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Elastomeric Impressions in Fixed Prosthodontics II

Procedure, Refractory Models and Dies, and Discussion

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. This article illustrates a protocol that will yield accurate impressions on a consistent and predictable basis. The principles discussed are applicable to simple as well as extensive restorations. Multiple cases from a routine private practice setting will be presented to underscore the predictability of the technique.

Impression Procedure

A sufficient quantity of high-viscosity tray material is loaded into the impression tray utilizing the cybernetic mixing unit. The light-bodied polyether is then dispensed into an intraoral impression syringe with an automix gun and cartridge system. Although both materials may be cooled to prolong the total working period when necessary, a minimum temperature of 65˚F should be maintained to avoid an increase in viscosity, which may affect the automatic mixing systems.

The secondary retraction cord is subsequently removed from around the teeth and the preparations are dried once again. The light-bodied polyether is then syringed around the finish lines until the preparations are completely covered. During application, the nozzle tip should remain immersed in the material without breaks in continuity to avoid entrapment of air bubbles. The low viscosity and outstanding wetting ability of this material are immediately evident in its capacity to readily adhere to the intraoral structures. In instances where soft tissue collapse has occurred following removal of the secondary cord, a gentle stream of air may be applied to force the material into potentially difficult areas. The impression tray loaded with the high-viscosity polyether is immediately inserted and seated until liberal amounts of excess material flow over the flanges and through the perforations of the tray. The increased hydraulic pressure that results from the high-viscosity tray material's rheological characteristics enhances the ability of the low-viscosity material to capture surface detail (Figures 1 and 2).

Firm and constant pressure must be exerted to avoid movement of the impression tray, which must remain in place for 7 minutes. Complete polymerization is essential in preventing deformation of the material during removal and ensuring adequate elastic recovery. Although additional force may be required, removal of polyether impressions from the mouth can be accomplished with relative ease by applying pressure on the handle of the tray while simultaneously stretching the oral mucosa. Once a break in adhesion occurs, the impression can be gently manipulated out of the mouth. A water spray may be utilized to moisten the mucosal tissues and any absorbent isolation aids, which further facilitates the removal of the impression while preventing soft tissue abrasions.

The impression is rinsed, dried, and thoroughly inspected with magnification aids to verify its accuracy. Once deemed satisfactory, it is sprayed with a disinfectant solution, positioned upside down to prevent pooling of the disinfectant, and allowed to dry. Although reports have shown no adverse effects to the accuracy or dimensional stability of polyethers, immersion techniques are not utilized by the author for disinfection.1

Refractory Models and Dies

Following disinfection, the impression is forwarded to the dental laboratory for fabrication of refractory models. Care must be exercised to ensure the production of an accurate, void-free model. In the selection of gypsum materials for die fabrication, it is important to remember that dental stones exhibit a range of linear expansion of approximately 0.04% to 0.3% after setting.2 Such a degree of variation in expansion--which occurs in all dimensions--may result in significant model distortion, even with the use of an extremely accurate impression material.3

Triple pouring the definitive impression is recommended in multiabutment cases. The first pour is utilized as the working master model, the second pour serves as a spare master model, and the third pour is kept unsectioned as a solid model on which metal frameworks can be assembled and interproximal contacts adjusted (Figure 3).

Each individual die must be inspected and trimmed under magnification. It is recommended that the restorative dentist perform this task in complex cases. Comparing the finish lines on the dies against the definitive impression may be of aid during this process. Once identified, the finish lines should be marked with pencil and the dies trimmed short of them so that a portion of the intact tooth surface remains on the die. This aids the technician in establishing proper emergence profiles during fabrication of the restorations (Figure 4).

Discussion

Application of these principles significantly increases the predictability of fixed prosthodontic impression procedures. The ability to consistently obtain accurate impressions during the first attempt possesses obvious practical value for patient and clinician. These advantages are compounded when dealing with more extensive and technically demanding cases.

The previously detailed guidelines may be modified as necessary to address the specific requirements of the prescribed restorations. A second retraction cord, for example, may not be necessary where supragingival or slightly subgingival finish lines are indicated (eg, nonaesthetic areas, all-ceramic margins) (Figures 5-6-7-8).4 Although it is obviously more difficult to capture margins placed more subgingivally, restorative dentists are frequently confronted with situations that require further extension into the sulcus (eg, short clinical crowns or subgingival caries and restorations).4,5 The former does not, however, preclude the need for surgical crown lengthening when indicated. Alternatively, extensive reconstructions and periodontal prosthetic patients that require splinting with a metal-ceramic framework and metal margins demand more subgingival placement in aesthetically sensitive regions (Figures 9-10-11-12). These cases invariably benefit from the application of the double-cord impression technique.4

Since proper cord placement should not impinge upon or tear the connective tissue attachment, minimal or no bleeding will be elicited, provided that adequate soft tissue health is present.6 The hemostatic agent utilized with this technique generally controls bleeding except for instances that exhibit severe gingival inflammation. In situations where compromised tissue health must be accepted due to existing systemic conditions or physical handicaps, judicious use of a radiosurgical, electrosurgical, or laser unit may be incorporated as an adjunct to the achievement of hemostasis.7,8 The former may also be utilized when cord retraction is rendered ineffective due to the presence of redundant gingival tissue, a scenario frequently encountered around malpositioned teeth, following adult orthodontic treatment, or as a result of drug-induced gingival hyperplasia.

The degree of tissue retraction following cord placement is dictated by the relationship between the osseous crest and the gingival margin. In the presence of adequate gingival health, this retraction will be proportional to the amount of supracrestal tissue present. Conversely, the degree of retraction may be inversely proportional to the anticipated rebound of the gingival margin following the impression procedure.

Numerical parameters that define the relationship between the different anatomical structures comprising the dentogingival and dentoalveolar complexes have been previously described in the literature.9 The variability in these dimensions introduced by the existence of different periodontal biotypes must also be taken into account.10 Although these factors have been considered primarily in crown-lengthening surgery, they may also be applied as guidelines in predicting ultimate gingival margin levels following cord retraction procedures.

Conclusion

Indirect restorations depend on an accurate impression in order to achieve clinical success. Complex situations that involve multiple abutments can be properly recorded when care is exercised in the selection of impression materials and techniques. This presentation has reviewed a series of clinical considerations that must be addressed in association with indirect restorations and has demonstrated the use of a protocol that allows accurate impressions to be achieved with success and consistency.

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 Chair, Postdoctoral Periodontal Prosthesis, University of Pennsylvania, Philadelphia, Pennsylvania; private practice, Bryn Mawr, PA

 

 References

  1. Block PL. Restorative margins and periodontal health: A new look at an old perspective. J Prosthet Dent 1987;57(6):683-689.
  2. Azzi R, Tsao TF, Carranza FA Jr, Kenney EB. Comparative study of gingival retraction methods. J Prosthet Dent 1983;50(4):561-565.
  3. Chiche GJ. Double-string technique for final impressions: Practical applications. Dent Econ 1995;85(1):72-73.
  4. Benson BW, Bomberg TJ, Hatch RA, Hoffman W Jr. Tissue displacement methods in fixed prosthodontics. J Prosthet Dent 1986;55(2):175-181.
  5. 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.
  6. Lepe X, Johnson GH. Accuracy of polyether and addition silicone after long-term disinfection. J Prosthet Dent 1997;78(3):245-249.
  7. American National Standards Institute/American Dental Association 1987. Specification No. 25 for Dental Gypsum Products. Council on Dental Materials. Gaithersburg, MD, USA.
  8. Millstein PL. Determining the accuracy of gypsum casts made from type IV dental stone. J Oral Rehabil 1992;19(3):239-243.
  9. Kois J. Altering gingival levels: The restorative connection. Part I: Biologic variables. J Esthet Dent 1994;6:3-9.
  10. Sanavi F, Weisgold AS, Rose LF. Biologic width and its relation to periodontal biotypes. J Esthet Dent 1998;10(3):157-163.
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