IM 103

Taking an Alginate Impression

• Explain the procedure to the patient:
  • The material will feel cold, there is no unpleasant taste, and the material will set quickly.
  • Breathe deeply through your nose to help relax and feel more comfortable.
  • Use hand signals to communicate any discomfort.   

  Never leave the patient alone during this procedure.

  Would we ask the patient whether he or she has a sensitive gag reflex? Why or why not?

An Acceptable Alginate Impression

• The impression tray is centered. 
• The “peripheral roll,” including all of the vestibular areas, is completed. 
• The tray is not “overseated.”
• The impression is free of tears or voids.
• Sharp anatomic detail of all teeth and soft tissues is captured.
• The retromolar area, lingual frenum, tongue space, and mylohyoid ridge are reproduced in the mandibular impression.
• The hard palate and tuberosities are recorded in the maxillary impression.     

If these criteria are not met, you must retake the impression.

How an impression must appear.

Elastomeric Impression Materials 

Elastomeric impression materials are used when an extremely accurate impression is essential. The term elastomeric means “having elastic or rubberlike qualities.” 

  Commonly used in dental offices.

  Easier to use than reversible hydrocolloids.

Characteristics of Elastomeric Impression Materials

• Base 
  • Packaged as a: 
    • Paste in a tube
    • Cartridge
    • Putty in a jar 
• Catalyst
  • “Accelerator”  packaged as a: 
    • Paste in a tube
    • Cartridge
    • Liquid in a bottle with a dropper top 

  Very accurate impressions are obtained with the use of this material.

Forms of Elastomeric Materials

• Light-bodied
  • Also referred to as syringe type or wash type 
  • Used because it can flow into and around the details of the prepared tooth
  • Placed on and immediately around the prepared teeth with the use of a special syringe or extruder

• Regular and heavy-bodied
  • Also referred to as “tray-type” materials, they are much thicker. As the name implies, they are used to fill the tray. Their stiffness helps force the light-bodied material into close contact with the prepared teeth and surrounding tissues to ensure a more accurate impression of the details of a preparation. 

  The syringe may be disposable or sterilizable and reusable.

  Extruder tips are disposed of after each use.

  Would this material be used to define details of the prepared tooth?

Basic Impression Technique

1. The material selected depends on the dentist’s preference and the type of impression required for the procedure.
2. The dentist prepares the tooth or teeth for the impression. 
3. The light-bodied material is prepared and loaded into the syringe and transferred to the dentist.
4. The dentist places the light-bodied material over and around the prepared teeth and on the surrounding tissues.
5. The heavy-bodied material is prepared and loaded into the tray and transferred to the dentist.
6. When the impression material has reached its final set, the impression is removed and inspected for accuracy.
7. The impression is disinfected, placed in a biohazard bag, labeled, and readied for the laboratory technician.    

  What is the process for tray selection?

Curing Stages of Elastomeric Materials

• Initial set 
  • The first stage results in stiffening of the paste without the appearance of elastic properties. The material may be manipulated only during this first stage. 
• Final set 
  • The second stage begins with the appearance of elasticity and proceeds through a gradual change to a solid rubberlike mass. The material must be in place in the mouth before the elastic properties of the final set begin to develop. 
• Final cure 
  • The last stage takes 1 to 24 hours. 

Do these impressions need to be disinfected before delivery to the dental laboratory?

Types of Elastomeric Materials

• Polysulfide
• Polyether
• Silicone
• Polyvinyl siloxane (PVS, most popular) 

Polysulfide Impression Material

• Chemical makeup
  • Base: mercaptan polysulfide
  • Crosslinking agent: sulfur, lead peroxide, or both
  • Catalysts: copper hydroxides, zinc peroxide, organic hydrogen peroxide
  • Fillers: zinc sulfate, lithopone, or calcium sulfate dihydrate    

Manipulation and Technique Considerations for Polysulfide Material 

• Dispense pastes at the top of the mixing pad. 
• Mix pastes with the tip of a spatula to incorporate the material first.
• Transfer the material to the fresh surface of the mixing pad. 
• Water, saliva, and blood affect polysulfide material. 
• The impression should be removed quickly after setting; do not rock the tray. 
• The adhesive must be thin and dry before the impression material is added. 
• Wait 20 to 30 minutes before pouring the impression to allow stress relaxation to occur in the material.
• Be careful to avoid glove-powder contamination. 

Has a long working time.

  Has a long setting time.

  Would this material be used on a patient with a sensitive gag reflex?

Polyether Impression Material

• Chemical makeup
  • Base: polyether
  • Crosslinking agent: sulfate
  • Catalysts: glycol-based plasticizers 
  • Filler: silica   

 A thinner can be used with this material.

  What is a thinner used for?

Manipulation and Technique Considerations for Polyether Material 

• The material is very stiff, making it difficult to remove without rocking.
• When removing the impression, break the seal and rock it slightly to prevent tearing. 
• Water, saliva, and blood affect polyether material. 
• Added moisture will increase the impression’s marginal discrepancy. 
• Increased water absorption occurs if a thinning agent is used.   

  Supplied in a two-paste system or cartridge system.

Silicone Impression Material

• Chemical makeup
  • Base: poly dimethyl siloxane
  • Crosslinking agent: alkyl orthosilicate or organo hydrogen siloxane
  • Catalyst: organo tin compounds
  • Filler: silica

Odor free.

  Does not stain.

  Very good dimensional stability.

Manipulation and Technique Considerations for Silicone Material 

• The material has a limited shelf life. 
• The tray requires a special tray adhesive. 
• Silicone material is not subject to syneresis or imbibition, but it does respond with shrinkage over time. 
• The material is more flexible, so the more chance of distortion during removal is greater. 
• Wait 20 to 30 minutes before pouring of models for stress relaxation to occur. 

Supplied in a tube for the base and a bottle or smaller tube for the catalyst.

Polyvinyl Siloxane Impression Material

• Chemical makeup
  • Base: silicone polymer
  • Catalyst: chloroplatinic acid
  • Filler: silica 

  Also known as polysiloxane.

  High dimensional stability.

  Low tear resistance.

Manipulation and Technique Considerations for Polyvinyl Siloxane Material

• For dimensional stability, this is the best impression material. 
• Pouring of the model can be delayed as long as 7 to 10 days. 
• Stiffness of the material makes removal of the tray difficult. 
• The material is dispensed with the use of an auto-mixing unit and mixing tips.

  Most commonly used impression material.

  No odor.

  Tasteless.

  Available in light-, regular-, and heavy-bodied forms.

  Supplied in cartridges or putty form.