Critiquing the Impression

The lab cannot stress enough the importance of a good impression, since it is a major factor in achieving a successful hearing aid fitting. It is important that you critique your own impressions before sending them to the lab – and ideally, before your patient leaves your office, in case a remake is necessary.

Click to enlarge

Look to be sure that your impression has these features:

  1. Is the impression smooth and complete?
  2. Is the canal long enough to show the beginning of the second bend?
  3. Is the helix clearly defined?
  4. Is the tragus clearly defined?
  5. Is the concha complete?

    ...and that it doesn't have these:

  1. Is the impression material itself unusually oily?
  2. Are there weld marks (caused by impression material setting up too quickly or by failing to keep the tip in the material)?
  3. Are the edges of the folds in the concha rounded and not sharp (due to mashing the material in the concha against the skull)?

Special attention should be paid to the impression's canal length and diameter. Ideally, the best impression is one which fills the canal about 5-7 mm beyond the second bend, but it may not always be possible to reach this depth. In the finished earmold, the canal length may vary depending on the degree of hearing loss.

For mild losses, the canal may be short and reach just the first bend in the canal, as in many open-fit molds.

For moderate losses, the canal should go beyond the first bend, and the diameter should be able to accommodate special acoustic options you may order.

In severe losses, the canal needs to be long and extend to or beyond the second bend, and the diameter should always be full.

But always take the most complete impression you can. Too much canal length on an impression is never a problem for the lab; too little is.

Remember also that in most cases the acoustic seal of an earmold occurs between the aperture and the first bend, not deep within the canal. Too much canal length on a mold may be uncomfortable for the wearer, and can make the mold very difficult to insert and remove. Correct canal length depends entirely on the requirements of the individual patient.

On a daily basis, labs receive impressions which cannot be made into earmolds. It's that simple! When this happens, they call and request that you obtain a new impression. This is expensive for both you and the lab. Please be assured that they would not go to this added expense and delay if they felt they could do a proper job from the original impression.

This illustrates various defects that can appear in impressions due to improper technique. Be sure to check your impression against the one at the upper left. If it isn't complete, please remake it.

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