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BDS2 Dental Materials Sciences > impression materials > Flashcards

impression materials Flashcards

1
Q

what is an impression

A

negative reproduction of tissues

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2
Q

what are the 3 necessary functions of impression materials

A

produce an accurate replica of the surface and shape of

hard and soft oral tissues
negative reproduction

only useful if accurate
- material property and
technique sensitive

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3
Q

what is the job dental stone

A

used to fill the impression and produce a positive replica (stone cast)

which is used :-

  • for evaluating dentition when orthodontic, occlusal or other problems involved
  • in laboratory fabrication of restorations and prostheses
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4
Q

dental stone aka

A

gypsum

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5
Q

impression materials can take an impression of….

A

single tooth
whole dentition
edentulous mouth

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6
Q

what factor does the tx outcome of impression hinge on

A

quality & accuracy of the initial impression

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7
Q

what is required to achieve a good quality and accurate initial impression

A

IM with right properties
- Alginate can be good enough but not the best

clinician with good technique

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8
Q

ways to classify IM

A

clinical

  • mucostatic
  • mucocompressive

properties

  • elastic
  • non-elastic
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9
Q

mucostatic

A

fluid materials that displace the soft tissues slightly

i.e. give an impression of the undisplaced mucosa.

zinc oxide eugenol, low viscosity alginates)

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10
Q

mucoccompressive

A

viscous materials that record an impression of the mucosa under load

ie give impression of displaced soft tissue.

eg impression compound, high viscosity alginates/elastomers

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11
Q

ideal elastic behaviour

A

recover to original shape after flaring over bulbosity of tooth

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12
Q

viscoelastic behaviour

A

closer to reality, none truly elastic, lower the viscoelasticity,

gradually stretches and gradually recovers,
- doesn’t recover to original dimension – deformed, start with imperfect representation

It’s advantageous to wait for a time (Tf –TL ) after removing the tray before you pour the cast – so as to
- minimise permanent strain (deformation)

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13
Q

how is applying the load quickly beneficial when taking impressions

A

minimises permanent strain

maximum amount of strain experienced is less than first time round, recovers to close to 0 strain (pull quickly)

If LOAD time is less - and impression removed with a sharp pull, there is less overall permanent strain (ie lower deformation)

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14
Q

elastic materials (2)

A

Hydrocolloids

elastomers

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15
Q

2 hydrocolloids

A

Agar
- reversible (lab duplicating) - no longer used!

Alginate – irreversible

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16
Q

3 elastomers

A

Polysulphides

Polyethers

Silicones (conventional & addition cured)

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17
Q

what material is more ideally used hydrocolloid or elastomers?

A

elastomers

- not truly elastic but greater

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18
Q

non-elastic materials

A

recover at all well

- Fractured, torn

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19
Q

impression process 6 stages

A

preparation

insertion

setting

removal

storage

cast preparation

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20
Q

preparation stage of impression process

A

mixing (2 pastes; paste + liquid, powder + liquid etc).

chemical reaction

heat

need a material which flows and take up shape

21
Q

insertion stage of impression process

A

tray type selected

adhesive required?

22
Q

setting stage of impression process

A

chemical

polymerisation

cooling

forms solid replica

23
Q

storage stage of impression process considerations

A

how long

before dimensional changes become significant (of storage, need to do correctly, store for no longer than needed)

should you wait to allow elastic recovery ?

24
Q

6 factors on accuracy of impression material

A

flow
- surface detail recorded, good close intimate contact

setting changes
- no dimensional change - shrink or expand (remain same dimension)

removal

  • no effect on oral tissues - tear
  • no change dimensionally
    (i. e. no thermal contraction, between mouth and bench)
  • complete elastic recovery

storage
- does not change dimensionally over time (moisture) – weak point for alginates

decontamination
- dimensions & surfaces unaltered

compatible with cast material (gypsum, dental stone)

25
Q

6 properties which affect accuracy of impression material

A

viscosity
- determines ability to flow over surface & so level of detail recorded

setting mechanism
- does physical or chemical process cause dimensional contraction
(expansion or contraction)

thermal expansion coefficient
- ideally low (to cope with mouth to room temperature transition)

hydrophobic / hydrophilic
- affects surface contact - need to remove saliva ?

elasticity
- elastic / viscoelastic behaviour

tear strength

  • withstand tearing when removed esp. from undercuts in mouth
  • —-flow into undercut regions, small strips in undecruts – vulnerable to tearing and fracture when remove the tray
26
Q

4 factors of pt comfort that need to be satisfied by impression material

A

non-toxic, non-irritant

acceptable taste and smell

short setting time (unpleasant experience for pt)

removable without damage to oral tissues

27
Q

4 operator considerations for impression materials

A

quick, simple technique

convenient working/ setting times

must be able to be decontaminated

cost is a consideration (not a property) -inexpensive

28
Q

2 non-elastic impression matierals

A

impression compound

impression paste

29
Q

2 types of elastic impression materials

A

hydrocolloids

elastomers

30
Q

what is a hydrocolloid

A

A colloid is a 2 phase system of fine particles (1-200nm) of one phase dispersed in another phase (normally a liquid)

if dispersing medium is water then HYDROCOLLOID

31
Q

2 hydrocolloid states

A

sol

gel

32
Q

sol state of hydrocolloid

A

viscous liquid state

33
Q

gel state of hydrocolloid

A

jelly like consistency

agglomeration of particles

entangled framework of solid particles enclosing liquid by capillary forces

undertakes reaction

34
Q

example of an irreversible hydrocolloid

A

alginate

35
Q

5 components of alginate and their roles (irreversible hydrocolloid)

A

Salt of alginic acid (e.g. Na alginate) 12%
- Reacts with Ca ions

Calcium sulphate 12%
- Provides Ca ions

Trisodium phosphate 2%
- Delays gel formation

Filler (chalk like powder) 70%
- Cohesion, strength,

Modifiers, flavourings, chemical indicators (sol to gel state)
- Improve surface, taste, pH colour change

36
Q

role of salt of alginic acid in alginate

A

reacts with Ca ion
calcium ions interact with trisodium phosphate – pause things
remaining Ca ions react with sodium alginate

37
Q

role of calcium sulphate in alginate

A

provides Ca ions

38
Q

role of trisodium phosphate in alginate

A

Delays gel formation

calcium ions interact with trisodium phosphate – pause things
remaining Ca ions react with sodium alginate

39
Q

role of filler in alginate

A

cohesion

strength

40
Q

role of modifies, flavourings and chemical indicators in alginate

A

Improve surface, taste, pH colour change

41
Q

alginate setting reaction

A

2 Nan Alg + n CaSO4 –> nNa2SO4 + Can Alg

long cross-linked fibril entangling undissolved particles
- calcium alginate (gel)
cross linking makes it more viscous

42
Q

what allows the alginate delay

A

Tri-Sodium phosphate preferentially react with Ca ions in CaSO4
2 Na3 PO4 + 3 CaSO4 –> Ca3 (PO4)2 + 3Na2 SO4

Then:
2 Na Alg reacts with Ca ions

43
Q

how to use alginate correctly

A

use correct powder/ liquid ratio
- shake container

use water at 18 - 24°C

use perforated tray and adhesive

remove impression with a sharp pull

  • elastic recovery
  • maximum tear strength

increased temperature speeds up setting
sets nearest tissue first (in - contact with warm soft tissue – reaction faster)

crosslinking continues after apparent set

  • greater elastic properties if you wait further minute or two
  • need set throughout whole volume not just extremities
44
Q

how can you minimise an impression materials tendency to tear

A

permanent distortion and tearing slightly reduced by using large bulk of materials
- (Typ. 5mm)

Minimizes tendency to tear – if tears need to retake, no saving time

45
Q

how to store alginate

A

avoid moisture

  • use a damp gauze/tissue to sit over it
  • —-guess as to how moist

prepare cast ASAP to avoid dimensional change, can do both:

  • syneresis - release of water
  • imbibition - uptake of water

both distorts so renders impression useless

46
Q

syneresis

A

release of water e.g. from alginate

47
Q

imbibition

A

uptake of water e.g. from alginate

48
Q

accuracy evaluation of alginate

A

flow - fine detail - OK

setting changes - OK

but movement leads to internal stresses leads to distortion

nearly elastic – not 100% elastic recovery

poor tear strength – avoid deep undercuts

storage – syneresis or imbibition may affect dimensions /accuracy

BDS2 Dental Materials Sciences (13 decks)

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