Impression materials Flashcards
Impression definition
Negative imprint of hard (teeth) & soft tissues in mouth from which positive reproduction (or cast) can be formed
Tray definition
Container which is designed to roughly fit over dental arches
Process of impression materials
First mixed: liquid or semi-solid
Set to solid in a few minutes
Leaving imprint in mough
Properties wanted in impression material by px
Neutral taste and colour Short setting time Small tray Easily removed impression No retakes Non-toxic
Properties wanted in impression by dentist
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
Factors
Impression material
Impression tray
Impression technique
Impression trays
Metal -stainless steel Plastic -nylon-based -polystyrene-based
Dentists need to side
application –>
Types of impression materials
Rigid -waxes -impression composition -zinc oxide/ eugenol plaster of Paris Flexible -hydrocolloids (agar and alginate) -elastomers (polysulphide, polyether and silicone polymers)
How do impression materials set?
Waxes, impression composition, agar: thermoplastic
zinc oxide/ eugenol, plaster of Paris: chemical reaction
Alginate, elastomers: crosslinking
Dental waxes
Lab-based material
Used in clinic only to limited degree
Not really for taking impressions –> high TEC
Composition of dental waxes
Oligomer: long-chain molecule with simple strucutre Ch3-[CH2]n-CH3 n=15-42 Sources: -mineral -paraffin wax -animal -beeswax -vegetable -carnauba and candelilla wax
Properties of dental waxes
Softening temp just above mouth T ~42 degrees C
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
Dimensional accuracy
Shrinkage –> bigger model, good
-microns, space for your cement
Expansion –> smaller model, bad news
Thermal expansion coefficient (TEC)
The change in length, when determined per unit length, for a 1 degree C change in T (α)
- low: ceramic (8)
- high: resin (80)
- enamel and dentine in between
- dental waxes expand a lot: not good as impression materials
Compo impression composition
Thermoplastic Sheet or stick --> water bath Shape it in px's mouth (special trays) Does not flow enough Sometimes does not provide very good fit
Components of Compo
Combination of
Natural or synthetic resins
-shellac, dammar, colophony, sandarac
Plasticisers
–stearic acis or gutta percha (avoid brittleness)
Fillers
-talc, calcium carbonate or limestone (avoid tackiness)
Properties of Compo
Softening T (Tg) 55-60 degrees C
Rigid
High coefficient of thermal expansion - cooling contraction
High viscosity - mucocompressive/ mucodisplacive
Poor thermal conductivity - subject to stress relief
Mucostatic impression technique
Impression material is fluid enough to flow and does not displace oral tissues
e.g. impression plaster, agar, zinc oxide eugenol
Mucocompressive impression technique
Impression material is viscous and is able to compress oral tissues on insertion in px’s mouth
e.g. impression compound, viscous alginate
Rheology
Very useful tool for study of dental materials (‘handling’)
Study of flow of materials
For liquids flow is measured by viscosity (η)
-shear stress/ shear rate
Units of viscosity: Pa.s
Newtonian viscous behavoiur
Constant viscosity, μ, across all shear rates and include many of most common fluids e.g. water
Dilatant viscous behaviour
Shear-thickening fluids increase rates in apparent viscosity at higher hear rate
Rarely encountered e.g. silly putty
Pseudoplastic
Shear-thinning fluids have lower apparent viscosity at higher shear rates e.g. silicone impression materials
Thixotropic behaviour
Time-dependent viscosity
- characteristic of many elastomeric impression materials and improves handling properties
- some degree of molecular rearrangement caused by mixing
Constituents of typical zinc oxide/ eugenol
Base paste -zinc oxide -inert oils (plasticiser) -hydrogenated resins (increases setting time and improves cohesion) Reactor paste -eugenol (or can be carboxylic acid) -zinc acetate (accelorator) -fillers (talc or kaolin) Mixed in 1:1 ratio until paste of even colour
Properties of zinc oxide/ eugenol
Mucostatic (depending on brand) Hydrophilic: accurate reproduction of surface details Dimensionally stable Causes burning sensation of lips Tendency to stick to skin Stable on storage and good shelf life
Why do we want good wetting?
< voids
< entrapment of oral fluids
Bubble-free dies and models
< retakes
Wettability
Good wetting is ability of liquid to cover the surface of the substrate completely
Small contact angle –> more hydrophilic
Surface tensions
mJ/ m^2
For perfect wetting critical surface tension of the solid has to be greater than surface tension of liquid
Advantages of zinc oxide/ eugenol
Dimensional stability
Good surface detail
Stable on storage and good shelf life
Disadvantages of zinc oxide/ eugenol
Cannot be used in very deep undercuts
Only sets quickly in thin section
Eugenol allergy in some pxs
Plaster of Paris (Gypsum)
Crystalline mineral of hydrated calcium sulphate
Colourless or white, is not highly water-soluble and is not at all hard
Mixture of gypsum and water can be poured - gypsum hardens as water evaporates
Plaster of Paris is partly dehydrated form of gypsum
Composition of impression plaster
Powder
-calcium sulphate β-hemohydrate
-borax (slow setting time)
-potassium sulphate (reduce expansion/ accelerates setting)
-starch (aid disintegration of impression on separation from model)
Liquid - water
Expands on setting
Working and setting times of plaster
Working time 2-3min
Setting time 2-3min
Final set 4-6min
Mechanism of setting expansion plaster
Interaction between growing gypsum crystals results in formation of stresses and ultimately in expansion of the material
Advantages of impression plaster
Easy to mix -working time 2-3min -setting time 2-3min Low viscosity -mucostatic Good dimensional stability and accuracy Cheap
Disadvantages of impression plaster
Low strength Rough surface finish Poor abrasion resistance Rigid once set Dry sensation in mouth
Basic concepts of hydrocolloids
Colloid is heterogeneous mixtuer of two phases, where the two phases are not readily differentiated
- colloidal silica in resin
- agar and alginate impression materials
Sol
Viscous liquid
States of hydrocolloids
Sol –> gelation –> gel (jelly-like material)
Agar impression materials
Reversible hydrocolloids
Gel –> heating –> sol –> cooling –> gel
Advantages of agar impression materials
Good surface detail
Reusable and relatively easily steralised
Disadvantages of agar impression materials
Need specials equipment (water bath) and special technique
Dimensional instability
Alginate impression materials
Irreversible hydrocolloids
Sol –> chemical reaction –> gel
Composition of alginate impression materials
Sodium alginate: hydrogel former
Calcium sulphate dihydrate: provides calcium ions
Calcium ions displace sodium ions in sodium alginate polymer: crosslinking reaction
Properties of alginate impressionn materials
Dust free powder Cheap Limited shelf life Well controlled working & setting times Mucostatic Hyprophilic Poor surface reproduction Poor storage stability -imbibition (water sorption) -syneresis (water loss) Low tear strength Excessive permanent deformation
Alginate tray
Perforations in tray required to enable impression fix to tray securely
-alginate has poor dimensional stability
Elastomeric impression materials
Polysulphides Polyethers Silicones -condensation cured -addition cured
Polysulphide impression material
Base paste -polysulphide -filler (TiO2) Activator paste -lead dioxide -sulphur -plasticiser (dibutyl phthalate)
Polysulphide molecular composition
Polymer with terminal and pendant mercaptan groups (-SH) –> chain lengthening and cross-linking (condense together) –> byproduct H2O
Polyether impression material
Base paste -polyether -filler (colloidal silica) Activator paste -aromatic sulphonate ester -filler -plasticiser (dibuttyl phthalate)
Polyether molecular composition
Structure of polyether –> crosslinking reaction by addition polymerisation with imine end groups (no end product)
Silicone impression material (psuedoplastics) - condensation cured
Base paste -silicone polymer -filler (colloidal silica) -organo-tin compound Activator paste -silicone polymer -filler (colloidal silica) tetra-ethyl silicate
Silicone impression material condensation cured molecular composition
Hydroxyl terminated polydimethyl siloxane –> crosslinking reaction (condensation)
Silicone impression material: addition cured
Base paste -silicone polymer -filler (colloidal silica) -silanol Activator paste -silicone polymer -filler (colloidal silica) Pt catalyst
Silicone impression material addition cured molecular composition
Vinyl terminated polydimethyl siloxane –> crosslinking reaction (no byproduct)
Impression techniques
Twin-mix technique -typical for silicone impression materials -removal can be difficult Two-stage with spacer technique Two-stage without spacer technique
Relative merits of elastomeric impression materials: handing
Ease of mixing
-polysulphides & condensation-cured silicones somewhat difficult to mix due to different amounts of base and catalyst paste required
-gun delivery system for addition-cured silicones makes them easier to handle
Working and setting times
-polysulphides have long working and setting times
-condensation cured silicones have good working and setting times but setting may be inhibited by latex gloves
Relative merits of elastomeric impression materials: mechanical properties
Stiffness: PSPE>CCS>ACS
Teat strength: PS»PE>CCS=ACS
Relative merits of elastomeric impression materials: surface details
All show excellent reproduction of surface detail on dry surfaces
- polyethers generally best as more hydrophilic
- surfactants have been added to addition-cured silicones to improve their wettability
Relative merits of elastomeric impression materials: dimensional accuracy and stability
Setting shrinkage: PE=ACS
Failures with impressions: Poor reproduction of surface detail
Rough or uneven surface -incomplete set -rapid set Air bubbles -rapid set -improper mixing -surface contamination with moisture Irregular shaped voids -surface contamination with moisture -premature movement
Failures with impressions: poor fit
Distortion -adhesive failure on tray -tray not rigid enough -excessive seating pressure --> too much permanent set -movement of tray during setting Casting too big -inapprppriate technique -model poured too late -impression stored under wrong conditions Casting too small -inappropriate impression technique -model poured too early -impression sotred under wrong conditions
Pros and cons intraoral scanners
Pros: -px comfort -dentist auto evaluation -< model time -favours clinic-lab communication Cons -cost investment -training -just surface registration -coating
Define imbibition
Shape change as water is absorbed by solid-colloids causing an increase in volume
Gel + H2O –> bigger gel
Define syneresis
Shape changes as loss of a liquid from a gel causing a reduction in volume
Gel –> smaller gel + H2O
