impression materials Flashcards
What is an impression?
Negative imprint of hard and soft tissues in the mouth from which a positive reproduction can be formed
What is a tray?
Container designed to roughly fit over the dental arched and hold impression material
How do impression materials work?
Liquid/semi-solid when first mixed and placed in tray
Quickly set to solid to leave imprint of mouth structures
What should be considered for the patient?
Neutral taste and odour Short setting time Small tray Easily removed No retakes Non-toxic
What should be considered for the dentist?
Easily mixed Short working times Good quality Low cost Easily disinfected Simple procedure
What makes a good impression?
Accurate reproduction of surface detail (viscosity and wettability)
Dimensional accuracy and stability (shrinkage on setting, cooling contraction, permanent set, storage stability, type of tray)
What types of trays are there?
Metal- stainless steel
Plastic- nylon-/polystyrene- based
What types of materials are there?
Rigid- waxes, impression composition, ZnO/Eu, plaster of Paris
Flexible- hydrocolloids (agar + alginate), elastomers (polysulphide, polyether, silicone polymers)
What is the composition of dental waxes?
Oligomer (long chain molecule w simple structure
CH3-[CH2]n-CH3
n=15-42
Sources- mineral (paraffin), animals (beeswax), veg (carnauba and candelilla wax)
What are the properties of dental waxes?
Softening temp- ~42 degrees (just above mouth temp)
High coefficient of thermal expansion (cooling contraction)
Rigid
Poor thermal conductor
Low viscosity
Why is shrinkage of impression material important?
If shrinks- slight space for cement
If expands- smaller model, teeth don’t fit
What is the thermal expansion coefficient?
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
What at dental waxes used for?
Not impressions- high TEC
Lab based material-
Modelling, inlay, sheet casting, sticky, carding and boxing in waxes
What is an impression composition?
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
What is the composition of an impression composition?
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)
What are the properties of impression composition?
Softening temp- 55-60C Rigid High TEC High viscosity (mucocompressive) Poor thermal conductivity- subject to stress relief
What is mucostatic?
Material is fluid enough to flow and doesn’t displace or tissues
Eg. Impression plaster, agar, ZnO/Eu, light body elastomers
What is mucocompressive?
Material is viscous and able to compress the oral tissues on insertion in mouth
Eg. impression compound, viscous alginate, some rigid elastomers
What are some common applications of impression compound?
Full/partial impressions
Sticks for copper band impressions
Adding to periphery of tray
What is rheology?
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
What is viscous behaviour?
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
What is thixotropic behaviour?
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
What is the composition of Zinc Oxide/Eugenol?
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
What are the advantages of ZnO/Eu?
Dimensional stability
Good surface detail
Stable of storage
Good shelf life
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
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
Why do we want good wetting?
Fewer voids
Less entrapment of oral fluids
Bubble free dies and models
Fewer retakes
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
What are the advantages of plaster of Paris?
Easy to mix
Low viscosity- mucostatic
Good dimensional stability and accuracy
Cheap
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
What are the disadvantages of plaster of Paris?
Low strength
Rough surface finish
Rigid once set
Dry sensation in mouth
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
What is a solution?
Homogenous mixture consisting of a single phase
Particles- less than 10^-7cm
What is a suspension?
Mixture of two phases
Particles- 10^-7cm and 10^-5cm
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
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
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)
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
What are the advantages of agar?
Good surface detail
Reusable and relatively easily sterilised
What are the disadvantages of agar?
Need special equipment- water bath
Dimensional instability
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
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
What is imbibition?
Shape changes (swells) as water is absorbed by solid-colloids causing an increase in volume
What is syneresis?
Shape changes (shrinks) as loss of a liquid from a gel causes a reduction in volume
What are elastomers?
Polysulphides
Polyethers
Silicones- condensation/addition cured
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
What are common applications on polysulphide?
Crown and bridge work
Also, partial dentures/overdentures/implants
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)
What are common applications of polyether?
Crown and bridge work
Also, partial dentures/overdentures/implants
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
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)
What are common applications of silicone?
Crown and bridge work
Also, partial dentures/overdentures/implants
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
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
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
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)
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
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)
