- Email: [email protected]
CAM crowns based on the presence or absence of common preparation errors
Predicting marginal fit of CAD/CAM crowns based on the presence or absence of common preparation errors Walter Renne, DMD,a Samuel T. McGill, DMD,b Kaitlyn VanSickle Forshee,c Michael R. DeFee,d and Anthony S. Mennito, DMDe Medical University of South Carolina College of Dental Medicine, Charleston, SC Statement of problem. Confusion exists as to what constitutes an ideal ceramic crown preparation and whether certain deviations from the ideal can affect the marginal fit of the milled restoration. Purpose. This study evaluated the marginal gap of E4D crowns fabricated on preparations completed by clinicians with varying levels of expertise to identify whether common errors affect marginal fit. Material and methods. The fit of 75 crowns fabricated with the E4D system on preparations of varying quality were examined for marginal fit by using the replica technique. These same preparations were then visually examined for common criteria for ceramic restorations and placed in one of 3 categories: excellent, fair, or poor. These visual examinations sought the presence of common preparation errors, particularly those involving the finish line. The average marginal gap values and standard deviations were calculated for each category, and the Kruskal-Wallis test was used to determine significance. Results. The results showed a statistically significant correlation between the marginal fit of a CAD/CAM fabricated crown and the quality of the preparation. The mean marginal gap of the crowns fabricated on ideal preparations was 38.5 µm, those considered fair had a mean marginal gap of 58.3 µm, while those categorized as poor averaged 90.1 µm. The fit differences among all 3 groups were statistically significant (P<.05). Conclusions. Within the limitations of this in vitro study, it can be concluded that preparation quality has a significant impact on marginal gap on crowns fabricated with a CAD/CAM system. (J Prosthet Dent 2012;108:310-315)
Clinical Implications
Research evidence shows that common preparation errors, when present, lead to poorer fitting restorations. Knowing what these common preparation errors are, and being able to recognize and eliminate them, will allow clinicians to improve the fit of their CAD/ CAM restorations. Dentistry is in the middle of a digital revolution. Recently 2 new chairside computer-aided design/computer-aided manufacturing (CAD/CAM) systems have been introduced: CEREC AC with Bluecam (Sirona, Bensheim, Germany) and E4D (D4D, Richard-
son, Texas). Both systems allow clinicians to fabricate restorations in 1 visit with intraoral digital impressions and in-office milling. Chairside CAD/ CAM systems simplify the fabrication of esthetic ceramic restorations.1-3 Despite the increasing popular-
ity of the current CAD/CAM laboratory systems and their continuing technical advances, some clinicians have remained reluctant to incorporate CAD/CAM technology into their practices. One of the main concerns relates to the size of the marginal gap
Assistant Professor, Department of Oral Rehabilitation. Assistant Professor, Department of Oral Rehabilitation. c Predoctoral student. d Predoctoral student. e Clinical Instructor, Department of Oral Rehabilitation. a
b
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November 2012 that occurred in many of the restorations produced by the early CEREC systems. The fit of earlier versions of CAD/CAM restorations has been criticized by some authors and clinicians.4,5 Perhaps some clinicians have injudiciously associated the poor fit of older systems with recently introduced products when, in fact, marginal adaptation has been vastly improved and can exceed that of conventional laboratory methods.6 Modern CAD/ CAM technology has been shown to allow the fabrication of better fitting partial fixed dental prostheses and implant frameworks than those made with the traditional lost wax techniques.7,8 It has been reported that the CEREC system yields reliable internal and marginal fit.9-12 Studies agree that the fit of the CAD/CAM restoration rivals that of conventionally fabricated restorations.13,9-12 Many studies have been conducted evaluating the marginal fit of non CAD/CAM restorations, with marginal gaps ranging from 0 µm to 313 µm with a mean marginal opening of 155 µm.14,15 In an in vivo study, McLean and von Fraunhofer16 examined more than 1000 crowns after a 5-year period and concluded that a marginal opening of less than 120 µm is clinically acceptable for conventionally cemented restorations, much greater than the suggested and possibly unrealistic marginal gap of 25 µm to 40 µm for cemented restorations.17 Other studies in agreement with McLean and von Fraunhofer have also suggested acceptable clinical longevity with marginal gaps of 100 µm to 200 µm.16,18-22 A retrospective study conducted in 2003 found the longevity of 2328 chairside CEREC inlays and onlays to be 95.5% at 10 years.23 Other studies show longevity approaching that of cast gold, with a posterior failure rate of 0% to 4.4% for CAD/CAM ceramic restorations.24 With more than 20 million restorations produced with CEREC, and an additional 7.5 million each year with the new CEREC AC system, CAD/CAM technology appears to be the future of dentistry.25 As new
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systems become commercially available, it is essential to systematically and objectively evaluate the marginal fit produced by the products. Studies indicate that a better marginal fit of the restoration leads to less microleakage, periodontal disease, and recurrent caries.26,27 Most authors agree that marginal openings of less than 120 µm are in the range of clinical acceptability with regard to longevity.28,13,16 For this study, the replica technique was used to measure marginal gap.29-31 Several authors have validated this technique as an accurate method of determining marginal misfit.32,33 Tsitrou et al13 compared the film thicknesses of a light body silicone and composite resin for crown cementation. In this study the authors determined that marginal gaps obtained with resin cement were not statistically different than marginal gaps obtained with vinyl polysiloxane (VPS) impression material, further validating the use of VPS to replicate marginal adaptation. Previous studies evaluating fit determined that the marginal gap increases after cementation.34-38,11,15 In the present study marginal gaps were measured after cementation with a light body VPS material, possibly increasing the value of the true marginal gap. Confusion exists among clinicians as to what aspects of an ideal ceramic crown preparation are particularly important for milled restorations. While they may understand that certain guidelines for tooth reduction must be adhered to, fine details such as finish line design and smoothness may be overlooked. This may be because clinicians previously used the more forgiving lost wax technique to fabricate restorations. It may be important to follow preparation guidelines in instances of milled restorations. Currently, milling systems use fairly large diameter diamond rotary cutting instruments that machine every surface of the restoration. The size of these diamond rotary cutting instruments may limit the forgiveness of the mill to accurately replicate certain preparation errors.
Although numerous studies have been conducted evaluating the longevity and fit of restorations fabricated with the CAD/CAM system, little research has been published about how, if at all, preparation design affects the marginal fit of a milled restoration. It is known that current chairside CAD/CAM systems can achieve adequate marginal fit on standardized dies, although little is known about how fit is affected by the less than ideal preparations often seen clinically. The null hypothesis was that there is no difference in the mean marginal gaps between the crowns fabricated on poor preparations and those on ideal preparations with the E4D system.
MATERIAL AND METHODS Preparation of Specimens Marginal gap was defined, as previously reported by Holmes et al,39 as the perpendicular measurement from the internal surface of the crown to the preparation closest to the finish line (Fig. 1). A priori power analysis was performed by using software (GPower v3.1.3; Faul, Erdfelder, Lang & Buchner, Dusseldorf, Germany) to determine the sample size, the parameters for which were a Cohen’s effect size of 0.4, an alpha error probability of .05, and a power of .8 with 3 groups. The results of the power analysis showed that a sample size of 66 was required to obtain a natural power of 0.81. Ten crowns were fabricated on standardized dies (LR62K; Kilgore Intl, Coldwater, Mich). Sixtytwo clinicians with various degrees of clinical experience were calibrated to ideal preparation parameters through a 1-hour lecture based on the most current textbooks of Rosenstiel et al and Shillingburg et al.40,41 This ideal preparation is depicted in Figure 2 and includes the following specifications: a smooth, 1 mm heavy chamfer or modified shoulder finish line that follows the rise and fall of the gingiva, a distinct and continuous finish line
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1 Measuring vertical marginal gaps in varying types of misfit situations.
2 Description of ideal ceramic posterior crown preparation.
3 Variations in finish line designs, both acceptable and unacceptable.
4 Dotted line represents spiked margin. Cannot be replicated by conical diamond rotary cutting instrument.
void of spikes and lips (Figs. 3, 4), 6 to10 degrees combined convergence angle, a functional cusp bevel, 1.5 to 2 mm of occlusal reduction, 1 to 1.5 mm of axial reduction and an overall rounded and smooth finish. The clinicians then prepared a ceramic preparation on a maxillary right first Typodont molar (Kilgore Intl), and each die was inserted into a 200 series Typodont (Model D85SDP-200), to simulate intraoral scanning. Dies were scanned into the E4D system (D4D, Richardson, Texas), and the crowns were fabricated from e.max CAD C14 A2 blocks (Ivoclar Vivadent, Mississauga, Canada) on the E4D mill (D4D) on standard mode. Mills were calibrated and new diamond rotary cutting instruments (Two Striper diamonds; Premier Dental, Plymouth Meeting, Pa) were inserted before the study and were replaced when worn or broken. A cement spacer of 100
µm and a marginal ramp of 25 µm were used for each crown fabricated. Preparations were visually evaluated by the evaluators and categorized as excellent, fair, or poor according to the number and level of severity of deviations from the ideal. Excellent preparations had no noticeable errors in preparation design. Fair preparations had one common error present, while poor preparations had multiple visible errors. The most common errors included lipped margins, sharp cervicoaxial line angles, and beveled and/or spiked and undulating finish lines. Milling The E4D mill was used to fabricate all of the crowns used in this study. Each was made on standard mill mode, which uses the ellipsoidal diamond rotary cutting instrument to mill the intaglio surface and the
The Journal of Prosthetic Dentistry
tapered Two Striper diamond rotary cutting instrument to mill the external surface. The E4D software was used to control the cement gap and marginal ramp to standardize each restoration. The default parameters used included a 100 μm cement gap and a 250 μm marginal ramp. Replica Technique and Measurement A green light body VPS impression material (Genie; Sultan Healthcare, Englewood, NJ), was used to replicate the marginal gap. A single operator, blind to the specimen groups, injected the crowns with light-bodied VPS material and placed them on the corresponding die with finger pressure. Crowns were then loaded onto a custom device with a constant defined load of 137 N for 5 min. This force was based on 2 studies: Kogawa et al42 determined the maximal occlusal
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November 2012 green filter was used to better delineate the separation between the light body and reflective occlusal material. The digital images were captured with a digital camera (Ueye, Obersulm, Germany) connected to the microscope. Measurements were made of the marginal gap with digital software (Omnimet 8.8.1; Buehler) (Fig. 6). The software was calibrated before the start of the project and evaluated at several intervals to ensure accuracy. Eight measurements were made for each crown: 2 buccal, 2 lingual, 2 mesial, and 2 distal (Fig. 6). A separate calibrated operator repeated the measurements; therefore, a total of 16 measurements was made for each crown (8 from each operator), which were averaged to obtain the marginal gap values.
5 Replica technique.
Statistical Analysis 6 Measurement locations for replica technique.
Table I. Result of marginal gap measurements for each category Visual Rating
Excellent
Fair
Poor
Average marginal fit (µm)
38.5
58.2
90.1
Number of specimens
25
34
15
Standard deviation
9
12
23
Statistical analysis was performed with statistical software (IBM SPSS v19, IBM Corporation, Chicago, Ill). Distribution of the data necessitated the use of the Kruskal-Wallis test for determining significance among group means, and the Wilcoxon Signed Rank test with the Bonferroni correction was applied for post hoc paired comparisons.
RESULTS force in control subjects to be 338 N; and Weaver et al43 reported a mean finger pressure seating force of 78.5 N with a standard deviation of 12.75 N. It was also estimated that the force of 137 N was similar to the force exerted when closing on a cotton roll during cementation. After polymerization, excess VPS material was trimmed, and the crowns were removed from the dies. A film of green light body VPS remained attached to the Typodont tooth preparation. This film represented the misfit of the restoration and was supported with a reflective blue VPS occlusal registration material (Flexitime Bite; Heraeus Kulzer, South Bend, Ind). This material was injected into a custom fabricated tray
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and placed on the die that had the light body VPS attached to it. Once polymerized, the reflective occlusal VPS bonded to the light body VPS, creating a stable replica of the misfit (Fig. 5). It was found in a pilot study that the reflective supporting VPS delineated more distinctly between the light body and heavy body materials when viewed under the a microscope (ViewMet Inverted Laboratory Metallograph Microscope (Buehler, Lake Bluff, Ill). After the supporting material had set, each silicone replica was removed from the die and sectioned with a razor blade in 4 locations (Fig. 6). Sections were placed on the microscope and viewed at ×100 magnification. A
The measured mean thickness and standard deviations of the replica film representing the marginal gap is displayed in Table I. The mean marginal gaps of the crowns fabricated on preparations considered ideal, which included standardized dies and clinical preparations with no errors, was 38.5 μm. Those made on dies characterized as fair had an average marginal gap of 58.3 μm, while restorations fabricated on preparations categorized as poor averaged 90.1 μm. By using the Kruskal-Wallis test, a statistically significant difference of the marginal fit among the graded groups (H(2)=52.809, P<.001) was demonstrated. Post hoc paired com-
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Volume 108 Issue 5 parison with the Wilcoxon Signed Rank test and the appropriate alpha correction showed significant differences for all pair-wise comparisons: Ideal-Fair (Z=-25.057, P<.001), IdealPoor (Z=-51.033, P<.001), Fair-Poor (Z=-25.976, P<.001).
DISCUSSION The null hypothesis was rejected as a statistically significant difference was found among all groups; therefore, preparation errors had a direct effect on the mean marginal gaps of restorations. Marginal fit is an essential aspect of restoration longevity. Within the limitations of this study, certain preparation design flaws led to consistently poorer fitting restorations. These flaws included lipped margins, sharp cervicoaxial line angles, and beveled and/or spiked and undulating finish lines (Fig. 5). In general, the authors were able to predict which restorations would have the poorest fit solely on whether the preparations exhibited these flaws. While preparing the teeth for ceramic crowns, the operators could choose an 847KR .018 (Brasseler USA, Savannah, Ga) diamond rotary cutting instrument to create the modified shoulder or an 878K .018 (Brasseler USA) to create a heavy chamfer. The 847KR .018 is an ideal instrument for a ceramic finish line design since the tip of the instrument naturally creates a 1 mm rounded shoulder as shown in Figure 5. A lipped finish line is created when the clinician uses greater than half the diameter of a chamfer instrument or greater than the entire diameter of a shoulder diamond rotary cutting instrument as shown in Figure 5. One potential reason for this poorer fit has to do with the diamond rotary cutting instruments that the current CAD/CAM mills use and their inability to replicate the features caused by these preparation errors. For example, a common error in finish line design was a spike in the cavosurface margin. In this situation, the instrument that is used in standard
mode is unable to mill accurately into the sharp spike as shown in Figure 6. The E4D system does have the option of milling in detail mode with a smaller diameter conical diamond instrument, although this instrument is usually reserved for inlays, onlays, and veneers. However, this increases the milling time and risk of instrument fracture. An increasing number of laboratories are using CAD/CAM technology to fabricate restorations and may also have similar issues with poor preparations. Errors in preparation design, particularly involving the margin, are easier to cope with when a laboratory uses the lost wax technique to fabricate alloy restorations. Potential problems in the design of this study include errors in visual inspection and characterization of the preparations. Although careful examination with ×2.5 magnification loupes was conducted, it is possible that errors were overlooked or were too minute to detect under these conditions. Furthermore, when milling the restorations, variables within the mill itself, such as diamond rotary cutting instrument wear and water quality, may affect the quality of the restoration. It is worth noting that all preparations were deemed clinically acceptable by experienced clinicians. However, it is the hope of the authors that clinicians will be able to recognize and correct these common preparation errors in order to improve the overall fit of their restorations.
CONCLUSION Within the limitations of this in vitro study, it can be concluded that preparation quality has a significant effect on marginal gap when the clinician uses a chairside CAD/CAM system. It was found that common errors in preparation design have a negative impact on the mean marginal gap and that these errors can be seen with the naked eye and should, therefore, be identified and corrected. The most common errors included lipped
The Journal of Prosthetic Dentistry
finish lines and finish line spikes. These are difficult for the milling system to replicate adequately because of the size and shape of the diamond rotary cutting instrument.
REFERENCES 1. Hulterström AK, Bergman M. Polishing systems for dental ceramics. Acta Odontol Scand 1993;51:229-34. 2. Fasbinder DJ. Clinical performance of chairside CAD/CAM restorations. J Am Dent Assoc 2006;Supplement 137: 22S-31S. 3. Wittneben JG, Wright RF, Weber HP, Gallucci GO. A systematic review of the clinical performance of CAD/CAM singletooth restorations. Int J Prosthodont 2009;22:466-71. 4. Hickel R, Dasch W, Janda R, Tyas M, Anusavice K. New direct restorative materials. FDI Commission Project. Int Dent J 1998;48:3-16. 5. Bayne SC, Heymann HO. CAD/CAM in dentistry: present and future applications. Quintessence Int 1996;27:431-3. 6. Castillo de Oyague R, Sanchez-Jorge MI, Sanchex Turrion A, Monticelli F, Osoria R. Influence of CAM vs. CAD/CAM scanning methods and finish line of tooth preparations in the vertical misfit of zirconia bridge structures. Am J Dent 2009;2:79-83. 7. Gonzalo, E , Odont,a Suárez, MJ, Odont, Serrano, B, Lozano, JF. A comparison of the marginal vertical discrepancies of zirconium and metal ceramic posterior fixed dental prostheses before and after cementation. J Pros Dent 2009;6:378-84. 8. Almasri R, Drago CJ, Siegel SC, Hardigan PC. Volumetric Misfit in CAD/CAM and Cast Implant Frameworks: A university laboratory study. J Prosthodont 2011;20:267-74. 9. Nakamura T, Dei N, Kojima T, Wakabayashi K. Marginal and internal fit of Cerec 3 CAD/CAM all-ceramic crowns. Int J Prosthodont 2003;16:244-8. 10.Bindl A, Mörmann WH. Marginal and internal fit of all-ceramic CAD/CAM crowncopings on chamfer preparations. J Oral Rehabil 2005;32:441-7. 11.Quintas AF, Oliveira F, Bottino MA. Vertical marginal discrepancy of ceramic copings with different ceramic materials, finish lines, and luting agents: an in vitro evaluation. J Prosthet Dent 2004;92:250-7. 12.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. 13.Tsitrou EA, Northeast SE, van Noort R. Evaluation of the marginal fit of three margin designs of resin composite crowns using CAD/CAM. J Dent 2007;35:68-73. 14.Adair PJ, Grossmann DG. The castable ceramic crown. Int J Periodont Restor Dent 1984;4:33-45. 15.Pera P, Gilodi S, Bassi F, Carossa S. In vitro marginal adaptation of alumina porcelain ceramic crowns. J Prosthet Dent 1994;72:585-90.
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November 2012 16.McLean JW, von Fraunhofer JA. The estimation of cement film thickness by an in vivo technique. Br Dent J 1971;131:107-11. 17.American Dental Association. 1978. Guide to dental materials and devices, 8th ed. Chicago, American Dental Association, pp. 135-7. 18.Fransson B, Øilo G, Gjeitanger R. The fit of metal-ceramic crowns, a clinical study. Dent Mater 1985;1:197-9. 19.Karlsson S. The fit of Procera titanium crowns: An in vitro and clinical study. Acta Odontol Scand 1993;51:129-34. 20.In-Sung Y, Jae-Ho Y, Jai-Bong L. In vitro marginal fit of three all-ceramic crown systems. J Prosthet Dent 2003;90:459-64. 21.Boening K, Reppel PD, Walter M. Non-cast titanium restorations in fixed prosthodontics. J Oral Rehabil 1992;19:281-7. 22.May KB, Russell MM, Razzoog ME, Lang BR. Precision of fit: the Procera AllCeram crown. J Prosthet Dent 1998;80:394-404. 23.Posselt A, Kerschbaum T. Longevity of 2328 chairside Cerec inlays and onlays. Int J Comp Dent 2003;3:231-48. 24.Hickel R, Manhart J. Longevity of restorations in posterior teeth and reasons for failure. J Adhes Dent 2001;1:45-64. 25.Poticny DJ, Klim J. CAD/CAM In-office technology: innovations after 25 years for predictable, esthetic outcomes. J Amer Dent Assoc 2010;141:5S-9S. 26.Sorensen JA. A rationale for comparison of plaque-retaining properties of crown systems. J Prostet Dent 1989;62:264-9. 27.Sorensen SE, Larsen IB, Jorgensen KD. Gingival and alveolar bone reaction to marginal fit of subgingival crown margins. Scand J Dent Res 1986;94:109-14.
28.Yeo I, Yang J, Lee J. In vitro marginal fit of three all-ceramic crown systems. J Prosthet Dent 2003;5:459-464.34. Boening KW, Wolf BH, Schmidt AE, Kästner K, Walter MH. Clinical fit of Procera AllCeram crowns. J Prosthet Dent 2000;84:419-24. 29.Beschnidt SM, Strub JR. Evaluation of the marginal accuracy of different all-ceramic crown systems after simulation in the artificial mouth J Oral Rehab 1999;26: 582–93. 30.Reich S, Kappe K, Teschner H, Schmitt J. Clinical fit of four-unit zirconia posterior fixed dental prostheses. Eur J Oral Sci 2008;116:579-84. 31.Kohorst P, Brinkmann H, Li J, Borchers L, Stiesch M. Marginal accuracy of four-unit zirconia fixed dental prostheses fabricated using different computer-aided design/ computer-aided manufacturing systems. Eur J Oral Sci 2009;117:319-25. 32.Rahme HY, Tehini GE, Adib SM, Ardo AS, Rifai KT. In vitro evaluation of the “replica technique” in the measurement of the fit of Procera crowns. J Cont Dent Pract 2008;9:25-32. 33.Laurent M, Scheer P, Dejou J, Laborde G. Clinical evaluation of the marginal fit of cast crowns-validation of the silicone replica method. J Oral Rehabil 2008;35:116-22. 34.White SN, Yu Z, Tom JF, Sangsurarak S. In vivo marginal adaptation of cast crowns luted with different cements. J Prosthet Dent 1995;74:25-32. 35.White SN, Kipnis V. Effect of adhesive luting agents on the marginal seating of cast restorations. J Prosthet Dent 1993;69:28-31. 36.White SN, Yu Z, Tom JF, Sangsurasak S. In vivo microleakage of luting cements for cast crowns. J Prosthet Dent 1994;71:333-8.
37.Beschnidt SM, Strub JR. Evaluation of the marginal accuracy of different all-ceramic crown systems after simulation in the artificial mouth. J Oral Rehabil 1999;26:582-93. 38.Kern M, Schaller HG, Strub JR. Marginal fit of restorations before and after cementation in vivo. Int J Prosthet 1993;6:585- 91. 39.Holmes JR, Bayne SC, Holland GA, Sulik WD. Considerations in measurement of marginal fit. J Prosthet Dent 1989;62:405-8. 40.Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics. 4th ed. St Louis: Mosby Elsevier; 2006. p. 325-7. 41.Shillingburg HT, Hobo S, Whitsett, LD, Jacobi R, Brackett SE. Fundamentals of fixed prosthodontics. 3rd ed. Chicago: Quintessence Publishing; 1997. p. 437. 42.Kogawa EM, Calderon PS, Laurus JRP, Araujo CRP, Conti PCR. Evaluation of maximal bite force in temporomandibular disorders patients. J Oral Rehabil 2006;33:559-65. 43.Weaver JD, Johnson GH, Bales DJ. Marginal adaptation of castable ceramic crowns. J Prosthet Dent 1991;66:747-53 Corresponding author: Dr Anthony S. Mennito 173 Ashley Ave CDM BSB 335 Charleston, SC 29425 E-mail: [email protected] Copyright © 2012 by the Editorial Council for The Journal of Prosthetic Dentistry.
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Renne et al
Clinical Implications
Research evidence shows that common preparation errors, when present, lead to poorer fitting restorations. Knowing what these common preparation errors are, and being able to recognize and eliminate them, will allow clinicians to improve the fit of their CAD/ CAM restorations. Dentistry is in the middle of a digital revolution. Recently 2 new chairside computer-aided design/computer-aided manufacturing (CAD/CAM) systems have been introduced: CEREC AC with Bluecam (Sirona, Bensheim, Germany) and E4D (D4D, Richard-
son, Texas). Both systems allow clinicians to fabricate restorations in 1 visit with intraoral digital impressions and in-office milling. Chairside CAD/ CAM systems simplify the fabrication of esthetic ceramic restorations.1-3 Despite the increasing popular-
ity of the current CAD/CAM laboratory systems and their continuing technical advances, some clinicians have remained reluctant to incorporate CAD/CAM technology into their practices. One of the main concerns relates to the size of the marginal gap
Assistant Professor, Department of Oral Rehabilitation. Assistant Professor, Department of Oral Rehabilitation. c Predoctoral student. d Predoctoral student. e Clinical Instructor, Department of Oral Rehabilitation. a
b
The Journal of Prosthetic Dentistry
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November 2012 that occurred in many of the restorations produced by the early CEREC systems. The fit of earlier versions of CAD/CAM restorations has been criticized by some authors and clinicians.4,5 Perhaps some clinicians have injudiciously associated the poor fit of older systems with recently introduced products when, in fact, marginal adaptation has been vastly improved and can exceed that of conventional laboratory methods.6 Modern CAD/ CAM technology has been shown to allow the fabrication of better fitting partial fixed dental prostheses and implant frameworks than those made with the traditional lost wax techniques.7,8 It has been reported that the CEREC system yields reliable internal and marginal fit.9-12 Studies agree that the fit of the CAD/CAM restoration rivals that of conventionally fabricated restorations.13,9-12 Many studies have been conducted evaluating the marginal fit of non CAD/CAM restorations, with marginal gaps ranging from 0 µm to 313 µm with a mean marginal opening of 155 µm.14,15 In an in vivo study, McLean and von Fraunhofer16 examined more than 1000 crowns after a 5-year period and concluded that a marginal opening of less than 120 µm is clinically acceptable for conventionally cemented restorations, much greater than the suggested and possibly unrealistic marginal gap of 25 µm to 40 µm for cemented restorations.17 Other studies in agreement with McLean and von Fraunhofer have also suggested acceptable clinical longevity with marginal gaps of 100 µm to 200 µm.16,18-22 A retrospective study conducted in 2003 found the longevity of 2328 chairside CEREC inlays and onlays to be 95.5% at 10 years.23 Other studies show longevity approaching that of cast gold, with a posterior failure rate of 0% to 4.4% for CAD/CAM ceramic restorations.24 With more than 20 million restorations produced with CEREC, and an additional 7.5 million each year with the new CEREC AC system, CAD/CAM technology appears to be the future of dentistry.25 As new
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systems become commercially available, it is essential to systematically and objectively evaluate the marginal fit produced by the products. Studies indicate that a better marginal fit of the restoration leads to less microleakage, periodontal disease, and recurrent caries.26,27 Most authors agree that marginal openings of less than 120 µm are in the range of clinical acceptability with regard to longevity.28,13,16 For this study, the replica technique was used to measure marginal gap.29-31 Several authors have validated this technique as an accurate method of determining marginal misfit.32,33 Tsitrou et al13 compared the film thicknesses of a light body silicone and composite resin for crown cementation. In this study the authors determined that marginal gaps obtained with resin cement were not statistically different than marginal gaps obtained with vinyl polysiloxane (VPS) impression material, further validating the use of VPS to replicate marginal adaptation. Previous studies evaluating fit determined that the marginal gap increases after cementation.34-38,11,15 In the present study marginal gaps were measured after cementation with a light body VPS material, possibly increasing the value of the true marginal gap. Confusion exists among clinicians as to what aspects of an ideal ceramic crown preparation are particularly important for milled restorations. While they may understand that certain guidelines for tooth reduction must be adhered to, fine details such as finish line design and smoothness may be overlooked. This may be because clinicians previously used the more forgiving lost wax technique to fabricate restorations. It may be important to follow preparation guidelines in instances of milled restorations. Currently, milling systems use fairly large diameter diamond rotary cutting instruments that machine every surface of the restoration. The size of these diamond rotary cutting instruments may limit the forgiveness of the mill to accurately replicate certain preparation errors.
Although numerous studies have been conducted evaluating the longevity and fit of restorations fabricated with the CAD/CAM system, little research has been published about how, if at all, preparation design affects the marginal fit of a milled restoration. It is known that current chairside CAD/CAM systems can achieve adequate marginal fit on standardized dies, although little is known about how fit is affected by the less than ideal preparations often seen clinically. The null hypothesis was that there is no difference in the mean marginal gaps between the crowns fabricated on poor preparations and those on ideal preparations with the E4D system.
MATERIAL AND METHODS Preparation of Specimens Marginal gap was defined, as previously reported by Holmes et al,39 as the perpendicular measurement from the internal surface of the crown to the preparation closest to the finish line (Fig. 1). A priori power analysis was performed by using software (GPower v3.1.3; Faul, Erdfelder, Lang & Buchner, Dusseldorf, Germany) to determine the sample size, the parameters for which were a Cohen’s effect size of 0.4, an alpha error probability of .05, and a power of .8 with 3 groups. The results of the power analysis showed that a sample size of 66 was required to obtain a natural power of 0.81. Ten crowns were fabricated on standardized dies (LR62K; Kilgore Intl, Coldwater, Mich). Sixtytwo clinicians with various degrees of clinical experience were calibrated to ideal preparation parameters through a 1-hour lecture based on the most current textbooks of Rosenstiel et al and Shillingburg et al.40,41 This ideal preparation is depicted in Figure 2 and includes the following specifications: a smooth, 1 mm heavy chamfer or modified shoulder finish line that follows the rise and fall of the gingiva, a distinct and continuous finish line
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Volume 108 Issue 5
1 Measuring vertical marginal gaps in varying types of misfit situations.
2 Description of ideal ceramic posterior crown preparation.
3 Variations in finish line designs, both acceptable and unacceptable.
4 Dotted line represents spiked margin. Cannot be replicated by conical diamond rotary cutting instrument.
void of spikes and lips (Figs. 3, 4), 6 to10 degrees combined convergence angle, a functional cusp bevel, 1.5 to 2 mm of occlusal reduction, 1 to 1.5 mm of axial reduction and an overall rounded and smooth finish. The clinicians then prepared a ceramic preparation on a maxillary right first Typodont molar (Kilgore Intl), and each die was inserted into a 200 series Typodont (Model D85SDP-200), to simulate intraoral scanning. Dies were scanned into the E4D system (D4D, Richardson, Texas), and the crowns were fabricated from e.max CAD C14 A2 blocks (Ivoclar Vivadent, Mississauga, Canada) on the E4D mill (D4D) on standard mode. Mills were calibrated and new diamond rotary cutting instruments (Two Striper diamonds; Premier Dental, Plymouth Meeting, Pa) were inserted before the study and were replaced when worn or broken. A cement spacer of 100
µm and a marginal ramp of 25 µm were used for each crown fabricated. Preparations were visually evaluated by the evaluators and categorized as excellent, fair, or poor according to the number and level of severity of deviations from the ideal. Excellent preparations had no noticeable errors in preparation design. Fair preparations had one common error present, while poor preparations had multiple visible errors. The most common errors included lipped margins, sharp cervicoaxial line angles, and beveled and/or spiked and undulating finish lines. Milling The E4D mill was used to fabricate all of the crowns used in this study. Each was made on standard mill mode, which uses the ellipsoidal diamond rotary cutting instrument to mill the intaglio surface and the
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tapered Two Striper diamond rotary cutting instrument to mill the external surface. The E4D software was used to control the cement gap and marginal ramp to standardize each restoration. The default parameters used included a 100 μm cement gap and a 250 μm marginal ramp. Replica Technique and Measurement A green light body VPS impression material (Genie; Sultan Healthcare, Englewood, NJ), was used to replicate the marginal gap. A single operator, blind to the specimen groups, injected the crowns with light-bodied VPS material and placed them on the corresponding die with finger pressure. Crowns were then loaded onto a custom device with a constant defined load of 137 N for 5 min. This force was based on 2 studies: Kogawa et al42 determined the maximal occlusal
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November 2012 green filter was used to better delineate the separation between the light body and reflective occlusal material. The digital images were captured with a digital camera (Ueye, Obersulm, Germany) connected to the microscope. Measurements were made of the marginal gap with digital software (Omnimet 8.8.1; Buehler) (Fig. 6). The software was calibrated before the start of the project and evaluated at several intervals to ensure accuracy. Eight measurements were made for each crown: 2 buccal, 2 lingual, 2 mesial, and 2 distal (Fig. 6). A separate calibrated operator repeated the measurements; therefore, a total of 16 measurements was made for each crown (8 from each operator), which were averaged to obtain the marginal gap values.
5 Replica technique.
Statistical Analysis 6 Measurement locations for replica technique.
Table I. Result of marginal gap measurements for each category Visual Rating
Excellent
Fair
Poor
Average marginal fit (µm)
38.5
58.2
90.1
Number of specimens
25
34
15
Standard deviation
9
12
23
Statistical analysis was performed with statistical software (IBM SPSS v19, IBM Corporation, Chicago, Ill). Distribution of the data necessitated the use of the Kruskal-Wallis test for determining significance among group means, and the Wilcoxon Signed Rank test with the Bonferroni correction was applied for post hoc paired comparisons.
RESULTS force in control subjects to be 338 N; and Weaver et al43 reported a mean finger pressure seating force of 78.5 N with a standard deviation of 12.75 N. It was also estimated that the force of 137 N was similar to the force exerted when closing on a cotton roll during cementation. After polymerization, excess VPS material was trimmed, and the crowns were removed from the dies. A film of green light body VPS remained attached to the Typodont tooth preparation. This film represented the misfit of the restoration and was supported with a reflective blue VPS occlusal registration material (Flexitime Bite; Heraeus Kulzer, South Bend, Ind). This material was injected into a custom fabricated tray
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and placed on the die that had the light body VPS attached to it. Once polymerized, the reflective occlusal VPS bonded to the light body VPS, creating a stable replica of the misfit (Fig. 5). It was found in a pilot study that the reflective supporting VPS delineated more distinctly between the light body and heavy body materials when viewed under the a microscope (ViewMet Inverted Laboratory Metallograph Microscope (Buehler, Lake Bluff, Ill). After the supporting material had set, each silicone replica was removed from the die and sectioned with a razor blade in 4 locations (Fig. 6). Sections were placed on the microscope and viewed at ×100 magnification. A
The measured mean thickness and standard deviations of the replica film representing the marginal gap is displayed in Table I. The mean marginal gaps of the crowns fabricated on preparations considered ideal, which included standardized dies and clinical preparations with no errors, was 38.5 μm. Those made on dies characterized as fair had an average marginal gap of 58.3 μm, while restorations fabricated on preparations categorized as poor averaged 90.1 μm. By using the Kruskal-Wallis test, a statistically significant difference of the marginal fit among the graded groups (H(2)=52.809, P<.001) was demonstrated. Post hoc paired com-
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Volume 108 Issue 5 parison with the Wilcoxon Signed Rank test and the appropriate alpha correction showed significant differences for all pair-wise comparisons: Ideal-Fair (Z=-25.057, P<.001), IdealPoor (Z=-51.033, P<.001), Fair-Poor (Z=-25.976, P<.001).
DISCUSSION The null hypothesis was rejected as a statistically significant difference was found among all groups; therefore, preparation errors had a direct effect on the mean marginal gaps of restorations. Marginal fit is an essential aspect of restoration longevity. Within the limitations of this study, certain preparation design flaws led to consistently poorer fitting restorations. These flaws included lipped margins, sharp cervicoaxial line angles, and beveled and/or spiked and undulating finish lines (Fig. 5). In general, the authors were able to predict which restorations would have the poorest fit solely on whether the preparations exhibited these flaws. While preparing the teeth for ceramic crowns, the operators could choose an 847KR .018 (Brasseler USA, Savannah, Ga) diamond rotary cutting instrument to create the modified shoulder or an 878K .018 (Brasseler USA) to create a heavy chamfer. The 847KR .018 is an ideal instrument for a ceramic finish line design since the tip of the instrument naturally creates a 1 mm rounded shoulder as shown in Figure 5. A lipped finish line is created when the clinician uses greater than half the diameter of a chamfer instrument or greater than the entire diameter of a shoulder diamond rotary cutting instrument as shown in Figure 5. One potential reason for this poorer fit has to do with the diamond rotary cutting instruments that the current CAD/CAM mills use and their inability to replicate the features caused by these preparation errors. For example, a common error in finish line design was a spike in the cavosurface margin. In this situation, the instrument that is used in standard
mode is unable to mill accurately into the sharp spike as shown in Figure 6. The E4D system does have the option of milling in detail mode with a smaller diameter conical diamond instrument, although this instrument is usually reserved for inlays, onlays, and veneers. However, this increases the milling time and risk of instrument fracture. An increasing number of laboratories are using CAD/CAM technology to fabricate restorations and may also have similar issues with poor preparations. Errors in preparation design, particularly involving the margin, are easier to cope with when a laboratory uses the lost wax technique to fabricate alloy restorations. Potential problems in the design of this study include errors in visual inspection and characterization of the preparations. Although careful examination with ×2.5 magnification loupes was conducted, it is possible that errors were overlooked or were too minute to detect under these conditions. Furthermore, when milling the restorations, variables within the mill itself, such as diamond rotary cutting instrument wear and water quality, may affect the quality of the restoration. It is worth noting that all preparations were deemed clinically acceptable by experienced clinicians. However, it is the hope of the authors that clinicians will be able to recognize and correct these common preparation errors in order to improve the overall fit of their restorations.
CONCLUSION Within the limitations of this in vitro study, it can be concluded that preparation quality has a significant effect on marginal gap when the clinician uses a chairside CAD/CAM system. It was found that common errors in preparation design have a negative impact on the mean marginal gap and that these errors can be seen with the naked eye and should, therefore, be identified and corrected. The most common errors included lipped
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finish lines and finish line spikes. These are difficult for the milling system to replicate adequately because of the size and shape of the diamond rotary cutting instrument.
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37.Beschnidt SM, Strub JR. Evaluation of the marginal accuracy of different all-ceramic crown systems after simulation in the artificial mouth. J Oral Rehabil 1999;26:582-93. 38.Kern M, Schaller HG, Strub JR. Marginal fit of restorations before and after cementation in vivo. Int J Prosthet 1993;6:585- 91. 39.Holmes JR, Bayne SC, Holland GA, Sulik WD. Considerations in measurement of marginal fit. J Prosthet Dent 1989;62:405-8. 40.Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics. 4th ed. St Louis: Mosby Elsevier; 2006. p. 325-7. 41.Shillingburg HT, Hobo S, Whitsett, LD, Jacobi R, Brackett SE. Fundamentals of fixed prosthodontics. 3rd ed. Chicago: Quintessence Publishing; 1997. p. 437. 42.Kogawa EM, Calderon PS, Laurus JRP, Araujo CRP, Conti PCR. Evaluation of maximal bite force in temporomandibular disorders patients. J Oral Rehabil 2006;33:559-65. 43.Weaver JD, Johnson GH, Bales DJ. Marginal adaptation of castable ceramic crowns. J Prosthet Dent 1991;66:747-53 Corresponding author: Dr Anthony S. Mennito 173 Ashley Ave CDM BSB 335 Charleston, SC 29425 E-mail: [email protected] Copyright © 2012 by the Editorial Council for The Journal of Prosthetic Dentistry.
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