A Comparative Evaluation of Marginal Accuracy of Co–Cr Metal Copings Fabricated Using Traditional Casting Techniques and Metal Laser Sintering

INTRODUCTION

In recent times, there has been a paradigm shift in restorative dentistry which may be attributed to the various technological advances in the discipline. One of the main factors for a successful prosthodontic treatment is to provide an accurately fitting restoration or prosthesis. In the case of fixed prosthodontics, the success of the prosthesis is mainly determined by the marginal gap and internal gap.^1–5 Fabrication techniques like computer-aided milling and selective laser sintering, developed recently have gained a lot of popularity among commercial laboratories. However, despite these technological advances, the lost-wax technique still remains the most widely used technique for the fabrication of metal castings. The accuracy of fit of the casting is dependent on multiple factors starting from impression procedures till the final finishing of the prosthesis.^6,7

Precious metal alloys have been used as the primary materials for dental restorations in the past but nowadays increase in the cost of such materials has encouraged the development of new dental alloys. Cobalt–chromium (Co–Cr) alloys have been suggested as novel alternatives because of their high mechanical strength, corrosion resistance, biocompatibility, and cost efficiency.^8–10

The purpose of this study is to evaluate and compare the marginal accuracy and internal fit of the conventionally cast Co–Cr alloy crowns with laser-sintered Co–Cr alloy crowns using stereomicroscope and optical microscope and to assess the efficacy of these techniques to fabricate the crowns for fixed restorations.

MATERIALS AND METHODS

Standardized stainless steel die with a cervical diameter of 13 mm, 1.5 mm cervical margin, 5° of uniform taper and height of 12 mm, two retentive grooves (1 mm depth) extending 2 mm short of the finish line was prepared for the study.

A total of 40 specimens were prepared which included 20 samples of each category. All the samples prepared were of specific dimensions as a means for standardization.

The specimens were divided into two categories as follows: (n = 20)

Group I: 20 Co–Cr crown specimens fabricated by conventional casting technique.

Group II: 20 Co–Cr crown specimens fabricated by direct metal laser-sintering (DMLS) technique.

The metal master die was cleaned with ethyl alcohol and distilled water and dried with a paper towel before making each impression. A metal impression tube with an inner diameter of 19 and 6 mm relief over the master die was fabricated for making impressions of the master die. The impression of the master die was made with polyvinyl siloxane impression material (Silagum DMG Germany). A two-step putty wash technique was used to make impressions of the master die. After impression procedures, all impressions were visually inspected for the reproduction of all surface details, as well as for the occurrence of any surface defect under magnification (Figs 1 and 2 and Tables 1 and 2).

High strength dental stone, type IV (Ultrarock Kalabhai Karson) was used to pour the impressions. To eliminate individual variability, the same operator poured a total of 40 impressions to make 40 working stone dies. The working stone dies were removed from the impressions after 1 hour and were randomly assigned identification numbers from 1 to 20 in each group and stored at room temperature.

RESULTS

Grouping of samples and mean measurements of each specimen were as follows:

DISCUSSION

The present study investigated the marginal accuracy and internal fit of Co–Cr alloy crowns fabricated with conventional casting technique and DMLS technique.

The fabrication of a cast restoration involves a series of laboratory procedures that are technique sensitive. The procedures utilized are complex involving various stages, each of which may affect the dimensions and therefore, the accuracy of the final product. However, the marginal fit and accuracy of the patterns made go a long way in deciding the fit of the final restorations.

Base metal alloys, such as Co–Cr and nickel–chromium (Ni–Cr), have been widely used in the fabrication of fixed and removable partial denture frameworks since being introduced to dentistry in 1929. These alloys have increasingly replaced gold alloys owing to improved mechanical properties and decreased cost.¹¹ Initially, gold alloys were selected because of their biocompatibility and ease of use. Due to an increase in gold prices substantially, less expensive alternatives like base metal alloys were used. However, the casting of base metal alloys is more technique sensitive compared to casting of noble alloys because of the high melting range and oxidation of base metal alloys during casting. In addition, due to their high hardness, grinding of cast base metal alloys to finish castings is time-consuming for dental laboratories.³ Cobalt–chromium alloys were preferred, the major advantages of this alloy are its lower thermal expansion coefficient and higher creeping strength in high temperatures during porcelain firing.¹² Nevertheless, Ni–Cr alloys despite their satisfactory clinical performance and high modulus of elasticity, give them rigidity even in small thickness, these alloys have limitations related to their high hardness, difficult laboratory processing¹³ and low flow rate, the high incidence of incomplete margins being the most frequently reported problem. According to Moffa et al.,¹⁴ the increase of casting temperature increases the flow of the Ni–Cr alloy and reduces casting failures; nevertheless, it also increases surface roughness, which poses additional difficulty to the finishing and polishing procedures.¹² Many of the Ni–Cr alloys contain beryllium, which can be toxic in uncontrolled dental laboratories.

Of various techniques for fixed prosthodontics, mainly, conventional lost-wax technique accelerated casting technique, CAD/CAM is being used nowadays. Two fabrication methods are primarily used with this new digitized technique, either by milling the frameworks from a block of Co–Cr or by using direct laser metal sintering (DLMS). Unlike the milling technique, DLMS sinters a metal powder in layers, which is then fused by laser welding.³ The newly developed DMLS system is an additive metal fabrication technology, based on information received from three dimensional CAD, in which metal powder is shot selectively using a data file and fused with a laser to laminate approximately a 20–60-μm-thick layer with each shooting to complete a metal structure. There is little data on the marginal fit of fixed dental prostheses (FDPs) fabricated by the DMLS system so far.¹⁵ Hence, the present study was designed.

An improperly adapted margin of the metal casting causes plaque retention leading to periodontal diseases and secondary caries.¹⁶ To achieve a clinically acceptable result for FDPs the fit of the construction is one important requisite for a good long-term prognosis. A study performed by Foster¹⁷ on 142 failed FDPs concluded that one important reason for this technical complication was an unacceptable fit. An unacceptable marginal gap could result in cement washout with subsequent biological complications, such as secondary caries, periodontal problems, and pulpitis.¹⁸ Accurate marginal adaptation is necessary for maintaining gingival health and to protect the tooth from physical, chemical, bacterial, and thermal injuries.¹⁶ Several methods for measuring the marginal fit of dental prostheses have been used in previous studies: (1) a direct measuring method measures the gap or amount of cement directly by microscope after seating the dental prosthesis on the tooth model and incising it (2) using silicone replica, the thickness of silicone located between the dental prosthesis and tooth model is measured by microscope. Shortcomings of this technique might be the defects of the silicone material in the area of measurement and inaccuracies in the assessment of the film thickness with a microscope (3) a recently-introduced technique using X-ray micro-computed tomography (micro CT) that involves shooting the inner side of the dental prosthesis.¹⁵ The cross-sectional evaluation of margins allows greater precision in the determination of measuring points and permits the determination of the degree of horizontal discrepancy.¹⁹

For testing, a standardized complete crown preparation stainless steel die was made with a cervical diameter of 13 mm, 1.5 mm cervical margin, 5° of uniform taper and height of 12 mm, two retentive grooves (1 mm depth) extending 1 mm short of the finish line in accordance with various studies reviewed.^20,21 Impressions of the master die were made in addition to the silicone by two-stage putty wash technique. The advantage of the two-step technique over the one-step impression technique is that it records the surface completely in light body impression material,²² which influences a better reproduction of surface topography. Impressions of the master die were then poured using type IV dental stone to obtain working dies.

In this study, total of two groups were made as follows: (n = 20)

Group I: Co–Cr alloy crowns fabricated by a conventional casting technique.

Group II: Co–Cr alloy crowns fabricated by laser sintering technique.

Wax patterns for group I crowns were prepared on working die with casting wax by using bulk placement technique by a standardized method. Patterns and crowns for group II crowns were made by scanning the die and converting it into standard CAD format known as STL. This 3D model on the computer was used to design the pattern of the coping using the software (3 M Lava design 7). The software then prepared the CAD model of the pattern. It was then exported to the Cambridge software to create slice (SLI format) file exported slice files to MLS machine of EOS company. The machine then built units by addition technique with each layer of 20 μm. The Co–Cr powder was used to build the crowns. So, the accuracy of patterns prepared by this technique depended upon many factors. First, it depended upon how accurately the die was scanned by the scanner. Second, it depended upon the operator who had designed the pattern in the software. Although the software located the margin by itself, it required modification by the operator. To avoid individual variability, the same operator designed all the patterns in this study.

For group I patterns, ringless casting technique was followed in this study. Lombardas et al.²³ found in their study that the castings produced by the ringless casting technique provided less vertical margin discrepancy than the castings produced with the conventional metal ring technique. All the castings were done with Co–Cr base metal alloy.

After fabrication, each Co–Cr crown obtained from the casting procedure, as well as metal laser sintering procedure, was seated on its previously assigned respective working stone die and subjected to a constant load of 2 kg for 10 minutes.²⁴ The marginal discrepancy between the working die and the copings was then measured with stereomicroscope. The crowns obtained were then cemented on their dedicated working dies using glass-ionomer cement. The dies were then embedded in autopolymerizing acrylic resin for sectioning. The embedded specimens were then sectioned longitudinally using a slow-speed diamond cutting saw with a coolant to prevent distortion of metal during sectioning. A similar method was used earlier for sectioning by Davis,²⁵ Teteruck and Mumford.²⁶ These sections were then measured under an optical microscope for evaluation of internal fit.

Holmes et al.²⁷ have described absolute marginal discrepancy which they have defined as the angular combination of marginal discrepancy and any extension error (under-extension or over-extension) or perhaps more simply described as the distance from the margin of casting to cavosurface angles of tooth preparation. They stated that this distance would always be the largest measurement of error at the margin and reflect the total crown misfit at that point, vertical and horizontal.

Ucar et al.³ in their study significantly higher mean (SD) light-body silicone weights (p < 0.001) were observed in the laser-sintered Co–Cr alloy group [14.34 (1.67) mg] compared to the conventionally cast Ni–Cr alloy group [9.36 (1.97) mg] and Co–Cr alloy group [7.85 (1.19) mg]. The mean value of internal fit of laser-sintered Co–Cr group was 62.6 μm, and for conventionally casted Co–Cr, Ni–Cr alloy group was 50.55, 58.21 μm. Ortorp et al.¹⁸ reported the mean value of internal fit of direct metal laser-sintered crowns to be 69 and 99 μm in premolar and molar region, respectively. The mean internal fit by the conventional lost-wax method was found to be 133 μm reported that the best fit of three-unit FPD’s based on the means in μm for all measurement points was in the DLMS group 84, followed by milled wax with lost-wax method (MW 117), conventional lost-wax method (LW) 133, and milled Co–Cr (MC) 166. Significant differences were present between milled Co–Cr (MC) and DLMS (p < 0.05). The reduced number of steps of DMLS^3,18 may contribute to explaining those findings.²⁸ The values in the present study for the metal laser sintering group are in accordance with the above studies. Kim et al.¹⁵ in their study found that the mean values of an absolute marginal discrepancy, marginal discrepancy, and internal gap were significantly larger in the three-unit FDPs fabricated using a newly developed DMLS system using Co–Cr alloy than in three-unit FDPs fabricated by a conventional lost-wax technique (LW) using Ni–Cr alloy. The marginal fit of the DMLS system appeared significantly inferior compared to that of the conventional LW method and slightly larger than the acceptable range. The result is inconsistent with a previous investigation that has reported a superior fit of the DMLS system compared to the conventional method.

The CAD/CAM technique contains fewer production steps compared to the lost-wax technique, however, three main factors could affect the fit: the precision of the scanner that reads the abutments, how the software can transform the scanning data into a 3D model in the computer, and the precision of the machine that will CAM the objects from the CAD data.²⁸ According to the manufacturer, the scanner has a precision of 20 μm. However, according to Persson et al.²⁸ A scanner using laser technique tends to make sharp edges rounder. Possible explanations for this could be the optical properties of the plaster or the camera used.¹⁸

Clinical implementation of the DMLS system seems to require further investigation because studies of restoration longevity are scarce.¹⁵

CONCLUSION

Within the limitations of this study, the following conclusions were drawn:

• The mean marginal discrepancy of Co–Cr alloy crowns fabricated with conventional casting technique was found to be more than the crowns fabricated with laser-sintered technique.
• The marginal accuracy of Co–Cr alloy crowns fabricated with the laser-sintered technique was significantly higher than Co–Cr alloy crowns fabricated with conventional casting technique.

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