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The “Second-Cord Technique” for Taking Great Impressions
By Thomas R. McDonald, DMD
There are many factorsinvolved in capturing a quality crown-and-bridge impression, but most clinicians would agree that the two most important are selecting the correct impression material and executing a precise clinical technique. Developing that technique, however, can take years of trial and error. Fortunately, the author discovered a process 25 years ago that has been serving him well ever since, delivering consistently excellent results. The procedure works on single-unit and multi-unit cases, as well as challenging situations with deep existing restorations and/or inflamed periodontal tissues. Since putting this technique into practice, many laboratory technicians have commented on the flawless impressions that result.
This technique relies on two components: a unique application of retraction cord, and the use of polyether impression material. As a long-time polyether user, the author has used the material in just about every possible clinical situation, and has found that it performs better than any other material, both in the office and in the dental laboratory. In his experience as a speaker for dental laboratories across the country, technicians often strongly express a preference for working with polyether impressions. They explain quite frequently that they have fewer remakes with polyether than with other impression materials.
In both the operatory and the laboratory, polyether provides significant benefits. Its superior performance can be attributed to several properties. First, its initial hydrophilicity helps to prevent voids, even in a moist environment.1 Additionally, its flow properties help facilitate the capture of the finest detail, resulting in better-fitting restorations.2 Polyether’s “snap set” characteristic means it will maintain a low viscosity until the end of the working time, and then set almost immediately at the culmination to provide a most accurate impression. Finally, its stiffness and dimensional stability make it most suitable for full-arch application with complex restorative procedures and implants. These properties, coupled with the proper impression technique, make polyether an ideal material for nearly any clinical situation.
Unique Use of Retraction Cord
In addition to polyether impression material, this technique necessitates the use of woven retraction cord that has been pre-treated with Hemodent™ (Premier Dental, Plymouth Meeting, PA). While most cords come already impregnated with a hemostatic agent, for this procedure they are additionally soaked in liquid Hemodent™ and dried before use. Two cords are used at different points in the procedure, and the tooth is prepared with a cord in place so that the tissue is neverabraded during tooth preparation.
During tooth preparation, occlusal reduction is performed and contact is broken as early in the procedure as possible. Before final margin placement, a 1-mm retraction cord is placed into the sulcus. The rationale is that if the tissue is not abraded while the tooth is being prepared, there will be no hemorrhage and fluid to complicate the impression procedure later. The first retraction cord is left in place while the margins are prepared and during fabrication of the provisional restoration.
The second component of this technique takes place immediately before the impression is recorded. At that point, the first cord is removed and replaced with a second cord into the sulcus while the impression material is being prepared. The second cord is in place for less than a minute and is removed before taking the impression. The author discovered the benefit of using the second cord quite accidentally. Often, the first cord provides inadequate tissue retraction, resulting in areas where the margins are not completely visible. Removing the first cord and briefly replacing it with a second cord provides an additional dilation of the sulcus. If there is any hemorrhage or sulcular fluid accumulation after tooth preparation, the second cord solves the problem by physically displacing and chemically treating the sulcular tissue with the astringent. The retraction cord provides both a mechanical and chemical dilation of the sulcus.
It is necessary to use woven retraction cords for this procedure, as opposed to braided cord. A braided cord will catch on the bur during tooth preparation if it is accidentally touched, and can be traumatically flung out of the sulcus. Generally, a woven cord is not prone to catching the bur and has the additional advantage of being less expensive. A 1-mm size is used, which allows tooth preparation to be completed without causing gingival trauma while allowing precise margin placement. The second cord is typically one size larger than the first. The clinician must be able to visualize 360° of the margin without tissue contact.
This simple technique can be used with single or multiple teeth, veneers, full crowns, inlays, or onlays. For multi-unit cases, after performing occlusal reduction and breaking contact, a cord is placed around each tooth. All preparations are completed and provisional restorations fabricated with this cord in place. Just before recording the impression, the first cords are removed and replaced with a second set.
This case demonstrates the technique in use on a single-crown procedure on tooth No. 5 (Figure 1). After performing the occlusal reduction(Figure 2) and breaking contacts with theadjacent teeth (Figure 3), a 1-mm retractioncord was placed into the sulcus, in a singlestrand with no overlap (Figure 4). The rest ofthe preparation was completed and the provisional restoration was fabricated (Figure 5 and Figure 6). The first cord was then removed and a second cord (1.5 mm) was placed before the impression step, as previously described (Figure 7, Figure 8, Figure 9, Figure 10, Figure 11).
A polyether impression material, Impregum™ Medium Body (3M ESPE, St. Paul, MN) was mixed in the Pentamix 3™ Automatic Mixing Unit (3M ESPE) and dispensed into a Crystal Clear™ plastic impression tray (Affordable Dental Products, Levit-town, NY), which had been painted with adhesive 24 hours in advance. Applying the adhesive a full 24 hours in advance is essential for these procedures; the adhesive will not predictably retain the impression material otherwise. It should be noted that, whenever possible, the author uses Directed Flow Impression Trays™ (3M ESPE) which contain a self-adhesive fleece strip and a rim-lock, eliminating the need for tray adhesive. Depending on the clinical situation, trays are fitted onto diagnostic casts or in the mouth before tooth preparation. Cases not appropriate for the Directed Flow trays can be managed by modifying the Crystal Clear trays with heat and/or reshaping.
The second retraction cord was removed and the impression material was injected around the preparation (Figure 12) and distributed with a stream of air from the air-water syringe (Figure 13). The impression material was re-injected around the tooth (Figure 14). Then, the filled tray was seated in the mouth and left in place for 4 minutes (Figure 15), after which it was removed and inspected, and the provisional was placed (Figure 16). The author has not found it necessary to use a different weight material for the syringe and tray. Therefore, a medium body material was used for both.
A Simple Solution
The final crown exhibits an excellent fit, highlighting the accuracy of the second-cord technique paired with the use of polyether impression material and excellentlaboratory technique. Additionally, dispensing from an automatic mixing unit, in the author’s opinion, allows for the best possible mix, and enables the tray to be quickly loaded without voids—two benefits that are key to capturing an excellent impression. Additionally, it gives the clinician an ability to use the precise amount of product needed for the procedure, thereby receiving the greatest value for theimpression material purchased.
From the dental laboratory’s perspective, the ability to pour the same impression multiple times is a desirable property. Impregum possesses a unique resistance to deformation during multiple pouring. Complex restorative, esthetic, and implant cases require multiple models to accomplish predictable results. Diagnostic wax-ups, provisional casts, refractory models, and master dies are often interchanged during restoration fabrication. The author has determined that Impregum is particularly well-suited for this type of dentistry.
Taking an accurate impression is a prerequisite for all restorative dentistry; therefore, it is vital for practitioners to perfect their technique. Using polyether impression material and the second-cord technique is a simple and cost-effective method for excellent impressioning. By taking the time to ensure accurate impressions, dentists can improve relationships with their patients and the dental laboratory. Re-takes and re-makes can be eliminated, adjustments are reduced, and clinicians can deliver consistent, high-quality restorations.
1. Michalakis KX,Bakopoulou A, Hirayama H, et al. Pre- andpost-set hydrophilicity of elastomeric impression materials. J Prosthodont. 2007;16(4):238-248.
2. Data on file. 3M ESPE
About the Author
Thomas R. McDonald, DMD