Impression Technique: Review of Survey Findings

Impressions can be considered one of the fundamentals for fixed prosthetic dental care. A superb impression ranks with tissue management and prep design. I took a survey of dental laboratories across the country to get an idea of the quality of impressions that the labs were receiving. This article is a review of the results of this survey, including some suggestions on impression techniques.

SURVEY OUTLINE

In the survey, the labs were asked to respond on a scale of 1 to 5, with 1 being rarely, and 5 being often, divided into 20% increments. Note that 20% would range from 0% to 20%; 40% range would be 21% to 40%; 60% range would be 41% to 60%; 80% would range from 61% to 80%; and 81+% would be 81% to 100%. The number accompanying each percentage represents a lab in total, which represents hundreds of impressions for each lab.

Not all questions have the same total responses because some labs chose to write notes for some questions instead of giving a ranking. These responses are discussed at the end of the article.

All the questions in the survey are variations on the same theme.

           

QUESTION 1
How often do you receive impressions with hard-to-read margins?

Lab Responses

•Four stated 20% of the time.

•Four stated 40% of the time.

•Eight stated 60% of the time.

•Eight stated 80% of the time.

•Two stated 81+% of the time.

Author Comments

The lab has only the impression and the model to aid in the construction of the prosthesis. If the crown fits the die, what more can we ask of our lab?

Are your margins clear and visible, are all internal line angles easy to read, are crown margins easy to delineate, and are all internal line angles for inlays and onlays easy to identify? Examine your impressions with the aid of magnification; if all the margins are not easily discerned, either trim the impression and take a wash (if your material allows this technique) or start over. You have the advantage of checking the tooth to see if it matches the impression. The lab technician has no such luxury in comparing the prepared tooth with the impression.

If you are not sure what the margins or line angles should look like for the restorative material you are planning to use, ask your lab technician for a guideline sheet. The sheet will have pictures or drawings of the ideal prep design for a given restorative material.

QUESTION 2
How often do you receive poor opposing arch impressions?

Lab Responses

•Nine stated 20% of the time.

•Eight stated 40% of the time.

•Five stated 60% of the time.

•Three stated 81+% of the time.

Author Comments

The opposing arch impression is important to achieve proper occlusion, an accurate mount of the cast, and a good view of the resident anatomy. Always examine the opposing arch with the same scrutiny as a prepared arch impression. Take a second impression if the first is not excellent. Consider taking the opposing arch in the same material as the master impression. If you use an A-silicone, polyether, or reversible hydrocolloid for your master impression, use the same for your opposing impression. If cost is a concern with an A-silicone or polyether, use a C-silicone; an excellent example would be Speedex by Coltene Whaledent. Speedex will give an accurate impression, can be taken in a minimum of time, and, if you prefer, can be sent to the laboratory to be poured in die stone. If you wear latex gloves there is no need to change to vinyl gloves while using Speedex as you do with an A-silicone.

(Questions 3, 4, and 5 are combined here, as there are some common solutions to the problems.)

QUESTION 3
How often do you receive impressions that lack a 360º marginal clarity on prepared teeth? 

Lab Responses                 

•Two stated 20% of the time.

•Five stated 40% of the time.

•Eight stated 60% of the time.

•Eleven stated 80% of the time.

•Six stated 81+% of the time.

QUESTION 4
How often do you receive impressions that lack adequate retraction to clearly read margins of prepared teeth?

Lab Responses

•Three stated 20% of the time.

•Five stated 40% of the time.

•Eight stated 60% of the time.

•Eleven stated 80% of the time.

•Six stated 81+% of the time.

QUESTION 5
How often do you receive impressions with voids present on prepared teeth and/or adjacent teeth?

       
Lab Responses                 

•Ten stated 20% of the time.

•Four stated 40% of the time.

•Four stated 60% of the time.

•Five stated 80% of the time.

•One stated 81+% of the time.

Author Comments

If you are not getting clear 360º margins, review the way you are handling the gingival tissue. Protect the tissue from laceration from the bur by either retracting with retraction cord before preparing the tooth, being careful not to catch the retraction cord during tooth preparation, or using an instrument like the Greg 4/5 by Hufriedy placed between the tooth and the tissue to protect the tissue during preparation. Of course, you can elect to do both: place cord and use some type of instrument to deflect the bur away from the tissue during preparation.

If the tissue does get cut, an excellent hemostatic agent is Viscostat or Viscostat Plus by Ultradent. There is an excellent CD by Ultradent called Tissue Management which can be  purchased for a minimal charge. The CD covers hemostatic agents as well as related topics. Remember, if you use a ferric sulfate base hemostatic agent such as Viscostat you must thoroughly clean the preparation before attempting an impression with an A-silicone impression material. The sulfate can interfere with the setting of an addition reaction silicone. Also, do not combine ferric sulfate agents and epinephrine, as precipitate will be hard to remove. If you cannot avoid tissue impingement on the prepared margins, consider the use of electrosurgery or laser to contour the tissue. Review instructions for either technique before proceeding.

If the margin is clear and clean but it is not showing in the impression, check for moisture. Make sure you dry parallel to the axial wall in a gingival direction. Make sure the air supply is clean and avoid over drying. If the tissue is dry and the margin is clearly visible, then review how the impression material is applied to the tooth. I have found the microsystem by Coltene Whaledent (with its small size) aids in placing wash material exactly where it needs to be in the intracrevicular space. The intraoral tip can be bent to the desired position, and it will hold that bend. You can place the tip at the tissue/tooth interface and get light back pressure as you place a 360º ring of impression material into the sulcus. The material is flowed in with a continuous motion without lifting the tip. Pick a material that flows easily and stays in place. Affinis or fast-set regular wash by Coltene Whaledent is my current choice. By keeping steady pressure and not lifting the tip in and out of the material, you should be able to avoid bubbles at the margins.

QUESTION 6
How often do you receive impressions with lack of wash material in two-step impression techniques, ie, putty separate wash or heavy-body separate wash?

       

Lab Responses

•Seven stated 20% of the time.

•Six stated 40% of the time.

•Five stated 60% of the time.

•Five stated 80% of the time.

Author Comments

When using a two-stage impression technique remember that the first impression must be completely and thoroughly trimmed, or at reseating the initial impression can be distorted. If you don’t want to use the trim technique, place a spacer over the dentition or place the loaded impression tray into a plastic baggie, and take the initial impression. The spacer or the baggie will make trimming the impression unnecessary. Make sure the initial impression is clean and dry. Don’t underfill or use too little wash material. The wash should cover all important and relevant surfaces. Make sure that at the second step of a two-stage impression technique, the impression is fully and completely seated and held in place with steady, consistent pressure.

QUESTION 7
How often do you receive poor bite registrations, ie, (a) bite registration material too flexible, (b) bite registration too hard, (c) bite registration taken before teeth prepared, (d) no bite registration, (e) distorted bites?

Lab Responses No. 7a

•Sixteen stated 20% of the time.

•Thirteen stated 40% of the time.

•Five stated 60%of the time.

•Two stated 80% of the time.

•Five stated 81+% of the time.

No. 7b

•Eleven stated 20% of the time.

•Ten stated 40% of the time.

•Six stated 60% of the time.

•Two stated 80% of the time.

•Two stated 81+% of the time.

No. 7c

•Twenty-five stated 20% of the time.

•Four stated 40% of the time.

•Six stated 60% of the time.

•Two stated 81+% of the time.

No. 7d

•Ten stated 20% of the time.

•Nine stated 40% of the time.

•Twelve stated 60% of the time.

•Five stated 80% of the time.

•Four stated 81+% of the time.

No. 7e

•Nine stated 20% of the time.

•Ten stated 40% of the time.

•Eleven stated 60% of the time.

•Six stated 80% of the time.

•Six stated 81+% of the time.

Author Comments

There are many fine bite registration materials on the market today. In the vinyl polysiloxanes, 3M ESPE, Coltene Whaledent, and Discus Dental are just a few companies that make exceptional bite registration materials. The best advice is to ask your technician what she/he wants/expects of a bite registration, and go from there.

Question 8
The last question was, for all intent, a comment section. What type of impression accounts for the highest number of remakes? Plastic triple trays were the most common answer by over 50% of the respondents. Other comments of interest, as written by the labs, include:

•“Two-stage impressions with too little wash material for the second stage.”

•“The same doctors continually send poor impressions.”

•“It isn’t the material, it isn’t the technique, it is the doctor.”

•“Too many doctors do not fully understand or care, ie, result—bad impressions.”

•“Partial arch two-stage impression with not enough body removed before wash material taken.”

•“Not following manufacturer’s recommended directions, ie, setting time, etc….”

•“The single biggest problem my laboratory has is inadequate and improper occlusal reduction, ie, improper preparations, period!”

•“We did a survey for a prominent clinician… We found over a period of thirty days, only 40% of the dies had perfect 360º margins that were easy to read.”

Here are a few suggestions regarding these comments.

(1) If you use a flexible triple tray, consider a metal tray for closed-mouth impressions. D&D Dental Trays manufactured by Superior Dental & Surgical Manufacturing Co, Inc, are the most rigid dual-arch impression trays on the market. They come in partial and full-arch trays. They will solve distortion problems on a closed-mouth impression system.

(2) The two-stage impressions must first be properly relieved; you almost cannot over trim, and do not be stingy with the wash material, as it should cover all appropriate surfaces.

(3) Read the directions on how materials are supposed to be used. Manufacturers include working/set times, recommend disinfecting materials and times, and sometimes state which die material works best with their products.

(4) Finally, review your preparation design. There are lots of new restorative materials on the market. Find out what margin design will optimize the strength and aesthetics of the material you are using.

Talk with your laboratory technician. The dialogue can be invaluable. Ask if your impressions are easy to read and if they help produce clean, crisp, clear dies. You may agree to disagree and move your work elsewhere, or you may find that with some simple changes you can make your lab technician’s life easier, and receive great work in return. With an open dialogue, I try to give my lab technician what he needs, and he makes my clinical life easier.

CONCLUSION

This article is a very basic review, but sometimes we have to look at the little details we take for granted. Remember that an impression is a way to show the excellent work that we do to someone who will appreciate our technical expertise, our lab technicians. If you are having problems with impressions, there are classes given by manufacturers on how to use their materials. You may wish to talk to your laboratory and find out if they are willing to sponsor one of the classes. I know that Coltene Whaledent participates in these courses and will send a speaker to the lab to troubleshoot and help on taking better impressions. Good luck.


Addendum

Figures 1 through 11 illustrate tips for taking better impressions.

Figure 1. Make sure all margins are easy to read.
Figure 2. A full-arch impression may provide more information to the lab. Take impressions that are lab friendly with easy-to-see and read margins.
Figure 3. Clean, crisp, easy-to-read margins are a must.
Figure 4. Make it easy for your lab technician to find the margins.
Figure 5. Don’t ask your lab to fabricate a restoration from a terrible impression.
Figure 6. D&D trays are rigid trays for closed- mouth impressions, sectional, or full-arch impressions.
Figure 7. Place cord to retract tissue.
Figure 8. If you can’t see the margin, neither can your technician.
Figure 9. Try the tray before taking the impression.
Figure 10. Viscostat can help if a hemostatic agent is needed.
Figure 11. Paint your impression tray with the proper adhesive to prevent distortion.

Acknowledgment

Thanks to all the laboratories that participated in this review; Sheryl for faxing, re-faxing, mailing, etc, to get the questionnaires out; and my  lab technician, Jim Ferrara, for his input on questions that should be asked.



Dr. LaMond is a general practitioner in Milford, Ohio, and is on staff at Miami Valley Hospital in Dayton, Ohio. He has given impression technique lectures to national audiences. He can be reached at (513) 248-0565.