Assessment of the Internal Fit of Lithium Disilicate Crowns Using Micro-CT David P. Alfaro, BSc, DMD, MSc, Dip Pros, N. Dorin Ruse, MSc, PhD, MCIC, FADM, Ricardo M. Carvalho, DDS, PhD, & Chris C. Wyatt, BSc, DMD, MSc, Dip Pros Faculty of Dentistry, The University of British Columbia, Vancouver, Canada
Keywords CAD/CAM; ceramic crowns; cement space. Correspondence N. Dorin Ruse, Faculty of Dentistry, The University of British Columbia, 2199 Wesbrook Mall, Vancouver BC V6T 1Z3 Canada. E-mail: [email protected] The authors deny any conflicts of interest. Accepted September 15, 2014 doi: 10.1111/jopr.12274
Abstract Purpose: The aim of this study was to compare the internal fit of lithium disilicate crowns fabricated using digital technology with those fabricated by conventional means. Materials and Methods: Forty-five lithium disilicate crowns were fabricated: 15 using digital impression and computer-aided design/computer-aided machining technique (group 1), 15 from the same digital impressions, but using a conventional die and laboratory fabrication process (group 2), and 15 using a conventional poly (vinyl siloxane) (PVS) impression and laboratory fabrication process (group 3). Tooth #15 was prepared for all-ceramic restoration on an ivorine typodont, which was digitized and a replica milled in zirconia to serve as master model. The master zirconia model was used for the impression procedures. Duplicate dies of the master zirconia die were made in polyurethane, enabling the internal fit of each crown to be evaluated using X-ray microcomputed tomography. The total volume of the internal space between the crown and die, the mean and maximum thickness of this space, and the percentage of the space that was at or below 120 μm thickness was calculated for each group and statistically tested for significant difference using one-way ANOVA, with post hoc Scheff´e analysis. Results: Group 1 crowns resulted in a smaller volume of internal space (12.49 ± 1.50 mm3 ) compared to group 2 (15.40 ± 2.59 mm3 ) and to those of group 3 (18.01 ± 2.44 mm3 ). The mean thickness of the internal space for group 1 (0.16 ± 0.01 mm) and for group 2 (0.17 ± 0.03 mm) was significantly lower than that of group 3 (0.21 ± 0.03 mm). The average percentage of the internal space of a thickness of 120 μm and below was different between the three groups: 46.73 ± 5.66% for group 1, 37.08 ± 17.69% for group 2, and 22.89 ± 9.72% for group 3. Three-dimensional renderings of the internal space were also created. Conclusions: The results of this study suggested that pressed and milled IPS e. max crowns from LAVA COS digital impressions had a better internal fit to the prepared tooth than pressed IPS e.max crowns from PVS impressions in terms of total volume of internal space, average thickness of internal space, and percentage of internal space at or below 120 μm.
The use of dental laboratory fabricated crowns to restore structurally weak or fractured teeth has been the standard of care for decades. The process, which involves taking an intraoral impression, making a gypsum cast, articulation, sectioning of dies, waxing and casting/pressing the crown, is subject to errors at each step, with the end result being a potentially unacceptable fit of the crown to the tooth. The introduction of digital technology has revolutionized dentistry, now allowing for the digital impression of the tooth preparation, along with computer-aided design (CAD) and computer-aided manufacturing (CAM) of
dental restorations, therefore decreasing the number of steps and potential sources of error. The first commercially available system for direct digital R impressions, CEREC 1 (Sirona Dental Systems, Charlotte, NC), was introduced over 25 years ago and is now in its fourth generation.1 The LAVA Chairside Oral Scanner (LAVA COS; 3M ESPE, Lexington, MA) has been available since 2008, and can be used for both the impression of abutments for indirect restorations and for diagnostic casts,1 unlike the CEREC system, which is primarily used for single-tooth indirect
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restorations. The LAVA COS system uses active wavefront sampling to capture information in video format. It is one of the fastest systems on the market, capturing 20 3-D images per second.2 Other systems, such as the CEREC system, use a series of still images to capture the impression. Digital impressions allow for the fabrication of accurate and durable dies, fabricated either by milling of polyurethane blocks or via stereolithography (SLA).3 Virtual casts from digital impressions have been evaluated, with no difference being found between the virtual cast and the original scanned model,4 allowing for restorations to be designed digitally on these virtual models and fabricated via CAM technology. The CAM technology comes in a range of options, from compact in-office machines3 to larger in-laboratory ones,5 both having demonstrated acceptable clinical accuracy.6,7 The internal adaptation of the crown to the tooth plays a significant role in the success of the restoration. For dental laboratory fabricated crowns, a die spacer (paint or foil8 ) is used to provide space for cement, in an attempt to improve retention and fit. The use of die spacer has been found to improve seating of cemented castings by almost 100 μm (from 143 to 45 μm) and to improve the post cementation retention of these crowns by 25%.9 The improvement in the seating of castings is greatest when the die spacer is applied over the entire die to within 0.5 mm of the margin, in comparison to techniques that paint the die spacer only in the occlusal 1/3-2/3 area.10 It has been recommended that the die spacer be a thickness in the range of 25 to 40 μm, to match the film thickness afforded by different luting cements.11,12 The use of die spacer less than 30 μm thick can result in the incomplete seating of the prosthesis during cementation.13 In addition to affecting the fit of the prosthesis, the die spacer and the resulting cement thickness may affect the physical properties of the restoration. Porcelain veneers bonded with resin cement were found to have maximum bond strengths with a die spacer thickness of 12.8 μm and decreased bonds if the space was thinner or thicker.14 Fractures of cement and porcelain have been found to increase with cement thicknesses over 100 μm, with optimal cement properties found to occur between 50 and 100 μm for resin cement.15 Digital systems allow for the incorporation of a digital die spacer into the milling process, which should be consistent and accurate. The internal gap, which has been defined as the perpendicular distance from the surface of the axial wall of a preparation to the internal surface of the casting,16 has been evaluated using various techniques. Unfortunately, this measurement has primarily been limited to analysis in a single dimension17 and thus, the results can only be used to describe the cement thickness at specific locations underneath crowns. Only a few studies have looked at the volume of the cement space by investigating the 3-D architecture of the internal space between an indirect restoration and its abutment. Therefore, strong evidence on the role of the volume and distribution of the cement layer on the success of these restorations can only be inferred from the prevalent cement thickness studies. X-ray microcomputed tomography (μCT) analysis has been used to assess the internal space of 3-unit fixed dental prostheses18 and of onlays19 but, to date, there are no published articles on the evaluation of the internal fit of full coverage ceramic crowns. The aim 2
of this study was, therefore, to evaluate the 3-D internal fit of ceramic crowns, fabricated from elastomeric impressions and digital impressions, using μCT, with the hypothesis that there would be no significant difference between the different fabrication procedures.
Materials and methods Tooth #15 was prepared for a ceramic crown on an ivorine typodont (Frasaco USA, Greenville, NC) as recommended by the manufacturer (Ivoclar Vivadent, Schaan, Lichtenstein) for full contour crowns, using either IPS e.max CAD or IPS e.max Press ceramics. Both arches of the typodont were then digitized using a 3Shape D700 lab scanner (3Shape Inc., New Jersey, NY), and duplicates were milled from a monolithic block of zirconia (Wieland Dental, Schwenninger, Germany) in a fiveaxis milling machine (DMG/Mori Seiki, Cypress, CA). The resultant zirconia models were to-scale replicas of the original ivorine typodont and included a removable zirconia die of the prepared #15. These zirconia models were then impressed using either a traditional (elastomeric) or digital (LAVA COS) technique. The traditional impressions were secured using a single-step dual viscosity poly(vinyl siloxane) (PVS) impression (Aquasil Ultra; Dentsply Canada, Woodbridge, Canada) in a custom tray, which was fabricated in acrylic resin (Ivolen; Ivoclar Vivadent) and R painted with adhesive (Caulk Tray Adhesive; Dentsply Caulk, Milford, DE). To standardize the custom trays, a single 3-mm thick master tray was fabricated with a 2-mm wax spacer,20 and then 15 copies of this master tray were made using a denture duplication flask and Ivolen resin. The opposing master arch was impressed in alginate, using stock plastic trays (Kromopan 100; LASCOD, Firenze, Italy). In total, 15 sets of impressions were made and poured with Type IV stone (Silky Rock; Whip Mix Corporation, Louisville, KY), and 15 e.max Press crowns (Ivoclar Vivadent) were then fabricated via the traditional lostwax technique for tooth #15 for each cast. LAVA COS impressions of the zirconia models were taken as per the manufacturer’s instructions. The scanning process involved a light powdering of the dentition with titanium dioxide powder followed by a computer-guided motion video scanning, which was displayed on a monitor. The software outlined which areas had been successfully scanned, and which areas still required further scanning. At any point in the digital impression, the scan of the preparation could be evaluated in 3-D on the monitor to ensure that it had been adequately captured prior to data transfer. A total of 15 scans of the zirconia models were taken, with each scan being used in two methods: one being for a completely digital design and manufacturing process of an IPS e.max CAD crown; and the other being for the fabrication of a polyurethane die for the creation of a traditional pressed IPS e.max Press crown. For the entirely digital process, upon completion of the maxillary, mandibular, and bite registration scans of the master zirconia models, the files were exported in .STL (Standard Tessellation Language) format for input into a digital design R workflow (Core3dcentres , Las Vegas, NV) for a full contour crown, to be five-axis milled (DMG, Bielefeld, Germany) in IPS e.max CAD. For the partially digital process, the same
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files were used for the five-axis milling of a polyurethane die, which was then used in the fabrication of an IPS e.max Press pressed crown, via the lost wax technique. Therefore, each set of digital impressions was used for both the fabrication of an entirely digitally designed and milled crown, and for a digitally impressed but traditionally fabricated pressed crown. In total, 15 milled (group 1–digital milled [DM]) and 15 pressed crowns (group 2–digital pressed [DP]) were fabricated from 15 digital impressions, in addition to the 15 pressed crowns fabricated from the traditional elastomeric impression (group 3–traditional pressed [TP]). A 20 μm virtual die spacer was prescribed for the milled crowns (DM), and a 20 μm die spacer application was prescribed for the pressed crowns (DP and TP). For all the pressed crowns (DP and TP), one layer of a latexbased die spacer (Rem-e-die; Ivoclar Vivadent) was applied by the same experienced technician to achieve the prescribed 20 μm thickness. The decision to use 20 μm in all the groups was made to limit any possible errors that may have arisen by placing two films of spacer and because, often, the digitally prescribed space is 20 μm. All 45 crowns were returned from the lab fully sintered, stained, glazed, and etched with 4.5% hydrofluoric acid. To perform the internal fit measurements using a μCT scanner (Scanco Medical μCT100, Br¨uttisellen, Switzerland), 45 duplicate dies of the master zirconia die were fabricated. For this, a single LAVA COS scan of the original zirconia die was exported for the milling of 45 duplicates in polyurethane via five-axis milling. Each crown was then seated on its respective duplicate die with finger pressure,21,22 was stabilized with utility wax,19 and scanned at a 20 μm resolution. The resultant μCT scan images were analyzed with the scanner’s proprietary software by isolating the internal space on the images. Upon the hand selection of the outline of the internal space on each slice of the scan of each crown, the images were reconstructed by the software to create a 3-D image of the internal space, complete with color-graded thickness maps. The software allowed for quantitative analysis of the internal space and yielded various data, including minimum, maximum, and mean thicknesses of the internal space, along with the total volume of the space for each crown. It was also possible to break down the distribution of the internal space by indicating what percentage of the total space was of a specific thickness, in increments of 20 μm. The exact location of the distribution of the fit was not available from the numerical data, but was qualitatively visible in the 3-D reconstructions. A sample size of 12 was calculated for a power of 0.8, using reported historical means for the internal space of lithium disilicate crowns.23 Consequently, 15 crowns per group were made, to ensure that power was not affected if any specimens were damaged or data were deemed unusable. Univariate ANOVA was used to evaluate differences between the three groups in various categories, including maximum thickness, mean thickness, and total volume of internal space. Furthermore, the percentage of the internal space that was at or below 120 μm was calculated for each crown. This value of 120 μm was selected because it correlates with recommended cement thicknesses,15 existing ranges for internal fit of lithium disilicate crowns,17 and maximum clinically acceptable values for marginal fit.24 After ANOVA, if warranted, multiple means comparisons (Scheff´e)
Table 1 Internal space dimension Digital pressed (DM)
Digital pressed (DP)
Traditional pressed (TP)
Total volume (mm3 ) 12.49 ± 1.50a 15.40 ± 2.59b 18.01 ± 2.44c Mean thickness (μm) 0.16 ± 0.01a 0.17 ± 0.03a 0.21 ± 0.03b Max thickness (μm) 0.37 ± 0.03a 0.36 ± 0.08a 0.41 ± 0.06a Percentage ࣘ 120 46.73 ± 5.66a 37.08 ± 17.69a 22.89 ± 9.72b μm∗∗ Values with same superscript letters are not significantly different within a specific measurement (row). **The percentage of the space at or below 120 μm thickness.
were performed. All analyses were performed at α = 0.05 using SPSS (SPSS for Windows, version 12.0; Chicago, IL).
Results The mean of the total volume of the internal space was calculated as 12.49 ± 1.50 mm3 for DM, 15.40 ± 2.59 mm3 for DP, and 18.01 ± 2.44 mm3 for TP (Table 1). Statistical analysis indicated significant differences between the groups and post hoc Scheff´e analysis demonstrated that there was a significant difference between the volume of the internal space for all three groups, with DM having a smaller volume than DP and TP, and DP having a smaller volume than TP (DM < DP < TP). The average percentage of the internal space of a thickness of 120 μm and below was calculated as 46.73 ± 5.66% for DM, 37.08 ± 17.69% for DP, and 22.89 ± 9.72% for TP (Table 1). Statistical analysis indicated significant differences between the groups and post hoc Scheff´e analysis demonstrated a significant difference between the averages of the percent of internal space at or below 120 μm for both of the digital crown groups in comparison to the “traditional” crowns. The two groups of digital crowns, DM and DP, were not found to be significantly different from each other, but both groups (DM, DP) had a significantly larger percentage of their internal space at or below 120 μm in comparison to TP (TP < DM = DP). The mean thickness of the internal space was calculated as 0.16 ± 0.01 mm for DM, 0.17 ± 0.03 mm for DP, and 0.21 ± 0.03 mm for TP (Table 1). Statistical analysis indicated significant differences between the groups and post hoc Scheff´e analysis demonstrated a significant difference between the average thicknesses of the internal space for both groups of the digital crowns in comparison to the “traditional” crowns. The two groups of digital crowns, DM and DP, were not found to be significantly different from each other, but both groups (DM, DP) had a significantly smaller average thickness of the internal space in comparison to TP (TP > DM = DP). The mean maximum thickness of the internal space was calculated as 0.37 ± 0.03 mm for DM, 0.36 ± 0.08 mm for DP, and 0.41 ± 0.06 mm for TP (Table 1). Statistical analysis did not identify any significant difference between the groups (DM = DP = TP). The 3-D images rendered by the μCT image analysis allowed for a qualitative analysis of the internal space of the crowns. The images could be rotated to view the internal space in any orientation, and the scales of the thickness gradient could be
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Figure 1 Sample standardized scale (460 μm maximum) 3-D reconstruction of the internal space of digital impression and CAD/CAM technique group (DM).
Figure 3 Sample standardized scale (460 μm maximum) 3-D reconstruction of the internal space of conventional PVS impression and laboratory fabrication process group (TP).
Figure 2 Sample standardized scale (460 μm maximum) 3-D reconstruction of the internal space of digital impression and conventional die group (DP).
Figure 4 Internal space of digital impression and CAD/CAM technique group (DM), scaled to demonstrate space of 120 μm and below.
altered to highlight areas of specific thicknesses. The internal space images were set to the same scale (maximum data point) to compare the distribution of the space for the three crown types. Sample images of the standardized scale images are presented in Figures 1–3. As can be seen in the reconstructions, TP visually has a larger portion of the internal space close to the maximum of the scale (460 μm), while the digital crowns have less of the space approaching the maximum. All three crown groups tended to have larger spaces at the line angles of the preparations. The 3-D reconstructions were also scaled to demonstrate where the internal space was in the range of 120 μm and 4
below, to visualize where the prosthesis would fit in this “ideal” range (Figs 4–6). The 3-D images indicated that TP was significantly poorer fitting than either of the digital crowns.
Discussion Under the experimental conditions of this study, the results showed that lithium disilicate crowns fabricated using digital impressions and CAD/CAM were superior with respect to internal fit to those fabricated using traditional techniques. Crowns fabricated based on digital impressions (DP and DM groups) demonstrated lower and more consistent values for internal space in volume, mean thickness, and better percent
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Figure 5 Internal space of digital impression and conventional die group (DP), scaled to demonstrate space of 120 μm and below.
Figure 6 Internal space of conventional PVS impression and laboratory fabrication process group (TP), scaled to demonstrate space of 120 μm and below.
clinically acceptable fit, in comparison to the TP crowns (TP group). These results support the findings of other authors who have compared digital and traditional workflows and have found that the digital workflow allows for the fabrication of restorations of equal or superior fit to those fabricated by traditional technologies.1,25 Moreover, the results obtained are in agreement with those of a study concurrent to this project, which has also demonstrated that the completely digital process had a more accurate marginal fit in comparison to traditional techniques.26 As demonstrated in the 3-D reconstructions, the milled crowns had larger internal spaces at the line angles of the preparations. This finding may be attributed to limitations in the milling accuracies due to the dimensions of the burs.27 This
discrepancy at the line angles, however, was also found for the pressed prostheses, regardless of impression technique, which was not expected due to the theoretical ability to cast the ceramic into these rounded areas. When the two digital crown groups (DM, DP) fabricated from the same digital impression were compared, the milled crowns were not any better fitting than the pressed crowns, indicating that the fabrication technique was not a major source of error. Furthermore, various studies have compared the accuracy of milled polyurethane dies and type IV stone dies and have indicated that they are comparable.4,28 Therefore, it is also unlikely that the type of working die or the type of die spacer used played a role in any fit discrepancy. The elastomeric impression is likely the cause of the poorer fit of the traditional prostheses in this project. The image analysis proved to be a time-consuming process that was not easily automated. One of the challenges that previous authors found was the presence of artifacts in the scans, which did not allow for an automated selection of the internal space.19,29 While our samples did not have very many artifacts or scatter, the software was challenged in automatically selecting contours based on contrast when the internal space became very small. This may have been a limitation of the scanning resolution. Also, the crowns were delivered after being etched with hydrofluoric acid in the laboratory, which creates micromechanical voids in the internal surface, in the range of a few μm.30 It is possible that this inherent surface roughness decreased the sharpness of the image at the internal surface of the restoration, therefore adding to the difficulty of automating the selection of the outline of the internal space. Regardless of the challenges of the technique, the 3-D analysis of the internal space is a novel method of directly evaluating the internal fit of indirect restorations and deserves further exploration for the clinical reliability of these laboratory measurements. One of the limitations of the study was the use of duplicate dies for the measurement process. This technique was selected because it allowed for all 45 crowns to have their own duplicate die of the original preparation for measurement, thus preventing damage to the original zirconia die due to repeated measurements; it also allowed for multiple specimens to be scanned at the same time. Furthermore, the difference in the radiopacity between the polyurethane die and the ceramic crown facilitated the image analysis. While the dies were not evaluated for accuracy in comparison to the original zirconia die, they were all fabricated from one digital scan in the same five-axis mill from fully polymerized polyurethane and were randomly assigned to the crowns. As such, any systematic error in the process was randomly distributed within the data set, thus not affecting the reliability of the measurement technique, but perhaps the accuracy. Validation studies of the reproducibility and accuracy of the replica die technique are in progress. The entirely digital process offers many advantages compared to the traditional techniques. The digital technique can reduce total treatment time by half, even in the hands of inexperienced users.31 The ability to have an immediate direct visualization of the prepared and unprepared teeth can be beneficial to the patient, in terms of oral health education, and to the dentist, in terms of being able to see if the preparation is adequate, especially considering that in the traditional technique,
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errors may not be seen until the impression is poured.32 Furthermore, the data file is available almost instantaneously for the lab technician to evaluate, and feedback can be given to the dentist without the delays of pouring a cast.33 Digital technology can also help reduce the environmental impact of a dental practice by reducing the waste associated with traditional impression materials,34 not requiring extra materials for remakes31 and by removing the need for disinfection and transportation of traditional impressions.25
Conclusion The results of this study suggest that pressed and milled IPS e.max crowns from LAVA COS digital impressions had a better internal fit to the prepared tooth than pressed IPS e.max crowns from PVS impressions in terms of total volume of internal space, average thickness of internal space, and percentage of internal space at or below 120 μm.
Acknowledgments The authors thank Aurum Ceramics Dental Laboratories Ltd, Vancouver, Canada, for their generous financial support and CAD/CAM milling of the master ceramic model.
References 1. Seelbach P, Brueckel C, W¨ostmann B: Accuracy of digital and conventional impression techniques and workflow. Clin Oral Invest 2013;17:1759-1764 2. Galhano GA, Pellizzer EP, Mazaro JV: Optical impression systems for CAD-CAM restorations. J Craniofac Surg 2012;23:e575-e579 3. Kachalia PR, Geissberger MJ: Dentistry a la carte: in-office CAD/CAM technology. J Calif Dent Assoc 2010;38:323-330 4. Hwang YC, Park YS, Kim HK, et al: The evaluation of working casts prepared from digital impressions. Oper Dent 2013;38:655-662 5. Miyazaki T, Hotta Y: CAD/CAM systems available for the fabrication of crown and bridge restorations. Aust Dent J 2011;56(Suppl 1):97-106 6. Akbar JH, Petrie CS, Walker MP, et al: Marginal adaptation of Cerec 3 CAD/CAM composite crowns using two different finish line preparation designs. J Prosthodont 2006;15:155-163 7. May KB, Russell MM, Razzoog ME, et al: Precision of fit: the Procera AllCeram crown. J Prosthet Dent 1998;80:394-404 8. Carter SM, Wilson PR: The effects of die-spacing on post-cementation crown elevation and retention. Aust Dent J 1997;42:192-198 9. Eames WB: Techniques to improve the seating of castings. J Am Dent Assoc 1978;96:432-437 10. Olivera AB, Saito T: The effect of die spacer on retention and fitting of complete cast crowns. J Prosthodont 2006;15:243-249 11. ISO: ISO 9917–1 Dentistry—water-based cements—Part 1: Powder/liquid acid-base cements. 2007 12. ISO: ISO 4049 Dentistry–polymer-based filling, restorative and luting materials. 2009 13. Fusayama T, Ide K, Kurosu A, et al: Cement thickness between cast restorations and preparation walls. J Prosthet Dent 1963;13:354-364 6
14. Cho SH, Chang WG, Lim BS, et al: Effect of die spacer thickness on shear bond strength of porcelain laminate veneers. J Prosthet Dent 2006;95:201-208 15. Molin MK, Karlsson SL, Kristiansen MS: Influence of film thickness on joint bend strength of a ceramic/resin composite joint. Dent Mater 1996;12:245-249 16. Conrad HJ, Seong WJ, Pesun IJ: Current ceramic materials and systems with clinical recommendations: a systematic review. J Prosthet Dent 2007;98:389-404 17. Schaefer O, Kuepper H, Sigusch BW, et al: Three-dimensional fit of lithium disilicate partial crowns in vitro. J Dent 2013;41:271-277 18. Borba M, Cesar PF, Griggs JA, et al: Adaptation of all-ceramic fixed partial dentures. Dent Mater 2011;27:1119-1126 19. Seo D, Yi Y, Roh B: The effect of preparation designs on the marginal and internal gaps in Cerec3 partial ceramic crowns. J Dent 2009;37:374-382 20. Gordon GE, Johnson GH, Drennon DG: The effect of tray selection on the accuracy of elastomeric impression materials. J Prosthet Dent 1990;63:12-15 21. Syrek A, Reich G, Ranftl D, et al: Clinical evaluation of all-ceramic crowns fabricated from intraoral digital impressions based on the principle of active wavefront sampling. J Dent 2010;38:553-559 22. Kokubo Y, Tsumita M, Kano T, et al: Clinical marginal and internal gaps of zirconia all-ceramic crowns. J Prosthodont Res 2011;55:40-43 23. Al-Rabab’ah MA, Macfarlane TV, McCord JF: Vertical marginal and internal adaptation of all-ceramic copings made by CAD/CAM technology. Eur J Prosthodont Restor Dent 2008;16:109-115 24. Mclean JW, von Fraunhofer JA: The estimation of cement film thickness by an in vivo technique. Br Dent J 1971;131:107111 25. Brawek PK, Wolfart S, Endres L, et al: The clinical accuracy of single crowns exclusively fabricated by digital workflow–the comparison of two systems. Clin Oral Investig 2013;17:2119-2125 26. Ng J, Ruse D, Wyatt C: A comparison of the marginal fit of crowns fabricated with digital and conventional methods. J Prosthet Dent 2014;112:555-560 27. Kurbad A, Reichel K: CAD/CAM-manufactured restorations made of lithium disilicate glass ceramics. Int J Comput Dent 2005;8:337-348 28. Kim SY, Kim MJ, Han JS, et al: Accuracy of dies captured by an intraoral digital impression system using parallel confocal imaging. Int J Prosthodont 2013;26:161-163 29. Rungruanganunt P, Kelly JR, Adams DJ: Two imaging techniques for 3D quantification of pre-cementation space for CAD/CAM crowns. J Dent 2010;38:995-1000 30. Zogheib LV, Bona AD, Kimpara ET, et al: Effect of hydrofluoric acid etching duration on the roughness and flexural strength of a lithium disilicate-based glass ceramic. Braz Dent J 2011;22:45-50 31. Lee SJ, Gallucci GO: Digital vs. conventional implant impressions: efficiency outcomes. Clin Oral Implants Res 2013;24:111-115 32. Wassell RW, Barker D, Walls AW: Crowns and other extra-coronal restorations: impression materials and technique. Br Dent J 2002;192:679-690 33. Craddock MR, Windhorn RJ: Is the US Army Dental Corps ready for the digital impression? US Army Med Dep J 2011;38-41 34. Christensen GJ: Impressions are changing: deciding on conventional, digital or digital plus in-office milling. J Am Dent Assoc 2009;140:1301-1304
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Keywords CAD/CAM; ceramic crowns; cement space. Correspondence N. Dorin Ruse, Faculty of Dentistry, The University of British Columbia, 2199 Wesbrook Mall, Vancouver BC V6T 1Z3 Canada. E-mail: [email protected] The authors deny any conflicts of interest. Accepted September 15, 2014 doi: 10.1111/jopr.12274
Abstract Purpose: The aim of this study was to compare the internal fit of lithium disilicate crowns fabricated using digital technology with those fabricated by conventional means. Materials and Methods: Forty-five lithium disilicate crowns were fabricated: 15 using digital impression and computer-aided design/computer-aided machining technique (group 1), 15 from the same digital impressions, but using a conventional die and laboratory fabrication process (group 2), and 15 using a conventional poly (vinyl siloxane) (PVS) impression and laboratory fabrication process (group 3). Tooth #15 was prepared for all-ceramic restoration on an ivorine typodont, which was digitized and a replica milled in zirconia to serve as master model. The master zirconia model was used for the impression procedures. Duplicate dies of the master zirconia die were made in polyurethane, enabling the internal fit of each crown to be evaluated using X-ray microcomputed tomography. The total volume of the internal space between the crown and die, the mean and maximum thickness of this space, and the percentage of the space that was at or below 120 μm thickness was calculated for each group and statistically tested for significant difference using one-way ANOVA, with post hoc Scheff´e analysis. Results: Group 1 crowns resulted in a smaller volume of internal space (12.49 ± 1.50 mm3 ) compared to group 2 (15.40 ± 2.59 mm3 ) and to those of group 3 (18.01 ± 2.44 mm3 ). The mean thickness of the internal space for group 1 (0.16 ± 0.01 mm) and for group 2 (0.17 ± 0.03 mm) was significantly lower than that of group 3 (0.21 ± 0.03 mm). The average percentage of the internal space of a thickness of 120 μm and below was different between the three groups: 46.73 ± 5.66% for group 1, 37.08 ± 17.69% for group 2, and 22.89 ± 9.72% for group 3. Three-dimensional renderings of the internal space were also created. Conclusions: The results of this study suggested that pressed and milled IPS e. max crowns from LAVA COS digital impressions had a better internal fit to the prepared tooth than pressed IPS e.max crowns from PVS impressions in terms of total volume of internal space, average thickness of internal space, and percentage of internal space at or below 120 μm.
The use of dental laboratory fabricated crowns to restore structurally weak or fractured teeth has been the standard of care for decades. The process, which involves taking an intraoral impression, making a gypsum cast, articulation, sectioning of dies, waxing and casting/pressing the crown, is subject to errors at each step, with the end result being a potentially unacceptable fit of the crown to the tooth. The introduction of digital technology has revolutionized dentistry, now allowing for the digital impression of the tooth preparation, along with computer-aided design (CAD) and computer-aided manufacturing (CAM) of
dental restorations, therefore decreasing the number of steps and potential sources of error. The first commercially available system for direct digital R impressions, CEREC 1 (Sirona Dental Systems, Charlotte, NC), was introduced over 25 years ago and is now in its fourth generation.1 The LAVA Chairside Oral Scanner (LAVA COS; 3M ESPE, Lexington, MA) has been available since 2008, and can be used for both the impression of abutments for indirect restorations and for diagnostic casts,1 unlike the CEREC system, which is primarily used for single-tooth indirect
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restorations. The LAVA COS system uses active wavefront sampling to capture information in video format. It is one of the fastest systems on the market, capturing 20 3-D images per second.2 Other systems, such as the CEREC system, use a series of still images to capture the impression. Digital impressions allow for the fabrication of accurate and durable dies, fabricated either by milling of polyurethane blocks or via stereolithography (SLA).3 Virtual casts from digital impressions have been evaluated, with no difference being found between the virtual cast and the original scanned model,4 allowing for restorations to be designed digitally on these virtual models and fabricated via CAM technology. The CAM technology comes in a range of options, from compact in-office machines3 to larger in-laboratory ones,5 both having demonstrated acceptable clinical accuracy.6,7 The internal adaptation of the crown to the tooth plays a significant role in the success of the restoration. For dental laboratory fabricated crowns, a die spacer (paint or foil8 ) is used to provide space for cement, in an attempt to improve retention and fit. The use of die spacer has been found to improve seating of cemented castings by almost 100 μm (from 143 to 45 μm) and to improve the post cementation retention of these crowns by 25%.9 The improvement in the seating of castings is greatest when the die spacer is applied over the entire die to within 0.5 mm of the margin, in comparison to techniques that paint the die spacer only in the occlusal 1/3-2/3 area.10 It has been recommended that the die spacer be a thickness in the range of 25 to 40 μm, to match the film thickness afforded by different luting cements.11,12 The use of die spacer less than 30 μm thick can result in the incomplete seating of the prosthesis during cementation.13 In addition to affecting the fit of the prosthesis, the die spacer and the resulting cement thickness may affect the physical properties of the restoration. Porcelain veneers bonded with resin cement were found to have maximum bond strengths with a die spacer thickness of 12.8 μm and decreased bonds if the space was thinner or thicker.14 Fractures of cement and porcelain have been found to increase with cement thicknesses over 100 μm, with optimal cement properties found to occur between 50 and 100 μm for resin cement.15 Digital systems allow for the incorporation of a digital die spacer into the milling process, which should be consistent and accurate. The internal gap, which has been defined as the perpendicular distance from the surface of the axial wall of a preparation to the internal surface of the casting,16 has been evaluated using various techniques. Unfortunately, this measurement has primarily been limited to analysis in a single dimension17 and thus, the results can only be used to describe the cement thickness at specific locations underneath crowns. Only a few studies have looked at the volume of the cement space by investigating the 3-D architecture of the internal space between an indirect restoration and its abutment. Therefore, strong evidence on the role of the volume and distribution of the cement layer on the success of these restorations can only be inferred from the prevalent cement thickness studies. X-ray microcomputed tomography (μCT) analysis has been used to assess the internal space of 3-unit fixed dental prostheses18 and of onlays19 but, to date, there are no published articles on the evaluation of the internal fit of full coverage ceramic crowns. The aim 2
of this study was, therefore, to evaluate the 3-D internal fit of ceramic crowns, fabricated from elastomeric impressions and digital impressions, using μCT, with the hypothesis that there would be no significant difference between the different fabrication procedures.
Materials and methods Tooth #15 was prepared for a ceramic crown on an ivorine typodont (Frasaco USA, Greenville, NC) as recommended by the manufacturer (Ivoclar Vivadent, Schaan, Lichtenstein) for full contour crowns, using either IPS e.max CAD or IPS e.max Press ceramics. Both arches of the typodont were then digitized using a 3Shape D700 lab scanner (3Shape Inc., New Jersey, NY), and duplicates were milled from a monolithic block of zirconia (Wieland Dental, Schwenninger, Germany) in a fiveaxis milling machine (DMG/Mori Seiki, Cypress, CA). The resultant zirconia models were to-scale replicas of the original ivorine typodont and included a removable zirconia die of the prepared #15. These zirconia models were then impressed using either a traditional (elastomeric) or digital (LAVA COS) technique. The traditional impressions were secured using a single-step dual viscosity poly(vinyl siloxane) (PVS) impression (Aquasil Ultra; Dentsply Canada, Woodbridge, Canada) in a custom tray, which was fabricated in acrylic resin (Ivolen; Ivoclar Vivadent) and R painted with adhesive (Caulk Tray Adhesive; Dentsply Caulk, Milford, DE). To standardize the custom trays, a single 3-mm thick master tray was fabricated with a 2-mm wax spacer,20 and then 15 copies of this master tray were made using a denture duplication flask and Ivolen resin. The opposing master arch was impressed in alginate, using stock plastic trays (Kromopan 100; LASCOD, Firenze, Italy). In total, 15 sets of impressions were made and poured with Type IV stone (Silky Rock; Whip Mix Corporation, Louisville, KY), and 15 e.max Press crowns (Ivoclar Vivadent) were then fabricated via the traditional lostwax technique for tooth #15 for each cast. LAVA COS impressions of the zirconia models were taken as per the manufacturer’s instructions. The scanning process involved a light powdering of the dentition with titanium dioxide powder followed by a computer-guided motion video scanning, which was displayed on a monitor. The software outlined which areas had been successfully scanned, and which areas still required further scanning. At any point in the digital impression, the scan of the preparation could be evaluated in 3-D on the monitor to ensure that it had been adequately captured prior to data transfer. A total of 15 scans of the zirconia models were taken, with each scan being used in two methods: one being for a completely digital design and manufacturing process of an IPS e.max CAD crown; and the other being for the fabrication of a polyurethane die for the creation of a traditional pressed IPS e.max Press crown. For the entirely digital process, upon completion of the maxillary, mandibular, and bite registration scans of the master zirconia models, the files were exported in .STL (Standard Tessellation Language) format for input into a digital design R workflow (Core3dcentres , Las Vegas, NV) for a full contour crown, to be five-axis milled (DMG, Bielefeld, Germany) in IPS e.max CAD. For the partially digital process, the same
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files were used for the five-axis milling of a polyurethane die, which was then used in the fabrication of an IPS e.max Press pressed crown, via the lost wax technique. Therefore, each set of digital impressions was used for both the fabrication of an entirely digitally designed and milled crown, and for a digitally impressed but traditionally fabricated pressed crown. In total, 15 milled (group 1–digital milled [DM]) and 15 pressed crowns (group 2–digital pressed [DP]) were fabricated from 15 digital impressions, in addition to the 15 pressed crowns fabricated from the traditional elastomeric impression (group 3–traditional pressed [TP]). A 20 μm virtual die spacer was prescribed for the milled crowns (DM), and a 20 μm die spacer application was prescribed for the pressed crowns (DP and TP). For all the pressed crowns (DP and TP), one layer of a latexbased die spacer (Rem-e-die; Ivoclar Vivadent) was applied by the same experienced technician to achieve the prescribed 20 μm thickness. The decision to use 20 μm in all the groups was made to limit any possible errors that may have arisen by placing two films of spacer and because, often, the digitally prescribed space is 20 μm. All 45 crowns were returned from the lab fully sintered, stained, glazed, and etched with 4.5% hydrofluoric acid. To perform the internal fit measurements using a μCT scanner (Scanco Medical μCT100, Br¨uttisellen, Switzerland), 45 duplicate dies of the master zirconia die were fabricated. For this, a single LAVA COS scan of the original zirconia die was exported for the milling of 45 duplicates in polyurethane via five-axis milling. Each crown was then seated on its respective duplicate die with finger pressure,21,22 was stabilized with utility wax,19 and scanned at a 20 μm resolution. The resultant μCT scan images were analyzed with the scanner’s proprietary software by isolating the internal space on the images. Upon the hand selection of the outline of the internal space on each slice of the scan of each crown, the images were reconstructed by the software to create a 3-D image of the internal space, complete with color-graded thickness maps. The software allowed for quantitative analysis of the internal space and yielded various data, including minimum, maximum, and mean thicknesses of the internal space, along with the total volume of the space for each crown. It was also possible to break down the distribution of the internal space by indicating what percentage of the total space was of a specific thickness, in increments of 20 μm. The exact location of the distribution of the fit was not available from the numerical data, but was qualitatively visible in the 3-D reconstructions. A sample size of 12 was calculated for a power of 0.8, using reported historical means for the internal space of lithium disilicate crowns.23 Consequently, 15 crowns per group were made, to ensure that power was not affected if any specimens were damaged or data were deemed unusable. Univariate ANOVA was used to evaluate differences between the three groups in various categories, including maximum thickness, mean thickness, and total volume of internal space. Furthermore, the percentage of the internal space that was at or below 120 μm was calculated for each crown. This value of 120 μm was selected because it correlates with recommended cement thicknesses,15 existing ranges for internal fit of lithium disilicate crowns,17 and maximum clinically acceptable values for marginal fit.24 After ANOVA, if warranted, multiple means comparisons (Scheff´e)
Table 1 Internal space dimension Digital pressed (DM)
Digital pressed (DP)
Traditional pressed (TP)
Total volume (mm3 ) 12.49 ± 1.50a 15.40 ± 2.59b 18.01 ± 2.44c Mean thickness (μm) 0.16 ± 0.01a 0.17 ± 0.03a 0.21 ± 0.03b Max thickness (μm) 0.37 ± 0.03a 0.36 ± 0.08a 0.41 ± 0.06a Percentage ࣘ 120 46.73 ± 5.66a 37.08 ± 17.69a 22.89 ± 9.72b μm∗∗ Values with same superscript letters are not significantly different within a specific measurement (row). **The percentage of the space at or below 120 μm thickness.
were performed. All analyses were performed at α = 0.05 using SPSS (SPSS for Windows, version 12.0; Chicago, IL).
Results The mean of the total volume of the internal space was calculated as 12.49 ± 1.50 mm3 for DM, 15.40 ± 2.59 mm3 for DP, and 18.01 ± 2.44 mm3 for TP (Table 1). Statistical analysis indicated significant differences between the groups and post hoc Scheff´e analysis demonstrated that there was a significant difference between the volume of the internal space for all three groups, with DM having a smaller volume than DP and TP, and DP having a smaller volume than TP (DM < DP < TP). The average percentage of the internal space of a thickness of 120 μm and below was calculated as 46.73 ± 5.66% for DM, 37.08 ± 17.69% for DP, and 22.89 ± 9.72% for TP (Table 1). Statistical analysis indicated significant differences between the groups and post hoc Scheff´e analysis demonstrated a significant difference between the averages of the percent of internal space at or below 120 μm for both of the digital crown groups in comparison to the “traditional” crowns. The two groups of digital crowns, DM and DP, were not found to be significantly different from each other, but both groups (DM, DP) had a significantly larger percentage of their internal space at or below 120 μm in comparison to TP (TP < DM = DP). The mean thickness of the internal space was calculated as 0.16 ± 0.01 mm for DM, 0.17 ± 0.03 mm for DP, and 0.21 ± 0.03 mm for TP (Table 1). Statistical analysis indicated significant differences between the groups and post hoc Scheff´e analysis demonstrated a significant difference between the average thicknesses of the internal space for both groups of the digital crowns in comparison to the “traditional” crowns. The two groups of digital crowns, DM and DP, were not found to be significantly different from each other, but both groups (DM, DP) had a significantly smaller average thickness of the internal space in comparison to TP (TP > DM = DP). The mean maximum thickness of the internal space was calculated as 0.37 ± 0.03 mm for DM, 0.36 ± 0.08 mm for DP, and 0.41 ± 0.06 mm for TP (Table 1). Statistical analysis did not identify any significant difference between the groups (DM = DP = TP). The 3-D images rendered by the μCT image analysis allowed for a qualitative analysis of the internal space of the crowns. The images could be rotated to view the internal space in any orientation, and the scales of the thickness gradient could be
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Figure 1 Sample standardized scale (460 μm maximum) 3-D reconstruction of the internal space of digital impression and CAD/CAM technique group (DM).
Figure 3 Sample standardized scale (460 μm maximum) 3-D reconstruction of the internal space of conventional PVS impression and laboratory fabrication process group (TP).
Figure 2 Sample standardized scale (460 μm maximum) 3-D reconstruction of the internal space of digital impression and conventional die group (DP).
Figure 4 Internal space of digital impression and CAD/CAM technique group (DM), scaled to demonstrate space of 120 μm and below.
altered to highlight areas of specific thicknesses. The internal space images were set to the same scale (maximum data point) to compare the distribution of the space for the three crown types. Sample images of the standardized scale images are presented in Figures 1–3. As can be seen in the reconstructions, TP visually has a larger portion of the internal space close to the maximum of the scale (460 μm), while the digital crowns have less of the space approaching the maximum. All three crown groups tended to have larger spaces at the line angles of the preparations. The 3-D reconstructions were also scaled to demonstrate where the internal space was in the range of 120 μm and 4
below, to visualize where the prosthesis would fit in this “ideal” range (Figs 4–6). The 3-D images indicated that TP was significantly poorer fitting than either of the digital crowns.
Discussion Under the experimental conditions of this study, the results showed that lithium disilicate crowns fabricated using digital impressions and CAD/CAM were superior with respect to internal fit to those fabricated using traditional techniques. Crowns fabricated based on digital impressions (DP and DM groups) demonstrated lower and more consistent values for internal space in volume, mean thickness, and better percent
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Figure 5 Internal space of digital impression and conventional die group (DP), scaled to demonstrate space of 120 μm and below.
Figure 6 Internal space of conventional PVS impression and laboratory fabrication process group (TP), scaled to demonstrate space of 120 μm and below.
clinically acceptable fit, in comparison to the TP crowns (TP group). These results support the findings of other authors who have compared digital and traditional workflows and have found that the digital workflow allows for the fabrication of restorations of equal or superior fit to those fabricated by traditional technologies.1,25 Moreover, the results obtained are in agreement with those of a study concurrent to this project, which has also demonstrated that the completely digital process had a more accurate marginal fit in comparison to traditional techniques.26 As demonstrated in the 3-D reconstructions, the milled crowns had larger internal spaces at the line angles of the preparations. This finding may be attributed to limitations in the milling accuracies due to the dimensions of the burs.27 This
discrepancy at the line angles, however, was also found for the pressed prostheses, regardless of impression technique, which was not expected due to the theoretical ability to cast the ceramic into these rounded areas. When the two digital crown groups (DM, DP) fabricated from the same digital impression were compared, the milled crowns were not any better fitting than the pressed crowns, indicating that the fabrication technique was not a major source of error. Furthermore, various studies have compared the accuracy of milled polyurethane dies and type IV stone dies and have indicated that they are comparable.4,28 Therefore, it is also unlikely that the type of working die or the type of die spacer used played a role in any fit discrepancy. The elastomeric impression is likely the cause of the poorer fit of the traditional prostheses in this project. The image analysis proved to be a time-consuming process that was not easily automated. One of the challenges that previous authors found was the presence of artifacts in the scans, which did not allow for an automated selection of the internal space.19,29 While our samples did not have very many artifacts or scatter, the software was challenged in automatically selecting contours based on contrast when the internal space became very small. This may have been a limitation of the scanning resolution. Also, the crowns were delivered after being etched with hydrofluoric acid in the laboratory, which creates micromechanical voids in the internal surface, in the range of a few μm.30 It is possible that this inherent surface roughness decreased the sharpness of the image at the internal surface of the restoration, therefore adding to the difficulty of automating the selection of the outline of the internal space. Regardless of the challenges of the technique, the 3-D analysis of the internal space is a novel method of directly evaluating the internal fit of indirect restorations and deserves further exploration for the clinical reliability of these laboratory measurements. One of the limitations of the study was the use of duplicate dies for the measurement process. This technique was selected because it allowed for all 45 crowns to have their own duplicate die of the original preparation for measurement, thus preventing damage to the original zirconia die due to repeated measurements; it also allowed for multiple specimens to be scanned at the same time. Furthermore, the difference in the radiopacity between the polyurethane die and the ceramic crown facilitated the image analysis. While the dies were not evaluated for accuracy in comparison to the original zirconia die, they were all fabricated from one digital scan in the same five-axis mill from fully polymerized polyurethane and were randomly assigned to the crowns. As such, any systematic error in the process was randomly distributed within the data set, thus not affecting the reliability of the measurement technique, but perhaps the accuracy. Validation studies of the reproducibility and accuracy of the replica die technique are in progress. The entirely digital process offers many advantages compared to the traditional techniques. The digital technique can reduce total treatment time by half, even in the hands of inexperienced users.31 The ability to have an immediate direct visualization of the prepared and unprepared teeth can be beneficial to the patient, in terms of oral health education, and to the dentist, in terms of being able to see if the preparation is adequate, especially considering that in the traditional technique,
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errors may not be seen until the impression is poured.32 Furthermore, the data file is available almost instantaneously for the lab technician to evaluate, and feedback can be given to the dentist without the delays of pouring a cast.33 Digital technology can also help reduce the environmental impact of a dental practice by reducing the waste associated with traditional impression materials,34 not requiring extra materials for remakes31 and by removing the need for disinfection and transportation of traditional impressions.25
Conclusion The results of this study suggest that pressed and milled IPS e.max crowns from LAVA COS digital impressions had a better internal fit to the prepared tooth than pressed IPS e.max crowns from PVS impressions in terms of total volume of internal space, average thickness of internal space, and percentage of internal space at or below 120 μm.
Acknowledgments The authors thank Aurum Ceramics Dental Laboratories Ltd, Vancouver, Canada, for their generous financial support and CAD/CAM milling of the master ceramic model.
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14. Cho SH, Chang WG, Lim BS, et al: Effect of die spacer thickness on shear bond strength of porcelain laminate veneers. J Prosthet Dent 2006;95:201-208 15. Molin MK, Karlsson SL, Kristiansen MS: Influence of film thickness on joint bend strength of a ceramic/resin composite joint. Dent Mater 1996;12:245-249 16. Conrad HJ, Seong WJ, Pesun IJ: Current ceramic materials and systems with clinical recommendations: a systematic review. J Prosthet Dent 2007;98:389-404 17. Schaefer O, Kuepper H, Sigusch BW, et al: Three-dimensional fit of lithium disilicate partial crowns in vitro. J Dent 2013;41:271-277 18. Borba M, Cesar PF, Griggs JA, et al: Adaptation of all-ceramic fixed partial dentures. Dent Mater 2011;27:1119-1126 19. Seo D, Yi Y, Roh B: The effect of preparation designs on the marginal and internal gaps in Cerec3 partial ceramic crowns. J Dent 2009;37:374-382 20. Gordon GE, Johnson GH, Drennon DG: The effect of tray selection on the accuracy of elastomeric impression materials. J Prosthet Dent 1990;63:12-15 21. Syrek A, Reich G, Ranftl D, et al: Clinical evaluation of all-ceramic crowns fabricated from intraoral digital impressions based on the principle of active wavefront sampling. J Dent 2010;38:553-559 22. Kokubo Y, Tsumita M, Kano T, et al: Clinical marginal and internal gaps of zirconia all-ceramic crowns. J Prosthodont Res 2011;55:40-43 23. Al-Rabab’ah MA, Macfarlane TV, McCord JF: Vertical marginal and internal adaptation of all-ceramic copings made by CAD/CAM technology. Eur J Prosthodont Restor Dent 2008;16:109-115 24. Mclean JW, von Fraunhofer JA: The estimation of cement film thickness by an in vivo technique. Br Dent J 1971;131:107111 25. Brawek PK, Wolfart S, Endres L, et al: The clinical accuracy of single crowns exclusively fabricated by digital workflow–the comparison of two systems. Clin Oral Investig 2013;17:2119-2125 26. Ng J, Ruse D, Wyatt C: A comparison of the marginal fit of crowns fabricated with digital and conventional methods. J Prosthet Dent 2014;112:555-560 27. Kurbad A, Reichel K: CAD/CAM-manufactured restorations made of lithium disilicate glass ceramics. Int J Comput Dent 2005;8:337-348 28. Kim SY, Kim MJ, Han JS, et al: Accuracy of dies captured by an intraoral digital impression system using parallel confocal imaging. Int J Prosthodont 2013;26:161-163 29. Rungruanganunt P, Kelly JR, Adams DJ: Two imaging techniques for 3D quantification of pre-cementation space for CAD/CAM crowns. J Dent 2010;38:995-1000 30. Zogheib LV, Bona AD, Kimpara ET, et al: Effect of hydrofluoric acid etching duration on the roughness and flexural strength of a lithium disilicate-based glass ceramic. Braz Dent J 2011;22:45-50 31. Lee SJ, Gallucci GO: Digital vs. conventional implant impressions: efficiency outcomes. Clin Oral Implants Res 2013;24:111-115 32. Wassell RW, Barker D, Walls AW: Crowns and other extra-coronal restorations: impression materials and technique. Br Dent J 2002;192:679-690 33. Craddock MR, Windhorn RJ: Is the US Army Dental Corps ready for the digital impression? US Army Med Dep J 2011;38-41 34. Christensen GJ: Impressions are changing: deciding on conventional, digital or digital plus in-office milling. J Am Dent Assoc 2009;140:1301-1304
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