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Predictable Impression Technique for Indirect Restorations

 

An accurate impression is the foundation for success with indirect restorations.1,2 Modern impression techniques used in restorative dentistry require displacement of gingival tissue to record the exact dimensions of the preparation, the architecture of the margins of the preparation, and the relationship of the prepared teeth to the soft tissue and surrounding dentition.3 Restorative and periodontal complications can occur from improperly positioned subgingival margins, thin tissue biotype, bulky fibrous papilla, and traumatic manipulation of the soft tissue.2 Achieving an accurate final impression is the result of properly integrating multiple interrelated steps throughout the process.

 

Criteria for an Accurate Impression

A myriad of factors influences the quality and predictability of a final impression, such as moisture control, gingival retraction method, displacement time, size and type of impression tray, volume of the impression material, and tissue management.1,4,5 According to the literature, the disparate adoption of the latest materials may not lead to clinical success, but the accuracy of the impression may be controlled by technique.6 An accurate impression should provide the following:

  • Adequate wash thickness to withstand distortion and tearing when intraorally removed;
  • No evidence of voids, bubbles, drags, or tears;
  • Displays a uniform homogenous mix of materials;
  • Uniform bond between the impression material, adhesive, and tray;
  • Reproduces fine surface details, free of debris such as saliva and blood; and
  • Distortion free, and completely set upon removal.1

Managing the Periodontal Complex

Healthy periodontal tissue is a prerequisite for success. Inflammation of gingival tissue prior to impression-taking can complicate the procedure. Bleeding and moisture from crevicular fluid can displace impression material, resulting in voids and rounded indistinct finish lines that can cause an inaccurate castand an improperly fitting final restoration.4 Furthermore, if a subgingival margin is placed in the presence of inflammation, there is potential for gingival recession and exposure of the restorative finish line.7,8 The soft tissue must, therefore, be properly managed.

The preoperative consideration during initial therapy is control and elimination of all sources of irritation and inflammation. Unfortunately, this may require delaying the impression procedure after tooth preparation to allow for improvement in the soft tissue condition. The provisional restoration is an essential component of this initial therapy and can improve the quality of the impression. It preserves the position, form, and color of the gingiva and maintains the periodontal health prior to impression-taking and while the definitive restoration is being fabricated.

Soft tissue management during the preparation and impression-making stages requires an understanding of the gingival tissue architecture. The most important determining factor to predicting how the tissue will respond to preparation and impression techniques begins with the relationship of the free gingival margin to the osseous crest. Preoperative recordings of facial and interproximal bone height, and determination and preservation of the biologic width, can provide predictable postrestorative gingival margin levels and periodontal health.

Clinical Guidelines for Impression Techniques

 Various impression techniques can be utilized for different restorative and prosthetic reconstructive procedures. These techniques include the putty-wash one- and two-step system, dual-arch method, and simultaneous dual-viscosity--designed procedures such as the one-step/double-mix impression method. Impression materials will only make accurate impressions of tooth surfaces that are clean, visible, and dry. Tooth preparations with subgingival finish lines, therefore, require displacement of gingival tissue. Several techniques have been proposed in the literature, such as electrosurgery, rotative gingival curettage, and diode laser-assisted gingival troughing.9-11 To achieve a gingival displacement with minimal trauma, the retraction cord technique is most reliable and has demonstrated a high degree of predictability in regards to control of gingival recession caused by the impression. One study indicates that a minimal crevicular width of 0.20 mm is required for successful impressions.3  

Two measurements on the facial and interproximal regions of the unprepared tooth can provide predictable categorization. These osseous crest positions can be divided into three categories: normal, low, and high. For a normal crest position (ie, 85% of patients), these two measurements for anterior teeth should be approximately 3 mm on the facial and 4 mm on the interproximal aspects when adjacent teeth are present.2 When the depth of the osseous crest to gingival margin is greater than these measurements, it is considered a low crest position.2 If the depths are less than these measurements, it is considered a high crest position.2 In a normal osseous crest position, the gingival complex will return to a normal crest relationship after tissue manipulation and impression-taking.1,12 In a high crest relationship, however, traumatic manipulation and placement of subgingival preparation margins will position the final restoration too close to the osseous crest, creating a violation in biologic width.1,13 The low crest position with a thin biotype is considered the most unstable clinical situation and can result in the most variation in final gingival position.2,14 For optimal restorative results, the low and high osseous positions should be corrected through osseous and/or orthodontic treatment prior to restorative treatment. In clinical situations where adjunctive therapy is not planned, careful and gentle manipulation is critical. During gingival displacement, therefore, the number of retraction cords and the size should be modified according to the osseous crest position and tissue biotype.

 

Double-Cord Gingival Displacement Technique

The following clinical protocol demonstrates the one-step/double-mix impression with a double-cord gingival displacement for a normal crest position. During the restorative phase and after the onset of anesthesia, the tooth is prepared relative to the osseous crest with the finish line following the scallop of the gingiva. A primary compression cord of small diameter is soaked in a plain buffered aluminum chloride solution and gently placed in the bottom of the sulcus around the preparation with light pressure from a cord-packing instrument (Figure 1A). The finish line of the preparation is extended to the coronal aspect of the cord, which places the finish line of the final restoration approximately 0.5 mm to 1 mm below the gingiva. This initial placement of the retraction cord provides a seal to the sulcus to prevent contamination of the margins by blood or crevicular fluid. The first cord layer is a sulcus liner, to prevent tearing of the sulcular epithelium and bleeding, which can be a concern with the single-cord technique. Additionally, it retracts the tissue so as to prevent contact of the diamond bur with the gingival epithelium during final margin placement(Figure 1B).9

A second retraction cord is then inserted to displace the tissue apically and laterally (Figure 2A). The gingival retraction is allowed to remain for five minutes to allow water absorption by the superficial cord. This generates expansion of the superficial cord and increases the crevicular width.Prior to taking the impression, any excess moisture is eliminated. The second retraction cord is removed, and a low-viscosity impression material is immediately injected into the sulcus (Figure 2B). The entire preparation is covered with the low-viscosity material and directly followed by the placement of the tray, which has been loaded with a more viscous material. The tray is removed along the path of insertion after inspection of the set material and upon reaching the specified setting time. The impression is then confirmed for accuracy with magnification. The ideal registration is created with a laterally deflected sulcus greater than 0.5 mm in width, and more than 0.5 mm of apical deflection sufficient to record an adequate amount of unprepared tooth structure apical to the margin.

Conclusion

The quality and accuracy of the final impression plays a significant role in restorative success. An understanding of gingival tissue architecture and how it influences restorative margin placement can provide insight into proper management of soft tissue and improved impression techniques. One should not discount, however, the influence of a clinician’s skill and experience with a specific material and technique on the quality of the final impression and restorative result.

*Assistant Professor, Department of Restorative Dentistry and Biomaterials, University of Texas Health Science Center Dental Branch, Houston, TX; private practice, Institute of Esthetic and Restorative Dentistry, Houston, TX.

 

References

  1. Vakay RT, Kois JC. Universal paradigms for predictable final impressions. Compend Contin Educ Dent 2005;26(3):199-206.
  2. Kois J, Vakay RT. Relationship of the periodontium to impression procedures. Compend Contin Educ Dent 2000;21(8):684-690.
  3. Baharav H, Laufer B, Langer Y, Cardash HS. The effect of displacement time on gingival crevice width. Int J Prosthodont. 1997;10(3):248-253.
  4. Wassell RW, Barker D, Walls AW. Crowns and other extra-coronal restorations: Impression materials and technique Br Dent J 2002;192(12):679-690.
  5. Boksman L. Eliminating variables in impression-taking. Ontario Dent 2005:22-25.
  6. Nissan J, Laufer BZ, Brosh T, Assif D. Accuracy of three polyvinyl siloxane putty-wash impression techniques. J Prosthet Dent 2000;83(2):161-165.
  7. Donovan TE, Cho GC. Soft tissue management with metal-ceramic and all-ceramic restorations. J Calif Dent Assoc 1998;26(2):107-112.
  8. Lee EA. Impression material selection in contemporary fixed prosthodontics: Technique, rationale, and indications. Compend Contin Educ Dent 2005;26(11):780-789.
  9. Perakis N, Belser UC, Magne P. Final impressions: A review of material properties and description of a current technique. Int J Periodont Rest Dent 2004;24(2):109-117.
  10. Lee EA. Laser-assisted gingival tissue procedures in esthetic dentistry. Pract Proced Aesthet Dent 2006;18(9):2-6.
  11. Azzi R, Tsao TF, Carranza FA Jr, Kenney EB. Comparative study of gingival retraction methods. J Prosthet Dent 1983;50(4):561-565.
  12. van der Velden U. Regeneration of the interdental soft tissues following denudation procedures. J Clin Periodontol 1982;9(6):455-459.
  13. Gunay H, Seeger A, Tschernitschek H, Geurtsen W. Placement of the preparation line and periodontal health–A prospective 2-year clinical study. Int J Periodont Rest Dent 2000;20(2):171-181.
  14. Weisgold AS. Contours of the full crown restoration. Alpha Omegan 1977;70(3):77-89.

 

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