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BDS2 - dental material science > impression materials > Flashcards

impression materials Flashcards

1
Q

what is the function of an impression

A
  • 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
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2
Q

what is dental stone used for

A
  • gypsum
  • used to fill the impression and produce a positive replica
  • it is poured into the impression
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3
Q

what does the dental stone represent

A
  • the shape, position, size and orientation of each tooth
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4
Q

what is dental stone used for

A
  • 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
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5
Q

what can impression materials be for

A
  • single tooth
  • whole dentition
  • edentulous area/ mouth
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6
Q

what do impression material require

A
  • good properties

- clinician with good technique

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7
Q

what are the classifications of impression materials

A
  • clinical = mucostatic, mucocompressive

- properties = elastic, non-elastic

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8
Q

what are mucostatic materials

A
  • zinc oxide eugenol, low viscosity alginates
  • fluid materials that displace soft tissues slightly
  • give an impression of the undisplaced mucosa
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9
Q

what are mucocompressive materials

A
  • impression compound, high viscosity alginates/elastomers
  • viscous material that record an impression of the mucosa under load
  • give impression of displaced soft tissue
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10
Q

why do you want a material to be truly elastic

A
  • 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
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11
Q

what is the ideal elastic behaviour

A
  • 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
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12
Q

what is viscoelastic

A
  • 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
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13
Q

how can you reduce the permanent strain/ deformation on impression

A
  • if load time is less

- impression removed with a sharp pull

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14
Q

what are the two types of elastic materials

A
  • hydrocolloids

- elastomers

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15
Q

what are types of hydrocolloids

A
  • agar = reversible (not used)

- alginate = irreversible (used in DH)

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16
Q

what are some types of elastomers

A
  • polysulphides
  • polyethers
  • silicones (conventional and addition cured)
  • even these aren’t truly elastic
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17
Q

why aren’t non-elastic materials used anymore

A
  • does not recover well at al

- can get worse deformation or even fracturing

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18
Q

what are the stages in impression process

A
  • preparation
  • insertion
  • setting
  • removal
  • storage
  • cast preparation
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19
Q

what is involved in the preparation stage of impression process

A
  • mixing = 2 pastes, paste and liquid, powder and liquid
  • chemical reaction
  • heat
  • material flows and takes up shape/space available
20
Q

what needs to be considered during insertion phase of impression

A
  • tray type needed

- do you need adhesive

21
Q

what occurs during setting phase of impression

A
  • chemical
  • polymerisation
  • cooling
  • forms solid replica
22
Q

how long should the material be stored

A
  • storing can have significant dimensional changes that may even mean having to redo the impression
23
Q

what are the ideal properties in terms of accuracy of an impression material

A
  • 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
24
Q

what are the properties that can affect accuracy of impression

A
  • 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
25
Q

what are ideal properties of impression material in terms of patient comfort

A
  • non toxic, non irritant
  • acceptable taste and smell
  • short setting time
  • removable without damage to oral tissues
26
Q

what are ideal properties of impression material in terms of operator convenience

A
  • quick, simple technique
  • convenient working/setting time
  • must be able to be decontaminated
  • cost (inexpensive) is a consideration but not a factor
27
Q

what are some non elastic impression materials

A
  • impression compound
  • impression paste
  • not used anymore
28
Q

what is a colloid

A
  • 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
29
Q

what are the states a hydrocolloid can be in

A
  • SOL = viscous liquid state
  • GEL = jelly like consistency, agglomeration of particles, entangled framework of solid particles enclosing liquid by capillary forces
30
Q

how does hydrocolloids become when they start to set

A
  • more sturdy and rigid
31
Q

what is an irreversible hydrocolloid

A
  • alginate
32
Q

what is the composition of alginate

A
  • 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
33
Q

what does trisodium phosphate do

A
  • 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
34
Q

what is the setting reaction of alginate

A

2NanAlg + nCaSO4 –> nNa2SO4 + CanAlg

  • long cross-linking fibril entangling undissolved particles
  • sodium alginate –> calcium alginate
35
Q

what does cross linking do

A
  • makes the material more viscous - forms a gel
36
Q

what is alginate delay

A
  • trisodium phosphate preferentially react with Ca ions in CaSO4

2Na3PO4 + 3CaSO4 –> Ca3(PO4)2 + 3Na2SO4

  • then 2NanAlg reacts with Ca ions
37
Q

what must you do during alginate manipulation

A
  • use correct powder/liquid ratio
  • use water 18-24oC
  • use perforated tray and adhesive
  • remove impression with a sharp pull
38
Q

where will the impression material set the fastest

A
  • warmer temperatures

- where the material sits on the soft tissues is where it will set the fastest

39
Q

why should you leave the impression in the mouth a few minutes longer than you think

A
  • 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
40
Q

how can you reduce tearing and distortion of alginate

A
  • by using bigger bulk of materials

- use at leat 5mm layer

41
Q

what are the properties of alginate in terms of patient comfort

A
  • non toxic, non irritant
  • acceptable taste and smell
  • setting time is OK
42
Q

what are the properties of alginate in terms of operator convenience

A
  • relatively easy to work with
  • setting time is OK
  • storage = avoid moisture, prepare cast ASAP
43
Q

why must you avoid moisture and prepare the cast ASAP with alginate impression

A
  • 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
44
Q

what are the properties of alginate in terms of accuracy

A
  • 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
45
Q

what material should be used for deep undercuts

A
  • elastomeric materials

BDS2 - dental material science (13 decks)

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