impression materials Flashcards
What are impression materials?
materials used to record accurately the shape and dimensions of the oral tissues
How is a positive reproduction made?
pouring mixed plaster or stone into the impression and allowing it to harden
What is a model?
positive reproduction when large areas of the oral tissues are involved
What is a die?
positive reproduction of a single tooth
What are the requirements for an ideal impression material?
- safe (not toxic or irritant)
- acceptable to patient (good odour, colour and taste)
- sufficiently fluid to reproduce all fine details
- suitable setting time
- sufficient mechanical properties (high tear strength, doesn’t permanently deform during removal from the undercut areas of the mouth
- dimensionally stable to allow at least enough time to pour plaster or stone
- compatibility with die and model materials (no need for a separating medium that obliterate the recorded fine details)
- long shelf life (no deterioration in the dental office)
- easy to manipulate and inexpensive
- disinfected without affecting its accuracy or stability
What are the most important requirements for impression materials?
- accuracy: ability to record fine details
- stability: ability to maintain the recorded fine details
What are the factors that affect the accuracy and stability of the impression materials?
- sufficient fluidity to record all fine details
- minimal dimensional changes during setting
- adhesion to tray
- elastic on removal from undercuts
- minimal dimensional changes after removal from the mouth (during storage)
- compatible with the model and die materials (should not require a separating medium before pouring which obliterate fine details)
What is the classification of impression materials according to their behaviour after setting?
- non elastic
- elastic
What are non-elastic (rigid) impression materials?
- either fracture or not recovered on removal from undercuts
- plastic impression (not used anymore)
- impression compound (not used anymore)
- zinc oxide and eugenol (widely used)
What happens to plaster impression on removal from undercuts?
fractures
What happens to impression compound and zinc oxide and eugenol impression on removal from undercuts?
deformation
When do we use rigid impression materials?
- absence of undercuts
- edentulous patients
How is zinc oxide-eugenol supplied?
as 2 pastes
What is the composition of zinc oxide-eugenol?
base paste
- zinc oxide (powder)
- inert oil to make paste (liquid)
catalyst paste
- eugenol (liquid in oil of cloves)
- silica or talc powder to make paste
- MgCl or zinc acetate (accelerators)
- gum resin to give body, coherence and thermoplastic properties of the set mass for easy removal from the cast by softening it in hot water (to help adhesion to the tray)
What is the colour of the catalyst paste in zinc oxide-eugenol?
brown
What is the colour of the base paste in zinc oxide-eugenol?
white
What is the source of the characteristic smell found in dental offices?
eugenol
Why are there alternatives to zinc oxide-eugenol (eugenol free zinc oxide impression material)?
some patients find eugenol irritating to the soft tissues
What is the setting reaction of zinc oxide-eugenol?
- zinc oxide + eugenol –> (in presence of water) zinc eugenolate matric + unreacted zinc oxide
- results from chetation reaction
What does the set material of zinc oxide-eugenol consist of?
- matrix of amorphous zinc eugenolate which holds the unreacted zinc oxide particles together
- the manufacturer incorporates some percent of water which is necessary for the material to set
How is zinc oxide-eugenol manipulated?
- equal lengths of base and catalyst pastes are mixed on an oil resistant paper pad until a homogenous colour is obtained
- mix is then applied in the prepared tray for impression making
- if the mixed impression paste acts too slowly, setting may be accelerated by incorporating of a drop of water into the paste
- a drop of glycerine will retard the setting time
What helps the cleaning process of zinc oxide-eugenol an easier process?
using light coating of Vaseline on patient’s lip before making the impression
What are the properties of zinc oxide-eugenol?
- fluidity: ZnOE are sufficiently fluid to record all fine details
- dimensional changes during setting: very little shrinkage of only 0.1% accompanies the setting
- adhesion: non-elastic impression material and permanently deformed on removal from undercut areas (the only disadvantage)
- dimensional changes during storage: the only impression material which is stable (the main advantage) however because it contains gum resin, high temperature may cause its distortion
- no separating medium is needed before the stone is poured however after the stone has set, it can be separated from the impression by immersion in hot water at 60 degrees for 10 minutes
What is colloid?
- glue-like physical character
- heterogenous mixture of 2 phases however the 2 phases are not easily differentiated
What does the colloidal state represent?
dispersed phase of fine particles within dispersion medium between a solution and a suspension
What is a solution?
homogenous mixture consisting of one phase
What is a suspension?
heterogenous mixture of 2 phases
What are the 2 forms that hydrocolloids can only exist as?
- sol state (viscous liquid consistency)
- gel state (jelly-like consistency)
What are the particles like in the gel state?
- there is an entanglement network of solid particles with the water trapped in the spaces
the solid particles are in the form of fibrils or chains of molecules
Are hydrocolloid materials reversible?
yes
Why is alginate the most widely used impression materials all over the world?
- needs minimum requirements therefore easily manipulate
- high flexibility of set material (elastic record undercut areas)
- high accuracy if properly handled
- low cost
What are elastic impression materials?
- recovered after removal from undercuts
- hydrocolloid
- rubbers or elastomers
What are hydrocolloid impression materials?
- agar agar (reversible)
- alginate (irreversible)
What are rubber impression materials?
- polysulfide
- silicone (condensation)
- silicone (addition)
- polyether
What are th main disadvantages of alginate?
- suffer from synersis and imbibition
- low tear strength
- can’t be electroplated
What is alginate based from and derived from?
- based on alginic acid
- derived from marine plants
What is the presentation of alginate?
suppled as powder in bulk or in divided container to be mixed with water
What is the composition of alginate and their function?
- sodium alginate: main ingredient but is dissolved in water
- calcium sulfate: to react with soluble sodium alginate producing insoluble calcium alginate (setting time is very short)
- sodium phosphate: to react preferentially with calcium sulfate and thus acts as a retarder (increases setting time)
- potassium sulfate: to counteract inhibiting effect o alginate on setting of gypsum model and die materials
- glycol: to coat powder particles which minimise dust during hand mixing
- chlorohexidine: to provide self disinfection against microbes
- colouring and flavouring agents: to be acceptable to patient
- chemical indicator: to indicate different manipulation steps
How does sol turn to gel in alginate?
when mixed with water a soft mass is formed which becomes an irreversible gel after few minutes of mixing
What are the setting reactions of alginate?
- retardation reaction to prolong working time (calcium sulfate + sodium phosphate –> calcium phosphate + sodium sulfate) (in presence of water)
- gelation reaction (calcium sulfate + sodium alginate –> calcium alginate + sodium sulfate) (in presence of water)
What can shorten working ad setting time of alginate?
increasing temperature of water used to prepare the mix
The proportion of the powder and water increase time required to change into gel inside the patient’s mouth when the mix is…?
thinner
What are the properties of alginate?
- fluidity: alginate in the sol state is sufficiently fluid to record all fine details
-dimensional changes during gelation: no changes or negligible shrinkage takes place during gelation - elasticity: viscoelastic (elastic recovery is about 97.3%)
- dimensional changes during storage: water evaporates if the impression is stored in air and hence the impression shrinks (syneresis) and if it is placed in water is absorbs water and swell (imbibition) therefore storage in either air or water results in serious changes in dimensions and a loss of impression accuracy
- need perforated trays (adhesive tray))
- no separating medium is needed because of their compatibility with model and die materials
How should alginate be stored?
- 100% relative humidity as it results in te least dimensional changes
- it is better if the impression is poured immediately but if that is not possible then the impression should be kept at relative humidity of 100% by wrapping it in a wet towel for not more than half an hour
How can alginate impressions be disinfected?
- by soaking in 5% sodium hypochlorite for 10 minutes or 2% diluted glutaraldehyde
What is the water/powder ratio of alginate manipulation?
- alginate powder should be shaken well before use (because ingredients might not be mixed together and some of it precipitates)
- the water/powder ratio is according to the manufacturer instructions
- water used is at room temperature
What is the mixing method of alginate manipulation?
- powder added to water in the rubber bowl and mixed with a stirring action to wet the powder
- after the powder has been moistened, the alginate should be mixed with a vigorous action that squeezes the material between the blade of the spatula and the side of the rubber bowl leading to a mix of smooth creamy consistency
What is the loading step of alginate manipulation?
- loading the mx=ix on a perforated tray
- should be done very quickly since the material will set 2 minutes from the time of loading a tray
How to make the impression during alginate manipulation?
- posterior portion if the tray is usually seated first and then the anterior portion and held gently in place until gelation
- it is advisable to allow the impression to remain in the mouth for 2 additional minutes because physical properties improve sharply during this period (tear strength increased and permanent deformation decreased)
How do remove the impression during alginate manipulation?
- impression and tray should be removed with a single firm motion (sharp-snap removal)
- the rapid removal, the rapidly alginates deforms, the higher the tear strength and the lower the permanent deformation they have (more accuracy)
- the probability that the tearing will occur in a thin secretion is reduced by rapid removal of the impression
Is there storage period for pouring on alginate impression?
no, immediate pouring is the best procedure
How is the impression poured during alginate manipulation?
- impression should be poured immediately to avoid syneresis and imbibition
- impression must first be rinsed and disinfected after removal from the patient’s mouth to remove any saliva as this will interfere with the setting of the gypsum model
- any surface water should be removed prior to pouring of model as residual water will dilute the model material and result in a soft surface which is easily damaged
- after pouring, alginate impression should not be left on plaster or stone set model as it becomes difficult to separate if allowed to dry
What are some notes about accuracy of hydrocolloids?
- sufficiently fluid to record fine details in the mouth
- during gelation, the first part of an agar impression to gel is that which is in contact with the tray since this is cooler than the tissues
- so material in contact with the tissues stays liquid for longer time and can flow to compensate for any inaccuracy due to dimensional changes
- on the contrary during setting of an alginate, the reaction is faster at higher temperatures and so the portion is contact with the tissues sets up stresses and so the portion in contact with the tissues sets up stresses within the material which will distort the alginate after its removal from the mouth
- need perforated trays
- sufficiently elastic to reproduce undercuts
- not dimensionally stable during storage after removal from patient’s mouth due to syneresis and imbibition
- no separating medium is needed because of their compatibility with model materials
What are the types of rubber base impression materials?
- polysulfide
- silicone (condensation and addition)
- polyether
When is rubber impression material used?
- when high degree of accuracy is needed
- implant works
- crown and bridge works
What are some characteristics of rubber base impression materials?
- high tear strength
- very expensive
- large elastic recovery
- can be electroformed
- adequate dimensional stability
What are rubbers?
polymers that are coiled
Which rubber material was first introduced in dentistry?
polysulfide rubber impression material
What is the polymerisation reaction of polysulfide?
cndensation
What is the presentation of polysulfide rubber?
- viscosity of base paste depends on the amount of filler present
- supplied in 3 consistencies
What are the different consistencies of rubber impression materials?
- heavy (used as tray)
- light (injection in syringe)
- intermediate (used alone with a special tray
- light and heavy bodies are used in combination
What is the base paste of polysulfide rubbers?
- 80% low MW polysulfide liquid polymer (bad odour) having reactive mercaptan group (-SH)
- 20% silica as filler to give required consistency to the paste
What is the activator paste for polysulfide rubber?
lead dioxide
What is the setting reaction of polysulfide rubber?
polysulfide (liquid polymer) + lead dioxide –> polysulfide rubber + H2O (condensation reaction)
Why was silicone rubber impression material made after polysulfide rubber was made?
- polysulfide disadvantages
- like their bad odour
- their stains in dental uniform by brown lead dioxide
- high effort required to mix base with accelerator in polysulfide and its long setting time
- high shrinkage on setting and permanent deformation
What are the types of silicone impression material?
- addition (vinyl)
- condensation
What is the presentation of condensation silicone?
- supplied in 2 pastes, base and accelerator
What is the base paste of condensation silicone?
- 80% low MW silicon liquid polymer having reactive (OH) group
- 20% silica as a filler t give the proper consistency to paste
What is the accelerator paste of condensation silicone?
tin octoate
What is the polymerisation reaction of condensation silicone?
tin octoate + silicone (liquid polymer) + ethyl silicate –> silicone (rubber) + ethyl alcohol (condensation reaction)
What is the most widely used rubber impression material in dental clinics all over the world?
addition silicon
What is the base of addition silicone?
- 80% low MW silicone liquid polymer having (vinyl) terminal group
- 20% filler as silica
What is the accelerator of addition silicone?
chloroplatinic acid
What is the setting reaction of addition silicone?
silicon (liquid polymer) + chloroplatinic acid –> silicon rubber (addition reaction)
What is the disadvantage of silicon impression material?
- hydrophobic and therefore not easily wetted by plaster or stone
- cannot wet oral tissues
- these effects affect the accuracy of resultant final restorations
What is the use of surfactant in rubber?
- material recently added by manufacturers to render rubber impression materials to be more hydrophilic to accurately wet the oral tissues and therefore record all fine details (advantage)
- hydrophilic silicones permit gypsum models and dies t be obtained with fewer air bubbles (with rough surface)
- however, new hydrophilic addition silicones are extremely stiff (disadvantage)
What are the 2 disadvantages of polyether rubber?
- short working time
- high stiffness
What are the advantages of polyether rubber?
- better mechanical properties than polysulfide
- less dimensional change than condensation silicon
- highly hydrophilic and therefore better wetting to soft tissues (fore details reproduction) as well as to wet gypsum products during pouring the model resulting in restorations with excellent surface details
What is the presentation of polyether rubber?
supplied in 2 pastes, base and catalyst
What is the base of polyether rubber?
- 80% low MW polyether liquid polymer containing (ethylene imine) terminal group
- 20% filler as silica
What is the catalyst of polyether rubber?
sulfonic acid ester
What is the setting reaction of polyether rubber?
polyether (liquid polymer) + sulfonic acid ester –> polyether rubber (addition reaction)
What is the fluidity of rubber impression materials?
depends on consistency
What is the dimensional changes during polymerisation of rubber impression materials (after 24 hours)?
- addition silicone has the least shrinkage (0.05%)
- polyether (0.1%)
- polysulfide (0.3%)
- condensation silicone has the most shrinkage (0.5%
What causes thermal contraction of rubber impression material and how to reduce it?
- condensation occurring on cooling rubber impressions from mouth to room temperature causes thermal contraction
- it can be reduced by adhesion of impression material to tray if tray is painted with an adhesive material
What is the adhesion of rubber impression materials?
non-perforated tray painted with an adhesive should be used to assure retention of impression to acrylic tray
What happens if a perforated tray is used for rubber impression material?
distortion of impression will take place as there is different shrinkage results due to uneven thickness in perforated tray
What is the elasticity of rubber impression materials?
- sufficiently elastic to be withdrawn over undercuts areas
Why do rubbers not show 100% elastic recovery?
- silicone (condensation and addition) has the best elastic recovery (99.5%)
- polyether (98.9%)
- polysulfide (least elastic recovery 98%)
What are the dimensional changes on storage of rubber impression materials?
- small contraction can occur due to further polymerisation shrinkage and evaporation of volatile constituents (if condensation types are used)
- should be poured 30-60 minutes after removal in order to allow highest elastic recovery and allow least polymerisation shrinkage
What is the cast compatibility of rubber impression materials?
compatible with gypsum products (no need for a separating medium)
What is the thickness of rubber impression materials?
2-3mm thickness to decrease its polymerisation shrinkage without possibility of rupture because of its high tear strength
What is the flexibility of rubber impression materials?
- polysulfide is the most flexible of all rubbers
- this allows easy removal of impression from undercut areas as well as easy removal from patient mouth
- followed by silicones while polyether is the least flexible one (stiff)
What can be a problem for polyethers during impression taking?
difficult to be removed from mouth with high possibility of removal of teeth if they were very loose
What are nano-filled rubber impression materials?
uses silica fillers in a nano size to have better flow and hence better details reproduction
What are some recently developed rubber impressions?
- nano-filled rubber impression
- novel hybrid silicon polyether impression
Why were novel hybrid silicon-polyether impression materials developed?
an attempt to combine high dimensional stability, high elastic recovery and high flexibility of addition silicone together with the high hydrophilicity of polyether
