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Marginal and internal adaptation of milled cobalt-chromium copings
RESEARCH AND EDUCATION
Marginal and internal adaptation of milled cobalt-chromium copings Lisa M. Kane, DDS, MS,a Dimitrios Chronaios, BDS, MS,b Marianella Sierraalta, DDS, MS,c and Furat M. George, BDS, MSd
ABSTRACT Statement of problem. The application of computer-aided design and computer-aided manufacturing (CAD/CAM) systems to produce complete coverage restorations with different materials continues to increase. To date, insufficient information is available regarding the adaptation of recently introduced milled cobalt-chromium (Co-Cr) copings for metal ceramic restorations. Purpose. The purpose of this in vitro study was to evaluate the marginal and internal fit of milled Co-Cr copings produced by CAD/CAM with 2 different marginal preparation designs. Material and methods. Four master dies were developed from 2 ivorine central incisors and 2 ivorine maxillary molars, 1 of each prepared with a 0.8-mm chamfer and a 1.2-mm rounded shoulder. These 4 groups of teeth were replicated with polyvinyl siloxane and used as templates to fabricate epoxy dies (n=10) for each of the 4 groups; a total of 40 epoxy resin dies. Cobalt-chromium copings of standard thickness (0.4 mm) were fabricated for each die with CAD/CAM technology. Next, the working dies were scanned with a 5-axis laser scanner to produce a 3-dimensional model. A thin layer of low-viscosity polyvinyl siloxane material was placed inside each coping and seated on the die until the material set. Copings were removed from the dies, leaving the polyvinyl siloxane intact, and these silicone-coated dies were scanned. The software superimposed the 2 scans, and the marginal openings and internal fit were measured at multiple locations. The marginal opening was determined at 4 locations: mid-buccal (mB), mid-lingual (mL), mid-mesial (mM), and mid-distal (mD), and the mean of these 4 measurement locations was referred to as the group variable “edge.” The internal occlusal adaptation was measured at the midpoint from buccal to lingual and mesial to distal locations and referred to as mid-occlusal (mO). Means and standard deviations for edge (marginal adaptation) and mO were calculated for each of the 4 groups. A 2-sample t test was performed to detect differences among groups. A regression analysis was done to evaluate the interaction between the variables mO and edge (a=.05). Results. Significantly smaller mean marginal openings (P=.017) were observed overall for the chamfer marginal design (anterior chamfer: 61 ±41 mm; posterior chamfer: 52 ±27 mm) compared with the shoulder design (anterior shoulder 103 ±49 mm, posterior shoulder 113 ±110 mm). The anterior chamfer had a statistically significant (P=.055) smaller mean marginal opening (61 ±41 mm) than the anterior shoulder (103 ±49 mm). No statistically significant differences (P=.119) were found between the posterior chamfer and posterior shoulder. The internal adaptation at the mO location was not significantly different among all 4 groups (P>.05). However, a regression analysis demonstrated a strong correlation (R=.842; P<.001) between the occlusal seat (mO) and marginal opening, with the smaller mean marginal opening of the chamfer design coinciding with the smaller occlusal seat values (61mm; mO: 182 mm) anterior chamfer; (52 mm; mO: 172 mm) posterior chamfer versus (103 mm; mO: 235 mm) anterior shoulder; (113 mm; mO: 242 mm) posterior shoulder. Conclusions. The milled Co-Cr copings produced with a CAD/CAM system in this study demonstrated clinically acceptable marginal fit in the range of 52 to 113 mm before ceramic application. (J Prosthet Dent 2015;-:---)
Historically, the alloys of choice for metal ceramic restorations have been based on gold. However, given the increased cost of traditional gold alloys, use of economic
alternatives such as base metal alloys is growing. Nickelchromium alloys have been the most popular base metal alloys for metal ceramic restorations.1,2 In response to
Partially supported by Delta Dental Master’s Thesis Research Fund. a Clinical Assistant Professor, Division of Prosthodontics, Department of Biologic and Materials Sciences, University of Michigan School of Dentistry, Ann Arbor, Mich. b Private practice, Athens, Greece. c Clinical Professor, Division of Prosthodontics, Department of Biologic and Material Sciences, University of Michigan School of Dentistry, Ann Arbor, Mich. d Clinical Assistant Professor, Division of Prosthodontics, Department of Biologic and Materials Sciences, University of Michigan School of Dentistry, Ann Arbor, Mich.
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Clinical Implications Given the challenges associated with the traditional lost wax technique and complex inherent casting variables, alternative fabrication techniques for base metal ceramic restorations may result in improved fit. The application of CAD/CAM technology to produce milled Co-Cr copings for metal ceramic restorations will consistently produce a coping with clinically acceptable marginal fit.
concerns about the toxicity of nickel and beryllium, cobalt-chromium (Co-Cr) alloys for use in metal ceramic restorations were developed.1 The current trend is to replace nickel-chromium alloys with Co-Cr alloys, which are more biocompatible.3 Cobalt-chromium alloys have been shown to be corrosion resistant and stable in biologic environments.4 Conventionally, production of base metal copings for metal ceramic restorations have centered on the lost-wax fabrication technique. Base metal alloys are generally considered more technique sensitive and more difficult to cast than noble metal alloys.5 Early researchers focused on ways to increase the precision of castings of Co-Cr alloy, 6-9 but castability challenges remain. Given the challenges associated with the traditional lost wax fabrication technique and the complex inherent casting variables that must be overcome, alternative techniques for metal ceramic restorations are of interest. Computeraided design/computer-aided manufacture (CAD/CAM) technology has recently been introduced (NobelProcera, Nobel Biocare) to produce Co-Cr copings for metal ceramic restorations from prefabricated metal blocks. Also, recently introduced is a CAD/CAM laser melting technology (Bego Medical; Phenix Systems) for the fabrication of metal copings from Co-Cr alloy powders.10,11 This selective laser-melting technology has demonstrated marginal fit of Co-Cr crowns that is improved compared with traditional casting.12 The hope is that application of CAD/CAM technology for metal ceramic restorations may result in restorations with improved adaptation. Marginal and internal accuracy of fit is an important consideration for the clinical acceptability and success of complete coverage crowns.13-15 Restoration misfits create a space between the restoration and preparation, exposing the luting material to the oral environment and causing plaque accumulation.16 This gap can be viewed as a physical roughness and may eventually lead to periodontal inflammation, caries, and loss of the restoration.17-19 The adaptation of a restoration can be defined in terms of misfit measured at various points of the restoration and the tooth along the internal surface, at
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the margin, or on the external surface of the casting.20 This misfit represents the available cement space. Early investigators recognized that the film thickness of cement along the axial walls of castings produced by the lost-wax technique influenced the seating.21 Various techniques such as die relief, internal acid etching, venting, use of disclosing agents, cementation process, and preparation geometry were investigated to improve seating.22-31 Current researchers, using CAD/CAM to produce copings, also appreciate the effect the cement or luting space variable has on the fit.32-35 Cement thickness has also been shown to influence the strength of ceramic crowns produced with various CAD/CAM systems36,37 and the bond strength of cement to cast metal.38 Although some CAD/CAM systems allow this space to be controlled, the system used in the current investigation did not allow the cement space to be altered, nor did the manufacturer state the dimensions of this space; this is an important variable to investigate. Controversy exists regarding how large a gap is considered clinically acceptable in marginal openings, but 120 mm is usually cited as the maximum.39 Marginal adaptation of cemented restorations of 25 mm has been suggested14 as an ideal clinical goal and is the American Dental Association Specification no. 96 for maximum film thickness40; however, marginal openings this size are seldom reported. Also, measurement methods varied41,42 widely in the different studies, depending on the system used. This may lead to ambiguities in the comparisons of values among studies or in the actual fit of the crown system in question.42 Investigators43-46 have compared the marginal accuracy of CAD/CAM processing of ceramic systems with that of the traditional lost wax technique. Some studies have demonstrated lower marginal opening values with CAD/CAM processing than with the lost wax technique43,44; however, all values were within clinically acceptable limits. Other studies15,35 using CAD/CAM to fabricate ceramic crowns show accuracy of fit similar to that of conventional techniques. Studies regarding marginal and internal fit of milled Co-Cr metal ceramic crowns were not found in our search. Therefore, the purpose of this study was to investigate the marginal and internal fit of milled Co-Cr copings for both the chamfer and the shoulder preparation designs with CAD/CAM technology. The null hypothesis was that no differences in marginal and internal adaptation would be found between the tested groups. MATERIAL AND METHODS Four master dies were developed from 2 ivorine maxillary central incisors and 2 ivorine maxillary molars (860 series; Columbia Dentoform Corp). One ivorine incisor and 1 molar were prepared with either a 0.8-mm chamfer or a
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1.2-mm rounded shoulder and a uniform 2.0-mm occlusal reduction. Preparations were made with diamond rotary instruments using a high-speed laboratory handpiece (Carv-aire; Jelenko) mounted on a milling machine (Fräsgerät F1; Degussa) following accepted principals of tooth preparation.47 The chamfer was prepared with a chamfer diamond rotary instrument (6878K.31.016; Brasseler) to one-half its diameter and the shoulder with a modified flat-end diamond rotary instrument (837KR.31.012; Brasseler). Impressions of the 4 groups of master dies (anterior chamfer, anterior shoulder, posterior chamfer, posterior shoulder) were made with polyvinyl siloxane (Extrude light-bodied; Kerr Corp) and used as templates for the fabrication of 10 epoxy resin dies (Epoxy Potting Resin; General Polymer Corp) for each of the 4 groups, for a total of 40 dies. A power calculation was performed based on previous studies with a similar methodolology35,48 with software (nQuery Advisor version 7.0; Statistical Solutions, Ltd). According to the analysis, a sample size of 4 to 5 in each group would have a 90% power to detect a difference in marginal fit.49 It was decided to use 10 specimens in each group to increase the statistical power of the results. The Co-Cr copings were fabricated with CAD/CAM technology (NobelProcera scanner; Nobel Biocare). Each epoxy die was digitized by the laser scanner, and the coping was designed virtually by using the manufacturer’s design software to a standard thickness of 0.4 mm. Data were sent electronically to a production facility (NobelProcera; Nobel Biocare) and used to fabricate the Co-Cr copings with a 5-axis milling of a solid base metal alloy Co-Cr monoblock (Dentaurum Renamium; Nobel Biocare) composed of cobalt (61 ±2%), chromium (28 ±2%), tungsten (9 ±1%), silicon (1.5 ±1%), iron, manganese, nitrogen, and niobium (each <1%). The manufacturer states the alloy is biocompatible and free of nickel and beryllium. The working dies were indexed (Pindex laser system; Coltène/Whaledent Inc) and mounted on a Type III dental stone base (Microstone Golden; Whip Mix Corp). Dies were scanned using a 5-axis laser scanner (activity 101; Smart Optics) to digitize the specimens and collect 3-dimensional (3D) data. The laser system has a reported accuracy of ±20 mm. To measure the precision of fit of the copings to the die, a thin layer of low-viscosity polyvinyl siloxane impression material (Extrude light-bodied; Kerr Corp) was used to simulate a luting agent and was placed inside the copings. Light-bodied elastomers have been used previously by investigators to simulate the luting agent and found to be reliable.39,50 The copings were seated on the die and held under a 49-N load until the polyvinyl siloxane was set. After setting, the copings were carefully removed from the dies, leaving the polyvinyl siloxane intact. The polyvinyl siloxane coat on the luting surface of the die Kane et al
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Figure 1. Measurements of polyvinyl siloxane-coated die. D, mean distance; Dx, distance in x axis; Dy, distance in y axis; Dz, distance in z axis.
provides the 3D space between the die and the coping and represents the available cement space. This siliconecoated die was mounted on the original stone base and scanned with the laser scanner. Software for measuring (Geomagic Qualify; Geomagic) was used to superimpose the 2 scans and to compare the surfaces of the epoxy dies with the silicone-coated dies, using the base of the die as a reference. From the 3D representations, the marginal discrepancies and internal fit were measured at multiple locations (Fig. 1). The marginal opening was determined at the point closest to the coping margin, and the die was measured in 4 locations: mid-buccal (mB), mid-mesial (mM), mid-distal (mD), and mid-lingual (mL). The group variable edge represents the mean of these 4 measurement locations. The occlusal adaptation, which was measured from the occlusal/incisal surface of the die to the internal surface of the coping at the mid-point from buccal to lingual and mesial to distal, was measured at 1 location and referred to as mid-occlusal (mO). All measurements were made by 1 operator (D.C.). Means and standard deviations (SD) were calculated for edge (marginal adaptation) and mO for all 4 groups. A 2-sample t test was used to detect differences among groups. A regression analysis evaluated the interaction between the variables mO and edge with software (StatView v5.0; SAS Institute, Inc). RESULTS Descriptive data for mean ±SD marginal opening for all 4 groups are shown in the bar graph of Figure 2. Mean ±SD mO gap dimensions are shown in Figure 3. Significantly smaller mean marginal openings (P=.017) were observed overall for the chamfer marginal design (anterior chamfer: 61 ±41 mm; posterior chamfer: 52 ±27 mm) than for the shoulder design (anterior shoulder: 103 ±49 mm; posterior shoulder: 113 ±110 mm). The anterior chamfer had a statistically significant (P=.055) smaller THE JOURNAL OF PROSTHETIC DENTISTRY
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Co-Cr Copings Figure 2. Mean marginal opening of milled Co-Cr copings.
mean marginal opening than the anterior shoulder. No statistically significant differences (P=.119) were found between the posterior chamfer and posterior shoulder. The internal adaptations at the mO location were not significantly different among the 4 groups (P>.05). A regression analysis demonstrated a strong correlation (R=.842; P<.001) between the variables occlusal seat and marginal opening, with the smaller mean marginal opening of the chamfer design coinciding with the smaller occlusal seat values (anterior chamfer: 61 mm; mO: 182 mm; posterior chamfer: 52 mm; mO: 172 mm; anterior shoulder: 103 mm; mO: 235 mm; and posterior shoulder: 113 mm; mO: 242 mm). DISCUSSION The mean marginal openings of the milled Co-Cr copings studied in this investigation were within the range (52 to 113 mm) considered clinically acceptable by most reports in the dental studies (<120 mm).39 The hypothesis that no differences in marginal and internal adaptation of the milled Co-Cr copings would be found among the 4 tested groups was rejected for marginal fit. In the present study, a significantly (P=.017) improved marginal overall adaptation was noted for the chamfer marginal design compared to that of the shoulder. Many studies have looked at the influence of finish-line geometry on the fit of cemented crowns, with conflicting results.29-31 Current research suggests that the expectation of improved fit is not associated with specific finish-line geometries.47 The results for better mean marginal adaptation for chamfer versus shoulder in both anterior and posterior groups may be explained as a result of the scanning, software, and/or milling capabilities of the system used. The scanner or milling unit used in the present investigation may be limited in its ability to reach the depths required of the master die in the shoulder design, or limitations may exist in the software for identifying the margins. Accordingly, the THE JOURNAL OF PROSTHETIC DENTISTRY
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manufacturer’s clinical instruction manual recommends a chamfer finish line with rounded internal line angles; thus, given the current trend toward increased application of CAD/CAM fabrication techniques for complete coverage crown restorations, a clinician would be wise to choose this margin design when choosing this fabrication technique for metal ceramic crowns. Kokubo et al33 used the GN-I CAD/CAM system to produce InCeram crowns when marginal and internal gaps were evaluated and stated that the manufacturer recommended a chamfer design because it is “difficult to mill a complicated curved surface that includes both concave and convex areas.” Hydrodynamic forces that occur during cementation dictate that an internal space be available to facilitate seating. Several studies have shown that the seating of crowns is greatly improved by providing internal relief.25-28 However, the film thickness should be maintained at an optimal level because it can affect the clinical performance and longevity of the restoration. An increase in cement thickness decreases the tensile bond strength of cement to cast metal,38 and the fracture strength of crowns has been shown to be highest when the cement thickness is lowest.36,37 The current investigation using Nobel Biocare’s NobelProcera system to mill Co-Cr copings for metal ceramic restorations did not allow for the size of the cement space to be modified, nor is the actual value of this space specified by the manufacturer. It was decided to measure the internal fit discrepancy at a point midway between the buccal and lingual and mesial and distal surfaces, referred to mO. The occlusal seat has been related to the marginal seal, and ideally, the space at the margin and the occlusal portion of the preparation after cementation should be identical.28,29 The amount of incomplete seating of a cemented crown is thus theoretically equal to the thickness of the cement at the Kane et al
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occlusal surface.28 In the present investigation, no significant differences (P>.05) were found in mO values among the 4 tested groups; however, the best marginal seal tended to coincide with the smallest occlusal seat for the chamfer design. These results are contrary to the findings of Bindl and Mormann,35 who concluded that a large internal gap width may have favored the smaller marginal gap dimensions in their study when they compared different CAD/CAM systems to fabricate ceramic crowns. They reported marginal gap widths of 17 ±16 mm compared with a mid-orobuccal space of 136 ±68 mm for the Procera system. An average occlusal space misfit between 100 and 200 mm is often reported in the dental studies,37 and the results of the present investigation are in this range. Nakamura et al34 looked at the marginal and internal fit of Cerec 3 CAD/CAM crowns and demonstrated that the mean marginal openings were significantly greater when the luting space was set at 10 mm (95 to 108 mm) than at 30 or 50 mm (53-67 mm). KoKubo et al33 investigated InCeram copings produced by CAD/CAM technology and noted that, if the predetermined cement space was smaller than 50 mm, the marginal gaps increased significantly. The authors explained this might be caused by “axial wall space” contacting the abutment tooth. In the present study, axial wall space was not included in the internal discrepancy measurements, only the occlusal seat was considered. This decision was based on the assumption that the occlusal seat was the most important predictor of marginal seal and the fact that the manufacturer apparently allows for a uniform cement space in the design software. The present study demonstrated both the highest occlusal seat of the posterior shoulder at 242 mm along with the largest marginal opening value at 113 mm. This finding suggests possible axial wall binding, which might have prevented further seating of the coping. Further investigations should include more data point analysis beyond that measured in our study to investigate whether impingement actually prevented further seating. Unlike the system used in this study, the Dentronic CAD/CAM system used in a study by Coli et al32 allowed the programming of a predetermined cement space thickness. Those investigators found that, generally, the space along the axial walls was smaller than that along the occlusal walls; a uniform thickness was not found everywhere. White et al22 defined a phenomenon which they called tilting, which refers to the binding of the casting against the axial wall of the tooth preparation. Tilting prevents seating of the binding side but adapts the margin better on the opposite side.22 Inaccurate placement of the coping to the master die can be a source of error because only 1 position will ultimately give an optimal fit. A minimal rotation of the coping on the die may result in an increased discrepancy on 1 side and a Kane et al
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smaller on another side; this is 1 possible source of error in the present investigation. Further investigation is warranted to determine whether the cement space allocated by the software used in the Nobel Biocare NobelProcera system is truly a uniform space. It is reasonable to assume that the optimum cement space present under a restoration would be the smallest that allowed for the best marginal fit26 without compromising strength or retention. In this study, a light-bodied polyvinyl siloxane was used to simulate the luting agent and to represent the cement space.39,50 The film thickness of the cement is 1 of the many factors that greatly influence adaptation of the restoration to the tooth. The film thickness of the light-bodied polyvinyl siloxane used in this investigation is unknown relative to the that of commonly used cements. McLean and von Fraunhofer39 measured a film thickness of 22 mm with a polyether compared with 20 mm with zinc phosphate cement when estimating the cement film thickness in their study. This is close to the ADA Specification no. 96 of 25 mm for a waterbased cement.40 The measured values only approximate the true or absolute value of the marginal and internal adaptation of the milled Co-Cr copings produced in this study. Further investigations to evaluate other factors involved in cementation technique, such as seating pressure, temperature, humidity, and their effect on marginal and internal adaptation, are warranted. CONCLUSIONS Within the limitations of this in vitro study, the milled Co-Cr copings produced with the Nobel Biocare NobelProcera CAD/CAM system demonstrated clinically acceptable marginal fit in the range of 52 to 113 mm before ceramic application. REFERENCES 1. Baran GR. Selection criteria for base metal alloys for use with porcelains. Dent Clin North Am 1985;29:779-87. 2. Kelly JR, Rose TC. Nonprecious alloys for use in fixed prosthodontics: a literature review. J Prosthet Dent 1983;49:363-70. 3. Bezzon OL, de Mattos Mda G, Ribero RF, Rollo JM. Effect of beryllium on the castability and resistance of ceramometal bonds in nickel-chromium alloys. J Prosthet Dent 1998;80:570-4. 4. Khamis E, Seddik M. Corrosion evaluation of recasting non-precious dental cast alloys. Int Dent J 1995;45:209-17. 5. Baran GR. The metallurgy of Ni-Cr alloys for fixed prosthodontics. J Prosthet Dent 1983;50:639-50. 6. Carter TJ, Kidd JN. The precision casting of cobalt-chromium alloy. 1. The influence of casting variables on dimensions and finish. Br Dent J 1965;118: 383-90. 7. Strandman E, Lockowandt P. An equipment for standardized casting of Co-Cr alloys in dentistry. Odontol Rev 1976;27:145-54. 8. Vermilyea SG, Kuffler MJ, Tamura JJ. Techniques for the accurate casting of base-metal alloys. Mil Med 1983;148:354-7. 9. Carreiro Ada F, Ribero RF, Mattos Mda G, Rodrigues RC. Evaluation of the castability of a Co-Cr-Mo-W alloy varying the investing technique. Braz Dent J 2005;16:50-5. 10. Quante K, Ludwig K, Kern M. Marginal and internal fit of metal-ceramic crowns fabricated with a new laser melting technology. Dent Mater 2008;24: 1311-5.
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11. Ucar Y, Akova T, Akyil MS, Brantley WA. Internal fit evaluation of crowns prepared using a new dental crown fabrication technique: laser-sintered CoCr crowns. J Prosthet Dent 2009;102:253-9. 12. Dan Xu, Nan Xiang, Bin Wei. The marginal fit of selective laser meltingfabricated metal crowns: an in-vitro study. J Prosthet Dent 2014;112:1437-40. 13. Moore JA, Barghi N, Bruki CE, Kaiser DA. Marginal distortion of cast restorations induced by cementation. J Prosthet Dent 1985;54:336-40. 14. Hung SH, Hung KS, Eick JD, Chappell RP. Marginal fit of porcelain-fusedto-metal and two types of ceramic crown. J Prosthet Dent 1990;63:26-31. 15. Biscaro L, Bonfiglioli R, Soattin M, Vigolo P. An In vivo evaluation of fit of zirconium-oxide based ceramic single crowns, generated with two CAD/ CAM systems, in comparison to metal ceramic single crowns. J Prosthodont 2013;22:36-41. 16. Sorensen JA. A rationale for comparison of plaque-retaining properties of crown systems. J Prosthet Dent 1989;62:264-9. 17. Felton DA, Kanoy BE, Bayne SC, Wirthman GP. Effect of in vivo crown margin discrepancies on periodontal health. J Prosthet Dent 1991;65:357-64. 18. Grasso JE, Nalbandian J, Sanford C, Bailit H. Effect of restoration quality on periodontal health. J Prosthet Dent 1985;53:14-9. 19. Walton JN, Gardner FM, Agar JR. A survey of crown and fixed partial denture failures: length of service and reasons for replacement. J Prosthet Dent 1986;56:416-21. 20. Holmes JR, Bayne SC, Holland GA, Sulik WD. Considerations in measurement of marginal fit. J Prosthet Dent 1989;62:405-8. 21. Fusayama T, Ide K, Kurosu A, Hosoda H. Cement thickness between cast restorations and preparation walls. J Prosthet Dent 1963;13:354-64. 22. White SN, Kipnis V. The three-dimensional effects of adjustment and cementation on crown seating. Int J Prosthodont 1993;6:248-54. 23. Jorgensen KD. Factors affecting the film thickness of zinc phosphate cements. Acta Odontol Scand 1960;18:189. 24. Pascoe DF. Analysis of the geometry of finishing lines for full crown restorations. J Prosthet Dent 1978;40:157-62. 25. Eames WB. Techniques to improve the seating of castings. Quintessence Dent Technol 1981;5:437-41. 26. Grajower R, Zuberi Y, Lewinstein I. Improving the fit of crowns with die spacers. J of Prosthet Dent 1989;61:555-63. 27. Ishikiriama A, de Freitas Oliveira J, Vieira DF, Mondelli J. Influence of some factors on the fit of cemented crowns. J Prosthet Dent 1981;45:400-4. 28. Pilo R, Cardash HS, Baharav H, Helft M. Incomplete seating of cemented crowns: A literature review. J Prosthet Dent 1988;59:429-33. 29. Gavelis JR, Morency JD, Riley ED, Sozio RB. The effect of various finish line preparations on the marginal seal and occlusal seat of full crown preparations 1981. J Prosthet Dent 2004;92:1-7. 30. Byrne G. Influence of finish-line form on crown cementation. Int J Prosthodont 1992;5:137-44. 31. Shillingburg HT, Hobo S Jr, Fisher DW. Preparation design and margin distortion in porcelain-fused-to-metal restorations. J Prosthet Dent 1973;29: 276-84. 32. Coli P, Karlsson S. Precision of a CAD/CAM technique for the production of zirconium dioxide copings. Int J Prosthodont 2004;17:577-80. 33. Kokubo Y, Nagayama Y, Tsumita M, Ohkubo C, Fukushima S, Vult von Steyern P. Clinical marginal and internal gaps of In-Ceram crowns fabricated using the GN-I system. J Oral Rehabil 2005;32:753-8. 34. Nakamura T, Nobuyoshi D, Kojima T, Wakabayashi K. Marginal and internal fit of cerec 3 CAD/CAM all-ceramic crowns. Int J Prosthodont 2003;16:244-8.
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35. Bindl A, Mormann WH. Marginal and internal fit of all-ceramic CAD/CAM crown-copings on chamfer preparations. J Oral Rehabil 2005;32:441-7. 36. Tuntiprawon M, Wilson PR. The effect of cement thickness on the fracture strength of all-ceramic crowns. Aust Dent J 1995;40:17-21. 37. May GL, Kelly RJ, Bottino MA, Hill T. Effects of cement thickness and bonding on the failure loads of CAD/CAM ceramic crowns: multi-physics FEA modeling and monotonic testing. Dent Mater 2012;28:99-109. 38. Rosenstiel SF, Land MF, Crispin BJ. Dental luting agents: a review of the current literature. J Prosthet Dent 1998;80:280-301. 39. McLean JW, von Fraunhofer JA. The estimation of cement film thickness by an in vivo technique. Br Dent J 1971;131:107-11. 40. ANSI/ADA. ANSI/ADA Specification No. 96dDental Water-based Cements: 2012. 41. Sorensen JA. A standardized method for determination of crown margin fidelity. J Prosthet Dent 1990;64:18-24. 42. Nawafleh NA, Mack F, Evans J, Mackay J, Hatamleh MM. Accuracy and reliability of methods to measure marginal adaptation of crowns and FDP’s: a literature review. J Prosthodont 2013;22:419-28. 43. Ural C, Burgaz Y, Sarac D. In vitro evaluation of marginal adaptation in five ceramic restoration fabricating techniques. Quintessence Int 2010;41:585-90. 44. Gonzalo E, Suarez MJ, Serrano B, Lozano J. A comparison of the marginal vertical discrepancies of zirconium and metal ceramic posterior fixed dental prostheses before and after cementation. J Prosthet Dent 2009;102:378-84. 45. Baig MR, Tan KB, Nicholls JI. Evaluation of the marginal fit of a zirconia ceramic computer-aided machined (CAM) crown system. J Prosthet Dent 2010;104:216-27. 46. Quintas AF, Oliveira F, Bottino MA. Vertical marginal discrepancy of ceramic copings with different ceramic materials, finish lines, and luting agents: an invitro evaluation. J Prosthet Dent 2004;92:250-7. 47. Goodacre CJ, Campagni WV, Aquilino SA. Tooth preparations for complete crowns: An art form based on scientific principals. J Prosthet Dent 2001;85: 363-76. 48. Lee KB, Park CW, Kim KH, Kwon TY. Marginal and internal fit of all-ceramic crowns fabricated with two different CAD/CAM systems. Dent Mater J 2008;27:422-6. 49. Moser BK, Stevens GR, Watts CL. The two-sample t-test versus Satterthwaite’s approximate f test. Commun Statist-theory Meth 1989;18: 3963-75. 50. Frannson B, Oilo G, Gjeitanger R. The fit of metal-ceramic crowns, a clinical study. Dent Mater 1985;1:197-9. Corresponding author: Dr Lisa Kane Division of Prosthodontics Department of Biologic and Materials Sciences University of Michigan School of Dentistry 1011 N. University Ann Arbor, MI 48109 Email: [email protected] Acknowledgment The authors thank Dr Rui-Feng Wang for assistance in the preparation of measurement data and performance of statistical tests used in this investigation. Copyright © 2015 by the Editorial Council for The Journal of Prosthetic Dentistry.
Kane et al
Marginal and internal adaptation of milled cobalt-chromium copings Lisa M. Kane, DDS, MS,a Dimitrios Chronaios, BDS, MS,b Marianella Sierraalta, DDS, MS,c and Furat M. George, BDS, MSd
ABSTRACT Statement of problem. The application of computer-aided design and computer-aided manufacturing (CAD/CAM) systems to produce complete coverage restorations with different materials continues to increase. To date, insufficient information is available regarding the adaptation of recently introduced milled cobalt-chromium (Co-Cr) copings for metal ceramic restorations. Purpose. The purpose of this in vitro study was to evaluate the marginal and internal fit of milled Co-Cr copings produced by CAD/CAM with 2 different marginal preparation designs. Material and methods. Four master dies were developed from 2 ivorine central incisors and 2 ivorine maxillary molars, 1 of each prepared with a 0.8-mm chamfer and a 1.2-mm rounded shoulder. These 4 groups of teeth were replicated with polyvinyl siloxane and used as templates to fabricate epoxy dies (n=10) for each of the 4 groups; a total of 40 epoxy resin dies. Cobalt-chromium copings of standard thickness (0.4 mm) were fabricated for each die with CAD/CAM technology. Next, the working dies were scanned with a 5-axis laser scanner to produce a 3-dimensional model. A thin layer of low-viscosity polyvinyl siloxane material was placed inside each coping and seated on the die until the material set. Copings were removed from the dies, leaving the polyvinyl siloxane intact, and these silicone-coated dies were scanned. The software superimposed the 2 scans, and the marginal openings and internal fit were measured at multiple locations. The marginal opening was determined at 4 locations: mid-buccal (mB), mid-lingual (mL), mid-mesial (mM), and mid-distal (mD), and the mean of these 4 measurement locations was referred to as the group variable “edge.” The internal occlusal adaptation was measured at the midpoint from buccal to lingual and mesial to distal locations and referred to as mid-occlusal (mO). Means and standard deviations for edge (marginal adaptation) and mO were calculated for each of the 4 groups. A 2-sample t test was performed to detect differences among groups. A regression analysis was done to evaluate the interaction between the variables mO and edge (a=.05). Results. Significantly smaller mean marginal openings (P=.017) were observed overall for the chamfer marginal design (anterior chamfer: 61 ±41 mm; posterior chamfer: 52 ±27 mm) compared with the shoulder design (anterior shoulder 103 ±49 mm, posterior shoulder 113 ±110 mm). The anterior chamfer had a statistically significant (P=.055) smaller mean marginal opening (61 ±41 mm) than the anterior shoulder (103 ±49 mm). No statistically significant differences (P=.119) were found between the posterior chamfer and posterior shoulder. The internal adaptation at the mO location was not significantly different among all 4 groups (P>.05). However, a regression analysis demonstrated a strong correlation (R=.842; P<.001) between the occlusal seat (mO) and marginal opening, with the smaller mean marginal opening of the chamfer design coinciding with the smaller occlusal seat values (61mm; mO: 182 mm) anterior chamfer; (52 mm; mO: 172 mm) posterior chamfer versus (103 mm; mO: 235 mm) anterior shoulder; (113 mm; mO: 242 mm) posterior shoulder. Conclusions. The milled Co-Cr copings produced with a CAD/CAM system in this study demonstrated clinically acceptable marginal fit in the range of 52 to 113 mm before ceramic application. (J Prosthet Dent 2015;-:---)
Historically, the alloys of choice for metal ceramic restorations have been based on gold. However, given the increased cost of traditional gold alloys, use of economic
alternatives such as base metal alloys is growing. Nickelchromium alloys have been the most popular base metal alloys for metal ceramic restorations.1,2 In response to
Partially supported by Delta Dental Master’s Thesis Research Fund. a Clinical Assistant Professor, Division of Prosthodontics, Department of Biologic and Materials Sciences, University of Michigan School of Dentistry, Ann Arbor, Mich. b Private practice, Athens, Greece. c Clinical Professor, Division of Prosthodontics, Department of Biologic and Material Sciences, University of Michigan School of Dentistry, Ann Arbor, Mich. d Clinical Assistant Professor, Division of Prosthodontics, Department of Biologic and Materials Sciences, University of Michigan School of Dentistry, Ann Arbor, Mich.
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Clinical Implications Given the challenges associated with the traditional lost wax technique and complex inherent casting variables, alternative fabrication techniques for base metal ceramic restorations may result in improved fit. The application of CAD/CAM technology to produce milled Co-Cr copings for metal ceramic restorations will consistently produce a coping with clinically acceptable marginal fit.
concerns about the toxicity of nickel and beryllium, cobalt-chromium (Co-Cr) alloys for use in metal ceramic restorations were developed.1 The current trend is to replace nickel-chromium alloys with Co-Cr alloys, which are more biocompatible.3 Cobalt-chromium alloys have been shown to be corrosion resistant and stable in biologic environments.4 Conventionally, production of base metal copings for metal ceramic restorations have centered on the lost-wax fabrication technique. Base metal alloys are generally considered more technique sensitive and more difficult to cast than noble metal alloys.5 Early researchers focused on ways to increase the precision of castings of Co-Cr alloy, 6-9 but castability challenges remain. Given the challenges associated with the traditional lost wax fabrication technique and the complex inherent casting variables that must be overcome, alternative techniques for metal ceramic restorations are of interest. Computeraided design/computer-aided manufacture (CAD/CAM) technology has recently been introduced (NobelProcera, Nobel Biocare) to produce Co-Cr copings for metal ceramic restorations from prefabricated metal blocks. Also, recently introduced is a CAD/CAM laser melting technology (Bego Medical; Phenix Systems) for the fabrication of metal copings from Co-Cr alloy powders.10,11 This selective laser-melting technology has demonstrated marginal fit of Co-Cr crowns that is improved compared with traditional casting.12 The hope is that application of CAD/CAM technology for metal ceramic restorations may result in restorations with improved adaptation. Marginal and internal accuracy of fit is an important consideration for the clinical acceptability and success of complete coverage crowns.13-15 Restoration misfits create a space between the restoration and preparation, exposing the luting material to the oral environment and causing plaque accumulation.16 This gap can be viewed as a physical roughness and may eventually lead to periodontal inflammation, caries, and loss of the restoration.17-19 The adaptation of a restoration can be defined in terms of misfit measured at various points of the restoration and the tooth along the internal surface, at
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the margin, or on the external surface of the casting.20 This misfit represents the available cement space. Early investigators recognized that the film thickness of cement along the axial walls of castings produced by the lost-wax technique influenced the seating.21 Various techniques such as die relief, internal acid etching, venting, use of disclosing agents, cementation process, and preparation geometry were investigated to improve seating.22-31 Current researchers, using CAD/CAM to produce copings, also appreciate the effect the cement or luting space variable has on the fit.32-35 Cement thickness has also been shown to influence the strength of ceramic crowns produced with various CAD/CAM systems36,37 and the bond strength of cement to cast metal.38 Although some CAD/CAM systems allow this space to be controlled, the system used in the current investigation did not allow the cement space to be altered, nor did the manufacturer state the dimensions of this space; this is an important variable to investigate. Controversy exists regarding how large a gap is considered clinically acceptable in marginal openings, but 120 mm is usually cited as the maximum.39 Marginal adaptation of cemented restorations of 25 mm has been suggested14 as an ideal clinical goal and is the American Dental Association Specification no. 96 for maximum film thickness40; however, marginal openings this size are seldom reported. Also, measurement methods varied41,42 widely in the different studies, depending on the system used. This may lead to ambiguities in the comparisons of values among studies or in the actual fit of the crown system in question.42 Investigators43-46 have compared the marginal accuracy of CAD/CAM processing of ceramic systems with that of the traditional lost wax technique. Some studies have demonstrated lower marginal opening values with CAD/CAM processing than with the lost wax technique43,44; however, all values were within clinically acceptable limits. Other studies15,35 using CAD/CAM to fabricate ceramic crowns show accuracy of fit similar to that of conventional techniques. Studies regarding marginal and internal fit of milled Co-Cr metal ceramic crowns were not found in our search. Therefore, the purpose of this study was to investigate the marginal and internal fit of milled Co-Cr copings for both the chamfer and the shoulder preparation designs with CAD/CAM technology. The null hypothesis was that no differences in marginal and internal adaptation would be found between the tested groups. MATERIAL AND METHODS Four master dies were developed from 2 ivorine maxillary central incisors and 2 ivorine maxillary molars (860 series; Columbia Dentoform Corp). One ivorine incisor and 1 molar were prepared with either a 0.8-mm chamfer or a
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1.2-mm rounded shoulder and a uniform 2.0-mm occlusal reduction. Preparations were made with diamond rotary instruments using a high-speed laboratory handpiece (Carv-aire; Jelenko) mounted on a milling machine (Fräsgerät F1; Degussa) following accepted principals of tooth preparation.47 The chamfer was prepared with a chamfer diamond rotary instrument (6878K.31.016; Brasseler) to one-half its diameter and the shoulder with a modified flat-end diamond rotary instrument (837KR.31.012; Brasseler). Impressions of the 4 groups of master dies (anterior chamfer, anterior shoulder, posterior chamfer, posterior shoulder) were made with polyvinyl siloxane (Extrude light-bodied; Kerr Corp) and used as templates for the fabrication of 10 epoxy resin dies (Epoxy Potting Resin; General Polymer Corp) for each of the 4 groups, for a total of 40 dies. A power calculation was performed based on previous studies with a similar methodolology35,48 with software (nQuery Advisor version 7.0; Statistical Solutions, Ltd). According to the analysis, a sample size of 4 to 5 in each group would have a 90% power to detect a difference in marginal fit.49 It was decided to use 10 specimens in each group to increase the statistical power of the results. The Co-Cr copings were fabricated with CAD/CAM technology (NobelProcera scanner; Nobel Biocare). Each epoxy die was digitized by the laser scanner, and the coping was designed virtually by using the manufacturer’s design software to a standard thickness of 0.4 mm. Data were sent electronically to a production facility (NobelProcera; Nobel Biocare) and used to fabricate the Co-Cr copings with a 5-axis milling of a solid base metal alloy Co-Cr monoblock (Dentaurum Renamium; Nobel Biocare) composed of cobalt (61 ±2%), chromium (28 ±2%), tungsten (9 ±1%), silicon (1.5 ±1%), iron, manganese, nitrogen, and niobium (each <1%). The manufacturer states the alloy is biocompatible and free of nickel and beryllium. The working dies were indexed (Pindex laser system; Coltène/Whaledent Inc) and mounted on a Type III dental stone base (Microstone Golden; Whip Mix Corp). Dies were scanned using a 5-axis laser scanner (activity 101; Smart Optics) to digitize the specimens and collect 3-dimensional (3D) data. The laser system has a reported accuracy of ±20 mm. To measure the precision of fit of the copings to the die, a thin layer of low-viscosity polyvinyl siloxane impression material (Extrude light-bodied; Kerr Corp) was used to simulate a luting agent and was placed inside the copings. Light-bodied elastomers have been used previously by investigators to simulate the luting agent and found to be reliable.39,50 The copings were seated on the die and held under a 49-N load until the polyvinyl siloxane was set. After setting, the copings were carefully removed from the dies, leaving the polyvinyl siloxane intact. The polyvinyl siloxane coat on the luting surface of the die Kane et al
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Figure 1. Measurements of polyvinyl siloxane-coated die. D, mean distance; Dx, distance in x axis; Dy, distance in y axis; Dz, distance in z axis.
provides the 3D space between the die and the coping and represents the available cement space. This siliconecoated die was mounted on the original stone base and scanned with the laser scanner. Software for measuring (Geomagic Qualify; Geomagic) was used to superimpose the 2 scans and to compare the surfaces of the epoxy dies with the silicone-coated dies, using the base of the die as a reference. From the 3D representations, the marginal discrepancies and internal fit were measured at multiple locations (Fig. 1). The marginal opening was determined at the point closest to the coping margin, and the die was measured in 4 locations: mid-buccal (mB), mid-mesial (mM), mid-distal (mD), and mid-lingual (mL). The group variable edge represents the mean of these 4 measurement locations. The occlusal adaptation, which was measured from the occlusal/incisal surface of the die to the internal surface of the coping at the mid-point from buccal to lingual and mesial to distal, was measured at 1 location and referred to as mid-occlusal (mO). All measurements were made by 1 operator (D.C.). Means and standard deviations (SD) were calculated for edge (marginal adaptation) and mO for all 4 groups. A 2-sample t test was used to detect differences among groups. A regression analysis evaluated the interaction between the variables mO and edge with software (StatView v5.0; SAS Institute, Inc). RESULTS Descriptive data for mean ±SD marginal opening for all 4 groups are shown in the bar graph of Figure 2. Mean ±SD mO gap dimensions are shown in Figure 3. Significantly smaller mean marginal openings (P=.017) were observed overall for the chamfer marginal design (anterior chamfer: 61 ±41 mm; posterior chamfer: 52 ±27 mm) than for the shoulder design (anterior shoulder: 103 ±49 mm; posterior shoulder: 113 ±110 mm). The anterior chamfer had a statistically significant (P=.055) smaller THE JOURNAL OF PROSTHETIC DENTISTRY
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Co-Cr Copings Figure 2. Mean marginal opening of milled Co-Cr copings.
mean marginal opening than the anterior shoulder. No statistically significant differences (P=.119) were found between the posterior chamfer and posterior shoulder. The internal adaptations at the mO location were not significantly different among the 4 groups (P>.05). A regression analysis demonstrated a strong correlation (R=.842; P<.001) between the variables occlusal seat and marginal opening, with the smaller mean marginal opening of the chamfer design coinciding with the smaller occlusal seat values (anterior chamfer: 61 mm; mO: 182 mm; posterior chamfer: 52 mm; mO: 172 mm; anterior shoulder: 103 mm; mO: 235 mm; and posterior shoulder: 113 mm; mO: 242 mm). DISCUSSION The mean marginal openings of the milled Co-Cr copings studied in this investigation were within the range (52 to 113 mm) considered clinically acceptable by most reports in the dental studies (<120 mm).39 The hypothesis that no differences in marginal and internal adaptation of the milled Co-Cr copings would be found among the 4 tested groups was rejected for marginal fit. In the present study, a significantly (P=.017) improved marginal overall adaptation was noted for the chamfer marginal design compared to that of the shoulder. Many studies have looked at the influence of finish-line geometry on the fit of cemented crowns, with conflicting results.29-31 Current research suggests that the expectation of improved fit is not associated with specific finish-line geometries.47 The results for better mean marginal adaptation for chamfer versus shoulder in both anterior and posterior groups may be explained as a result of the scanning, software, and/or milling capabilities of the system used. The scanner or milling unit used in the present investigation may be limited in its ability to reach the depths required of the master die in the shoulder design, or limitations may exist in the software for identifying the margins. Accordingly, the THE JOURNAL OF PROSTHETIC DENTISTRY
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Co-Cr Copings Figure 3. Mean mid-occlusal internal gap measurement of milled Co-Cr copings.
manufacturer’s clinical instruction manual recommends a chamfer finish line with rounded internal line angles; thus, given the current trend toward increased application of CAD/CAM fabrication techniques for complete coverage crown restorations, a clinician would be wise to choose this margin design when choosing this fabrication technique for metal ceramic crowns. Kokubo et al33 used the GN-I CAD/CAM system to produce InCeram crowns when marginal and internal gaps were evaluated and stated that the manufacturer recommended a chamfer design because it is “difficult to mill a complicated curved surface that includes both concave and convex areas.” Hydrodynamic forces that occur during cementation dictate that an internal space be available to facilitate seating. Several studies have shown that the seating of crowns is greatly improved by providing internal relief.25-28 However, the film thickness should be maintained at an optimal level because it can affect the clinical performance and longevity of the restoration. An increase in cement thickness decreases the tensile bond strength of cement to cast metal,38 and the fracture strength of crowns has been shown to be highest when the cement thickness is lowest.36,37 The current investigation using Nobel Biocare’s NobelProcera system to mill Co-Cr copings for metal ceramic restorations did not allow for the size of the cement space to be modified, nor is the actual value of this space specified by the manufacturer. It was decided to measure the internal fit discrepancy at a point midway between the buccal and lingual and mesial and distal surfaces, referred to mO. The occlusal seat has been related to the marginal seal, and ideally, the space at the margin and the occlusal portion of the preparation after cementation should be identical.28,29 The amount of incomplete seating of a cemented crown is thus theoretically equal to the thickness of the cement at the Kane et al
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occlusal surface.28 In the present investigation, no significant differences (P>.05) were found in mO values among the 4 tested groups; however, the best marginal seal tended to coincide with the smallest occlusal seat for the chamfer design. These results are contrary to the findings of Bindl and Mormann,35 who concluded that a large internal gap width may have favored the smaller marginal gap dimensions in their study when they compared different CAD/CAM systems to fabricate ceramic crowns. They reported marginal gap widths of 17 ±16 mm compared with a mid-orobuccal space of 136 ±68 mm for the Procera system. An average occlusal space misfit between 100 and 200 mm is often reported in the dental studies,37 and the results of the present investigation are in this range. Nakamura et al34 looked at the marginal and internal fit of Cerec 3 CAD/CAM crowns and demonstrated that the mean marginal openings were significantly greater when the luting space was set at 10 mm (95 to 108 mm) than at 30 or 50 mm (53-67 mm). KoKubo et al33 investigated InCeram copings produced by CAD/CAM technology and noted that, if the predetermined cement space was smaller than 50 mm, the marginal gaps increased significantly. The authors explained this might be caused by “axial wall space” contacting the abutment tooth. In the present study, axial wall space was not included in the internal discrepancy measurements, only the occlusal seat was considered. This decision was based on the assumption that the occlusal seat was the most important predictor of marginal seal and the fact that the manufacturer apparently allows for a uniform cement space in the design software. The present study demonstrated both the highest occlusal seat of the posterior shoulder at 242 mm along with the largest marginal opening value at 113 mm. This finding suggests possible axial wall binding, which might have prevented further seating of the coping. Further investigations should include more data point analysis beyond that measured in our study to investigate whether impingement actually prevented further seating. Unlike the system used in this study, the Dentronic CAD/CAM system used in a study by Coli et al32 allowed the programming of a predetermined cement space thickness. Those investigators found that, generally, the space along the axial walls was smaller than that along the occlusal walls; a uniform thickness was not found everywhere. White et al22 defined a phenomenon which they called tilting, which refers to the binding of the casting against the axial wall of the tooth preparation. Tilting prevents seating of the binding side but adapts the margin better on the opposite side.22 Inaccurate placement of the coping to the master die can be a source of error because only 1 position will ultimately give an optimal fit. A minimal rotation of the coping on the die may result in an increased discrepancy on 1 side and a Kane et al
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smaller on another side; this is 1 possible source of error in the present investigation. Further investigation is warranted to determine whether the cement space allocated by the software used in the Nobel Biocare NobelProcera system is truly a uniform space. It is reasonable to assume that the optimum cement space present under a restoration would be the smallest that allowed for the best marginal fit26 without compromising strength or retention. In this study, a light-bodied polyvinyl siloxane was used to simulate the luting agent and to represent the cement space.39,50 The film thickness of the cement is 1 of the many factors that greatly influence adaptation of the restoration to the tooth. The film thickness of the light-bodied polyvinyl siloxane used in this investigation is unknown relative to the that of commonly used cements. McLean and von Fraunhofer39 measured a film thickness of 22 mm with a polyether compared with 20 mm with zinc phosphate cement when estimating the cement film thickness in their study. This is close to the ADA Specification no. 96 of 25 mm for a waterbased cement.40 The measured values only approximate the true or absolute value of the marginal and internal adaptation of the milled Co-Cr copings produced in this study. Further investigations to evaluate other factors involved in cementation technique, such as seating pressure, temperature, humidity, and their effect on marginal and internal adaptation, are warranted. CONCLUSIONS Within the limitations of this in vitro study, the milled Co-Cr copings produced with the Nobel Biocare NobelProcera CAD/CAM system demonstrated clinically acceptable marginal fit in the range of 52 to 113 mm before ceramic application. REFERENCES 1. Baran GR. Selection criteria for base metal alloys for use with porcelains. Dent Clin North Am 1985;29:779-87. 2. Kelly JR, Rose TC. Nonprecious alloys for use in fixed prosthodontics: a literature review. J Prosthet Dent 1983;49:363-70. 3. Bezzon OL, de Mattos Mda G, Ribero RF, Rollo JM. Effect of beryllium on the castability and resistance of ceramometal bonds in nickel-chromium alloys. J Prosthet Dent 1998;80:570-4. 4. Khamis E, Seddik M. Corrosion evaluation of recasting non-precious dental cast alloys. Int Dent J 1995;45:209-17. 5. Baran GR. The metallurgy of Ni-Cr alloys for fixed prosthodontics. J Prosthet Dent 1983;50:639-50. 6. Carter TJ, Kidd JN. The precision casting of cobalt-chromium alloy. 1. The influence of casting variables on dimensions and finish. Br Dent J 1965;118: 383-90. 7. Strandman E, Lockowandt P. An equipment for standardized casting of Co-Cr alloys in dentistry. Odontol Rev 1976;27:145-54. 8. Vermilyea SG, Kuffler MJ, Tamura JJ. Techniques for the accurate casting of base-metal alloys. Mil Med 1983;148:354-7. 9. Carreiro Ada F, Ribero RF, Mattos Mda G, Rodrigues RC. Evaluation of the castability of a Co-Cr-Mo-W alloy varying the investing technique. Braz Dent J 2005;16:50-5. 10. Quante K, Ludwig K, Kern M. Marginal and internal fit of metal-ceramic crowns fabricated with a new laser melting technology. Dent Mater 2008;24: 1311-5.
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11. Ucar Y, Akova T, Akyil MS, Brantley WA. Internal fit evaluation of crowns prepared using a new dental crown fabrication technique: laser-sintered CoCr crowns. J Prosthet Dent 2009;102:253-9. 12. Dan Xu, Nan Xiang, Bin Wei. The marginal fit of selective laser meltingfabricated metal crowns: an in-vitro study. J Prosthet Dent 2014;112:1437-40. 13. Moore JA, Barghi N, Bruki CE, Kaiser DA. Marginal distortion of cast restorations induced by cementation. J Prosthet Dent 1985;54:336-40. 14. Hung SH, Hung KS, Eick JD, Chappell RP. Marginal fit of porcelain-fusedto-metal and two types of ceramic crown. J Prosthet Dent 1990;63:26-31. 15. Biscaro L, Bonfiglioli R, Soattin M, Vigolo P. An In vivo evaluation of fit of zirconium-oxide based ceramic single crowns, generated with two CAD/ CAM systems, in comparison to metal ceramic single crowns. J Prosthodont 2013;22:36-41. 16. Sorensen JA. A rationale for comparison of plaque-retaining properties of crown systems. J Prosthet Dent 1989;62:264-9. 17. Felton DA, Kanoy BE, Bayne SC, Wirthman GP. Effect of in vivo crown margin discrepancies on periodontal health. J Prosthet Dent 1991;65:357-64. 18. Grasso JE, Nalbandian J, Sanford C, Bailit H. Effect of restoration quality on periodontal health. J Prosthet Dent 1985;53:14-9. 19. Walton JN, Gardner FM, Agar JR. A survey of crown and fixed partial denture failures: length of service and reasons for replacement. J Prosthet Dent 1986;56:416-21. 20. Holmes JR, Bayne SC, Holland GA, Sulik WD. Considerations in measurement of marginal fit. J Prosthet Dent 1989;62:405-8. 21. Fusayama T, Ide K, Kurosu A, Hosoda H. Cement thickness between cast restorations and preparation walls. J Prosthet Dent 1963;13:354-64. 22. White SN, Kipnis V. The three-dimensional effects of adjustment and cementation on crown seating. Int J Prosthodont 1993;6:248-54. 23. Jorgensen KD. Factors affecting the film thickness of zinc phosphate cements. Acta Odontol Scand 1960;18:189. 24. Pascoe DF. Analysis of the geometry of finishing lines for full crown restorations. J Prosthet Dent 1978;40:157-62. 25. Eames WB. Techniques to improve the seating of castings. Quintessence Dent Technol 1981;5:437-41. 26. Grajower R, Zuberi Y, Lewinstein I. Improving the fit of crowns with die spacers. J of Prosthet Dent 1989;61:555-63. 27. Ishikiriama A, de Freitas Oliveira J, Vieira DF, Mondelli J. Influence of some factors on the fit of cemented crowns. J Prosthet Dent 1981;45:400-4. 28. Pilo R, Cardash HS, Baharav H, Helft M. Incomplete seating of cemented crowns: A literature review. J Prosthet Dent 1988;59:429-33. 29. Gavelis JR, Morency JD, Riley ED, Sozio RB. The effect of various finish line preparations on the marginal seal and occlusal seat of full crown preparations 1981. J Prosthet Dent 2004;92:1-7. 30. Byrne G. Influence of finish-line form on crown cementation. Int J Prosthodont 1992;5:137-44. 31. Shillingburg HT, Hobo S Jr, Fisher DW. Preparation design and margin distortion in porcelain-fused-to-metal restorations. J Prosthet Dent 1973;29: 276-84. 32. Coli P, Karlsson S. Precision of a CAD/CAM technique for the production of zirconium dioxide copings. Int J Prosthodont 2004;17:577-80. 33. Kokubo Y, Nagayama Y, Tsumita M, Ohkubo C, Fukushima S, Vult von Steyern P. Clinical marginal and internal gaps of In-Ceram crowns fabricated using the GN-I system. J Oral Rehabil 2005;32:753-8. 34. Nakamura T, Nobuyoshi D, Kojima T, Wakabayashi K. Marginal and internal fit of cerec 3 CAD/CAM all-ceramic crowns. Int J Prosthodont 2003;16:244-8.
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35. Bindl A, Mormann WH. Marginal and internal fit of all-ceramic CAD/CAM crown-copings on chamfer preparations. J Oral Rehabil 2005;32:441-7. 36. Tuntiprawon M, Wilson PR. The effect of cement thickness on the fracture strength of all-ceramic crowns. Aust Dent J 1995;40:17-21. 37. May GL, Kelly RJ, Bottino MA, Hill T. Effects of cement thickness and bonding on the failure loads of CAD/CAM ceramic crowns: multi-physics FEA modeling and monotonic testing. Dent Mater 2012;28:99-109. 38. Rosenstiel SF, Land MF, Crispin BJ. Dental luting agents: a review of the current literature. J Prosthet Dent 1998;80:280-301. 39. McLean JW, von Fraunhofer JA. The estimation of cement film thickness by an in vivo technique. Br Dent J 1971;131:107-11. 40. ANSI/ADA. ANSI/ADA Specification No. 96dDental Water-based Cements: 2012. 41. Sorensen JA. A standardized method for determination of crown margin fidelity. J Prosthet Dent 1990;64:18-24. 42. Nawafleh NA, Mack F, Evans J, Mackay J, Hatamleh MM. Accuracy and reliability of methods to measure marginal adaptation of crowns and FDP’s: a literature review. J Prosthodont 2013;22:419-28. 43. Ural C, Burgaz Y, Sarac D. In vitro evaluation of marginal adaptation in five ceramic restoration fabricating techniques. Quintessence Int 2010;41:585-90. 44. Gonzalo E, Suarez MJ, Serrano B, Lozano J. A comparison of the marginal vertical discrepancies of zirconium and metal ceramic posterior fixed dental prostheses before and after cementation. J Prosthet Dent 2009;102:378-84. 45. Baig MR, Tan KB, Nicholls JI. Evaluation of the marginal fit of a zirconia ceramic computer-aided machined (CAM) crown system. J Prosthet Dent 2010;104:216-27. 46. Quintas AF, Oliveira F, Bottino MA. Vertical marginal discrepancy of ceramic copings with different ceramic materials, finish lines, and luting agents: an invitro evaluation. J Prosthet Dent 2004;92:250-7. 47. Goodacre CJ, Campagni WV, Aquilino SA. Tooth preparations for complete crowns: An art form based on scientific principals. J Prosthet Dent 2001;85: 363-76. 48. Lee KB, Park CW, Kim KH, Kwon TY. Marginal and internal fit of all-ceramic crowns fabricated with two different CAD/CAM systems. Dent Mater J 2008;27:422-6. 49. Moser BK, Stevens GR, Watts CL. The two-sample t-test versus Satterthwaite’s approximate f test. Commun Statist-theory Meth 1989;18: 3963-75. 50. Frannson B, Oilo G, Gjeitanger R. The fit of metal-ceramic crowns, a clinical study. Dent Mater 1985;1:197-9. Corresponding author: Dr Lisa Kane Division of Prosthodontics Department of Biologic and Materials Sciences University of Michigan School of Dentistry 1011 N. University Ann Arbor, MI 48109 Email: [email protected] Acknowledgment The authors thank Dr Rui-Feng Wang for assistance in the preparation of measurement data and performance of statistical tests used in this investigation. Copyright © 2015 by the Editorial Council for The Journal of Prosthetic Dentistry.
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