Dental Impression Materials

Impression Materials

A dental impression is a negative record of the tissues of the mouth. The material used for taking impressions is called impression material.

Read And Learn More: Dental Materials Question And Answers

Classification of Impression Material

• Impression materials are classified on the basis of:
• On the basis of mode of setting and elasticity (Skinner)
• On the basis of tissue displacement during the impression procedure
  On the basis of use in dentistry.
• Based on the type of tray.

On the Basis of Mode of Setting and Elasticity (Skinner):

On the Basis of Tissue Displacement:

According to the tissue displacement impression materials are mucostatic and mucocompressive.

• Mucostatic materials: They produce minimum displacement of tissue during the impression.
  • For example:  Impression plaster, zinc oxide eugenol, low viscosity alginates, high viscosity elastomers, etc.
• Mucocompressive materials: They are more viscous and displace the tissues while recording them.
  • For example: Impression compounds, high-viscosity alginates, high-viscosity elastomers, etc.

On the Basis of their use in Dentistry Impression material used for complete denture prosthesis:

Impression plaster, impression compound, and impression paste. These materials have rigid mass and are not removed from undercuts without being fractured. Hence, these materials are used for an edentulous mouth.

Impression material used for the dentulous mouth: Alginate and rubber base impression materials are elastic and can easily be withdrawn from undercuts. These impression materials are suitable for impression for fabrication of removable and field partial denture prostheses.

Based on the Type of Tray:

• Perforated metal tray: Alginate hydrocolloid
• Water-cooled metal tray: Agar hydrocolloid
• Custom tray: Zinc oxide eugenol, impression plaster, elastomeric impression material

Alginate of Impression Material:

• Irreversible hydrocolloid.
• The word alginate comes from ‘Algin’ which is a peculiar
  mucous extract yielded by certain brown seaweeds (Algae).

Composition of Impression Material:

Chemistry of Setting / Setting Reaction:

When alginate powder is mixed with water a sol is formed which later sets to a gel by a chemical reaction. The final gel, i.e. insoluble calcium alginate is produced when soluble sodium alginate reacts with calcium sulfate. However, this reaction proceeds too fast. There is not enough working time. So the reaction is delayed by the addition of a retarder (trisodium phosphate)First sodium phosphate reacts with the calcium sulphate to provide adequate working time.

Next, after the sodium phosphate is used up, the remaining calcium sulphate reacts with sodium alginate to form insoluble calcium alginate, which forms a gel with water.

• First reaction:

2 Na₃PO₄ + 3CaSO₄→ Ca₃(PO₄)₂ + 3 Na₂SO₄

• Second reaction:

Sod. alginate + CaSO₄+ H₂O → Ca alginate + Na₂SO₄

(Powder)                                (Gel)

Gel Structure:

The final gel consists of a brush heal of calcium alginate fibril network enclosing unreacted sodium alginate sol, excess water, filer particles, and reaction by-products. It is a cross-linked structure and calcium leads to cross-linking.

Manipulation: For manipulation, the requirements are:

• A clean flexible plastic bowl.
• A clean wide-bladed, reasonably stif metal spatula (curved).

The steps for the manipulation are:

Fluf the powder by inverting the cane several time, this ensures uniform distribution of filer before mixing.

• The proper water/powder ratio should be used.
• The measured quantity of the powder is sprinkled in the measured amount of water in the rubber-mixing bowl.
• The mixing is started with a stirring motion to wet the powder with water.
• Once the powder has been moistened rapid speculation by swiping or stropping against the side of the bowl is done.
• A vigorous force of eight motion can be used with the mix being swiped against the wall of the bowl while rotating the bowl 180 degree.
• A proper mix is smooth and creamy with minimum voids and does not drip of the spatula when it is raised from the bowl.

Finally, the formed gel structure consists of brush heap of calcium alginate firil network which consists of unreacted sodium alginate sol, excess water, filer particles and reaction by-products.

Question 2. What are hydrocolloids 
Answer:

Hydrocolloids are polysaccharide materials such as agar and alginate which form cross-link networks by hydrogen bonding. Both agar and alginate are sensitive to water gain or loss. For composition and manipulation of irreversible hydrocolloid.