- Email: [email protected]
Three-dimensional evaluation of the reproducibility of presintered zirconia single copings fabricated with the subtractive method
RESEARCH AND EDUCATION
Three-dimensional evaluation of the reproducibility of presintered zirconia single copings fabricated with the subtractive method Chong-Myeong Kim, MSc, PhD,a Jin-Hun Jeon, MSc, PhD,b Ji-Hwan Kim, MPH, PhD,c Hae-Young Kim, DDS, PhD,d and Woong-Chul Kim, MPH, PhDe Previous methods of fabriABSTRACT cating dental prostheses relied Statement of problem. Reproducibility is an important factor determining the success of a prosmostly on the lost wax techthesis. However, no studies have focused on identifying the location of errors on prostheses nique. However, this technique fabricated with the subtractive method, leading to a lack of standards for reproducibility is complicated and time conevaluations. suming, results in errors that are Purpose. The purpose of this in vitro study was to evaluate the reproducibility of the subtractive difficult to control, and exhibits method by conducting 3-dimensional assessments of presintered single-tooth zirconia copings for low reproducibility.1,2 Recent different teeth. advances in technology have Material and methods. Acrylic resin tooth molds for the canine (CAN), premolar (PRE), and molar led to the introduction of the (MOL) were used to prepare stone casts, and copings were designed and fabricated with the subsubtractive method, which is tractive method. The intaglio surfaces of corresponding presintered zirconia copings were scanned based on dental computerwith a blue light scanner. Initial scan data were used as a reference for comparisons with subsequent data for the measurement of errors. Nine color-difference maps were created for each of the aided design/computer-aided 3 groups and used to calculate root-mean-square (RMS) error values. One-way analysis of variance manufacture (CAD/CAM) techand the Tukey honestly significant difference tests were used for statistical evaluations (a=.05). nology and is relatively simple and less time-consuming comResults. MOL copings exhibited the highest RMS error value (9.22 ±1.56 mm), which was significantly different from values for CAN (3.33 ±2.65 mm) and PRE (4.00 ±2.40 mm; P<.001) copings. pared with the lost wax techColor-difference maps revealed maximum errors in the line angles. nique. Moreover, machine fabrication of the prostheses Conclusions. The highest reproducibility was observed for the CAN copings. The clinical reproducibility of the subtractive method can be improved by avoiding sharp angles during abutment results in high reproducibility, preparation and careful reproduction of angles during prosthesis fabrication. (J Prosthet Dent as it is less dependent on the 2016;-:---) skills of the dental technician.3-6 Previous studies have shown that dental CAD/CAM systems3 not only improve the the reproducibility of the subtractive method for fabrireproducibility of the fabrication technique but also overcating dental prostheses. come the other limitations of conventional methods.1,7-14 The reproducibility of CAD/CAM systems can be Reproducibility is defined in ISO 12836 guidelines. objectively analyzed by using 3-dimensional (3D) asFrom these perspectives, it is also essential to evaluate sessments of the prostheses, which have been used in Supported by Institute of Health Science grant K1422631, Korea University. a Doctoral student, Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Seoul, South Korea. b Assistant professor, Department of Dental Technology, Medical Campus, KyungDong University, Gangwon-do, South Korea. c Professor, Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Seoul, South Korea. d Associate Professor, Department of Dental Laboratory Science and Engineering, College of Health Science, and Department Public Health Sciences, Korea University, Seoul, South Korea. e Professor, Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Seoul, South Korea.
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Clinical Implications The subtractive method based on computer-aided design/computer-aided manufacture technology can generate considerable errors in the fabricated prosthesis. These errors can be minimized by avoiding abutment preparations with sharp angles and careful reproduction of angles during fabrication.
numerous recent studies to measure prosthesis precision.3,16-19 The technique involves the use of multifocal points for analysis, thereby facilitating the determination of errors over a wide area. Furthermore, 3D evaluation prevents damage to the test specimens during the experiment, and the data obtained are easier to analyze.8,15,16,20,21 Errors during the fabrication of a prosthesis with a CAD/CAM system include errors during cast fabrication, scanning errors, and errors during the subtractive fabrication process.7,17,22 Errors during cast fabrication (10-12 mm) and scanning (14-16 mm) have been analyzed.17 Other critical errors in the subtraction process have also been investigated, such as errors in the machine itself and errors due to tool abrasion, rapid fabrication, and tool wear.17 However, no study has focused on identifying the location of errors on the fabricated prosthesis, leading to the lack of standards during reproducibility evaluations. Therefore, 3D evaluations were used to precisely locate errors on presintered single-tooth zirconia copings fabricated with the subtractive method based on dental CAD/CAM technology for different teeth. The null hypothesis was that the reproducibility of presintered zirconia copings fabricated with the subtractive method would not differ for the canine (CAN), premolar (PRE), and molar (MOL) teeth. MATERIAL AND METHODS Acrylic resin abutments with standard chamfer margins (AG-3 ZPVK; Frasaco GmbH) were used in this study. Molds for the maxillary CAN, PRE, and MOL teeth were fabricated with impression material (Aquasil Ultra; Dentsply Intl) and a custom tray7 and were used to prepare casts from Type 4 dental stone (GC Fujirock EP; GC Corp). The stone casts were scanned with a blue light scanner (Identica Blue; Medit) for use with the CAD program (DentCAD; Delcam Ltd). Copings were designed according to the following parameters: 30-mm thickness for the cement on the occlusal surface; 0-mm distance between the coping margin and cement;
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0.5-mm thickness over the remaining surfaces; and 0.15mm thickness at the finishing margin. The initial primary form remained constant and was used for all subsequent procedures (Fig. 1). The designed copings were then milled from presintered zirconia blocks (ZirPremium-DS; Acucera, Inc) by using the subtractive method (DWX-50; Roland DG Corp). The equipment was calibrated before milling to minimize errors. The milling machine used diamondcoated rotary instruments with diameters of 2 mm for rough cutting, 1 mm for finishing, and 0.6 mm for finishing. After the fabrication of each prosthesis, the tools were replaced. Ten test specimens were prepared for each tooth and sequentially numbered from 1 to 10 according to the order of fabrication. To evaluate the reproducibility of the fabricated copings, their intaglio surfaces were scanned with a blue light scanner. The images were cropped to eliminate unnecessary structures and structures beyond the margins. The initial set of scan data for each of the 3 groups was used as a reference for comparisons with the subsequent 9 sets of data. The scanned stereolithography files were transformed into a point-cloud American Standard Code for Information Interchange (ASCII) file with matching software (CopyCAD v7.350 SP3 software; Delcam Ltd) and superimposed with inspection software (PowerINSPECT 2012; Delcam Ltd), creating a total of 9 color-difference maps for each tooth (total of 27 maps). To measure reproducibility, the specimens were replaced in the digital equipment, where they were fabricated and removed.23 The reproducibility analysis method recommended by ISO 1283623 is well known for objective evaluation of the reproducibility of a fabricated prosthesis and involves the calculation of rootmean-square (RMS) ±standard deviation (SD) values. Accordingly, RMS ±SD values were calculated from the color-difference maps as the average of the positive and negative values of coexistence, according to the following formula: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ffi Pn −x x 2;i i=1 1;i pffiffiffi RMS= ; n where n is the total number of specimens, x1;i is the measuring point in the reference scan data, and x2;i is the measurement point in the subsequent scan data. Errors were evaluated on the basis of the derived RMS values, with lower values representing higher reproducibility.23 One-way ANOVA with Tukey honest significant differences (HSD) post hoc tests were used for statistical evaluations of the measurements. Software (SPSS Statistics, v21.0; IBM Corp) was used for all statistical analyses (a=.05).
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Figure 1. Cross-sectional views of representative specimens. A, Cement thickness. Green arrows indicate thickness at crown margin; blue arrows indicate margin-cement distance; orange arrows indicate thickness on side; and purple arrows indicate thickness on occlusal surface. B, Coping thickness. Green arrows indicate thickness at finishing margin, and blue arrows indicate thickness on occlusal surface.
RESULTS Table 1 shows RMS ±SD values for the 3 groups, which were 3.33 ±2.65 mm (95% confidence interval [CI]), 1.305.37 mm for the CAN copings, 4.00 ±2.40 mm (95% CI, 2.16-5.84 mm) for the PRE copings, and 9.22 ±1.56 mm (95% CI, 8.02-10.42 mm) for the MOL copings. Thus, the MOL copings exhibited the highest value, which was significantly different from the values for the CAN and PRE copings (P<.001) (Table 1). One-way ANOVA revealed statistically significant differences in reproducibility values among the 3 groups (P<.001). In a color-difference map (Fig. 1), red depicts a positive error (+) with a value higher than normal, blue depicts a negative error (−) with a value lower than normal, and green depicts the absence of errors (0), that is, the absence of a difference between the reference prosthesis and the test prosthesis in that particular area. Thus, this map enables the visualization of errors on a prosthesis. The color-difference maps for the CAN copings showed positive errors (red) in the region of the cusp tip and vertical negative errors (blue), but most areas had no errors (green). However, the color-difference maps for the PRE copings showed negative errors (blue) on the lingual cusp and positive errors (red) on the buccal cusp, although most areas were free of errors (green). The color-difference maps for the MOL copings showed positive errors (red) in the disto-occlusal line angle (junction between the distal and occlusal surfaces) and negative errors (blue) on the distal surface (Fig. 2). DISCUSSION The reproducibility of the subtractive method based on dental CAD/CAM technology was evaluated with 3D assessments of presintered single-tooth zirconia copings. Kim et al
Table 1. RMS and SD values for presintered canine, premolar, and molar copings fabricated with subtractive method Group RMS ±SD (mm) 95% CI P Canine
3.33 ±2.65A
Premolar
4.00 ±2.40A
2.16-5.84
Molar
9.22 ±1.56B
8.02-10.42
1.30-5.37 <.001
CI, confidence interval; RMS, root mean square; SD, standard deviation. Different superscript letters indicate significant differences (P<.05).
The highest RMS error value was for the MOL copings; thus, the null hypothesis tested in this study was rejected. Moreover, the CAN copings showed the lowest RMS error value, which indicated the highest reproducibility. The use of the subtractive method has increased with the development of dental CAD/CAM systems, necessitating objective assessment of the reliability (errors) of this method. Reproducibility tests are the best indicators of reliability, and the procedure for these tests, which involves the calculation of RMS and SD values, is outlined in the ISO 12836 guidelines.23 The guidelines also mandate that reference data are required for comparisons with test data for the evaluation of reproducibility.23 Jeon et al24,25 assessed precision by conducting 3D assessments of the intaglio surfaces of CAN, PRE, and MOL copings. These assessments and the resulting color-difference maps enable easy error visualization and correction.3,13,15,16,19,20,24,26 In addition, color-difference maps facilitate the identification of the exact site of error. We successfully used similar techniques to determine errors caused by the subtractive method. The CAN copings exhibited the lowest error value (3.33 ±2.65 mm), with positive errors in the cusp tip area, whereas the MOL copings exhibited the highest error value (9.22 ±1.56 mm), with positive errors in the distoocclusal line angle. With regard to the PRE copings, the error value was 4.00 ±2.40 mm, with most of the positive THE JOURNAL OF PROSTHETIC DENTISTRY
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Figure 2. Color difference maps indicate reliability of subtractive method for fabrication of presintered single-tooth zirconia copings. CAN, canine; MOL, molar; PRE, premolar.
errors on the occlusal surface. These findings collectively suggest that errors tend to occur in angular areas. The primary reason for this phenomenon is that the diameter of the tool used in the subtractive method is larger than the angle made with the specimen surface, resulting in errors during the fabrication process (Fig. 2). Therefore, the use of tools with a smaller diameter would be beneficial. However, some studies have suggested that a smaller tool diameter may slow the process and necessitate frequent tool replacement.17,27 Nevertheless, the benefit of increased precision outweighs the limitation of increased fabrication time. A previous study stated that CANs, PRE, and MOL are adequate for assessing the reproducibility of the subtractive method.27 Specifically, CANs have the sharpest form, whereas PREs are relatively rounded with narrow surfaces and sharp cusps. MOLs, however, are rounded with broad surfaces. Accordingly, maxillary CANs, PREs, and MOLs were selected in the present study. The selection of appropriate equipment is essential to ensure a credible reproducibility analysis. Therefore, we selected a 5-axis dental mill with a spring motor that firmly holds the milling burr and minimizes vibration, THE JOURNAL OF PROSTHETIC DENTISTRY
and with the ability to eliminate undercuts.28 Furthermore, the rotary instruments were carefully selected according to strict recommendations from the company and replaced after each prosthesis was fabricated. Calibration was performed before milling to minimize errors. All these measures were based on efforts made in previous studies to ensure the credibility of the results.12,22,28,29 This study had some limitations. First, acrylic resin tooth forms, which imitate an ideal tooth preparation, were used to prepare the stone casts. Second, the zirconia copings were not definitively sintered; therefore, the reproducibility findings do not consider the effects of sintering contraction. Further studies should assess the effects of sintering contraction on the reproducibility of the subtractive method. CONCLUSIONS Results of this study demonstrated the highest reproducibility for the CAN copings. Furthermore, analysis of color-difference maps revealed maximum errors in angular areas. Thus, the clinical reproducibility of the Kim et al
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subtractive method can be improved by avoiding the creation of angles during abutment preparation and by the careful reproduction of angles during prosthesis fabrication. REFERENCES 1. Miyazaki T, Hotta Y, Kunii J, Kuriyama S, Tamaki Y. A review of dental CAD/ CAM: current status and future perspectives from 20 years of experience. Dent Mater J 2009;28:44-56. 2. Petteno D, Schierano G, Bassi F, Bresciano ME, Carossa S. Comparison of marginal fit of 3 different metal-ceramic system: an in vitro study. Int J Prosthodont 2000;13:405-8. 3. Kim KB, Kim JH, Kim WC, Kim JH. Three-dimensional evaluation of gaps associated with fixed dental prostheses fabricated with new technologies. J Prosthet Dent 2014;112:1432-6. 4. Ng J, Ruse D, Wyatt C. A comparison of the marginal fit of crowns fabricated with digital and conventional methods. J Prosthet Dent 2014;112:555-60. 5. Van Noort R. The future of dental devices is digital. Dent Mater 2012;28:3-12. 6. Beuer F, Schweiger J, Edelhoff D. Digital dentistry: an overview of recent developments for CAD/CAM generated restorations. Br Dent J 2008;204: 505-11. 7. Abdel-Azim T, Rogers K, Elathamna E, Zandinejad A, Metz M, Morton D. Comparison of the marginal fit of lithium disilicate crowns fabricated with CAD/CAM technology by using conventional impressions and two intraoral digital scanners. J Prosthet Dent 2015;114:554-9. 8. Lebon N, Tapie L, Vennat E, Mawussi B. Influence of CAD/CAM tool and material on tool wear and roughness of dental prostheses after milling. J Prosthet Dent 2015;114:236-47. 9. Seydler B, Schmitter M. Clinical performance of two different CAD/CAMfabricated ceramic crowns: 2-year results. J Prosthet Dent 2015;114:212-6. 10. Anadioti E, Aquilino SA, Gratton DG, Holloway JA, Denry IL, Thomas GW, et al. Internal fit of pressed and computer-aided design/computer-aided manufacturing ceramic crowns made from digital and conventional impressions. J Prosthet Dent 2015;113:304-9. 11. Lins L, Bemfica V, Queiroz C, Canabarro A. In vitro evaluation of the internal and marginal misfit of CAD/CAM zirconia copings. J Prosthet Dent 2015;113: 205-11. 12. Lei XB, Xie F, Zhao JW. Design of a 5-axis CNC machine tool applied to dental restoration. Key Eng Mater 2013;568:75-80. 13. Moldovan O, Lut’t RG, Corcodel N, Rudolph H. Three-dimensional fit of CAD/CAM-made zirconia copings. Dent Mater 2011;27:1273-8. 14. Grenade C, Mainjot A, Vanheusden A. Fit of single tooth zirconia copings: comparison between various manufacturing processes. J Prosthet Dent 2011;105:249-55. 15. Schaefer O, Watts DC, Sigusch BW, Kuepper H, Guentsch A. Marginal and internal fit of pressed lithium disilicate partial crowns in vitro: a three-dimensional analysis of accuracy and reproducibility. Dent Mater 2012;28:320-6. 16. Schaefer O, Schmidt M, Goebel R, Kuepper H. Qualitative and quantitative three-dimensional accuracy of a single tooth captured by elastomeric impression materials: an in vitro study. J Prosthet Dent 2012;108:165-72.
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17. Schaefer O, Kuepper H, Thompson GA, Cachovan G, Hefti AF, Guentsch A. Effect of CNC- milling on the marginal and internal fit of dental ceramics: a pilot study. Dent Mater 2013;29:851-8. 18. Schaefer O, Kuepper H, Sigusch BW, Thompson GA, Hefti AF, Guentsch A. Three- dimensional fit of lithium disilicate partial crowns in vitro. J Dent 2013;41:271-7. 19. Schaefer O, Decker M, Wittstock F, Kuepper H, Guentsch A. Impact of digital impression techniques on the adaption of ceramic partial crowns in vitro. J Dent 2014;42:677-83. 20. Persson A, Andersson M, Oden A, Sndborgh-Englund G. A three-dimensional evaluation of a laser scanner and a touch-probe scanner. J Prosthet Dent 2006;95:194-200. 21. Keshvad A, Hooshmand T, Asefzadeh F, Khalilinejad F, Alihemmati M, Van Noort R. Marginal gap, internal fit, and fracture load of leucite-reinforced ceramic inlays fabricated by CEREC inLab and hot-pressed techniques. J Prosthodont 2011;20:535-40. 22. Hamza TA, Ezzat HA, El-Hossary MMK, El Megid Katamish HA, Shokry TE, Rosenstiel SF. Accuracy of ceramic restorations made with two CAD/CAM systems. J Prosthet Dent 2013;109:83-7. 23. International Organization for Standardization. ISO-12836. Dentistry Digitizing devices for CAD/CAM systems for indirect dental restorationsdtest methods for assessing accuracy. Geneva: ISO; 2015. Available at: http://www. iso.org/iso/store.htm. Accessed March 2, 2016. 24. Jeon JH, Lee KT, Kim HY, Kim JH, Kim WC. White light scanner-based repeatability of 3- dimensional digitizing of silicon rubber abutment teeth impressions. J Adv Prosthondont 2013;5:452-6. 25. Jeon JH, Kim HY, Kim JH, Kim WC. Accuracy of 3D white light scanning of abutment teeth impressions: evaluation of trueness and precision. J Adv Prosthondont 2014;6:468-73. 26. Luthardt RG, Bornemann G, Lemelson S, Walter MH, Huls A. An innovative method for evaluation of the 3-D internal fit of CAD/CAM crowns fabricated after direct optical versus indirect laser scan digitizing. Int J Prosthodont 2004;17:680-6. 27. Nakamura T, Dei N, Kojima T, Wakabayashi K. Marginal and internal fit of Cerec 3 CAD/CAM all-ceramic crown. Int J Prosthodont 2003;16:244-8. 28. Abduo J, Lyons K, Swain M. Fit of zirconia fixed partial denture: a systematic review. J Oral Rehabil 2010;37:866-76. 29. Addison O, Cao X, Sunnar P, Fleming GJ. Machining variability impacts on the strength of a ‘chair-side’ CADeCAM ceramic. Dent Mater 2012;28:880-7. Corresponding author: Dr Woong-Chul Kim Department of Dental Laboratory Science and Engineering College of Health Science, Korea University 145 Anam-ro, Seongbuk-gu Seoul 136-713 SOUTH KOREA Email: [email protected] Acknowledgments The authors thank the staff of Delcam Ltd, and Medit Inc, who have supported the inspection program and assisted with the blue light scanner measuring. Copyright © 2016 by the Editorial Council for The Journal of Prosthetic Dentistry.
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Three-dimensional evaluation of the reproducibility of presintered zirconia single copings fabricated with the subtractive method Chong-Myeong Kim, MSc, PhD,a Jin-Hun Jeon, MSc, PhD,b Ji-Hwan Kim, MPH, PhD,c Hae-Young Kim, DDS, PhD,d and Woong-Chul Kim, MPH, PhDe Previous methods of fabriABSTRACT cating dental prostheses relied Statement of problem. Reproducibility is an important factor determining the success of a prosmostly on the lost wax techthesis. However, no studies have focused on identifying the location of errors on prostheses nique. However, this technique fabricated with the subtractive method, leading to a lack of standards for reproducibility is complicated and time conevaluations. suming, results in errors that are Purpose. The purpose of this in vitro study was to evaluate the reproducibility of the subtractive difficult to control, and exhibits method by conducting 3-dimensional assessments of presintered single-tooth zirconia copings for low reproducibility.1,2 Recent different teeth. advances in technology have Material and methods. Acrylic resin tooth molds for the canine (CAN), premolar (PRE), and molar led to the introduction of the (MOL) were used to prepare stone casts, and copings were designed and fabricated with the subsubtractive method, which is tractive method. The intaglio surfaces of corresponding presintered zirconia copings were scanned based on dental computerwith a blue light scanner. Initial scan data were used as a reference for comparisons with subsequent data for the measurement of errors. Nine color-difference maps were created for each of the aided design/computer-aided 3 groups and used to calculate root-mean-square (RMS) error values. One-way analysis of variance manufacture (CAD/CAM) techand the Tukey honestly significant difference tests were used for statistical evaluations (a=.05). nology and is relatively simple and less time-consuming comResults. MOL copings exhibited the highest RMS error value (9.22 ±1.56 mm), which was significantly different from values for CAN (3.33 ±2.65 mm) and PRE (4.00 ±2.40 mm; P<.001) copings. pared with the lost wax techColor-difference maps revealed maximum errors in the line angles. nique. Moreover, machine fabrication of the prostheses Conclusions. The highest reproducibility was observed for the CAN copings. The clinical reproducibility of the subtractive method can be improved by avoiding sharp angles during abutment results in high reproducibility, preparation and careful reproduction of angles during prosthesis fabrication. (J Prosthet Dent as it is less dependent on the 2016;-:---) skills of the dental technician.3-6 Previous studies have shown that dental CAD/CAM systems3 not only improve the the reproducibility of the subtractive method for fabrireproducibility of the fabrication technique but also overcating dental prostheses. come the other limitations of conventional methods.1,7-14 The reproducibility of CAD/CAM systems can be Reproducibility is defined in ISO 12836 guidelines. objectively analyzed by using 3-dimensional (3D) asFrom these perspectives, it is also essential to evaluate sessments of the prostheses, which have been used in Supported by Institute of Health Science grant K1422631, Korea University. a Doctoral student, Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Seoul, South Korea. b Assistant professor, Department of Dental Technology, Medical Campus, KyungDong University, Gangwon-do, South Korea. c Professor, Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Seoul, South Korea. d Associate Professor, Department of Dental Laboratory Science and Engineering, College of Health Science, and Department Public Health Sciences, Korea University, Seoul, South Korea. e Professor, Department of Dental Laboratory Science and Engineering, College of Health Science, Korea University, Seoul, South Korea.
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Clinical Implications The subtractive method based on computer-aided design/computer-aided manufacture technology can generate considerable errors in the fabricated prosthesis. These errors can be minimized by avoiding abutment preparations with sharp angles and careful reproduction of angles during fabrication.
numerous recent studies to measure prosthesis precision.3,16-19 The technique involves the use of multifocal points for analysis, thereby facilitating the determination of errors over a wide area. Furthermore, 3D evaluation prevents damage to the test specimens during the experiment, and the data obtained are easier to analyze.8,15,16,20,21 Errors during the fabrication of a prosthesis with a CAD/CAM system include errors during cast fabrication, scanning errors, and errors during the subtractive fabrication process.7,17,22 Errors during cast fabrication (10-12 mm) and scanning (14-16 mm) have been analyzed.17 Other critical errors in the subtraction process have also been investigated, such as errors in the machine itself and errors due to tool abrasion, rapid fabrication, and tool wear.17 However, no study has focused on identifying the location of errors on the fabricated prosthesis, leading to the lack of standards during reproducibility evaluations. Therefore, 3D evaluations were used to precisely locate errors on presintered single-tooth zirconia copings fabricated with the subtractive method based on dental CAD/CAM technology for different teeth. The null hypothesis was that the reproducibility of presintered zirconia copings fabricated with the subtractive method would not differ for the canine (CAN), premolar (PRE), and molar (MOL) teeth. MATERIAL AND METHODS Acrylic resin abutments with standard chamfer margins (AG-3 ZPVK; Frasaco GmbH) were used in this study. Molds for the maxillary CAN, PRE, and MOL teeth were fabricated with impression material (Aquasil Ultra; Dentsply Intl) and a custom tray7 and were used to prepare casts from Type 4 dental stone (GC Fujirock EP; GC Corp). The stone casts were scanned with a blue light scanner (Identica Blue; Medit) for use with the CAD program (DentCAD; Delcam Ltd). Copings were designed according to the following parameters: 30-mm thickness for the cement on the occlusal surface; 0-mm distance between the coping margin and cement;
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0.5-mm thickness over the remaining surfaces; and 0.15mm thickness at the finishing margin. The initial primary form remained constant and was used for all subsequent procedures (Fig. 1). The designed copings were then milled from presintered zirconia blocks (ZirPremium-DS; Acucera, Inc) by using the subtractive method (DWX-50; Roland DG Corp). The equipment was calibrated before milling to minimize errors. The milling machine used diamondcoated rotary instruments with diameters of 2 mm for rough cutting, 1 mm for finishing, and 0.6 mm for finishing. After the fabrication of each prosthesis, the tools were replaced. Ten test specimens were prepared for each tooth and sequentially numbered from 1 to 10 according to the order of fabrication. To evaluate the reproducibility of the fabricated copings, their intaglio surfaces were scanned with a blue light scanner. The images were cropped to eliminate unnecessary structures and structures beyond the margins. The initial set of scan data for each of the 3 groups was used as a reference for comparisons with the subsequent 9 sets of data. The scanned stereolithography files were transformed into a point-cloud American Standard Code for Information Interchange (ASCII) file with matching software (CopyCAD v7.350 SP3 software; Delcam Ltd) and superimposed with inspection software (PowerINSPECT 2012; Delcam Ltd), creating a total of 9 color-difference maps for each tooth (total of 27 maps). To measure reproducibility, the specimens were replaced in the digital equipment, where they were fabricated and removed.23 The reproducibility analysis method recommended by ISO 1283623 is well known for objective evaluation of the reproducibility of a fabricated prosthesis and involves the calculation of rootmean-square (RMS) ±standard deviation (SD) values. Accordingly, RMS ±SD values were calculated from the color-difference maps as the average of the positive and negative values of coexistence, according to the following formula: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ffi Pn −x x 2;i i=1 1;i pffiffiffi RMS= ; n where n is the total number of specimens, x1;i is the measuring point in the reference scan data, and x2;i is the measurement point in the subsequent scan data. Errors were evaluated on the basis of the derived RMS values, with lower values representing higher reproducibility.23 One-way ANOVA with Tukey honest significant differences (HSD) post hoc tests were used for statistical evaluations of the measurements. Software (SPSS Statistics, v21.0; IBM Corp) was used for all statistical analyses (a=.05).
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Figure 1. Cross-sectional views of representative specimens. A, Cement thickness. Green arrows indicate thickness at crown margin; blue arrows indicate margin-cement distance; orange arrows indicate thickness on side; and purple arrows indicate thickness on occlusal surface. B, Coping thickness. Green arrows indicate thickness at finishing margin, and blue arrows indicate thickness on occlusal surface.
RESULTS Table 1 shows RMS ±SD values for the 3 groups, which were 3.33 ±2.65 mm (95% confidence interval [CI]), 1.305.37 mm for the CAN copings, 4.00 ±2.40 mm (95% CI, 2.16-5.84 mm) for the PRE copings, and 9.22 ±1.56 mm (95% CI, 8.02-10.42 mm) for the MOL copings. Thus, the MOL copings exhibited the highest value, which was significantly different from the values for the CAN and PRE copings (P<.001) (Table 1). One-way ANOVA revealed statistically significant differences in reproducibility values among the 3 groups (P<.001). In a color-difference map (Fig. 1), red depicts a positive error (+) with a value higher than normal, blue depicts a negative error (−) with a value lower than normal, and green depicts the absence of errors (0), that is, the absence of a difference between the reference prosthesis and the test prosthesis in that particular area. Thus, this map enables the visualization of errors on a prosthesis. The color-difference maps for the CAN copings showed positive errors (red) in the region of the cusp tip and vertical negative errors (blue), but most areas had no errors (green). However, the color-difference maps for the PRE copings showed negative errors (blue) on the lingual cusp and positive errors (red) on the buccal cusp, although most areas were free of errors (green). The color-difference maps for the MOL copings showed positive errors (red) in the disto-occlusal line angle (junction between the distal and occlusal surfaces) and negative errors (blue) on the distal surface (Fig. 2). DISCUSSION The reproducibility of the subtractive method based on dental CAD/CAM technology was evaluated with 3D assessments of presintered single-tooth zirconia copings. Kim et al
Table 1. RMS and SD values for presintered canine, premolar, and molar copings fabricated with subtractive method Group RMS ±SD (mm) 95% CI P Canine
3.33 ±2.65A
Premolar
4.00 ±2.40A
2.16-5.84
Molar
9.22 ±1.56B
8.02-10.42
1.30-5.37 <.001
CI, confidence interval; RMS, root mean square; SD, standard deviation. Different superscript letters indicate significant differences (P<.05).
The highest RMS error value was for the MOL copings; thus, the null hypothesis tested in this study was rejected. Moreover, the CAN copings showed the lowest RMS error value, which indicated the highest reproducibility. The use of the subtractive method has increased with the development of dental CAD/CAM systems, necessitating objective assessment of the reliability (errors) of this method. Reproducibility tests are the best indicators of reliability, and the procedure for these tests, which involves the calculation of RMS and SD values, is outlined in the ISO 12836 guidelines.23 The guidelines also mandate that reference data are required for comparisons with test data for the evaluation of reproducibility.23 Jeon et al24,25 assessed precision by conducting 3D assessments of the intaglio surfaces of CAN, PRE, and MOL copings. These assessments and the resulting color-difference maps enable easy error visualization and correction.3,13,15,16,19,20,24,26 In addition, color-difference maps facilitate the identification of the exact site of error. We successfully used similar techniques to determine errors caused by the subtractive method. The CAN copings exhibited the lowest error value (3.33 ±2.65 mm), with positive errors in the cusp tip area, whereas the MOL copings exhibited the highest error value (9.22 ±1.56 mm), with positive errors in the distoocclusal line angle. With regard to the PRE copings, the error value was 4.00 ±2.40 mm, with most of the positive THE JOURNAL OF PROSTHETIC DENTISTRY
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Figure 2. Color difference maps indicate reliability of subtractive method for fabrication of presintered single-tooth zirconia copings. CAN, canine; MOL, molar; PRE, premolar.
errors on the occlusal surface. These findings collectively suggest that errors tend to occur in angular areas. The primary reason for this phenomenon is that the diameter of the tool used in the subtractive method is larger than the angle made with the specimen surface, resulting in errors during the fabrication process (Fig. 2). Therefore, the use of tools with a smaller diameter would be beneficial. However, some studies have suggested that a smaller tool diameter may slow the process and necessitate frequent tool replacement.17,27 Nevertheless, the benefit of increased precision outweighs the limitation of increased fabrication time. A previous study stated that CANs, PRE, and MOL are adequate for assessing the reproducibility of the subtractive method.27 Specifically, CANs have the sharpest form, whereas PREs are relatively rounded with narrow surfaces and sharp cusps. MOLs, however, are rounded with broad surfaces. Accordingly, maxillary CANs, PREs, and MOLs were selected in the present study. The selection of appropriate equipment is essential to ensure a credible reproducibility analysis. Therefore, we selected a 5-axis dental mill with a spring motor that firmly holds the milling burr and minimizes vibration, THE JOURNAL OF PROSTHETIC DENTISTRY
and with the ability to eliminate undercuts.28 Furthermore, the rotary instruments were carefully selected according to strict recommendations from the company and replaced after each prosthesis was fabricated. Calibration was performed before milling to minimize errors. All these measures were based on efforts made in previous studies to ensure the credibility of the results.12,22,28,29 This study had some limitations. First, acrylic resin tooth forms, which imitate an ideal tooth preparation, were used to prepare the stone casts. Second, the zirconia copings were not definitively sintered; therefore, the reproducibility findings do not consider the effects of sintering contraction. Further studies should assess the effects of sintering contraction on the reproducibility of the subtractive method. CONCLUSIONS Results of this study demonstrated the highest reproducibility for the CAN copings. Furthermore, analysis of color-difference maps revealed maximum errors in angular areas. Thus, the clinical reproducibility of the Kim et al
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subtractive method can be improved by avoiding the creation of angles during abutment preparation and by the careful reproduction of angles during prosthesis fabrication. REFERENCES 1. Miyazaki T, Hotta Y, Kunii J, Kuriyama S, Tamaki Y. A review of dental CAD/ CAM: current status and future perspectives from 20 years of experience. Dent Mater J 2009;28:44-56. 2. Petteno D, Schierano G, Bassi F, Bresciano ME, Carossa S. Comparison of marginal fit of 3 different metal-ceramic system: an in vitro study. Int J Prosthodont 2000;13:405-8. 3. Kim KB, Kim JH, Kim WC, Kim JH. Three-dimensional evaluation of gaps associated with fixed dental prostheses fabricated with new technologies. J Prosthet Dent 2014;112:1432-6. 4. Ng J, Ruse D, Wyatt C. A comparison of the marginal fit of crowns fabricated with digital and conventional methods. J Prosthet Dent 2014;112:555-60. 5. Van Noort R. The future of dental devices is digital. Dent Mater 2012;28:3-12. 6. Beuer F, Schweiger J, Edelhoff D. Digital dentistry: an overview of recent developments for CAD/CAM generated restorations. Br Dent J 2008;204: 505-11. 7. Abdel-Azim T, Rogers K, Elathamna E, Zandinejad A, Metz M, Morton D. Comparison of the marginal fit of lithium disilicate crowns fabricated with CAD/CAM technology by using conventional impressions and two intraoral digital scanners. J Prosthet Dent 2015;114:554-9. 8. Lebon N, Tapie L, Vennat E, Mawussi B. Influence of CAD/CAM tool and material on tool wear and roughness of dental prostheses after milling. J Prosthet Dent 2015;114:236-47. 9. Seydler B, Schmitter M. Clinical performance of two different CAD/CAMfabricated ceramic crowns: 2-year results. J Prosthet Dent 2015;114:212-6. 10. Anadioti E, Aquilino SA, Gratton DG, Holloway JA, Denry IL, Thomas GW, et al. Internal fit of pressed and computer-aided design/computer-aided manufacturing ceramic crowns made from digital and conventional impressions. J Prosthet Dent 2015;113:304-9. 11. Lins L, Bemfica V, Queiroz C, Canabarro A. In vitro evaluation of the internal and marginal misfit of CAD/CAM zirconia copings. J Prosthet Dent 2015;113: 205-11. 12. Lei XB, Xie F, Zhao JW. Design of a 5-axis CNC machine tool applied to dental restoration. Key Eng Mater 2013;568:75-80. 13. Moldovan O, Lut’t RG, Corcodel N, Rudolph H. Three-dimensional fit of CAD/CAM-made zirconia copings. Dent Mater 2011;27:1273-8. 14. Grenade C, Mainjot A, Vanheusden A. Fit of single tooth zirconia copings: comparison between various manufacturing processes. J Prosthet Dent 2011;105:249-55. 15. Schaefer O, Watts DC, Sigusch BW, Kuepper H, Guentsch A. Marginal and internal fit of pressed lithium disilicate partial crowns in vitro: a three-dimensional analysis of accuracy and reproducibility. Dent Mater 2012;28:320-6. 16. Schaefer O, Schmidt M, Goebel R, Kuepper H. Qualitative and quantitative three-dimensional accuracy of a single tooth captured by elastomeric impression materials: an in vitro study. J Prosthet Dent 2012;108:165-72.
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