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 Table of Contents  
ORIGINAL RESEARCH
Year : 2021  |  Volume : 13  |  Issue : 4  |  Page : 386-392

Evaluation of the marginal discrepancy of cobalt chromium metal copings fabricated with additive and subtractive techniques


Operative Department, College of Dentistry, Mustanseriah University, Baghdad, Iraq

Date of Submission18-Feb-2021
Date of Decision21-Feb-2021
Date of Acceptance22-Apr-2021
Date of Web Publication19-Aug-2021

Correspondence Address:
Dr. Safa Thaer Noori
Operative Department, College of Dentistry, Mustanseriah University, Al-Mansure District, Alyarmook Area, Alley 2, Baghdad.
Iraq
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/JIOH.JIOH_41_21

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  Abstract 

Aim: The aim of this article was to compare the marginal discrepancy of CoCr metal coping fabricated by three techniques, namely (A) direct CAD/CAM milling, (B) casted milled CAD/CAM wax, and (C) direct metal laser sintering (DMLS) technique. Materials and Methods: A comparative study between additive and subtractive manufacturing techniques was made. The study involves three groups; 15 metal copings were made for each group. Standardization was done by using the same STL file for all groups. With group A, direct milling was done to the CoCr blank. Group B follows the conventional casting method for wax framework milled by a CAD/CAM machine. Group C metal coping was made by the DMLS technique. The measurement of marginal discrepancy was done to four aspects: buccal, palatal, mesial, and distal using a digital light traveling microscope with magnification power of 200×. Statistical analysis used was one-way analysis of variance test and Tukey’s test. Results: The mean vertical marginal discrepancies for groups A, B, and C were 19.070, 23.470, and 38.533 μm, respectively. Statistical analysis shows a very high significant difference among groups (P < 0.001). Conclusion: All the tested groups showed an acceptable marginal discrepancy. The direct CAD/CAM metal milling method results in minimum marginal gaps when compared with additive and subtractive techniques.

Keywords: Additive Manufacturing, CAD/CAM, Cobalt Chromium, Direct Metal Laser Sintering, Vertical Marginal Discrepancy


How to cite this article:
Noori ST, Gholam MK. Evaluation of the marginal discrepancy of cobalt chromium metal copings fabricated with additive and subtractive techniques. J Int Oral Health 2021;13:386-92

How to cite this URL:
Noori ST, Gholam MK. Evaluation of the marginal discrepancy of cobalt chromium metal copings fabricated with additive and subtractive techniques. J Int Oral Health [serial online] 2021 [cited 2022 Jun 26];13:386-92. Available from: https://www.jioh.org/text.asp?2021/13/4/386/324144




  Introduction Top


Metal ceramic restorations marked a high degree of success, as they join the power in the function of metal with the esthetics of porcelain. Alloys used in porcelain fused to metal (PFM) include both precious and non-precious alloys. Commonly, precious alloys are used to produce the metal coping because of their biocompatibility, together with their optimum casting properties and bonding strength. However, base metals (Ni Cr and Co Cr) are often preferred more than precious alloys because they are better in mechanical properties and have lower cost.[1] The degree of success and durability of dental restorations depend on marginal fitness, which determined the space between the tooth structure and restoration. Marginal discrepancy tends to accelerate cement degradation inside the oral cavity and causes periodontal diseases, recurrent caries, pulpal lesions, hypersensitivity, and marginal staining. Thus, researches studying the marginal fitness of dental restoration play an important role.[2]

Lost wax technique is routinely used to construct PFM restorations. But recently, the digital manufacturing system has been introduced as a computer-aided design/computer-aided manufacturing system, in which the restoration to be manufactured was typically submitted to a process of scanning, designing, and computer manufacturing. Computer-aided manufacturing systems involve two types: subtractive and additive techniques. Using the subtractive method, the restoration framework is manufactured by milling solidified blocks with a diamond rotary instrument.[3] The advantages of the subtractive method are time-saving because of the ability to produce multi-restoration at the same time with the easiness of procedure, but the waste of materials and the fatigue of the milling burs are considered their disadvantages.[4]

Additive manufacturing builds up the restoration layer-by-layer. The newly introduced direct metal laser sintering (DMLS) technique is a progressed laser-based additive manufacturing technique that uses virtual 3D design data to construct a component using a layer-by-layer merging technique. The process started by the application of a thin layer of powdered material on the build platform. A beam of high-power laser scans over the described pattern is formed by the designer. Repetitively, after every single layer, laser beam fuses the powder particles at precise positions using a laser scanning optic.[5]

Many types of researches have been carried out to evaluate and compare the marginal fitness of restoration fabricated with the CAD/CAM technique and the traditional lost wax technique, and some have shown bitter marginal fitness with the CAD/CAM technique.[5],[6]

The purpose of this study was to evaluate the effect of CAD/CAM fabrication techniques on the marginal fitness and to measure and compare marginal gaps in CoCr copings manufactured by using subtractive and additive methods.

Null hypothesis was that there are no differences in the marginal fitness of CoCr metal copings fabricated by CAD/CAM additive and subtractive techniques.


  Materials and Methods Top


Setting and design

The study is an in vitro study, with no exclusion involved. The samples were randomly selected on the master die, no bias, no drop. The study location was in the UAE and Dubai. It took about 6–8 months.

Fabrication of master die

A brass die model was placed vertically into a rubber mold using dental stone to be fixed during the preparation, and then the mold with the attached die was seated at the base unit of the surveyor. The die model has the features of a premolar tooth with 1 cm in height and 8 mm in width. The preparation was done by a high-speed turbine (Sirona T3, Germany), which was mounted on the vertical arm of the surveyor using a cross-like pipe holder specially designed to ensure that the bur is kept parallel to the longitudinal axis of the die and in a vertical way to the finishing line in order to assure appropriate degree of axial tapering and to remove the undercuts.[7]

The completed prepared die had a planar occlusal surface reduction, 0.5 mm shoulder finishing line depth, 6° convergence angle, and 5 mm height from the occlusal level to the intended finish line.

Fabrication of coping design

The die model was scanned using a model scanner (Imes-icore® GmbH, I3d Scan, Germany) after applying a scanning powder (hi-tech, Korea) to remove luster and to get the precise picture [Figure 1]. The STL format file was generated. Coping was designed by using dental CAD software (Exocad DentalCAD; Exocad GmbH, Germany). The coping is designed to have a thickness of about 0.5 mm with an internal cement gap of 50 µm beginning from 0 mm at the finishing line. The position of connectors was designed to be away from the finishing line [Figure 2]. Then the image was saved in the STL file format.[8]
Figure 1: Prepared die on the table of scanner

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Figure 2: Copy designing with Exocad dental program

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Fabrication of copings

Group A: Direct CAD/CAM milling copings

The STL file was sent to the CAM milling equipment (CORITEC 350i; Imes-icore GmbH, Germany). Pre-sintered metal blank (KERA®-DISC, Eisenbacher Dentalwaren ED GmbH, Germany) was set in the milling machine, and 15 CoCr copings were manufactured using the subtractive technique.

Group B: Casted milled wax copings

The same STL file format was uploaded to the milling machine (CORITEC 350i; Imes-icore GmbH, Germany). Wax blank was set (NHT High Technology, Korea) in the milling part of the CAM machine. Fifteen wax patterns were then milled and separated from the blank, and then the sprue was attached, invested, and subjected to the lost wax casting technique.

Group C: DMLS

The STL file of the die model was sent to a 3D printer machine (EOS M 100, GmbH, Germany). The machine utilizes a laser beam with a power of 200 W, the beam diameter of about 40 μm, and CoCr powder (EOS cobalt chrome SP2, GmbH, Germany). In a layer of 30 µm thickness, metal powder printing was performed, and 15 metal copings were gained after removing the supports carefully [Figure 3]. Finally, there were 45 metal copings samples ready for measurement [Figure 4].
Figure 3: Fifteen metal coping fabricated with DMLS

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Figure 4: Forty-five metal copings were fabricated with three different methods

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Measurement of the marginal discrepancy

A non-destructive method was used to measure the marginal gap by placing three mark points on the center of each surface of the metal die. Twelve points were determined on the four surfaces: buccal, palatal, mesial, and distal surfaces. A special holding device was used to seat each one metal coping on the brass die.[9] The holder utilized a standard force of about 50 N to seat the copings on the brass die.[10] The vertical marginal discrepancy was measured on each point that was marked with a digital light traveling microscope (Cooling Tech Microscope, China) with 200× magnification power [Figure 5]A–[C].
Figure 5: View of microscopic image of the marginal gap between metal coping and brass metal die under magnification of about 200× (A: direct CAD/CAM metal milling, B: casted CAD/CAM milled wax, C: direct metal laser sintering)

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Statistical analysis

The images captured by the microscope were analyzed by a software program (Image J 1.50i, U.S. National Institutes of Health, Bethesda, MA, USA) to measure the discrepancy in pixel and then converted to micrometers. The statistical analysis uses the mean values of these measurements. Analysis of variance (ANOVA) (one-way) and Tukey’s test were performed for multiple comparisons between different pairs of the groups with a significance level set at 5%.


  Result Top


The averages of vertical marginal discrepancy for the three tested groups A, B, and C were 19.070, 23.470, and 38.533 µm, respectively. [Table 1] shows the mean of vertical marginal gaps, minimum and maximum values for all groups, and the standard deviations [Table 2]. One-way ANOVA test was implemented among the tested groups and shows highly significant differences at P ≤ 0.05. [Table 3] shows the difference among the three groups on all the four surfaces. Tukey’s test has revealed a highly significant difference among the groups (P ≤ 0.05).
Table 1: Descriptive statistics of mean marginal gaps (µm) of all groups

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Table 2: One-way ANOVA test for mean marginal gaps among the groups

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Table 3: Tukey’s test for mean marginal gap between groups

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  Discussion Top


Marginal accuracy is a major important factor that should be evaluated to keep the biological and mechanical feedback of prosthetic restoration in an acceptable range.[3]

Holmes et al.[11] establish the definition of the vertical marginal gap as the distance between margins of the crown and finish line vertically. Therefore, this study was performed to measure the vertical marginal discrepancy with different fabrication techniques.

A digital light traveling microscope was used for the measurement because it provides a direct view and is considered a non-destructive method. The magnification used was 200× which is enough to view and measure the vertical marginal gap.

Different values of an acceptable range of marginal discrepancy were evaluated and described by several authors. According to McLean and Fraunhofer and Fransson et al., the acceptable clinical marginal gap is between 100 and 120 µm.[12],[13]

According to Assif et al., the mean of the marginal gap should be closer to 140 μm, whereas Hung et al. determined a range value of 50–75μm.[14] The measurement of marginal fitness shows a mean vertical gap that ranges from 17 to 45 µm. The result was within the acceptable range according to the previous studies.

The statistical analysis of vertical marginal gap values of CoCr coping presents a highly significant difference in marginal fitness among the three groups.

With the CAD/CAM system, three main factors affect the fitness[15]:

  1. Reliability of the scanner.


  2. Accuracy of software in transforming the scanning data into a 3D model.


  3. Accuracy of machine that will manufacture the object from the design data.


The mean marginal discrepancy was 19.070, 23.470, and 38.533 µm in groups A, B, and C, respectively. The DMLS group shows the highest values of marginal gap when compared with the other two groups and this agreed with studies made by Ness et al. and Kim et al., in which the additive group shows the highest marginal gap of about 156 and 239 µm, respectively.[3],[16]

With the DMLS group, the machine used was EOS M 100 that builds up the metal coping layer by layer with 30 µm layer thickness; the wavelength ranges from 900 to 1200 nm, maximum power was 200 W, laser scan speed was 7000 mm/s, and the focus diameter of the laser beam was around 0.04 mm.[17] SP2 powder (CoCr powder that is used with DMLS) has a melting point about 1410–1450°C.[18] Sintering temperature is about 900–1150°C.[19] The powder has a lower content of molybdenum and another ingredient, which has a high melting temperature of 2623°C.[20] The greatest marginal gap may be due to the high temperature of laser sintering, which resulted in the warpage, and distortion of metal core ended with the marginal gap in dental restoration.[21]

The laser beam performs high temperatures to melt the powder particle surface and this will affect the marginal area by metal core cooling followed by shrinkage.[22] Although the shrinkage that follows the 3D printing technique results in decreasing the fitness, this result disagrees with the studies made by Chang et al. and Yildirim et al., in which the laser sintering group shows the best marginal fitness with a mean of marginal gap of about 116 and 71.6 µm, respectively.[13],[23] This may be attributed to the type of machine used in these studies as the machine used was EOS M 270 which is different in the powder layer thickness and laser beam focussing diameter. In EOS M 270, the powder layer thickness is about 20 µm but with EOS M100 the powder layer thickness is about 30 µm.

Increasing thickness of the powder layer will have a negative effect on the final properties of the object including marginal fitness,[24] and this agrees with a study made by Ekren et al. and Kalili et al.[25],[26]

Kalili et al. compared the marginal fitness of laser-printed metal coping using powder layer thicknesses of 25 and 50 µm, and the results show a highly significant difference between groups with a mean of marginal gap about 58 µm for the group with 25 µm layer thickness and about 65 µm for the group with 50 µm layer thickness. The group with less powder layer thickness shows more fitness and less marginal gap, and the reason behind that was the laser beam penetrating energy decreased as the layer thickness increased ended with a balling effect.[27] The balling effect is a kind of porosity that makes a poor interlayer bonding between the sintered layer and the new fresh layer, which may explain the marginal discrepancy. The statistical analysis of this study shows a significant difference between the casted milled CAD/CAM wax group and the other two groups. Although the wax framework was constricted by the digital CAD/CAM system in group B, casting was done with the conventional lost wax technique and this may contribute to the marginal discrepancy and the resultant increase in the marginal gap.

Besides the shrinkage and stress relaxation of the investing procedure, the heating in the burn-out procedure melts alloy at high temperatures than melting range and that may cause to lose the low melting point of a compositional element which results in increased viscosity that affects the flow of metal. Delay time to melt alloy in the electrical machine also results in modifying and distortion of the wax framework that affects the marginal fitness.[28] The result agrees with the comparative study made by Lövgren et al., which shows a mean of 104 µm of a marginal gap in the casted milled wax group, and this was greater than that of the direct CAD/CAM milling group (91 µm). However, the result also disagrees with that of Lövgren et al. in the 3D-printed metal group which shows best marginal fitness and a lower mean of a marginal gap of about 53 µm and this may be due to different types of machines with their specification used in these studies.[29]

The study has few limitations as CoCr was not from the same manufacturer as in group A pre-sintered CoCr blank was used which is different from the casting CoCr alloy used in group B and the powder CoCr used in group C.


  Conclusion Top


The best marginal fitness was with the direct CAD/CAM milling group, followed by the casted milled CAD-CAM wax group and the DMLS group. The DMLS technique had the highest marginal discrepancy among the tested groups. All the marginal gap values were clinically acceptable in all groups.

Acknowledgements

The authors are grateful to the Ministry of Higher Education, Mustansiriyah University for its support in the research.

Financial support and sponsorship

Not applicable.

Conflicts of interest

The authors declare no conflicts of interest.

Author contributions

All the data for this in vitro study was collected from laboratory work and then analyzed. Finally, all the authors had given their consent for publication.

Ethical policy and Institutional Review board statement

Not applicable.

Patient declaration of consent: (if in-vivo study/case reports)

An in vitro study samples involve metal coping of a die model.

Data availability statement

Data are available on reasonable request with the corresponding author’s mail.



 
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Kim SR, Koak J-Y, Heo S-J, Kim S-K, Kim M-J. Influence of the accuracy of abutment tooth preparation on the marginal adaptation of Co-Cr alloy copings fabricated with a selective laser sintering technology. J Korean Acad Prosthodont 2015;53:337-44.  Back to cited text no. 4
    
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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