Taking Accurate Impressions: The Classic Approach

Posted by Dr. Patrice Smith on Wed, Jan 20, 2016 @ 02:00 PM


Image source: John Syrbu, Dental Art & Humor

The field of dentistry has experienced rapid growth and change throughout the years. There have been advances in technology, which has vastly improved our modes of operation. We have seen our equipment and materials get better, which in turn has made us more efficient and has simplified our lives.

One such area of growth is impression taking. Every so often a manufacturer will pose a claim regarding the superiority and accuracy of its equipment and impression materials, whether conventional or digital. Regardless of the equipment or material of choice, the bottom line in taking an impression is that it provides the likeness of the structures being captured, that it captures detail, and (most importantly) that it is accurate.

Taking an accurate impression can be very technique sensitive and has been proven to be one of the most difficult parts of a procedure for some dentists. An accurate impression is critical in ensuring an accurate diagnosis, the proper fit of appliances, prostheses, and restorations, and for minimizing chairside adjustments (Figure 1).

Figure 1. Proper retraction is key to obtaining an accurate impression that captures all the details of the prepared tooth or teeth. (Courtesy: E. Lee)

There are several techniques and certainly several different types of impression materials that have been introduced to dentists over the years. We can place impression materials--at least conventional options--into elastic and non-elastic categories. The elastic materials include alginate (irreversible hydrocolloid), agar (reversible hydrocolloid), polysulfide, condensation silicone, addition silicone (polyvinyl siloxane/PVS), and polyether. The non-elastic materials include plaster, dental compound, and zinc oxide eugenol pastes. (The properties of which have all been discussed ad nauseam in your dental materials classes (smile)). When evaluating these impression materials, most dentists tend to be more concerned with hydrophilicity, setting time, and cost – and rightly so.

However, there are other characteristics that beg to be considered. Such characteristics include viscosity, dimensional stability, detail reproducibility, tear resistance, ease of removal, shelf life, acceptable taste/odor, gypsum compatibility, among others. Simple research into the properties and characteristics of different materials, with consideration given to the kind of impression you are taking, should be very beneficial. For example, taking an impression for a study model versus one of a tooth prepped to receive a crown with margins placed subgingivally will call for completely different impression materials. Needless to say, the latter requires material that is able to capture far greater detail than that of a study model.

There Is More Than One Right Option

Figure 2. Illustration conveys the application of light-body and heavy-body materials for impression making.

The keys to successfully taking accurate impressions are one's impression-taking technique, material, and choice of impression trays. There is no "one size fits all" when it comes to technique, and most practitioners develop their own based on what works well for them through a series of trial and error. Some systems utilize a gun/cartridge, are automated, or are hand mixed. There are pros and cons to each system depending on the intended use of the material, and it is up to the practitioner to find what works best for him or her. Implementing a retraction technique that will allow for visualization of the preparation margins or for retraction to capture the vestibule and muscles in that area should also be a consideration when thinking about honing technique (Figure 2).

Choices of impression trays are also vital. They can be metal, plastic, or Styrofoam, and are sterilizable or disposable. Many come with retention grooves or perforations and/or can be painted with a coat of adhesive to retain the material inside of the tray. There exist full-arch trays, sectional trays, dual-arch trays; one can even make trays customized for each patient (i.e., custom trays). The proper size tray for the particular patient should also be selected keeping in mind that all structures should be covered (including the last molar(s) in the arch), it should not impinge on the soft tissues, and the tray should not interfere with the dentition.  

Regardless of the technique or material that is used, a good impression should ensure:

  • Visible muscle definition
  • Clear representation of the midline
  • Smooth anatomical contours
  • Proper depths showing all gingival tissue margins
  • Molars on distal segment are fully captured
  • Impression material rolled over the edge of the tray (for strength and retention)

Impression taking seems a mundane topic and a pretty simple concept to grasp, but it is surprisingly a source of frustration in some dental practices. To avoid this frustration, improve your knowledge of the different materials on the market, or consider solidifying and honing good technique. Not only will this make you an efficient, detailed-oriented dentist, but it will prove to be very cost effective as well.  


Need more help taking an accurate impression? THE NEXTDDS has many other resources and step-by-step technique guides available to support you. What are you waiting for? Enroll today!

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Tags: dental impressions, impression making, impressions

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