then tempered for a minimum of 2 minutes in water at 46+/- 1 degrees celsius
How is the syringe manipulated for agar use?
The syringe, loaded with a cylinder, is placed in boiling water for 10 minutes
No tempering is required before use
The agar is then injected directly into the tooth preparation
After removal, the impression is rinsed, disinfected, superficially dried and then poured
How is agar manipulated for impression taking?
It is heated in water at 100 degrees celsius for 8 - 12 minutes
Stored in water at 65 degrees celsius
Placed in the tray at 65 degrees celsius
Tempered in 45 degrees in water for 2 - 3 minutes
Impression is taken and cooler (20 degree) water is circulated through the impression mould until gelation takes place
What is the laminate technique?
It is a combined alginate/agar technique where the hydrocolloid in the tray is replaced with a mix of chilled alginate that bonds with the agar from the syringe and the agar sets in the cool alginate rather than the water circulating through the tray.
What are the advantages of the laminate technique?
Maximum details are reproduced
Lower equipment cost and less preparation time
Most cost effective way of producing an impression with adequate details
What are the disadvantages of the laminate technique?
The bond between agar and alginate is not always sound
The alginate has a higher viscosity displacing the agar hydrocolloid during setting
The dimensional inaccuracy of the alginate hydrocolloid limits its use to single units
Additional applications of using hydrocolloids:
Duplicating materials
Construction of prosthetic appliances and orthodontic models
How is the hydrocolloid material of duplicating material different to that of impression material?
It has a higher water content
Agar and alginate content is lower which affects compressive strength and percent permanent set.
What are the types of modified alginates?
Traditional = 2 component system consisting of water and a powder
Recent = 2 component system consisting of alginate sol and Ca++ reactor
What is the biocompatibility of hydrocolloids like?
No known chemical or allergic reactions associated with hydrocolloid impressions
Most likely side effect is thermal injury from reversible agar hydrocolloid as a result of improper tempering or faulty equipment during impression
Inhalation of fine airborne particles from alginate material can cause silicosis and pulmonary hypersensitivity. Dustless alginate is preferred
How are hydrocolloids disinfected?
Since the hydrocolloid impression must be poured within a short time after removal from the mouth disinfection should be relatively rapid to prevent dimensional change
Most manufacturers recommend disinfectants such as iodophor, bleach or glutaraldehyde
Are all hydrocolloids compatible with gypsum?
No, agar hydrocolloid materials are more compatible with gypsum than alginates.
Impression should be washed of saliva and any traces of blood will retard the setting of gypsum.
If the agar impression was stored it should be rinsed with cool water to remove any exudate formed from syneresis from puring up the gypsum model
What are the effects that could arise from mishandling hydrocolloids?
Grainy material
Separation of tray and syringe materials
External bubbles
Irregularly shaped voids
Rough or chalky stone model
Distortion
What are rubber base impression materials?
Non aqueous elastomeric impression materials, that are rubber-like, and are made-up from long chain coiled molecules
What are the advantages to elastomers?
High accuracy
More dimensional stability than agar and alginate
Ease of manipulation
Can be used with undercuts
Can be electroplated
What are the consistencies of elastomeric impressions?
Low (Syringe/wash)
Medium (regular)
High (tray)
Extra high (Putty)
How can rubber based impressions be mixed?
3 types of mixing:
Hand mixing
Auto-mixing
Dynamic mechanical mixing
What are the types of elastomeric impression techniques?
Simultaneous dual viscosity technique
Single viscosity (mono-phase technique)
Putty wash technique
What are the types of elastomeric impression materials?
Polysulfides
Silicone (condensation and addition polymers)
Polyether
What is the composition of polysulfide rubber impression material?
2 pastes: A base and a catalyst.
Base consists of 80% low molecular weight organic polymer with reactive mercaptan group and 20% reinforcing agents such as titanium dioxide, zinc sulfate, copper carbonate and silica
Catalyst consists of lead dioxide and copper hydroxide
What is the setting reaction for polysulfide rubber impression material?
Mercaptan + lead dioxide -> Polysulfide + H2O
THIS REACTION IS SENSITIVE TO MOISTURE AND TEMPERATURE
What is the working time for polysulfide rubber impression material?
5 - 7 minutes final setting time is 8 - 12 minutes
What is the permanent deformation rate of polysulfide rubber impression material?
2 - 3%
What is the dimensional accuracy and stability of polysulfide rubber impression material like?
Dimensional accuracy: Highly accurate
Dimensional stability: Polymerization shrinkage of 0.25% takes place within the 1st 24 hours
What is the flow of polysulfide rubber based impression material?
An average flow of 0.3 - 0.9% occurs within the 1st 15 minutes
What is the flexibility, elasticity, and tear resistance of polysulfide rubber based impression material like?
Most flexible of all rubbers
Viscoelastic with elastic recovery of 98%
It is also highly tear resistant
How compatible is polysulfide rubber based impression material with die materials?
Compatible
It can also be disinfected by most disinfectants
What are the advantages of polysulfide rubber impression material?
High dimensional stability
High strength and resistance to tear
Can be electroplated
What are the disadvantages of polysulfide rubber impression material?
Can be distorted
Hydrophobic
Bad odor (sulfide odor)
Staining of clothes
Relatively long setting time
What are condensation silicone rubber impression materials made of?
A base and a catalyst
Base: Paste of low molecular weight silicone liquid with reinforcing agents as silica.
Catalyst: Paste or liquid of tin organic ester (Tin octoate) and alkyl silicate.
What is the condensation setting reaction of silicone rubber impression materials?
Dimethyl siloxane + Ortho-ethyl silicate + tin octoate -> Silicone rubber + ethyl alcohol
EVAPORATION OF ETHYL ALCOHOL LEADS TO INCREASE IN SHRINKAGE WITHIN THE 1ST 24 HOURS
What is the composition of addition silicone rubber impression material?
2 pastes:
1: Low molecular weight silicone with terminal vinyl group, chloroplaticic acid catalyst, and reinforcing fillers.
2: Paste or liquid of low molecular weight silicone with silane hydrogens. reinforcing fillers
What is the addition setting reaction of silicone rubber impression materials?
Silane - containing siloxane + vinyl terminal siloxane + Chloroplatinic acid -> Silicone rubber
What is the accuracy of addition silicone like?
Highly accurate
What is the flow like in addition silicone?
Low flow
Which is more dimensionally stable condensation or addition silicone?
Addition silicone
How is flexibility and elasticity different between silicone and polysulfide impression materials?
Silicone is less flexible and more elastic than polysulfides.
Is silicone compatible with die materials?
Silicone is highly compatible with die materials
What is tear strength like in silicone?
Relatively low but rapid rate of removal gives high tear strength
What are the advantages of silicone based rubber impression materials?
Highly accurate
Elastic
Campatible with most die materials
Can be electroplated
Can be disinfected by most disinfectants
What is the composition of polyether impression material?
Present only in medium consistency as base and catalyst:
Base contains moderately low molecular weight polyether with ethyline imime terminal group
Catalyst contains aromatic sulfonic acid ester
What is the setting reaction for polyether impression materials?
Polyether with ethylene imine groups + sulfonic acid ester -> Cross linked high molecular weight polyether rubber
What are the advantages of using polyether impression materials?
Highly accurate
Very compatible with die materials
Good tear strength
Can be disinfected by various antimicrobials
Better mechanical properties than polysulfides
Less dimensional changes than condensation silicone
What is the dimensional stability like in polyether impression material?
Polymerization shrinkage of 0.25% within 24 hours
What is the flexibility of polyether impression material like?
Least flexibility of all rubbers
What is the elasticity of polyether like?
Viscoelastic with elastic recovery 98.9%
What is the tear strength of polyether like?
Better than silicone less than polysulfide
What are the disadvantages to using polyether impression materials?
Increased working time
High stiffness
High cost
What are the most recent advances in impression materials?
Light curing impression materials (polyether urethane dimethacrylate impression material)
Addition silicone
Soft polyether impression material
Optical impression
Alginate substitute
Automatic mixing devices
What are the advantages and disadvantages to using light curing impression materials?
Adv:
Allows operator to work for a longer time
Extraordinary tear resistance compared to other impression materials
Strain in compression is higher than that of condensation silicone but lower than that of addition silicones
Disadv:
Expansion rate of light-curing material about 0.07% one day after curing whereas other impression materials have negative dimensional changes after 24 hours mixing.
The surface material that set against teeth and gums was tacky (moisture inhibits setting)
What are the features of addition silicone?
Most accurate of elastomeric impression materials
Supplied in different viscosities: Extra-light, light, medium, mono-phase, heave and putty
It is modification of original condensation silicones
It is hydrophilic impression material
What is the reaction for addition silicone?
Base material: Polymethyl hydrogen siloxane copolymer which is moderately low molecular mass polymer with silane terminal group
Accelerator material: Vinyl-terminated polydimethyl siloxane. On mixing addition reaction occurs between silane adn vinyl group
What are the benefits to using additional silicone?
Minimal dimensional change during polymerization reaction and no by-products
No longer necessary to wait 1 hour before pouring. (addition silicone has least viscoelastic qualities thus requiring least time for recovery from viscoelastic deformation)
Addition silicone is not susceptible to changes in humidity and doesn’t undergo any further chemical reactions or release byproducts
Low dimensional change on setting
High tear strength
Hydrophilic (excellent wetting of oral tissues and isn’t affected by moisture and doesn’t form bubbles or voids)
Ability to repour impression without loss of accuracy (opposite to polyether)
Very accurate with great detail reproduction
Flexible and elastic so easily removed from the mouth when set
Elastic recovery; impression returns to its original dimensions upon removal from undercut areas in the mouth without distortion.
Colourized thermo-chromic impression material can allow dentist to know when it has set
High tear strength
How are hydrogen gas bubbles removed from surface of addition silicone when pouring gypsum dies?
Palladium is added which absorbs hydrogens
Why is addition silicone able to recover elastically so well?
Due to highly cross-linked structure of addition silicone
What are the disadvantages of using addition silicone?
Setting time can be longer than other materials
Hydrogen gas released can lead to bubbles in gypsum
Expensive
Gloves containing sulfur inhibit setting
What are the advantages to using soft polyether impression material?
Excellent wettability
Good dimensional stability
Excellent detail reproduction without distortion
What are the disadvantages to using soft polyether impression material?
Impression cannot be repoured
Some products are rigid
Poor taste and colour
What are the advantages and disadvantages to an optical impression?
Advantages:
No need for impression material or trays
Impression can be transmitted and stored electronically
No mess
3D, 360 degree view of preparation that can be moved and turned every angle
Adjustments can be made to the virtual impression
No disinfection required
Disadvantages:
Expensive start up cost
Technique sensitive
More equipment needed
What is the alginate substitute?
Alginate modified by adding silicone polymers without hand mixing
What are the advantages and disadvantages to using alginate substitute?
Advantages:
Good hydrophilic properties
Shorter setting time
Easy removal from the mouth
Excellent detail reproduction, elastic recovery and dimensional stability can be stored and reused
Dust-free and compatible with gypsum die materials
Higher patient acceptance than traditional alginate
No hand mixing or powder/liquid mix
Disadvantages:
Much more expensive than traditional alginate.
The material sets very hard & can be a challenge to separate from the model
What are the advantages and disadvantages to automixing device?
Advantages:
Consistent, homogeneously mixed material.
Less waste dispenses, only the amount of material needed.
The mix obtained is free of voids.
Automatic mixing decrease the strain for dental assistants.
Allow the dentist to use quick setting material.
Higher viscosity materials extrude quickly.
Disadvantages:
Takes up counter space.
Not all impression materials are available in poly-bags.
