Hydrocolloids:

Reversible - Agar:

Main Constituents:

• Seaweed: extracted constituents
• Borax: strengthen (can be detrimental to the cast)
• Potassium Sulphate: prevents cast inhibition

Setting:

• Liquefaction phase - heat (water baths needed)
• Transformation phase - just above body temp, ready for imp to be taken

The Good:

• Non-toxic and non-irritant

The Bad:

• Low Tear Resistance
• Low Dimensional stability

Irreversible - Alginates:

Main Constituents:

• Seaweed: Alginic Acid: Sodium and Potassium Salts (alginates)
• Calcium Sulphate
• Fillers e.g. silicate powder
• Sodium sulphate - Increase working time
• Potassium sulphate or borate - again to reduce inhibition of the casting process (gypsum based models)

The active ingredients are the sodium and potassium alginates alongside calcium sulphate.

Setting:

• Chemical: irreversible reaction - Sol phase to gel phase = decomposition reaction
• Mix: Potassium sulphate (alginate) + Calcium Sulphate (dihydrate) + Water
• Set: Calcium alginate + potassium sulphate
• pH Changes: Colour change seen (11-7)

The Good:

• Fast Set
• Cheap
• Minimal tissue displacement
• Good fluidity - can record detail just not accurate enough or stable enough for lab made restorations e.g. margins for a crown prep
• Indications: Study casts, Opposing casts, Splints

The Bad:

• Distortion: In areas of compression and increased stress, alginates can show permanent distortion. A test for alginates to prove adequate is to show a 95% recoil in areas compressed over 20%. Alginates under stress will have some form of distortion <5%. Alginates ideally should be poured the same day, but allowing adequate time for recoil could decrease the distortion seen.
• 3mm thickness required to limit distortion.
• Poor Tear Resistance.
• Poor Dimensional stability:
  Imbibition (absorbs water) - Swells. Therefore smaller negative space - Hydrophilic nature.
  Syneresis (loses water) - Shrink. Larger negative space.
• Poor mix: incorporate air and/or particles (grainy)
• Single pour

Elastomers:

Elastomers include Polysulphides, Polyethers, Condensation and Addition Silicones. They can come in a range of viscosities, with the addition silicones covering a wide variety; providing: putties, heavy body, medium body and light body materials. Having these options can be useful in making cutting guides, and can provide methods of impression taking, using a combination of materials, e.g. a light body wash to pick up fine detail in a heavy body surrounding to provide support (see banner photo above). Clinically, these materials can take a few minutes to become set and dimensionally stable so appropriate distraction methods make need to be required during the impression taking process.

Knowing the conditions required for impression stability is crucial when sending to the lab, leading to accurate casting. Some materials are at an increased risk of swelling or dehydrating. Material shrinkage can be occurring and when it does it is usually toward the centre of mass of the impression material itself (this essentially means towards the tray). The can lead to slightly larger negative space.

Another general property of the materials below is that they are viscoelastic. This means that under strain, they can flow and become distorted. Straining of the material is most likely to occur on removal of the impression. They can be very stiff to remove, and a short snapping action, rather than a long tension pull is more desired.

Polysulphides:

Main Constituents:

• Base: Polysulphide polymer and fillers
• Accelerator: Lead dioxide and sulphur

Setting:

• Condensation Polymerisation (oxidisation and cross linkage) - releases water

The Good:

• Excellent Tear Strength

The Bad:

• Stain (clothing)
• Smelly
• Taste horrible
• Require excellent moisture control
• Can be related to latex allergies.
• Contraction occurs with water loss (condensation)

These materials have been since been superseded

Polyethers:

Main Constituents:

• Base: Polyether polymer
• Accelerator: Alkyl aromatic sulphonate + fillers and plasticiser

Setting:

• Cross linkage via cationic polymerisation

The Good:

• Hydrophilic - accurate takes with less than ideal moisture control
• Limited shrinkage on set - accurate
• Non-toxic and non-irritant
• Good dimensional stability - VERY stiff
• Monophase imp technique

The Bad:

• Difficult to remove from the mouth - awareness to block out undercuts
• Die construction in the lab (when crowns are made) if thin margins are present, due to the stiffness of the material it could fracture the mould.
• Absorb water - send dry

Example: Impragum (penta soft)

Silicones:

General point: Silicone materials are naturally hydrophobic

Condensation Silicones:

Main Constituents:

• Base: Dimethyl Siloxane
• Accelerator: Stannous octoate and alkyl silicate
• Catalyst: Tin compound

Setting:

• Condensation reaction: releasing ethyl alcohol

The Good:

• Can be accurate if casted up as soon as possible and the mix is good
• Tolerated

The Bad:

• High shrinkage on set due to loss of ethyl alcohol
• Cross-linkage is susceptible to hydrolysis
• Hard to accurately mix

Addition Silicones

Main Constituents:

• Base: Poly Methyl Hydrogren Siloxane + Siloxane prepolymers
• Accelerator: Divinyl polydimethyl Siloxane + Chloroplatinic salt
• Catalyst: Platinum
• + Fillers and surfactants.

Setting:

• Addition polymerisation (hydrogen cross linkage)

The Good:

• No/ limited byproduct production - less shrinkage
• High dimensional stability
• Good elastic recovery
• Reasonable tear strength as cross linkage occurs (don't take out mouth too soon)
• Accurate
• Poured more than once
• Variety of forms: heavy, Medium and light body materials.

The Bad:

• Hydrophobic - even with surfactants, moisture control should be obtained
• Latex gloves can contain sulphur - inhibits the setting reaction
• Potential for hydrogen gas release which can lead to bubbles forming in the casting process and increase porosity of the models.
• React with oxygen inhibition layer of composites

Vinyl Polyether Silicone (VPS):

The main aim of these impression materials is to incorporate the higher tear resistance and hydrophilic properties of the polyethers, alongside the high elastic properties of the addition silicones. These materials can react to latex and retraction cord constituents. At the moment, the general feel during my reading and whilst on clinic, is that addition silicones seem to be a popular choice when accurate impressions are required.

Rigid Materials:

These materials are mainly used for edentulous patients/ edentulous areas. A common method of warming the material is to use water baths.

Compound:

Main Constituents:

• A combination of resin and wax
• Calcium carbonate as a filler
• Plasticisers such as Gutta Percha used.

Setting:

• Thermoplastic

The Good:

• Mucocompressive/ displacement - obtain accurate recording of underlying ridge - If used as a primary imp - a close fitting special tray can then be constructed and used with a mucostatic material.
• Green stick can be used for border moulding.

The Bad:

• Poor dimensional stability
• Green stick hard to use (heating methods)

ZOE:

Main Constituents:

• Base: Zinc Oxide
• Catalyst: Eugenol
• + various accelerators, fillers and binders

Setting:

• Zinc oxide and eugenol reaction (as for cements)
• Sets in the presence of water to form zinc eugenolate.

The Good:

• Mucostatic - accurate at obtaining soft tissue areas
• Good dimensional stability
• Can be used in thin section - 1mm
• Close fitting non perforated special tray should be used

The Bad:

• Rigid therefore can break, crack, distort - Awareness of undercuts.
• Mucosa sensations during imp taking.

Trays:

Stock Trays:

• Plastic or Metal
• Dentate or edentulous fitting
• Perforated or non-perforated
• Sectional trays

Requirements:

• Rigid (limit distortion)
• Extensions and Arch fit correctly (may need to adjust accordingly)

Special Trays

• Custom Fit
• Stops incorporated = On seating, enables homogenous thickness (note: these stops aren't placed aware of recording need e.g. over a prepped rest seat)

Requests:

Spaced: Usually used in the dentate patient
Close Fitting:Usually used in the edentulous patient

Tray Retention:

Rim locks, or perforations can provide mechanical retention; however, on set the material is still likely to pull away from the tray towards the tooth as it shrinks. Therefore, additional adhesives are required.

Adhesives:

• Alginate: Toluene + isopropanol
• Polysulfide: Butyle rubber/ styrene acrylonitrile + chloroform/ketone
• Polyether: Ethyl acetate + propanol/ acetone
• Silicones: poly dimethyl silicone + ethyl silicate

Example Indications:

The list is not the be all and end all, but an idea of where the materials can be used:

• Alginate: Primary Imps
• Polyether: Crown/ Bridge
• Addition Silicone putty: Cutting guide/ stents/ border mould
• Addition Silicone Medium Body: CoCr/ Acrylic secondary imps
• Addition Silicone Wash + Heavy body: Crown/ Bridge
• Compound + Alginate ("wash"): F/F Primary Imps
• ZOE: F/F secondary Imps