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Dental alloys for direct metal laser sintering DMLS are available on the market today, but there is little scientific evidence reported on their characteristics. One of them is the release of ions, as an indicator of the corrosion characteristics of a dental alloy.
Is obtained results show that the metal elution in artificial saliva from the DMLS alloy was lower than the elution from the CM alloy. The release of all investigated metal ions was influenced by the acidity, both from the DMLS and CM alloy, throughout the investigated period of 30 days.
The change in acidity from a pH of 6. The greatest release out of Co, Iiso and Mo was for Co for both tested alloys.
BS EN ISO 22674:2016
Further, the iao release of all ions was measured at pH 2. In saliva of pH 2. Both alloys showed a safe level of elution according to the ISO definition in all investigated acidic environments. 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 kso 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 [ 12 ].
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. 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 [ 1112 ]. 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 ]. It is most important to prove that dental devices manufactured by this new and promising technology are not harmful for the patient.
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 Coare 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. The concentration of the released ions is calculated according to the intensity of light [ 16 ].
Zhang Biao et al. 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 [ 13192021 ].
The results of these investigations suggest that DMLS LS dental devices performed better than the conventional devices, except the investigation of Kim et al. 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: 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 [ 2526 ].
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.
Specimens for the investigation of metal release from dental Co-Cr-Mo alloy into the artificial saliva were prepared to simulate the preparation of denture frameworks for clinical cases. The number of samples in each test group was above the ISO standard, which requires at least two parallel sets.
The size of a specimen was slightly larger than the final specimen size in order to allow for the reduction of the size to precise dimensions and the elimination of any curling and delaminating that may have occurred during the DMLS fabrication process.
Processing parameters for the Yb-fibre laser are given in Table 1. The composition by mass of SP2, according to the manufacturer, in percentages, is as follows: The specimens were finally prepared according to ISO [ 27 ].
The plates were prepared by removing at least 0.
ISOD – ISO Metallic materials for fixed and removable restorations
All specimens were subsequently polished with abrasive rubber wheels and finally polished with Co-Cr-Mo polishing paste Bego, Bremen, Germany Figure 2. The dimensions were measured with a micrometre, according to ISO [ 27 ], and carefully examined for porosity.
Afterwards, patterns were spruced with 4 mm diameter wax and invested in phosphate bonded precision investment material for RPD frameworks Rema dynamic S, Dentaurum, Ispringen, Germany. The powder liquid mixing ratio was g of powder to 16 mL of expansion fluid. The investment muffles were heated gradually in a furnace and cast in a Nautilus CC vacuum pressure casting machine Bego, Bremen, Germany. The composition of the alloy by mass according to the manufacturer is given in percentages as follows: After casting, the muffles were left to cool and the plates were divested and blasted Figure 3.
Final preparation of the specimens was carried out according to ISO [ 27 ], following the same procedure described for the final preparation of DMLS specimens. Specimens were ultrasonically cleaned for 2 min in ethanol, rinsed with distilled water and dried with oil- and water-free compressed air.
Each specimen was placed in a separate glass container and suspending on a nylon thread Figure 4. The specimens were completely immersed in a solution of artificial saliva NaCl, Lactic acid of pH 2. The testing period was 1, 7, 14 and 30 days. The quantity of the artificial saliva in every glass container was such to produce a ratio of 1 mL of solution per 1 cm 2 of sample surface area. The volume of solution was recorded to an accuracy of 0. Additional glass containers were used for two reference solutions without immersed specimens for each test group.
They were tightly sealed and maintained in the same conditions with the solutions containing the specimens until the end of a test period. A total of 24 different glass containers were prepared. The advantages of ICP-MS overcome all of the shortcomings of conventional methods, making the analysis simple and controllable. With conventional methods, trace elements could not be detected. When working with a complex material, the matrix could influence the accuracy of the results, and the linearity of the conventional methods could not be presented in as wide of a concentration range as with ICP-MS from ppt to ppm [ 16 ].
The ICP-MS technique provides excellent sensitivity, requires minimal sample size, affords minimal elemental interferences and provides a means to perform rapid and automated multi-elemental analyses, and there is no dependence of the various chemical functionalities contained in the sample matrices on the individual element recoveries.
Running in reaction mode, ICP-MS enabled detection of Cr with the ultimate detection limit, as Cr is difficult to detect, due to interferences [ 16 ]. Density measurement was performed according to instructions from ISO [ 28 ]. For density determination, a pycnometer Rajas Enterprises, Haryana, India was used.
The highest release of Co was found in the most acidic environment. The greatest difference was measured after 30 days of immersion at pH of 2. The release of Cr was the lowest compared to all other measured metal release Table 3.
The total release of metals in artificial saliva of different pH is presented in Table 5 and Figure 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. The increase for the DMLS alloy was higher: The greatest release out of Co, Cr and Mo was for Co.
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 5Figure 6Figure 7 and Figure 8.
The presented results are in accordance with the results of Alifui-Segbaya et al.
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 2674 with the results of other investigations.
The results of density 222674 are presented in Table 6. The microstructure of the CM alloy is shown in Figure 9with the characteristic dendrite microstructure dark portion in the figureformed 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. The kso microstructure indicates that metal particles melt sio and sufficiently. 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.