Taking an Alginate Impression
What is it?
An alginate impression is used in dentistry to make an exact replica of the patient’s teeth, gingiva, and surrounding tissues in the mouth.
The alginate dental impression forms an imprint (i.e. a ‘negative’ mould) of those teeth and gums, which can then be used to make a cast or ‘positive’ model of the patient’s dentition.
Alginate impression material begins as a powder. When mixed with water, a thick pasty material, similar to cake frosting, appears. The dental impression tray is filled with the viscous alginate impression material and placed over the teeth, one arch at a time.
The alginate impression material sets after 30 to 60 seconds in the patient’s mouth. Cold water prolongs the setting time of alginate, while warm water shortens the set time considerably. Once the alginate impression material has set, it becomes solid.
When ready to create the final study models, the operator will fill the set alginate impression material with a stone mixture.
Why do we take impressions?
Alginate impressions are taken for the fabrication of study models. Once we have a model poured in stone we can use them to make whitening trays, appliances, mouthguards and more.
How to take an alginate impression:
• Start by selecting a tray size that is slightly larger than arch.
• Fluff or aerate the Jeltrate powder before dispensing. Pour measured amount of water into a clean mixing bowl and then add powder. This method is how we learned is school However it is not very practical when setting up for procedures ahead of time.
• The number of recommended scoops depends on the procedure and size of the tray.
• Fill scoop with powder, tap side with spatula then level off do not pack powder to tightly in scoop you will have to much powder to water ratio for mix.
• Two scoops are generally used for full arch impressions. Measure the matching amount of water with cup provided.
• The higher the water temperature the faster the impression material will set up. Use cooler water to allow more working time.
• If hand-mixing, rapidly mix alginate with a spatula, pressing the mixture against the side of the bowl. Mixing time is approximately1 minute. At this point, the mix should be a smooth, creamy consistency that does not fall off spatula.
• Collect alginate from sides of bowl with spatula and place the impression material evenly in the tray.
• Set time is three minutes from start of mix. Fast set alginate is two minutes.
• It is recommended to wipe alginate on occlusal surfaces before seating tray to help prevent occurrence of trapping bubbles in impression.
Lower tray impression technique:
For taking lower impressions the operator’s position should be at 8 to 9 o’clock, your elbow. Should be the same height as the patients shoulders.
• Take the impression from in front of the patient. Roll it in the mouth one side then the other, seat in a even motion pulling lips out around the tray, once seated, ask patient to lift their tongue, hold tray with your fingers until material is set.
• To remove tray pull out the cheeks to break seal, protect the opposing teeth , snap it out rolling it out the same as you put it in one side then the other.
• You should have a lip roll and all of the teeth and no bubbles in the impression.
• Wrap impression in a wet paper towel until you can pour it up.
Spray the impression with a disinfectant and drain, before pouring with stone or plaster.
Maxillary tray impression technique:
For taking upper impressions the operator’s position should be at 9 or 12 o’clock if you are right handed and dental chair should be raised so you are not bending over.
To help prevent gagging:
• Use fast set alginate or water that is not to cold
• Do not overfill the tray
• Standing behind the patient enter with one corner of tray, seat posterior first then roll anterior. Ask patient to breath thru their nose and till chin down. Hold with finger till set.
• Once set, retract cheeks to loosen and roll out.
• make sure there are no voids in impression material.
• Wrap in a wet paper towel.
Spray the impression with a disinfectant and drain, before pouring with stone or plaster.
Do not allow the disinfectant to lay in the impression longer than the recommended “kill time” as the solution can distort the alginate.
For best results, the model should be poured within 30 minutes after the impression is obtained prevent distortion.
For more information:
References: Elements of Dental Materials for Dental Hygienist and Assistants by Phillips, Fourth Edition
https://www.greatlakesortho.com/content/Content-GLO/Library-Resources/Manuals-Guides-Appliances/Impression-Work-Model-Instr-S218.pdf
https://www.youtube.com/watch?v=wnFfKVUzR0A
