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Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs
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Journal of Prosthodontic Research journal homepage: www.elsevier.com/locate/jpor
Original article
Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs Nawal Alharbia,b,* , Saud Alharbic , Vincent M.J.I. Cuijpersd, Reham B. Osmane,f , Daniel Wismeijerg a Department of Oral Implantology and Prosthetic Dentistry, Academic Centre for Dentistry Amsterdam (ACTA), University of Amsterdam and Vrije Universiteit Amsterdam, Gustav Mahlerlaan 3004, 1081LA Amsterdam, Netherlands b Department of Prosthetic Dental Science, King Saud University, College of Dentistry, Riyadh, Saudi Arabia c Prince Sultan Military Medical City, Riyadh, Saudi Arabia d Department of Biomaterials, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands e Removable Prosthodontics Department, Faculty of Dentistry, Cairo University, Egypt f Department of Oral Implantology and Prosthetic Dentistry, Academic Centre for Dentistry Amsterdam (ACTA), Universiteit van Amsterdam and Vrije Universiteit Amsterdam, The Netherlands g Department of Oral Implantology and Prosthetic Dentistry, Academic Centre for Dentistry Amsterdam (ACTA), Universiteit van Amsterdam and Vrije Universiteit Amsterdam, The Netherlands
A R T I C L E I N F O
Article history: Received 15 February 2017 Received in revised form 10 July 2017 Accepted 23 September 2017 Available online xxx Keywords: 3D-SLA printing Finish line design Marginal and internal fit Micro-CT Additive manufacturing
A B S T R A C T
PurposeTo evaluate the influence of fabrication method and finish line design on marginal and internal fit of full-coverage interim restorations. MethodsFour typodont models of maxillary central-incisor were prepared for full-coverage restorations. Four groups were defined; knife-edge (KE), chamfer (C), rounded-shoulder (RS), rounded-shoulder with bevel (RSB). All preparations were digitally scanned. A total of 80 restorations were fabricated; 20 per group (SLA/3D-printed n = 10, milled n = 10). All restorations were positioned on the master die and scanned using micro-computed tomography. The mean gaps were measured digitally (ImageJ). The results were compared using MANOVA (a = .05). ResultsInternal and marginal gaps were significantly influenced by fabrication method (P = .000) and finish-line design (P = .000). 3D-Printed restorations showed statistically significant lower mean gap compared to milled restorations at all points (P = .000). The mean internal gap for 3D-printed restorations were 66, 149, 130, 95 mm and for milled restorations were 89, 177, 185, 154 mm for KE, C, RS, RSB respectively. The mean absolute marginal discrepancy in 3D-printed restorations were (30, 41, 30, 28 mm) and in milled restorations were (56, 54, 52, 38 mm) for KE, C, RS, RSB respectively. ConclusionsThe fabrication methods showed more of an influence on the fit compared to the effect of the finish-line design in both milled and printed restorations. SLA-printed interim restorations exhibit lower marginal and internal gap than milled restorations. Nonetheless, for both techniques, all values were within the reported values for CAD/CAM restorations. Significance3D-printing can offer an alternative fabrication method comparable to those of milled restorations. © 2017 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.
1. Introduction
* Corresponding author at: Department of Oral Implantology and Prosthetic Dentistry, Academic Centre for Dentistry, Amsterdam (ACTA), University of Amsterdam and Vrije Universiteit Amsterdam, Gustav Mahlerlaan 3004, 1081 LA Amsterdam, The Netherlands. E-mail address: [email protected] (N. Alharbi).
In contemporary prosthodontics, interim restorations are used as a transitional phase to evaluate the functional and esthetical outcomes [1–4]. The restorations should offer adequate fit to ensure mechanical stability and durability of the restoration and thus the health of the surrounding tissues [5,6]. Lack of adequate fit can result in plaque accumulation, cement microleakage, marginal
https://doi.org/10.1016/j.jpor.2017.09.002 1883-1958/ © 2017 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: N. Alharbi, et al., Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs, J Prosthodont Res (2017), https://doi.org/10.1016/j.jpor.2017.09.002
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discoloration, poor esthetics, teeth sensitivity, caries and periodontal diseases [5]. Wide variation in the definition of adequate fit and a clinically acceptable marginal gap exists in the literature and can be attributed to different study designs including different restorative materials, examination method, finish line designs and fabrication methods [7]. A recent systematic review on the fit of CAD/CAM restorations fabricated from different materials reported an absolute marginal gap (AMG) of crowns to be in the range from 10–110 mm with the majority of values less than 80 mm. In the same context, the internal gap at axial part was 23–154 mm and at occlusal part was 45–219 mm [8]. With the ongoing developments in the field of data acquisition and manufacturing processes, digital technologies are gaining more acceptances. Additive manufacturing (AM) is being increasingly applied in the field of prosthodontics. Dental restorations can be fabricated using resin based AM techniques both directly and indirectly; stereolithography and or digital light processing (SLA/ DLP) [2,4,9–13]. Both techniques use light/laser to cure a photosensitive liquid polymer layer by layer, following a specific path of the designed model. Limited studies have evaluated the marginal and/or internal fit of AM copings fabricated using SLA/DLP techniques. Kim et al. [10] found that the mean marginal gap of Co–Cr crowns was significantly larger when fabricated indirectly using SLA technique (96 mm) compared to traditional, lost-wax fabrication method (67 mm). The authors also reported an axial and an occlusal gap of 84 mm and 114 mm respectively for the indirectly manufactured SLA crowns [10]. In contrast, Park et al. [2] found PMMA DLPfabricated implant restorations to have better marginal fit compared to milled and conventionally fabricated counterparts fabricated from PEEK and PMMA respectively. Nevertheless, the authors found the fit of the three manufacturing methods to be within the clinically acceptable range. Munoz et al. [12] used DLP technique to fabricate wax patterns for indirect manufacturing of cast gold crowns and revealed that the marginal gap was significantly larger for DLP fabricated patterns compared to the milled or manually fabricated wax patterns. In the same context, different finish line designs have been shown to influence the marginal gap of the dental restorations. Comlekoglu et al. [14] compared the marginal gap of zirconia crowns with knife-edge, mini-chamfer, chamfer and rounded
shoulder finish line designs and found the lowest absolute marginal discrepancy values with knife-edge design (87 mm) compared to mini-chamfer (114 mm), chamfer (144 mm) and rounded shoulder (114 mm) finish line designs. Euán et al. [15] found lower mean marginal gap values for Lava All-Ceramic System with round shoulder finish-line design (52 mm) compared to chamfer counterpart (64 mm). In contrast, Tsitrou et al. [16] and Akbar et al. [17] revealed no significant difference in margnial gap of dental restorations with shoulder and chamfer finish line designs. The margin gap of CEREC 3 system composite resin crowns was 94 mm in chamfer finish line and 91 mm with shoulder finish line [16]. To the authors’ best knowledge, no previous study has evaluated the internal and marginal adaptation of 3D-printed directly fabricated restorations using the SLA technique with different finish line designs. Therefore, the aim of this study was to evaluate the influence of fabrication method and finish line design on the marginal and internal fit of full coverage interim dental restorations in two digital workflows (3D-printing versus milling). Our null hypothesis is that there is no difference in marginal and internal fit between 3D-printing and milled restorations in different finish line designs. 2. Materials and methods 2.1. Study design Four typodont models of maxillary central incisors were prepared to receive full coverage dental restorations; each with a different finish line design. The four finish line designs were; knife-edge (KE), chamfer (C), rounded shoulder (RS), rounded shoulder with bevel (RSB). In both digital workflows, the prepared models were digitally scanned and then the data was exported as a standard tessellation language (STL) format to a CAD software for the design and the fabrication of full coverage restorations (n = 80). For each finish line design, the restorations were fabricated using 3D-printing (n = 10) and milling (n = 10) techniques. All restorations were fitted on the master die without cementation and scanned using micro-computed tomography (Skyscan 1072, Bruker microCT, Kontich) [18]. The marginal and internal gaps were measured using digital imaging processing software (ImageJ 1.51; NIH). The flow of the study design is shown in Fig. 1.
Fig. 1. Flow of study design.
Please cite this article in press as: N. Alharbi, et al., Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs, J Prosthodont Res (2017), https://doi.org/10.1016/j.jpor.2017.09.002
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2.2. Crown preparation The models were prepared following the basic principles of crown preparation [19]. All models were prepared with 2 mm incisal reduction. Four finish lines designs were used; KE, 1 mm (C), 1 mm (RS) and 1 mm (RSB) [19]. All teeth were prepared with the use of silicon index [20], together with the use of magnifying loupe (30) for better visualization. The preparations were done using diamond burs kit (LOT:22483; Horico) under continuous watercooling. The prepared models were scanned using desktop optical scanner (Medit Identica BlueTM; Renishaw). The scanner was calibrated following the manufacturer’s instructions. A thin layer of anti-reflective powder coating was sprayed at a fixed distance of 20 cm for all the specimens to enhance the scanning procedure (Helling 3D Scan Spray; Helling GmbH) [21,22]. The digital files were exported in STL format, which were used for the design of restorations. 2.3. Restorations design and CAD/CAM fabrication The restorations were designed using 3-Shape Dental SystemTM CAD solution version 2015. The design parameters were standardized for all groups. A cement gap of 30 mm was defined [23]. Additional vertical space of 80 mm and a horizontal space of 30 mm was added as recommended by an experienced dental technician. One trained technician printed the restorations using stereolithography-based 3D-printer (DW028D, DWS) in a hybrid composite resin material (Temporis1, shade A2, LOT: 040725; DWS). The thickness of build layer was .05 mm and the maximum laser speed was 5000 mm/s [24]. The printed specimens were cleaned with 95% ethanol for one minute and post-cured using ultraviolet curing unit (S2; DWS) for 30 min as per the manufacturer’s instructions [24]. The milled restorations were fabricated using 5-axis milling machine (Wissner Ltd.; Germany) in PMMA-based acrylate resin (Polycon1 ae; Straumann; shade A2) with a bur diameter of 1 and 3 mm following manufacturer’s recommendation [25]. All specimens were then stored in a dry, lightproof box and tested within ten days of the manufacturing process. 2.4. Micro-CT scanning and analysis Before scanning, all restorations were visually examined for any manufacturing defects, using optical magnification loupe 3.5. Each restoration was manually placed on the master die without cementation and without any manual adjustment [18]. The position of each crown was visually checked before scanning; where the crown was stable without any rotation or displacement [18]. The specimens were scanned using a commercially available desktop X-ray micro-tomography system scanner (Skyscan 1072,
Fig. 3. Slicing protocol for all scanned restorations.
Bruker micro-CT, Kontich). A custom holder was fabricated to standardize the position of the specimens during scanning and to ensure that the long axis of the tooth was perpendicular to the scanning beam. The specimen holder involves a transverse cylindrical hole, which allows the X-ray beam to pass through. The specimen position was verified on the screen by a visible well defined, sharp and clear circle formed when the X-ray beam pass and run parallel to the transverse circular hole within the holder (Fig. 2). A scan resolution of 11 mm was defined for all specimens where the whole crown and coronal 1/3 of the root were included in one scan. Scans were recorded at X-ray source set to 100 kV and 98 mA, a 180 sample rotation and 10-frames averaging, a rotation step of .90 , an exposure time of 3.8 s and a 1 mm thick aluminum filter to optimize image contrast. After scanning, a cone beam reconstruction was performed to obtain 11 mm pixels, 2D-images with NRecon v.1.6.9.18 software. The ring artifact correction was set to 20 and beam hardening correction to 4%. For the analysis, six slices were selected from each specimen; three in mesio-distal (MD) and three in labio-palatal (LP) direction. The distance between each of the selected measuring points was 60 slices in LP and 110 slices in MD direction (Fig. 3). The slicing of the images was performed with CTanalyzer v.1.14 software. A total of 480 images were transferred to Image processing software for analysis. One blinded and trained examiner performed the measurements of all images (n = 480) by using Image processing software (ImageJ 1.51; NIH). A second examiner was consulted for any
Fig. 2. Custom-made specimen holder with visible circle indicating that X-rays run parallel to the transverse hollow cylinder.
Please cite this article in press as: N. Alharbi, et al., Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs, J Prosthodont Res (2017), https://doi.org/10.1016/j.jpor.2017.09.002
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Fig. 4. Internal and marginal fit measurement points on; (A) labio-palatal (LP) sections, (B) mesio-distal (MD) sections, (C) marginal fit measurement points on both (LP) and (MD) sections.
difficulty encountered. Prior to each reading, the measurement tool was calibrated by measuring the pixel size; one pixel was equal to 11 mm. All measurements were taken under the magnification of 400. The internal and marginal fit was measured at several defined points on MD and LP sections as shown in Fig. 4. The internal fit involves both the incisal gap and mid-axial gap measured at the middle of mid-axial wall. Further for better interpretation of the results, the data of the incisal gap and mid-axial gap were reported separately. The marginal fit was reported by using the vertical gap (VG), horizontal gap (HG) and absolute marginal discrepancy (AMD). Each point was measured three times and the average was used. For each tooth, a total of 51 measurements (15 for internal fit and 36 for marginal fit) were collected as follows; on each BL slice: (2 midaxial) + (1 incisal) + (2 VG) + (2 HG) + (2 AMD) = 9. Measurement on each MD slice: (2 mid-axial) + (2 VG) + (2 HG) + (2 AMD) = 8. Thus, the total measurements per tooth: (9 3 slices) + (8 3 slices) = 51. 2.5. Examiner calibration Examiner calibration was done prior to the study. The blinded examiner (A.S) was calibrated against an experienced examiner (A. N). A total of 20 2D-images were used for the calibration procedure. The internal and marginal gaps were measured on all 2D-images. After 10 days, the gaps were re-measured. Intra-examiner and inter-examiner reliability were analyzed using interclass correlation (ICC). 2.6. Statistical analysis The mean and standard deviations of internal and marginal gaps were analyzed using descriptive statistics. The data were checked for normality distribution and equivalence of variances. The results were compared using multivariate analysis of variance (MANOVA). A follow-up simple main effect test with Bonferroni corrections was used to evaluate the difference between the fabrication methods in each finish line design (a = .05). One-way ANOVA was used to analyze the influence of finish line design on the marginal and internal gap of the restorations in each fabrication method separately. 3. Results The intra-examiner reliability was .98 and the absolute agreement between the two examiners was .97 and thus eliminating any possible measurement or examiner bias.
The mean and standard deviation of the marginal and internal gap of all specimens are shown in Tables 1 and 2. The result of MANOVA was reported using Pillai’s trace multivariate test as the assumption of homogeneity of variance-covariance matrices was violated (Box’s M test, P = .000) [26]. The mean internal and marginal gap were significantly influenced by fabrication method (P = .000) and finish line design (P = .000). Yet the influence of fabrication method was more as shown by the Partial Eta squared value of (.96) compared to (.66) for finish line design. MANOVA showed statistically significant interaction effect between fabrication methods and finish line design, F (18,207) = 8.15, P = .000; Pillai’s trace = 1.245. Step-down univariate analysis showed that the interaction between fabrication methods and finish line designs was statistically significant at incisal, internal, mid-axial, VG and HG. On the contrary, at AMD, the fabrication method showed similar effect in different finish line designs (P = .133). The 3D-Printed restorations showed lower estimated mean marginal and internal gap values compared to milled restorations. The difference was statistically significant at all points (P = .000). Amongst all points, the incisal gap was the largest in both 3D-printed and milled restorations (Tables 1 and 2) (Fig. 5). MANOVA simple main effect showed that the mean internal gap, mid-axial and AMD of 3D-printed restorations were significantly lower than that of milled restorations for all finish lines designs (P <.05). The difference at incisal gap was significant in all finish lines except in KE design (P = .662), whereas for the HG the difference was significant in all finish line designs except in C design (P = .219). The difference at VG was significant in all finish lines except RSB design (P = .142). The results of one-way ANOVA revealed that the mean internal gap and AMD were significantly different between different finish line designs (P = .000) in 3D-printed restorations, whereas in milled restorations only mean internal gap was significantly different in different finish lines (Table 3). Tukey post hoc showed that in 3D-printed restorations the lowest mean internal gap was in KE design, the difference between different finish lines was statistically significant (P <.05) (Fig. 6A). Similarly, in milled restorations the lowest mean internal gap was in KE design. The difference was statistically significant (P = .000) between all designs except S and C (P = .249). The lowest AMD for 3D-printed restorations was with RSB design (Fig. 6B). The difference between the groups was not significant except for C design (P <.05). Likewise, the lowest mean AMD in milled restorations was with RSB finish line (P >.05) (Fig. 7A and B).
Please cite this article in press as: N. Alharbi, et al., Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs, J Prosthodont Res (2017), https://doi.org/10.1016/j.jpor.2017.09.002
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Table 1 Mean and standard deviation (SD) of internal gap (sum of incisal gap + mid-axial gap).
Incisal gap
Fabrication methods
Finish line
Specimens number (N)
Mean (mm)
Standard deviation (SD)
3D-printed
KE C RS RSB Total KE C RS RSB Total KE C RS RSB Total
10 10 10 10 40 10 10 10 10 40 20 20 20 20 80
93 213 210 161 169 97 271 259 208 209 95 242 235 184 189
14 31 6 15 52 17 18 19 11 71 15 38 29 27 65
KE C RS RSB Total KE C RS RSB Total KE C RS RSB Total
10 10 10 10 40 10 10 10 10 40 20 20 20 20 80
39 45 51 29 41 80 84 111 99 94 59 64 81 64 67
5 3 4 2 9 6 7 9 7 14 22 20 32 36 29
KE C RS RSB Total KE C RS RSB Total KE C RS RSB Total
10 10 10 10 40 10 10 10 10 40 20 20 20 20 80
66 149 130 95 110 89 177 185 154 151 77 163 158 124 131
8 15 4 7 33 8 10 11 7 39 14 19 29 31 42
Milling
Total
Mid-axial gap
3D-printed
Milling
Total
Internal gap
3D-printed
Milling
Total
Knife edge (KE), chamfer (C), rounded shoulder (RS), rounded shoulder with bevel (RSB).
4. Discussion This in vitro study evaluated the effect of fabrication methods in two digital workflows; 3D-printing versus milling and four finish line designs (KE, C, RS, RSB) on the marginal and internal fit of full coverage interim dental restorations. Based on the results, the null hypothesis that no difference exists in marginal and internal fit between 3D-printing and milled restorations in different finish line designs was rejected. In this study, only a maxillary central incisor model was evaluated to eliminate the influence of different tooth geometries [27]. In addition, all restorations were analyzed without cementation, which is not a true representation of the actual clinical situation. Nonetheless, it has been shown that the use of cement may lead to improper seating of the restoration and thus increases the marginal and internal gap [7]. Furthermore, the radiopaque nature of the cement may result in radiation artifacts during the procedure of micro-CT scanning, an artifact that may impede the measurements of the internal and marginal gaps [18,28].
In the literature, several methods are reported to evaluate the marginal and internal fit of the restorations [7]. In this study, micro-CT was used. This approach offers several advantages of being non-destructive, and allows for quantitative measurements in three dimensions [18,28]. However, shortcoming of this method is the radiation artifacts resulting from the difference in coefficient of radiation absorption among different materials [28]. A custommade holder was fabricated to standardize the position of the specimens during the scanning procedures. The maximum magnification to visualize the complete crown resulted in pixel size of 11 mm. During the reslicing, the locations of the 2D-slices were selected so that the measurement points were widely distributed on the tooth without any distortion of the image. (Fig. 3) The measurements of the selected points on the 2D-slices allowed for 3Danalysis of the fit of the restorations. Marginal fit was evaluated by measuring AMD, VG and HG to have an in-depth view of the marginal misfit of the restorations [29]. The number of measurement points for marginal gap was 36, which has been shown to maintain the precision level of the measurements [7,28–30].
Please cite this article in press as: N. Alharbi, et al., Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs, J Prosthodont Res (2017), https://doi.org/10.1016/j.jpor.2017.09.002
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Table 2 Mean and standard deviation (SD) of marginal gap.
Vertical gap (VG)
Fabrication methods
Finish line
Specimens number (N)
Mean (mm)
Standard deviation (SD)
3D-printed
KE C RS RSB Total KE C RS RSB Total KE C RS RSB Total
10 10 10 10 40 10 10 10 10 40 20 20 20 20 80
16 25 21 26 22 39 40 34 19 33 27 33 27 23 28
10 6 4 10 8 21 5 13 4 15 20 9 12 8 13
KE C RS RSB Total KE C RS RSB Total KE C RS RSB Total
10 10 10 10 40 10 10 10 10 40 20 20 20 20 80
25 22 20 25 23 31 25 41 33 32 28 23 30 30 28
9 4 3 4 6 8 3 8 4 8 9 4 12 6 8
KE C RS RSB Total KE C RS RSB Total KE C RS RSB Total
10 10 10 10 40 10 10 10 10 40 20 20 20 20 80
30 41 30 28 32 56 54 52 38 50 43 48 41 33 41
10 5 5 2 8 25 5 16 3 16 23 8 5 6 15
Milling
Total
Horizontal gap (HG)
3D-printed
Milling
Total
Absolute marginal discrepancy (AMD)
3D-printed
Milling
Total
Knife edge (KE), chamfer (C), rounded shoulder (RS), rounded shoulder with bevel (RSB).
Internal fit was evaluated by measuring the incisal and mid-axial gaps, yet data of the mid-axial and incisal gaps were reported separately to provide details of the internal misfit of the restorations. The results were analyzed using MANOVA rather than multiple two-way ANOVA’s to minimize the inflation of type I error [26,31]. The results showed that 3D-printed restorations exhibit significantly lower marginal and internal gap values than their milled counterparts. The inferior marginal and internal fit of milled restorations may be attributed to errors resulting from the tolerance of milling burs [32,33]. Nevertheless, the mean AMG marginal gaps of both 3D-printed and milled restorations were within the reported values for the CAD/CAM restorations (10– 110 mm) [8]. Among all measurement points, the largest gap in both fabrication methods was found at the incisal part (169 mm for 3D-printing and 209 mm for milling). This finding was in accordance with previous studies [2,27,34–37]. In the present study, the measured incisal gap was 169 mm and 209 mm which were greater than the programmed incisal cement space by
1.5 and 1.8 for both the printed and milled restorations respectively. This finding was different than the results of other reported studies. Kokubo et al. [34] reported an incisal gap of 170 mm, which is 3 to 4 larger than the programmed cement space. Other reports revealed an incisal gap and an occlusal gap that ranged from 110 to 204 mm, that was 5 to 6 more than the programmed cement space [2,35–37]. The reason for the increased incisal gap and whether it is related to incorporated spaces during the design process still merits further investigation. Several design parameters including programmed cement gaps are proposed based on the required cement thickness and are mostly recommended based on operators’ and technicians’ experience and the available software [9,33,37]. The programmed cement thickness reported in literature varied from no gap [11], to 10 mm [3], till 15 mm with an additional vertical space of 65 mm and a horizontal space of 50 mm [38], 30 mm [2,10,23], 50 mm [34], 60 mm [1], till 85 mm [9]. Hoang et al. [9] evaluated the reproducibility of 25 mm, 45 mm, 65 mm, 85 mm and 105 mm cement gap of DLP 3D-printed resin copings and found that the 85 mm cement gap was the most reproducible [9]. In the same
Please cite this article in press as: N. Alharbi, et al., Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs, J Prosthodont Res (2017), https://doi.org/10.1016/j.jpor.2017.09.002
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Fig. 5. Estimated mean of internal and marginal gap in 3D-printed and milled restorations.
context, the results of the current study revealed that the mid-axial gap in 3D-printed restorations was 41 mm, which is lower than the programmed cement space (60 mm). Thus, it may be recommended that the programmed axial gap be increased to enhance the marginal fit though this still requires to be confirmed with further studies (Fig. 6A and B). This lack of consistency between measured axial gaps and programed incorporated cement space may be due to an error occurring during the slicing procedure of the STL file. Such an error can minimize the accuracy of the fitting surface of printed restorations compared to the designed model. On the contrary, the larger mid-axial gap of the milled restorations was attributed to errors from the tolerance of milling burs [32]. Similarly, the incisal gap in KE design was lower than the programmed incisal space in both fabrication methods. Vertical displacement of the restoration is unlikely to be the reason considering the values of the measured vertical gap (Table 1). The results of MANOVA showed that the finish line design is less influential on the marginal and internal fit of interim restorations compared to the fabrication method. Several studies evaluated the influence of finish line design on marginal and internal fit of milled
dental restorations with controversial findings [14–17]. This can be viewed in the light of different experimental designs, different material, fabrication and analysis methods. The findings of this study revealed that in both fabrication methods KE design has the least internal gap and RSB design showed the least AMD. The difference in AMD between different finish line designs was not significant in milled restorations. In 3Dprinted restorations, the C design showed significantly larger AMD than the other finish line designs. This may be attributed to the incremental layer pattern of build process in the 3D printing technology. The curved surface of the axio-gingival line angle in C design may cause an increased stair-stepping error during the slicing of the STL file. Therefore, the resultant increase in the values of the AMD recorded with the C design in the printed group. Although the results revealed that KE design exhibit the least internal gap in both fabrication methods, it cannot be recommended for current clinical practice. KE design represents a very conservative approach, which may limit the choice of restorative material. In addition, the visibility of KE finish line on digital or analogue model might not be optimal. Gingival bulk has been
Table 3 The influence of finish line designs on internal and AMD (mm) in both fabrication methods using One-way ANOVA. Fabrication methods 3D-printing
Internal gap
Absolute marginal discrepancy
Milling
Internal gap
Absolute marginal discrepancy
Sum of squares
df
Mean square
F
Significance
Between groups Within groups Total Between groups Within groups Total
.040 .003 .044 .001 .001 .003
3 36 39 3 36 39
.013 .000
145
.000
.000 .000
9.43
.000
Between groups Within groups Total Between groups Within groups Total
.057 .003 .061 .002 .008 .010
3 36 39 3 36 39
.019 .000
209
.000
.001 .000
2.83
.052
Please cite this article in press as: N. Alharbi, et al., Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs, J Prosthodont Res (2017), https://doi.org/10.1016/j.jpor.2017.09.002
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Fig. 6. Three dimensional reconstructed micro-CT image of the 3D-printed restoration in; (A) KE finish-line, (B) RSB finish-line.
Fig. 7. Influence of finish line design on internal fit and AMD in; (A) 3D-printing, (B) in milling. Different letters indicate statistical significance between different finish line design.
reported in some cases with KE design due to insufficient room for the restorative material [14]. On the contrary, RSB design showed the lowest AMD values in both fabrication methods, as well as an internal gap following the KE design (Fig. 6A and B). Improved marginal adaptation associated with reduced AMD values combined with acceptable internal fit values observed may be an indication for recommending RSB design for SLA-3D printed restorations. However, generalizability of the results should be regarded with caution. In this study, only one clinically available material and one system for each fabrication method was applied. Whether the interpretation given for the findings of this study can be applied to different materials and different systems still needs to be evaluated especially considering that such an interim restoration would still be followed with a definitive restoration. In addition, more studies are still needed to evaluate the influence of cementation procedure as well as the single versus multi-unit fixed partial dentures. The optimal cement gap, extra vertical and horizontal space values that should be incorporated during
designing of the restoration are also parameters that still need to be further assessed. 5. Conclusion Within the limitation of this in vitro study, fabrication methods showed more of an influence on the marginal and internal fit of dental restorations compared to the effect of the design of the finish line. Restorations fabricated using 3D-printing techniques exhibited lower marginal and internal gap than restorations fabricated using milling techniques. Nevertheless, in both fabrication methods, all values were within the reported values for CAD/ CAM restorations.
Acknowledgments The authors would like to thank DWS, Italy for supplying the specimens used in this study. Additionally, the authors would like
Please cite this article in press as: N. Alharbi, et al., Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs, J Prosthodont Res (2017), https://doi.org/10.1016/j.jpor.2017.09.002
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to thank Irene H.A. Aartman (Assistant Professor in statistics at ACTA) for her statistical advice. This study was supported by a scholarship grant number 2/302626 from King Saud University, Riyadh, Kingdom of Saudi Arabia. References [1] Khng KYK, Ettinger RL, Armstrong SR, Lindquist T, Gratton DG, Qian F. In vitro evaluation of the marginal integrity of CAD/CAM interim crowns. J Prosthet Dent 2016;115:617–23. [2] Park J-Y, Lee J-J, Bae S-Y, Kim J-H, Kim W-C. In vitro assessment of the marginal and internal fits of interim implant restorations fabricated with different methods. J Prosthet Dent 2016;116:536–42. [3] Abdullah AO, Tsitrou EA, Pollington S. Comparative in vitro evaluation of CAD/ CAM vs conventional provisional crowns. J Appl Oral Sci 2016;24:258–63. [4] Joo H-S, Park S-W, Yun K-D, Lim H-P. Complete-mouth rehabilitation using a 3D printing technique and the CAD/CAM double scanning method: a clinical report. J Prosthet Dent 2016;116:3–7. [5] Burns DR, Beck DA, Nelson SK. A review of selected dental literature on contemporary provisional fixed prosthodontic treatment: report of the committee on research in fixed prosthodontics of the academy of fixed prosthodontics. J Prosthet Dent 2003;90:474–97. [6] Tuntiprawon M, Wilson PR. The effect of cement thickness on the fracture strength of all-ceramic crowns. Aust Dent J 1995;40:17–21. [7] Nawafleh NA, Mack F, Evans J, Mackay J, Hatamleh MM. Accuracy and reliability of methods to measure marginal adaptation of crowns and FDPs: a literature review. J Prosthodont 2013;22:419–28. [8] Boitelle P, Mawussi B, Tapie L, Fromentin O. A systematic review of CAD/CAM fit restoration evaluations. J Oral Rehabil 2014;41:853–74. [9] Hoang LN, Thompson GA, Cho S-H, Berzins DW, Ahn KW. Die spacer thickness reproduction for central incisor crown fabrication with combined computeraided design and 3D printing technology: an in vitro study. J Prosthet Dent 2015;113:398–404. [10] Kim K-B, Kim J-H, Kim W-C, Kim J-H. In vitro evaluation of marginal and internal adaptation of three-unit fixed dental prostheses produced by stereolithography. Dent Mater J 2014;33:504–9. [11] Kim D-Y, Jeon J-H, Kim J-H, Kim H-Y, Kim W-C. Reproducibility of different arrangement of resin copings by dental microstereolithography: evaluating the marginal discrepancy of resin copings. J Prosthet Dent 2017;117:260–5, doi:http://dx.doi.org/10.1016/j.prosdent.2016.07.007. [12] Munoz S, Ramos V, Dickinson DP. Comparison of margin discrepancy of complete gold crowns fabricated using printed, milled, and conventional hand-waxed patterns. J Prosthet Dent 2017;118:89–94, doi:http://dx.doi.org/ 10.1016/j.prosdent.2016.09.018. [13] Sancho-Puchades M, Fehmer V, Hämmerle C, Sailer I. Advanced smile diagnostics using CAD/CAM mock-ups. Int J Esthet Dent 2014;10:374–91. [14] Comlekoglu M, Dundar M, Özcan M, Gungor M, Gokce B, Artunc C. Influence of cervical finish line type on the marginal adaptation of zirconia ceramic crowns. Oper Dent 2009;34:586–92. [15] Euán R, Figueras-Álvarez O, Cabratosa-Termes J, Oliver-Parra R. Marginal adaptation of zirconium dioxide copings: influence of the CAD/CAM system and the finish line design. J Prosthet Dent 2014;112:155–62. [16] Tsitrou EA, Northeast SE, van Noort R. Evaluation of the marginal fit of three margin designs of resin composite crowns using CAD/CAM. J Dent 2007;35:68–73. [17] Akbar JH, Petrie CS, Walker MP, Williams K, Eick JD. Marginal adaptation of Cerec 3 CAD/CAM composite crowns using two different finish line preparation designs. J Prosthodont 2006;15:155–63.
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[18] Borba M, Cesar PF, Griggs JA, Della Bona Á. Adaptation of all-ceramic fixed partial dentures. Dent Mater 2011;27:119–26. [19] Goodacre CJ, Campagni WV, Aquilino SA. Tooth preparations for complete crowns: an art form based on scientific principles. J Prosthet Dent 2001;85:363–76. [20] Martínez-Rus F, Suárez MJ, Rivera B, Pradíes G. Evaluation of the absolute marginal discrepancy of zirconia-based ceramic copings. J Prosthet Dent 2011;105:108–14. [21] Alharbi N, Osman R, Wismeijer D. Factors influencing the dimensional accuracy of 3D-printed full coverage dental restorations using stereolithography technology. Int J Prosthodont 2016;29:503–10. [22] Osman R, Alharbi N, Wismeijer D. Build angle, does it have an influence on the accuracy of 3D-printed dental restorations using digital light-processing technology. Int J Prosthodont 2017;30:182–8. [23] Wu JC, Wilson PR. Optimal cement space for resin luting cements. Int J Prosthodont 1994;7:209–15. [24] DWS. Dental lab additive manufacturing systems. Italy: DWS; 2013. [25] AG IS. Straumann CARES tooth-born prosthetic procedures. Switzerland: Straumann; 2012. p. 10–30. [26] Tabachnick BG, Fidell LS. Using multivariate statistics. 5th ed. United States: Allyn and Bacon; 2008. p. 243–68. [27] Boening KW, Wolf BH, Schmidt AE, Kästner K, Walter MH. Clinical fit of Procera All Ceram crowns. J Prosthet Dent 2000;84:419–24. [28] Mously HA, Finkelman M, Zandparsa R, Hirayama H. Marginal and internal adaptation of ceramic crown restorations fabricated with CAD/CAM technology and the heat-press technique. J Prosthet Dent 2014;112:249–56. [29] Holmes JR, Bayne SC, Holland GA, Sulik WD. Considerations in measurement of marginal fit. J Prosthet Dent 1989;62:405–8. [30] Groten M, Axmann D, Pröbster L, Weber H. Determination of the minimum number of marginal gap measurements required for practical in vitro testing. J Prosthet Dent 2000;83:40–9. [31] Haase RF, Ellis MV. Multivariate analysis of variance. J Couns Psychol 1987;34:404–13. [32] Örtorp A, Jönsson D, Mouhsen A, Vult von Steyern P. The fit of cobalt– chromium three-unit fixed dental prostheses fabricated with four different techniques: a comparative in vitro study. Dent Mater 2011;27:356–63. [33] 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. [34] Kokubo Y, Nagayama Y, Tsumita M, Ohkubo C, Fukushima S, Steyern P. Clinical marginal and internal gaps of In-Ceram crowns fabricated using the GN-I system. J Oral Rehabil 2005;32:753–8. [35] Martins LM, Lorenzoni FC, Melo AOd, Silva LMd, Oliveira JLGd, Oliveira PCGd, et al. Internal fit of two all-ceramic systems and metal-ceramic crowns. J Appl Oral Sci 2012;20:235–40. lu E, Özkan YK, Yildiz C. Comparison of marginal and internal fit of [36] Bayramog press-on-metal and conventional ceramic systems for three-and four-unit implant-supported partial fixed dental prostheses: an in vitro study. J Prosthet Dent 2015;114:52–8. [37] Shamseddine L, Mortada R, Rifai K, Chidiac JJ. Marginal and internal fit of pressed ceramic crowns made from conventional and computer-aided design/ computer-aided manufacturing wax patterns: an in vitro comparison. J Prosthet Dent 2016;116:242–8. [38] Anunmana C, Charoenchitt M, Asvanund C. Gap comparison between single crown and three-unit bridge zirconia substructures. J Adv Prosthodont 2014;6:253–8.
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journal of prosthodontic research xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
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Original article
Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs Nawal Alharbia,b,* , Saud Alharbic , Vincent M.J.I. Cuijpersd, Reham B. Osmane,f , Daniel Wismeijerg a Department of Oral Implantology and Prosthetic Dentistry, Academic Centre for Dentistry Amsterdam (ACTA), University of Amsterdam and Vrije Universiteit Amsterdam, Gustav Mahlerlaan 3004, 1081LA Amsterdam, Netherlands b Department of Prosthetic Dental Science, King Saud University, College of Dentistry, Riyadh, Saudi Arabia c Prince Sultan Military Medical City, Riyadh, Saudi Arabia d Department of Biomaterials, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands e Removable Prosthodontics Department, Faculty of Dentistry, Cairo University, Egypt f Department of Oral Implantology and Prosthetic Dentistry, Academic Centre for Dentistry Amsterdam (ACTA), Universiteit van Amsterdam and Vrije Universiteit Amsterdam, The Netherlands g Department of Oral Implantology and Prosthetic Dentistry, Academic Centre for Dentistry Amsterdam (ACTA), Universiteit van Amsterdam and Vrije Universiteit Amsterdam, The Netherlands
A R T I C L E I N F O
Article history: Received 15 February 2017 Received in revised form 10 July 2017 Accepted 23 September 2017 Available online xxx Keywords: 3D-SLA printing Finish line design Marginal and internal fit Micro-CT Additive manufacturing
A B S T R A C T
PurposeTo evaluate the influence of fabrication method and finish line design on marginal and internal fit of full-coverage interim restorations. MethodsFour typodont models of maxillary central-incisor were prepared for full-coverage restorations. Four groups were defined; knife-edge (KE), chamfer (C), rounded-shoulder (RS), rounded-shoulder with bevel (RSB). All preparations were digitally scanned. A total of 80 restorations were fabricated; 20 per group (SLA/3D-printed n = 10, milled n = 10). All restorations were positioned on the master die and scanned using micro-computed tomography. The mean gaps were measured digitally (ImageJ). The results were compared using MANOVA (a = .05). ResultsInternal and marginal gaps were significantly influenced by fabrication method (P = .000) and finish-line design (P = .000). 3D-Printed restorations showed statistically significant lower mean gap compared to milled restorations at all points (P = .000). The mean internal gap for 3D-printed restorations were 66, 149, 130, 95 mm and for milled restorations were 89, 177, 185, 154 mm for KE, C, RS, RSB respectively. The mean absolute marginal discrepancy in 3D-printed restorations were (30, 41, 30, 28 mm) and in milled restorations were (56, 54, 52, 38 mm) for KE, C, RS, RSB respectively. ConclusionsThe fabrication methods showed more of an influence on the fit compared to the effect of the finish-line design in both milled and printed restorations. SLA-printed interim restorations exhibit lower marginal and internal gap than milled restorations. Nonetheless, for both techniques, all values were within the reported values for CAD/CAM restorations. Significance3D-printing can offer an alternative fabrication method comparable to those of milled restorations. © 2017 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.
1. Introduction
* Corresponding author at: Department of Oral Implantology and Prosthetic Dentistry, Academic Centre for Dentistry, Amsterdam (ACTA), University of Amsterdam and Vrije Universiteit Amsterdam, Gustav Mahlerlaan 3004, 1081 LA Amsterdam, The Netherlands. E-mail address: [email protected] (N. Alharbi).
In contemporary prosthodontics, interim restorations are used as a transitional phase to evaluate the functional and esthetical outcomes [1–4]. The restorations should offer adequate fit to ensure mechanical stability and durability of the restoration and thus the health of the surrounding tissues [5,6]. Lack of adequate fit can result in plaque accumulation, cement microleakage, marginal
https://doi.org/10.1016/j.jpor.2017.09.002 1883-1958/ © 2017 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: N. Alharbi, et al., Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs, J Prosthodont Res (2017), https://doi.org/10.1016/j.jpor.2017.09.002
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discoloration, poor esthetics, teeth sensitivity, caries and periodontal diseases [5]. Wide variation in the definition of adequate fit and a clinically acceptable marginal gap exists in the literature and can be attributed to different study designs including different restorative materials, examination method, finish line designs and fabrication methods [7]. A recent systematic review on the fit of CAD/CAM restorations fabricated from different materials reported an absolute marginal gap (AMG) of crowns to be in the range from 10–110 mm with the majority of values less than 80 mm. In the same context, the internal gap at axial part was 23–154 mm and at occlusal part was 45–219 mm [8]. With the ongoing developments in the field of data acquisition and manufacturing processes, digital technologies are gaining more acceptances. Additive manufacturing (AM) is being increasingly applied in the field of prosthodontics. Dental restorations can be fabricated using resin based AM techniques both directly and indirectly; stereolithography and or digital light processing (SLA/ DLP) [2,4,9–13]. Both techniques use light/laser to cure a photosensitive liquid polymer layer by layer, following a specific path of the designed model. Limited studies have evaluated the marginal and/or internal fit of AM copings fabricated using SLA/DLP techniques. Kim et al. [10] found that the mean marginal gap of Co–Cr crowns was significantly larger when fabricated indirectly using SLA technique (96 mm) compared to traditional, lost-wax fabrication method (67 mm). The authors also reported an axial and an occlusal gap of 84 mm and 114 mm respectively for the indirectly manufactured SLA crowns [10]. In contrast, Park et al. [2] found PMMA DLPfabricated implant restorations to have better marginal fit compared to milled and conventionally fabricated counterparts fabricated from PEEK and PMMA respectively. Nevertheless, the authors found the fit of the three manufacturing methods to be within the clinically acceptable range. Munoz et al. [12] used DLP technique to fabricate wax patterns for indirect manufacturing of cast gold crowns and revealed that the marginal gap was significantly larger for DLP fabricated patterns compared to the milled or manually fabricated wax patterns. In the same context, different finish line designs have been shown to influence the marginal gap of the dental restorations. Comlekoglu et al. [14] compared the marginal gap of zirconia crowns with knife-edge, mini-chamfer, chamfer and rounded
shoulder finish line designs and found the lowest absolute marginal discrepancy values with knife-edge design (87 mm) compared to mini-chamfer (114 mm), chamfer (144 mm) and rounded shoulder (114 mm) finish line designs. Euán et al. [15] found lower mean marginal gap values for Lava All-Ceramic System with round shoulder finish-line design (52 mm) compared to chamfer counterpart (64 mm). In contrast, Tsitrou et al. [16] and Akbar et al. [17] revealed no significant difference in margnial gap of dental restorations with shoulder and chamfer finish line designs. The margin gap of CEREC 3 system composite resin crowns was 94 mm in chamfer finish line and 91 mm with shoulder finish line [16]. To the authors’ best knowledge, no previous study has evaluated the internal and marginal adaptation of 3D-printed directly fabricated restorations using the SLA technique with different finish line designs. Therefore, the aim of this study was to evaluate the influence of fabrication method and finish line design on the marginal and internal fit of full coverage interim dental restorations in two digital workflows (3D-printing versus milling). Our null hypothesis is that there is no difference in marginal and internal fit between 3D-printing and milled restorations in different finish line designs. 2. Materials and methods 2.1. Study design Four typodont models of maxillary central incisors were prepared to receive full coverage dental restorations; each with a different finish line design. The four finish line designs were; knife-edge (KE), chamfer (C), rounded shoulder (RS), rounded shoulder with bevel (RSB). In both digital workflows, the prepared models were digitally scanned and then the data was exported as a standard tessellation language (STL) format to a CAD software for the design and the fabrication of full coverage restorations (n = 80). For each finish line design, the restorations were fabricated using 3D-printing (n = 10) and milling (n = 10) techniques. All restorations were fitted on the master die without cementation and scanned using micro-computed tomography (Skyscan 1072, Bruker microCT, Kontich) [18]. The marginal and internal gaps were measured using digital imaging processing software (ImageJ 1.51; NIH). The flow of the study design is shown in Fig. 1.
Fig. 1. Flow of study design.
Please cite this article in press as: N. Alharbi, et al., Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs, J Prosthodont Res (2017), https://doi.org/10.1016/j.jpor.2017.09.002
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2.2. Crown preparation The models were prepared following the basic principles of crown preparation [19]. All models were prepared with 2 mm incisal reduction. Four finish lines designs were used; KE, 1 mm (C), 1 mm (RS) and 1 mm (RSB) [19]. All teeth were prepared with the use of silicon index [20], together with the use of magnifying loupe (30) for better visualization. The preparations were done using diamond burs kit (LOT:22483; Horico) under continuous watercooling. The prepared models were scanned using desktop optical scanner (Medit Identica BlueTM; Renishaw). The scanner was calibrated following the manufacturer’s instructions. A thin layer of anti-reflective powder coating was sprayed at a fixed distance of 20 cm for all the specimens to enhance the scanning procedure (Helling 3D Scan Spray; Helling GmbH) [21,22]. The digital files were exported in STL format, which were used for the design of restorations. 2.3. Restorations design and CAD/CAM fabrication The restorations were designed using 3-Shape Dental SystemTM CAD solution version 2015. The design parameters were standardized for all groups. A cement gap of 30 mm was defined [23]. Additional vertical space of 80 mm and a horizontal space of 30 mm was added as recommended by an experienced dental technician. One trained technician printed the restorations using stereolithography-based 3D-printer (DW028D, DWS) in a hybrid composite resin material (Temporis1, shade A2, LOT: 040725; DWS). The thickness of build layer was .05 mm and the maximum laser speed was 5000 mm/s [24]. The printed specimens were cleaned with 95% ethanol for one minute and post-cured using ultraviolet curing unit (S2; DWS) for 30 min as per the manufacturer’s instructions [24]. The milled restorations were fabricated using 5-axis milling machine (Wissner Ltd.; Germany) in PMMA-based acrylate resin (Polycon1 ae; Straumann; shade A2) with a bur diameter of 1 and 3 mm following manufacturer’s recommendation [25]. All specimens were then stored in a dry, lightproof box and tested within ten days of the manufacturing process. 2.4. Micro-CT scanning and analysis Before scanning, all restorations were visually examined for any manufacturing defects, using optical magnification loupe 3.5. Each restoration was manually placed on the master die without cementation and without any manual adjustment [18]. The position of each crown was visually checked before scanning; where the crown was stable without any rotation or displacement [18]. The specimens were scanned using a commercially available desktop X-ray micro-tomography system scanner (Skyscan 1072,
Fig. 3. Slicing protocol for all scanned restorations.
Bruker micro-CT, Kontich). A custom holder was fabricated to standardize the position of the specimens during scanning and to ensure that the long axis of the tooth was perpendicular to the scanning beam. The specimen holder involves a transverse cylindrical hole, which allows the X-ray beam to pass through. The specimen position was verified on the screen by a visible well defined, sharp and clear circle formed when the X-ray beam pass and run parallel to the transverse circular hole within the holder (Fig. 2). A scan resolution of 11 mm was defined for all specimens where the whole crown and coronal 1/3 of the root were included in one scan. Scans were recorded at X-ray source set to 100 kV and 98 mA, a 180 sample rotation and 10-frames averaging, a rotation step of .90 , an exposure time of 3.8 s and a 1 mm thick aluminum filter to optimize image contrast. After scanning, a cone beam reconstruction was performed to obtain 11 mm pixels, 2D-images with NRecon v.1.6.9.18 software. The ring artifact correction was set to 20 and beam hardening correction to 4%. For the analysis, six slices were selected from each specimen; three in mesio-distal (MD) and three in labio-palatal (LP) direction. The distance between each of the selected measuring points was 60 slices in LP and 110 slices in MD direction (Fig. 3). The slicing of the images was performed with CTanalyzer v.1.14 software. A total of 480 images were transferred to Image processing software for analysis. One blinded and trained examiner performed the measurements of all images (n = 480) by using Image processing software (ImageJ 1.51; NIH). A second examiner was consulted for any
Fig. 2. Custom-made specimen holder with visible circle indicating that X-rays run parallel to the transverse hollow cylinder.
Please cite this article in press as: N. Alharbi, et al., Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs, J Prosthodont Res (2017), https://doi.org/10.1016/j.jpor.2017.09.002
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Fig. 4. Internal and marginal fit measurement points on; (A) labio-palatal (LP) sections, (B) mesio-distal (MD) sections, (C) marginal fit measurement points on both (LP) and (MD) sections.
difficulty encountered. Prior to each reading, the measurement tool was calibrated by measuring the pixel size; one pixel was equal to 11 mm. All measurements were taken under the magnification of 400. The internal and marginal fit was measured at several defined points on MD and LP sections as shown in Fig. 4. The internal fit involves both the incisal gap and mid-axial gap measured at the middle of mid-axial wall. Further for better interpretation of the results, the data of the incisal gap and mid-axial gap were reported separately. The marginal fit was reported by using the vertical gap (VG), horizontal gap (HG) and absolute marginal discrepancy (AMD). Each point was measured three times and the average was used. For each tooth, a total of 51 measurements (15 for internal fit and 36 for marginal fit) were collected as follows; on each BL slice: (2 midaxial) + (1 incisal) + (2 VG) + (2 HG) + (2 AMD) = 9. Measurement on each MD slice: (2 mid-axial) + (2 VG) + (2 HG) + (2 AMD) = 8. Thus, the total measurements per tooth: (9 3 slices) + (8 3 slices) = 51. 2.5. Examiner calibration Examiner calibration was done prior to the study. The blinded examiner (A.S) was calibrated against an experienced examiner (A. N). A total of 20 2D-images were used for the calibration procedure. The internal and marginal gaps were measured on all 2D-images. After 10 days, the gaps were re-measured. Intra-examiner and inter-examiner reliability were analyzed using interclass correlation (ICC). 2.6. Statistical analysis The mean and standard deviations of internal and marginal gaps were analyzed using descriptive statistics. The data were checked for normality distribution and equivalence of variances. The results were compared using multivariate analysis of variance (MANOVA). A follow-up simple main effect test with Bonferroni corrections was used to evaluate the difference between the fabrication methods in each finish line design (a = .05). One-way ANOVA was used to analyze the influence of finish line design on the marginal and internal gap of the restorations in each fabrication method separately. 3. Results The intra-examiner reliability was .98 and the absolute agreement between the two examiners was .97 and thus eliminating any possible measurement or examiner bias.
The mean and standard deviation of the marginal and internal gap of all specimens are shown in Tables 1 and 2. The result of MANOVA was reported using Pillai’s trace multivariate test as the assumption of homogeneity of variance-covariance matrices was violated (Box’s M test, P = .000) [26]. The mean internal and marginal gap were significantly influenced by fabrication method (P = .000) and finish line design (P = .000). Yet the influence of fabrication method was more as shown by the Partial Eta squared value of (.96) compared to (.66) for finish line design. MANOVA showed statistically significant interaction effect between fabrication methods and finish line design, F (18,207) = 8.15, P = .000; Pillai’s trace = 1.245. Step-down univariate analysis showed that the interaction between fabrication methods and finish line designs was statistically significant at incisal, internal, mid-axial, VG and HG. On the contrary, at AMD, the fabrication method showed similar effect in different finish line designs (P = .133). The 3D-Printed restorations showed lower estimated mean marginal and internal gap values compared to milled restorations. The difference was statistically significant at all points (P = .000). Amongst all points, the incisal gap was the largest in both 3D-printed and milled restorations (Tables 1 and 2) (Fig. 5). MANOVA simple main effect showed that the mean internal gap, mid-axial and AMD of 3D-printed restorations were significantly lower than that of milled restorations for all finish lines designs (P <.05). The difference at incisal gap was significant in all finish lines except in KE design (P = .662), whereas for the HG the difference was significant in all finish line designs except in C design (P = .219). The difference at VG was significant in all finish lines except RSB design (P = .142). The results of one-way ANOVA revealed that the mean internal gap and AMD were significantly different between different finish line designs (P = .000) in 3D-printed restorations, whereas in milled restorations only mean internal gap was significantly different in different finish lines (Table 3). Tukey post hoc showed that in 3D-printed restorations the lowest mean internal gap was in KE design, the difference between different finish lines was statistically significant (P <.05) (Fig. 6A). Similarly, in milled restorations the lowest mean internal gap was in KE design. The difference was statistically significant (P = .000) between all designs except S and C (P = .249). The lowest AMD for 3D-printed restorations was with RSB design (Fig. 6B). The difference between the groups was not significant except for C design (P <.05). Likewise, the lowest mean AMD in milled restorations was with RSB finish line (P >.05) (Fig. 7A and B).
Please cite this article in press as: N. Alharbi, et al., Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs, J Prosthodont Res (2017), https://doi.org/10.1016/j.jpor.2017.09.002
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Table 1 Mean and standard deviation (SD) of internal gap (sum of incisal gap + mid-axial gap).
Incisal gap
Fabrication methods
Finish line
Specimens number (N)
Mean (mm)
Standard deviation (SD)
3D-printed
KE C RS RSB Total KE C RS RSB Total KE C RS RSB Total
10 10 10 10 40 10 10 10 10 40 20 20 20 20 80
93 213 210 161 169 97 271 259 208 209 95 242 235 184 189
14 31 6 15 52 17 18 19 11 71 15 38 29 27 65
KE C RS RSB Total KE C RS RSB Total KE C RS RSB Total
10 10 10 10 40 10 10 10 10 40 20 20 20 20 80
39 45 51 29 41 80 84 111 99 94 59 64 81 64 67
5 3 4 2 9 6 7 9 7 14 22 20 32 36 29
KE C RS RSB Total KE C RS RSB Total KE C RS RSB Total
10 10 10 10 40 10 10 10 10 40 20 20 20 20 80
66 149 130 95 110 89 177 185 154 151 77 163 158 124 131
8 15 4 7 33 8 10 11 7 39 14 19 29 31 42
Milling
Total
Mid-axial gap
3D-printed
Milling
Total
Internal gap
3D-printed
Milling
Total
Knife edge (KE), chamfer (C), rounded shoulder (RS), rounded shoulder with bevel (RSB).
4. Discussion This in vitro study evaluated the effect of fabrication methods in two digital workflows; 3D-printing versus milling and four finish line designs (KE, C, RS, RSB) on the marginal and internal fit of full coverage interim dental restorations. Based on the results, the null hypothesis that no difference exists in marginal and internal fit between 3D-printing and milled restorations in different finish line designs was rejected. In this study, only a maxillary central incisor model was evaluated to eliminate the influence of different tooth geometries [27]. In addition, all restorations were analyzed without cementation, which is not a true representation of the actual clinical situation. Nonetheless, it has been shown that the use of cement may lead to improper seating of the restoration and thus increases the marginal and internal gap [7]. Furthermore, the radiopaque nature of the cement may result in radiation artifacts during the procedure of micro-CT scanning, an artifact that may impede the measurements of the internal and marginal gaps [18,28].
In the literature, several methods are reported to evaluate the marginal and internal fit of the restorations [7]. In this study, micro-CT was used. This approach offers several advantages of being non-destructive, and allows for quantitative measurements in three dimensions [18,28]. However, shortcoming of this method is the radiation artifacts resulting from the difference in coefficient of radiation absorption among different materials [28]. A custommade holder was fabricated to standardize the position of the specimens during the scanning procedures. The maximum magnification to visualize the complete crown resulted in pixel size of 11 mm. During the reslicing, the locations of the 2D-slices were selected so that the measurement points were widely distributed on the tooth without any distortion of the image. (Fig. 3) The measurements of the selected points on the 2D-slices allowed for 3Danalysis of the fit of the restorations. Marginal fit was evaluated by measuring AMD, VG and HG to have an in-depth view of the marginal misfit of the restorations [29]. The number of measurement points for marginal gap was 36, which has been shown to maintain the precision level of the measurements [7,28–30].
Please cite this article in press as: N. Alharbi, et al., Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs, J Prosthodont Res (2017), https://doi.org/10.1016/j.jpor.2017.09.002
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Table 2 Mean and standard deviation (SD) of marginal gap.
Vertical gap (VG)
Fabrication methods
Finish line
Specimens number (N)
Mean (mm)
Standard deviation (SD)
3D-printed
KE C RS RSB Total KE C RS RSB Total KE C RS RSB Total
10 10 10 10 40 10 10 10 10 40 20 20 20 20 80
16 25 21 26 22 39 40 34 19 33 27 33 27 23 28
10 6 4 10 8 21 5 13 4 15 20 9 12 8 13
KE C RS RSB Total KE C RS RSB Total KE C RS RSB Total
10 10 10 10 40 10 10 10 10 40 20 20 20 20 80
25 22 20 25 23 31 25 41 33 32 28 23 30 30 28
9 4 3 4 6 8 3 8 4 8 9 4 12 6 8
KE C RS RSB Total KE C RS RSB Total KE C RS RSB Total
10 10 10 10 40 10 10 10 10 40 20 20 20 20 80
30 41 30 28 32 56 54 52 38 50 43 48 41 33 41
10 5 5 2 8 25 5 16 3 16 23 8 5 6 15
Milling
Total
Horizontal gap (HG)
3D-printed
Milling
Total
Absolute marginal discrepancy (AMD)
3D-printed
Milling
Total
Knife edge (KE), chamfer (C), rounded shoulder (RS), rounded shoulder with bevel (RSB).
Internal fit was evaluated by measuring the incisal and mid-axial gaps, yet data of the mid-axial and incisal gaps were reported separately to provide details of the internal misfit of the restorations. The results were analyzed using MANOVA rather than multiple two-way ANOVA’s to minimize the inflation of type I error [26,31]. The results showed that 3D-printed restorations exhibit significantly lower marginal and internal gap values than their milled counterparts. The inferior marginal and internal fit of milled restorations may be attributed to errors resulting from the tolerance of milling burs [32,33]. Nevertheless, the mean AMG marginal gaps of both 3D-printed and milled restorations were within the reported values for the CAD/CAM restorations (10– 110 mm) [8]. Among all measurement points, the largest gap in both fabrication methods was found at the incisal part (169 mm for 3D-printing and 209 mm for milling). This finding was in accordance with previous studies [2,27,34–37]. In the present study, the measured incisal gap was 169 mm and 209 mm which were greater than the programmed incisal cement space by
1.5 and 1.8 for both the printed and milled restorations respectively. This finding was different than the results of other reported studies. Kokubo et al. [34] reported an incisal gap of 170 mm, which is 3 to 4 larger than the programmed cement space. Other reports revealed an incisal gap and an occlusal gap that ranged from 110 to 204 mm, that was 5 to 6 more than the programmed cement space [2,35–37]. The reason for the increased incisal gap and whether it is related to incorporated spaces during the design process still merits further investigation. Several design parameters including programmed cement gaps are proposed based on the required cement thickness and are mostly recommended based on operators’ and technicians’ experience and the available software [9,33,37]. The programmed cement thickness reported in literature varied from no gap [11], to 10 mm [3], till 15 mm with an additional vertical space of 65 mm and a horizontal space of 50 mm [38], 30 mm [2,10,23], 50 mm [34], 60 mm [1], till 85 mm [9]. Hoang et al. [9] evaluated the reproducibility of 25 mm, 45 mm, 65 mm, 85 mm and 105 mm cement gap of DLP 3D-printed resin copings and found that the 85 mm cement gap was the most reproducible [9]. In the same
Please cite this article in press as: N. Alharbi, et al., Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs, J Prosthodont Res (2017), https://doi.org/10.1016/j.jpor.2017.09.002
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Fig. 5. Estimated mean of internal and marginal gap in 3D-printed and milled restorations.
context, the results of the current study revealed that the mid-axial gap in 3D-printed restorations was 41 mm, which is lower than the programmed cement space (60 mm). Thus, it may be recommended that the programmed axial gap be increased to enhance the marginal fit though this still requires to be confirmed with further studies (Fig. 6A and B). This lack of consistency between measured axial gaps and programed incorporated cement space may be due to an error occurring during the slicing procedure of the STL file. Such an error can minimize the accuracy of the fitting surface of printed restorations compared to the designed model. On the contrary, the larger mid-axial gap of the milled restorations was attributed to errors from the tolerance of milling burs [32]. Similarly, the incisal gap in KE design was lower than the programmed incisal space in both fabrication methods. Vertical displacement of the restoration is unlikely to be the reason considering the values of the measured vertical gap (Table 1). The results of MANOVA showed that the finish line design is less influential on the marginal and internal fit of interim restorations compared to the fabrication method. Several studies evaluated the influence of finish line design on marginal and internal fit of milled
dental restorations with controversial findings [14–17]. This can be viewed in the light of different experimental designs, different material, fabrication and analysis methods. The findings of this study revealed that in both fabrication methods KE design has the least internal gap and RSB design showed the least AMD. The difference in AMD between different finish line designs was not significant in milled restorations. In 3Dprinted restorations, the C design showed significantly larger AMD than the other finish line designs. This may be attributed to the incremental layer pattern of build process in the 3D printing technology. The curved surface of the axio-gingival line angle in C design may cause an increased stair-stepping error during the slicing of the STL file. Therefore, the resultant increase in the values of the AMD recorded with the C design in the printed group. Although the results revealed that KE design exhibit the least internal gap in both fabrication methods, it cannot be recommended for current clinical practice. KE design represents a very conservative approach, which may limit the choice of restorative material. In addition, the visibility of KE finish line on digital or analogue model might not be optimal. Gingival bulk has been
Table 3 The influence of finish line designs on internal and AMD (mm) in both fabrication methods using One-way ANOVA. Fabrication methods 3D-printing
Internal gap
Absolute marginal discrepancy
Milling
Internal gap
Absolute marginal discrepancy
Sum of squares
df
Mean square
F
Significance
Between groups Within groups Total Between groups Within groups Total
.040 .003 .044 .001 .001 .003
3 36 39 3 36 39
.013 .000
145
.000
.000 .000
9.43
.000
Between groups Within groups Total Between groups Within groups Total
.057 .003 .061 .002 .008 .010
3 36 39 3 36 39
.019 .000
209
.000
.001 .000
2.83
.052
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Fig. 6. Three dimensional reconstructed micro-CT image of the 3D-printed restoration in; (A) KE finish-line, (B) RSB finish-line.
Fig. 7. Influence of finish line design on internal fit and AMD in; (A) 3D-printing, (B) in milling. Different letters indicate statistical significance between different finish line design.
reported in some cases with KE design due to insufficient room for the restorative material [14]. On the contrary, RSB design showed the lowest AMD values in both fabrication methods, as well as an internal gap following the KE design (Fig. 6A and B). Improved marginal adaptation associated with reduced AMD values combined with acceptable internal fit values observed may be an indication for recommending RSB design for SLA-3D printed restorations. However, generalizability of the results should be regarded with caution. In this study, only one clinically available material and one system for each fabrication method was applied. Whether the interpretation given for the findings of this study can be applied to different materials and different systems still needs to be evaluated especially considering that such an interim restoration would still be followed with a definitive restoration. In addition, more studies are still needed to evaluate the influence of cementation procedure as well as the single versus multi-unit fixed partial dentures. The optimal cement gap, extra vertical and horizontal space values that should be incorporated during
designing of the restoration are also parameters that still need to be further assessed. 5. Conclusion Within the limitation of this in vitro study, fabrication methods showed more of an influence on the marginal and internal fit of dental restorations compared to the effect of the design of the finish line. Restorations fabricated using 3D-printing techniques exhibited lower marginal and internal gap than restorations fabricated using milling techniques. Nevertheless, in both fabrication methods, all values were within the reported values for CAD/ CAM restorations.
Acknowledgments The authors would like to thank DWS, Italy for supplying the specimens used in this study. Additionally, the authors would like
Please cite this article in press as: N. Alharbi, et al., Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs, J Prosthodont Res (2017), https://doi.org/10.1016/j.jpor.2017.09.002
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Please cite this article in press as: N. Alharbi, et al., Three-dimensional evaluation of marginal and internal fit of 3D-printed interim restorations fabricated on different finish line designs, J Prosthodont Res (2017), https://doi.org/10.1016/j.jpor.2017.09.002