Mastering the Art of Impression Making
Robert A. Lowe, DDS
The excellence and marginal fit of the definitive laboratory restorations can only be as good as the master dies from which they are created. The precision of the master impression is something that cannot be compromised. Marginal detail and tooth structure apical to the restorative margin are both necessary elements of an acceptable final impression. Without precision, the definitive restoration is doomed to clinical failure. Who remembers hearing, “Let’s pour it up and see what we’ve got” in dental school? If the margins cannot be seen in the impression, they will not “magically” appear when the impression is poured. It is important for the dentist to have a critical eye and reject all but the “perfect” master impression. Techniques will be described in this article to help the dentist to achieve this result.
Restorative Material Dictates Restorative Margin Placement
Retraction techniques for master impressions will vary depending on restorative marginal placement. With the esthetic materials options available today, the restorative margin can be located supracrevicular (above the gingival tissues), equicrevicular (at the free gingival margin), or intracrevicular (in the gingival sulcus). Porcelain-fused-to-metal crowns are often more esthetic with intracrevicular margin placement. All-ceramic restorations often can be placed at the free gingival margin, or in the case of “contact lens” porcelain veneers, slightly supragingival.This is the ideal location for dentin and enamel bonding procedures.
Intracrevicular Impressions: The Two-Cord Technique
A two-cord impression technique can be used to capture most master impressions for full coverage (circumcoronal) and facial veneer restorations with both intracrevicular and equicrevicular margins (at the free gingival margin). First, a #0 cord (GingiBRAID 0a, Dux Dental, Oxnard, CA) is packed around each preparation margin starting from the lingual-proximal to the facial aspect, then back through the remaining proximal area to the lingual aspect (Figure 1). The excess at both lingual ends is trimmed, and the ends of the cord are tucked into the lingual gingival sulcus so that the ends butt against one another. For a facial veneer preparation, the retraction cord is packed through the proximal aspect toward the lingual proximal line angle where the excess cord is cut and the remaining end is tucked into the gingival sulcus. If desired, the cords may be soaked in a hemostatic solution (Styptin, Dux Dental) and dried with a 2 x 2 roll before placement, or soaked after placement (Figure 2). Next, a #1 cord (GingiBRAID 1a, Dux Dental) is placed on top of the #0 cord in the same fashion (Figure 3 and Figure 4). The preparation is cleansed with dentin desensitizer, AcquaSeal Dentin Desensitizer (AcquaMed™ Technologies, Inc, Batavia, IL), on a cotton pledget. When ready, the #1 cord is teased out of the sulcus using an explorer (Figure 5) from the facial aspect of each preparation and the amount of retraction is evaluated. The impression should capture not only the entire restorative margin but also about 0.5 mm of the tooth-root surface apical to the margin. If the marginal gingiva adjacent to any restorative margin rebounds to contact the tooth/margin, a small piece of a larger diameter cord (#2) should be placed into the affected area for an additional minute and then removed. This added retraction should be sufficient to create a space between the tooth surface and the inner lining of the gingival sulcus. The goal of retraction is to “create a moat (space in which to inject light-bodied impression material) around the castle (tooth preparation)” (Figure 6). To capture a precision impression, light-bodied impression material should be injected not only around the prepared teeth but also over all occlusal and incisal surfaces so that the stone models can be accurately articulated. The impression tray with the heavy-bodied impression material is then placed in the mouth for the appropriate time based on the manufacturer’s recommendations. When inspecting the master impression, all preparation margins should be readily visible and a cuff of impression material must appear around all marginal areas (Figure 7). This will help to ensure proper marginal trimming of dies by the laboratory and correct restorative emergence profiles.
The Single-Cord Impression Technique
Use of a single-cord technique is most effective for equicrevicular margins where the retraction cord can be left in place while taking the impression. There is always a risk of laceration of the sulcular epithelial lining when removing a single-cord technique for intracrevicular preparations. We know that the 2 main “enemies” of impression materials are blood and crevicular fluid. The importance of having healthy tissue before making a master impression cannot be overemphasized. Using a technique where a cord is left in the sulcus will help to maintain a dry environment while the impression material is injected into the sulcus with a syringe. Although many of the current impression materials are “hydrophilic” and have a low contact angle, impressions are always more predictable when taken in a dry sulcus. If, after placement of the single cord—usually a #0—there is a need for further tissue deflection, a material such as Expasyl (Kerr Corporation, Orange, CA) can be used over the cord and rinsed away before injecting the sulcular impression material. Expasyl also is an excellent astringent and drying agent, which helps to promote a dry field.
There is no “almost” in making the perfect master impression. Control of the gingival tissues through precise provisionalization and proper retraction management will ensure repeatable excellence in this most critical step of dental reconstruction. When properly done, the dental ceramist can create dental restorations that defy detection (Figure 8).
About the Author
Robert A. Lowe, DDS
Charlotte, North Carolina