Impression materials 203 Flashcards
What is an impression?
negative imprint of hard
(teeth) and soft tissues in the mouth from which a positive reproduction (or cast) can be formed.
What things does a patient want from an impression material?
• neutral taste and odour • short setting time • small tray • easily removed impression • no retakes • non-toxic
What things does a dentist want from an impression material?
- easily mixed
- short working times
- good quality impressions
- low cost
- easily disinfected
- simple procedure
What makes a good impression?
Accurate reproduction of surface detail
Viscosity
Wettability
Dimensional accuracy and stability Shrinkage on setting Cooling contraction Permanent set Storage stability Type of tray
What are the rigid types of impression materials? (four)
●Waxes
●Impression Composition
●Zinc Oxide/Eugenol
●Plaster of Paris
What are the flexible types of impression materials? (four)
●Hydrocolloids ( agar & alginate)
●Elastomers (polysulphide, polyether & silicone
polymers)
Which impression materials would you use for full dentures?
Plaster of Paris: Single stage Stock tray / special tray
ZnO-Eug: Single stage Special tray
Compo/ZnO-Eug: Two stage/ Stock tray
Which impression materials would you use for partial dentures?
Alginate: Single stage/ stock tray / special tray
Elastomers: Single stage/ Special tray
Which impression materials would you use for fixed prosthodontics?
Compo/wax: Single stage/ Copper ring
Elastomers: Single stage (M) Special tray
Twin mix (H+L) Special tray
What is the structure of a dental wax?
Long-chain Molecule with simple structure - Oligomer
CH3 - CH2 (n) - CH3
What are some sources of dental wax?
• Mineral: paraffin wax • Animal: beeswax • Vegetable: carnauba and candelilla wax
What are the properties of dental waxes?
Softening temperature (Tg ~ 42oC) just above mouth temperature High coefficient of thermal expansion cooling contraction Rigid; (Good for single tooth) Poor thermal conductor: (surface cools faster than inside (stress)) subject to stress relief Low viscosity
Why is shrinkage an important thing to consider for impression models?
If the
What is the thermal expansion coefficient? (def)
The change in length, when determined
per unit length, for a 1
oC change in temperature (a)
What’s the TEC for enamel, dentine, resin and ceramic?
Enamel - 12
Dentine - 14
Resin - 80
Ceramic - 8
Why should TEC be low for restorative materials?
TEC should be low for restorative material because you don’t want large changes in length (shrinkage and expansion) when the mouth is exposed to a broad range of temperatures
Why are dental waxes not good as impression materials? (TEC)
Waxes have high TEC so tend to expand a lot under changes in temperature
hence limited use and is a lab based material
What are some uses of dental waxes?
• Modelling waxes registration blocks and try-in dentures • Inlay waxes inlay patterns in the mouth (direct) or on a model or die (indirect) • Sheet casting wax construction of partial dentures • Sticky wax joining metal parts prior to soldering joining broken dentures prior to repair • Carding and boxing-in wax boxing-in impressions mounting teeth in sets
What is impression composition? (compo)
Thermoplastic material comes in sheets/sticks, use a water bath to soften and ten shape it in the patients mouth (special trays)
doesn’t flow enough and sometimes doesn’t provide a very good fit
What is the composition of a compo? (3 things - N, P, F)
●Natural or synthetic resins Shellac, dammar, colophony, sandarac (careful with allergies) ●Plasticisers Stearic acid or gutta percha (avoid brittleness) ●Fillers Talc, calcium carbonate or limestone (avoid tackiness)
What are the properties of compo?
●Softening temperature (Tg) 55-60oC ●Rigid ●High coefficient of thermal expansion - bad for impressions ●High Viscosity (thicker) ●mucocompressive ●Poor thermal conductivity ●subject to stress relief
What is a mucostatic impession technique?
the impression
material is fluid enough to flow and does not displace the oral tissues
What are the mucostatic impression materials?
1) Impression plaster
2) Agar mpression material
3) Zinc Oxide Eugenol impression pastes
4) Light body Elastomers
What is a mucocompressive impession technique?
theimpression material is viscous and is able to compress the oral tissues on insertion in the
patient ‘s mouth.
What are the mucocompressive impression materials?
1) Impression compound
2) Viscous alginate
3) Some rigid elastomers
What are some uses of compo?
●Full or partial impressions ●Sticks for copper band impressions ●For adding to the periphery of an impression tray
What is rheology?
the study of flow of materials
What is flow of liquids measured by?
their viscosity
What is the formula for viscosity?
Viscosity = n = shear stress /shear rate
Units of viscosity: Pa.s
What is a dilatant fluid/ shear thickening?
a non-Newtonian fluid where the shear viscosity increases with applied shear stress. This behavior is only one type of deviation from Newton’s Law, and it is controlled by such factors as particle size, shape, and distribution.
Shear-thickening fluids increase in apparent viscosity at higher shear rates (more pressure becomes more thick).
They are rarely encountered E.g. Silly Putty
What is a pseudoplastic/ shear thinning fluid?
Shear-thinning fluids have a lower apparent viscosity at higher shear rates (more pressure, becomes more fluid).
E.g. Silicone Impression Materials
What is a Newtonian fluid?
a fluid in which the viscous stresses arising from its flow, at every point, are linearly proportional to the local strain rate
These fluids have a constant viscosity, μ,
across all shear rates and include many of
the most common fluids (e.g. water)
What is thixotrophy?
the progressive
decrease in viscosity (less thick) with time for a constant applied shear stress, followed by a
gradual recovery when the stress is removed
• Characteristic of many elastomeric impression
materials and improves handling properties
What are the constituents of a typical zinc oxide eugenol paste?
Base paste:
zinc ocide, intert oils (plasticider), hydrogenated resins (increases setting time and improves cohesion)
reactor paste:
eugenol, zinc acetate (accelerator), fillers (talc or kaolin)
What are the advantages and disadvantages of zinc oxide eugenol?
ADVANTAGES - Dimensional stability - Good surface detail - Stable on storage and good shelf life
DISADVANTAGES - Cannot be used in very deep undercuts - Only sets quickly in thin section - Eugenol allergy in some patients
What are the properties of ZnO?
Mucostatic Hydrophilic - Accurate reproduction of surface details Dimensionally stable Causes burning sensation of lips Tendency to stick to the skin Stable on storage and good shelf life
Why do we want good wetting in an impression material?
●Fewer voids
●Less entrapment of oral fluids
●Bubble-free dies and models
●Fewer retakes
What is wetting?
the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces.
What is the relationship between contact angle of the liquid on the solid and how hydrophilic it is?
Smaller contact angle = more wetting - more hydrophilic
How does perfect wetting occur?
the critical surface tension of the solid has to be greater than the surface tension of the liquid
What is plaster of paris? (gypsum)
a crystalline mineral of
hydrated calcium sulphate (chemical
formula CaSO4 • 2H2O (this is heated to drive off water to give calcium sulphate hemihydrate and 3 waters).
2 CaSO4.2H2O –> (CaSO4)2.H2O + 3H2O
colourless or white, is not
highly water-soluble and is not at all
hard.
mainly used in the partly
dehydrated form of plaster of Paris
(2CaSO4 • H2O)
What is the composition of impression plaster?
●Powder
●calcium sulphate -hemihydrate
(CaSO4)2.H2O
●borax (slow setting rate) ●potassium sulphate (reduce expansion/accelerates setting) ●starch (aid disintegration of impression on separation from model) ●Liquid ●wate
What causes the expansion of gypsum?
The interaction between growing gypsum crystals results in formation of stresses and ultimately in the expansion of the material
Expansion causes inaccuracy of the impression so the plaster needs to be mixed with anti-expansion solution (4% potassium sulphate and 0.4% borax) - reduces expansion to 0.05% from 0.3-0.6%
however potassium sulphate also accelerates the setting reaction so borax is added to slow it down so more time to take the impression
What are the advantages and disadvantages of impression plaster?
Advantages
Easy to mix
●Working time 2-3 min ●Setting time 2-3 min Low viscosity
●mucostatic
Good dimensional stability & accuracy
Cheap
Disadvantages
Low strength
Rough surface finish Rigid once set
Dry sensation in the mouth
What are some common applications of impression plaster?
- Useful for making impressions of patients with excessively mobile soft tissue
- Commonly used as the model material set in impression mould
What is a hydrocolloid?
a heterogeneous mixture of two phases, where the two phases are not readily differentiated
e.g.
Colloidal silica in resin Agar and Alginate impression material
What is a suspension?
A mixture of two phases
What is a solution?
a homogeneous mixture consisting of a single phase
What type of hydrocolloid is agar and alginate?
Agar - reversible hydrocolloid (heat gel - viscous liquid, cool liquid = gel)
Alginate - irreversible hydrocolloid (goes from sol to gel and stays as gel)
How are the hydrocolloids first introduced intra-orally and in what form are they removed?
Introduced in sol (viscous liquid) form and removed in gel form from patient’s mouth
What are the properties of hydrocolloids?
•Good surface detail •Can be used on undercuts, but liable to tear on deep undercuts •Non toxic and non irritant •Slow setting time •Poor tear resistance •Adequate shelf life •Can be sterilised by an aqueous solution of hypochlorit
What are the advantages and disadvantages of hydrocolloids?
ADVANTAGES
- Good surface detail
- Reusable and relatively easily sterilised
DISADVANTAGES special equipment (water bath) and special technique 2. Dimensional instability
What is the composition of alginate?
- Sodium alginate hydrogel former
- Calcium sulphate dihydrate provides calcium ions (ca ions displace the sodium ions in the sodium alginate polymer)
- Sodium phosphate controls working time
- Potassium sulphate enhances setting of model
- Fillers controls consistency
- Sodium silicofluoride controls pH
What are the properties of alginate impression materials?
Dust free powder
Cheap
Limited shelf life
Well controlled working and setting times
Mucostatic
Hydrophilic
Poor surface reproduction Poor storage stability –Imbibition (water sorption) –Syneresis (water loss) Low tear strength Excessive permanent deformation
Why are perforations in the tray needed with alginate?
enable the impression material to fix to the tray securely (Mechanical Retention): only tray adhesive is required for other impression materials.
this is needed because alginate has poor dimensional stability
What is Imbibition?
Shape change as water is absorbed by solid-colloids causing an increase in volume - the gel swells
What is syneresis?
Shape change as loss of a liquid from a gel causing a reduction in volume
What are some elastomeric impression materials?
Polysulphides
Polyethers
Silicones
Condensation cured Addition cure
What is the composition of polysulphide impression material?
Base paste: polysulphide and filler (TiO2)
Activator paste : Lead dioxide, sulphur, plasticiser (dibutyl phthalate)
What are some common applications of polysulphide impression material?
Mainly crown and bridge work
also used for partial dentures, overdentures, implants
What is the composition of polyether impression material?
Base paste: polyether, filler (colloidal silica), plasticiser
Activator paste - aromatic sulphonate ester, filler, plasticiser (dibutyl phthalate)
What are some common applications of polyether impression material?
Crowns, bridge work, partial dentures, implants, overdentures
What is the composition of silicone impression material (condensation cured)?
Base paste: Silicone polymer, Filler (colloidal silica), Organo-tin compound
Activator paste:
Silicone polymer, Filler (colloidal silica), Tetra-ethyl silicate
By what reaction does silicone set?
Condensation polymerisation
What is the composition of silicone impression material (addition cured - activation, initiation, propagation, termination))?
Base paste: silicone polymer, filler (colloidal silica, Silanol)
Activator paste: silicone polymer, filler (colloidal silica), Pt catalyst
What are some common applications of silicone impression material?
crown, bridge work, partial dentures, implants, overdentures
What are the relative merits of elastomeric impression materials?
• Ease of mixing
– polysulphides and condensation-cured silicones are somewhat difficult to mix due to different amounts of base and catalyst paste required. – Gun delivery system for addition-cured silicones makes them easiest to handle
Working and setting times
– polysulphides are susceptible to environmental variations with long working (6min) and setting times (12min).
– Condensation cured silicones can show erratic setting due to moisture contamination.
– Polyethers have very consistent working and setting times
– Addition-cured silicones have good working and setting times but setting may be inhibited by latex gloves.
- Stiffness - PSPE>CCS>ACS
- Tear strength - PS»PE>CCS=AC
- All show excellent reproduction of surface detail on dry surfaces.
- Polyethers are generally best as these are more hydrophilic than the other impression materials.
- Surfactants have been added to the addition-cured silicones to improve their wettability
What are the failures with impressions?
- Poor reproduction of surface detail
- Rough or uneven surface Incomplete set (premature removal, improper mixing, contamination)
Rapid set (humidity, temperature, wrong mix) -
Air bubbles
Rapid set
Improper mixing
Surface contamination (moisture)
Irregular shaped voids Surface contamination (moisture)
Premature movement
Poor fit
●Distortion
Adhesive failure between tray and impression material
Tray not sufficiently rigid Excessive seating pressure resulting in too much permanent set Movement of tray during setting
●Casting too big Inappropriate impression technique
Model poured too late (excessive shrinkage on storage)
Impression stored under wrong conditions
●Casting too small Inappropriate impression technique
Model poured too early (insufficient time for elastic recovery)
Impression stored under wrong conditions
- Extra chair-side time
- Repeat visits for the patient
- Patient distress
- Waste of expensive materials
- Remake bill from the laboratory
- Spoil relationship with laboratory
What are the pros and cons of intraoral scanners?
Patient comfort
Dentist auto evaluation Reduced model time Favours clinic-lab communication
Cost investment
Training
Just surface registration Coating (Not in the modern ones)
