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Elastomeric Impressions in Fixed Prosthodontics I

Material Selection and Soft Tissue Management

The technical complexity inherent in fixed prosthodontic treatment is compounded by the necessity to utilize an indirect approach, where fabrication occurs extraorally at the dental laboratory and the completed restorations are subsequently retrofitted in the oral cavity. One fundamental aspect in the fabrication of indirect restorations is the procurement of an accurate impression from which a refractory model can be created to precisely replicate the intraoral environment. Although numerous materials and techniques have been advocated, predictable impression taking remains a challenge for a majority of practitioners--particularly in cases that involve multiple abutments.

Whereas the companion presentation to this will discuss the impression procedure and the role of refractory dies and models, the following will address material selection and soft tissue management in elastomeric impressions.

Review of Material Properties

Elastomeric materials currently set the standard for definitive impressions in fixed prosthodontic applications. While several materials are available, polyethers and addition reaction silicones (or "polyvinylsiloxanes") are used with the greatest frequency.1 While not an elastomer, reversible hydrocolloid is often utilized due to its inherent hydrophilic nature and unmatched wetting ability. Although this material is relatively inexpensive, it exhibits low tear strength and a lack of dimensional stability that requires the impression to be poured immediately. In addition, accurate multiple pours cannot be obtained with reversible hydrocolloid.1 Mercaptan-based polysulfides are used infrequently as a result of their highly unpleasant odor and taste as well as difficult handling characteristics. Although accurate restorations can be produced from this material, it does not rival the physical properties of alternative elastomers.1

As a group, addition reaction silicones are currently the most popular category of impression material. They are available in different viscosities and can accommodate several techniques. These materials are extremely accurate, and exhibit high tear strength, adequate dimensional stability, and neutral odor and taste. Disadvantages include their hydrophobic nature and susceptibility to inadequate polymerization as a result of latex contamination. In addition, methylmethacrylate acrylics and ferric sulfate hemostatic agents have been reported to inhibit the setting of polyvinylsiloxanes.1-3 While surfactants have been added in attempts to improve their wetting ability, true hydrophilic polyvinylsiloxanes are yet to be developed.

Condensation reaction silicones have decreased in popularity as a result of their poor dimensional stability and elastic recovery. Significant volumetric changes during polymerization require that a two-step putty tray and wash technique be utilized in order to maintain a minimum thickness of the low-viscosity material. This elastomer offers no advantage over the polyvinylsiloxanes and its use is difficult to justify.1

Polyether impression materials are often utilized as a result of their inherent hydrophilic nature and excellent wetting ability. Polyethers also exhibit favorable accuracy, elastic recovery, dimensional stability, and high tear strength. These materials are also characterized by a higher degree of stiffness, which requires a greater effort during removal, and an unpleasant taste. Since polyethers are available in different consistencies, they can be utilized with either a monophase or a dual-viscosity technique.1-3

Rationale for Material Selection

In order to be utilized in fixed prosthetic applications, an impression material must fulfill specific physical requirements (eg, accuracy, dimensional stability, elastic recovery, and tear resistance). The ability to store the impression prior to model fabrication and to subsequently produce multiple accurate pours from an individual impression are also important considerations in contemporary restorative practice. In the intraoral environment, tolerance to moisture and contamination, wetting ability, and flow characteristics significantly impact the material's ability to accurately record surface detail. The impression material should also ensure the patient's comfort while satisfying practical clinical considerations (eg, easy handling, convenient setup and cleaning, and expense). Since polyethers satisfy these requirements while also exhibiting wetting ability unsurpassed by other elastomers, they constitute the author's material of choice. Although satisfactory results can be achieved with alternative materials, evidence has suggested that polyethers display the most consistent accuracy for full-arch applications.4-7

A high-viscosity tray material  and a light-bodied syringeable polyether are specifically utilized by the author in a one-step dual-viscosity technique with the aid of a cybernetic automatic mixing system. The cybernetic unit is a precise, convenient mixing system that provides a void-free, even material with extended working periods when compared to manually mixed polyethers. In addition, the higher viscosity polyether tray material exhibits superior rheological characteristics that result in unmatched predictability. Accurate, wrinkle-free impressions can be consistently achieved even in full-arch applications and under less-than-optimal circumstances (Figures 1-2-3-4).

Soft Tissue Preparation

Achieving an accurate definitive impression is one of the most technically demanding aspects of fixed prosthodontic treatment. Its procurement cannot solely depend on the selection of materials or procedures, but rather in a protocol that encompasses adequate tooth preparation and soft tissue management. The principles that govern tooth preparation for various types of fixed restorations have been extensively covered in the literature and are not within the scope of this article. It must be stated, however, that adequate tooth reduction and finish line placement in particular may directly affect the accuracy of the impression (Figure 5).8 While the location of a finish line may vary within the sulcus, it must never impinge upon the biological width. Clinical judgment must be exercised to identify situations where surgical crown-lengthening procedures are indicated.

Concomitantly, periodontal health must be accomplished so that adequate moisture control and predictable postimpression gingival margin levels can be achieved (Figure 6).9 This is not only limited to the treatment of periodontitis and the incorporation of a proper oral hygiene regimen, but also includes surgical procedures necessary to increase the quantity and improve the quality of the mucogingival tissues.

Gingival Retraction Technique

Once the aforementioned requirements have been satisfied, the impression procedure is initiated with tissue retraction. This process has been previously described as a prepacking or double-cord impression technique.10 Nonimpregnated knitted cord is soaked in a 21.3% buffered aluminum chloride hemostatic solution, which is considered to be an effective chemical agent for tissue displacement and bleeding control.14 A thin initial cord is atraumatically packed into the sulcus around the circumference of the preparation. Any excess is trimmed to avoid overlapping (Figure 7). This cord provides hemostasis as well as vertical (and to a lesser degree horizontal) displacement of the tissue. It is allowed to remain in place lining the sulcus during the impression procedure, thus protecting the sulcular epithelium from tears.11

The tissue retraction achieved by the placement of the first cord may expose the finish lines, which may then be re-prepared to the level of the retracted gingival margin depending on specific aesthetic and functional requirements (Figure 8).10-12 Caution must be exercised at this stage to avoid excessive tissue injury as well as to prevent catching the cord with the diamond bur. The second retraction-cord layer is then carefully packed into the sulcus and allowed to remain in place for 5 minutes. Sufficient excess length of cord must be preserved to facilitate its removal, which will occur immediately prior to taking the impression (Figure 9). In the presence of gingival health, cord packing procedures should not elicit bleeding as long as they are performed atraumatically and avoid tearing of the connective tissue attachment.9,11

The diameter of the second cord is selected according to the thickness of the soft tissue wall and the degree of lateral displacement required. Generally speaking, it should be of the maximum thickness that can be atraumatically accommodated within the sulcus.11,12 Once packed into place, the secondary cord should be visible from the occlusal aspect along the full extent of the preparation (Figure 10). This is an indication that adequate lateral displacement of the soft tissues has been achieved, which results in impression margins of sufficient thickness. These margins provide increased tear-resistance upon removal and prevent distortion during pouring (Figures 11-12-13).13

Impression Tray Considerations

A rigid stock impression tray of adequate size is selected and beaded with utility rope wax on a study model to control and limit the flow of the elastomeric material. The wax also serves as a soft tissue stop during seating of the loaded impression tray to ensure that adequate material thickness is preserved. A polyether adhesive is applied to the tray and allowed to dry for 60 seconds.

Although it is generally accepted that accuracy is enhanced with the use of custom trays,14,15 polyethers utilized in conjunction with stock trays achieve or exceed results obtained with alternative materials.16 The author has experienced no clinically discernible difference in accuracy between custom or stock trays when utilizing the recommended technique. Second only to the silicone putty materials in viscosity and stiffness, the high-viscosity polyether tray material is an accurate space filler that effectively supports the syringeable material and captures surface detail when used with stock trays while retaining the desirable physical characteristics of polyethers.

Mucosal Tissue Management

The corresponding area is thoroughly rinsed and dried, followed by isolation of the prepared teeth. An analysis of the requirements for adequate management of the mucosal tissues is essential in the achievement of accurate impressions. Anatomical and physiological considerations (eg, degree of mucosal hydration, character of salivary secretion, vestibular depth, and tongue position) determine the proper selection and strategic placement of isolation aids. When bilateral or full-arch impressions are necessary, the anticipated behavior and deformation of the oral tissues upon tray insertion must be clearly understood. The biomechanical considerations are such that a path of insertion must be predetermined for proper placement and seating of the impression tray.

Conclusion

For the clinician involved in complex reconstructions, the ability to consistently attain accurate impressions acquires added significance due to the additional time and effort involved in preparation for the impression procedure, as well as the increased expenses associated with remakes that result from inaccuracies.  The technique for the clinical impression procedure and the role of refractory dies and models are also important considerations that should be readily understood by the savvy clinician.

The author mentions his gratitude to Alfred Nelson CDT, Matt Roberts, and Juan Rego, CDT for the restorations presented in this article.

*Clinical Assistant Professor and Chairman, Postdoctoral Periodontal Prosthesis, University of Pennsylvania, Philadelphia, Pennsylvania; private practice, Bryn Mawr, PA.

References

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  3. Lacy AM, Fukui H, Bellman T, Jendresen MD. Time-dependent accuracy of elastomer impression materials. Part II: Polyether, polysulfides, and polyvinylsiloxane. J Prosthet Dent 1981;45(3):329-333.
  4. Henry PJ, Harnist DJ. Dimensional stability and accuracy of rubber impression materials. Aust Dent J 1974;19(3):162-166.
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  11. Benson BW, Bomberg TJ, Hatch RA, Hoffman W Jr. Tissue displacement methods in fixed prosthodontics. J Prosthet Dent 1986;55(2):175-181.
  12. Nemetz H, Donovan T, Landesman H. Exposing the gingival margin: A systematic approach for the control of hemorrhage. J Prosthet Dent 1984;51(5):647-651.
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