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ORIGINAL ARTICLE
Year : 2020  |  Volume : 23  |  Issue : 9  |  Page : 1274-1280

Marginal discrepancy of 3-unit Co-Cr metal copings fabricated with additive and subtractive manners: A comparative study


Department of Prosthetic Dentistry, Faculty of Dentistry, Near East University, Nicosia, Mersin-10, Turkey

Date of Submission23-Jan-2020
Date of Acceptance31-Mar-2020
Date of Web Publication10-Sep-2020

Correspondence Address:
Dr. O Onoral
Department of Prosthetic Dentistry, Faculty of Dentistry, Near East University, Near East Boulevard, Post Code: 99138, Nicosia, Mersin-10
Turkey
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/njcp.njcp_33_20

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   Abstract 


Background: Poor marginal adaptation may give rise to a series of biological complications. Despite its importance, comparative studies on marginal adaptation of metal-ceramic fixed restorations fabricated with newer methods are limited. Aim: Therefore, it was aimed to assess the marginal accuracy of copings fabricated with subtractive and additive manners used in contemporary dentistry. Materials and Methods: For a typodont model, 60 three-unit cobalt-chrome metal copings were fabricated by selective laser sintering (SLS), hard alloy milling (HAM), and soft alloy milling (SAM) in comparison to casting (C). Marginal discrepancy assessment was performed by using replication technique. Mesiodistal and buccopalatal cross-sections of silicone replicas were investigated under a stereomicroscope at × 80 magnification. A total of 960 measurements were subsequently made by means of corresponding image-review software on captured images after calibration of the software to μm scale. Obtained data were subjected to nonparametric Kruskal Wallis and Tamhane post-hoc tests (α =0.05). Results: Marginal adaptation of C group was significantly inferior to test groups in both canine- and premolar-teeth measurements (P < 0.05). Marginal fit was found to be tooth dependent (P < 0.001). HAM, SAM, and SLS groups exhibited analogous marginal discrepancy values on canine-tooth measurements. Differences among these groups were not statistically significant (P > 0.05). SAM and SLS groups demonstrated better marginal adaptation than others on premolar-tooth measurements. Also, no statistically significant difference was detected between SAM and SLS groups (P > 0.05). Conclusions: SAM group demonstrated superior marginal accuracy. All groups had clinically acceptable marginal discrepancy values (<120 μm), except cast group.

Keywords: Additive manufacturing, CAD/CAM, Marginal discrepancy


How to cite this article:
Onoral O. Marginal discrepancy of 3-unit Co-Cr metal copings fabricated with additive and subtractive manners: A comparative study. Niger J Clin Pract 2020;23:1274-80

How to cite this URL:
Onoral O. Marginal discrepancy of 3-unit Co-Cr metal copings fabricated with additive and subtractive manners: A comparative study. Niger J Clin Pract [serial online] 2020 [cited 2020 Sep 27];23:1274-80. Available from: http://www.njcponline.com/text.asp?2020/23/9/1274/294680




   Introduction Top


Precise fit is a prominent characteristic for the successful service of prosthetic restorations[1],[2],[3],[4],[5] as the deterioration of the marginal fit tends to cause bacterial accumulation and results in a variety of biological complications including microleakage, hypersensitivity, secondary decay formation, infection, periodontal diseases, and bone resorption.[6],[7],[8],[9],[10] Other than these, internal fit disorders may result in malocclusion and loss of retention.[11] Thus, restorations should present small discrepancy values for extended oral lifespan.[12],[13],[14],[15]

At present, literature offers a wide array of factors that can influence the discrepancy values of metal-ceramic restorations such as taper of preparation,[16] type of marginal configuration,[7] cementation,[17] software parameters,[18] fabrication technique of wax replicas,[13] die spacer application,[19] gap assessment technique,[20] porcelain firing,[21] alloy type,[11] and manufacturing technique.[22] However, despite intense studies, considerable controversy still exists with regards to the optimal fit. McLean and von Fraunhaufer reported that the maximum marginal gaP value to be compensated was 120 micrometers and that the values above this would lead to aforementioned detrimental consequences.[23]

In the fabrication of metal-ceramic restorations, casting with the lost-wax technique described by Taggart is adopted as the golden standard but, although well established, is challenging, time consuming, and involving excessive manual steps.[24],[25] It is, therefore, being rapidly replaced by computerized techniques in order to provide automation for the production cycle, to ensure minimization of the error, and to achieve standardized repeated accuracy in the fabrication of prosthetic restorations.[26],[27] Such computerized techniques are divided into those using subtractive manner and those using additive manner.[1],[6],[9],[11],[13] Selective laser sintering, an additive manufacturing modality, layers metal powder by irradiating with high-temperature carbon dioxide laser beam on the basis of sliced data from the 3-dimensional (3D) design.[5],[14],[22] The powder dispersed by the roller is selectively melted when exposed to the beam. This significantly shortens the fabrication time. Moreover, it does not require supportive unit during fabrication, has a superior fabrication capacity, presents better mechanical properties, and allow to reuse residual raw material.[1],[6],[21] Based on these advantages, it is currently being used widely in dental practice.[9] Hard alloy milling (HAM), a subtractive manufacturing modality, allows subtraction of metal copings from a hard metal (fully sintered) solid blocks by cutting with the aid of computer numerically controlled system. However, residual material cannot be reused, and cutting of post-sintered hard blocks leads to excessive stress on the computer-aided milling (CAM) equipment and end mill.[8],[11] Also, recently introduced is a soft alloy milling (SAM), a subtractive manufacturing modality, for the fabrication of metal-ceramic restoration. Unlike HAM, metal copings are subtracted from soft metal (pre-sintered) blocks and thereby, all shortcomings of HAM are circumvented.[9],[11],[21] Its clinical utility in terms of fit compared to other techniques is conflict of issue as this technique necessitates sintering process in an argon gas atmosphere which results in a contraction of approximately 11%.[8],[22]

The marginal fit of metal copings fabricated with these newer technologies has not been studied to the same extent as the marginal fit of copings fabricated with preceding techniques, despite its importance for life expectancy of prosthetic restorations. Therefore, it was aimed to assess and compare the marginal gaP values of 3-unit metal copings fabricated with C, HAM, SAM, and SLS technologies. The null hypotheses were that there would be no difference in terms of marginal adaptation between test groups (SLS, HAM, and SAM) and control group (C), and that marginal discrepancy values exhibited by all groups would be in the range of clinical acceptability (≤120 μm).


   Materials and Methods Top


The schematic setup of the experiment is presented in [Figure 1]. To fabricate three-unit fixed metal-ceramic restoration, maxillary canine and maxillary second premolar abutmen teeth in a typodont model (AG-3; Frasaco GmbH, Tettnang, Germany) were subjected to 360° 1.0 mm chamfer preparation, 6° axial taper, and 1.52.0 mm incisal/occlusal reduction with the aid of rotary instruments and subsequently, were scrutinized under a stereomicroscope (Leica S8 APO; Leica Microsystems GmbH) to ensure that preparations were free from undercuts and were parallel to each other for the path of insertion. The master model was sprayed with a powder spray (Cerec Optispray; Dentsply Sirona, Bensheim, Germany) to attain a scannable surface, and then digitized by an intraoral scanner (CEREC Omnicam, Sirona Dental Systems, Bensheim, Germany). After transfer of acquired data to the computer-aided design (CAD) software (InLab 15; Sirona Dental Systems, Bensheim, Germany), virtual design of metal copings was conducted and thereby, a file in a format of standard tessellation language (STL) was generated. Fabrication parameters were set as 500 μm coping thickness and 50 μm cement space.
Figure 1: The schematic setup of the experiment

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During the fabrication of cast-specimens (n = 15), STL file was uploaded to the 5-axis CAM equipment (inLab MC X5; Sirona Dental Systems, Bensheim, Germany) and the wax replicas of the metal copings were obtained from prefabricated wax blocks (Alliance Wax; Turkuaz Dental) using subtractive manner. The replicas with wax sprue attachments were vacuum invested (Sherafina-Rapid; Shera Werkstoff-Technologie GmbH and Co KG) in a casting ring and then baked in a burnout furnace (MFX-1010; Dental Mikrotek, Ankara, Turkey). Casting was conducted with the aid of an electric induction furnace (INF-2010; Mikrotek Dental, Ankara, Turkey) by using Co-Cr alloy ingots (Co, 53.8%; CR, 25%; W, 10.5%; Fe, 7% Mo, 2.5%; Other, 1.2%/Wirobond® easy; Bego GmbH, Bremen, Germany). The cast specimens were made free from residues by using 125 μm aluminum-oxide abrasives and carbide discs with low-speed handpiece. The sprues were eliminated by using a separating disc (Dentaurum GmbH and Co KG). Specimens of soft-milled group (n = 15) were subtracted from pre-sintered Co-Cr blocks (CopraSintec K; Whitepeaks Dental Solutions GmbH and Co. KG, Germany) with the aid of the 5-axis milling device (inLab MC X5; Sirona Dental Systems GmbH, Germany) and then sintered in an argon gas atmosphere according to the manufacturer's recommendations. For this purpose, exclusive sinter furnace (Calidia Sintec 770; Whitepeaks Dental Solutions GmbH and Co KG, Germany) was used. Specimens of hard-milled group (n = 15) were subtracted from post-sintered Co-Cr blocks (CopraBond Co-Cr; Whitepeaks Dental Solutions GmbH and Co KG, Germany) using 5-axis milling device (inLab MC X5; Sirona Dental Systems GmbH, Germany). To produce the specimens of the SLS group (n = 15), the same STL file was sent electronically to a SLS printer (EOSINT M 270; EOS GmbH, Krailling, Munich). Co-Cr-Mo powder material (Co, 61.8-65.8%; Cr, 23.7-25.7%; Mo, 4.6-5.6%; W, 4.9-5.9%; Si, max. 0.8-1.2%; Mn, max. 0.50%; Fe, max. 0.1%/CobaltChrome SP2; EOS GmbH, Krailling, Munich) was used.

Silicone replica technique was chosen for the assessment of marginal openings. After the fabrication of all metal copings, light-body silicone impression material (Elite HD + Light body Fast Setting; Zhermack, Polesine, Italy) was applied into the intaglio surface of metal copings with the aid of a dispensing gun in a ratio of 1:1 for base and catalyst, respectively. Copings were then seated on the respective abutment teeth on the typodont model in order to simulate the cementation process and finger pressure was applied vertically. After the light-body silicone hardened, the metal copings were carefully separated from the master model. However, as the replica obtained at this stage was very thin, it was reinforced with a strong heavy-body silicone impression material (Elite HD + Putty Soft Fast Setting; Zhermack, Polesine, Italy) in order to achieve sufficient stabilization while cutting with the scalpel. Accordingly, the molded silicones were cut along the mesiodistal and buccopalatal directions, and examined under a stereomicroscope (Leica S8 APO; Leica Microsystems GmbH, Wetzlar, Germany) at ×80 magnification. Four images for each abutment tooth of each specimen were captured. Eight points on each abutment tooth were predefined, and in total, 960 measurements were made by means of corresponding image-review software on captured images after the calibration of the software to μm scale. All measurements were performed by a single investigator (Ö.Ö.).

As the data obtained for marginal discrepancies on canine and premolar abutments were shown not to fit normal distribution with respect to Kolmogorov–Smirnov Test, and as the hypothesis regarding the homogeneity of variances was rejected with respect to Levene Test; non-parametric Kruskal–Wallis test was conducted by using a statistical software package (IBM SPSS Statistics v23; IBM Corp) (α = 0.05).


   Results Top


Descriptive data for marginal discrepancy values of all groups are depicted in [Table 1] and [Table 2]. The measurements on canine abutment tooth indicated that the SAM group had the best marginal fit, followed by HAM, SLS, and cast groups, respectively. On the other hand, the measurements on premolar abutment tooth showed that SAM group followed by SLS, HAM, and cast groups, respectively. Statistically significant differences (P < 0.05) among all groups in terms of marginal discrepancy are shown in [Table 3] with the aid of Tamhane T2 Multiple Comparison Test. Also, a statistically significant difference (P < 0.001) was found between the measurements carried on canine and second premolar teeth in accordance with Mann–Whitney U Test.
Table 1: Marginal discrepancy values measured on canine abutment for 4 fabrication methods with results of Tamhane T2 multiple comparison test (unit: μm)

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Table 2: Marginal discrepancy values measured on premolar abutment for 4 fabrication methods with results of Tamhane T2 multiple comparison test (unit: μm)

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Table 3: Tamhane T2 multiple comparison test results (95% confidence interval) (unit: μm)

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   Discussion Top


The results of the present study revealed significant differences among groups in terms of marginal adaptation. In addition, it was found that all groups were not below the clinically acceptable threshold (120 μm). Therefore, null hypotheses were rejected.

It was found that the highest marginal gap values belonged to the cast group; therefore, cast group exhibited vulnerability in terms of marginal accuracy when compared with test groups. Cast group also displayed higher standard deviations. These findings correspond with other studies in the literature[21],[22],[28] and are attributed to the workflow of casting that increases the risk of error accumulation from a multistep procedure.[1] Moreover, deformation of the wax replicas and technician's skill may be other contributing factors for the inferiority of cast group.[22]

SAM, SLS, and HAM groups demonstrated superior marginal adaptation. This can translate to decreased processing errors as multiple error-introducing steps are omitted in these computerized techniques. SAM group presented better marginal adaptation on both canine and premolar measurements. This finding is in accordance with the studies in literature[28] and can be correlated with the hardness of the blocks. Soft-type blocks are used in this technology which tends to increase machinability and thereby decrease stress on CAM unit.[8],[11],[21],[28] Moreover, the contraction amount of metal has been predefined and took into account in order to minimize sinter-related errors in SAM.[22],[28] However, in SLS, deformations might have been introduced during the contraction of the metal as it cooled.

As a statistically significant difference in terms of marginal accuracy was observed between canine and premolar teeth, it can be concluded that tooth-type significantly influenced the marginal fit. Although this provides consistency with the study by Nakamura et al.[29]; it is inconsistent with a previous research by Huang et al.[30] The measurements conducted on canine tooth also presented higher gap values. Possible explanation for this result might be that maxillary canine is positioned at where the dental arch extremely rotates. This makes the span longer, increases weight of the prosthesis, causes permanent deformation (sag) during heat treatments, and thereby results in an ill-fitting prosthetic restoration.[31]

The precision of the milling operation is greatly influenced by the number of axes of the milling machine.[32] In today's CAM technology, systems are categorized as 3-, 4-, or 5-axis.[16] Although 3-axis systems allow for short production time and have simple operating system,[16] the lack of rotation axis may affect the precision of milling procedure at the inner surface of the restoration and this may be expressed as a limitation.[32] It is thought that there will be a higher production accuracy in 4-axis systems and 5-axis systems. This hypothesis was reinforced with a study by Bosch et al.[33] which reported that 5-axis milling devices allow more accurate and precise production compared to 4-axis milling machines. Accordingly, in order to increase the precision of the production, 5-axis CAM device was preferred during the production of wax replicas, metal copings of HAM group, and metal copings of SAM group in this study.

Although Ni-Cr alloy has been dominantly used in contemporary dentistry for the manufacture of metal-ceramic restorations,[6],[7] it is well documented that Co-Cr alloy offers superior mechanical and biological characteristics.[1] It has been proved that Co-Cr alloy has less allergic reactions than Ni-Cr alloy.[6],[11],[34] This is explained by the absence of nickel, one of the most well-known allergens, in the composition of Co-Cr alloy.[5],[9] In addition, Co-Cr alloy contains more than 25 wt.% chromium. This increases corrosion resistance and enhances biocompatibility.[34] It is, furthermore, well-known that this alloy exhibits better mechanical properties than Ni-Cr alloy in terms of sag resistance. The high sag resistance of the alloy is crucial during baking of ceramic. If the alloy cannot resist flow, permanent deformation of the metal coping may occur during porcelain firing, and the compatibility of the restoration may be detrimentally affected. Moreover, the specific weight of Co-Cr alloy is quite low. This is beneficial to clinicians because it is a fact that the greater the weight of the metal coping, the more distortion will occur.[31] Considering these abovementioned properties, in the present study, use of Co-Cr alloy can be justified. However, the cast ability of this alloy is quite difficult as it has a high melting range.[7],[9] In addition, its manipulation is very low.[1] Newer CAD/CAM technologies have been developed in order to address these shortcomings and to enhance machinability,[7] but there is still no consensus that these technologies can be considered as an alternative to traditional casting technique.

It is well-known that dimensional stability of the impression can be influenced by the changes in humidity and temperature, while transferring to the laboratory. Likewise, the application of disinfection solution may cause an additional distortion. All these may adversely affect the harmonization of prosthetic restorations. In the traditional impression technique, gag reflex and damage to the gingival tissues may also occur, leading to patients' dissatisfaction.[12] In the light of these, a digital scanner was preferred in this study in order to circumvent aforementioned shortcomings.

As stated, poor seals may give rise to a series of complications.[3],[4],[6] It is, therefore, very important to assess the marginal accuracy of prosthetic restorations.[7] For this purpose, several invasive and non-invasive techniques have been suggested in the literature, including direct-view technique, replication technique, cross-section technique, micrometric computed tomography (micro CT) technique, and 3D mapping technique.[7],[8],[9],[14],[21],[24],[26] Despite being easy, repeatable, and non-invasive;[28] the direct-view technique has the shortcoming of not facilitating measurement of the points planned for this study. Although it offers very precise results,[24] the cross-sectioning technique was also inappropriate for this study due to its invasive nature. In this technique, it is inevitable that specimens have to be destroyed due to the sectioning at observation. In such a study where a large number of measurements were performed, preservation of the specimens is of paramount importance. Otherwise, a large number of specimens were required. Moreover, it does not allow the fit evaluation from various points.[8],[28] The micro-CT examines the intaglio of the FDPs. However, it requires an advanced technology and presents low capacity of discrimination.[9] Just lately, a new computerized mapping technique offering 3D measurement has been developed. It has a noninvasive nature and provides very reliable measurements from various directions.[7],[26],[28] It is, nevertheless, expensive and requires advanced experimental setup.[21] The replication, which yields an advantage of allowing multiple measurements from various points is a cheap, non-invasive technique.[8],[21],[28] Accordingly, this study used silicone replica technique to conduct marginal discrepancy measurements.

Different microscope types can be used to evaluate the compatibility of prosthetic restorations. By scanning electron microscopy (SEM), highly accurate and reliable results can be obtained.[2] However, no statistically significant difference was reported between SEM and light microscopy in terms of marginal discrepancy values.[35] SEM is subject to certain drawbacks, for it necessitates fixation and carbon coating of specimens prior scanning, thus increasing the chance of error accumulation from a multi-step procedure.[2] Moreover, use of light microscope is limited by projection errors. Care should be taken for the affixation of the specimens 90° to the optical axis of light microscope.[3] In this study, stereomicroscope was preferred because of being user-friendly and having its own software for image acquisition and discrepancy measurement.

This study has some shortcomings. Only one additive and two subtractive manufacturing systems were compared. Future studies should also compare different additive and subtractive production technologies. In addition, chamfer margin configuration was preferred for standardization. It is a fact that different results can be achieved with different margin designs. The 3D measurement technology which offers very sensitive results is not preferred in this study. Therefore, the current literature is still insufficient in terms of adaptation of the metal-ceramic restorations produced with new additive and subtractive techniques, and further studies are needed.


   Conclusions Top


Within the scope of this comparative study, following conclusions can be drawn: (1) all groups had clinically acceptable marginal discrepancy values (<120 μm), except cast group; (2) soft alloy milling group provided a better service in terms of marginal accuracy; (3) cast group demonstrated inferiority when compared with all test groups; (4) marginal fit was found to be tooth dependent.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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