The Journal of Contemporary Dental Practice
Volume 23 | Issue 10 | Year 2022

Comparative Evaluation of Retention and Vertical Marginal Accuracy of Co–Cr Copings Fabricated Using Three Different Techniques: An In Vitro Study

Bhagyashree G Kalsekar1, Paresh V Gandhi2, Rupali Patil3, Ajay V Sabane4, Pankaj P Kadam5, Nishita S Bhosale6

1–5Department of Prosthodontics and Crown and Bridge, Bharati Vidyapeeth Deemed University Dental College and Hospital, Pune, Maharashtra, India

6Department of Periodontology, Bharati Vidyapeeth Deemed University Dental College and Hospital, Pune, Maharashtra, India

Corresponding Author: Bhagyashree G Kalsekar, Department of Prosthodontics and Crown and Bridge, Bharati Vidyapeeth Deemed University Dental College and Hospital, Pune, Maharashtra, India, Phone: +91 9890876681, e-mail: bhagyashree.kalsekar@bharatividyapeeth.edu

How to cite this article: Kalsekar BG, Gandhi PV, Patil R, et al. Comparative Evaluation of Retention and Vertical Marginal Accuracy of Co–Cr Copings Fabricated using Three Different Techniques: An In Vitro Study. J Contemp Dent Pract 2022;23(10):991–997.

Source of support: Nil

Conflict of interest: None


Aim: This study was conducted to comparatively assess the retention and vertical marginal fit of cobalt–chromium copings fabricated by the conventional casting technique, 3D-printed resin pattern, and with direct metal laser sintering (DMLS) technique.

Materials and methods: Out of the total 60 test samples, 20 copings were obtained from inlay-casting wax and 20 from casting of 3D-printed resin patterns. In total, 20 copings were obtained from the laser sintering technique.

All 60 test samples were then cemented serially on the prepared maxillary-extracted premolars and were evaluated for vertical marginal gap in 8 pre-established reference areas. Retention was evaluated using a universal testing machine.

Results: Results obtained for both marginal gap and retention were statistically analyzed, and the values fall within the clinically acceptable range. The DMLS technique proved precedence over the other two techniques used, as it exhibited maximum retention and marginal accuracy, which is an area of prime concern.

Conclusion: The results from this study encourage further research with different pattern-forming materials and techniques and the need to identify the factors that facilitate better marginal fit and retention of cast restorations.

Clinical significance: This study has myriad of applications in clinical dentistry mainly in decision-making for casting procedure to provide better retention and marginal accuracy for fabrication of Co–Cr crowns. It also aims to aid the clinician to minimize errors by using different techniques for fabrication of wax pattern as well as the coping, keeping abreast with the recent technology to evaluate the accuracy of 3D-printed resin pattern over conventional wax pattern.

Keywords: Co–Cr copings, Direct metal laser sintering, Marginal fit, Retention, 3D-printed resin pattern.


The fundamental goal of fixed prosthodontic treatment involves replacement and restoration of teeth by artificial substitutes ranging from the restoration of a single tooth to the rehabilitation of the entire occlusion.1

Based on literature review, the largest prevalent cause of failure in crown and fixed partial dentures is the lack of retention, amounting to 45% of the total failures, which can be attributed to various causes like improper preparation of tooth and lack of resistance form. Another most common cause of failure is poor marginal adaptation leading to microleakage.2,3

The survival of fixed prosthodontics particularly depends on the marginal integrity and retention.4 This is principally balanced by manual and laboratory factors, the latter playing an enormous role. Manual errors being inaccuracy in impression-making, inappropriate oral environment for impression-making, whereas laboratory factors comprising of incompatibility of dental materials such as wax, die stone and casting investments, die spacer, and the casting techniques.

To overcome the manual and laboratory challenges, the advent of recent technologies like digitizing and automation has gained an important place in fixed prosthodontic treatment plan.5 Several techniques of using Co–Cr for fixed prosthesis are available today after the lost-wax technique that was introduced by William Taggart in 1907.6 In recent times, introduction of computer-aided design and computer-aided manufacturing (CAD–CAM) technology has given an opportunity for fabricating metal prosthesis using alternate methods like milling, 3D printing, and DMLS.7,8 The intraoral scanner helps in reducing the manual errors associated with impression-making, while the CAD–CAM and rapid prototyping techniques help reduce the laboratory errors affecting the fit of the restorations.

The proponents of these newer methodologies claim superiority over the conventional casting technique, but additional studies to check the fit of these restorations prepared by these new techniques are required.

Literature states studies conducted on fabrication of Co–Cr crowns and bridges using different techniques that have been performed using metal and stone dies.

This study was planned to focus on the effect of conventional and rapid prototyping techniques (3D printing and direct metal laser sintering) on the vertical marginal fit and retention of Co–Cr copings fabricated on extracted teeth which gains utmost importance for application in clinical dentistry.


An in vitro study was undertaken at the Department of Prosthodontics, Bharati Vidyapeeth Deemed University Dental College and Hospital, Pune, Maharashtra. The measurements were done at Praj Metallurgical Laboratory, Kothrud, Pune, Maharashtra.

Materials Used in the Study

Sixty sound extracted human maxillary premolars for orthodontic treatment:

  • a) Acrylic block for mounting teeth

  • b) Pattern wax

  • c) Resin pattern for 3D printing

  • d) Investment materials

  • e) Luting agent

  • f) Testing equipment (Praj Metallurgical Laboratory, Kothrud, Pune, Maharashtra)

    • Universal testing machine (computerized, software-based) Company: ACME, India, Model no. UNITEST 10.

    • Stereomicroscope: Make: Wuzhou New Found Instrument Co. Ltd., China, Model: XTL 3400E, magnification: 20×


For the evaluation of vertical marginal fit and retention, preparation of 60 extracted human maxillary premolars was carried out to receive an all-metal crown. The preparation was standardized by using an airotor attachment to the surveyor. The teeth were mounted in an acrylic block using a metal mold and were randomly divided into three groups of 20 samples each to receive Co–Cr copings fabricated by three techniques (Flowchart 1).

Flowchart 1: Flowchart of the methodology of the study

Group A (samples A1–A20) – Conventional wax pattern fabricated through lost wax technique.

Group B (samples B1–B20) – 3D-printed resin pattern using the (FORM LABS) 3D printer, and cast conventionally.

Group C (samples C1–C20) – Direct metal laser sintering using the LAVA 3M EOSINT M270.

The methodology is divided into four steps:

Tooth Preparation

A stainless-steel rod was inserted through the roots of each tooth and 60 resin blocks of dimension 20(L) × 20(W) × 20(H) were made using a metal mold. All the tooth preparations had the following features:

  • Occluso-cervical dimension was maintained between 6 and 7 mm as customized with the plaster base on the surveyor to engage the acrylic block with 1-mm-wide chamfer finish line and a convergence angle of 6 degrees.

  • The tooth preparations were standardized by using an attached instrument with clamp that was able to maintain a constant relationship of high-speed airotor to a surveyor (Fig. 1A). A protractor was affixed perpendicular to the horizontal long axis of the airotor on the horizontal bar, which was attached to the vertical spindle of the surveyor (Fig. 1B).

  • Each tooth preparation had an inbuilt taper due to the round-end-tapered diamond point, which is 3 degrees on each axial surface, therefore, total occlusal convergence of 6 degrees.

  • Standardization was maintained in proximal (Fig. 1C) and occlusal preparations (Fig. 1D).

Figs 1A to D: (A) Surveyor with airotor attachment; (B) Protractor attached to the vertical spindle of the surveyor; (C) Completed tooth preparation with 6-degree occlusal convergence (proximal view); (D) Occlusal view of completed tooth preparation

Wax Pattern, Casting (Group A)

The conventional wax patterns were directly fabricated on the prepared teeth (Fig. 2A). Investment of these wax patterns was done in phosphate-bonded investment material (Bellasum, Bego). The castings were then inspected for any irregularities, which, if present, were removed using a sintered diamond bur. A total of 40 castings were fabricated, respectively, for group A and group B.

Figs 2A to C: (A) Conventional wax patterns for group A; (B) Group B 3D-printed resin patterns; (C) Copings of all the groups ready for cementation and testing

Three-dimensional Printing of Resin Patterns Followed by Conventional Casting (Group B)

Scanning using EXO-CAD software and designing of copings for 3D printing: Group B was scanned using 3M/LAVA scanner and scans were processed to dental CAD software (EXOCAD). A digital model selection option was selected to export the data into STL format. The print option of the model was selected along with selection of the layer thickness of the resin which was 0.05 mm of thickness to achieve clinically relevant accuracy. The resin tank, resin cartridge, and build platform were inserted into the printer and the copings were printed onto the build platform. Printing of the 20 copings (group B) in pattern resin was completed (Fig. 2B) and the copings were then conventionally casted in Co–Cr using similar steps as used for casting the copings of group A.

Fabrication of Laser-sintered Co–Cr Copings (Group C)

Twenty prepared maxillary premolars were scanned using LAVA scanner and were processed to dental CAD software (EXOCAD). The designing involved die spacer thickness set at 30 μm. The designs were uploaded into the sintering unit. High-powered (200 W) laser beam was focused onto a powdered metal bed that started production of the copings by fusion of these particles into a thin solid layer. Parts were built up additively layer by layer, of about 0.02 mm of thickness. When all the layers had been built up, the solid copings were retrieved from the machine, sandblasted, polished, inspected, and ultrasonically cleaned. Copings of all the groups (groups A, B, and C) were ready for cementation and testing (Fig. 2C).


Glass ionomer cement was used for the cementation of each coping. Each coping was then seated on the tooth and held under compression load test under 5000 gm (50 N) for 5 minutes in order to simulate the maximum biting force generated by the masticatory muscles that vary from 42 N to 1245 N.


Testing for vertical marginal fit of the CoCr copings: Measurements were made using a digital optical stereomicroscope (Fig. 3A) (Make: Wuzhou New Found Instrument Co. Ltd., China Model: XTL 3400E, magnification: 20×), with an accuracy of 0.1 μm. The casting was seated onto the tooth and was clamped horizontally in position, using the metallic jig fixture of the digital microscope, at two-and-a half turns of the thumb-screw (Fig. 3B).

Figs 3A to D: (A) Digital stereomicroscope used for testing vertical marginal fit; (B) Testing of vertical marginal gap of each sample using a metallic jig; (C) Marginal gap measurement for group A Co–Cr copings; (D) Marginal gap measurement for group C Co–Cr copings

Measurements were made between the margins of the casting (Figs 3C and D).

The reference marks were scribed on the acrylic block at four points, i.e., 0°, 90°, 180°, and 270°. All measurements were performed by the same operator without knowledge of the identity of the testing groups.

Testing for retention of the CoCr copings: Universal testing machine was used for the evaluation of retention of all the samples (Fig. 4). Coping retention was measured by applying a tensile force to the circular ring attached to the cast coping in a universal testing machine at a cross-head speed of 0.5 mm/minute. The machine’s software recorded the tensile force values required to separate the copings (N).

Fig. 4: Testing for retention of copings under the universal testing machine

Statistical Analysis

The results were tabulated and subjected to unpaired t-test and one-way analysis of variance (ANOVA) to detect statistically significant differences. Between- and within-group differences in tensile bond strength were analyzed using one-way ANOVA test of significance with Tukey’s post hoc analysis. Statistical analysis was done using the IBM Corporation SPSS version 20 (Chicago, USA) program at a significant level of p ≤ 0.05.


The mean values of marginal fit were tabulated and a highly significant difference was observed with the p>0.001* (Table 1). The Co–Cr copings obtained by conventional casting technique (group A) (216.54 ± 36.682) showed the highest marginal discrepancy followed by Co–Cr copings fabricated from 3D-printed resin pattern (group B) (100.82 ± 12.516) and the Co–Cr copings fabricated from direct metal laser sintering (group C) (100.82 ± 12.516) showed the least marginal discrepancy (Fig. 5). Therefore, the present study has rejected the null hypothesis as there is a significant difference in the marginal discrepancy of Co–Cr copings fabricated by group A and Co–Cr copings fabricated from group B and group C.

Table 1: Comparison of the marginal gap in terms of {mean (SD)} among all the groups using the ANOVA test
Group N Mean Std. deviation F-value p-value
Group A 20 216.54 36.682 265.500 <0.001**
Group B 20 100.82 12.516
Group C 20 49.21 12.535
Total 60 122.19 74.246
**p < 0.001 – highly significant

Fig. 5: Comparison of the marginal gap in terms of {mean (SD)} among all the groups using ANOVA test

Comparison of the retention force among all the groups showed significant difference present in the retention of the three groups, with the p < 0.05* (Table 2). Laser sintering technique (group C) (186.08 ± 74.121) of coping fabrication showed the highest retention of the Co–Cr copings followed by 3D-printed resin printed technique (group B) (169.62 ± 81.653) (Fig. 6).

Table 2: Comparison of the retention force in terms of {mean (SD)} among all the groups using ANOVA test
Group N Mean Std. deviation F-value p-value
Group A 20 120.53 74.613 3.936 0.025*
Group B 20 169.62 81.653
Group C 20 186.08 74.121
Total 60 158.74 80.608
*p< 0.05 – significant

Fig. 6: Comparison of the retention force in terms of {mean (SD)} among all the groups


Successful clinical outcome of fixed prosthodontic treatment depends on the creation of mechanically, biologically, and aesthetically sound tooth preparations.9 The principal objective of this study was to evaluate whether there is any influence of the fabrication technique on marginal integrity and retention factors on the longevity of the final prosthesis in the oral cavity.10 In this study, Co–Cr copings fabricated from three different techniques – conventional casting, 3D-printed pattern resin, and direct metal laser sintering – were evaluated for vertical marginal accuracy and retention. Group A Co–Cr copings were fabricated using conventional casting of manually fabricated wax patterns. Group B Co–Cr copings were fabricated using conventional casting of 3D-printed resin patterns. Group C Co–Cr copings were fabricated using direct metal laser sintering. There are various studies discussing the effect of different fabrication techniques on retention and marginal fit of Co–Cr copings conducted on metal and stone dies. This study has been performed on extracted human maxillary premolars to add to the clinical relevance of the study.

While evaluating vertical marginal fit parameter, null hypothesis of the present study was rejected as group C showed highly significant marginal accuracy as compared to group A and group B. Diverse values for clinically acceptable range of marginal discrepancy have been stated in literature. The frontiers like Fransson et al. and McLean and von Fraunhofer have given the clinically acceptable marginal gap after cementation as <150 and 120 μm, respectively.11,12

Akova et al. reported a marginal gap in the range of less than or upto 76–93 μm and 62.6 μm, respectively, for laser-sintered copings.8 Thus, in accordance to the different marginal gap values quoted by different authors, the present study is in congruence with the clinically acceptable marginal gap value as stated by McLean et al. and von Fraunher for groups B and C and Moldovan et al. for group A, respectively.13

In the present study, group A demonstrated the highest mean marginal discrepancy among all the groups. The precision of the scanner used to digitize the working models, precision of designing via computer software, and precision of the machine used to fabricate the 3D design attributed to this difference in the marginal discrepancy.

The findings of the present study are in consensus with the work conducted by Vojdani et al.,14 who observed the lowest marginal discrepancy for DMLS copings (group C) and highest discrepancy for conventional lost-wax technique.15 The reason for the highest marginal discrepancy for group A attributes to multiple steps in the production of copings, which maximizes discrepancies in the definitive product. Wax has a tendency to distort or warp when allowed to stand unrestrained. The use of 3D printing software in group B compensated for the polymerization shrinkage and increased the precision without any chance of manual errors during fabrication process, which led to a better marginal fit of group B when compared with group A.

The results of retention parameter in the present study state that the technique of fabrication does have an effect on the retentive properties of the copings, irrespective of the cement used. Our findings are in accordance with Suleiman and Vult von Steyern,16 who concluded there was no significant difference in strength between Co and Cr crowns produced using production technologies: casting, milling, or laser-sintering. In a study conducted by Krug et al., there were no statistically significant differences in the retentive forces between conventionally cast, metal-sintered and milled FDPs.17

In the present study, GIC was used to cement milled metal and all ceramic copings. Although resin cements have maximum retention, previous studies assessed that GIC has also got adequate retention.18 Recently, the study conducted by Choi et al. evaluated the mechanical properties for Co–Cr alloy fabricated by conventional casting, 3D printing laser-sintered, and CAD-/CAM-milled techniques. The results were in congruence with the present study showing that the Co–Cr alloy fabricated by laser sintering had the highest retention.19

The significant difference in the retention values between the Co–Cr copings fabricated from lost-wax technique and direct metal laser-sintered technique is because of the errors involved in technique-sensitive conventional casting procedure, precision involved in the addition technique for designing of the DMLS copings, and manual errors involved in wax pattern fabrication. However, there are very few studies to evaluate the retention for Co–Cr copings fabricated with different fabrication techniques. Further studies are required for better evaluation and verification of the values of retention obtained in the present study.

Limitations of the Study

  • Marginal fit of copings was not determined before cementation.

  • Cemented copings were not subjected to thermal cycling. Thermal cycling is one of the important factors that affect the long-term marginal fit of crown.

  • All the copings were produced and tested under ideal conditions, which may not reflect conditions in daily clinical practice.

  • Polishing and finishing of the casting were done manually, which may also create some discrepancy in the casting.

Future Directions of the Study

  • Help the clinician decide which casting procedure provides better retention and marginal accuracy for fabrication of Co–Cr crowns.

  • Help the clinician get an idea about the errors that can be minimized by using different techniques for fabrication of wax pattern as well as the coping.

  • Help the clinician evaluate the effect of laser sintering on the retention and marginal fit of direct metal laser-sintered Co–Cr crowns.

  • Help the clinician evaluate the accuracy of 3D-printed resin patterns over conventional wax pattern.


In this in vitro study, the Co–Cr copings obtained from DMLS technique show the highest marginal accuracy when compared with conventionally cast copings obtained from inlay casting wax and 3D-printed resin pattern. The retention was least for copings obtained from the lost-wax technique followed by copings fabricated from 3D-printed resin pattern and copings from DMLS technique.

Within the limitations of the study, we can conclude that the results encourage further research with different pattern-forming materials and techniques and the need to identify the factors that facilitate better marginal fit and retention of cast restorations. Further studies that better simulate oral conditions are recommended for optimized clinical success.


1. Rosenstiel SF, Land MF, Fujimoto J. Contemporary Fixed Prosthodontics. 2nd ed. St. Louis, MO: Mosby Year Book; 1995. pp. 137–138, 170–173, 184–185, 229.

2. Goodacre CJ, Bernal G, Rungcharassaeng K, et al. Clinical complications in fixed prosthodontics. J Prosthet Dent 2003;90(1):31–41. DOI: 10.1016/s0022-3913(03)00214-2.

3. Selby A. Fixed prosthodontic failure: A review and discussion of important aspects. Aust Dent J 1994;39(3):150–156. DOI: 10.1111/j.1834-7819.1994.tb03083.x.

4. Rai R, Kumar SA, Prabhu R, et al. Evaluation of marginal and internal gaps of metal ceramic crowns obtained from conventional impressions and casting techniques with those obtained from digital techniques. Indian J Dent Res 2017;28(3):291–297. DOI: 10.4103/ijdr.IJDR_81_17.

5. Abduo J, Lyons K, Bennamoun M. Trends in computer-aided manufacturing in prosthodontics: A review of the available streams. Int J Dent 2014;2014:783948. DOI: 10.1155/2014/783948.

6. Venkatesh KV, Nandini VV. Direct metal laser sintering: A digitised metal casting technology. J Indian Prosthodont Soc 2013;13(4):389–392. DOI: 10.1007/s13191-013-0256-8.

7. Örtorp A, Jönsson D, Mouhsen A, et al. The fit of cobalt-chromium three-unit fixed dental prostheses fabricated with four different techniques: A comparative in vitro study. Dent Mater 2011;27(4):356–363.DOI: 10.1016/j.dental.2010.11.015.

8. Akova T, Ucar Y, Tukay A, et al. Comparison of the bond strength of laser-sintered and cast base metal dental alloys to porcelain. Dent Mater 2008;24(10):1400–1404. DOI: 10.1016/j.dental.2008.03.001.

9. Narula S, Punia V, Khandelwal M, et al. Retention in conventional fixed partial dentures. J Clin Diagn Res 2011;5(5):1128–1133. DOI: 10.7860/JCDR/2011/.1582.

10. Junner RE, Stevens L. Anisotropic expansion of phosphate-bonded investment. Aust Dent J 1986;31:6434–6439. DOI: 10.1111/j.1834-7819.1986.tb01246.x.

11. Bezzon OL, Pedrazzi H, Zaniquelli O, et al. Effect of casting technique on surface roughness and consequent mass loss after polishing of NiCr and CoCr base metal alloys: A comparative study with titanium. J Prosthet Dent 2004;92(3):274–277. DOI: 10.1016/j.prosdent.2004.04.021.

12. Fransson B, Oilo G, Gjeitanger R. The fit of metal-ceramic crowns, a clinical study. Dent Mater 1985;1(5):197–199. DOI: 10.1016/s0109-5641(85)80019-1.

13. McLean JW, von Fraunhofer JA. The estimation of cement film thickness by an in vivo technique. Br Dent J 1971;131(3):107–111. DOI: 10.1038/sj.bdj.4802708.

14. Vojdani M, Torabi K, Farjood E, et al. Comparison the marginal and internal fit of metal copings cast from wax patterns fabricated by CAD/CAM and conventional wax up techniques. J Dent (Shiraz) 2013;14(3):118–129. PMID: 24724133.

15. Bhaskaran E, Azhagarasan NS, Miglani S, et al. Comparative evaluation of marginal and internal gap of Co-Cr copings fabricated from conventional wax pattern, 3D printed resin pattern and DMLS tech: An in vitro study. J Indian Prosthodont Soc 2013;13(3):189–195. DOI: 10.1007/s13191-013-0283-5.

16. Suleiman SH, Vult von SteyernP. Fracture strength of porcelain fused to metal crowns made of cast, milled or laser-sintered cobalt-chromium. Acta Odontol Scand 2013;71(5):1280–1289. DOI: 10.3109/00016357.2012.757650.

17. Krug KP, Knauber AW, Nothdurft FP. Fracture behavior of metal ceramic fixed dental prostheses with frameworks from cast or newly developed sintered cobalt-chromium alloy. Clin Oral Investig 2015;19(2):401–411. DOI: 10.1007/s00784-014-1233-2.

18. Biscaro L, Bonfiglioli R, Soattin M, et al. An in vivo evaluation of fit of zirconiumoxide based ceramic single crowns, generated with two CAD/CAM systems, in comparison to metal ceramic single crowns. J Prosthodont 2013;22(1):3641. DOI: 10.1111/j.1532-849X.2012.00907.x.

19. Choi Y-J, Koak J-Y, Heo S-J, et al. Comparison of the mechanical properties and microstructures of fractured surface for Co-Cr alloy fabricated by conventional cast, 3-D printing laser-sintered and CAD/CAM milled techniques. J Korean Acad Prosthodont 2014;52(2):67. DOI: 10.4047/jkap.2014.52.2.67.

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