impression materials Flashcards
what is the function of an impression
- produce a negative replica of the surface and shape of hard and soft oral tissues
- negative replica of tissues
- accuracy depends on material and technique
what is dental stone used for
- gypsum
- used to fill the impression and produce a positive replica
- it is poured into the impression
what does the dental stone represent
- the shape, position, size and orientation of each tooth
what is dental stone used for
- for evaluating the dentition when orthodontic, occlusal or other problems involved
- in laboratory fabrication of restorations and prosthesis = lab needs study cast to produce RPD framework
what can impression materials be for
- single tooth
- whole dentition
- edentulous area/ mouth
what do impression material require
- good properties
- clinician with good technique
what are the classifications of impression materials
- clinical = mucostatic, mucocompressive
- properties = elastic, non-elastic
what are mucostatic materials
- zinc oxide eugenol, low viscosity alginates
- fluid materials that displace soft tissues slightly
- give an impression of the undisplaced mucosa
what are mucocompressive materials
- impression compound, high viscosity alginates/elastomers
- viscous material that record an impression of the mucosa under load
- give impression of displaced soft tissue
why do you want a material to be truly elastic
- because material has to strain/change to get over bulbosity of tooth during removal and you want it to be able to fully recover afterwards
- no impression material is truly elastic though and there will always be permanent deformation from removal
what is the ideal elastic behaviour
- needs to be able to snap back to original shape after going under strain
- needs to react in an elastic manner
- would return to 0% strain if truly elastic
what is viscoelastic
- no materials are truly elastic, they are all viscoelastic
- lower viscoelasticity = more truly elastic
- there is a permanent strain, deformed, doesn’t recover to original dimensions
- doesn’t truly reflect patients dentition
how can you reduce the permanent strain/ deformation on impression
- if load time is less
- impression removed with a sharp pull
what are the two types of elastic materials
- hydrocolloids
- elastomers
what are types of hydrocolloids
- agar = reversible (not used)
- alginate = irreversible (used in DH)
what are some types of elastomers
- polysulphides
- polyethers
- silicones (conventional and addition cured)
- even these aren’t truly elastic
why aren’t non-elastic materials used anymore
- does not recover well at al
- can get worse deformation or even fracturing
what are the stages in impression process
- preparation
- insertion
- setting
- removal
- storage
- cast preparation
what is involved in the preparation stage of impression process
- mixing = 2 pastes, paste and liquid, powder and liquid
- chemical reaction
- heat
- material flows and takes up shape/space available
what needs to be considered during insertion phase of impression
- tray type needed
- do you need adhesive
what occurs during setting phase of impression
- chemical
- polymerisation
- cooling
- forms solid replica
how long should the material be stored
- storing can have significant dimensional changes that may even mean having to redo the impression
what are the ideal properties in terms of accuracy of an impression material
- flow = needs to flow over dentition to make good contact to get good detail
- setting changes = want no dimensional change
- removal = want no effect on oral tissues and no dimensional change (can be difficult due to change in temp out of mouth)
- storage = no dimensional change
- decontamination = dimensions and surfaces unaltered
- compatible with cast material = gypsum
what are the properties that can affect accuracy of impression
- viscosity = determines ability to flow over dentition
- setting mechanism = does it cause dimensional contraction
- thermal expansion coefficient = ideally low to cope with change
- hydrophobic/hydrophilic = affects surface contact, does saliva need removed before impression
- elasticity = elastic/viscoelastic behaviour
- tear strength = withstand tearing when removed especially under undercuts as most fragile
what are ideal properties of impression material in terms of patient comfort
- non toxic, non irritant
- acceptable taste and smell
- short setting time
- removable without damage to oral tissues
what are ideal properties of impression material in terms of operator convenience
- quick, simple technique
- convenient working/setting time
- must be able to be decontaminated
- cost (inexpensive) is a consideration but not a factor
what are some non elastic impression materials
- impression compound
- impression paste
- not used anymore
what is a colloid
- a 2 phase system of fine particles (1-200nm) of one phase dispersed in another phase
- if dispersing medium is water then it is a hydrocolloid
what are the states a hydrocolloid can be in
- SOL = viscous liquid state
- GEL = jelly like consistency, agglomeration of particles, entangled framework of solid particles enclosing liquid by capillary forces
how does hydrocolloids become when they start to set
- more sturdy and rigid
what is an irreversible hydrocolloid
- alginate
what is the composition of alginate
- salt of alginic acid (Na alginate) = 12%, reacts with Ca ions
- calcium sulphate =12%, provides Ca ions
- trisodium phosphate = 2%
- filler = 70%, cohesion and strength
- modifiers, flavourings, chemical indicators = low %, improve surface, taste, ph colour change
what does trisodium phosphate do
- delays gel formation
- Ca ions react with this and then only the excess Ca react with Na alginate
- main functions to causes things to happen
what is the setting reaction of alginate
2NanAlg + nCaSO4 –> nNa2SO4 + CanAlg
- long cross-linking fibril entangling undissolved particles
- sodium alginate –> calcium alginate
what does cross linking do
- makes the material more viscous - forms a gel
what is alginate delay
- trisodium phosphate preferentially react with Ca ions in CaSO4
2Na3PO4 + 3CaSO4 –> Ca3(PO4)2 + 3Na2SO4
- then 2NanAlg reacts with Ca ions
what must you do during alginate manipulation
- use correct powder/liquid ratio
- use water 18-24oC
- use perforated tray and adhesive
- remove impression with a sharp pull
where will the impression material set the fastest
- warmer temperatures
- where the material sits on the soft tissues is where it will set the fastest
why should you leave the impression in the mouth a few minutes longer than you think
- cross linking continues after apparent set
- even when it seems set on the outer surface, cross linking is still going on
- will get greater elastic properties if wait further 2 minutes before removal
how can you reduce tearing and distortion of alginate
- by using bigger bulk of materials
- use at leat 5mm layer
what are the properties of alginate in terms of patient comfort
- non toxic, non irritant
- acceptable taste and smell
- setting time is OK
what are the properties of alginate in terms of operator convenience
- relatively easy to work with
- setting time is OK
- storage = avoid moisture, prepare cast ASAP
why must you avoid moisture and prepare the cast ASAP with alginate impression
- avoid dimensional change
- typically you place a damp towel or gauze over impression but alginate can release or absorb water so don’t know how damp a tissue to use
- syneris =release of water
- inhibitor = uptake of water
- so want to prepare cast quickly to prevent impression distorting from releasing or up taking water
what are the properties of alginate in terms of accuracy
- flow = can get fine detail OK
- setting changes = OK
- movement of patient = causes internal stresses and distortion, takes 5 mins t set roughly
- nearly elastic = not 100% elastic recovery
- poor tear strength = avoid deep undercuts
- storage = syneris or inhibition may affect dimensions/accuracy
what material should be used for deep undercuts
- elastomeric materials
