IM 102

Tray-Selection Criteria

• Must feel comfortable to the patient 
• Should extend slightly beyond the facial surfaces of the teeth 
• Should extend approximately 2 to 3 mm beyond the third molar, retromolar, or tuberosity area of the arch 
• Depth to allow 2 to 3 mm of material between the tray and incisal or occlusal edges of the teeth 

  Try the tray in the patient’s mouth before mixing material.

  Explain the procedure to the patient.

Tray Adhesives

• VPS adhesives (blue)
  • For polyvinyl siloxane and polyether impression materials. 
• Rubber base adhesive (brown) 
  • Used with rubber base impression materials. 
• Silicone adhesive (orange-pink)
  • Used with silicone impression materials. 

 Tray adhesives should be applied after the tray has been tried in the patient’s mouth.

  How long should the tray adhesive be applied to the tray before the impression material is added?

Hydrocolloid Impression Materials

• These materials are used to obtain preliminary and final impressions.
• Hydro mean “water.”
• Colloid means “gelatin substance.”

What can alginate impressions be used for?

Irreversible Hydrocolloid

• Material cannot return to a solid state after it becomes a gel.
  • Alginate is the irreversible hydrocolloid most widely used to take preliminary impressions.  

Makeup of Alginate

• Potassium alginate
  • Comes from seaweed; also used in foods such as ice cream as a thickening agent  
• Calcium sulfate
  • Reacts with the potassium alginate to form the gel
• Trisodium phosphate
  • Added to slow the reaction time for mixing.
• Diatomaceous earth
  • A filler that adds bulk to the material
• Zinc oxide
  • Adds bulk to the material
• Potassium titanium fluoride
  • Added so as not to interfere with the setting and surface strength 

No taste.

  Flavors are available. (Cherry is the most common.)

  Odor-free.

Physical Phases of Alginate

• First phase 
  • In the sol (as in solution) phase, the material is in a liquid or semiliquid form. 
• Second phase 
  • In the gel phase, the material is semisolid, similar to a gelatin dessert. 

On completion of both phases, alginate is susceptible to tearing and stretching.

Packaging and Storing of Alginate*

• Containers 
• Premeasured packages 

* The shelf life of alginate is approximately one year

  Alginate should be stored at room temperature.

Packaging of Alginate

Alginate impression materials come in a reusable tub with a measuring scoop. This is the most common form of alginate.

Premeasured foil packets are available at a slightly higher cost.

Cause for Distortion and Dimensional Change

• Imbibition  
  • Alginate impression stored in water or a very wet paper towel will absorb the additional water and expand. 
• Syneresis
  • If an alginate impression remains in the open air, its moisture will evaporate, causing the impression to shrink and distort. 

 How long after the impression is obtained should we proceed to pouring?

  Storing the impression in a plastic bag can help prevent distortion.

Alginate Settings

• Normal-set alginate
  • Working time of 2 minutes and a setting time of up to 4½ minutes after mixing 
• Fast-set alginate
  • Working time of 1¼ minutes and a setting time of 1 to 2 minutes 
• Working time
  • The time allowed for mixing the alginate, loading the tray, and positioning the tray in the patient’s mouth
• Setting time 
  • The time required for the chemical action to be completed.

  Which setting time would be preferred for a patient who has a sensitive gag reflex?

Altering the Setting Time of Alginate

• Cooler water can increase the setting time if additional time is needed for the procedure. 
• Warmer water can reduce or shorten the setting time of the procedure. 

  Which water temperature would be preferred for a patient who has a sensitive gag reflex? Why?

Water-to-Powder Ratio

• Adult mandibular impression 
  • Two scoops of powder and two measures of water
• Adult maxillary impression 
  • Three scoops of powder and three measures of water  

  These ratios are only guidelines for measurement. 

  Check the manufacturer’s directions before measuring. 

  Adjustments may have to be adjusted, because not all patients are the same.

A plastic scoop and plastic cylinder are supplied with alginate. 

  Do not pack powder into scoops.

  Use a spatula to level the top of the scoop.