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impression materials Flashcards

1
Q

What is an impression?

A

Negative imprint of hard and soft tissues in the mouth from which a positive reproduction can be formed

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2
Q

What is a tray?

A

Container designed to roughly fit over the dental arched and hold impression material

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3
Q

How do impression materials work?

A

Liquid/semi-solid when first mixed and placed in tray

Quickly set to solid to leave imprint of mouth structures

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4
Q

What should be considered for the patient?

A 
Neutral taste and odour
Short setting time
Small tray
Easily removed
No retakes
Non-toxic
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5
Q

What should be considered for the dentist?

A 
Easily mixed
Short working times
Good quality
Low cost
Easily disinfected
Simple procedure
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6
Q

What makes a good impression?

A

Accurate reproduction of surface detail (viscosity and wettability)

Dimensional accuracy and stability (shrinkage on setting, cooling contraction, permanent set, storage stability, type of tray)

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7
Q

What types of trays are there?

A

Metal- stainless steel

Plastic- nylon-/polystyrene- based

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8
Q

What types of materials are there?

A

Rigid- waxes, impression composition, ZnO/Eu, plaster of Paris
Flexible- hydrocolloids (agar + alginate), elastomers (polysulphide, polyether, silicone polymers)

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9
Q

What is the composition of dental waxes?

A

Oligomer (long chain molecule w simple structure

CH3-[CH2]n-CH3
n=15-42

Sources- mineral (paraffin), animals (beeswax), veg (carnauba and candelilla wax)

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10
Q

What are the properties of dental waxes?

A

Softening temp- ~42 degrees (just above mouth temp)
High coefficient of thermal expansion (cooling contraction)
Rigid
Poor thermal conductor
Low viscosity

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11
Q

Why is shrinkage of impression material important?

A

If shrinks- slight space for cement

If expands- smaller model, teeth don’t fit

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12
Q

What is the thermal expansion coefficient?

A

The change in length, when determined per unit length for a 1C change in temp

Mouth is exposed to broad range of temps and therefore materials should be able to too

Materials should have a lower TEC so they don’t expand so much

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13
Q

What at dental waxes used for?

A

Not impressions- high TEC

Lab based material-
Modelling, inlay, sheet casting, sticky, carding and boxing in waxes

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14
Q

What is an impression composition?

A

Thermoplastic
Sheet or stick, warmed in water bath
Shaped in patients mouth in special tray

Doesn’t flow much
Sometimes doesn’t provide a v good fit

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15
Q

What is the composition of an impression composition?

A

Combination of-
~natural/synthetic resins (shellac, dammar, colophony, sandarac)

~plasticisers (stearic acid or gutta percha)- avoids brittleness

~fillers (talc, calcium carbonate, limestone)- avoids tackiness

Careful w allergies (resins)

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16
Q

What are the properties of impression composition?

A 
Softening temp- 55-60C
Rigid
High TEC
High viscosity (mucocompressive)
Poor thermal conductivity- subject to stress relief
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17
Q

What is mucostatic?

A

Material is fluid enough to flow and doesn’t displace or tissues

Eg. Impression plaster, agar, ZnO/Eu, light body elastomers

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18
Q

What is mucocompressive?

A

Material is viscous and able to compress the oral tissues on insertion in mouth

Eg. impression compound, viscous alginate, some rigid elastomers

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19
Q

What are some common applications of impression compound?

A

Full/partial impressions
Sticks for copper band impressions
Adding to periphery of tray

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20
Q

What is rheology?

A

Study of flow of materials
Liquid flow measured by viscosity

Viscosity=shear stress/rate
Unit of viscosity= Pa.s

Rheometer- material loaded between plates, shear force applied under known conditions, force measured

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21
Q

What is viscous behaviour?

A

Shear stress-shear rate

Dilatant- shear thickening, increase in apparent viscosity at higher shear rates, eg. Silly putty

Newtonian- constant viscosity, across all shear rates eg. water

Pseudoplastic- shear thinning, lower apparent viscosity at higher shear rates

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22
Q

What is thixotropic behaviour?

A

Time dependant shear thinning property

Progressive decrease in viscosity with time for a constant applied shear stress, followed by a gradual recovery when the stress is removed

Characteristic of many elastomeric impression materials

Some degree of molecular rearrangement caused by mixing

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23
Q

What is the composition of Zinc Oxide/Eugenol?

A

Base paste- Zinc oxide, inert oils (plasticiser), hydrogenated resins (increases setting time and improves cohesion)

Reactor paste- Euganol, zinc acetate (accelerator), fillers (talc/kaolin)

1:1 (red and white)
Used w special tray

Some pasted contain eugenol substitute eg. Carboxylic acid

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24
Q

What are the advantages of ZnO/Eu?

A

Dimensional stability
Good surface detail
Stable of storage
Good shelf life

25
What are the disadvantages of ZnO/Eu?
Can’t be used in v deep undercuts Only sets quickly in thin section Eugenol allergy in some patients
26
What are the properties of ZnO/Eu?
``` Mucostatic Hydrophilic- accurate reproduction of surface details Dimensionally stable Causes burning sensation on lips Sticks to skin Stable storage, good shelf life ```
27
Why do we want good wetting?
Fewer voids Less entrapment of oral fluids Bubble free dies and models Fewer retakes
28
What is the composition of plaster of Paris (gypsum)?
Crystalline mineral of hydrated calcium sulphate CaSO4•2H2O White/colourless Comes as powder- calcium sulphate beta-hemihydrate (CaSO4)2•H2O, 0.4% borax (retarder- slows setting rate), 4% potassium sulphate (reduce expansion/accelerates setting), starch (aid disintegration of impression on separation from model) Potassium sulphate and borax- anti-expansion solution to 0.05% Liquid- water
29
What are the advantages of plaster of Paris?
Easy to mix Low viscosity- mucostatic Good dimensional stability and accuracy Cheap
30
What are the working and setting times of plaster of Paris?
Working time- 2-3 mins Setting time- 2-3 mins Final set- 4-6 mins Plaster expands on setting (0.3-0.6%)- interaction between growing gypsum crystals forms stresses
31
What are the disadvantages of plaster of Paris?
Low strength Rough surface finish Rigid once set Dry sensation in mouth
32
What are the common applications of plaster of Paris?
Useful for patients w excessively mobile soft tissue | Commonly used as model material set in impression moulds
33
What is a solution?
Homogenous mixture consisting of a single phase | Particles- less than 10^-7cm
34
What is a suspension?
Mixture of two phases | Particles- 10^-7cm and 10^-5cm
35
What is a colloid?
Heterogenous mixture of two phases where the two phases aren’t readily differentiated Particles- more than 10^-5cm There is colloidal silica in resin Colloids in agar and alginate
36
What are the states of hydrocolloids?
Agar- reversible (drop in temp) Alginate- irreversible (chemical) Both introduced in sol form intra orally Removed in gel form Sol—>gel via gelation
37
What is agar?
Gel heated into viscous liquid which cools into a gel Supplied sealed- prevent evaporation of water Tubes heated in boiling water for 10-45mins Often used to duplicate models as can be reused many times Agar, borax (strengthen gel), potassium sulphate and water (dispersion medium)
38
What are the properties of agar?
Good surface detail Can be used on undercuts but can tear if too deep Non toxic/irritant Slow setting time Poor tear resistance Adequate shelf life Can be sterilised by aq solution of hypochlorite
39
What are the advantages of agar?
Good surface detail | Reusable and relatively easily sterilised
40
What are the disadvantages of agar?
Need special equipment- water bath | Dimensional instability
41
What is alginate?
Viscous liquid into gel via chemical reaction Sodium alginate (forms hydrogel) Calcium sulphate dihydrate (provides Ca ions) Sodium phosphate (controls working time) Potassium sulphate (enhances setting of model) Fillers (controls consistency) Sodium silicofluoride (controls pH) Ca ions displace Na ions in sodium alginate polymer- cross linking reaction CaSO4•2H2O->Ca2+ + SO4- + 2H2O
42
What are the properties of alginate?
``` Dust free powder Cheap Limited shelf life Well controlled working/setting times Mucostatic Hydrophilic Poor surface reproduction Poor storage stability (imbibition/syneresis) Low tear strength Excessive permanent deformation ``` Perforations in trays for mechanical retention Only tray adhesive for other materials
43
What is imbibition?
Shape changes (swells) as water is absorbed by solid-colloids causing an increase in volume
44
What is syneresis?
Shape changes (shrinks) as loss of a liquid from a gel causes a reduction in volume
45
What are elastomers?
Polysulphides Polyethers Silicones- condensation/addition cured
46
What is a polysulphide?
Base paste- ~Polysulphide ~Filler (TiO2) Activator paste- ~Lead dioxide ~Sulphur ~Plasticiser (dibutyl phthalate) Polymer w terminal and pendant mercapton groups (-SH) -SH responsible for cross linking and condensation polymerisation By product is H2O
47
What are common applications on polysulphide?
Crown and bridge work Also, partial dentures/overdentures/implants
48
What is a polyether?
Base paste- ~Polyether ~Filler (colloidal silica) Activator paste- ~Aromatic sulphonate ester ~Filler ~Plasticiser (dibutyl phthalate) Cross linking with imine end groups- addition polymerisation (activation, initiation, propagation, termination)
49
What are common applications of polyether?
Crown and bridge work Also, partial dentures/overdentures/implants
50
What is a condensation cured silicone?
Base paste- ~Silicone polymer ~Filler (colloidal silica) ~Organo-tin compound Activator paste- ~Silicone polymer ~Filler (colloidal silica) ~Tetra-ethyl silicate Condesation cured Eg. Hydroxyl terminated polydimethyl siloxane- cross linking reaction requiring catalyst Byproduct- ethanol
51
What is an addition cured silicone?
Base paste- ~Silicone polymer ~Filler (colloidal silica) ~Silanol Activator paste- ~Silicone polymer ~Filler (colloidal silica) ~Pt catalyst Eg. Vinyl terminated polydimethyl siloxane- cross linking reaction, driven by radical (activation, initiation, propagation, termination)
52
What are common applications of silicone?
Crown and bridge work Also, partial dentures/overdentures/implants
53
What are relative merits of elastomers?
Ease of mixing- ~Polysulphides and condensation cured silicones- difficult to mix (different amounts of paste ) ~Addition cured silicones- gun delivery system- easiest Working/setting times- ~Polysulphides- environmental variation and long (W-6mins, S-12mins) ~condensation silicones- erratic due to moisture contamination ~polyether- consistent ~addition silicones- good times but setting may be inhibited by latex gloves
54
What are the mechanical properties of elastomers?
PS- polysulphide PE- polyether CCS- condensation silicone ACS- addition silicone Stiffness PSPE>CCS>ACS Tear strength PS>>PE>CCS=ACS
55
What is the surface detail like for elastomers?
All v good reproduction of detail on dry surfaces Polyethers best- more hydrophilic Surfactants added to ACS to improve wettability
56
Why might an impression fail?
POOR REPRODUCTION PF SURFACE DETAIL ~Rough or uneven surface- incomplete set, rapid set (premature removal, improper mixing, contamination, humidity, temp) ~Air bubbles- rapid set, improper mixing, surface contamination ~Irregular shaped voids- surface contamination, premature movement POOR FIT ~distortion (adhesive fail, tray not rigid, excessive seating pressure, movement during setting) ~casting too big (inappropriate technique, model poured late, wrong storage) ~casting too small (inappropriate technique, mode poured early, wrong storage)
57
Why do we need to avoid poor impressions?
``` Extra chair side time Repeat patient visits Patient distress Waste of expensive materials Remake bill form lab Spoil relationship w lab ```
58
What are intra oral scanners?
3D laser scanning Pros- patient comfort, dentist auto evaluation, reduced model time, favours clinic-lab communication Cons- cost investment, training, just surface registration, coating (not in modern ones)

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