Properties of the tray adhesive of an addition silicone to impression tray materials Mohd. Zainal Abidin
Mohd Sulong, MSc,* and Derrick
J. Setchell,
polymerizing
MSb
University of Malaya, Faculty of Dentistry, Kuala Lumpur, Malaysia, and Institute of Dental Surgery, London, England Adhesive bond strength studies for the tray adhesive of an addition vinyl polysiloxane (President) impression material were conducted with an acrylic resin, chromium-plated brass, and plastic trays. Tensile and shear stress studies were performed on the Instron Universal testing machine. Acrylic resin specimens roughened with SO-grit silicon carbide paper exhibited appreciably higher bond strengths compared with different types of tray material and methods of surface preparation. (J PROSTAET DENT 1991;66:743-7.)
N
umerous dental impression materials and trays are available for various clinical situations to obtain accurate impressions. Elastomeric impression materials are popular because of their proven accuracy and reliability.l, 2 However, there are differences in accuracy among the four types of materials, which include condensation silicones, addition polymerizing silicones (AS), polysulfides, and polyether.3-5 In general, the AS are more consistently accurate than the other types of elastomers. The dimensional stability of elastomeric impression materials is substantially elevated when they are bonded to an acrylic resin tray.6 Permanent distortion occurs when the impression material does not adhere to the tray. An effective adhesive is especially indicated when the impression material has a high tear-resistance, so that it can effectively be removed from undercuts. Ideally, the impression material tears instead of undergoing considerable distortion before it is released from the undercut. Metal and plastic stock trays are used routinely for dental impressions, especially with the putty-wash systems. However, heat-cured trays with an adhesive are recommended for AS because the adhesives for AS are less effective than those for polysulfides, polyether, and condensation silicones.5 Clinical custom trays are commonly made of autopolymerizing acrylic resins and are used for elastomeric impressions. The custom acrylic resin trays with “programmed” occlusal stops ensure a uniform distribution of impression material around the teeth to the recommended thickness of approximately 3 mm.7 However, it has been reported that there were no statistically significant differences between two-dimensional changes in impressions with stock trays and custom trays.$ This study compared the effect of three different methods of surface preparation of an acrylic resin tray material
aLecturer, Department of Conservation, University of Malaya, Faculty of Dentistry. bHead, Department of Conservation, Institute of Dental Surgery. 10/l/31339
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
on the adhesive bond strengths of an AS. A comparison was also made of the adhesive bond strengths of an AS on acrylic resin, chromium-plated brass, and stock “plastic” polystyrene tray specimens. MATERIAL
AND
METHODS
Round tray resin specimens 3 cm in diameter and rectangular specimens 2 by 3.5 cm were constructed for tensile and shear strength tests, respectively. Both tray specimens had a test surface of approximately 7 cm.2 Acrylic resin trays (Formatray, Sybron/Kerr, Romulus, Mich.) were made from silicone rubber (Silastic 9161 RTW, Dow Corning, Midland, Mich.) molds of master patterns. The surface preparation of the resin trays was accomplished as follows: 1. The test surface was cured against aluminum foil. 2. The test surface was abraded with BO-grit silicon carbide (Sic) paper on rotary equipment under running water, rinsed thoroughly, and allowed to dry overnight. 3. Fifteen evenly distributed perforations 2.5 mm in diameter were made in a resin block previously roughened with Sic paper to simulate a perforated tray. The following tray specimens were also selected for this study: (1) smooth chromium-plated brass, (2) chromiumplated brass roughened with BO-grit Sic paper, and (3) polystyrene roughened with BO-grit Sic paper. President (Coltene AG, Alstatten, Switzerland) heavybodied polyvinyl siloxane impression material with its tray adhesive was used throughout the study. The surface of each tray specimen was coated with a thin, uniform layer of tray adhesive with the brush supplied. The adhesive was confined to the tested surface to prevent contact of the impression material elsewhere. The adhesive was allowed to bench dry for 15 minutes to provide the highest bond strength.s A rigid 3 mm thick brass plate with multiple perforations secured the impression material for testing. A circular plate was selected for the tensile test while a rectangular plate was used for the shear test. A thin coat of tray adhesive was 743
SULONG
AND
SETCHELL
Fig. 2. Shear test specimen. a, Compressive load cell of Instron machine; b, brass plate; c, impression material; d, acrylic resin tray; e, plastic mounting platform.
(Fig. 2). The tests were accomplished at a crossheadspeed of 50 cm per minute, and each test was performed on five samples for each tray specimen. RESULTS
Fig. I. Tensile test specimen. a, Steel chain secured to :nstron machine; b, acrylic resin tray; c, impression mat ;e1ial on retentive brass plate. applied to the brass plates for additional retention of the impression material. The impression material was manipulated according to the manufacturer’s instructions. A thorough homogenous mix was made by using the double-spatula technique. The surface of the tray to be tested was carefully covered with the impression material, and the remainder of the material was applied to the brass plate sustaining the material during testing. The tray specimen and the brass plate were then securely joined in a fixed position and the entire assembly was placed in a humidor at 37O C and 100% humidity during the set of the impression material. Both the tensile and the shear stress testings were conducted 12 minutes after the start of mixing of the impression material. A sharp scalpel blade was used to remove the excessimpression material from the sides of the trays before they were mounted on the Instron Universal testing machine (Instron Corp., Canton, Mass.). The opposite ends of the tensile test specimens were attached to the grips of the Instron machine (Fig. 1). A special plastic platform was used to mount the shear test specimens and the compression load cell was lowered to the protruding brass plate
744
Fig. 3 demonstrates the tensile and shear stresses for President tray adhesive in relation to acrylic resin surface preparations. The surface roughened with 80-grit Sic paper created the best bond in both tensile and shear tests. The results among the three variables were statistically significant, and the lowest values were recorded for the resin blocks cured against aluminum foil, whereas the perforated tray specimens registered intermediate values. The pilot study on perforated tray specimens revealed that failure in tension occurred at 0.05 MPa when the impression material did not flow completely through the perforations. However, when the extruded material coalesced on the external surface of the tray, 0.16 MPa was recorded for failure in tension. The difference was statistically significant (p < 0.05). Fig. 4 lists the tensile and shear stresses for President tray adhesive on different types of trays. Acrylic resin roughened with go-grit Sic produced the highest bond strengths compared with chromium-plated brass and plastic trays. The differences between smooth and roughened chromium-plated brass tray specimens was not significant; both appeared to create better retention for the President tray adhesive compared with polystyrene. DISCUSSION The result of roughening the acrylic resin trays with Sic paper was a higher adhesive bond strength compared with specimens cured against aluminum foil. This finding was
DECEMBER
1991
VOLUME
66
NUMBER
6
PROPERTIES
OF TRAY
ADHESIVE
qit B-pmfaatul c--fofi
sic w
A - 80
-
0-1
0.16
0.02 tA
B
Fig. 3. Adhesive strength as function of surface preparation. the opposite of that reported on the joint strength of a polysulfide adhesive on acrylic resin abraded with Sic paper against that cured against tin foil.lO In this study, small patches of adhesive remained on the acrylic resin specimens cured against aluminum foil whereasthe greater bulk of the President tray adhesivebecamecompletely detached from the smooth surfaces of the tray specimens. Conversely, large patches were observed on the roughened specimens, which could be attributed to the rough surface texture of the resin blocks providing a greater surface area to retain the adhesive. The tensile tests did not exhibit rupture within the impression material, indicating that the bond strength of the adhesive was less than the rupture strength of the impression material. It is a common laboratory practice to complete the preparation of an acrylic resin tray with sandpaper becausethis procedure not only eliminates sharp edgesbut rounds and smoothes the borders of the tray. This study proved that the bond strength of President adhesive is substantially improved by roughening the resin tray with 80-grit Sic paper. The finishing of acrylic resin trays should also include the fitting surface. The use of aluminum foil reduces wax contamination of the acrylic resin tray and, combined with a final finishing with sandpaper, enhances the retentive properties of the tray adhesive. Acrylic resin trays roughened with Sic paper recorded a higher tensile bond strength for President adhesive com-
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
pared with trays prepared against aluminum foil. In addition, the shear bond strength was elevated by more than three times, which was important becausethe shear bond strengths of tray adhesiveswere consistently lower than the tensile bond strengths. Perforating the tray actually resulted in an appreciably lower bond strength despite roughening of the test surfaces with Sic paper before application of the tray adhesive.Two types of failures were observed: (1) rupture of the impression material occurred randomly at the ends within the perforations and (2) failure of the adhesive was confined to the surface of the resin blocks without perforations. Rupture of the impression material can be attributed to a rapid and high concentration of stresseswithin limited material at the perforation, which resulted in an elevation of stresses between the perforations. This caused the detachment of impression material from the trays. Attempts to secure the impression by perforating the tray were not as successful as roughening the tray. Despite the lower tensile and shear bond strengths of the perforated trays compared with the roughened acrylic resin trays, they were more retentive than trays cured against aluminum foil. If a perforated tray is used, the impression material should flow completely through the perforation& and the extruded material should coalesceon the outer surface of the tray. Failure to accomplish this results in diminished retention for the impression as was observed during the pi-
745
SULONG
AND
SETCHELL
0.26 0.24 0.22 0-M 0.1s ?IIIIILEL m
0.16
(ml
0.14 0.12
0.10 0.06 0.06 0.04 0.01
A
B
D
C
F ig. 4. Adhesive strength as function of tray material.
lot study. Adequate retention of a perforated tray is also determined by the diameter and the distance between perforations.’ “. *‘j The differences between acrylic resin, Cr-plated brass, and plastic tray specimens was attributed to the characteristics of the roughened surfaces. The homogenous Crplat.ed brass and plastic materials did not contribute to an appreciable increase in surface area after being roughened with 80 grit SiC paper. The acrylic resin was transformed into a heterogenous structure of resinous matix with varying sizes of filler part.icles. The variation in surface energies of materials could then have accounted for the variable adhesive bond strengths; thus it can be concluded that different tray materials possessdissimiliar bonding affinities to tray adhesives. Failure in adhesion and rupture of the impression material in this study following shear tests resulted from a unidirectional load. Forces inflicted on an impression tray during removal from the mouth originate from various angles becauseof the anatomic variations of teeth and jaws. The forces required to detach the impression material from the tray could easily exceed those in this study. Hence, in the normal clinical situation? the bond of tray adhesives to the impression tray is considered adequate when the tray is perforat,ed and the impression material extends to the outer surface of the tray.
Clinical
impEcations
Roughening the acrylic resin tray enhancesthe retentive properties of tray adhesive. This procedure is accomplished 746
with Sic paper or sandpaper during the finishing of a custom acrylic resin tray. If a perforated tray is selected, it is imperative that the impression material flows through the perforations to coalesce on the external surface of the tray. Although putty material was not included in this study, it is reasonable to postulate that the putty material should also flow through all the perforations for maximal retention. The use of metal trays is usually limited to specific applications such as an impression for transfer copings, and the trays should be roughened by sandblasting. Conversely, plastic trays should be used sparingly becausethe bonding to tray adhesives is weaker and inconsistent.
SUMMARY The properties of tensile and shear strength of President tray adhesive were determined for different methods of acrylic resin tray preparations by use of an Instron machine. A comparison was also drawn between three different types of tray materials. This study revealed that acrylic resin trays roughened with 80-grit Sic created the highest tensile and shear values while plastic tray specimens recorded the least favorable adhesion to the tray adhesive.
CONCLUSION 1. The greatest adhesive bond strength was recorded for acrylic resin roughened with 80-grit Sic paper. 2. Perforated acrylic resin trays were not as adhesive as those roughened with go-grit Sic paper. DECEMBER
1991
VOLUME
66
NUMBER
6
PROPERTIES
OF TRAY
ADHESIVE
3. Acrylic resin cured against aluminum foil resulted in the weakest bond for the President adhesive. 4. The adhesives did not adhere well to chromiumplated metal or plastic stock tray material. 5. The tensile bond strengths of tray adhesives consistently exceeded the shear bond strengths. REFERENCES 1. Yeh CL, PowersJM, Craig RG. Properties of addition-type silicone im-
pression materials. J A m Dent Asaoc 1980,101:482-4. 2. Willis PT, Jackson DG, Bergman W . An evaluation of the time-dependent dimensional stability of elevenelastomericimpression materials. J PROSTHET DENT 19&1;52:120-5. 3. Johnson GH, Craig GC. Accuracy of four types of rubber impression materials compared with time of pour and a repeat of models. J PROS-
DENT 19&1,52:514-7.
9. Mohd Zainal AMS. An investigation of the bonding properties of tray adhesivesfor addition polymerising vinyl polysilonanes.MSc Thesis, London: University of London, 1985. 10. Davis GB, Moser JB. Brinsden GI. The bonding properties of elastomer tray adhesives.J PR~~THET DENT 1976;36:278-85. 11. Samman JM, Fletcher AM. A study of impression tray adhesives. QuintessenceInt 1985;4:305-9. 12. FusayamaT, Nakazato M. The designsof stock trays and the retention of irreversible hydrocolloid impressions. J PROSTHET DENT 1969; 21:136-42. 13. Anderson JN. Applied dental materials. 5th ed. London: Blackwell Sci-
entific Publications, 1976223. Reprint requests to:
THET DENT 1985;53:484-90.
4. Clancy MS, Sandrett FR, Ettinger RL. Long-term dimensional stability of three current elastomers.J Oral Rehabil 1983;10:324-33. 5. Phillips RW. Scienceof dental materials. 8th ed. St Louis: CV Mosby Co, 1982:137-56. 6. Sandrik JL, Vacco JL. Tensile and bond strength of putty-wash elastomeric impression materials. J PROSTHET DENT 1983;50:358-61.
Marginal
7. Eames WB, SiewekeJC, Wallace SW. Elastomeric impression materials: effect of bulk on accuracy.J PROSTHET DENT 1979;41:304-7. 8. Valderhaug J, Floystrand F. Dimensional stability of elastomeric impression materials in custom-made and stock trays. J PROSTHET
adaptation
of castable
DR. M. Z. A. MOHD SULONG FACULTV OF DENTISTRY UNIVEFWT~ OF MALAYA 59100 KUALA LUMPLIR MALAYSIA
ceramic
crowns
James D. Weaver, DDS,a Glen H. Johnson, DDS, MS,b and David J. Bales, DDS, MS0 University of Washington, School of Dentistry, Seattle, Wash. Tooth preparations and seating techniques of castable ceramic crowns diier from metal ceramic crowns. This study evaluated the variable effects of cementation on the marginal adaptation of Dicer, Cerestore, and porcelain-fused-to-metal crowns. The shoulder preparation was maintained for ceramic crowns, and a cavosurface bevel was designed for metal ceramic crowns. Crowns were made with a replication size of 10, placed on master dies, and the marginal openings measured with a Nikon Measurescope 20 instrument. Thirty crowns were cemented with zinc phosphate cement and the recommended clinical force. Marginal adaptation was not improved with a gingival bevel preparation or an increased seating force. The best marginal adaptation was recorded for Cerestore crowns. (J PROSTHET DENT 1991;66:747-63.)
P
atients are demanding improved esthetics and desire natural-appearing teeth. All-ceramic crowns have increased in popularity and are widely used in dentistry. The absence of a metal collar or metal substrate makes ceramic crowns an esthetic alternative to porcelain-fused-tometal (PFM) crowns. One of the prerequisites for crowns is precise marginal adaptation. The differences in preparation design and
Presented at the American Association for Dental Research meeting, San Francisco, Calif.
aAssistant Professor, Department of Restorative Dentistry. bAssociate Professor, Department of Restorative Dentistry. cAssociate Professor and Chairman, Department of Restorative Dentistry. 10/l/31368 THE JOURNAL
OF PROSTHETIC
DENTISTRY
variation in seating forces must be considered in comparing the marginal fit of castable ceramic crowns and PFM crowns. A beveled shoulder, in which a go-degree shoulder with a 45degree bevel surrounds the entire tooth preparation, is the recommended tooth preparation for PFM crowns.l, 2 Rosner3 and Shillingburg et a1.4demonstrated that a suitably placed bevel on a tooth preparation reduces the marginal opening of a seated casting, but this concept has been disputed. Grajower and Lewinstein5 demonstrated mathematically a superior marginal fit without a beveled preparation and Belser et aL6 illustrated no difference in the marginal adaptation of seated castings before and after cementation with beveled or nonbeveled tooth preparations. The seating forces during cementation for PFM and 747
Mohd Sulong, MSc,* and Derrick
J. Setchell,
polymerizing
MSb
University of Malaya, Faculty of Dentistry, Kuala Lumpur, Malaysia, and Institute of Dental Surgery, London, England Adhesive bond strength studies for the tray adhesive of an addition vinyl polysiloxane (President) impression material were conducted with an acrylic resin, chromium-plated brass, and plastic trays. Tensile and shear stress studies were performed on the Instron Universal testing machine. Acrylic resin specimens roughened with SO-grit silicon carbide paper exhibited appreciably higher bond strengths compared with different types of tray material and methods of surface preparation. (J PROSTAET DENT 1991;66:743-7.)
N
umerous dental impression materials and trays are available for various clinical situations to obtain accurate impressions. Elastomeric impression materials are popular because of their proven accuracy and reliability.l, 2 However, there are differences in accuracy among the four types of materials, which include condensation silicones, addition polymerizing silicones (AS), polysulfides, and polyether.3-5 In general, the AS are more consistently accurate than the other types of elastomers. The dimensional stability of elastomeric impression materials is substantially elevated when they are bonded to an acrylic resin tray.6 Permanent distortion occurs when the impression material does not adhere to the tray. An effective adhesive is especially indicated when the impression material has a high tear-resistance, so that it can effectively be removed from undercuts. Ideally, the impression material tears instead of undergoing considerable distortion before it is released from the undercut. Metal and plastic stock trays are used routinely for dental impressions, especially with the putty-wash systems. However, heat-cured trays with an adhesive are recommended for AS because the adhesives for AS are less effective than those for polysulfides, polyether, and condensation silicones.5 Clinical custom trays are commonly made of autopolymerizing acrylic resins and are used for elastomeric impressions. The custom acrylic resin trays with “programmed” occlusal stops ensure a uniform distribution of impression material around the teeth to the recommended thickness of approximately 3 mm.7 However, it has been reported that there were no statistically significant differences between two-dimensional changes in impressions with stock trays and custom trays.$ This study compared the effect of three different methods of surface preparation of an acrylic resin tray material
aLecturer, Department of Conservation, University of Malaya, Faculty of Dentistry. bHead, Department of Conservation, Institute of Dental Surgery. 10/l/31339
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
on the adhesive bond strengths of an AS. A comparison was also made of the adhesive bond strengths of an AS on acrylic resin, chromium-plated brass, and stock “plastic” polystyrene tray specimens. MATERIAL
AND
METHODS
Round tray resin specimens 3 cm in diameter and rectangular specimens 2 by 3.5 cm were constructed for tensile and shear strength tests, respectively. Both tray specimens had a test surface of approximately 7 cm.2 Acrylic resin trays (Formatray, Sybron/Kerr, Romulus, Mich.) were made from silicone rubber (Silastic 9161 RTW, Dow Corning, Midland, Mich.) molds of master patterns. The surface preparation of the resin trays was accomplished as follows: 1. The test surface was cured against aluminum foil. 2. The test surface was abraded with BO-grit silicon carbide (Sic) paper on rotary equipment under running water, rinsed thoroughly, and allowed to dry overnight. 3. Fifteen evenly distributed perforations 2.5 mm in diameter were made in a resin block previously roughened with Sic paper to simulate a perforated tray. The following tray specimens were also selected for this study: (1) smooth chromium-plated brass, (2) chromiumplated brass roughened with BO-grit Sic paper, and (3) polystyrene roughened with BO-grit Sic paper. President (Coltene AG, Alstatten, Switzerland) heavybodied polyvinyl siloxane impression material with its tray adhesive was used throughout the study. The surface of each tray specimen was coated with a thin, uniform layer of tray adhesive with the brush supplied. The adhesive was confined to the tested surface to prevent contact of the impression material elsewhere. The adhesive was allowed to bench dry for 15 minutes to provide the highest bond strength.s A rigid 3 mm thick brass plate with multiple perforations secured the impression material for testing. A circular plate was selected for the tensile test while a rectangular plate was used for the shear test. A thin coat of tray adhesive was 743
SULONG
AND
SETCHELL
Fig. 2. Shear test specimen. a, Compressive load cell of Instron machine; b, brass plate; c, impression material; d, acrylic resin tray; e, plastic mounting platform.
(Fig. 2). The tests were accomplished at a crossheadspeed of 50 cm per minute, and each test was performed on five samples for each tray specimen. RESULTS
Fig. I. Tensile test specimen. a, Steel chain secured to :nstron machine; b, acrylic resin tray; c, impression mat ;e1ial on retentive brass plate. applied to the brass plates for additional retention of the impression material. The impression material was manipulated according to the manufacturer’s instructions. A thorough homogenous mix was made by using the double-spatula technique. The surface of the tray to be tested was carefully covered with the impression material, and the remainder of the material was applied to the brass plate sustaining the material during testing. The tray specimen and the brass plate were then securely joined in a fixed position and the entire assembly was placed in a humidor at 37O C and 100% humidity during the set of the impression material. Both the tensile and the shear stress testings were conducted 12 minutes after the start of mixing of the impression material. A sharp scalpel blade was used to remove the excessimpression material from the sides of the trays before they were mounted on the Instron Universal testing machine (Instron Corp., Canton, Mass.). The opposite ends of the tensile test specimens were attached to the grips of the Instron machine (Fig. 1). A special plastic platform was used to mount the shear test specimens and the compression load cell was lowered to the protruding brass plate
744
Fig. 3 demonstrates the tensile and shear stresses for President tray adhesive in relation to acrylic resin surface preparations. The surface roughened with 80-grit Sic paper created the best bond in both tensile and shear tests. The results among the three variables were statistically significant, and the lowest values were recorded for the resin blocks cured against aluminum foil, whereas the perforated tray specimens registered intermediate values. The pilot study on perforated tray specimens revealed that failure in tension occurred at 0.05 MPa when the impression material did not flow completely through the perforations. However, when the extruded material coalesced on the external surface of the tray, 0.16 MPa was recorded for failure in tension. The difference was statistically significant (p < 0.05). Fig. 4 lists the tensile and shear stresses for President tray adhesive on different types of trays. Acrylic resin roughened with go-grit Sic produced the highest bond strengths compared with chromium-plated brass and plastic trays. The differences between smooth and roughened chromium-plated brass tray specimens was not significant; both appeared to create better retention for the President tray adhesive compared with polystyrene. DISCUSSION The result of roughening the acrylic resin trays with Sic paper was a higher adhesive bond strength compared with specimens cured against aluminum foil. This finding was
DECEMBER
1991
VOLUME
66
NUMBER
6
PROPERTIES
OF TRAY
ADHESIVE
qit B-pmfaatul c--fofi
sic w
A - 80
-
0-1
0.16
0.02 tA
B
Fig. 3. Adhesive strength as function of surface preparation. the opposite of that reported on the joint strength of a polysulfide adhesive on acrylic resin abraded with Sic paper against that cured against tin foil.lO In this study, small patches of adhesive remained on the acrylic resin specimens cured against aluminum foil whereasthe greater bulk of the President tray adhesivebecamecompletely detached from the smooth surfaces of the tray specimens. Conversely, large patches were observed on the roughened specimens, which could be attributed to the rough surface texture of the resin blocks providing a greater surface area to retain the adhesive. The tensile tests did not exhibit rupture within the impression material, indicating that the bond strength of the adhesive was less than the rupture strength of the impression material. It is a common laboratory practice to complete the preparation of an acrylic resin tray with sandpaper becausethis procedure not only eliminates sharp edgesbut rounds and smoothes the borders of the tray. This study proved that the bond strength of President adhesive is substantially improved by roughening the resin tray with 80-grit Sic paper. The finishing of acrylic resin trays should also include the fitting surface. The use of aluminum foil reduces wax contamination of the acrylic resin tray and, combined with a final finishing with sandpaper, enhances the retentive properties of the tray adhesive. Acrylic resin trays roughened with Sic paper recorded a higher tensile bond strength for President adhesive com-
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
pared with trays prepared against aluminum foil. In addition, the shear bond strength was elevated by more than three times, which was important becausethe shear bond strengths of tray adhesiveswere consistently lower than the tensile bond strengths. Perforating the tray actually resulted in an appreciably lower bond strength despite roughening of the test surfaces with Sic paper before application of the tray adhesive.Two types of failures were observed: (1) rupture of the impression material occurred randomly at the ends within the perforations and (2) failure of the adhesive was confined to the surface of the resin blocks without perforations. Rupture of the impression material can be attributed to a rapid and high concentration of stresseswithin limited material at the perforation, which resulted in an elevation of stresses between the perforations. This caused the detachment of impression material from the trays. Attempts to secure the impression by perforating the tray were not as successful as roughening the tray. Despite the lower tensile and shear bond strengths of the perforated trays compared with the roughened acrylic resin trays, they were more retentive than trays cured against aluminum foil. If a perforated tray is used, the impression material should flow completely through the perforation& and the extruded material should coalesceon the outer surface of the tray. Failure to accomplish this results in diminished retention for the impression as was observed during the pi-
745
SULONG
AND
SETCHELL
0.26 0.24 0.22 0-M 0.1s ?IIIIILEL m
0.16
(ml
0.14 0.12
0.10 0.06 0.06 0.04 0.01
A
B
D
C
F ig. 4. Adhesive strength as function of tray material.
lot study. Adequate retention of a perforated tray is also determined by the diameter and the distance between perforations.’ “. *‘j The differences between acrylic resin, Cr-plated brass, and plastic tray specimens was attributed to the characteristics of the roughened surfaces. The homogenous Crplat.ed brass and plastic materials did not contribute to an appreciable increase in surface area after being roughened with 80 grit SiC paper. The acrylic resin was transformed into a heterogenous structure of resinous matix with varying sizes of filler part.icles. The variation in surface energies of materials could then have accounted for the variable adhesive bond strengths; thus it can be concluded that different tray materials possessdissimiliar bonding affinities to tray adhesives. Failure in adhesion and rupture of the impression material in this study following shear tests resulted from a unidirectional load. Forces inflicted on an impression tray during removal from the mouth originate from various angles becauseof the anatomic variations of teeth and jaws. The forces required to detach the impression material from the tray could easily exceed those in this study. Hence, in the normal clinical situation? the bond of tray adhesives to the impression tray is considered adequate when the tray is perforat,ed and the impression material extends to the outer surface of the tray.
Clinical
impEcations
Roughening the acrylic resin tray enhancesthe retentive properties of tray adhesive. This procedure is accomplished 746
with Sic paper or sandpaper during the finishing of a custom acrylic resin tray. If a perforated tray is selected, it is imperative that the impression material flows through the perforations to coalesce on the external surface of the tray. Although putty material was not included in this study, it is reasonable to postulate that the putty material should also flow through all the perforations for maximal retention. The use of metal trays is usually limited to specific applications such as an impression for transfer copings, and the trays should be roughened by sandblasting. Conversely, plastic trays should be used sparingly becausethe bonding to tray adhesives is weaker and inconsistent.
SUMMARY The properties of tensile and shear strength of President tray adhesive were determined for different methods of acrylic resin tray preparations by use of an Instron machine. A comparison was also drawn between three different types of tray materials. This study revealed that acrylic resin trays roughened with 80-grit Sic created the highest tensile and shear values while plastic tray specimens recorded the least favorable adhesion to the tray adhesive.
CONCLUSION 1. The greatest adhesive bond strength was recorded for acrylic resin roughened with 80-grit Sic paper. 2. Perforated acrylic resin trays were not as adhesive as those roughened with go-grit Sic paper. DECEMBER
1991
VOLUME
66
NUMBER
6
PROPERTIES
OF TRAY
ADHESIVE
3. Acrylic resin cured against aluminum foil resulted in the weakest bond for the President adhesive. 4. The adhesives did not adhere well to chromiumplated metal or plastic stock tray material. 5. The tensile bond strengths of tray adhesives consistently exceeded the shear bond strengths. REFERENCES 1. Yeh CL, PowersJM, Craig RG. Properties of addition-type silicone im-
pression materials. J A m Dent Asaoc 1980,101:482-4. 2. Willis PT, Jackson DG, Bergman W . An evaluation of the time-dependent dimensional stability of elevenelastomericimpression materials. J PROSTHET DENT 19&1;52:120-5. 3. Johnson GH, Craig GC. Accuracy of four types of rubber impression materials compared with time of pour and a repeat of models. J PROS-
DENT 19&1,52:514-7.
9. Mohd Zainal AMS. An investigation of the bonding properties of tray adhesivesfor addition polymerising vinyl polysilonanes.MSc Thesis, London: University of London, 1985. 10. Davis GB, Moser JB. Brinsden GI. The bonding properties of elastomer tray adhesives.J PR~~THET DENT 1976;36:278-85. 11. Samman JM, Fletcher AM. A study of impression tray adhesives. QuintessenceInt 1985;4:305-9. 12. FusayamaT, Nakazato M. The designsof stock trays and the retention of irreversible hydrocolloid impressions. J PROSTHET DENT 1969; 21:136-42. 13. Anderson JN. Applied dental materials. 5th ed. London: Blackwell Sci-
entific Publications, 1976223. Reprint requests to:
THET DENT 1985;53:484-90.
4. Clancy MS, Sandrett FR, Ettinger RL. Long-term dimensional stability of three current elastomers.J Oral Rehabil 1983;10:324-33. 5. Phillips RW. Scienceof dental materials. 8th ed. St Louis: CV Mosby Co, 1982:137-56. 6. Sandrik JL, Vacco JL. Tensile and bond strength of putty-wash elastomeric impression materials. J PROSTHET DENT 1983;50:358-61.
Marginal
7. Eames WB, SiewekeJC, Wallace SW. Elastomeric impression materials: effect of bulk on accuracy.J PROSTHET DENT 1979;41:304-7. 8. Valderhaug J, Floystrand F. Dimensional stability of elastomeric impression materials in custom-made and stock trays. J PROSTHET
adaptation
of castable
DR. M. Z. A. MOHD SULONG FACULTV OF DENTISTRY UNIVEFWT~ OF MALAYA 59100 KUALA LUMPLIR MALAYSIA
ceramic
crowns
James D. Weaver, DDS,a Glen H. Johnson, DDS, MS,b and David J. Bales, DDS, MS0 University of Washington, School of Dentistry, Seattle, Wash. Tooth preparations and seating techniques of castable ceramic crowns diier from metal ceramic crowns. This study evaluated the variable effects of cementation on the marginal adaptation of Dicer, Cerestore, and porcelain-fused-to-metal crowns. The shoulder preparation was maintained for ceramic crowns, and a cavosurface bevel was designed for metal ceramic crowns. Crowns were made with a replication size of 10, placed on master dies, and the marginal openings measured with a Nikon Measurescope 20 instrument. Thirty crowns were cemented with zinc phosphate cement and the recommended clinical force. Marginal adaptation was not improved with a gingival bevel preparation or an increased seating force. The best marginal adaptation was recorded for Cerestore crowns. (J PROSTHET DENT 1991;66:747-63.)
P
atients are demanding improved esthetics and desire natural-appearing teeth. All-ceramic crowns have increased in popularity and are widely used in dentistry. The absence of a metal collar or metal substrate makes ceramic crowns an esthetic alternative to porcelain-fused-tometal (PFM) crowns. One of the prerequisites for crowns is precise marginal adaptation. The differences in preparation design and
Presented at the American Association for Dental Research meeting, San Francisco, Calif.
aAssistant Professor, Department of Restorative Dentistry. bAssociate Professor, Department of Restorative Dentistry. cAssociate Professor and Chairman, Department of Restorative Dentistry. 10/l/31368 THE JOURNAL
OF PROSTHETIC
DENTISTRY
variation in seating forces must be considered in comparing the marginal fit of castable ceramic crowns and PFM crowns. A beveled shoulder, in which a go-degree shoulder with a 45degree bevel surrounds the entire tooth preparation, is the recommended tooth preparation for PFM crowns.l, 2 Rosner3 and Shillingburg et a1.4demonstrated that a suitably placed bevel on a tooth preparation reduces the marginal opening of a seated casting, but this concept has been disputed. Grajower and Lewinstein5 demonstrated mathematically a superior marginal fit without a beveled preparation and Belser et aL6 illustrated no difference in the marginal adaptation of seated castings before and after cementation with beveled or nonbeveled tooth preparations. The seating forces during cementation for PFM and 747
