inyl siloxane impression properties and techniques Winston
W. L. Chee, BDSpa and Terry
materials:
E. Donovan,
A review
of
DDSb
University of SouthernCalifornia, Los Angeles,Calif. Polyvinyl siloxane impression materials have been shown to have excellent properties as impression materials; however, they are sensitive to manipulative variables. Several methods of using very high viscosity (putty) materials to form “trays” to obtain uniform bulk of the wash impression are described, and the disadvantages of each of these techniques is pointed out. It is recommended that for best results acrylic resin custom trays should be used routinely. The interaction of polyvinyl siloxane materials with latex products is also discussed and problems that this inhibition can cause are stated. Suggestions to avoid this interaction are outlined. One of the disadvantages of the impression materials is that it has a relatively short working time. Refrigerating the material will increase working time without affecting accuracy. (J PROSTHET DENT 1992;68:728-32.)
olyvinyl siloxane impression materials, also known as addition reaction silicones, became extremely popular during the past decade. Although they are among the most expensive impression materials, they are now used in a wide variety of situations in fixed prosthodontics, operative dentistry, removable prosthodontics, and implant dentistry. The popularity of these materials is understandable, given the combination of excellent physical properties, handling characteristics, and unlimited dimensional stability. Despite all of the advantages that the polyvinyl siloxanes possess, a thorough understanding of the composition, physical properties, and manipulative variables of these materials is essential to achieve predictable success.This article will review these factors and give guidelines for techniques that will result in optimum performance. PHYSICAL
PROPERTIES
Addition reaction silicone impression materials have excellent physical properties. Their accuracy is unsurpassed and they can record fine detail. They also have the best elastic recovery of all available impression materia1s.l Becausethere is virtually no by-product to the polymerization reaction, impressions are dimensionally stable and can be poured at the convenience of the operator. They also allow the opportunity to make a second pour.2 This may also be a factor with the use of silver-plated dies, which are most
Presentedat the PacificCoastSocietyof Prosthodonticsmeeting, Victoria, British Columbia. aAssistantProfessor,Departmentof RestorativeDentistry, Director of Implant Dentistry. bAssociateProfessor,Chairman,Sectionof Biomaterials,and Director of Clinical RestorativeR.esearch. lo/1140360
728
accurate when recovered from polyvinyl siloxane impressions.3 The handling characteristics of addition reaction silicones are also favorable. They are supplied in a number of viscosities, ranging from very low for use with a syringe or wash material to medium, high, and very high. This allows these materials to be used for a host of applications. Polyvinyl siloxane materials are supplied by many manufacturers in the automix system, which is convenient, provides a consistent mix, and is cost-effective.* The number of bubbles incorporated in the mix is reduced with the automix system. From a patient comfort standpoint, these materials are ideal because they are clean, odorless, and tasteless. They polymerize quickly and, especially when used with a custom tray, the amount of bulk of material can be kept to a minimum. The polymerized impression is somewhat rigid, but this rigidity is nowhere that of the polyether materials and is not a major problem. Polyvinyl siloxane impression materials require an absolutely dry field for an acceptable impression, as is true of all elastomeric impression materials. Some contemporary polyvinyl siloxanes have been modified with the addition of certain nonionic surfactants and are described by the manufacturers as hydrophylic. This designation is a misnomer, and at best they are slightly less hydrophobic than their predecessors.The clinical advantage of the modification is that the polymerized impression is somewhat easier to pour. The term hydrophylic incorrectly implies that impressions can be made in a wet or moist environment. Manufacturers of some polyvinyl materials recommend delaying pouring the impression for a period of time becausethe material emits hydrogen gas, which can create voids in the cast. Most of the newer products contain small amounts of palladium, which prevents this emission, and allows the impression to be poured at the convenience of the operator.
NOVEMBER1992
VOLUME68
NUMBER6
POLYVINYL
SILOXANE
IMPRESSION
TECHNIQUES
Fig. 1. Polyvinyl siloxane impression completely separated from acrylic resin custom tray (impression surface). Fig. 2. Polyvinyl siloxane impression completely separated from acrylic resin custom tray (tray side view of impression in Fig. I). Fig. 3. Diagnostic cast with appropriate amount of spacer for an elastomeric impression. Fig. 4. Acrylic resin tray made on diagnostic cast with spacer (Fig. 3). Fig. 5. Putty custom tray made on diagnostic cast with spacer (Fig. 3). Fig. 6. Putty is arbitrarily removed from preoperative overimpression of arch to make space for wash material.
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
729
CNEE
AND
DONOVAN
Fig. 7. Thin polyethylene sheet is interposed between putty and teeth to prevent penetration of putty material interproximally and to create space for wash material. Fig. 8. Completed 2-step putty wash impression shows bottoming out of the putty custom tray. Fig. 9. Close-up of the preparations recorded with the impression (Fig. 8). Fig. 10. Some of the margins are recorded in putty with the one-step putty wash technique.
Three areas deserve special consideration in the discussion of these materials. First is the interaction of latex rubber with polyvinyl siloxane. Second is the putty-wash technique recommended by some of the manufacturers. The third topic is extending the working times with these materials.
Interaction of latex siloxane materials
rubber
and polyvinyl
Some latex products have a definite inhibitory effect on the polymerization of addition reaction silicones. This inhibition was originally observed when the putty materials were mixed with gloved hands or hands that had previously been wearing gloves.5 It was also noted when the impression material was allowed to set in contact with a rubber dam.6 Recently, the inhibition of polymerization intraorally
730
as a result of casual contact of tooth preparations and surrounding soft tissues with latex gloves before impression making has been documented.7-s The inhibition of polymerization is usually subtle and is limite‘d to the surface of the impression in contact with the contaminated tissues. Even though the inhibition is minimal, the gypsum cast in contact with unpolymerized material will definitely be distorted and hence unacceptable. The mechanism of inhibition is thought to be contamination of the chloroplantinic acid catalyst in the impression material with free sulphur remaining in the glove.lO It is clear that the powder is not a factor and not all latex gloves produce this inhibition. Vinyl gloves and synthetic latex gloves do not seem to cause problems. Natural latex, the material in most of the gloves currently on the market, contains residual sulphur and hence these gloves are the
NOVEMBER
1992
VOLUME
68
NUMBER
5
POLYVINYL
SILOXANE
IMPRESSION
TECHNIQUES
main offender. Contact of the internal surface of the impression tray with gloved hands can also result in failure of the material adjacent to the tray to polymerize, and separation of the tray from the impression material can occur (Figs. 1 and 2). The potential for additional interactions, for example with certain medicaments used for gingival retraction procedures, also exists.ll IMPRESSION POLYVINYL
TECHNIQUES SILOXANE
WITH
Elastomeric impression materials are most accurate when used with a uniform bulk of 1.5 to 2.5 mm.12-14This has led to the routine use of acrylic resin custom trays. It is generally advocated that one,or two layers of baseplate wax on the diagnostic cast will create the desired relief and that the occlusal stops be utilized to correctly orient the tray so that this optimum thickness of material is obtained. Trays should be made a minimum of 24 hours before use in order to insure stability. They should be rigid and painted with an appropriate adhesive.15It is our strong opinion that the optimum technique for the use of polyvinyl siloxane impression materials is to construct an acrylic resin custom tray as described and make the impression by using low-viscosity material in the syringe and medium- or heavy-bodied material in the tray. One technique often used is the putty-wash system. Putty materials were originally developed as simply another way to make a custom tray, with the major objective again being control of bulk of material. Advocates of some of the techniques currently used seem to have lost sight of this fact. The best way to use the putty-wash system is to make a putty tray in the same manner as an acrylic resin tray. A stock tray is painted with adhesive and a putty impression is made of the spaced diagnostic cast. This will produce a putty tray that is identical to one made of acrylic resin and is used in the same fashion (Figs. 3 through 5). One important precaution in using this technique is to select the stock tray to closely fit the arch form. This will reduce the amount of impression material required to make the impression and will facilitate seating the loaded impression tray intraorally. Several techniques for the intraoral fabrication of putty trays have also been advocated. One method is to make a preliminary intraoral putty impression, selectively relieving the putty, and make the final impression with a wash material (Fig. 6). Yet another technique creates space for the wash material by interposing a polyethylene sheet between the teeth and oral structures. This sheet prevents penetration of the putty into the interproximal areas (Fig. 7). There are some potential difficulties with both of these techniques. First, it is practically impossible to control bulk so that an even amount of wash material (1.5 to 2.5 mm) is used. It is not unusual to see areas of the impression, even
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Fig. 11. Silver Mylar film strips are used to verify complete closing in intercuspal position with the impression material in place. in critical marginal locations, where the putty shows through (Figs. 8 and 9). Second, becausemost putty materials are somewhat resilient, hydraulic pressure creates undetectable distortions in the impression. These distortions are not apparent until the castings made from the impression fail to seat. Placing the tray in the mouth with minimal pressure is essential, but cannot, by itself, guarantee that no harmful pressure will be created. Thus these techniques, although they can be used successfully, are not recommended. The worst method of making a putty-wash impression is the “simultaneous technique” currently advocated by some manufacturers. With this technique the putty material and the syringe material are mixed simultaneously. The syringe material is then injected around the prepared teeth, and the tray loaded with putty is inserted into the mouth. There are several deficiencies with this approach. First, there is absolutely no control of bulk. Moreover, in most situations, parts of the prepared teeth, including margins, are duplicated with putty instead of the syringe materials (Fig. 10). Most putties on the market cannot reproduce fine enough detail to meet the American Dental Association (ADA) specification 19, which is necessary for impression materials used in the fabrication of precision castings.K l7 More bubbles are produced and included in the set impression with this technique than with the use of a custom tray. l8 One final disadvantage is that by mixing the putty material at the same time as the syringe material, the setting distortion of the putty is included in the overall distortion of the impression. Although this distortion is relatively small, it is desirable to eliminate it if possible. The dual arch impression
technique
One of the more popular techniques used with polyvinyl siloxane is the dual arch or double bite technique. This
731
CHEE
procedure can be used when the number of prepared teeth is limited to one or two, when the patient can repeatedly close in maximum intercuspation with the tray in place, and when the patient has existing anterior guidance. In the absence of the latter, it is difficult to avoid creating nonworking (balancing) contacts in the final restorations. Plastic trays should not be used with this technique becausethey are far too flexible and are narrower than the buccolingual arches of most patients. This results in flexure during impression making and distortion on removal. More rigid metal trays should be utilized. Care should be taken with the choice of impression material when the dual arch techn.ique is used. Monophasic materials (Imprint, 3M Mfg., Minneapolis, Minn.) are best because of their resistance to slumping. The ability of the patient to close in maximum intercuspation with the tray in place is verified with silver Mylar strips (Du Pont Co., Wilmington, Del.) by having the patient close in this position without the tray and noting the nature of the contact in the cuspid area on the opposite side. This procedure is repeated with the tray in place, verifying that the contacts are the same. The silver Mylar strips are used again when the impression for occlusal registration is made (Fig. 11). The impression can be made in one step or two steps, if desired, by using the hydrodynamic or laminated techniqueSi Extending
working
time
with
polyvinyl
siloxanes
In general, polyvinyl siloxanes have adequate working time, especially when used with the automix system.20In some situations it is advantageous to extend working time. With the automix system, there is no way to do this by altering the base/catalyst ratio. Refrigeration of the syringe material will extend working time approximately 1% minutes without affecting the accuracy or working characteristics (unpublished data). SUMMARY
AND
CONCLUSIONS
Polyvinyl siloxane impression materials have becomethe impression material of choice in many clinical situations. They possess excellent physical properties and handling characteristics. These materials are somewhat techniquesensitive, and care must be taken to insure optimum results: 1. A complete dry field is essential when impressions are made with addition reaction silicone. 2. These materials are best used in conjunction with an acrylic resin custom tray. 3. Polymerization of these materials is inhibited by
732
AND
DONOVAN
many brands of latex gloves. Care must be taken to avoid contact with critical oral structures or the use of noncontaminating gloves is essential. 4. The simultaneous impression technique should be avoided. REFERENCES 1. Phillips RW. Skinner’s science of dental materials. 9th Ed. Philadelphia: WB Saunders, 1991:145-7. 2. Lacy AM, Fukui H, Bellman T, Jendresen MD. Time-dependent accuracy of elastomeric impression materials. Part II: polyether, polysulfides, and polyvinylsiloxane. J PROSTHET DENT 1981;45:329-33. 3. Crispin BJ, Watson JF, Frawley KR. Silver-plated dies. Part II: Marginal accuracy of cast. restorations. J PROSTHET DENT 1984;51:768-73. 4. Craig RG. Evaluation of an automatic mixing system for an addition silicone impression material. J Am Dent Assoc 1985;110:213-5. 5. Reitz CD, Clark NP. The setting of vinyl polysiloxane and condensation silicone putties when mixed with gloved hands. J Am Dent Assoc 1988;116:371-5. 6. Noonan JE, Goldfogel MH, Lambert RL. Inhibited set of the surface of addition silicones in contact with rubber dam. Oper Dent 1986;2:46-8. 7. Chee WWL, Donovan TE, Kahn RL. Indirect inhibition of polymerization of a polyvinyl impression material: a case report. Quint .Int 1991;22:133-5. 8. Kahn RL, Donovan TE. A pilot study of polymerization inhibition of poly (vinyl siloxane) materials by latex gloves. Int J Prosthodont 1989;2:128-30. 9. Kahn RL, Donovan TE, Chee WWL. Interaction of latex gloves and poly (vinyl siloxane) impression materials: a screening survey. Int J Prosthodont 1989;2:342-6. 10. Cook WD, Thomas F. Rubber gloves and addition silicone impression materials. Aust Dent J 1986;31:140-4. 11. Phillips RW. Skinner’s science of dental materials. 9th ed. Philadelphia: WB Saunders, 1991:154-f.% 12. Eames WB, Sieweke C, Wallace GW, Rogers LB. Elastomeric impression materials: effect of bulk on accuracy. J PFXJSTHET DENT 1979; 41:304-7. 13. Williams PT, Jackson GD, Bergman W. An evaluation of the time-dependent dimensional stability of eleven elastomeric impression materials. J PROSTHET DENT 1984,52:120-5. 14. Donovan TE. A review of contemporary impression materials. California Institute for Continuing Education 1988;27:9-19. 15. Eames WB, Sieweke JC. Seven acrylic resins for trays and five putty wash systems compared. Oper Dent 1980;5:162-7. 16. Council on Dental Materials and Devices, ADA. Revised American Dental Association specification 19 for nonaqueous, elastomeric impression materials. J Am Dent Assoc 1977;94:733-41. 17. Chee WWL, Donovan TE. Fine detail reproduction of very high viscosity poly (vinyl silorane) impression materials. Int J Prosthodont 1989;2:368-70. 18. Johnson GH, Drennon DG. Clinical evaluation of detail reproduction of elastomeric impression materials [Abstract]. J Dent Res 1987;66(special issue):331. 19. Lococo MP. The hydrodynamic impression technique. J Can Dent Assoc 1986;52:1001-03. 20. Sy JT, Munoz CA, Schnell RJ, Moore BK, Goodacre CJ. Some effects of cooling and chemical retarders on five elastomeric impression materials. Int J Prosthodont 198&1:252-S. Reprint requests to: DR. WINSTON W. L. CHEE SCHOOL OF DENTISTRY ROOM 4374 UNIVERSITY OF SOUTHERN CALIFORNIA Los ANGELES, CA 90089-0641
NOVEMBER
1992
VOLUME
68
NUMBER
5
W. L. Chee, BDSpa and Terry
materials:
E. Donovan,
A review
of
DDSb
University of SouthernCalifornia, Los Angeles,Calif. Polyvinyl siloxane impression materials have been shown to have excellent properties as impression materials; however, they are sensitive to manipulative variables. Several methods of using very high viscosity (putty) materials to form “trays” to obtain uniform bulk of the wash impression are described, and the disadvantages of each of these techniques is pointed out. It is recommended that for best results acrylic resin custom trays should be used routinely. The interaction of polyvinyl siloxane materials with latex products is also discussed and problems that this inhibition can cause are stated. Suggestions to avoid this interaction are outlined. One of the disadvantages of the impression materials is that it has a relatively short working time. Refrigerating the material will increase working time without affecting accuracy. (J PROSTHET DENT 1992;68:728-32.)
olyvinyl siloxane impression materials, also known as addition reaction silicones, became extremely popular during the past decade. Although they are among the most expensive impression materials, they are now used in a wide variety of situations in fixed prosthodontics, operative dentistry, removable prosthodontics, and implant dentistry. The popularity of these materials is understandable, given the combination of excellent physical properties, handling characteristics, and unlimited dimensional stability. Despite all of the advantages that the polyvinyl siloxanes possess, a thorough understanding of the composition, physical properties, and manipulative variables of these materials is essential to achieve predictable success.This article will review these factors and give guidelines for techniques that will result in optimum performance. PHYSICAL
PROPERTIES
Addition reaction silicone impression materials have excellent physical properties. Their accuracy is unsurpassed and they can record fine detail. They also have the best elastic recovery of all available impression materia1s.l Becausethere is virtually no by-product to the polymerization reaction, impressions are dimensionally stable and can be poured at the convenience of the operator. They also allow the opportunity to make a second pour.2 This may also be a factor with the use of silver-plated dies, which are most
Presentedat the PacificCoastSocietyof Prosthodonticsmeeting, Victoria, British Columbia. aAssistantProfessor,Departmentof RestorativeDentistry, Director of Implant Dentistry. bAssociateProfessor,Chairman,Sectionof Biomaterials,and Director of Clinical RestorativeR.esearch. lo/1140360
728
accurate when recovered from polyvinyl siloxane impressions.3 The handling characteristics of addition reaction silicones are also favorable. They are supplied in a number of viscosities, ranging from very low for use with a syringe or wash material to medium, high, and very high. This allows these materials to be used for a host of applications. Polyvinyl siloxane materials are supplied by many manufacturers in the automix system, which is convenient, provides a consistent mix, and is cost-effective.* The number of bubbles incorporated in the mix is reduced with the automix system. From a patient comfort standpoint, these materials are ideal because they are clean, odorless, and tasteless. They polymerize quickly and, especially when used with a custom tray, the amount of bulk of material can be kept to a minimum. The polymerized impression is somewhat rigid, but this rigidity is nowhere that of the polyether materials and is not a major problem. Polyvinyl siloxane impression materials require an absolutely dry field for an acceptable impression, as is true of all elastomeric impression materials. Some contemporary polyvinyl siloxanes have been modified with the addition of certain nonionic surfactants and are described by the manufacturers as hydrophylic. This designation is a misnomer, and at best they are slightly less hydrophobic than their predecessors.The clinical advantage of the modification is that the polymerized impression is somewhat easier to pour. The term hydrophylic incorrectly implies that impressions can be made in a wet or moist environment. Manufacturers of some polyvinyl materials recommend delaying pouring the impression for a period of time becausethe material emits hydrogen gas, which can create voids in the cast. Most of the newer products contain small amounts of palladium, which prevents this emission, and allows the impression to be poured at the convenience of the operator.
NOVEMBER1992
VOLUME68
NUMBER6
POLYVINYL
SILOXANE
IMPRESSION
TECHNIQUES
Fig. 1. Polyvinyl siloxane impression completely separated from acrylic resin custom tray (impression surface). Fig. 2. Polyvinyl siloxane impression completely separated from acrylic resin custom tray (tray side view of impression in Fig. I). Fig. 3. Diagnostic cast with appropriate amount of spacer for an elastomeric impression. Fig. 4. Acrylic resin tray made on diagnostic cast with spacer (Fig. 3). Fig. 5. Putty custom tray made on diagnostic cast with spacer (Fig. 3). Fig. 6. Putty is arbitrarily removed from preoperative overimpression of arch to make space for wash material.
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
729
CNEE
AND
DONOVAN
Fig. 7. Thin polyethylene sheet is interposed between putty and teeth to prevent penetration of putty material interproximally and to create space for wash material. Fig. 8. Completed 2-step putty wash impression shows bottoming out of the putty custom tray. Fig. 9. Close-up of the preparations recorded with the impression (Fig. 8). Fig. 10. Some of the margins are recorded in putty with the one-step putty wash technique.
Three areas deserve special consideration in the discussion of these materials. First is the interaction of latex rubber with polyvinyl siloxane. Second is the putty-wash technique recommended by some of the manufacturers. The third topic is extending the working times with these materials.
Interaction of latex siloxane materials
rubber
and polyvinyl
Some latex products have a definite inhibitory effect on the polymerization of addition reaction silicones. This inhibition was originally observed when the putty materials were mixed with gloved hands or hands that had previously been wearing gloves.5 It was also noted when the impression material was allowed to set in contact with a rubber dam.6 Recently, the inhibition of polymerization intraorally
730
as a result of casual contact of tooth preparations and surrounding soft tissues with latex gloves before impression making has been documented.7-s The inhibition of polymerization is usually subtle and is limite‘d to the surface of the impression in contact with the contaminated tissues. Even though the inhibition is minimal, the gypsum cast in contact with unpolymerized material will definitely be distorted and hence unacceptable. The mechanism of inhibition is thought to be contamination of the chloroplantinic acid catalyst in the impression material with free sulphur remaining in the glove.lO It is clear that the powder is not a factor and not all latex gloves produce this inhibition. Vinyl gloves and synthetic latex gloves do not seem to cause problems. Natural latex, the material in most of the gloves currently on the market, contains residual sulphur and hence these gloves are the
NOVEMBER
1992
VOLUME
68
NUMBER
5
POLYVINYL
SILOXANE
IMPRESSION
TECHNIQUES
main offender. Contact of the internal surface of the impression tray with gloved hands can also result in failure of the material adjacent to the tray to polymerize, and separation of the tray from the impression material can occur (Figs. 1 and 2). The potential for additional interactions, for example with certain medicaments used for gingival retraction procedures, also exists.ll IMPRESSION POLYVINYL
TECHNIQUES SILOXANE
WITH
Elastomeric impression materials are most accurate when used with a uniform bulk of 1.5 to 2.5 mm.12-14This has led to the routine use of acrylic resin custom trays. It is generally advocated that one,or two layers of baseplate wax on the diagnostic cast will create the desired relief and that the occlusal stops be utilized to correctly orient the tray so that this optimum thickness of material is obtained. Trays should be made a minimum of 24 hours before use in order to insure stability. They should be rigid and painted with an appropriate adhesive.15It is our strong opinion that the optimum technique for the use of polyvinyl siloxane impression materials is to construct an acrylic resin custom tray as described and make the impression by using low-viscosity material in the syringe and medium- or heavy-bodied material in the tray. One technique often used is the putty-wash system. Putty materials were originally developed as simply another way to make a custom tray, with the major objective again being control of bulk of material. Advocates of some of the techniques currently used seem to have lost sight of this fact. The best way to use the putty-wash system is to make a putty tray in the same manner as an acrylic resin tray. A stock tray is painted with adhesive and a putty impression is made of the spaced diagnostic cast. This will produce a putty tray that is identical to one made of acrylic resin and is used in the same fashion (Figs. 3 through 5). One important precaution in using this technique is to select the stock tray to closely fit the arch form. This will reduce the amount of impression material required to make the impression and will facilitate seating the loaded impression tray intraorally. Several techniques for the intraoral fabrication of putty trays have also been advocated. One method is to make a preliminary intraoral putty impression, selectively relieving the putty, and make the final impression with a wash material (Fig. 6). Yet another technique creates space for the wash material by interposing a polyethylene sheet between the teeth and oral structures. This sheet prevents penetration of the putty into the interproximal areas (Fig. 7). There are some potential difficulties with both of these techniques. First, it is practically impossible to control bulk so that an even amount of wash material (1.5 to 2.5 mm) is used. It is not unusual to see areas of the impression, even
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Fig. 11. Silver Mylar film strips are used to verify complete closing in intercuspal position with the impression material in place. in critical marginal locations, where the putty shows through (Figs. 8 and 9). Second, becausemost putty materials are somewhat resilient, hydraulic pressure creates undetectable distortions in the impression. These distortions are not apparent until the castings made from the impression fail to seat. Placing the tray in the mouth with minimal pressure is essential, but cannot, by itself, guarantee that no harmful pressure will be created. Thus these techniques, although they can be used successfully, are not recommended. The worst method of making a putty-wash impression is the “simultaneous technique” currently advocated by some manufacturers. With this technique the putty material and the syringe material are mixed simultaneously. The syringe material is then injected around the prepared teeth, and the tray loaded with putty is inserted into the mouth. There are several deficiencies with this approach. First, there is absolutely no control of bulk. Moreover, in most situations, parts of the prepared teeth, including margins, are duplicated with putty instead of the syringe materials (Fig. 10). Most putties on the market cannot reproduce fine enough detail to meet the American Dental Association (ADA) specification 19, which is necessary for impression materials used in the fabrication of precision castings.K l7 More bubbles are produced and included in the set impression with this technique than with the use of a custom tray. l8 One final disadvantage is that by mixing the putty material at the same time as the syringe material, the setting distortion of the putty is included in the overall distortion of the impression. Although this distortion is relatively small, it is desirable to eliminate it if possible. The dual arch impression
technique
One of the more popular techniques used with polyvinyl siloxane is the dual arch or double bite technique. This
731
CHEE
procedure can be used when the number of prepared teeth is limited to one or two, when the patient can repeatedly close in maximum intercuspation with the tray in place, and when the patient has existing anterior guidance. In the absence of the latter, it is difficult to avoid creating nonworking (balancing) contacts in the final restorations. Plastic trays should not be used with this technique becausethey are far too flexible and are narrower than the buccolingual arches of most patients. This results in flexure during impression making and distortion on removal. More rigid metal trays should be utilized. Care should be taken with the choice of impression material when the dual arch techn.ique is used. Monophasic materials (Imprint, 3M Mfg., Minneapolis, Minn.) are best because of their resistance to slumping. The ability of the patient to close in maximum intercuspation with the tray in place is verified with silver Mylar strips (Du Pont Co., Wilmington, Del.) by having the patient close in this position without the tray and noting the nature of the contact in the cuspid area on the opposite side. This procedure is repeated with the tray in place, verifying that the contacts are the same. The silver Mylar strips are used again when the impression for occlusal registration is made (Fig. 11). The impression can be made in one step or two steps, if desired, by using the hydrodynamic or laminated techniqueSi Extending
working
time
with
polyvinyl
siloxanes
In general, polyvinyl siloxanes have adequate working time, especially when used with the automix system.20In some situations it is advantageous to extend working time. With the automix system, there is no way to do this by altering the base/catalyst ratio. Refrigeration of the syringe material will extend working time approximately 1% minutes without affecting the accuracy or working characteristics (unpublished data). SUMMARY
AND
CONCLUSIONS
Polyvinyl siloxane impression materials have becomethe impression material of choice in many clinical situations. They possess excellent physical properties and handling characteristics. These materials are somewhat techniquesensitive, and care must be taken to insure optimum results: 1. A complete dry field is essential when impressions are made with addition reaction silicone. 2. These materials are best used in conjunction with an acrylic resin custom tray. 3. Polymerization of these materials is inhibited by
732
AND
DONOVAN
many brands of latex gloves. Care must be taken to avoid contact with critical oral structures or the use of noncontaminating gloves is essential. 4. The simultaneous impression technique should be avoided. REFERENCES 1. Phillips RW. Skinner’s science of dental materials. 9th Ed. Philadelphia: WB Saunders, 1991:145-7. 2. Lacy AM, Fukui H, Bellman T, Jendresen MD. Time-dependent accuracy of elastomeric impression materials. Part II: polyether, polysulfides, and polyvinylsiloxane. J PROSTHET DENT 1981;45:329-33. 3. Crispin BJ, Watson JF, Frawley KR. Silver-plated dies. Part II: Marginal accuracy of cast. restorations. J PROSTHET DENT 1984;51:768-73. 4. Craig RG. Evaluation of an automatic mixing system for an addition silicone impression material. J Am Dent Assoc 1985;110:213-5. 5. Reitz CD, Clark NP. The setting of vinyl polysiloxane and condensation silicone putties when mixed with gloved hands. J Am Dent Assoc 1988;116:371-5. 6. Noonan JE, Goldfogel MH, Lambert RL. Inhibited set of the surface of addition silicones in contact with rubber dam. Oper Dent 1986;2:46-8. 7. Chee WWL, Donovan TE, Kahn RL. Indirect inhibition of polymerization of a polyvinyl impression material: a case report. Quint .Int 1991;22:133-5. 8. Kahn RL, Donovan TE. A pilot study of polymerization inhibition of poly (vinyl siloxane) materials by latex gloves. Int J Prosthodont 1989;2:128-30. 9. Kahn RL, Donovan TE, Chee WWL. Interaction of latex gloves and poly (vinyl siloxane) impression materials: a screening survey. Int J Prosthodont 1989;2:342-6. 10. Cook WD, Thomas F. Rubber gloves and addition silicone impression materials. Aust Dent J 1986;31:140-4. 11. Phillips RW. Skinner’s science of dental materials. 9th ed. Philadelphia: WB Saunders, 1991:154-f.% 12. Eames WB, Sieweke C, Wallace GW, Rogers LB. Elastomeric impression materials: effect of bulk on accuracy. J PFXJSTHET DENT 1979; 41:304-7. 13. Williams PT, Jackson GD, Bergman W. An evaluation of the time-dependent dimensional stability of eleven elastomeric impression materials. J PROSTHET DENT 1984,52:120-5. 14. Donovan TE. A review of contemporary impression materials. California Institute for Continuing Education 1988;27:9-19. 15. Eames WB, Sieweke JC. Seven acrylic resins for trays and five putty wash systems compared. Oper Dent 1980;5:162-7. 16. Council on Dental Materials and Devices, ADA. Revised American Dental Association specification 19 for nonaqueous, elastomeric impression materials. J Am Dent Assoc 1977;94:733-41. 17. Chee WWL, Donovan TE. Fine detail reproduction of very high viscosity poly (vinyl silorane) impression materials. Int J Prosthodont 1989;2:368-70. 18. Johnson GH, Drennon DG. Clinical evaluation of detail reproduction of elastomeric impression materials [Abstract]. J Dent Res 1987;66(special issue):331. 19. Lococo MP. The hydrodynamic impression technique. J Can Dent Assoc 1986;52:1001-03. 20. Sy JT, Munoz CA, Schnell RJ, Moore BK, Goodacre CJ. Some effects of cooling and chemical retarders on five elastomeric impression materials. Int J Prosthodont 198&1:252-S. Reprint requests to: DR. WINSTON W. L. CHEE SCHOOL OF DENTISTRY ROOM 4374 UNIVERSITY OF SOUTHERN CALIFORNIA Los ANGELES, CA 90089-0641
NOVEMBER
1992
VOLUME
68
NUMBER
5
