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Taking the Perfect Impression

July 30, 2008
Author
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Robert A. Lowe, DDS, FAGD, FICD, FADI, FACD, FIADFE, FASDA
Taking the Perfect Impression

Introduction: Masterful Final Impressions

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. Remember in dental school hearing, “Let’s pour it up and see what we’ve got”? If you can’t see the margins in the impression, they won’t “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 to aid the dentist in achieving this result.

Restorative Margin Placement Is Dictated by the Restorative Material Chosen

Retraction techniques for master impressions will vary depending on restorative marginal placement. With today’s esthetic material options, 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 can often 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

Figure 1

Figure 1: The #00 cord is being placed in the gingival sulcus using a PFI A-6 (Hu-Friedy; Chicago, Ill.). Note that the blade of the instrument is parallel with and aligned against the root surface while exerting apical pressure to prevent tearing of the free gingival tissue.

Figure 2

Figure 2: The excess #00 cord is cut on the lingual aspect so that the ends of the cord when placed do not overlap.

Figure 3

Figure 3: If an intracrevicular margin is desired, the margin can be adjusted to the retracted level of the first cord. After cord removal the tissues will rebound, leaving an intracrevicular margin placement.

Figure 4

Figure 4: The #1 cord is shown after placement. It should be circumferentially visible from the occlusal view.

Figure 5

Figure 5: The preparation is cleansed and desensitized using AcquaSeal Dentin Desensitizer prior to taking the master impression.

Figure 6

Figure 6: The #1 cord is teased out of the gingival sulcus and the quality of the retraction is evaluated.

Figure 7

Figure 7: “Ring around the collar” (360 degree sulcular patency) is visible from the occlusal aspect.

Figure 8

Figure 8: The light bodied impression material is introduced into the retracted sulcus.

A two-cord impression technique is utilized to capture most master impressions for full coverage (circumcoronal) restorations, with both intracrevicular and equicrevicular margins (at the free gingival margin). First, a #00 cord is packed around each preparation starting from the lingual, around the proximal to the facial aspect, then back through the remaining proximal area to the lingual aspect. 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. Next, a #1 cord is placed on top of the #00 in the same fashion. If desired, the cords may be soaked in a hemostatic solution, then dried with a 2×2 prior to placement. The preparation is cleansed with AcquaSeal™ Dentin Desensitizer (AcquaMed Technologies, Inc.; West Chicago, Ill.) on a cotton pledget. When ready, the #1 cord is teased out of the sulcus using an explorer 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 millimeters 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) is 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).” 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 manufacturers’ recommendations. In the mandibular anterior region, it may be necessary to start with a #000 and place a #00 or #0 as the top cord due to the constraints of the gingival sulcus around these anatomically small teeth.

The Single-Cord Impression Technique

Figure 9

Figure 9: A view of the master impression (Aquasil [DENTSPLY Caulk; Milford, Del.]).

Figure 10

Figure 10: The resulting fixed bridge is shown from a buccal view.

Figure 11

Figure 11: The #1 cord has been teased from the facial portion of the gingival sulcus on this mandibular anterior segment. Note the visible root surface beyond the margin and the absence of sulcular fluid.

Figure 12

Figure 12: The #1 cord is fully removed showing the retracted tissue just prior to impression registration.

Figure 13

Figure 13: The master impression is shown (Honigum® [Zenith/DMG; Englewood, N.J.]).

Figure 14

Figure 14: The definitive porcelain reconstruction is shown from a frontal retracted view. Porcelain restorations fabricated using IPS d.SIGN® (Ivoclar Vivadent; Amherst, N.Y.) by DAL Signature Laboratory in Peoria, Illinois.

Figure 15

Figure 15: An occlusal view is shown of a prepared quadrant with a combination of equicrevicular and slight intracrevicular margins.

Figure 16

Figure 16: After placement of the #00 cord, Expasyl is placed on top of the #00 cord to gain further tissue deflection and drying of the sulcular environment.

Figure 17

Figure 17: The Expasyl is rinsed away and the #00 cords are left in the sulcus prior to the registration of the master impression.

Figure 18

Figure 18: The master impression is shown using StandOut (Kerr Corporation; Orange, Calif.).

Figure 19

Figure 19: The definitive restorations are shown from the occlusal view. BelleGlass™ NG (Kerr Corporation) fabricated by DAL Signature Laboratory.

Figure 20

Figure 20: Preparations for esthetic porcelain inlays are shown from the occlusal aspect. After placement of the #00 cords, 0.5 millimeters of the tooth surface apical to the restorative margin is clearly visible. The #00 cord will be left in place during impression making.

Figure 21

Figure 21: The master impression taken with Imprint™ II (3M™ ESPE™; St. Paul, Minn.).

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 two main “enemies” of impression materials are blood and crevicular fluid. Having healthy tissue before taking a master impression cannot be overemphasized. Using a technique where a cord is left in the sulcus will help maintain a dry environment to syringe impression material into. Even though 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. After placement of the single cord, usually a #00, if there is a need for further tissue “deflection,” a material such as Expasyl™ (Kerr Corporation; Orange, Calif.) can be used on top of the cord and then rinsed away prior to sulcular impression material injection. Expasyl also is an excellent astringent and drying agent, which helps promote a dry field.

The “No-Cord” Impression Technique

Figure 22

Figure 22: Placement of light body without tissue retraction is shown on this porcelain veneer case. Because it is the function of the light bodied material to capture marginal detail, first syringe the marginal area of all preparations before covering the rest of the preparations incisal or occlusal to the margins. The tray material will record these areas accurately if time does not permit coverage with light bodied material.

Figure 23

Figure 23: The double arch impression is removed from the mouth (D&D Double Bite Trays [Superior Dental and Surgical Manufacturing Company, Inc.; Port St. Lucie, Fla.]).

Figure 24

Figure 24: The master impression with Honigum.

Figure 25

Figure 25: The porcelain laminates are shown from a 1x facial view. Note the slight supragingival margin placement and the absence of gingival inflammation.

The “no-cord” impression technique is only indicated for supragingival margins where there is clearly tooth/root surface apical to the margin. A prime example would be for the supragingivally placed porcelain laminate veneers. Once the restorative margin reaches the free gingival margin, retraction is mandatory. Remember, the laboratory technician depends on tooth surface apical to the margin to create the proper emergence angle to the restoration. Without this critical information, most restorations will have improper cervical contours (overcontoured or undercontoured) leading to possible periodontal sequelae.

The “no-cord” impression technique is only indicated for supragingival margins where there is clearly tooth/root surface apical to the margin.

Conclusion: 100% or 0%

There is no “almost” in taking 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.

If you would like to contact Dr. Robert Lowe, call 704-364-4711, visit destinationsmile.com, or email boblowedds@aol.com.