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Table of Contents
Year : 2020  |  Volume : 8  |  Issue : 4  |  Page : 74-77

Comparison of marginal accuracy and internal fit of cast nickel chromium and metal laser-sintered crowns: An in vitro study

1 Department of Prosthodontics, AME’S Dental College and Hospital, Raichur, Karnataka, India
2 Department of Prosthodontics and Implantology, HKE’S SN Dental College and Hospital, Kalaburgi, Karnataka, India
3 Department of Prosthodontics, HKE’S SN Dental College and Hospital, Kalaburgi, Karnataka, India
4 Department of Oral Medicine and Radiology, Rajiv Gandhi University of Health Sciences, Bengaluru, Karnataka, India
5 Department of Conservative and Endodontics, Bangalore Institute of Dental Science and Hospital, Bengaluru, Karnataka, India
6 Department of Conservative and Endodontics, Aditya Dental College and Hospital, Beed, Maharashtra, India

Date of Submission12-Sep-2020
Date of Acceptance03-Oct-2020
Date of Web Publication27-Nov-2020

Correspondence Address:
Dr. Priyanka Konin
Department of Prosthodontics, AME’S Dental College and Hospital, H. No: 6-2-70/50A, Manikprabhu Layout, Raichur 584103, Karnataka.
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/INJO.INJO_40_20

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Purpose: The aim of this study was to evaluate and compare the marginal accuracy and internal fit of crowns fabricated by direct metal laser sintering, induction casting, and centrifugal casting. Materials and Methods: An acrylic resin analog of the right maxillary first molar was prepared with a total convergence angle of 6° and an occlusal reduction of 1.5 mm and 0.8 mm shoulder finish line. Thirty impressions of the analog tooth were obtained from the definitive die, n = 10 for each group. Wax patterns were fabricated and casting was done using an induction casting machine and centrifugal gas torch method, and for the direct metal laser-sintering group the dies were scanned and crown fabricated using EOS sintering machine. The marginal and internal gaps were estimated by measuring the cement thickness using a stereomicroscope. The mean value was determined and P values were obtained using a one-way analysis of variance (ANOVA) test. Student’s t test was used to determine the differences between the three groups. Results: The mean marginal and internal gaps were 59.82 ± 5.21 μm and118.69 ± 20.23 μm in the direct metal laser-sintering group, 116.13 ± 7.88 μm and 136.94 ± 13.50 μm in induction casting group, and 116.87 ± 7.46 μm and 133.77 ± 10.63 μm in centrifugal casting group, respectively. Conclusion: Under the limitation of this in vitro study, the marginal and internal gaps of DMLS group are better than centrifugal and induction casting groups.

Keywords: Casting machine, internal fit, marginal fit, metal laser sintered crowns

How to cite this article:
Konin P, Moldi A, Ingleshwar N, Nawabadkar S, Subhashini R, Anjum MG. Comparison of marginal accuracy and internal fit of cast nickel chromium and metal laser-sintered crowns: An in vitro study. Int J Oral Care Res 2020;8:74-7

How to cite this URL:
Konin P, Moldi A, Ingleshwar N, Nawabadkar S, Subhashini R, Anjum MG. Comparison of marginal accuracy and internal fit of cast nickel chromium and metal laser-sintered crowns: An in vitro study. Int J Oral Care Res [serial online] 2020 [cited 2021 Aug 6];8:74-7. Available from: https://www.ijocr.org/text.asp?2020/8/4/74/301703

  Introduction Top

Margins are one of the most important and weakest links in the success of cast restorations. Precise marginal fit is essential for a successful cast restoration because intraoral degradation of cements can result in loss of marginal seal and promote retention of plaque. Marginal fit of castings is one factor that leads directly or indirectly to secondary dental caries, adverse pulpal reactions, and periodontal disease.[1] The marginal fit of castings basically relies on perceptive tooth preparation, accurate impressions, precision casting, and careful finishing procedures. Hollenback[2] stated that well-fitting castings unless they were relieved an optimum 25 µm might fail to completely seat by as much as 100 µm.

There has been substantial disagreement about the acceptable marginal gap for dental crowns and fixed partial dentures. McLean and von Frauhofer[3] stated that a gap of 120 μm is tolerable, and that marginal discrepancies of <80 μm are difficult to detect under clinical conditions. Kashani et al.[4] considered that marginal openings >100 μm were unacceptable, whereas Blackman et al.[5] reported that an acceptable gap should be no >50 μm.

The traditional method of fabrication of dental restoration is lost wax technique. Although the “lost wax” process has been used since ancient times, it has become common practice in dentistry after it was introduced by Taggart in 1907.[6] There have been numerous reports on attempts to perfect the casting procedure in dentistry by improving materials and techniques. The majority of these efforts deal with the so-called “conventional” investing and casting techniques, which usually require at least 1h bench set for the investment, followed by a one- or two-stage wax elimination procedure before casting takes place. The whole process is time-consuming and requires approximately 2–4h for completion.

Modern precision laboratory procedures have a profound edge over traditional laboratory procedures in fabricating more ideal and precise restorations. Recently there have been many efforts to supplant traditional processes by using computer-aided design/manufacturing (CAD/CAM) systems. The advent of CAD/CAM has enabled the dentists and laboratories to harness the power of computers to design and fabricate aesthetic and durable restorations.[7] When CAD/CAM burst onto the restorative dentistry scene a few years ago it brought with it a new range of exciting features; precision fit reduced marginal adjustments. Direct metal laser-sintering (DMLS) is a well-known method for e-Manufacturing, the fast, flexible, and cost-effective production directly from 3D CAD data. Using the conventional casting production process, a dental technician can currently produce only about 20 dental frames per day. Laser sintering is a significantly superior method: one fully automated laser-sintering system can produce approximately 450 high-quality units of dental crowns and bridges within 24h. The technological centerpiece of this is the EOSINT M 270––the only system of its kind that produces cost-effective, high-quality dental prostheses using DMLS. To be able to manufacture dental prostheses using this additive layer manufacturing method, the 3D-CAD data are sliced into layers. Using these as a model, the desired geometry is produced in layers by selectively fusing cobalt-chromium powder using a laser. DMLS is believed to produce high-quality dental restoration which is cost-effective. Therefore, this study was carried out to evaluate the marginal integrity and internal fit of metal laser sintering crown and cast nickel-chromium crown. The null hypothesis was that the fabrication method would have no effect on the marginal accuracy and internal fit.

  Materials and Methods Top

An acrylic resin analog of the right maxillary first molar (Columbia Dentoform, Long Island City, New York) was prepared with a round-end diamond rotary cutting instrument (MANI, diaburs, Suzhou Welcome Dental Instrument Co. Ltd) in an aerotar handpiece (NSK). The total convergence angle was 6°. An occlusal reduction of 1.5 mm was accomplished to produce a complete crown preparation. The definitive die had a 0.8 mm shoulder finish line. A flat-end diamond rotary cutting instrument, 1.6 mm in diameter, at the tip (MANI, diaburs, Suzhou Welcome Dental Instrument Co. Ltd) was used for the shoulder margin. Thirty impressions of the analog tooth were obtained from the definitive die by using a putty and light viscosity vinylpolysiloxane (Aquasil Soft Putty/Light Set, Dentsply, Germany).

All impressions were poured with type IV die stone (Neelkanth Healthcare, Rajasthan, India). The dies were kept dry for at least 2 days before further processing. For the cast group, two coats of die spacer (Heart-Man Colour Spacer, CS-1000) were applied within 1 mm of the margin with a brush system and according to the manufacturer’s instruction the bottles were kept closed between applications, and the brush was cleaned frequently with thinner. The mean thickness of two coats of die spacers used in this experiment was approximately 25 μm according to a previous study.[1] All stone dies were sealed with a die hardener (Stone die and plaster hardener resin; George Taub and Fusion), and a wax separator (GC Sep; GC Corporation) was applied. Inlay wax (Blue inlay, Kerr Corp) was used to produce wax patterns. Dip method was used to fabricate wax pattern. When the pattern had cooled, the marginal excess was carved and the margin was burnished with the beavertail burnisher. The margin was examined with a stereomicroscope (Vardhan, India, Model M300) at ×40 magnification. The thickness of the crown was confirmed with a thickness gauge (Iwanson crown wax caliper; Surgidental Instruments, Deer Park, New York). Three millimeter round wax sprues were attached to the distopalatal cusp. The wax patterns were invested with a phosphate-based investment (BEGO Wilhelm-HerbstStraBe, Germany) for nickel-chromium casting at a 38mL/160g (water/powder) mixing ratio. Wax elimination was achieved by heating the mold to 850°C at 6°C/min according to the manufacturer’s instructions. After casting, the investment on the crowns was removed by using airborne-particle abrasion with 50-μm aluminum oxide particles (Basic master; Renfert GmbH, Hilzingen, Germany) at a pressure of 0.3MPa and with an ultrasonic cleaner. The casting sprues adjacent to the crown were removed by using a low-speed handpiece with a disc.

For DMLS crowns, the die was sprayed with a contrast spray and was scanned using 3M ESP and a design module, The simulated die spacer was programmed at 25 μm, starting 1 mm from the margin. Data was sent for production of the frameworks with the Co–Cr powder in a laser sintering machine. Each crown was luted to the original stone die with zinc phosphate cement (Harvard) and firm finger pressure for 5min until the hydraulic pressure was relieved. They were sectioned longitudinally in a mesiodistal direction with a diamond disc [Figure 1]. Sectioned specimens were polished using 1000-grit Al2O3 abrasive paper to remove the metal particles that adhered to the cement region. The marginal gap and internal gap at the occlusal fossa [Figure 2] area was estimated by measuring the cement thickness using the stereomicroscope (Vardhan India, Model M300). Each point was measured three times by a single investigator and the mean value was determined. P Values were obtained using a one-way analysis of variance (ANOVA) test. A Student’s t test was used to determine the differences between the three groups: DMLS, induction, and centrifugal groups.
Figure 1: Longitudinal cut specimen of crown cemented over the die

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Figure 2: Schematic diagram showing measuring points for marginal gap (A), internal gap (B), (C) crown, and (D) die

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  Results Top

The mean marginal gap was 116.13 ± 7.88 µm in induction casting (IC) group, 116.87 ± 7.46 µm in centrifugal casting (CC) group, and 59.82 ± 5.21 µm in DMLS group, respectively. The mean marginal gap in DMLS group was significantly smaller than that in induction and centrifugal groups. The mean internal gap was 136.94 ± 13.50 µm in IC group, 133.77 ± 10.63 µm in CC group, and 118.69 ± 20.23 µm in DMLS group, respectively. Internal gap was significantly smaller for the DMLS group than IC and CC groups. Comparison of three groups by Student’s t test [Table 1] shows that there is a significant difference between the DMLS group and IC group, and between DMLS and CC groups. No significant difference was found in the marginal and internal gaps between IC and CC groups. The mean marginal and internal gaps are significantly smaller in DMLS than IC and CC groups.
Table 1: Comparison by Student’s t test

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  Discussion Top

The null hypothesis states that the fabrication method would have no effect on the marginal accuracy and internal fit is rejected. The success of a restoration is determined by a range of factors. Marginal fit is one of the most important criteria when evaluating the clinical acceptability of the crowns. Lack of adequate fit is potentially detrimental to both the tooth and supporting periodontal tissues due to cement solubility or plaque retention. According to a study evaluating the service life of crowns in a 15-year period,[8] the primary reason for crown failure was caries, 36.8%; followed by uncemented crowns, 12.1%; and defective margins, 11.3%.

Various determinants of marginal adaptation of cast restorations are geometry of tooth preparation, dental materials such as impression materials, wax, die stone and casting investment, cement film thickness. Cement film thickness is related to preparation dimensions, occlusal perforations, die spacing, internal relief of the crowns, type of finish line, and type of cement. Other factors include power–liquid (P/L) ratio, cementation pressure, temperature and rate of mixing the cement, the powder particle size, and the liquid composition.[9]

However, the descriptive terminology defining the fit varies considerably among investigators. Moreover, the same term is used for different measurements, or different terms are used for the same measurement. In this study, the marginal gap and internal gap was defined according to the terminology reported by Holmes et al.[10] Two common methods of measuring the marginal gap are measurements of embedded and sectioned specimens and measurements of the specimens by direct visualization. The latter method is nondestructive and provides several measuring points. However, it is difficult to obtain accurate measurements and the internal fit cannot be measured. Therefore, the former method was used in this study.

The report of this study supports rejection of the null hypothesis that there would be no difference in the marginal and internal fit of crowns produced by DMLS group, the IC group and CC group. There has been substantial disagreement about the acceptable marginal gap for dental crowns and fixed partial dentures. McLean and von Fraunhofer[3] concluded that for single-tooth restorations to be clinically acceptable, the maximum marginal gap should be 120 µm (10–160 µm).

In this study, the mean marginal gaps in all the three groups are within an acceptable marginal gap of 120 µm with a smaller marginal gap in DMLS group than in induction and centrifugal groups.

The few published studies on the fit of constructions fabricated in Co–Cr have shown marginal discrepancies of 74–99 µm, with internal gaps ranging from 250 to 350 µm using laser melting technology on single crowns.[11] Bindl and Mormann[12] reported internal gap widths of 81–136 μm for different all-ceramic CAD/CAM crown copings. Örtorp et al.[13] in his study reported a mean internal gap of 144 ± 67 for conventional lost wax group and mean internal gap of 151 ± 58 for DMLS group. In this study, the mean internal gap of 118 ± 69 in DLMS, 136.94 ± 13.50 in induction group and 133.77 ± 10.63 in centrifugal group which were within the range compared to previous studies.

Laurent et al.[14] showed that in vitro studies present a better quality of fit in a controlled laboratory environment with optimal circumstances, than in a clinical setting. However, the production procedures for dental restorations in the dental laboratory do affect the fit more than the study design.[15] The main clinical and laboratory variables are impression making, master cast fabrication, die spacing, fitting procedures, and cementation.[16] Accordingly, future CAD/CAM techniques could minimize some of these risk factors proven in the DLMS group where few critical manual steps are present. This study is done with a smaller sample size; therefore, further study with a larger sample size is suggested.

  Conclusion Top

DMLS, a newer CAD/CAM technology, provides us with the crown of higher marginal accuracy and internal fit than the conventional casting method.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

Ushiwata O, de Moraes JV, Bottino MA, da Silva EG Marginal fit of nickel-chromium copings before and after internal adjustments with duplicated stone dies and disclosing agent. J Prosthet Dent2000;83:634-43.  Back to cited text no. 1
Hollenback GM Precision gold inlays made by a simple technique. J Am Dent Assoc 1943;30:99.  Back to cited text no. 2
McLean JW, von Fraunhofer JA The estimation of cement film thickness by an in vivo technique. Br Dent J 1971;131:107-11.  Back to cited text no. 3
Kashani HG, Khera SC, Gulker IA The effects of bevel angulation on marginal integrity. J Am Dent Assoc 1981;103:882-5.  Back to cited text no. 4
Blackman R, Baez R, Barghi N Marginal accuracy and geometry of cast titanium copings. J Prosthet Dent 1992;67:435-40.  Back to cited text no. 5
Taggart WH A new and accurate method of making gold inlays. Dental Cosmos 1907;11:1117-21.  Back to cited text no. 6
Mantri SS, Bhasin AS CAD/CAM in dental restorations: An overview. Annal Essen Dent2010;2:123-8.  Back to cited text no. 7
Schwartz NL, Whitsett LD, Berry TG, Stuart JL: Unserviceable crowns and fixed partial dentures-life span and causes for loss of serviceability. J Am Dent Assoc 1970;81:1395.  Back to cited text no. 8
Schwartz IS A review of methods and techniques to improve fit of cast restorations. JPD 1986;56:279-83.  Back to cited text no. 9
Holmes JR, Bayne SC, Holland GA, Sulik WD Considerations in measurement of marginal fit. J Prosthet Dent 1989;62:405-8.  Back to cited text no. 10
Quante K, Ludwig K, Kern M Marginal and internal fit of metal–ceramic crowns fabricated with a new laser melting technology. Dent Mater 2008;24:1311-5.  Back to cited text no. 11
Bindl A, Mormann WH Marginal and internal fit of all-ceramic CAD/CAM crown copings on chamfer preparations. J Oral Rehabil 2005;32:441-7.  Back to cited text no. 12
Örtorp A, Jonsson 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 Mater2011;27:356-63 .  Back to cited text no. 13
Laurent M, Scheer P, Dejou J, Laborde G Clinical evaluation of the marginal fit of cast crowns-validation of the silicone replica method. J Oral Rehabil 2008;35:116-22.  Back to cited text no. 14
Fransson B, Oilo G, Gjeitanger R The fit of metal–ceramic crowns: A clinical study. Dent Mater 1985;1:197-9.  Back to cited text no. 15
Papazoglou E, Brantley W, Johnston W Evaluation of high-temperature distortion of high-palladium metal–ceramic crowns. J Prosthet Dent 2001;85:133-40.  Back to cited text no. 16


  [Figure 1], [Figure 2]

  [Table 1]


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