In providing professional services for D&S clients, I have the opportunity to visit a number of dental offices to provide mini-workshops for staff on a variety of topics. One of the most popular requests is on taking and pouring an alginate impression. Due to this interest, I have decided to address this topic. Please feel free to print it and share it with your staff or tailor it to your specific needs.
Jane Busch, DDS

HOW TO TAKE AND POUR AN ALGINATE IMPRESSION

TAKING AN ALGINATE IMPRESSION
1) Select the proper tray.

2) Mix the alginate impression material.

• use a clean mixing bowl.
• use room-temperature distilled water.
• thoroughly mix the powder before opening.
  
• add the powder to the liquid.
  
• use the manufacturer’s recommended measurements.
  
• be sure the mixing bowl is cool before starting a second mix.
  

3) Load the tray.

• use a dry tray.
  
• push the alginate firmly into the tray.
  
• over-fill the lower tray in the posterior.
  
• over-fill the upper tray in the anterior and in the palate area.

4) Take the lower impression.

• when seating the lower tray, first partially seat the tray evenly and have the patient put
  their tongue over the tray, then seat the posterior until material extrudes distally, finally
  seat the anterior until the mucolabial fold is filled with alginate.
  
• muscle trim the lower
  
  1. pull and massage the facial and buccal muscles.
     
  2. place finger on tongue and ask patient to push tongue up and against your finger.
     
  3. have patient lick their upper lip corner-to-corner.
     
  4. massage again.
     
  5. hold both under and over the mandible with equal pressure at the premolar region.
• remove the lower tray by breaking the seal with your finger.

5) Taking the upper impression.

• insert the tray parallel to the teeth, then seat the anterior first, then the posterior area until you see material extruding distally, complete the anterior seating until the mucobuccal fold is full.
  
• muscle trim the upper.
  1. massage the buccinator muscle.
     
  2. pull and massage the upper lip to clear the labial and buccal frenae.
  3. ask patient to move jaw right-to-left to clear the coronoid process.
     
  4. have patient open wide and close to clear the pterygomandibular raphe.
     
  5. massage again.
  6. hold with equal pressure at the premolar area until set.
• remove tray by breaking seal with your finger (or have patient blow air through mouth).

POURING A STONE MODEL FROM AN ALGINATE IMPRESSION
1) Rinse impression under water to remove blood and saliva.

2) Examine the impression. If in doubt, take a new impression.

3) Spray impression with a chloride-based disinfectant and place in a zip-lock bag for 3 minutes.

4) Remove from bag and rinse again.

5) Pour impression with dental stone.

• Use a clean dry bowl and spatula. A bowl or spatula with stone imbedded will cause the mix
  to set up too fast.
  
• Tumble stone contents for uniformity. Store stone in a sealed container in a cool, dry room (moisture affects setting expansion).
  
• Use room-temperature distilled water.
  
• Proportions are important. The ratio of water to powder governs the resulting compressive strength, resistance to abrasion, and brittleness.

PLASTER 55-65 cc water to 100 grams of powder by weight.
MODEL STONE 27-33 cc water to 100 grams of powder by weight.
DIE STONE 18-25 cc water to 100 grams of powder by weight.

• Add the powder to the liquid in a clean plaster bowl.
  
• Mix well, creamy with no air bubbles. Do not beat the mixture. Scrape sides of bowl often.
• Hold either the tray handle or your finger on the vibrator, not the tray itself.
  
• Add the stone mixture slowly starting at the most posterior tooth on one side. Allow the stone portion to push the excess water out the other side ahead of the mix.
  
• If the impression is vibrated loose from the tray, take a new impression.
  
• Pour the model and base in one mix of stone.

6) For the lower impression, place a wet paper towel in the tongue area of the tray.

7) Handle set casts properly. Erosion occurs rapidly if casts are in water for more than 1 minute.

Information compiled by Jane Busch, DDS and Director of Education at D&S Dental Laboratory, Inc. 1-800-236-3859.