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, Powers JM, 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 eleven elastomeric impression 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 adhesives for 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. Quintessence Int 1985;4:305-9. 12. Fusayama T, Nakazato M. The designs of 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. Science of 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, Sieweke JC, 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 a1.4 demonstrated 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

WEAVER,

JOHNSON,

AND

BALES

signs of tooth preparations and seating forces were representative of acceptable clinical procedures.

MATERIAL

AND METHODS

Thirty standardized central incisor preparations were used in this study (Fig. 1). These identical preparations were made on Ivorine teeth (Columbia Dentoform Corp., New York, N.Y.) and constructed by Dentsply International. The cavosurface margin was a go-degree shoulder for the castable ceramic crowns and a 0.5 mm, 45-degree bevel was prepared for the cavosurface of the PFM crowns. A lingual groove was placed in the cingulum of the preparations as an antirotational mechanism during cementation. Ten Dicer crowns, 10 Cerestore crowns, and 10 PFM crowns were constructed.

Dicor crowns

1. 1. Ivorine central incisor tooth preparation ree shoulder cavosurface margin.

with 90-

A light-bodied polyvinyl siloxane (Exallex, GC Dental Industrial Corp., Tokyo, Japan) impression of the prepared tooth was made and cast with type IV gypsum (Die Keen, Modern Materials Manufacturing Co., Division of Columbia Dental, Edison, N.J.). After the die was trimmed, two even layers of Dicer die spacer were applied to within 1 mm of the margin; the crown was waxed to appropriate contour and cast with the Dicer casting machine. The glass castings were divested, cleaned, and heat-treated (cerammed) to develop their physical properties and fit. The casting was then finished and cleaned, and the Dicer shading porcelain was fired on the surface for the selected shade.

Cerestore

F ig. 2. A, Completed Cerestore crown on master Ivorine B, Epoxy die for direct molding of ceramic core.

die.

castable ceramic crowns differ because PFM crowns are seated with a dynamic force.7 The patient is instructed to occlude on an orangewood stick that is rocked back and forth verticallv and horizontally, whereas manufacturers recommend that ceramic crowns should be seated with only static finger pressure. This study compared the marginal adaptation of two types of castable ceramic crowns and PFM crowns before and after cementation. The all-ceramic restorations were Cerestore (Johnson & Johnson, E. W indsor, N.J.) and Dicar (Dentsply International, York, Pa.). The Cerestore crowns are now marketed under the name of Alceram (Innotek Dental Corp., Lakewood, Colo.). The different de-

Y-48

crowns

A polyvinyl siloxane (Exaflex) impression of the preparation was made and an epoxy (Cerestore Epoxy, Johnson & Johnson, E. W indsor, N.J.) resin designed for the shrink-free ceramic system was poured into the impression for an epoxy die. The epoxy die was cured and a separating medium and die spacer placed. A coping was waxed to a uniform thickness of 0.5 mm and tapered to finish flush with the margins of the die. This coping was then invested with the epoxy die, and the wax was boiled out. The ceramic core was pressed directly over the epoxy die. The coping was then trimmed and cured for 9 hours in a Cerestore furnace to sinter the aluminum-oxide coping and develop the physical properties to complete the crown. A porcelain veneer was added to the coping for the proper contour and color (Fig. 2).

Porcelain-fused-to-metal

crowns

A polyvinyl siloxane (Exaflex) impression of the prepared tooth was made and a master die poured (Die Keen, Modern Materials). Two uniform layers of die spacer were applied to within 1 mm of the margin. Copings were then waxed with a uniform thickness of 0.5 mm, with a 1 mm waxed collar around the cavorsurface margin, and then the waxed copings were cast in gold (Olympia, J. F. Jelenko & Co., Armonk, N.Y.). The gold coping was shaped, sand-

DECEMBER

1991

VOLUME

66

NUMBER

6

blasted, and cleaned to receive the porcelain. Opaque, body, and enamel porcelains were then baked to the gold coping to the appropriate contour and color. The completed crowns were fit to the dies and Ivorine teeth, and each preparation was marked with a small dot on the buccal, lingual, mesial, and distal surfaces (Fig. 3). Precementation measurements of the marginal opening were recorded at these four reference points by using a measuring microscope (Nikon Measurescope 20, Nikon, Inc., Instrument Division, Garden City, N.Y.) with an accuracy of 0.001 mm and precision of measurement of 0.002 mm (Figs. 4 and 5). The recommended seating force for the castable ceramic Dicer and Cerestore crowns is not the same force used for PFM crowns. Castable ceramic crowns can only be seated with static force of finger pressure, and this recommendation was followed to prevent fracture during cementation. Eight dentists recorded their finger pressure while seating a crown on an Instron Universal testing machine (Instron Corporation, Canton, Mass.). The average force was 8 kg with a standard deviation of 1.3 kg. The metal ceramic crowns were seated with a dynamic seating force. All 30 crowns were seated with zinc phosphate cement (Flecks, Mizzy, Inc., Clifton Forge, Va.). The cement was mixed in stages for 2 minutes on a chilled glass slab according to the manufacturer’s instructions, and the crowns were seated on the Ivorine teeth with a calibrated seating instrument (Rimac Tools, Dumont, N.J.) (Fig. 6). The castable ceramic crowns were seated with 8 kg force and the PFM crowns were tapped in place with an orangewood stick and mallet, and then seated with a dynamic force and sustained at a static force of 25 kg simulating clinical conditions. After 10 minutes of bench set, the excess cement was removed, verified under magnification, and the crowns were remeasured at the same reference dots before cementation (Fig. 7). Data

Fig. 3. Dicer crown on master die with dots on mesial and labial surfaces for before-and-after cementation measurements.

analysis

The data were analyzed by three-factor analysis of variance (ANOVA) to identify significant changes in location of measurement, type of crown, and before-versus-after cementation values. Of these three factors, the location of measurement was not significant, and because the crossproduct interactions involving location were also not significant, the mean of the four was used as the dependent variable. A single-factor ANOVA was then computed in conjunction with the Newman-Keuls method for paired comparison of means to determine mean differences. The null hypothesis was that the mean marginal openings for the three different crowns did not differ, either before or after cementation. All hypothesis testing was conducted at the 95% level of confidence.

RESULTS A graphicplot depictingthe meanmarginal openingfor the three different types of estheticcrownsbefore and afTHE JOURNAL

OF PROSTHETIC

DENTISTRY

Fig. 4. Nikon Measurescope 20 instrument.

ter cementationis shown in Fig. 8, The meansandstandard deviationsare also presentedwith bars connectingmeans that do not differ at alpha 0.05. Before cementation,the marginal openings were the 749

Fig. 5. Cerestore crown under Xl00 magnification. (P) Porcelain and (T) tooth preparation noted with marginal opening.

Fig. 6. A, Rimac seating instrument seating Cerestore crown. B, Close-up of Cerestore crown cemented and maintained at 8 kg.

least for Cerestore and PFM crowns with means of 21.6 and 30.5 pm, respectively, compared with 44.4 pm for Dicer crowns. After cementation, the smallest mean openings were recorded for Cerestore crowns at 31.7 pm compared with Dicer at 57 and PFM crowns with 59 pm. The best marginal adaptation after cementation was observed with &restore C~OW~IS, Another resuh with clinical significance is the degree of variation of measurements for the 10 samples of each type of crown. The coefficient, of variation (or CV = 100 X standard deviation/mean) is an indication of the precision of

;

,Y,( I

)

measurement of marginal openings. This measurement was low for Cerestore crowns before and after cementation (1% and 2 % ), moderate for Dicer crowns (7% and 6%), and relatively large for PFM crowns (13% and 25% ). Because there was no difficulty in locating reference points for measurements of the samples, these results indicated that the injection molding process associated with Cerestore crowns provided the most precise, consistent method of making crowns, followed by the Dicer and PFM techniques. The contribution of the cementation process on marginal

DECEMBER1991VOLUME66 NUMBER6

CASTABLE

CERAMIC

CROWNS

Fig. 7. Dicer crown after cementation under Xl00 magnification. (0) Dicer and (T) tooth preparation with cement thickness.

Fig. 8. Graph demonstrates marginal discrepancy before and after cementation for Cerestore, PFM, and Dicer crowns.

opening was determined by calculating the differences between before-and-after cementation reference points on each sample. The smallest opening resulted after cementation of the ceramic crowns (10 to 13 pm), whereas the mean increase for PFM crowns was two to three times greater, at 28.3 wrn (Fig. 9). These differences could be explained by the amount of relief with each type of crown, so the crowns were imbedded in acrylic resin and sectioned buccolingually (Fig. 10) with an ultraflne diamond disk. The cement thickness measured at the midpoint of the buccal and incisal surfaces revealed that the mean cement thickness at the incisal surface exceeded 100 pm for all three crowns (Fig. 11). Although the Cerestore crowns demonstrated the greatest incisal thickness, adequate relief was provided so that

THE

JOURNAL

OF

PROSTHETIC

DENTISTRY

complete seating of the crowns was not hampered incisally. The mean thickness of the axial cement was 25 pm for PFM crowns, 37 pm for Cerestore crowns, and 61 Frn for Dicer crowns. DISCUSSION The marginal adaptation for all three crown systems was clinically acceptable, but an objective method to measure this adaptation clinically has not been established. Dentists have used 70 f 10 pm as a suitable marginal adaptation for complete veneer crowns. Kurosu and Ides reported marginal cement thicknesses between 30 and 40 pm on inlays but 90 km on crowns. The largest marginal openings after cementation recorded in this study were 59 pm with PFM crowns and 57 pm with Dicer crowns. Cerestore

751

WEAVER,

JOHNSON,

AND

BALES

Fig. 9. Graph illustrates increase in marginal opening after cementation.

Fig. 10. Midlabial to lingual section of PFM crown imbedded in acrylic.

crowns demonstrated the lowest marginal opening, 31.7 m. The best marginal adaptation in this study was observed with the Cerestore crowns. Sato et aLg also reported excellent fit of Cerestore crowns. The casting technique is completely different for this system, because the wax pattern is not removed from the epoxy die and the core is molded directly to the die. This study also demonstrated that improved marginal adaptation was not realized with a gingival bevel preparation and increased seating force. However, the amount of die relief appeared to be a significant factor. Eames et al.‘O and Fusayama et, al.ll confirmed that castings made from

752

relieved dies diminished the film thickness and improved the marginal adaptation. The maximal allowable film thickness for zinc phosphate cement has been established at 25 prn,12and all three types of crowns provided relief at or greater than the maximal film thickness. The relief associated with the PFM crowns was minimal in comparison to the maximal film thickness, and this was attributed to the increased seating pressure used with the metal ceramic crowns compared with the ceramic crowns. Jorgensen13 concluded that film thickness diminshed as seating pressure increased; therefore, two layers of die spacer have been advocated in making metal castings. However, this study indicated that additional die

DECEMBER

1991

VOLUME

96

NUMBER

6

CASTABLE

CERAMIC

CROWNS

Fig.

11. Graph demonstrates mean thickness of cement at in&al

spacer exceeding minimal cement film thickness would be advisable for a cement relief space. CONCLUSIONS 1. The marginal adaptation of all three esthetic crowns was clinically acceptable after cementation. 2. Cerestore crowns exhibited the best marginal adaptation after cementation. 3. Marginal adaptation was not improved with a gingival bevel preparation and an increased seating force. W e gratefully acknowledge the excellent technical assistance of Mr. Monte Toruville, CDT, for the P F M and Cerestore crowns and Mr. Rory McCallum for the Dicer crowns. REFERENCES 1. Shilliigburg HE Sr, Hobo S, Fisher DW. Preparation design and margin distortion in porcelain-fused-to-metal restorations. J PROSTHET DENT 19’73;29:276-84. 2. Faucher RR, Nicholls JI. Distortion related to margin design in porcelain-fused-to-metal restorations. J PROSTHETDENT 1980;43:149-55. 3. Rosner D. Function placement and reproduction of bevels for gold castings. J PROSTHETDENT 1963;13:1166-6.

THE

JOURNAL

OF

PROSTHETIC

DENTISTRY

and axial positions.

4. Shillingburg HT, Hobo S, Whitsett LW. Fundamentals of fixed prosthodontics. 2nd ed. Chicago: Quintessence, 1981:117-25. 5. Grajower R, Lewinstein IA. A mathematical treatise on the fit of crown castings. J PROSTHIZT DENT 1983;49:663-74. 6. Belser VC, MacEntee MI, Richter WA. Fit of three porcelain-fused-tometal marginal designs in viva: a scanning electron microscope study. J PROSTHETDENT 1985;53:24-9. 7. Rosenstill SF, Gegauff AG. Improving the cementation of complete cast crowns: a comparison of static and dynamic seating methods. J A m Dent Assoc 1988,117:845-8. 8. Kurosu A, Ide K. Cement thickness between cast restorations and preparation walls [Abstract]. Bull Tokyo Med Dent Univ 1961;8:337-8. 9. Sate T, Wohlwend A, Scharer P. Marginal fit in a “shrink-free” ceramic crown system. Int J Perio Rest Dent 1986,3:8-21. 10. Eames WB, O ’Neal SJ, Montiero J, Miller C, Roan JD, Cohen KS. Techniques to improve seating of castings. J A m Dent Assoc 197896: 432-7. 11. Fusayama T, Ide K, Hosada H. Relief of resistance of cement of full-cast crowns. J PROSTHETDENT 1964;14:95-106. 12. ANS/ADA Specification No. 8-1977. Chicago: American Dental Association, 1977. 13. Jorgensen KD. Factors affecting the film thickness of zinc phosphate cements. Acta Odontol Stand 1960,18:479-90. Reprint

requests to:

DR. JAMEYD. WEAVER SCHOLLO F DENTISTRY,SM-56 UNIVERSITYO F WASHINGTON SEATIZE,W A 98195

753