Optimal marginal adaptation and internal fit are among the main criteria for long-term success of all types of crowns, including metal-ceramic restorations. ¹ , ² , ³ Poor marginal adaptation can lead to plaque accumulation, microleakage, cement washout and degradation and subsequent leakage of bacteria and saliva through the gap, which may eventually result in pulpal inflammation, recurrent caries, and periodontal disease. ⁴

At present, cobalt-chromium alloys are more commonly used than nickel–chromium alloys for the fabrication of metal frameworks of fixed partial dentures. ⁵ Electrochemical studies have shown that cobalt-chromium alloys are more resistant to corrosion than nickel–chromium alloys. Moreover, nickel-based alloys are more allergen. ⁶

Marginal adaptation of metal-ceramic crowns highly depends on their fabrication process. The conventional method for the fabrication of metal frameworks includes the lost-wax technique and the use of metal alloys for casting. ⁷ Wax has many inherent limitations such as fineness, heat sensitivity, and high elastic memory and high coefficient of thermal expansion. ⁸

The newly introduced computer-aided design/computer-aided manufacturing (CAD/CAM) laser melting is an additional fabrication process for metal copings of metal-ceramic crowns. ⁹ , ¹⁰ , ¹¹ , ¹² , ¹³ This technique does not have the limitations of the conventional waxing and lost-wax technique. Selective laser melting (SLM) uses high-temperature laser beams for selective heating of the metal framework, which is in the form of powder and uses the CAD data obtained from designing the framework for this purpose. ¹⁴ SLM has many advantages including the fabrication of restorations with a more homogeneous quality, standardization of the process of restoration shaping, less production costs, requiring fewer human resources and shorter time and the potential to yield higher accuracy due to the elimination of multiple procedural steps such as wax-up, flasking, and casting. However, the SLM scanners have disadvantages as well. For instance, the scanner systems have limited resolution, which can lead to slightly rounder margins. ¹⁵ , ¹⁶ , ¹⁷ , ¹⁸

Only a few studies have compared marginal adaptation/internal fit of restorations fabricated by the SLM technique with metal copings or crowns fabricated by the conventional technique. ¹¹ , ¹² , ¹⁵ , ¹⁹ However, comparison of the results of these studies is difficult due to variations in sample size, measurement methods, cement space, and laser systems.

Among the available tools for the measurement of gap and marginal adaptation, profilometry is a noninvasive method, which visualizes the die and sample in the same focal plane on a monitor and thus, enables more accurate focus. ⁷ , ²⁰

Considering the advantages and disadvantages of both the conventional waxing and SLM techniques, the significance of optimal marginal adaptation and internal fit and since a profilometer has not been previously used for this purpose, this study aimed to assess the marginal adaptation and internal fit of cobalt-chromium copings fabricated by the SLM and conventional techniques using a profilometer.

The null hypothesis stated that the samples fabricated using the two methods would have no difference with each other in terms of marginal and internal fit.

In thisin vitro study, the sample size was calculated to be five samples for each technique (a total of 10).

A brass model Figure 1along with its counterpart milled by a milling machine (CNC 350; Arix Co., Taiwan) was used as a model of prepared tooth for this study. This model had a circular cross-section with 8 mm diameter at the tip and 10 mm diameter at the base along with a round shoulder margin at the periphery with 0.5 mm thickness and axial walls with 10 mm length and 6° taper. The counterpart surrounded all the walls with 0.5 mm distance. ¹⁴ Figure 1

Brass die along with its counterpart

As shown in Table 1, a significant difference existed in the marginal gap between the casting and SLM groups in that the mean marginal gap was lower in SLM group (P < 0.05). According to Bootstrap test, the mean marginal gap in the SLM group was not within the confidence interval of the casting group. Furthermore, the mean marginal gap of the casting group was not within the confidence interval of the SLM group. Thus, nonparametric test (Mann–Whitney test) also confirmed that the difference in this respect was significant between the two groups (P < 0.05).{Table 1}

As shown in Table 1, the difference between the casting and SLM groups in the internal gap was not significant. (P > 0.05) According to Bootstrap test, the mean internal gap (= weight of the internal silicone material) of the SLM group was within the confidence interval of the casting group. The mean internal gap of the casting group was within the confidence interval of the SLM group as well. Thus, nonparametric test (Mann–Whitney test) also failed to show a significant difference between the two groups (P > 0.05).

As shown in Table 2, according to the Pearson and Spearman correlation tests, there was no correlation between the internal gap and marginal gap in the SLM and casting groups.{Table 2}

The current results rejected the null hypothesis of this study since there was a significant difference in marginal adaptation between the two methods of fabrication of copings. Marginal adaptation in the SLM group was superior to that of the conventional casting group, but the internal fit of the two groups was not significantly different.

In the present study, a brass die was used as abutment. Many researchers have used metal, or acrylic resin dies for the measurement of the marginal gap. ³ , ¹⁴ , ²⁵ , ²⁶ The advantages of metal die include standardized preparation and no wear during the measurement and fabrication process. Some studies have shown that due to differences in the seating of crown in different areas, the magnitude of marginal gap may vary at different points on a die. ²⁷ , ²⁸ , ²⁹ However, Holmes et al. ¹ hypothesized that the crown seating in different areas was not significantly different, and our findings supported their statement.

The sample size varied from 5 to 10 samples in each group in previous studies. ¹⁴ , ¹⁹ , ³⁰ , ³¹ Thus, five samples were fabricated by each method. The low volume of sample was due to financial problems in the project, but it has no effect on the results of the project because in the statistical analyses, in addition to parametric analyzes, nonparametric analyzes have also been used and the results of all analyzes are in concordant with each other, and the results have been confirmed by each other.

Measurements were made at 12 points on each sample in our study. Therefore, the mean discrepancy value could be generalized to the entire margin. The measurement techniques often involve sectioning of the crown and die and measuring the marginal gap under a light or scanning electron microscope. These techniques have the potential to cause distortion artifact and provide limited number and sites of measurements. Moreover, these techniques are considered invasive. ³² However, the use of a contact profilometer is noninvasive and allows multiple measurements between the finish line and final coping. The profilometer visualizes the die and the sample in the same plane and therefore, allows accurate focusing. In the present study, the marginal gap was quantified using profilometry, and the results showed that the marginal gap in the SLM group was significantly smaller than the casting group.

In this study, the terminology suggested by Holmes et al., ¹ for the assessment of marginal adaptation was used. Accordingly, the absolute marginal discrepancy, which is defined as the vertical distance between the coping margin and finish line was measured at 12 points for each coping, and the mean of 12 values was considered as the mean marginal gap of the entire crown.

The majority of researchers follow the principles of McLean and von Fraunhofer ³³ to describe the greatest width of the clinical marginal gap. The two authors in a 5-year study on 1000 restorations concluded that marginal gap by 120 μm yields the best clinical results. ³³ Accordingly, only the marginal gap in the SLM group was <120 μm and within the clinically acceptable range in our study. However, according to Moldovan et al., ³⁴ marginal gap by up to 100 μm is considered good and marginal discrepancy between 200–300 μm is considered acceptable. Accordingly, the marginal gap in our study (132.93 μm in the conventional casting group and 68.94 μm in the SLM group) was clinically acceptable and in accordance with the ranges reported by McLean and von Fraunhofer, White et al., Dedmon and Hung et al. ³⁰ , ³³ , ³⁵ , ³⁶

In the present study, slightly higher marginal gap in the casting group can be attributed to the shrinkage and stress release by the inlay wax and inherent inaccuracy of the investment material. The composition of the cobalt-chromium alloy powder used in the SLM technique differs from the casting material. The molybdenum content of the former is less than that in the alloy used for casting. This enhances the laser melting of the alloy because molybdenum has a higher melting point than the cobalt and chromium.

Moreover, the SLM technique completely eliminates the casting processes and the related human errors and yields superior results. Furthermore, since cast restorations are fabricated on a stone die obtained from polyvinyl siloxane impressions, marginal gap may be affected by the shrinkage of impression material or dental stone, deformation of wax pattern and differences in the thickness of spacer. However, these steps are eliminated in the SLM technique, which would allow the crown to be directly transferred to the patient's mouth without requiring such steps.

Several techniques have been employed to assess the internal fit of restorations such as scanning electron microscopy, stereomicroscope, resin replica technique, and weighing the light-body silicon. All these techniques, except for the weighing method, enable a 2D assessment only in a plane obtained by longitudinal sectioning of the coping. ¹ , ³ , ⁴ , ³⁷ However, the amount of light-body silicone entrapped between the coping and die indicates the entire space that exists between the coping and die and therefore, enables a 3D assessment of the fit of coping.

In the current study, the light-body silicone was weighed to assess the internal fit of restorations. According to the results, the SLM group showed a higher mean weight than the casting group but not significantly. In a study by Ucar et al., the internal fit of metal crowns fabricated by the casting and SLM techniques was evaluated and compared by two methods of weighting the light-body silicone and sectioning. The results showed a difference between the two methods in that the weight of light-body in the casting method was significantly less than that in the SLM technique. However, when the internal fit of crowns was evaluated using the sectioning method, the casting group had no significant difference with the SLM group although the mean value was lower in the casting method. Variations in values reported in our study and previous investigations may be attributed to the method of measurement, type of die, accuracy of scanners, and measurement of gap in noncemented copings.

Zeng et al. ¹⁸ stated that the SLM alloy shows a homogeneous microstructure while the cast alloy shows a typical dendritic microstructure. Highly different microstructure of cobalt-chromium alloys fabricated by the SLM and casting techniques results in a totally different corrosion behavior. Due to the fine scale microstructure, the SLM alloy shows better anti-corrosion properties, which helps in maintaining the marginal integrity of SLM metal copings in the long-term. ¹⁸ Thus, SLM metal copings are preferred for clinical application. In addition, in the present study, the mean internal gap in the SLM group was higher than that in the casting group, which may possibly be due to the fact that during scanning of the original die and fabrication of 3D model, the margin was manually adjusted while the external die surface was scanned by the offsetting algorithm of the scanning software.

The CAD/CAM process benefits from automated scanning and a strong CAD software for the fabrication of copings using SLM. This has several advantages such as complete control over the coping design and frameworks, marginal integrity, maintaining the cement space, coping thickness, pontic design, and elimination of casting processes.

The difference in elastic modulus of brass die and dentin was a limitation of this study.In vitro design was another limitation of this study since oral conditions cannot be perfectly simulated in vitro.

The copings fabricated by the SLM technique showed a smaller vertical marginal gap compared to the casting group. However, the two groups were not significantly different in terms of internal fit.

Acknowledgment

This article was written based on a dissertation for a specialty degree in prosthodontics. The authors would like to extend their gratitude to the Deputy of Research at Hamadan University of Medical Sciences, and the Dental Research Center for the financial support provided.

Financial support and sponsorship

This study was supported by Hamadan University of Medical Sciences, Hamadan, Iran.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or non-financial in this article.