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Current Controversies in Final Impressions

Making an accurate final impression is the initial procedure in the fabrication of the definitive restoration. In addition to the fabrication of a master model, a second unsectioned model should be constructed in order to establish interproximal contact points and to provide information regarding the desired emergence profile of the restoration in relation to the soft tissues. The following discussion provides guidelines that will enable the clinician to determine if an additional final impression is necessary, and whether to pour this impression chairside or at the dental laboratory.

How Many Final Impressions are Necessary?

The answer is directly related to the ability of the clinician to successfully recover the model from the impression without destroying any of the abutments, and depends on the rigidity of the impression material. Although the most rigid impression material is high viscosity polyvinylsiloxane (putty),1 this material is never utilized alone, and is accompanied by the low viscosity polyvinylsiloxane (wash), which has low rigidity.1 Thus, with the exception of extremely divergent abutments, the master model can be successfully recovered, and the impression utilized for a second pour. Medium viscosity polyethers provide superior tear resistance in comparison with polyvinylsiloxane low viscosity (ie, wash),1 and therefore enable the clinician to be less aggressive in the management of tissue. These materials are also more hydrophilic, which, in the author's opinion, renders them an optimal alternative for patients with thin and friable tissues, or for patients in which minor recessions can jeopardize the aesthetic results (eg, anterior restorations). Conversely, this material is far more rigid than polyvinylsiloxane wash,1 and can be difficult to recover from the oral cavity and the master model. It has been the author's experience that cases with thin abutments (particularly in maxillary and mandibular anterior teeth) often produce a high risk of abutment fracture when attempting to recover the master model. Since this material should be ideally utilized in a custom tray, a simple solution is to carefully section the custom tray and to subsequently peel the remaining impression material off the model. This eliminates the ability to repour the final impression, and therefore requires an additional impression.

Pour Chairside or at the Dental Laboratory?

Assuming that the accurate pouring of the final impression is within the comfort zone of the clinician, the decision to pour chairside or at the dental laboratory relies primarily on the effect of transportation conditions on the accuracy of the final impression. Corso et al demonstrated that the storage of unpoured polyvinylsiloxane and polyether impressions in the temperature range of 4°C to 40°C did not affect the accuracy of the master model.2 It is easy to imagine, however, that the temperature in the vehicle transporting the impression can easily surpass this range of temperatures. Purk et al studied the effect of storage conditions in the range of 10°C to 66°C, and revealed significant distortions in the materials following an extended period of storage at 66°C.3

Summary

Achieving an accurate final impression is the initial step in the fabrication of the definitive restoration (Figure 1).  If polyether is to be utilized for the final impression, obtaining an additional impression may minimize the risk of fractured abutments. In addition, transportation conditions to the dental laboratory that may expose the impression to extreme temperatures should necessitate the pouring of the impression prior to shipping.

*Dean, University of Southern California, Ostrow School of Dentistry, Los Angeles, CA.

References

  1. Chai J. Takahashi Y. Lautenschlager EP. Clinically relevant mechanical properties of elastomeric impression materials. Int J Prosthodont 1998;11(3):219-223.
  2. Corso M, Abanomy A, Canzio JD, et al. The effect of temperature changes on the dimensional stability of polyvinylsiloxane and polyether impression materials. J Prosthet Dent 1998;79(6):626-631.
  3. Purk JH, Willes MG, Tira DE, et al. The effects of different storage conditions on polyether and polyvinylsiloxane impressions. J Am Dent Assoc 1998;129:1014-1021.
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