|Year : 2015 | Volume
| Issue : 1 | Page : 23-26
Comparison of the marginal gap of zirconia-fabricated copings generated by CAD/CAM and Copy-Milling methods
Abdolhamid Alhavaz1, Ladan Jamshidy2
1 Department of Prosthodontics, School of Dentistry, Babol University of Medical Sciences, Mazandaran, Iran
2 Department of Prosthodontics, Kermanshah University of Medical Sciences, Kermanshah, Iran
|Date of Web Publication||5-Feb-2015|
Department of Prosthodontics, School of Dentistry, Kermanshah University of Medical Sciences, Shariati St., Kermanshah - 67139 54658
Source of Support: None, Conflict of Interest: None
Introduction: This study was conducted to compare the marginal fit of single-tooth zirconia-based copings fabricated by CAD/CAM process (Cercon; Degudent) and Copy Milling (Dentium). Materials and Methods: One in vitro prepared abutment from one mandibular molar model served as a template for replication of 40 epoxy resin (highly filled) dies, which had been taken by polyvinyl siloxane impression material. Copings were manufactured on epoxy replicas by two processes: the CAD/CAM (Cercon; Degudent) (n = 20) and the Copy-Milling (Dentium) (n = 20) processes. Four measurements were performed for each surface coping (Buccal, Lingual, Mesial, and Distal). Data were analyzed by SPSS 16 statistical software using independent t-test. Results: There was a significant difference in the marginal gap values between CAD/CAM and Copy-Milling groups (P < 0.001). The mean value of the marginal gap (SD) for CAD/CAM Copings (56.87 μm) was significantly less than that of Copy-Milling (136.12 μm) (P < 0.001). Conclusion: The marginal fit of CAD/CAM copings was better than that of Copy-Milling copings.
Keywords: CAD/CAM, marginal gap, zirconia
|How to cite this article:|
Alhavaz A, Jamshidy L. Comparison of the marginal gap of zirconia-fabricated copings generated by CAD/CAM and Copy-Milling methods. Dent Hypotheses 2015;6:23-6
| Introduction|| |
Zirconia restorations have been widely used in the past few decades. This may be due to the optimal esthetic characteristics as well as the zirconia's bio-compatibility. ,
Ceramic restorations have a long-term success, which depends on the materials. The mechanical and bonding characteristics  are also significantly influenced by the marginal fit. ,,,,,,,, Inaccurate marginal fit causes the plaque accumulation, micro-leakage, and cement breakdown. Subsequently, the risk of carious lesions, periodontal disease, and endodontic inflammation is increased, and adverse consequences that affect the underlying health of abutments may occur. ,,,,,,
Several studies have evaluated the maximal marginal gap values. ,,,,,,, McLean et al. , evaluated more than 1000 crowns in 5 years and concluded that a marginal gap of less than 120 μm is clinically acceptable; however, in vitro studies have reported values of 100 μm. ,,,,,
To fabricate the zirconia-based copings, different computer-aided design (CAD)/computer-aided manufacturing (CAM) processes are commercially available. The Cercon system is a CAD/CAM process that can use one of two available coping design methods. This coping is designed either virtually on the scanned die or on a scanned wax pattern and a scanned die (double-scan technique). By scanning, all information about the die and wax is digitized and the data are transmitted to a manufacturing center. To compensate the shrinkage, the zirconia blank is milled on an enlarged replica. The internal and external contour and the finishing line are all milled; the final sintering is also performed in the manufacturing center.
The Copy-Milling system is a developed fabrication technique in which the copings are fabricated using an auto-polymerized acrylic resin (Rainbow, Dentium; Korea) pattern previously adapted to the die. This pattern is read by a technician-guided sensor, and the copings are concurrently milled out of a partially sintered zirconia blank and sintered in the laboratory furnace, afterward.
The results of the two studies related to the Zeno (Wieland-Imes, Eiterfeld, Germany) and Denzir (Dentronic, Aarhus, Denmark) indicated the mean ± SD marginal gaps of 13 ± 12 μm and 22-24 ± 25-36 μm, respectively. , To evaluate the Cercon zirconia single-crowns fit based on the studies, moderate absolute marginal discrepancy (AMD; the distance from the internal edge of the coping's margin to the preparation finishing line) varied from 17 μm to 83 μm.
The aim of the present study was to evaluate and compare the zirconia single-crowns marginal fit manufactured by a CAD/CAM process (Cercon, Deguden) and a Copy Milling (Dentium).
| Materials and Methods|| |
A standard preparation was performed on the mandibular molar model with the following characteristics (W&H turbine, Allegra): 1.5 mm occlusal reduction and 1 mm axial reduction with 6-8 convergence (3-4 for each side). 
Round shoulder finishing line with at least 1 mm width was prepared on the lingual and facial surfaces, which were reduced in two planes, , and all of the line angles were rounded to reduce stress concentrations.
The prepared model served as a master die for preparation of 40 other dies using a highly filled epoxy resin (shoots dental Germany) of similar elastic modulus as human dentin (12.9 GPa). To this end, 40 impressions were taken from the prepared model by polyvinylsiloxane (Panacil, Kettenbach, Germany). Then, these impressions were poured under the vacuum with epoxy resin to reproduce void free dies.
Radiographic images (Eastman Kodak, America) were taken to ensure void free dies.  Dies were randomly divided to two groups (n = 20). In the first group, copings were manufactured on epoxy replicas by Cercon CAD/CAM machine (n = 20), and in the other group, Copy-Milling machine was used (n = 20). Each of the dies of group one was scanned and the obtained data were transformed to the CAD data. The blanks of zirconia ceramic (yttrium oxide tetragonal) were milled by the CAM device to produce a coping with 1 mm thickness. It should be noted that in this software, 30 μm virtual thickness was intended under each coping (cement space). Then, copings were sintered in the porcelain furnace at 1350°C for 7 hours and 30 minutes. All of the dies were covered with two layers of spacer (Vita Inceram interspace varnish) with resultant thickness of 30 μm. Care must be taken that the varnish is 0.5 mm short of finish line.  After each application of varnish, 5 minutes are required for drying and after the second layer, 20 minute time is required. Next, the autopolymerizing acrylic resin (rain bow, dentium) was used to construct acrylic resin copings. Thickness was assessed in different areas (midbuccal, midlingual, midmesial and middistal) by Iwanson gauge (Odontomed Establishment Medical Equipments & Supplies, Al Ain, Emirates). Reduction was performed in thick areas until the optimum thickness of 1 mm was attained. Then, the patterns were put on the dies and were read by a sensor guided by the technician, and the copings were concurrently milled out of partially sintered zirconia blanks. Copings were then sintered in the furnace. To ensure fully seated copings on the dies, fit checker (anti-spray, boush) was used on the intaglio surface of copings and if any pressure points were detected, the respective coping was excluded from the study and a new one was made. A stereo microscope was used for identification of marginal gap on four points of buccal, lingual, mesial and distal surfaces, and mean marginal gap of groups was compared and assessed.
Data were analyzed by (SPSS Inc. Released 2007. SPSS for Windows, Version 16.0. Chicago, SPSS Inc) software using independent t-test. P-value less than 0.05 was considered significant.
| Results|| |
The mean marginal gap in the mesial-distal-facial-lingual surfaces of copings manufactured by CAD/CAM was 56.87 μm and for Copy-Milling system was 136.12 μm, which was statistically significant (P < 0.001). The mean marginal gap values for mesial, distal, facial, lingual surfaces were calculated and compared with each other. In the mesial surface, the mean marginal gap in the CAD/CAM group was 57 μm, and for Copy-Milling group was 149.5 μm. In the distal surface, the mean marginal gap for CAD/CAM group was 54.5 μm and for Copy-Milling was 111 μm. In the facial surface, the mean marginal gap in the CAD/CAM group was 69 μm and for Copy-Milling was 147 μm. In the lingual surface, the mean marginal gap for CAD/CAM group was 47 μm, and for Copy-Milling group the mean marginal gap was 137 μm, which was statistically significant (P < 0.001) [Table 1].
|Table 1: Mean ± SD marginal gap of mesial-distal-facial-lingual surfaces in CAD/CAM and Copy-Milling|
Click here to view
| Discussion|| |
The null hypothesis, which stated manufacturing process does not influence the single-tooth zirconia copings marginal fit, was rejected.
In most in vitro studies similar to the present study, the prepared and standardized dies have been desirably used to evaluate the accuracy of copings.  Then, the prepared dies were generated using an impression material. So, an accurate controlling was performed on the parameters such as the convergence rate of axial walls and the finishing line. Therefore, the uniform size of these epoxy resin dies and consequently fabricated coping were resulted.
In some studies, the marginal gap and the AMD have been compared. , These two measurements have been performed to compare the results of numerous studies. ,, The accuracy of copings was measured with no veneering, because the finished crown's fit could be assessed by evaluating the copings. ,,,,
In the present study, the mean marginal gap values for CAD/CAM and Copy-Milling copings were 56.87 μm and 136.12 μm, respectively. Based on 100 μm criterion as the limit of the clinical acceptability, the mean marginal gap of the CAD/CAM system was acceptable. ,,,
The CAD/CAM system provides copings with a marginal fit significantly better than the Copy-Milling system. The results of the present study are in accordance with those of other authors that reported the Copy-Milling method had less accuracy in comparison with other methods.  There are some influential factors: first, this difference may be from the spacer thickness of the die and second, the reference for the coping manufacturing. The coping manufacturing by the Cercon system is based on the die, while it is based on the epoxy resin patterns' intaglio by the Copy-Milling system. Other factors that may influence the accuracy of the fit are related to the type of the zirconia used and the management of the sintering process, because in Cercon CAD/CAM system the rate of shrinkage after sintering is calculated and intended for manufacturing larger copings to compensate this shrinkage. This study did not use dissection of the specimens that may result in distortion of the copings and may reduce the measurement accuracy by producing heating on the sectioning.
One of the limitations of this study was not being able to entirely reflect the final marginal fit of the restoration. In fact, the veneering influencing procedure was not evaluated. Finally, selecting the manufacturing process depends on several factors. For example, the manufacturing process impacts the health of the dentistry laboratory technician that must be considered as a main factor. The risk of technician who has to inhale zirconia dust during the milling process is a concern accompanied by this system.
| Conclusion|| |
Marginal gap and AMD values for the Cercon CAD/CAM were smaller than those for the Copy-Milling. So, based on the criterion of 100 μm as the limit of the clinical acceptability, the mean marginal gap of the CAD/CAM system was acceptable and preferred to fabricate the restoration.
| Acknowledgement|| |
This study has been approved by Babol University of Medical Sciences (No. 19T).
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