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During the routine practice of restorative dentistry, clinicians rely on elastomeric impressions and digital scanning to fabricate diagnostic casts and for final impressions so that indirect restorations can be produced by the dental laboratory.1 Despite the growing popularity of chairside digital scanning, impression materials still provide the accuracy required to capture the anatomy of the teeth and soft tissues.1-3
The earliest impression materials were used for complete dentures and were rigid. They included plaster of Paris, impression compound, zinc-oxide and eugenol pastes, and impression waxes. The earliest elastomeric impression materials were irreversible and reversible hydrocolloids, condensation silicones, and rubber base (polysulfide rubber). In the 1970s an addition-reaction silicone, vinyl polysiloxane (VPS), was introduced. VPS impression materials offered considerable advantages when compared to the other elastomeric material.3-5 Currently, VPS impression materials are the standard for routine fixed prosthodontic treatment and for definitive impressions for orthodontic diagnosis and aligner design.6,7
Critical to the success of VPS impression materials are the physical properties and characteristics of this class of material.5,8,9 These properties include:
• clinically relevant working time; reasonably fast setting time
• excellent handling characteristics
• hydrophilic (will work in a moist environment) through the addition of surfactants
• multiple viscosities for applications in different clinical techniques
• color contrast for impression readability; variety of colors for better visibility when evaluating impressions
• accuracy and good reproduction of surface detail
• dimensional stability over time
• excellent tear strength when removed from sulcus elastic recovery from deformation, such as when the impression is removed over undercuts
• absence of unpleasant taste or odors (for better patience acceptance)
• convenient delivery (automixing cartridges, putty tubes, magnum cartridges)
Chemically, VPS impression materials are hydrophobic. Many manufacturers have modified the chemistry of VPS materials by adding surface surfactants and hydrophilic modifiers that change the ability of the vinyl polysiloxanes to record detail on slightly moist surfaces accurately. Through the addition of surfactants, the impression material is able to wet the tooth preparation, reducing remakes.4 Typically, the contact angle of vinyl polysiloxane without modifiers is around 140 degrees. With the addition of modifiers, vinyl polysiloxane has an improved wetting ability.
VPS materials provide a wide variety of clinical choices in viscosities that allow for an array of clinical techniques.4,10,11 Viscosities available from most manufacturers include light-body (low viscosity), medium- or regular-body (medium viscosity), heavy viscosity (tray), and very high viscosity (putty). Additional options include regular set and fast set with regard to working time and setting time. For a single crown, a fast-setting impression material saves time, which is conducive for patients being more tolerant of the impression-making process. Once placed around the tooth and in the mouth, the latest generation of fast-setting VPS materials use the intraoral temperature to accelerate their set. In fact, the limit on “fast setting” has probably been reached when considering the time allowed for the material to be mixed, the adequate working time to be placed in a tray, and then insertion in the mouth without compromising accuracy. For multiple tooth preparation impressions, extended working times are desired.
VPS impression materials are accurate and dimensionally stable. It is critical that the clinician prepare the tooth to fulfill the requirements of the restorative material being prescribed. Also, most cases require soft-tissue management during impression-making so the material can capture the margins of the tooth preparation. Errors due to misuse of the impression material have been reported.12,13 Two separate studies evaluating the quality of impressions sent by dentists to commercial laboratories for the fabrication of fixed partial dentures noted observable errors of voids or tears at finish lines over 50% and 44.2% of the time.13,14 The definitive restoration can only be as good as the impression materials and the impression captured by the dentist, who is responsible for evaluating its quality before sending it to the laboratory.
VPS Materials Beyond Impressions
Typically, clinicians use materials based upon manufacturers’ recommendations. However, they are also taught to think “outside of the box” for circumstances that may require a combination of clinical techniques. Because of their elasticity, durability, accuracy, and ease of use—along with varied viscosities and working and setting times—VPS materials are able to be used clinically beyond impression-making. In some cases, manufacturers have modified formulations to introduce an innovative use for the material based upon how clinicians were using the VPS materials, eg, as bite registration materials.15,16
VPS Bite Registration Materials
One of the most common uses of specialized VPS materials is for bite registrations.17,18 Clinicians and dental laboratories use a variety of techniques to articulate casts, including direct interdigitation of casts (“hand articulation”), use of interocclusal records with posterior and anterior teeth, interocclusal records of posterior teeth only, and use of occlusal rims on record bases when posterior teeth are missing. A critical element of any technique for articulating casts is its reproducibility. Of all the techniques used, “hand articulation” is the most problematic. Without using an accurate index, casts that are hand articulated when arbitrarily interdigitated can lead to errors.19,20 Use of an accurate index with rigid fixation of casts provides for the most reproducible mounting for diagnostic evaluation and for the fabrication of indirect restorations.19,20
Interocclusal registration materials include waxes, wax-on-resin baseplates, zinc-oxide eugenol (ZOE) pastes, acrylic resin, VPS impression materials, and VPS bite registration materials. While waxes are easy to work with for bite registration, they do not record tooth surfaces accurately, are dimensionally unstable, and can be inconsistent due to interference with passive and active movements of the mandible.5,20 ZOE pastes have the advantage of accuracy in reproducing the tooth surfaces and are dimensionally stable. Because of the rigid nature of ZOE paste when set, interferences can occur when interdigitating casts within the set paste. In addition, these materials have a long setting time and are brittle.17,18 Acrylic resins have been used for interocclusal records. Usually, they are of limited use in the fabrication of a single occlusal stop, and their rigid nature can interfere with seating of casts. Acrylic resins can lead to inaccuracies when mounting casts due to high polymerization shrinkage, and they are dimensionally unstable because of continued polymerization shrinkage after removal from the mouth.
Among bite registration materials, VPS materials have significant advantages. VPS bite registration materials are the most accurate in duplicating tooth surfaces and are dimensionally stable after setting. In addition, because VPS is elastomeric, it does not interfere with cast interdigitation, and because it flows, it will not interfere with mandibular closure. VPS bite registration materials do not require a special carrier, and the registration is rigid and holds its shape. Unlike very rigid materials, the elastic nature of VPS material allows it to withdraw over undercuts, and it can be easily trimmed to fit casts.17,20 VPS bite registration materials can also be used when there is a need to create an opposing occlusal relationship that is being captured for digital imaging and CAD/CAM restoration fabrication. Regardless of which bite registration material is selected, the casts being used must be accurate.
VPS Carrier/Stent for Provisional Restoration Fabrication
While bite registration is the predominant use of VPS materials, they can also be utilized in problem-solving ways that enhance the practice of restorative dentistry. One of this author’s favorite techniques for fabrication of bis-acryl provisional restorations uses a fast-setting medium-body VPS impression material.21,22 With this technique for both single-unit and multiple-unit provisional restorations, a carrier is made before starting the crown preparations. As part of diagnosis, the clinician must determine if any changes are needed to the tooth being prepared before producing the carrier for the fabrication of the temporary restoration. For bis-acryl provisional restorations, one can typically use a bite impression tray with a fast-setting regular-bodied or heavy-bodied VPS impression material. The benefit of a bite impression tray over a quadrant tray is that the complete seating can be verified by observing the patient’s occlusion. This technique leads to minimal, if any, additional occlusal and proximal contact adjustment, because the provisional is fabricated with the patient biting in maximum intercuspation (MIP).
First, the VPS impression material is syringed into both upper and lower portions of the tray. The tray is inserted into the patient’s mouth in the quadrant to be treated, and the patient bites into MIP. When the material is set, the tray is removed. The VPS impression material accurately captures the detail of the tooth contours and occlusion. If problems arise when attempting to capture the detail of the impression with a biting-down–only technique, an automixing tip can be used to syringe material around the tooth before preparation and “sandwich” the VPS on the tooth with the impression material in the tray. Using this technique for carrier fabrication provides a carrier that can fabricate a well-fitted and occlusion-accurate temporary restoration with minimal time and effort for adjustment (Figure 1 through Figure 5).22
For multiple units it may be necessary to use an impression of a diagnostic wax-up to fabricate the template/stent for the provisional restoration. In these cases a quadrant tray with a fast-setting regular-bodied VPS impression material can be used to capture an impression of the diagnostic wax-up (Figure 6 through Figure 11).
VPS Template for Esthetic Dentistry and Tooth Preparation Evaluation
While much of the practice of dentistry is demanding due to the critical nature of virtually any clinical procedure, anterior esthetic procedures can be the most scrutinized when final results are evaluated. Unlike procedures for posterior teeth, restorations in the esthetic zone require that not only the clinician be pleased with the completed result, but the patient must also approve the esthetic and functional outcome. The use of a VPS custom lingual matrix created from a diagnostic wax-up on a study cast can simplify the routine restoration of an incisal edge fracture (Figure 12 through Figure 14).23 In the anterior, the shape and dimensions of the tooth being restored are more easily duplicated from the wax-up to the completed restoration.
Also, the use of a diagnostic wax-up for misaligned anterior teeth treatment-planned for porcelain veneers allows for the fabrication of a preparation guide to take into account tooth rotations and malpositioning. Therefore, the tooth to be restored will not be over- or under-prepared (Figure 15 through Figure 17). Additionally, the physical requirements of the newest ceramic materials necessitate tooth preparations with adequate reduction. The use of VPS templates can help verify that the tooth preparation has fulfilled the requirements of the restorative material prior to making an impression.
Use of VPS for Blockout During Restorative Procedures
During highly involved restorative procedures, the use of VPS impression material to blockout undercuts or minimize excess composite resin can save time and result in less trauma to the patient. Gerrow described the use of polyvinyl siloxane as a blockout material during indirect placement of overdenture attachments.24 In much the same way, to make an elastomeric impression when the patient has increased open gingival embrasures that would lock the impression onto the teeth, using irreversible hydrocolloid (alginate) impression material syringed into the gingival embrasures provides a blockout before making the VPS or polyether impression.25
Fiber-reinforced splints are a clinically effective technique to stabilize and splint periodontally involved teeth.26,27 The use of periodontal splinting as recommended therapy to stabilize periodontally compromised teeth has been demonstrated to improve long-term prognosis.28,29 In the past, wedges were placed to minimize excess composite in the gingival interproximal embrasure areas. However, with wedges the potential exists for highly mobile teeth to be splinted in a different position. With a VPS impression material, a novel technique for minimizing excessive composite resin in these areas has been described.30 The technique requires the placement of a fast-setting medium- or heavy-viscosity VPS impression material or a bite registration VPS material using an impression syringe in the gingival embrasure areas of the teeth to be splinted. The impression material must be placed after tooth etching, rinsing, and drying to avoid the trapping of moisture that can occur if the technique is done earlier. This use of elastomeric impression material assures a passive placement of the blockout. After the fiber splint has been placed, the impression material is easily removed, minimizing composite removal in these difficult-to-access areas (Figure 18 through Figure 23).
VPS impression materials provide the clinician with an extremely accurate impression for single- and multiple-tooth preparations for the fabrication of indirect restorations. VPS materials also have expanded uses that enhance the way clinicians practice dentistry. These materials can be used for interocclusal records; as a template for provisional resin restorations; for blockout of gingival embrasure spaces when restoring with implants, fiber splints, and precision attachments; as a guide for preparation reductions for crowns and veneers; and as a template for direct composite resin restorations. The only limitation in the use of VPS materials is the dentist’s imagination.
ABOUT THE AUTHOR
Howard E. Strassler, DMD
Professor and Director of Operative Dentistry, University of Maryland School of Dental Medicine, Baltimore, Maryland
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