EXA’lence in Impression Making
In a perfect world, making impressions would be simple and predictable. All margins would be supragingival or slightly subgingival and the gingiva would be healthy. The dentist would avoid inadvertently touching the gingiva with rotary instruments and the patient would be able to fully open his or her mouth. Patients would not salivate during dental appointments. They would take impeccable care of their mouths and be free from medications altering the oral flora. There would be no calculus or plaque present and the ideal periodontal biologic width would be present around each tooth. Unfortunately, ideal conditions are rarely present in the majority of patients. Therefore, we look for impression materials which help to compensate for less than ideal conditions.
Early impression materials, such as compound, used the physical reactions of heating to flow and cooling to set. Although effective, the techniques were difficult to master and were best suited for single-unit impressions. Irreversible hydrocolloid impression materials were developed that allowed one impression to be made for multiple teeth. Although excellent for general use, they do not have the accuracy, stability, or detail required for most fixed prostheses.1 Reversible hydrocolloid impression materials suitable for fixed impressions were developed but require elaborate preparation before impression making. Bulky water-cooled trays are used to cool the material. Although initially accurate, only one model can be made from each impression and gypsum casts must be poured within 60 minutes. They have poor tear strength and do not perform well in narrow subgingival marginal areas.
With the introduction of the elastomeric impression materials, a chemical reaction occurred to harden the material. Polysulfide rubber impressions offer improved ease of use and exhibit good tear strength. They have a terrible taste and are slow-setting. Not available in auto-mixing cartridges, they can be difficult to thoroughly mix to a uniform consistency and can be messy. They are not very patient friendly and can ruin clothes. Long-term dimensional stability is not good and impressions should be poured within an hour to prevent significant change.
Polyether impression materials were developed that set quicker than polysulfide rubber but still have an objectionable taste. They have excellent accuracy, give good surface detail, and are the most hydrophilic elastomeric material available.2 They are very rigid and difficult to remove from the mouth when set and often loosened previously placed fixed prostheses (Figure 1). Modifications have been made through the years to improve on the negative aspects of polyether materials but the taste is still poor and they are relatively slow setting.
The introduction of the polyvinyl siloxane impressions improved the taste and setting speed of the impression material. Unfortunately, they are generally hydrophobic and do not absorb water. Hydrophobic impression materials divert fluids ahead of them and often interfere with the union of impression material that has been syringed 360° around the tooth. The impression material does not unite, resulting in fins, voids, and folds often around the marginal area (Figure 2). This effect has been exacerbated by the development of faster-setting materials that may actually begin to set before the impression material has been fully placed. Surfactants were added to reduce the contact angle between the impression material and any liquid. The relative humidity in the mouth affects the accuracy and surface detail reproduction of the impression. The challenge of taking accurate impressions in a less than ideal wet or blood-contaminated environment has not been fully solved and may be based on the experience and preferences of the operator.
Recently a new material has been introduced that incorporates the chemistry and ideal characteristics of both polyvinyl siloxane and polyether impression materials. It is called vinyl polyether silicone (VPES) impression material (Figure 3). It is not repelled by moisture and better records the surface of a moist tooth, giving improved detail. The material flows well into moist crevicular areas to better capture subgingival areas. The working time allows adequate time to place the material before it begins to set, but the setting time is not excessive. It is available in multiple setting times and viscosities in convenient auto-mix cartridges. The tear strength has been improved, decreasing the likelihood of separation in the sulcus. The set material is rigid but easily removable from the mouth. Finally, the taste has improved and is not as bitter as traditional polyether materials.
Laboratory tests may indicate improved physical characteristics for a new material but the benefits must be demonstrated by superior clinical performance. Dentists report the material to be forgiving in deep crevicular areas in the presence of contaminants (Figure 4). With the increased use of metal-free crowns, margins with 90° butt joints are more commonly being prepared. Moisture along the sulcus is more likely to interfere with accurate impressions of these margins than with beveled or chamfer margins. Figure 5 demonstrates four maxillary teeth prepared for porcelain-fused-to-zirconia crowns. Accurate impressions were easily made of the teeth and sent to the dental laboratory for crown fabrication (Figure 6).
Obviously success depends on the skill of the operator and laboratory. Dentists often rely too heavily on a material to compensate for poor technique. It appears that this blended impression material is an evolution of existing technologies that produces a more forgiving and predictable impression material.
1. Chen SY, Liang WM, Chen FN. Factors affectingthe accuracy of elastomeric impression materials. J Dent.2004;32(8): 603-609.
2. Michalakis KX, Bakopoulou A, Hirayama H, et al. Pre-and post-set hydrophilicity of elastomeric impression materials. J Prosthodont. 2007; 16(4): 238-248.
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