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BDS2 Dental Materials Sciences > impression materials > Flashcards

impression materials Flashcards

(48 cards)

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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

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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

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A

tray type selected

adhesive required?

22
Q

setting stage of impression process

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A

chemical

polymerisation

cooling

forms solid replica

23
Q

storage stage of impression process considerations

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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

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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
6 properties which affect accuracy of impression material
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
4 factors of pt comfort that need to be satisfied by impression material
non-toxic, non-irritant acceptable taste and smell short setting time (unpleasant experience for pt) removable without damage to oral tissues
27
4 operator considerations for impression materials
quick, simple technique convenient working/ setting times must be able to be decontaminated cost is a consideration (not a property) -inexpensive
28
2 non-elastic impression matierals
impression compound impression paste
29
2 types of elastic impression materials
hydrocolloids elastomers
30
what is a hydrocolloid
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
2 hydrocolloid states
sol gel
32
sol state of hydrocolloid
viscous liquid state
33
gel state of hydrocolloid
jelly like consistency agglomeration of particles entangled framework of solid particles enclosing liquid by capillary forces undertakes reaction
34
example of an irreversible hydrocolloid
alginate
35
5 components of alginate and their roles (irreversible hydrocolloid)
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
role of salt of alginic acid in alginate
reacts with Ca ion calcium ions interact with trisodium phosphate – pause things remaining Ca ions react with sodium alginate
37
role of calcium sulphate in alginate
provides Ca ions
38
role of trisodium phosphate in alginate
Delays gel formation calcium ions interact with trisodium phosphate – pause things remaining Ca ions react with sodium alginate
39
role of filler in alginate
cohesion | strength
40
role of modifies, flavourings and chemical indicators in alginate
Improve surface, taste, pH colour change
41
alginate setting reaction
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
what allows the alginate delay
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
how to use alginate correctly
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
how can you minimise an impression materials tendency to tear
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
how to store alginate
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
syneresis
release of water e.g. from alginate
47
imbibition
uptake of water e.g. from alginate
48
accuracy evaluation of alginate
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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