Dental Impression Materials

• Impression materials record the shape and dimensions of oral tissues accurately.
• Mixed material in a soft state is placed against oral tissues to set when making an impression.
• The impression gives a negative reproduction of the oral tissues after setting.
• A positive reproduction of oral tissues is called a model when large areas are involved or a die when a single tooth is prepared, and is obtained by pouring mixed plaster or stone into the impression and allowing it to harden.

Requirements for an Ideal Impression Material

• Safe (not toxic or irritant).
• Acceptable to the patient (good odor, color, and taste).
• Fluid enough to reproduce all fine details.
• Has a suitable setting time.
• Has sufficient mechanical properties.
• Is dimensionally stable.
• Is compatible with die and model materials.
• Long shelf life (no deterioration in the dental office).
• Easy to manipulate and inexpensive.
• Can be disinfected without affecting its accuracy or stability.
• Accuracy: the ability to record fine details.
• Stability: the ability to maintain the recorded fine details.

Factors Affecting Accuracy and Stability

• Sufficient fluidity to record all fine details.
• Minimal dimensional changes during setting.
• Adhesion to the tray.
• Elastic on removal from the undercuts.
• Minimal dimensional changes after removal from the mouth.
• Compatible with the model and die materials.

Classification According to Behavior After Setting

• Non-elastic (rigid).
  • Either fracture or it is not recovered on removal from undercuts.
  • Includes plaster impression, impression compound, and zinc oxide-eugenol.
  • Fracture while compound and zinc oxide eugenol are permanently deformed on removal from undercuts.
• Elastic.
  • It is recovered after removal from undercuts.
• Hydrocolloid.
  • Includes agar (reversible) and alginate (irreversible).
• Rubbers or (Elastomers):
  • Includes polysulfide, silicone (condensation), silicone (addition), and polyether
• Rigid impression materials are restricted to use in areas where no undercuts for impressions for edentulous patients.
• Of the non-elastic impressions, only ZnO & eugenol material is still used.

Non-Elastic Impression Materials

• Plaster impression.
  • Plaster impression is supplied as powder to which water is added to produce a paste.
  • Powder: based on calcium sulfate hemihydrates mixed with modifiers.
• Setting Reaction: Ca SO4 ½ H2O + H2O → Ca S O4 2 H2O + heat.

Properties of Plaster Impression

• Fluidity: the mixed material has a high fluidity.
• Dimensional changes during setting: little setting expansion is observed (0.06%).
• Adhesion to the tray: adheres to the tray.
• Elasticity: rigid once set, fractures on removal, but can be reassembled.
• Dimensional stability: small degree of drying shrinkage during storage may occur.
• Cast compatibility: not compatible with model plaster or stone, therefore a separating medium must be used.
• Impression compound Presentation: supplied in several forms: sheets, cones, sticks, and cakes. It is a thermoplastic material.
• Above 55°C, it becomes soft and records details.
• On cooling to mouth (37°C) temperature, it hardens and can be removed.

Composition of an impression compound

• Resin and wax: give the material its thermoplastic property with modifiers to regulate the material properties.
• Hard at 55°C and soft at 37°C.
• Heated in a hot water bath at 55°C, then placed in the tray and inserted in the patient's mouth.
• During cooling in the oral cavity, time must be allowed till the entire impression becomes rigid.

Properties of an Impression Compound

• Fluidity: Very low fluidity and does not record fine details.
• Dimensional changes during setting: contracts 0.4% on cooling from 55°C to mouth temperature (37°C).
• Adheres well to the tray.
• Elasticity: Permanently deformed on removal from the undercuts.
• During storage: The release of residual stress in the impression and its high coefficient of thermal expansion and contraction cause warpage.
• Cast compatibility: Does not need a separating medium.

Zinc Oxide-Eugenol Impression Material

• Zinc oxide-eugenol is supplied as two pastes.
• The catalyst paste is usually colored brown, whereas the base paste is white.
• Some patients find eugenol irritating to the soft tissues, therefore, some eugenol-free products are available.

Composition of Zinc Oxide-Eugenol Impression Material

• Base paste
  • zinc oxide powder.
  • inert oil to make paste.
• Catalyst paste:
  • eugenol (in oil of cloves).
  • silica or talc powder to make paste.
  • magnesium chloride or zinc acetate.
  • moisture (H2O) which is essential for the reaction.
  • gum resin to gives body and coherence..
• Setting is a chelation reaction of eugenol with zinc oxide.
• The setting equation is: Zinc oxide + Eugenol -->(H2O)--> Zinc eugenolate matrix + Unreacted Zinc oxide.

Manipulation of Zinc Oxide-Eugenol Impression Material:

• Set material consists of a matrix of amorphous zinc eugenolate, which holds unreacted zinc oxide particles together.
• Equal lengths of the two pastes are mixed on an oil resistant paper pad until a homogenous color is obtained.
• The mix is then applied in the prepared tray.
• Vaseline on the patient's lips before making the impression makes cleaning easier.
• Setting may be accelerated by incorporating a drop of water into the paste.
• A drop of glycerin will retard the setting times.

Properties of Zinc Oxide-Eugenol Impression Material

• Fluidity: Sufficiently fluid to record all fine details.
• Dimensional changes during setting: Very little shrinkage of only 0.1% accompanies the setting.
• Adhesion to the tray: It adheres well to the tray (contains gum).
• Elasticity: Non-elastic; permanently deformed on removal from undercut areas.
• Dimensional changes during storage: Stable as it contains gum resins; high temperature may cause its distortion.
• Separating medium not needed before pouring.
• After the stone has set, it can be separated from the impression by immersion in hot water at 60°C for 10 minutes.

Elastic Impression Materials

• Hydrocolloids:
  • The word colloid means glue-like, thus a colloid has a glue-like physical character.
  • The colloidal state represents a dispersed phase of fine particles within the dispersion medium between an a solution and a suspension.
  • Solution: a homogenous mixture consisting of one phase.
  • Suspension: a heterogeneous mixture of two phases.
  • Colloid: a heterogeneous mixture of two phases.
• Hydrocolloids are materials that can exist in only two forms:
  • Sol state: viscose liquid consistency.
  • Gel state: Jelly-like consistency.
• In the gel state, it is an entanglement network of solid particles with the water trapped in the spaces as fibrils or chains of molecules.
• Hydrocolloid impression materials: reversible like Agar Agar and irreversible like Alginate.

Agar Agar Impression Material

• Supplied as a gel in tubes or jars.
• Composition: Agar as a dispersed phase and H2O as a dispersion medium
• Setting reaction: physical and reversible.
• Hard at 100°C to soft at 37°C.

Properties

• Fluidity: Can record all fine details.
• Dimensional changes during gellation: No changes or negligible shrinkage.
• Mechanical Interlocking: It needs perforated trays
• Elasticity: Elastic (98%) and therefore can easily record undercut areas.
• Dimensional changes during storage: Not stable and shrinks when stored in air (syneresis), and expands stored in water (imbibition).
• For impression keeping, keep at a relative humidity of 100% by wrapping it in a wet towel for less than 30min.
• Cast compatibility: Does not need a separating medium.
• It is 15% flexible at the time of removal from the mouth
• Alginate is one of the most widely used impression materials all over the world.
  • It needs minimum equipment, is easily manipulated, has high flexibility with the set material, has high accuracy if properly handled, and is low in cost.

Disadvantages of Alginate

• Sufers from synersis and imbibition.
• Low tear strenght.
• Cannot be electroplated.
• It is derived from marine plants.

Properties

• Sodium alginate dissolves in water as the main ingredient.
• Calcium sulfate reacts to produce insoluble calcium alginate.
• Sodium phosphate reacts with calcium sulfate acting as a retarder.
• Potassium sulfate counteracts the inhibiting effect of alginate on the setting of gypsum model.
• Glycol coats the powder particles.
• Chlorohexidine provides protection against microbes.
• Coloring agents make it acceptable .
• Adding water makes soft mass and then becomes an irreversible gel.
• Reaction: Sol --> Gel
• Alginate reacts in 2 steps :
• Retardation reaction Ca sulfate= Na Phosphate H2O= CA phosphate +Na sulfate
• gellation reaction Ca sulfate+Na alginate H20= ca alignate +Na sulfate.
• The proportions of powder and water can affect setting times.
• Use thinner mixes to increase the time required to change inside the mouth. Alginate is sufficiently fluid.
• No changes in negligible shrinkage take place during gellation. - Impression material is viscoelastic with its elastic recovery being about 97.3%.
• Perforated trays are needed.
• No separating medium is needed due to compatibility with model and die materials.
• Alginate impressions can be disinfected by soaking in 5% sodium hypochlorite for 10 minutes or 2% diluted glutaraldehyde.

Alginate Manipulation

• Shake well before use
• W/P ratio should be according to the manufacturer instructions.
• Water should be at room temperature.
• Powder should be added in the rubber bowl and mixed with stirring to wet the powder.
• Should be moved with vigorous action leading to a smooth consistent mix.
• Use perforated trays.
• Loading of the tray should be done quickly since the material only takes 2 minutes
• The impression remains in the mouth for2 additional minutes so that the physical properties can improve.
• The higher the strength ,the permanent deformation decreases.
• Pour immediately to avoid imbibition
• Impression rinsed to remove saliva to prevent interference with setting of the gypsum model.
• Remove surface water to avoid diluting the model.
• Avoid contact with stone to avoid it sticking

Notes about accuracy of hydrocolloids

• They are sufficiently fluid to record fine details in the mouth.
• the material in contact with the tissues stays liquid for longer time and can flow for any inaccuracy.

Setting and impression tips

• use an alginate during setting.
• Faster results at higher temperatures
• Use perforated trays and be sufficiently elastic
• No dimensional changes from the mouth due to syneresis.
• Doesnt need separating medium

Rubber base or elastomeric impression materials:

• The non elastic (rigid) impression materials cannot be removed from deep undercut unless they are either fractured as plaster impression or permanently deformed as ZnO/E and impression compound.
• Need accurate impression material. - Rubber impression materials: rubber base and elastomeric
• There are 4 rubber impression materials including a hybrid
• Rubbers are polymers that are coiled.

Polysulfide materials

• First rubber to be used is dentistry
• Provided as abase paste and an accelerator
• the base material depends on the filler Three viscosoities: heavy,medium and light heavy material is used alone

Composition

• base paste
• 80 low molecular weight
• mercaptan group
• 20s silica as filler
• activator paste
• lead dioxide setting reaction
• polysufide liquid+ ledd dioxide = polysulfide+H20

Silicone material

• Developed by criticism of polysufide
• Types of silicones: addition and condensation
• condensation silicone:
• Presentation:
• supplied in two pastes
• base and accelerator composition: base paste contains

Composition Silicones

base pace includes M.W silicon liquid reactive groups and slica with accelerator paste containing Tin octoate Polymerization reaction is;

• reaction is polysulfide
• ethyl silicate b. addition silicone: widley use in dental industry presentation and compision: base contains silicon with silica with accelerator w chloro acid and is hydrophobic

Polyether rubber impression materials

Offer combination of better mechanical properties/ Polyether are short working time with high stiffness Composition:

• two pastes
• M.W containi polymer containing termins 20% fillers with sulfunic acid reaction the fluidity dpeens on consistenscy

properties

• dimensional changes depend on potermezation
• silcone shrinking with condensa and polymer shrinking with heat
• Adheasion with trays

properties elasticity

silicone rubber and polyether dimensoa changes on storage is small shrinkage

Storage Properties

Rubber impressions shrink overtime, therefore its best to Allow least polymerization compatibility witg gypsum products impression can give accurate polysufied is the most flexible of rubbes allow easy removal with undercut Very recently, nanofilled rubber impression materials were developed uses silica fillers better flow ##New Materials

• A novel hybrid silicon recently