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The Key to the Perfect Impression
Gary M. Radz, DDS
There appears to be universal agreement that far too many inadequate and unreadable impressions are being sent to dental laboratories.1-3 This creates daily frustration for the technicians as they try to fabricate a clinically acceptable restoration with less-than-adequate information.
All too often, the dentist will blame the impression material. However, impression materials are among the most developed and reliable of all dental materials.1 Currently, many excellent choices are available.
More often than not, the fault lies with us, the dentists. We may be overlooking essential details, such as the proper handling of the soft tissue during tooth preparation or management of the soft tissue immediately before taking the impression. If correct technique is used and appropriate attention paid to the management of the soft tissue, the chance of capturing a clinically acceptable impression is dramatically increased.
Many simple techniques and beneficial materials can be applied to help the dentist properly manage the soft tissue, thereby helping to obtain a more ideal impression.
Soft-Tissue Management During Tooth Preparation
In an ideal world, excellent soft-tissue health would be a prerequisite for predictable impressions. Inflamed tissues will bleed more readily and exhibit increased crevicular fluid flow, rendering moisture control more difficult.4 However, the reality of private practice does not always provide the opportunity to consistently have ideal soft-tissue health. Despite having less-than-ideal conditions, the capture of an excellent impression is still possible.
During tooth preparation, it is critical to minimize, if not eliminate, soft-tissue trauma. This trauma will create fluid flow, making it more difficult to manage the area when the time comes to take the final impression. There is general agreement that during tooth preparation, the soft tissue should be mechanically displaced using a gingival retraction cord.2,3,5 A single cord of the appropriate size is placed to deflect the soft tissue from the path of the rotary instrument, thereby decreasing the opportunity to mistakenly touch the tissue. Inevitably, this is not possible 100% of the time, but this technique will certainly minimize the amount of trauma created during tooth preparation.
Tissue Management for Final Impression
The use of a two-cord technique is a time-proven and effective way to properly deflect and control the soft tissue in order to capture the margin of the tooth preparation in its entirety.1-7 During the tooth preparation, the clinician will leave in place the initial cord (or replace it if damaged during preparation), and then position a second cord on the first. Research has demonstrated that this second cord should remain for 4 minutes before the final impression is taken.8 After this time span, the top cord is pulled and the final impression taken. Use of this technique consistently produces excellent impressions.
Another option is the use of a single-cord technique. This method can work well with tooth preparations that terminate supragingivally or at the tissue height. In today’s age of all-ceramic restorations, we frequently find that the placement of subgingival margins is not always necessary.
With this technique, the cord used to displace the tissue during tooth preparation is kept in place (or replaced if damaged) for the final impression. Often, this technique will work very well, provided the clinician maintains control of the soft tissues and the related fluids in the area.
The placement of retraction cord is frequently uncomfortable for the patient. In areas of less-than-ideal soft-tissue health, it can lead to more bleeding. Recently, the introduction of gingival retraction pastes has provided dentists with a more comfortable and less traumatic option. These pastes are placed in the gingival sulcus and are stiff enough to physically displace the soft tissue and allow for better exposure of the preparation margin. Also, these products have aluminum chloride, which will provide for localized hemostasis.
These materials work best with preparations located at the height of the tissue. Another application that can function well is the use the retraction paste instead of placement of a second retraction cord. Virtually, this is the two-cord technique without the second cord.
Clinical situations can arise in which the soft tissue is too bulky or too inflamed, hindering visualization of the preparation margin. In these situations, mechanical removal of the soft tissue may be indicated.
The use of electrosurgery or laser surgery provides the opportunity to remove excess soft tissue, which will enable the clinician to see where to place the margin of the final restoration. The tissue removal will also allow for exposure of the preparation margin when it is time to take the final impression.
Gingivectomies with either an electrosurge or laser can be practical ways of dealing with excessive and/or irritated soft tissue. However, the dentist needs to be aware of the concept of biological width.5 Impinging on the biological width can lead to long-term failure of the final restoration.
Fluid Management for the Final Impression
Crevicular fluids and blood can and will lead to an inaccurate final impression. The dentist must have the area controlled before attempting to take the impression. Several techniques are available for the management of the fluids.
The use of a retraction cord is the first line of defense to control fluid flow. When placed in the gingival sulcus, the retraction cord will physically block crevicular fluids from the preparation margin. In addition, retraction cords can be impregnated with epinephrine, which is an excellent hemostatic agent. It can minimize any bleeding that may be in the preparation area.
The use of electrosurgery or laser surgery is not only effective in eliminating excess soft tissue but also can be used to provide an area of hemostasis. A slight alteration in the settings of these devices can change from a “cutting” setting to a “coagulation” setting. These tools provide a predictable and quick option to control bleeding areas.
Most commonly, bleeding is managed chemically. Ferric sulfate, aluminum chloride, and epinephrine are the most common options. These materials will cause constriction of peripheral blood vessels, resulting in a transient shrinkage of the surrounding tissues.4
For years, ferric sulfate has been the most frequently used hemostatic and has been proven to be highly effective in stopping sulcular bleeding.9 The only issue is its potential to leave an organic black residue on the tooth preparation.2,4 This is an adverse effect if placing an all-ceramic restoration.
Aluminum chloride, while not quite as effective as ferric sulfate, is another popular option for controlling localized bleeding. Its benefit is that no dark residue remains on the restoration. This makes aluminum chloride the chemical of choice when the final restoration is made of an all-ceramic or indirect composite material.
Another option is epinephrine, which stops localized bleeding through vasoconstriction. While effective, the potential for systemic interactions4 makes it the least desirable choice. However, when used in combination with a local anesthetic in a 1:50,000 concentration, it can be highly successful at controlling localized bleeding for a short period.
Digital impression devices have recently became commercially available and are proving to be very effective clinically. While providing a dramatic step in helping to create even better impressions through digital accuracy, these new tools require the clinician to continue applying precise soft-tissue management.
These devices work by capturing digital images of the preparation and surrounding area. If the device cannot “see” the preparation margin, it cannot capture it.
A bad impression is rarely caused by the material itself. If great care is not taken to manage the soft tissue during the preparation and in preparation of taking the final impression, inadequate impressions will continue to be sent to the dental laboratories. The good news is that we have many time-proven materials and techniques to help us create great impressions.
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2. Christensen GJ. Laboratories want better impressions. J Am Dent Assoc. 2007;138(4):527-529.
3. Miller MB. Impression taking–is it a lost art? Gen Dent. 2007;55(5): 392-393.
4. Lee E. Impression-taking considerations for predictable indirect restorations. Pract Proced Aesthet Dent. 2003;15(6):454-457.
5. Vakay RT, Kois JC. Universal paradigms for predictable final impressions. Compend Contin Educ Dent. 2005;26(3):199-206.
6. Terry DA. The impression process: part III–technique. Pract Proced Aesthet Dent. 2007;19(1):27-29.
7. Lowe RA. Predictable fixed prosthodontics: technique is the key to success. Compend Contin Educ Dent. 2002;23(3 suppl 1):4-12.
8. Baharav H, Laufer BZ, Langer Y, et al. The effect of displacement time on gingival crevice width. Int J Prosthodont. 1997;10(3):248253.
9. Bailey JH, Fischer DE. Procedural hemostatis and sulcular fluid control: fulcrum to modern dentistry. Ultradent Restorative Monograph. 1995:31-41.
About the Author
Gary M. Radz, DDS
Associate Clinical Professor
University of Colorado School of Dentistry