FLEXIBLE IMPRESSION MATERIALS

what are the types of flexible impression materials

• hydrocolloids

• elastomers

give examples of hydrocolloids

hydrocolloids

• agar

• alginate

give examples of elastomers

elastomers

• polysulphide

• polyether

• silicone polymers

what is a solution

solution: a homogenous mixture consisting of a single phase 

what is a suspension

suspension: a heterogenous mixture of two phases

what is a colloid

colloid: a heterogenous mixture of two phases where the two phases are not readily differentiated

e.g. colloidal silica in resin

how does agar become alginate

sol form = non-set form

what does an agar impression material consist of

• agar (colloid)

• borax (strengthen gel)

• potassium sulphate

• water (dispersion medium)

outline how agar is reversible

what is the clinical relevance of agars

• used for complex impressions for advanced restorative work

• often used in labs to duplicate models because they can be reused many times

how is agar supplied in dentistry

• supplied in sealed tubes to prevent evaporation of water

what are properties/ advantages of agar impression materials

• good surface detail

• can be used on undercuts but liable to tear on deep undercuts

• non-toxic and non-irritant

• slow setting time

• adequate shelf life

• can be sterilised by an aqueous solution of hypochlorite 

what are the main advantages of agar impression materials

• good surface detail

• reusable and relatively easily sterilised

what are disadvantages of agar impression materials

• need special equipment (water bath)

• dimensional instability

• poor tear resistance

how does alginate become a solid

outline the composition of alginate impression materials

• sodium alginate (hydrogel former)

• calcium sulphate dihydrate (provides calcium ions)

• sodium phosphate (controls working time)

• potassium sulphate (enhances setting of model)

• fillers (controls consistency)

• sodium silicofluoride (controls pH)

diagram of the alginate reaction (what kind of reaction is the alginate reaction)

why are perforated trays needed when using alginate

• this enables alginate to be fixed to the tray securely i.e. mechanical retention

  • needed because alginate has poor dimensional stability

• only tray adhesive is required for other impression materials

what are advantages of alginate impression materials

• dust free powder

• cheap

• well controlled working and setting times

• hydrophilic

• mucostatic technique

what are disadvantages of alginate impression materials

• limited shelf life

• poor surface reproduction

• poor storage stability

  • imbibition (water absorption)

  • syneresis (water loss)

• low tear strength

• excessive permanent deformation/ poor dimensional stability

define imbibition

imbibition: shape change as water is absorbed by solid-colloids causing an increase in volume

define syneresis

syneresis: shape change as loss of a liquid from a gel causing a reduction in volume

what are the types of elastomeric impression materials

• polysulphides

• polyethers

• silicones

  • condensation cured

  • addition cured

what does a polysulphide impression material consist of

• base paste

  • polysulphide

  • filler (TiO₂)

• activator paste

  • lead dioxide

  • sulphur

  • plasticiser (dibutyl phthalate)

is there a byproduct in the polysulphide impression material reaction, if so, what is it

• yes

• the byproduct is H₂O

what are the common applications for polysulphide impression materials

mainly:

• crown

• bridge work

also used for:

• partial dentures

• overdentures

• implants

what is a polyether impression material made up of

• base paste

  • polyether

  • filler (colloidal silica)

• activator paste

  • aromatic sulphonate ester

  • filler

  • plasticiser (dibutyl phthalate)

is there a byproduct in a polyether impression material reaction, if so what is it

• no there is no byproduct

image of the structure of polyether

what are the common applications for polyether

• crowns

• bridge work

• partial dentures

• implants

• overdentures

what does a condensation cured silicone impression material consist of 

• base paste

  • silicone polymer

  • filler (colloidal silica)

  • oregano-tin compound

• activator paste

  • silicone polymer

  • filler (colloidal silica)

  • tetra-ethyl silicate

what does an addition cured silicone impression material consist of 

• base paste

  • silicone polymer

  • filler (colloidal silica)

  • silanol

• activator paste

  • silicone polymer

  • filler (colloidal silica)

  • platinum (Pt) catalyst

what are common applications for silicone impression materials

mainly:

• crown

• bridge work

also used for:

• partial dentures

• implants

• overdentures

which types of elastomeric impression materials are difficult to mix and why

• polysulphides

• condensation-cured silicones

» due to different amounts of base and catalyst paste required

(disadvantage)

what makes addition-cured silicones easy to handle

gun delivery system

relative working and setting times of elastomeric impression materials

relative mechanical properties of elastomeric impression materials

which elastomeric impression material shows the best surface detail

• polyethers are generally best as they are more hydrophilic than the other impression materials

• but all show excellent reproduction of surface detail on dry surfaces

what has been added to addition-cured silicones to improve their wettability

surfactants

why may there be poor reproduction of surface detail in an impression

• rough or uneven surface

• air bubbles

• irregular shaped voids

why may an impression have a rough/ uneven surface

• incomplete set - premature removal, improper mixing, contamination

• rapid set - humidity, temp, wrong mix

why may an impression have air bubbles

• rapid set

• improper mixing

• surface contamination (moisture)

why may an impression have irregular shaped voids

• surface contamination (moisture)

• premature movement

why may an impression have poor fit

• distortion 

  • adhesive failure between tray and impression material

  • movement of tray during setting

  • tray not sufficiently rigid

  • excessive seating pressure

• 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 (not enough time for elastic recovery)

  • impression stored under wrong conditions

what are the consequences of bad impressions

• extra chair-side time

• repeat visits for patient

• patient distress

• waste of expensive materials

• remake bill from the laboratory

• spoil relationship with laboratory

what are advantages of intraoral scanners

• patient comfort

• dentist auto evaluation

• reduced model time

• favours clinic-lab communication

what are disadvantages of intraoral scanners

• cost investment

• training

• just surface registration

• coating needed - this is when IOS were new so a coating had to be applied in patient’s mouth to improve the scan quality