July 1, 2012
by Robert A. Lowe, DDS, FAGD, FICD, FADI, FACD
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. We often think of this procedure as “basic”, but dental laboratories will many times report that “only 20-30% of the master impressions that they receive are clinically excellent!” Because of the nature of the oral environment, moisture control and crevicular bleeding are two things that can make even the most “simple” impression hard to make properly. Marginal detail and tooth structure apical to the restorative margin are both necessary elements of an acceptable master 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 aesthetic materials 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.
EQUICREVICULAR AND INTRACREVICULAR IMPRESSIONS: THE TWO-CORD TECHNIQUE
A two-cord impression technique is utilized 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 #00 cord 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 (Fig. 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. If desired, the cords may be soaked in a hemostatic solution then dried with a 2X2 prior to placement. At this point, fine margin correction is accomplished with the rounded, tapered diamond bur using a plastic or cord packing instrument to protect the gingival tissues (Fig. 2). This is a critical correction since it is not uncommon to accidently cut too far axially with the rounded diamond creating a “J” margin that leaves a “lip” of tooth structure at the outward extreme of the prepared margin. This “lip” can break off or abrade on the die during the fabrication process resulting in an open margin on the definitive restoration. The use of a minimum of 4X magnification (Orascoptic Prisms: Kerr Corporation) is also critical to visualize any minor discrepancies or “nicks” in the margin that must be corrected prior to impression making. If these minor imperfections are not seen and corrected, they will be present on the dies and can create problems with the marginal integrity of the definitive restoration. Next, a #1 cord is placed on top of the #0 in the same fashion (Fig. 3). After placement of the #1 cord at the level of the restorative margin, no deeper, the operator should be able to see the “blue” of the cord 360 degrees circumferentially around the preparation. (Fig. 4). If there is an area, no matter how small, where tissue overlaps the cord, this must be addressed! Do not rely on what we were told in dental school, that the heavy body would “magically”, hydraulically push the light body into the subgingival area. That same force can just as easily push the marginal gingival against the preparation margin and not let the light body get into the intracrevicular space! If there is some tissue overlap of the top cord either 1) use a dental laser, or electrosurgery device (Sensimatic Electrosurge 600SE: Parkell, Inc) to do a “conservative” gingivoplasty above the level of the #1 retraction cord, or 2) Place a small “third” piece of cord, either #1 or #2, on top of the originally placed #1 cord to gain further retraction in that localized area.
The preparation is then cleansed with a dentin desensitizer 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 (Fig. 5). The impression should capture not only the entire restorative margin, but also about .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 after the #1 cord is removed, 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).” When injecting the light bodied material, make sure to “push” the material in front of the tip as you move the impression material around the sulcus (Fig. 6 to 8). This will avoid the potential of trapping air between the impression material and floor of the sulcus, which can happen when “pulling” the material around the tooth. To capture a precision impression, light bodied impression material (Fig. 9) 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 filled and placed in the mouth for the appropriate time based on manufacturers’ 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. This will help to ensure proper marginal trimming of dies by the laboratory and correct restorative emergence profiles (Fig. 10).
BITE REGISTRATION, PROVISIONALIZATION, PROVISIONAL CEMENTATION
Once the master impression is made, a bite registration in centric occlusion position is taken using a PVS bite registration material. Next, using a provisional stent made preoperatively, a provisional crown is constructed out of a provisional material. Upon completion, the provisional restoration is cemented using a temporary cement. After checking for proximal and marginal fit, the provisional crown is filled with cement, and then seated with finger pressure. Any small remaining amounts of resin temporary cement can be easily removed using an explorer. Finally, the #00 cord is removed after cementation of the provisional restoration to ensure complete removal of temporary cement from the intracrevicular area. Leaving this cord in place during provisional cementation also helps to prevent bleeding from interfering with the cementation process.
DELIVERY OF THE DEFINITIVE RESTORATION
When the restoration is delivered from the laboratory, marginal fit and accuracy of the dies are checked (Fig. 11). The provisional restoration is removed after administration of local anesthetic. The preparati
ons are cleansed with an antimicrobial solution (Fig. 12) and the restoration is tried on the preparations to check marginal fit, proximal and occlusal contacts. Adjustments are made as needed, the restoration is radiographed in place to verify seat, then, it is ready for cementation. For this case, Ceramir (Doxa Dental) was chosen to cement the case. Ceramir has shown increased retention to Zirconia restorations when compared to traditional resin modified glass ionomer cements. After the cement is set, it is cleaned from the margins using an explorer tip (Fig. 13). Another advantage of Ceramir is that it is very easy to clean up. Figure 14 shows the restoration after delivery.
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. When properly done, the dental ceramist can create dental restorations that defy detection. OH
Dr. Robert A. Lowe graduated magna cum laude from Loyola University School of Dentistry in 1982 and was an Assistant Professor in Operative Dentistry until its closure in 1993. Since January of 2000, Dr. Lowe has been in private practice in Charlotte, North Carolina. Dr. Lowe lectures internationally and publishes in well-known dental journals on esthetic and restorative dentistry. He is a clinical evaluator of materials and products with many prominent dental manufacturers.
Dr. Lowe can be reached at: firstname.lastname@example.org.
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