The Impression Process--Application
Douglas A. Terry, DDS
An accurate
impression is a primary determinant for the precise fitting of an indirect
restoration and the key to clinical success. The impression process depends
upon material selection as well as proper application and technique. The three most
popular elastomeric impression materials include polyvinyl siloxanes (PVS),
polyethers, and vinyl-polyether hybrids.1 Part I of this article discussed
the most significant physical properties of these materials and how a working
knowledge of these properties can be beneficial in the selection of the proper
impression material for a specific clinical situation. This discussion will
review the unique characteristics of each type of elastomeric impression
material and will illustrate the advantages and disadvantages for clinical
application; Part III will describe the clinical criteria and technique utilized
for attaining an accurate impression.
Polyether Impression Materials
Polyethers are
supplied as two-paste systems (ie, base, catalyst) that result in a reaction,
converting the paste to a polymer when the base and catalyst are mixed. This polymer is formed through ionic polymerization
and an opening of the terminal imine rings. The open rings become cations,
opening rings of adjacent polyether chains and producing a cascade reaction
that continues until polymerization is complete. Since no volatile byproduct is
formed, the reaction results in unusually low polymerization shrinkage, which
improves long-term dimensional stability.2 The most significant
attributes of polyethers are their hydrophilic nature, enhanced wetting ability,5
and ability to reproduce surface detail.3
Because of their ability to absorb water, however, the storage and disinfection
of polyethers is critical. Distortion can occur if stored in water or high
humidity. Thus, polyether impressions should be disinfected with glutaraldehyde
for 10 minutes and will remain dimensionally stable for up to 7 days if kept
dry.3,4 Some examples of polyethers include Impregum F/Penta, Polygel NF, and P2 (Table 1).1,2,4,5
Polyvinyl Siloxane (PVS) Impression Materials
Addition-reaction
silicones, or PVS, constitute the most popular category of impression material.
These are supplied as two-paste systems (ie, base, catalyst) and upon mixing,
an addition-reaction occurs between the silane and the vinyl groups, producing a
cross-linked silicone rubber. In contrast to condensation-reaction silicones,
during polymerization no volatile by-products are produced from the set
impression, resulting in less polymerization shrinkage and, therefore, minimal
dimensional change.1,4 Furthermore, the addition-reaction is fast
and the conversion goes almost to completion--unlike the stepwise reactions of
the polysulfide, condensation silicones, and polyethers that require several
hours.6,7 This characteristic provides dimensional stability once
the impression is removed, which allows the impression to be poured at any time
after removal from the mouth, whereas other elastomeric impression materials
require 20 to 30 minutes for viscoelastic recovery.
Several
consideration factors with addition-reaction silicones include the release of
hydrogen gas, the inhibition of polymerization from latex gloves, and their
limitation with moisture control. The release of hydrogen gas from a side
reaction unrelated to the setting, a condition known as outgassing, can result
in voids in the cast.5,8 Most of the contemporary PVS materials,
however,contain scavengers, small amounts of platinum or palladium,
which prevent the release of hydrogen gas that could cause bubbles in the final
model.6 The mechanism of inhibition of polymerization is a sulfur-containing
compound released from certain latex gloves that contaminates the platinum
catalyst (ie, chloroplatinic acid) in the PVS impression material.9,10
The use of vinyl or nitrile gloves, or newer generation gloves that do not use
sulfur as the accelerator, is recommended during the impression process.11
Residues from methacrylates, acrylics, and petroleum jelly lubricants can also
interfere with the setting reaction of PVS and may interfere with the material’s
ability to capture detail. It is thus important to clean the preparation and
surrounding structures prior to final impression taking.12 The final
concern with PVS impression materials is how their hydrophobic nature limits
control of moisture. Although some manufacturers claim that the addition of
surfactants has rendered these materials hydrophilic, it has only made them
less hydrophobic.13 Surfactants lower the contact angle and improve
the wetting characteristics against set material; however, in freshly mixed
material the surfactant has not completely migrated to the surface and thus is
not hydrophilic upon initial contact with intraoral moisture.1
Therefore, the hydrophobic nature of these materials can compromise their
application in areas where moisture cannot be controlled.14 Addition-reaction
silicone impressions can be easily disinfected without loss of accuracy and may
remain dimensionally stable for up to 7 days (Table 2).1,4-11
Vinyl-Polyether Hybrid Impression Materials
Vinyl-polyether
hybrids represent the newest class of elastomeric impression materials. The
vinyl-polyether hybrid impression material combines the characteristics from
the addition-reaction silicones and polyether materials. The vinyl-polyether
impression material is supplied as a two-paste system (ie, base, catalyst). When
the base and catalyst are mixed, it becomes a polymer chain with polyether and
siloxane groups through the use of two addition-reactions. The first involves a
reaction similar to that of the PVS, while the second reaction involves the addition
of a vinyl-terminated polyether to a siloxane.15 In this system, the
polysiloxane and polyether become linked as copolymers and the hybrid material
combines elements of each of these elastomeric systems. Thus, the polyether
groups on the polymer provide a hydrophilic material that can be attained
without the addition of surfactants while the siloxane groups on the polymer
provide a material that is dimensionally stable and recovers from deformation. These
transformations in the hybrid’s properties, however, are dependent upon the ratio
of polyether/polysiloxane in the elastomer. Therefore, the copolymer will take
on the characteristics of the elastomer that is greater in the copolymer. The
material has a platinum catalyst and the polymerization reaction can be
contaminated with sulfur from latex gloves. These hybrid materials are
available in different viscosities (eg, putty, heavy, medium, wash materials)
to accommodate different impression techniques. The potential advantages of
this system are that it combines the excellent elastic characteristics
associated with PVS with the hydrophilic characteristics of the polyethers, although
further independent evaluation is required to determine these benefits. The
most significant reported attributes of vinyl-polyether hybrid impression
materials include hydrophilicity during the setting and after polymerization,
dimensional stability, and elastic recovery.1,15
Conclusion
The impression
process is the most challenging multifaceted procedure in dentistry due to the
number of variables that can affect the success and long-term outcome of the
indirect restoration. This process begins with knowledge of the properties of
the impression materials and must be integrated with the proper application of
materials. The polyether and polysiloxane impression materials provide the
standard for achieving an accurate impression in operative and prosthetic
dentistry. While future hybrids may complement these materials, their potential
benefits will require further investigation. Whereas this part provided an overview
of the most commonly used elastomeric impression materials and compared their
advantages and disadvantages, the third and concluding part of this discussion will
describe the final element of the impression process: technique.
*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.
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Tables
Table 1: Advantages and Disadvantages of Polyether Impression Materials
Advantages | Disadvantages |
Low polymerization
shrinkage | Rigid and sets to a stiff
consistency |
Inherently hydrophilic,
with excellent wetting ability | Unpleasant taste and odor |
Highly accurate surface
detail | High cost |
Minimal distortion on
removal | Can be distorted by water
absorption |
No negative affects on polymerization
due to latex gloves | Difficult intraoral
removal and cast separation |
Table 2: Advantages and Disadvantages of PVS Impression Materials
Highly accurate with less
polymerization shrinkage | Inherently hydrophobic
nature |
Neutral odor and taste | Sensitive to sulfur/latex
contamination |
Superior tear resistance | Potential for hydrogen
gas by-product |
Less rigid on setting
with easier intraoral removal and exhibits excellent elastic recovery | May wrinkle with delay in
the insertion of impression tray material |