fidelity and microleakage wo techniques John A. Sorensen, DMD,a Judith M. Strutz, Sean P. Avera, BS,” and Daniel Materdominid University
of California,
School of Dentistry,
o
orcelain
v
DDS,b
Los Angeles, Calif.
This study evaluated the marginal fidelity and microleakage of porcelain veneers made with the platinum foil and refractory die techniques. Maxillary incisors, matched for size and amount of enamel, were prepared with 0.5 mm uniform intraenamel reduction. The indirectly made veneers were etched, treated with silane, and luted with a composite resin, and the margins were finished and polished. The restored teeth were stored in 37’ C water, thermocycled 1000 times, stained with silver nitrate, embedded, sectioned buccolingually and mesiodistally, and measured at X250 magnification. The platinum foil veneers had significantly better vertical marginal fidelity but significantly more overcontouring than had the refractory die veneers. Universal microleakage at the tooth-composite resin interface and negligible microleakage at the porcelain-composite resin interface were observed. No relationship was found between the amount of vertical marginal opening and the amount of microleakage. (J PROSTHET DENT 1992;67:16-22.)
entistry has experienced a dramatic increase in the use of porcelain veneers for esthetic restoration of anterior teeth. Porcelain veneers hold advantages over direct composite resin restorations in color stability, wear resistance, lifelike esthetics, and minimization of composite resin.l When indicated, porcelain veneers also have many advantages over ceramic crowns, such as minimal tooth reduction (0.5 mm), less involvement of surface area of the tooth, and better control of shade for superior esthetics. Like any laboratory-made restoration that is cemented with a margin near the gingivae, there is the potential for iatrogenic periodontal disease.2For any cemented restoration the weak link is at the restoration-cement-tooth interface. In porcelain veneers the composite resin luting agent is the weak link in the system. Although the use of etchants3s4 and silane coupling agents5 increases the bond strength of composite resin to etched porcelain, the marginal seal is established with composite resin. When used as a luting agent, the bulk of composite resin is greatly reduced. However, there is still polymerization volumetric shrinkage in the amount of 2.6% to 5.7% ,6 which may create a marginal opening or loss of the marginal seal. The polymerization shrinkage stresses create a com-
Supported in part by BRSG SO7 RR05304 awarded by the Biomedical Research Support Grant Program, Division of Research Resources, National Institutes of Health, Bethesda, Md. Presented at the Pacific Coast Society of Prosthodontists meeting, Napa, Calif. aAssistant Professor and Director, Graduate Prosthodontics. bStudent, Graduate Prosthodontics. CCertified Dental Technician and Research Assistant. dDa Vinci Dental Studio, Canoga Park, Calif. 10/l/27301
16
petition between the bond to tooth structure and the bond to porcelain,7 possibly resulting in a marginal gap.7-9Additionally, a number of studies have demonstrated a dissolution of the resin matrix of composite resin in oral fluids.1°-14 Therefore it is desirable to minimize the composite component and maximize the porcelain component by having as close an adaption of the porcelain veneer as possible. There are many ways to make porcelain veneers. The two most common methods are the platinum foil technique and the refractory die technique.15 This study compared the marginal fidelity of porcelain veneers made with the platinum foil and refractory die techniques. The relationship between marginal fidelity and microleakage of porcelain veneers placed entirely on enamel margins was also evaluated.
MATERIAL
AND METHODS
Freshly extracted maxillary incisors were collected, scaled, cleaned with pumice, and stored in physiologic saline solution. The teeth were examined for defects, cracks, or inadequate thickness of enamel. Twenty central incisors were paired for size and amount of enamel and assigned to two groups: group 1, platinum foil technique using 0.001inch thick platinum foil (Jelenko, Armonk, N.Y.) and Ceramco II porcelain (Johnson and Johnson, East Windsor, N.J.); group 2, refractory die technique using HiCeram refractory die material and Vitadur-N porcelain (Vita, Bad Sackingen, Germany). Veneer preparations were standardized to 0.5 mm uniform reduction with a self-limiting diamond bur (No. LVS-1, Brassler USA, Savannah, Ga.) and finished at the margins with a l2-fluted carbide finishing bur (No. RCBIIK-24, Brassler USA). Impressions were made with a vinyl polysiloxane (Reprosil, L.
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Fig. 1. Diagram of incisogingival and mesiodistal sectioning and measurement points.
D. Caulk Co., Division of Dentsply, Int., Inc., Milford, Del.) and the impressions were poured in a vacuum-mixed improved stone (Die Keen, Modern Materials, Inc., St. Louis, MO.) with standardized water/powder ratios and mixing techniques. Two dental technicians made 10 veneers each in the method of their expertise. The veneers were then tried on the teeth and adjusted with Fit Checker (G.C. International, Tokyo, Japan) tape. The glazed side of the veneer was covered with wax and etched for 4 minutes with 20% hydrofluoric acid (Stripit, National Keystone Products, Philadelphia, Pa.). The sticky wax was removed and veneers were steam cleaned. The etched porcelain surfaces were treated with silane (Luminbond, Vident, Baldwin Park, Calif.) for 20 seconds and allowed to dry, and a second layer was applied and dried. The teeth were cleaned with pumice for 10 seconds, rinsed for 10 seconds, etched with phosphoric acid for 60 seconds, rinsed for 40 seconds, and dried with air. Photopolymerized composite resin shade Al,BP (Luminbond, Vident) was applied, and the veneer was seated with gentle finger pressure. The excess resin was then removed with a brush dipped in unfilled resin to minimize postpolymerization composite resin finishing.16 The composite resin was cured with a composite curing light (Heliolux II, Vivadent, Tanawanda, N.Y.) for 60 seconds at each aspect of the veneer. The margins were finished with Esthetic Trimming carbide burs (Brassler USA) and an ETU 30 fluted finishing bur (Brassler USA), and polished with Sof-Lex disks (Nos. 1982M, 198233,and 1982SF, 3M Dental Products, St. Paul, Minn.), followed by diamond polishing paste (Luminbond, Vident) to finalize the finish. The teeth were stored in 37O C distilled water for 21 days and thermocycled between 5” and 50’ C for 1000 cycles with a 30-second dwell time and a 20-second traveling time. All tooth surfaces except for 1 mm around the veneer margin were coated with fingernail polish, and tooth preparations were placed in a 50 % solution of silver nitrate for 45 minutes, after a modified technique of Wu et a1.17,I8 The
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Fig. 2. Diagram of measuring technique for vertical marginal discrepancy.
specimens were then rinsed with distilled water, placed in a developer solution (T-MAX Developer, Kodak, Rochester, N.Y.), and exposed under a 150-W photo flood lamp for 6 hours, causing the areas of silver nitrate penetration to turn black. The specimens were imbedded in clear epoxy resin (Hastings Plastics Co., Santa Monica, Calif.) and allowed to cure for 24 hours. The teeth were sectioned in two cuts, incisogingivally in the center of the tooth and mesiodistally at the gingivoproximal line angle (Fig. l), with a low-speed diamond saw (Isomet, Buehler Ltd., Evanston, Ill.). The sectioned sides of the samples were again exposed under the 150-W flood lamp for 5 minutes to ensure complete silver oxidation. The sectioned teeth were analyzed at x250 magnification on a metallurgical microscope model BHMJ (Olympus Optical Co., Ltd., Tokyo, Japan). A Mitutoyo Digimatic Head 164 series (Mitutoyo Mfg. Co. Ltd., Tokyo, Japan) digital traveling micrometer with an accuracy of 23 pm was used to make measurements. The method described by Sorensenlg was used for measuring vertical and horizontal marginal discrepancies. When the samples were viewed in cross section, the vertical marginal discrepancy was determined by lining the vertical cross hair with the correct emergence profile (Fig. 2). The horizontal cross hair was then moved from the tooth preparation margin to the porcelain margin (Fig. 2). To measure the horizontal marginal discrepancy, the vertical cross hair was again lined up with the emergence profile and the horizontal cross hair was lined up with the tooth margin (Fig. 3). When the veneer was overcontoured, the vertical cross hair was moved to the edge of the porcelain and a number recorded with a plus sign (Fig. 3). When the veneer was undercontoured, the vertical cross hair was moved axially in relation to the porcelain and a number recorded with a minus sign. Microleakage was recorded in terms of distance of penetration of silver nitrate stain from the enamel finish line axially. The cross-sectional samples were examined for mi17
BQRENSEN
ET AL.
Fig. 3. Diagram of measuring technique for horizontal marginal discrepancy.
Fig. 4. Diagram of measuring technique for silver nitrate stain penetration.
croleakage at both the tooth-composite resin and composite resin-porcelain interfaces (Fig. 4). Fig. 5 shows an example of a typical margin with microleakage at the toothcomposite resin interface, and Fig. 6 demonstrates an overcontoured margin. Three investigators made three measurements at each point on every margin, yielding 180 measurements at each position, for a total of 520 measurements per veneer technique group for the vertical and horizontal marginal discrepancies and extent of microleakage. Descriptive statistics, such as the mean and standard deviation, were calculated. The differentials in marginal fidelity and microleakage at various points on the tooth were analyzed and compared. A Kruskal-Wallis analysis of variance (ANOVA) was used to evaluate for statistical differences between fabrication methods and different locations on the tooth.
ble II). Veneers made with the refractory die method were significantly less overcontoured overall than the platinum foil veneers (p < 0.0001) at the lingual 0, < 0.0007) and gingivoproximal (p < 0.0013) locations. For both techniques the facial margin had by far the greatest amount of overcontouring, whereas the lingual aspect bad the least. Table III shows microleakage values for the various interfaces of tooth, composite resin, and porcelain. A KruskalWallis ANOVA showed no significant difference in microleakage between fabrication techniques, but for toothcomposite resin microleakage there was a significant difference for the platinum foil and combined groups between facial and lingual aspects as well as lingual and proximal aspects (p < 0.0001). Microleakage at the toothcomposite resin interface was universal, whereas microleakage at the porcelain-composite resin interface was negligible, with a significant difference between the two (p < 0.001) (Table III). A regression analysis revealed no relationship between the amount of vertical marginal opening and the amount of internal microleakage. No correlation between vertical opening and microleakage was demonstrated (r = 0.036).
ESULTS The proportion of measurement variance resulting from interobserver variability in error-free scores was calculated to determine the accuracy of the measurements by the three observers. A low degree of interobserver variability was found, with an estimated reliability of 0.91; that is, 9 % of the overall variability was due to interobserver variance. Table I shows the mean vertical marginal discrepancies of porcelain veneers. The mean vertical marginal discrepancy for all positions combined for platinum foil veneers (187 pm) was significantly less than that for veneers made with the refractory die technique (242 wrn) (Kruskal-Wallis ANOVA, p < 0.0001). Similarly, for the three positions of facial (p < O.Ol), lingual (p < O.OOOl),and gingivoproximal Cp< O.OOOl),the platinum foil veneers had significantly smaller mean vertical marginal discrepancies than bad the refractory die fabricated veneers. For both techniques the gingivoproximal margin had two to four times the vertical marginal opening of the facial margin. Veneer margins in both groups were overcontoured (Ta18
BISCUSSIQN The platinum foil method was expected to have a greater marginal opening because of porcelain shrinkage toward the greatest bulk of porcelain in the center of the veneer, and also because of the inherent thickness of the platinum foil itself (about 25 wm). The porcelain should shrink toward the die material during firing with the refractory die method. Porcelain veneers made directly on a refractory die are less likely to warp and distort during the firing process, thus resulting in a better fit.15 Perhaps the gap discrepancy found with the refractory die technique is due to the aluminum oxide abrasive used to remove the refractory die material from the porcelain veneer. The thin and delicate porcelain veneer margins may be inadvertently damaged with aluminum oxide abrasion. Wohlwend et al.2Q
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Fig. 5. Example of veneer margin with microleakage at tooth-composite resin interface. (Original magnification X150.)
Fig.
6. Example of overcontoured veneer margin. (Original magnification x150.)
recommended a 0.1 mm layer of fine-grained quartzbonded investment material be used to cover the entire surface of the impression and the remainder of the impression be filled with refractory die material. This layering technique aids removal of the refractory die material from the porcelain veneer by abrasive spraying and therefore helps to protect the fragile porcelain margins.20 The 187 to 242 pm vertical marginal discrepancy of porcelain veneers in this study seemed quite large, especially in comparison to values cited in published studies on crown margin fidelity. Our previous work, using a similar methodology of indirect fabrication, cementation, embedding, sectioning, and viewing in cross section at high magnification, determined vertical marginal discrepancies of 36 pm
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for metal-ceramic margins21,22and ranging from 43 to 139 pm for all-ceramic crown systems.23-25Tay et al-r6 measured marginal discrepancies in the range of 400 Frn for porcelain veneers. Studies evaluating marginal fidelity of an overcontoured and apically overextended veneer margin by the directview technique would underreport the dimension of the marginal gaplg (Fig. 6). When the marginal gap of a curved surface is measured with the direct-view method, the surfaces that reflect light under the microscope are,used by the observer to determine the margins.lg Tooth-colored luting agents make accurate measurement of marginal fidelity infeasible unless cross-sectional inspection is used (Fig. 6). The measurement technique we introduced, in which mar19
SORENSEN
ET AL.
Fig. 7. Scanning electron micrograph of cross section of veneer at porcelain-composite resin-tooth interface. (Original magnification x260.) Table I. Mean vertical marginal discrepancy [in microns (SD)]
Method
Platinum foil Refractory die
Facial
108 (236) 114 (105)
Lingual
146 (64) 304 (181)
Gingivoproximal
248 (82) 374 (303)
Table II. Mean horizontal marginal discrepancy [in microns (SD)] GingivoOverall
187 (148) 292 (260)
ginal fidelity is divided into vertical and horizontal components, is ideal for the ceramic and composite margins observed in these veneer specimens.lg The vertical marginal openings at the gingivoproximal corners were two to four times larger than that at the facial position. The margins at the corners of the restorations may be more difficult to make. There was less horizontal overcontouring of three of four margin locations with the refractory die technique compared to the platinum foil technique. The presence of the platinum foil may obscure the emergence profile of the tooth. The margins of the veneers made by both methods were consistently overcontoured. There are several possible reasons for this finding. Because of the frail nature of the porcelain veneers, technicians and dentists may be overly cautious in finishing the margins to the proper contours, in fear of fracturing the veneer. When the margins are finished, it is difficult to differentiate between the color of the composite resin and tooth structure. Additionally, during the finishing procedure, selective removal of excess porcelain was technically challenging. This study observed that the veneers were overcontoured by 62 to 71 pm. Again, comparing these findings to our previous work on crown margins,22~25the veneers generally had a greater amount of overcontouring than was measured with the crowns.
20
Method
Platinum foil Refractory die
Facial
i-132 (61) +112 (73)
Lingual
proximal
Overall
+31 (41) +25 (62)
+61 (106) +37 (68)
+71 (91) +53 (76)
The conditions of the in vitro study were ideal because the extracted teeth were held in the hand, which provided excellent accessfor finishing from all aspects of the tooth. In the clinical situation the gingivae and adjacent teeth greatly hinder access, causing greater overcontouring and a poorer surface finish at the marginal area. Universal microleakage occurring at the tooth-composite resin interface and negligible microleakage at the porcelain-composite resin interface were remarkable observations. Tjan et a1.26found marginal leakage ranging from 200 to 1200 pm in 40 % of the tooth-composite resin interfaces and 200 to 1100 pm in 20% of the porcelain-composite resin interfaces in preparations with all-enamel margins. In this study the greatest microleakage was found at the cervical enamel margin. The microleakage was even more marked in margins prepared in dentin. Tooth preparations were ground flat on the enamel surface and tooth preparation was minimal.26 Tjan et a1.26suggested that the deviation in enamel rod orientation, especially at the cervical area, may have contributed to the bond strengtb of the acid-etched enamel. In our study, veneer preparations were made by incorporation of this change in enamel rod orientation, just as in the clinical situation. This may account for the improved microleakage results of the present study. In vitro bond strengths of composite resin to etched enamel are quite high, in excess of 20 MPa.27,28Porcelaincomposite resin bond strengths were so high that when
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Mean interfacial silver nitrate stain penetration [in microns (SD)]
III.
Facial
Interface
Tooth-composite resin Platinum foil Refractory die Combined Porcelain-composite resin Platinum foil Refractory die Combined
Lingual
683 (1602) 334 (376) 500 (1147)
232 (445) 256 (398) 245 (420)
7 (34) 22 (124) 15 (93)
0 0 0
tested for shear stress, cohesive fractures of the porcelain occurred.* Yet when restricted, the polymerizing composite resin set up tremendous contraction stresses of 20 MPa.7y2g In fact, these contraction stresses are strong enough to cause cuspal flexure during polymerization of
class II composites resins. 3oA cross-sectional view of the porcelain-composite resin-tooth interface (Fig. 7) reveals why microleakage
occurred predominately
at the tooth-
composite resin interface. The irregularities created in the microtopography
of etched porcelain
Gingivoproximal
are rougher by many
orders of magnitude compared to the etched enamel. Therefore the micromechanical interlocking of the porcelain won in the competition for bond strength during composite resin polymerization shrinkage over the bond to enamel.7 Feilzer et a1.31demonstrated that when the thickness of composite resin is thinned down, as when used as a luting agent, the wall-to-wall polymerization shrinkage may be three times the normal linear contraction of composite resin. The polymerization contraction stresses could then be expected to be even greater than those reported for composite resin restorations7, 2gA regression analysis revealed no relationship between amount of vertical marginal opening and the amount of microleakage in the present study. The elimination of polymerization shrinkage of composite resins presents a great challenge to the dental profession. Additional problems with these restorations are presented by the difference in coefficients of thermal expansion between tooth structure and dental materials and by microleakage occurring because of thermocycling. Previous studies of porcelain margins have often failed to include thermocycling in their experimental design, or when included the thermocycling regimen was significantly less rigorous than that in the present study. The thermocycling regimen in this study may have been too rigorous since the clinical bond of composite resin to etched enamel is reliable.32y33 For both fabrication techniques, a significant difference was found in the vertical marginal openings at the gingivo-
Overall
566 (708) 452 (442) 506 (586)
511 (980) 373 (423) 439 (745)
9 (48) 40 (141) 25 (109)
7 (38) 25 (118) 16 (90)
fidelity as a result of the shrinkage of the porcelain toward the region of greatest bulk (the center) and the geometry of the margins. Clinically this poorer fit at the gingivoproximal corner of the veneers would be further compounded by the difficulty in accessfor finishing of the luted veneers in these regions. These large vertical gingivoproximal marginal openings underline the importance of supragingival, hygienically accessible veneer margins. The increased marginal openings seen at the toothveneer interface may facilitate a washout of the composite resin luting agent. Clinical research is needed for comparison of the dissolution rate of conventional crown luting agents and composite resin luting agents. CONCLUSIONS 1. The vertical marginal discrepancy of porcelain veneers made with the platinum foil technique (187 km) was significantly better than that of the refractory die method (292 pm). 2. For both methods the vertical discrepancy was significantly
greater at the gingivoproximal
position.
3. Both fabrication techniques resulted in veneers that were overcontoured, with refractory die veneers being significantly less overcontoured. 4. The facial margins produced by both methods had significantly greater overcontouring than other aspects. 5. Microleakage at the tooth-composite resin interface was universal.
6. Microleakage at the porcelain-composite resin interface was negligible. 7. There was no relationship between the amount of vertical marginal discrepancy and the amount of microleakage. We thank Sue Ingles, MS, for the statistical analysis.
REFERENCES
gual marginal openings. It was expected that the mesial and
1. Calamia JR. Etched porcelain veneers: the current state of the art. Quintessence Int 1985;16:5-12. 2. Sorensen JA. A rationale for comparisonof plaque retaining properties
distal gingivoproximal aspects of the veneers made with either technique would have a much poorer marginal
3. Simonsen RJ, Calamia JR. Tensile bond strength [Abstract]. J Dent Res 1983;62:297.
proximal
THE
aspects of the veneers compared to facial and lin-
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of crown systems.J PROSTHETDENT 1989;62:264-9. of etched porcelain
21
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4. Stangel I, Nathanson D, Hsu CS. Shear strength of the composite bond to etched porcelain. J Dent Res 1987;66:1460-5. 5. Calamia JR, Simonsen RJ. Effect of coupling agents on bond strength of etched porcelain [Abstract]. J Dent Res 1984;63:179. 6. Bausch JR, De Lange K, Davidson CL, Peters A, De Gee AJ. Clinical significance of polymerization shrinkage of composite resins. J PROSTHET DENT 1982;48:59-67. 7. Davidson CL, De Gee AJ, Feilzer AJ. The competition between the composite-dentin bond strength and the polymerization contraction stress. J Dent Res 1984;63:1396-9. 8. Asmussen E, Jorgensen KD. A microscopic investigation of the adaptation of some plastic filling materials to dental cavity walls. Acta Odonto1 Stand 1972;30:3-21. 9. Kidd EAM. Polymerization shrinkage and microleakage. In: Vanherle G, Smith DC, eds. International symposium on posterior composite resin dental restorative materials. Netherlands: Peter Szulz Publishing Co, ‘1985:263-8. 10. Leinfelder KF, Sluder TB, Sockwell WD, Strickland WD, Wall JT. Clinical evaluation of composite resins as anterior and posterior materials. J PROSTHETDENT 1975;33:407-16. 11. Osborne JW, Gale EN. A three-year clinical assessment of a composite resin and its radiopaque counterpart. J PROSTHETDENT 1980;44:164-6. 12. Roulet JF, Walti C. Influence of oral fluids on composite resin and glass-ionomer cement. J PROSTHETDENT 1984;52:182-9. 13. Vrijhoef MMA, Hendriks FHJ, Letzel H. Loss of substance of dental composite restorations. Dent Mater 1985;1:101-5. 14. McKinney JE, Wu W. Chemical softening and wear of dental composites. J Dent Res 1985;64:1326-31. 15. Hunt PR. Porcelain laminate systems. In: Tay WM, ed. General dental treatment. Brentford, England: Kluwer Pub1 LTD, 1986:1-14. 16. Tay WM, Lynch E, Auger D. Effects of some finishing techniques on cervical margins of porcelain laminates. Quintessence Int 1987;18:599609. 17. Wu W, Cobb E. A silver staining technique for investigating wear of restorative dental composites. J Biomed Mater Res 1981;15:343-8. 18. Wu W, Cobb E, Dermann K, Rupp NW. Detecting margin leakage of dental composite restorations. J Biomed Mater Res 1983;17:37-43. 19. Sorensen JA. A standardized method for determination of crown margin fidelity. J PROSTHETDENT 1990;64:18-24. 20. Wohlwend A, Strub JR, Scharer P. Metal ceramic and all-porcelain restorations: current considerations. Int J Prosthodont 1989;2:13-26.
Availability
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ET AL.
21. Sorensen JA, Okamoto SK, Miller R, Yarovesky U. Marginal fidelity of four methods of Renaissance crown fabrication [Abstract]. J Dent Res 1987;66:283. 22. Sorensen JA, Okamoto SK, Seghi R, Yarovesky U, Miller R. Marginal fidelity of four methods of swaged metal matrix crown fabrication, J PROSTHETDENT (IN PRESS). 23. Sorensen JA, Okamoto SK. Comparison of marginal fit of all-ceramic m-own systems [Abstract]. J Dent Res 1987;66:283. 24. Sorensen JA, Okamoto SK. Marginal fidelity of Optec crown system [Abstract]. J Dent Res 1989;68:200. 25. Sorensen JA, Okamoto SK, Seghi R. Marginal fidelity of all-ceramic crown systems. (Submitted for publication.) 26. Tjan AHL, Dunn JR, Sanderson IR. Microleakage patterns of porcelain castable ceramic laminate veneers. J PROSTHETDENT 1989;61:276-82. 27. Retief DH, Gross JD, Bradley EL, Denys FR. Tensile bond strengths of dentin bonding agents to dentin. Dent Mater 1986;2:72-7. 28. Tao L, Pashley DH, Boyd L. Effect of different types of smear layers on dentin and enamel shear bond strengths. Dent Mater 1988;4:208-16. 29. Davidson CL, De Gee AJ. Relaxation of polymerization contraction stresses by flow in dental composites. J Dent Res 1984;63:146-8. 30. Pearson GJ, Hegarty SM. Cusp movement in molar teeth using dentine adhesives and composite filling materials. Biomaterials 1987;8:473-6. 31. Feilzer AJ, De Gee AJ, Davidson CL. Increased wall-to-wall curing contraction in thin bonded resin layers. J Dent Res 1989;68:48-50. 32. Jordan RE, Suzuki M, Gwinnet AJ, Hunter JK. Restoration of fractured and hypoplastic incisors by the acid etch technique. J Am Dent Assoc 1977;95:795-803. 33. Jordan RE, Suzuki M, Gwinnet AJ. Conservative applications of acid etch resin techniques. Dent Clin North Am 1981;25:307-24. Reprint requests to: DR. JOHN A. SORENSEN CHS 33-041 SCHOOLOF DENTISTRY UNIVERSITYOF CALIFORNIA Los ANGELES,CA 90024
Contributing Tony
back
J. Torres,
issues,
author CDT,
Research Assistant
19851990
Back issues of THE JOURNAL OF PROSTHETIC DENTISTRY are available for purchase from the publisher, Mosby-Year Book, Inc., at a cost of $6.50 per issue. (Foreign postage is not included.) The following quantity discounts are available: 25% off on quantities of 12 to 23, and one third off on quantities of 24 or more. Please write to Mosby-Year Book, Inc.: Subscription Services, 11830 Westline Industrial Drive, St. Louis, MO 63146-3318, or call (314)453-4351 for information on availability of particular issues for that period from 1979 to 1990. If unavailable from the publisher, photocopies of complete issues are available from University Microforms International, 300 N. Zeeb Rd., Ann Arbor, MI 48106, (313)7614700.
22
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1
of California,
School of Dentistry,
o
orcelain
v
DDS,b
Los Angeles, Calif.
This study evaluated the marginal fidelity and microleakage of porcelain veneers made with the platinum foil and refractory die techniques. Maxillary incisors, matched for size and amount of enamel, were prepared with 0.5 mm uniform intraenamel reduction. The indirectly made veneers were etched, treated with silane, and luted with a composite resin, and the margins were finished and polished. The restored teeth were stored in 37’ C water, thermocycled 1000 times, stained with silver nitrate, embedded, sectioned buccolingually and mesiodistally, and measured at X250 magnification. The platinum foil veneers had significantly better vertical marginal fidelity but significantly more overcontouring than had the refractory die veneers. Universal microleakage at the tooth-composite resin interface and negligible microleakage at the porcelain-composite resin interface were observed. No relationship was found between the amount of vertical marginal opening and the amount of microleakage. (J PROSTHET DENT 1992;67:16-22.)
entistry has experienced a dramatic increase in the use of porcelain veneers for esthetic restoration of anterior teeth. Porcelain veneers hold advantages over direct composite resin restorations in color stability, wear resistance, lifelike esthetics, and minimization of composite resin.l When indicated, porcelain veneers also have many advantages over ceramic crowns, such as minimal tooth reduction (0.5 mm), less involvement of surface area of the tooth, and better control of shade for superior esthetics. Like any laboratory-made restoration that is cemented with a margin near the gingivae, there is the potential for iatrogenic periodontal disease.2For any cemented restoration the weak link is at the restoration-cement-tooth interface. In porcelain veneers the composite resin luting agent is the weak link in the system. Although the use of etchants3s4 and silane coupling agents5 increases the bond strength of composite resin to etched porcelain, the marginal seal is established with composite resin. When used as a luting agent, the bulk of composite resin is greatly reduced. However, there is still polymerization volumetric shrinkage in the amount of 2.6% to 5.7% ,6 which may create a marginal opening or loss of the marginal seal. The polymerization shrinkage stresses create a com-
Supported in part by BRSG SO7 RR05304 awarded by the Biomedical Research Support Grant Program, Division of Research Resources, National Institutes of Health, Bethesda, Md. Presented at the Pacific Coast Society of Prosthodontists meeting, Napa, Calif. aAssistant Professor and Director, Graduate Prosthodontics. bStudent, Graduate Prosthodontics. CCertified Dental Technician and Research Assistant. dDa Vinci Dental Studio, Canoga Park, Calif. 10/l/27301
16
petition between the bond to tooth structure and the bond to porcelain,7 possibly resulting in a marginal gap.7-9Additionally, a number of studies have demonstrated a dissolution of the resin matrix of composite resin in oral fluids.1°-14 Therefore it is desirable to minimize the composite component and maximize the porcelain component by having as close an adaption of the porcelain veneer as possible. There are many ways to make porcelain veneers. The two most common methods are the platinum foil technique and the refractory die technique.15 This study compared the marginal fidelity of porcelain veneers made with the platinum foil and refractory die techniques. The relationship between marginal fidelity and microleakage of porcelain veneers placed entirely on enamel margins was also evaluated.
MATERIAL
AND METHODS
Freshly extracted maxillary incisors were collected, scaled, cleaned with pumice, and stored in physiologic saline solution. The teeth were examined for defects, cracks, or inadequate thickness of enamel. Twenty central incisors were paired for size and amount of enamel and assigned to two groups: group 1, platinum foil technique using 0.001inch thick platinum foil (Jelenko, Armonk, N.Y.) and Ceramco II porcelain (Johnson and Johnson, East Windsor, N.J.); group 2, refractory die technique using HiCeram refractory die material and Vitadur-N porcelain (Vita, Bad Sackingen, Germany). Veneer preparations were standardized to 0.5 mm uniform reduction with a self-limiting diamond bur (No. LVS-1, Brassler USA, Savannah, Ga.) and finished at the margins with a l2-fluted carbide finishing bur (No. RCBIIK-24, Brassler USA). Impressions were made with a vinyl polysiloxane (Reprosil, L.
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Fig. 1. Diagram of incisogingival and mesiodistal sectioning and measurement points.
D. Caulk Co., Division of Dentsply, Int., Inc., Milford, Del.) and the impressions were poured in a vacuum-mixed improved stone (Die Keen, Modern Materials, Inc., St. Louis, MO.) with standardized water/powder ratios and mixing techniques. Two dental technicians made 10 veneers each in the method of their expertise. The veneers were then tried on the teeth and adjusted with Fit Checker (G.C. International, Tokyo, Japan) tape. The glazed side of the veneer was covered with wax and etched for 4 minutes with 20% hydrofluoric acid (Stripit, National Keystone Products, Philadelphia, Pa.). The sticky wax was removed and veneers were steam cleaned. The etched porcelain surfaces were treated with silane (Luminbond, Vident, Baldwin Park, Calif.) for 20 seconds and allowed to dry, and a second layer was applied and dried. The teeth were cleaned with pumice for 10 seconds, rinsed for 10 seconds, etched with phosphoric acid for 60 seconds, rinsed for 40 seconds, and dried with air. Photopolymerized composite resin shade Al,BP (Luminbond, Vident) was applied, and the veneer was seated with gentle finger pressure. The excess resin was then removed with a brush dipped in unfilled resin to minimize postpolymerization composite resin finishing.16 The composite resin was cured with a composite curing light (Heliolux II, Vivadent, Tanawanda, N.Y.) for 60 seconds at each aspect of the veneer. The margins were finished with Esthetic Trimming carbide burs (Brassler USA) and an ETU 30 fluted finishing bur (Brassler USA), and polished with Sof-Lex disks (Nos. 1982M, 198233,and 1982SF, 3M Dental Products, St. Paul, Minn.), followed by diamond polishing paste (Luminbond, Vident) to finalize the finish. The teeth were stored in 37O C distilled water for 21 days and thermocycled between 5” and 50’ C for 1000 cycles with a 30-second dwell time and a 20-second traveling time. All tooth surfaces except for 1 mm around the veneer margin were coated with fingernail polish, and tooth preparations were placed in a 50 % solution of silver nitrate for 45 minutes, after a modified technique of Wu et a1.17,I8 The
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Fig. 2. Diagram of measuring technique for vertical marginal discrepancy.
specimens were then rinsed with distilled water, placed in a developer solution (T-MAX Developer, Kodak, Rochester, N.Y.), and exposed under a 150-W photo flood lamp for 6 hours, causing the areas of silver nitrate penetration to turn black. The specimens were imbedded in clear epoxy resin (Hastings Plastics Co., Santa Monica, Calif.) and allowed to cure for 24 hours. The teeth were sectioned in two cuts, incisogingivally in the center of the tooth and mesiodistally at the gingivoproximal line angle (Fig. l), with a low-speed diamond saw (Isomet, Buehler Ltd., Evanston, Ill.). The sectioned sides of the samples were again exposed under the 150-W flood lamp for 5 minutes to ensure complete silver oxidation. The sectioned teeth were analyzed at x250 magnification on a metallurgical microscope model BHMJ (Olympus Optical Co., Ltd., Tokyo, Japan). A Mitutoyo Digimatic Head 164 series (Mitutoyo Mfg. Co. Ltd., Tokyo, Japan) digital traveling micrometer with an accuracy of 23 pm was used to make measurements. The method described by Sorensenlg was used for measuring vertical and horizontal marginal discrepancies. When the samples were viewed in cross section, the vertical marginal discrepancy was determined by lining the vertical cross hair with the correct emergence profile (Fig. 2). The horizontal cross hair was then moved from the tooth preparation margin to the porcelain margin (Fig. 2). To measure the horizontal marginal discrepancy, the vertical cross hair was again lined up with the emergence profile and the horizontal cross hair was lined up with the tooth margin (Fig. 3). When the veneer was overcontoured, the vertical cross hair was moved to the edge of the porcelain and a number recorded with a plus sign (Fig. 3). When the veneer was undercontoured, the vertical cross hair was moved axially in relation to the porcelain and a number recorded with a minus sign. Microleakage was recorded in terms of distance of penetration of silver nitrate stain from the enamel finish line axially. The cross-sectional samples were examined for mi17
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ET AL.
Fig. 3. Diagram of measuring technique for horizontal marginal discrepancy.
Fig. 4. Diagram of measuring technique for silver nitrate stain penetration.
croleakage at both the tooth-composite resin and composite resin-porcelain interfaces (Fig. 4). Fig. 5 shows an example of a typical margin with microleakage at the toothcomposite resin interface, and Fig. 6 demonstrates an overcontoured margin. Three investigators made three measurements at each point on every margin, yielding 180 measurements at each position, for a total of 520 measurements per veneer technique group for the vertical and horizontal marginal discrepancies and extent of microleakage. Descriptive statistics, such as the mean and standard deviation, were calculated. The differentials in marginal fidelity and microleakage at various points on the tooth were analyzed and compared. A Kruskal-Wallis analysis of variance (ANOVA) was used to evaluate for statistical differences between fabrication methods and different locations on the tooth.
ble II). Veneers made with the refractory die method were significantly less overcontoured overall than the platinum foil veneers (p < 0.0001) at the lingual 0, < 0.0007) and gingivoproximal (p < 0.0013) locations. For both techniques the facial margin had by far the greatest amount of overcontouring, whereas the lingual aspect bad the least. Table III shows microleakage values for the various interfaces of tooth, composite resin, and porcelain. A KruskalWallis ANOVA showed no significant difference in microleakage between fabrication techniques, but for toothcomposite resin microleakage there was a significant difference for the platinum foil and combined groups between facial and lingual aspects as well as lingual and proximal aspects (p < 0.0001). Microleakage at the toothcomposite resin interface was universal, whereas microleakage at the porcelain-composite resin interface was negligible, with a significant difference between the two (p < 0.001) (Table III). A regression analysis revealed no relationship between the amount of vertical marginal opening and the amount of internal microleakage. No correlation between vertical opening and microleakage was demonstrated (r = 0.036).
ESULTS The proportion of measurement variance resulting from interobserver variability in error-free scores was calculated to determine the accuracy of the measurements by the three observers. A low degree of interobserver variability was found, with an estimated reliability of 0.91; that is, 9 % of the overall variability was due to interobserver variance. Table I shows the mean vertical marginal discrepancies of porcelain veneers. The mean vertical marginal discrepancy for all positions combined for platinum foil veneers (187 pm) was significantly less than that for veneers made with the refractory die technique (242 wrn) (Kruskal-Wallis ANOVA, p < 0.0001). Similarly, for the three positions of facial (p < O.Ol), lingual (p < O.OOOl),and gingivoproximal Cp< O.OOOl),the platinum foil veneers had significantly smaller mean vertical marginal discrepancies than bad the refractory die fabricated veneers. For both techniques the gingivoproximal margin had two to four times the vertical marginal opening of the facial margin. Veneer margins in both groups were overcontoured (Ta18
BISCUSSIQN The platinum foil method was expected to have a greater marginal opening because of porcelain shrinkage toward the greatest bulk of porcelain in the center of the veneer, and also because of the inherent thickness of the platinum foil itself (about 25 wm). The porcelain should shrink toward the die material during firing with the refractory die method. Porcelain veneers made directly on a refractory die are less likely to warp and distort during the firing process, thus resulting in a better fit.15 Perhaps the gap discrepancy found with the refractory die technique is due to the aluminum oxide abrasive used to remove the refractory die material from the porcelain veneer. The thin and delicate porcelain veneer margins may be inadvertently damaged with aluminum oxide abrasion. Wohlwend et al.2Q
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Fig. 5. Example of veneer margin with microleakage at tooth-composite resin interface. (Original magnification X150.)
Fig.
6. Example of overcontoured veneer margin. (Original magnification x150.)
recommended a 0.1 mm layer of fine-grained quartzbonded investment material be used to cover the entire surface of the impression and the remainder of the impression be filled with refractory die material. This layering technique aids removal of the refractory die material from the porcelain veneer by abrasive spraying and therefore helps to protect the fragile porcelain margins.20 The 187 to 242 pm vertical marginal discrepancy of porcelain veneers in this study seemed quite large, especially in comparison to values cited in published studies on crown margin fidelity. Our previous work, using a similar methodology of indirect fabrication, cementation, embedding, sectioning, and viewing in cross section at high magnification, determined vertical marginal discrepancies of 36 pm
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for metal-ceramic margins21,22and ranging from 43 to 139 pm for all-ceramic crown systems.23-25Tay et al-r6 measured marginal discrepancies in the range of 400 Frn for porcelain veneers. Studies evaluating marginal fidelity of an overcontoured and apically overextended veneer margin by the directview technique would underreport the dimension of the marginal gaplg (Fig. 6). When the marginal gap of a curved surface is measured with the direct-view method, the surfaces that reflect light under the microscope are,used by the observer to determine the margins.lg Tooth-colored luting agents make accurate measurement of marginal fidelity infeasible unless cross-sectional inspection is used (Fig. 6). The measurement technique we introduced, in which mar19
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Fig. 7. Scanning electron micrograph of cross section of veneer at porcelain-composite resin-tooth interface. (Original magnification x260.) Table I. Mean vertical marginal discrepancy [in microns (SD)]
Method
Platinum foil Refractory die
Facial
108 (236) 114 (105)
Lingual
146 (64) 304 (181)
Gingivoproximal
248 (82) 374 (303)
Table II. Mean horizontal marginal discrepancy [in microns (SD)] GingivoOverall
187 (148) 292 (260)
ginal fidelity is divided into vertical and horizontal components, is ideal for the ceramic and composite margins observed in these veneer specimens.lg The vertical marginal openings at the gingivoproximal corners were two to four times larger than that at the facial position. The margins at the corners of the restorations may be more difficult to make. There was less horizontal overcontouring of three of four margin locations with the refractory die technique compared to the platinum foil technique. The presence of the platinum foil may obscure the emergence profile of the tooth. The margins of the veneers made by both methods were consistently overcontoured. There are several possible reasons for this finding. Because of the frail nature of the porcelain veneers, technicians and dentists may be overly cautious in finishing the margins to the proper contours, in fear of fracturing the veneer. When the margins are finished, it is difficult to differentiate between the color of the composite resin and tooth structure. Additionally, during the finishing procedure, selective removal of excess porcelain was technically challenging. This study observed that the veneers were overcontoured by 62 to 71 pm. Again, comparing these findings to our previous work on crown margins,22~25the veneers generally had a greater amount of overcontouring than was measured with the crowns.
20
Method
Platinum foil Refractory die
Facial
i-132 (61) +112 (73)
Lingual
proximal
Overall
+31 (41) +25 (62)
+61 (106) +37 (68)
+71 (91) +53 (76)
The conditions of the in vitro study were ideal because the extracted teeth were held in the hand, which provided excellent accessfor finishing from all aspects of the tooth. In the clinical situation the gingivae and adjacent teeth greatly hinder access, causing greater overcontouring and a poorer surface finish at the marginal area. Universal microleakage occurring at the tooth-composite resin interface and negligible microleakage at the porcelain-composite resin interface were remarkable observations. Tjan et a1.26found marginal leakage ranging from 200 to 1200 pm in 40 % of the tooth-composite resin interfaces and 200 to 1100 pm in 20% of the porcelain-composite resin interfaces in preparations with all-enamel margins. In this study the greatest microleakage was found at the cervical enamel margin. The microleakage was even more marked in margins prepared in dentin. Tooth preparations were ground flat on the enamel surface and tooth preparation was minimal.26 Tjan et a1.26suggested that the deviation in enamel rod orientation, especially at the cervical area, may have contributed to the bond strengtb of the acid-etched enamel. In our study, veneer preparations were made by incorporation of this change in enamel rod orientation, just as in the clinical situation. This may account for the improved microleakage results of the present study. In vitro bond strengths of composite resin to etched enamel are quite high, in excess of 20 MPa.27,28Porcelaincomposite resin bond strengths were so high that when
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Mean interfacial silver nitrate stain penetration [in microns (SD)]
III.
Facial
Interface
Tooth-composite resin Platinum foil Refractory die Combined Porcelain-composite resin Platinum foil Refractory die Combined
Lingual
683 (1602) 334 (376) 500 (1147)
232 (445) 256 (398) 245 (420)
7 (34) 22 (124) 15 (93)
0 0 0
tested for shear stress, cohesive fractures of the porcelain occurred.* Yet when restricted, the polymerizing composite resin set up tremendous contraction stresses of 20 MPa.7y2g In fact, these contraction stresses are strong enough to cause cuspal flexure during polymerization of
class II composites resins. 3oA cross-sectional view of the porcelain-composite resin-tooth interface (Fig. 7) reveals why microleakage
occurred predominately
at the tooth-
composite resin interface. The irregularities created in the microtopography
of etched porcelain
Gingivoproximal
are rougher by many
orders of magnitude compared to the etched enamel. Therefore the micromechanical interlocking of the porcelain won in the competition for bond strength during composite resin polymerization shrinkage over the bond to enamel.7 Feilzer et a1.31demonstrated that when the thickness of composite resin is thinned down, as when used as a luting agent, the wall-to-wall polymerization shrinkage may be three times the normal linear contraction of composite resin. The polymerization contraction stresses could then be expected to be even greater than those reported for composite resin restorations7, 2gA regression analysis revealed no relationship between amount of vertical marginal opening and the amount of microleakage in the present study. The elimination of polymerization shrinkage of composite resins presents a great challenge to the dental profession. Additional problems with these restorations are presented by the difference in coefficients of thermal expansion between tooth structure and dental materials and by microleakage occurring because of thermocycling. Previous studies of porcelain margins have often failed to include thermocycling in their experimental design, or when included the thermocycling regimen was significantly less rigorous than that in the present study. The thermocycling regimen in this study may have been too rigorous since the clinical bond of composite resin to etched enamel is reliable.32y33 For both fabrication techniques, a significant difference was found in the vertical marginal openings at the gingivo-
Overall
566 (708) 452 (442) 506 (586)
511 (980) 373 (423) 439 (745)
9 (48) 40 (141) 25 (109)
7 (38) 25 (118) 16 (90)
fidelity as a result of the shrinkage of the porcelain toward the region of greatest bulk (the center) and the geometry of the margins. Clinically this poorer fit at the gingivoproximal corner of the veneers would be further compounded by the difficulty in accessfor finishing of the luted veneers in these regions. These large vertical gingivoproximal marginal openings underline the importance of supragingival, hygienically accessible veneer margins. The increased marginal openings seen at the toothveneer interface may facilitate a washout of the composite resin luting agent. Clinical research is needed for comparison of the dissolution rate of conventional crown luting agents and composite resin luting agents. CONCLUSIONS 1. The vertical marginal discrepancy of porcelain veneers made with the platinum foil technique (187 km) was significantly better than that of the refractory die method (292 pm). 2. For both methods the vertical discrepancy was significantly
greater at the gingivoproximal
position.
3. Both fabrication techniques resulted in veneers that were overcontoured, with refractory die veneers being significantly less overcontoured. 4. The facial margins produced by both methods had significantly greater overcontouring than other aspects. 5. Microleakage at the tooth-composite resin interface was universal.
6. Microleakage at the porcelain-composite resin interface was negligible. 7. There was no relationship between the amount of vertical marginal discrepancy and the amount of microleakage. We thank Sue Ingles, MS, for the statistical analysis.
REFERENCES
gual marginal openings. It was expected that the mesial and
1. Calamia JR. Etched porcelain veneers: the current state of the art. Quintessence Int 1985;16:5-12. 2. Sorensen JA. A rationale for comparisonof plaque retaining properties
distal gingivoproximal aspects of the veneers made with either technique would have a much poorer marginal
3. Simonsen RJ, Calamia JR. Tensile bond strength [Abstract]. J Dent Res 1983;62:297.
proximal
THE
aspects of the veneers compared to facial and lin-
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of crown systems.J PROSTHETDENT 1989;62:264-9. of etched porcelain
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4. Stangel I, Nathanson D, Hsu CS. Shear strength of the composite bond to etched porcelain. J Dent Res 1987;66:1460-5. 5. Calamia JR, Simonsen RJ. Effect of coupling agents on bond strength of etched porcelain [Abstract]. J Dent Res 1984;63:179. 6. Bausch JR, De Lange K, Davidson CL, Peters A, De Gee AJ. Clinical significance of polymerization shrinkage of composite resins. J PROSTHET DENT 1982;48:59-67. 7. Davidson CL, De Gee AJ, Feilzer AJ. The competition between the composite-dentin bond strength and the polymerization contraction stress. J Dent Res 1984;63:1396-9. 8. Asmussen E, Jorgensen KD. A microscopic investigation of the adaptation of some plastic filling materials to dental cavity walls. Acta Odonto1 Stand 1972;30:3-21. 9. Kidd EAM. Polymerization shrinkage and microleakage. In: Vanherle G, Smith DC, eds. International symposium on posterior composite resin dental restorative materials. Netherlands: Peter Szulz Publishing Co, ‘1985:263-8. 10. Leinfelder KF, Sluder TB, Sockwell WD, Strickland WD, Wall JT. Clinical evaluation of composite resins as anterior and posterior materials. J PROSTHETDENT 1975;33:407-16. 11. Osborne JW, Gale EN. A three-year clinical assessment of a composite resin and its radiopaque counterpart. J PROSTHETDENT 1980;44:164-6. 12. Roulet JF, Walti C. Influence of oral fluids on composite resin and glass-ionomer cement. J PROSTHETDENT 1984;52:182-9. 13. Vrijhoef MMA, Hendriks FHJ, Letzel H. Loss of substance of dental composite restorations. Dent Mater 1985;1:101-5. 14. McKinney JE, Wu W. Chemical softening and wear of dental composites. J Dent Res 1985;64:1326-31. 15. Hunt PR. Porcelain laminate systems. In: Tay WM, ed. General dental treatment. Brentford, England: Kluwer Pub1 LTD, 1986:1-14. 16. Tay WM, Lynch E, Auger D. Effects of some finishing techniques on cervical margins of porcelain laminates. Quintessence Int 1987;18:599609. 17. Wu W, Cobb E. A silver staining technique for investigating wear of restorative dental composites. J Biomed Mater Res 1981;15:343-8. 18. Wu W, Cobb E, Dermann K, Rupp NW. Detecting margin leakage of dental composite restorations. J Biomed Mater Res 1983;17:37-43. 19. Sorensen JA. A standardized method for determination of crown margin fidelity. J PROSTHETDENT 1990;64:18-24. 20. Wohlwend A, Strub JR, Scharer P. Metal ceramic and all-porcelain restorations: current considerations. Int J Prosthodont 1989;2:13-26.
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21. Sorensen JA, Okamoto SK, Miller R, Yarovesky U. Marginal fidelity of four methods of Renaissance crown fabrication [Abstract]. J Dent Res 1987;66:283. 22. Sorensen JA, Okamoto SK, Seghi R, Yarovesky U, Miller R. Marginal fidelity of four methods of swaged metal matrix crown fabrication, J PROSTHETDENT (IN PRESS). 23. Sorensen JA, Okamoto SK. Comparison of marginal fit of all-ceramic m-own systems [Abstract]. J Dent Res 1987;66:283. 24. Sorensen JA, Okamoto SK. Marginal fidelity of Optec crown system [Abstract]. J Dent Res 1989;68:200. 25. Sorensen JA, Okamoto SK, Seghi R. Marginal fidelity of all-ceramic crown systems. (Submitted for publication.) 26. Tjan AHL, Dunn JR, Sanderson IR. Microleakage patterns of porcelain castable ceramic laminate veneers. J PROSTHETDENT 1989;61:276-82. 27. Retief DH, Gross JD, Bradley EL, Denys FR. Tensile bond strengths of dentin bonding agents to dentin. Dent Mater 1986;2:72-7. 28. Tao L, Pashley DH, Boyd L. Effect of different types of smear layers on dentin and enamel shear bond strengths. Dent Mater 1988;4:208-16. 29. Davidson CL, De Gee AJ. Relaxation of polymerization contraction stresses by flow in dental composites. J Dent Res 1984;63:146-8. 30. Pearson GJ, Hegarty SM. Cusp movement in molar teeth using dentine adhesives and composite filling materials. Biomaterials 1987;8:473-6. 31. Feilzer AJ, De Gee AJ, Davidson CL. Increased wall-to-wall curing contraction in thin bonded resin layers. J Dent Res 1989;68:48-50. 32. Jordan RE, Suzuki M, Gwinnet AJ, Hunter JK. Restoration of fractured and hypoplastic incisors by the acid etch technique. J Am Dent Assoc 1977;95:795-803. 33. Jordan RE, Suzuki M, Gwinnet AJ. Conservative applications of acid etch resin techniques. Dent Clin North Am 1981;25:307-24. Reprint requests to: DR. JOHN A. SORENSEN CHS 33-041 SCHOOLOF DENTISTRY UNIVERSITYOF CALIFORNIA Los ANGELES,CA 90024
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back
J. Torres,
issues,
author CDT,
Research Assistant
19851990
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