Impression Techniques

An important part of the impression taking technique is the ability of the hearing aid professional to put the patient at ease. Always explain the procedures, as people fear the unknown. Inform the patient that you are going to take an impression of the ear with a soft material that must set up for as much as ten minutes (it is usually half that time with modern silicone materials). At this point also tell the patient that if the impression you are now taking is not perfect, you will be making a second one to assure the success and comfort of the fitting. In some instances, two impressions may be required by the lab.

Poor impressions will cause problems. The causes of poor impressions can be either the fitter's technique or the improper use of the impression material.

Historically there have been three widely practiced methods of taking an ear impression – the hand packing method, the syringe method and the cartridge-gun method. Earmold labs' experience shows that better impressions result from the syringe method, and we endorse their findings in our recommendations.

Once you are proficient with the syringe, you may wish to try the cartridge-gun systems, but the odds say you will get better impressions with the syringe. Several models are available, ranging from inexpensive acrylic to permanent stainless steel construction.

Set up your tools and materials in advance. Wash your hands. Examine the patient's ear with the otoscope, and trim hair if necessary. A canal block should then be set in the ear canal just beyond the second bend, and must occlude the full diameter of the ear canal. The canal block serves these vital functions:

  1. It prevents impression material from reaching the eardrum.
  2. It acts as a safety catch if impression material sticks in the canal.
  3. It assures a complete canal cross-section.

DO NOT use a canal block that does not compress against the canal wall. Too small a diameter plug may permit impression material to flow beyond the block and encroach on the eardrum.

Canal blocks must be trimmed with scissors for narrow canals and enlarged for big canals by using more than one. Canal blocks are available in cotton and foam, either of which is quite satisfactory when used properly. Specially sized foam blocks are available for CIC (deep-canal) impressions and should be used to protect the eardrum from possible damage.

Certain patients have ears that have been surgically altered. The plugs for these ears must be placed by a physician prior to taking impressions.

Once the canal blocks are properly positioned, the impression material should be mixed.

If you are using powder and liquid material, place the liquid in the mixing cup first, then the powder. Mix thoroughly with a spatula to remove all lumps of powder. As soon as it is mixed smoothly throughout, place the material in the syringe with the spatula. Work efficiently because of cure time, but never use more liquid than the instructions call for. Don't rush this material; let it cure the full 10 minutes.

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With materials having a tube catalyst, overfill the scoop with base material and then level it, pressing it with your finger to remove air bubbles. Put the excess material back in the jar for future use. Roll the base material out of the scoop and flatten it in the palm of your hand. Make an indentation in it with the overturned scoop.

Squeeze out one line of catalyst the length of the indentation. Fold the edges of the base material over the catalyst and blend the material with your fingers for 20-30 seconds, folding and kneading until the material is of consistent color throughout. Roll the material between the palms of your hands, insert into the syringe and inject into the ear. Allow this material to cure for 10 minutes.

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If you are using hand-mixed A+B polysiloxane (silicone-addition) impression materials, please take note of the following cautions:

  1. Two plastic spoons accompany these materials. Never alternate spoons between the materials. Always use the white spoon in the white material and the colored spoon in the colored material. Never store the spoons in the jars.

  2. Do not use latex gloves with silicone impression materials, as they can cause the material not to set up. Purple nitrile gloves are OK, though.

To use, scoop out one level spoonful of each "A" and "B" material. Transfer them to your hand, taking great care not to contaminate a spoon with its opposite material. Knead the material together with your fingers for 20-30 seconds, until it is of consistent color throughout. DO NOT roll the material vigorously between your palms; this creates heat that will shorten working time. Roll it gently instead, insert into the syringe, and proceed with the impression procedure. Allow the material to set up 5-6 minutes before removing the impression.

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Let's look at proper syringe technique:

Step 1

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A cotton or foam block is an absolute necessity when using the syringe. Set a tight block just past the second bend. Foam blocks must be compressed to insure proper results. Be sure to use the correct size foam block even though it may appear to be larger in diameter than the ear canal.

Step 2

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Mix the impression material according to instructions and place in the barrel of the syringe. The quicker you can use the material the better the impression.

Step 3

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Insert the plunger and gently push the material into the nozzle to remove air pockets.

Step 4

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Place the nozzle into the canal and fill the canal. Keep the syringe tip buried in the material at all times.

Step 5

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As material fills the canal, slowly withdraw the syringe and fill the helix and concha areas completely. Then cover the tragus. Never allow the tip to come out of the material until you are done.

Step 6

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When the external ear has been filled completely, touch your finger gently in the concha and helix areas. Be careful not to press hard as the impression will distort.

Step 7

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Allow a full cure time before removing. The impression will be distorted if removed too soon. To remove, gently press the ear away from the impression. Remove the helix curl slightly. Ask the patient to yawn or flex the jaw to help break the seal. Bring the impression straight out while holding (not pulling) the thread from the canal block. Take your time. Don't strain the impression with a long steady pull. Long fingernails can be a serious problem in impression removal.

Step 8 Re-inspect the ear with the otoscope to be certain that all the material is out and that there has been no tissue damage.

Helpful Tips for Better Impressions

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  1. If the patient wears glasses or dentures, make sure these are in place while taking the impression.

  2. Never flatten or smooth out the finished impression with the palm of your hand while impression material is in the patient's ear.

  3. Use the open-jaw impression technique, particularly if you detect cartilage movement within the canal walls. We'll talk about this technique in a bit.

  4. Children are sometimes fearful and can be hard to work with. Let the child watch you take an impression of Mother's ear to alleviate fears. Let him play with a piece of the "dough."
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