1. Introduction
Cobalt-Chromium-Molybdenum (Co-Cr-Mo) alloy is widely used alloy for the fabrication of removable partial dentures (RPDs) and porcelain-fused-to-metal crowns in dentistry today. It has increasingly replaced noble metal alloys, because of better mechanical properties and lower cost, although non-precious metal alloys are more difficult to cast. Certain inaccuracies may occur in casting Co-Cr-Mo alloys due to their higher melting range, limited ductility and potential for oxidation [
1,
2]. The time and labour required for traditional casting of Co-Cr-Mo alloy makes fabrication expensive, and efforts have been made to improve the technological process to overcome the shortcomings and to reduce costs.
Direct metal laser sintering (DMLS) is a promising technology that may enable the fabrication of dental devices, overcoming some of the imperfections of casting [
3]. The process of DMLS fabrication is an additive manufacturing (AM) process in which 3D parts are fabricated by the layered addition of material directly on the basis of computer aided design (CAD) data. In that way, DMLS enables a quick fabrication of 3D parts of any complex shape [
4,
5]. A high power laser is used to melt a powder feedstock to form fully-dense metallic parts [
6]. Thus, objects fabricated by DMLS are extremely dense, and the mechanical properties are comparable to, or better than, those of cast or machined parts. Accordingly, they can often replace parts produced by traditional methods in many applications [
7]. In dental applications, DMLS is a technique that could replace conventional metal casting procedures, as it can be used as a tool for the production of customized dental parts from biocompatible alloys directly.
Dental alloys for DMLS are available on the market today, but there is little scientific evidence reported on their characteristics, although the manufacturers state that the alloys are compliant with the standards of the International Organisation for Standardisation (ISO) [
8]. Nevertheless, the data are not fully available for academic scrutiny. It may also be worth noting that the DMLS process itself may affect the mechanical and chemical properties of the alloy. Materials for dental applications have to meet unique requirements, including suitable mechanical properties [
9] and acceptable biocompatibility, as they are to be placed in the oral environment with a variable pH [
10].
The release of ions from dental devices is an indicator of the corrosion characteristics of dental alloy. The corrosion characteristics, as well as other properties of the alloy are influenced by its microstructure. It is known that the technological procedure has an influence on the structure of the alloy [
11,
12]. DMLS is a completely different technological procedure from traditional casting that has been used for decades for manufacturing Co-Cr-Mo alloy dental devices [
13]. The dental device structure is built by layers created as cross-sections from a 3D CAD file that contains the framework design generated on the virtual model of the patient’s dental arch (
Figure 1). It is most important to prove that dental devices manufactured by this new and promising technology are not harmful for the patient.
Figure 1.
Removable partial denture (RPD) framework; (a) virtual design; (b) digital model with all relevant features for direct metal laser sintering (DMLS) manufacturing; (c) RPM framework fabricated on DMLS; (d) the final RPD framework after the finishing test fitted to the original patient cast.
Figure 1.
Removable partial denture (RPD) framework; (a) virtual design; (b) digital model with all relevant features for direct metal laser sintering (DMLS) manufacturing; (c) RPM framework fabricated on DMLS; (d) the final RPD framework after the finishing test fitted to the original patient cast.
Both the type of the released element and its quantity have an influence on the corrosion characteristics of an alloy. Many metals, such as chromium (Cr) and cobalt (Co), are toxic, even at low levels of exposure. Although Co is an essential element, most studies emphasize the fact that Co at higher concentrations is toxic and carcinogenic, with oxidative stress playing a crucial role [
14]. Experimental evidence points out that hexavalent Cr exposure, by either inhalation or ingestion, can have systemic effects that are distant from the site of exposure. Since hexavalent Cr is isostructural with sulphate and phosphate at physiological pH, it can be carried throughout the body and even into the brain [
15]. The previously noted points show the importance of determining the quantity of metal released in the oral cavity from DMLS dental alloy, as a new technology.
Metals released from dental alloys in artificial saliva are present in trace amounts. Their detection should be accomplished using a methodology that has a low detection limit and high specificity. Inductively-coupled plasma mass spectrometry (ICP-MS) is a suitable method for the detection of trace metals. This technique is a type of mass spectrometry with a very low detection limit for metals, which can be lower than one part per trillion (ppt). At certain points in the process of ICP-MS, the temperature is raised to 6000 °C, enough to excite and ionize all metals, which then emit their own spectrum. The concentration of the released ions is calculated according to the intensity of light [
16].
The corrosive effect of laser-sintered (LS) Co-Cr-Mo alloy has so far only been investigated by Alifui-Segbaya
et al. [
17], Takaichi
et al. [
12] and Zhang
et al. [
18]. Alifui-Segbaya
et al. [
17] investigated the dissolution of metal ions from rapid manufactured (RM) and cast Co-Cr-Mo alloy in artificial saliva of pH 2.3 and found that RM alloy performed better. Takaichi
et al. [
12] investigated the characteristics of selective laser melted (SLM, another type of metal RM process) Co-Cr-Mo alloy of experimental composition. The dissolution of metals from experimental Co-Cr-Mo SLM alloy in one mass % lactic acid was lower than from the cast Co-Cr-Mo alloy. Zhang Biao
et al. [
18] in a preliminary study on some properties of Co-Cr-Mo dental alloy formed by SLM investigated some of its properties and ion release behaviour after porcelain fused firing. The preliminary result indicated that SLM alloy performed better for Co, but the detection limit was not low enough to detect Cr release in this investigation.
Other researchers that investigated DMLS Co-Cr-Mo alloy focused their interest on the accuracy and misfit of fixed dental restorations, comparing them to cast Co-Cr-Mo devices [
13,
19,
20,
21]. The results of these investigations suggest that DMLS (LS) dental devices performed better than the conventional devices, except the investigation of Kim
et al. [
22], who found that the marginal fit of the DMLS system appeared significantly inferior to the conventional method.
The necessity of investigating the ion release from dental alloy in artificial saliva of different acidity exists because the oral cavity is a very demanding environment with high humidity, bacteria presence and variable pH. Bacteria significantly reduce the pH of the oral environment by the production of organic acids during sugar catabolism [
23]. To maintain a non-harmful pH in the oral cavity, the salivary system employs buffer systems: bicarbonate, phosphate and protein that maintain a pH of 6.0–7.5 [
24]. It is confirmed that high acidity can occur in small entrapped spaces that can be formed near dental devices where the buffering capacity of the saliva is diminished due to the reduced flow (fluctuation) or when gastric juice is ejected from the oesophagus into the oral cavity [
25,
26].
There is little evidence of the dissolution of metals of DMLS in saliva-like media in the scientific literature, while the influence of pH variations has not yet been reported. The research presented here is a comparative study of the dissolution of metals of two Co-Cr-Mo alloys for a removable partial denture framework, fabricated using DMLS and casting technologies in artificial saliva of three different pH.
3. Results and Discussion
The released Co from the CM and DMLS alloys in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8 in μg per cm
2 is presented in
Table 2. The release of Co was the highest compared to the release of Cr and Mo, both from the CM and DMLS alloys. The highest release of Co was found in the most acidic environment. The difference between the release of Co from the CM and DMLS alloys is shown in
Figure 5. The greatest difference was measured after 30 days of immersion at pH of 2.3. The Co release was 7 μg per cm
2 more from the CM alloy than from the DMLS alloy. The released quantity of Co from the DMLS alloy compared to the cast alloy at pH of 2.3 was 1.5-times less after one day, about 2.5-times less after seven and 14 days and 10-times less after 30 days of immersion (
Table 2,
Figure 5). At a pH of 4, the release of Co from the DMLS alloy was two- to four-times lower compared to the cast alloy, while at a pH of 6.8, the release was 3.8- to 5.8-times lower.
The release of Cr was the lowest compared to all other measured metal release (
Table 3). The difference between the release from the CM and DMLS alloys was the highest after 30 days of immersion in the most acid environment for Cr (
Figure 6) and Mo (
Table 4,
Figure 7).
Table 2.
The released Co from the cast (CM) and DMLS alloys (μg per cm2) in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8.
Table 2.
The released Co from the cast (CM) and DMLS alloys (μg per cm2) in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8.
pH | Sample | Time of the Exposure |
---|
1 Day | 7 Days | 14 Days | 30 Days |
---|
2.3 | CM | 0.500 | 1.685 | 1.819 | 8.244 |
DMLS | 0.324 | 0.685 | 0.759 | 0.881 |
4 | CM | 0.605 | 0.620 | 1.781 | 5.297 |
DMLS | 0.302 | 0.392 | 0.897 | 1.297 |
6.8 | CM | 0.162 | 0.166 | 0.179 | 0.183 |
DMLS | 0.031 | 0.035 | 0.038 | 0.044 |
Figure 5.
The difference between the release of Co from the CM and DMLS alloys (μg per cm2) in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8.
Figure 5.
The difference between the release of Co from the CM and DMLS alloys (μg per cm2) in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8.
Table 3.
The released Cr from the CM and DMLS alloys (μg per cm2) in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8.
Table 3.
The released Cr from the CM and DMLS alloys (μg per cm2) in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8.
pH | Sample | Time of the Exposure |
---|
1 Day | 7 Days | 14 Days | 30 Days |
---|
2.3 | CM | 0.012 | 0.363 | 0.437 | 0.606 |
DMLS | 0.007 | 0.026 | 0.031 | 0.036 |
4 | CM | 0.011 | 0.033 | 0.061 | 0.209 |
DMLS | 0.003 | 0.006 | 0.005 | 0.002 |
6.8 | CM | 0.004 | 0.029 | 0.034 | 0.041 |
DMLS | 0.000 | 0.000 | 0.000 | 0.001 |
The total release of metals in artificial saliva of different pH is presented in
Table 5 and
Figure 8. After seven days of immersion, the total release of Co, Cr and Mo from the CM alloy was three-times higher compared to the release from the DMLS alloy at a pH of 2.3, though both values were far below 200 μg per cm
2, the limit prescribed by the ISO standard. In saliva of pH 2.3 and pH 4.5, the longer the investigated period, the higher the difference between the total metal ion release from the CM and DMLS alloys.
Figure 6.
The difference between the release of Cr from the CM and DMLS alloys (μg per cm2) in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8.
Figure 6.
The difference between the release of Cr from the CM and DMLS alloys (μg per cm2) in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8.
Table 4.
The released Mo from the CM and DMLS alloys (μg per cm2) in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8.
Table 4.
The released Mo from the CM and DMLS alloys (μg per cm2) in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8.
pH | Sample | Time of the Exposure |
---|
1 Day | 7 Days | 14 Days | 30 Days |
---|
2.3 | CM | 0.059 | 0.279 | 1.133 | 2.431 |
DMLS | 0.015 | 0.061 | 0.093 | 0.113 |
4 | CM | 0.077 | 0.092 | 0.330 | 1.417 |
DMLS | 0.014 | 0.051 | 0.073 | 0.073 |
6.8 | CM | 0.092 | 0.113 | 0.130 | 0.179 |
DMLS | 0.092 | 0.008 | 0.034 | 0.039 |
Figure 7.
The difference between the release of Mo from the CM and DMLS alloys (μg per cm2) in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8.
Figure 7.
The difference between the release of Mo from the CM and DMLS alloys (μg per cm2) in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8.
Table 5.
Total release of Co, Cr and Mo from the CM and DMLS alloys (μg per cm2) in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8.
Table 5.
Total release of Co, Cr and Mo from the CM and DMLS alloys (μg per cm2) in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8.
pH | Sample | Time of the exposure |
---|
1 Day | 7 Days | 14 Days | 30 Days |
---|
2.3 | CM | 0.571 | 2.327 | 3.389 | 11.281 |
DMLS | 0.346 | 0.772 | 0.883 | 1.030 |
4 | CM | 0.693 | 0.745 | 2.172 | 6.923 |
DMLS | 0.319 | 0.449 | 0.975 | 1.372 |
6.8 | CM | 0.258 | 0.308 | 0.343 | 0.403 |
DMLS | 0.039 | 0.069 | 0.077 | 0.085 |
Figure 8.
The difference between total release of Co, Cr and Mo from the CM and DMLS alloys (μg per cm2) in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8.
Figure 8.
The difference between total release of Co, Cr and Mo from the CM and DMLS alloys (μg per cm2) in artificial saliva of pH = 2.3, pH = 4 and pH = 6.8.
The release of all investigated metal ions was influenced by the acidity. The greatest release was measured in the most acidic environment. A change in acidity from pH of 6.8 to pH of 2.3 for the cast alloy led to an increase of three- to 45-times for Co, three- to 15-times for Cr and up to 13-times for Mo. The increase for the DMLS alloy was higher: eight- to 25-times for Co, up to 18-times for Cr and 1.8- to 2.8-times for Mo. The greatest release out of Co, Cr and Mo was for Co. Further, the greatest release of all ions was measured at pH 2.3 after 30 days of immersion (
Table 2,
Table 3 and
Table 4). When comparing two technological procedures used to produce CM and DMLS samples, the greatest difference between the released ions occurred in the most acidic environment, where DMLS samples showed superior properties (
Figure 5,
Figure 6,
Figure 7 and
Figure 8). The presented results are in accordance with the results of Alifui-Segbaya
et al. [
17],
et al. [
12], as well as of Vandenbroucke and Kruth [
31], who reported a lower release of metal ions from RM samples than from the cast Co-Cr-Mo alloy in the environment of pH 2.3. As there is no report on the influence of the acidity on the release of Co, Cr and Mo from the DMLS alloy, it is not possible to compare the results presented here with the results of other investigations.
The results of density analysis are presented in
Table 6. The density of DMLS samples was found to be higher than for CM samples, about 4.5% on average. This is in line with research results of AM Co-Cr-Mo alloys reported by Vandenbroucke and Kruth [
31] and Monroy
et al. [
32].
Table 6.
Results of density analysis.
Table 6.
Results of density analysis.
Sample | Density ρ (g/cm3) |
---|
ρaverage | ρmedian | ρstdev | ρmin. | ρmax. |
---|
CM | 8.22 | 8.22 | 0.02 | 8.20 | 8.24 |
DMLS | 8.60 | 8.59 | 0.03 | 8.52 | 8.66 |
The microstructure of the CM alloy is shown in
Figure 9, with the characteristic dendrite microstructure (dark portion in the figure), formed due to the different solidification temperatures of Co and Cr. The intermetallic phase varies in size and distribution, and it is presented by the light portion in the figure. Carbide precipitates can be seen as dark grey points of different sizes and shapes in the intermetallic phase, while endogenous porosity can be seen as black points.
Figure 9.
Microstructure of a cross-section of the CM sample. Magnification: 60×.
Figure 9.
Microstructure of a cross-section of the CM sample. Magnification: 60×.
The microstructure of the DMLS alloy (
Figure 10) was observed in the transverse cross-section along the building direction of the DMLS process. The DMLS sample exhibited a more homogeneous structure than the CM sample. The fine microstructure indicates that metal particles melt evenly and sufficiently. In
Figure 10a (magnification of 60×) and even better in
Figure 10b (magnification of 120×), crystals can be seen as dark black points that are imbibed in yellow brownish intermetallic compounds. The bright yellow colour can be seen in a form of circular arch-shaped boundaries and waves, which correspond to laser scan tracks, or so-called melt pools, and they seem to be all oriented in the same direction. This is typical of the building direction of the DMLS process. The homogeneous microstructure of AM Co-Cr-Mo alloy is in accordance with the results of other corresponding investigations [
12,
32,
33].
Considering the results of the metals’ dissolution analysis, higher density and a more homogeneous microstructure can be correlated to a lower metal elution, i.e., to better corrosion resistance.
The elution of Co from dental devices made of Co-Cr-Mo alloy should be suppressed, as Co is known as a toxic and carcinogenic agent in higher concentrations [
14]. Using DMLS technology, the release of Co can be significantly decreased. The results presented indicate the superior behaviour of the alloy made by this technology.
Figure 10.
The microstructure of the cross-section of the DMLS sample: (a) magnification of 60×; (b) magnification of 120×.
Figure 10.
The microstructure of the cross-section of the DMLS sample: (a) magnification of 60×; (b) magnification of 120×.
The results show that the release of Co, Cr and Mo from the DMLS alloy was lower than the release of ions from the CM alloy in artificial saliva of all investigated pH concentrations. The results show that the characteristics of the cast Co-Cr-Mo alloy are less favourable than the DLMS alloy in all tests. The corrosion characteristics of the cast Co-Cr-Mo alloy are well documented, as it has been used for decades for the manufacturing of dental devices [
11]. Its apposite biological properties originate from the formation of a surface oxide layer consisting of oxides of Co and Cr [
34].
There are a number of reasons why DMLS alloys demonstrate improved performance. The increased corrosion resistance of the DMLS alloy could be due to the addition of tungsten (W), which is known to improve the corrosion properties of Co-Cr alloys and to reduce chromium-depleted inter-metallic areas [
35]. Furthermore, DMLS manufacturing is a completely different technology compared to conventional casting. While the casting procedure is difficult to control, the DMLS process parameters are computer controlled and, therefore, more repeatable. Finally, materials melted by laser show better homogeneity [
31].
The average pH of human saliva is 6.8 [
25], but in the oral cavity, pH can decrease even to 2.3. Acid environments in the oral cavity can be found in the small entrapped spaces near the dental devices, in the case of gastric juice regurgitation in the patients’ mouth, or it can be caused by repeated vomiting in pregnant women, or in the case of other gastrointestinal disorders, like bulimia. For investigating the influence of pH variations on the metal release, the most acidic (pH 2.3), the average (almost neutral pH 6.8) and one in between value (pH 4) were chosen.
According to the ISO standard mentioned above, the release of metallic ions over a seven-day immersion must be below 200 μg per cm
2 of the tested sample under prescribed conditions [
27]. The results of the investigation presented here show much lower amounts of ion release. The significantly lower release that occurred from the DMLS samples compared to cast samples can be seen in light of the present knowledge of the role of Co and Cr incorporated in cell metabolism, with oxidative stress playing a crucial role. Saliva is continuously excreted and swallowed within the oral cavity, so the released ions travel further through the digestive system. As these ions pass through the epithelial barrier in the intestines, it is possible that they can affect other organs and may have a systemic effect [
15]. Although there is evidence that the passive oxide layer increases in passivity over time [
17], a question arising is how much Co, Cr and Mo is released and swallowed by a patient wearing a partial denture metal alloy framework for five to 10 years. From that point of view, the results of many times lower release of metals from DMLS alloys may have more significance. This issue could be considered by future investigations. The measured quantity of the released ions indicate that both the technological procedures of casting and DMLS building of Co-Cr-Mo alloy devices are safe for use in the oral cavity according to the ISO standard, although encouragingly, DMLS specimens released many times less Co, Cr and Mo than CM specimens. The superiority of the DMLS alloy increases with the decrease of pH of the solution. The greater corrosion resistance of the DMLS-built alloy shows an excellent potential for dental applications.