Introduction

Beryllium (Be) is a chemical element and naturally occurring lightweight metal that finds industrial application in electronics, aerospace, and defense equipment1,2,3. In the field of dentistry, Be has been used in nickel–chromium4,5 alloys with contents of up to 2.05 mass.%6 for the fabrication of prosthetic reconstructions. Be reduces the melting temperature, decreases the surface tension, and increases the bond strength between metals and ceramics7. Furthermore, it improves the castability and polishing of non-precious alloys.

Manufacturing and processing of Be-containing materials is highly toxic, and workers are exposed to the inhalation of Be particles, fumes, or solutions8. Brief exposure can lead to the development of a rare condition called acute berylliosis9, while long-term contact can cause Be sensitization (BeS)10 and chronic Be disease (CBD), also known as chronic berylliosis10,11. An official statement of the American Thoracic Society assessed the prevalence of BeS between 0.9 and 14.6% and of CBD between 0.0 and 7.8%1. BeS represents an immunologically-mediated response to the metallic element without evidence of disease, while CBD is considered an incurable occupational lung condition and is often misdiagnosed with sarcoidosis or other granulomatous lung disorder10. Symptoms of CBD are cough, dyspnea, fatigue, fever, night sweats, and weight reduction3,8 with potential progression to the loss of respiratory function12. A history of occupational exposition to Be, positivity to the beryllium lymphocyte proliferation test (BeLPT) and a bioptic examination, confirming a granulomatous inflammation of the lungs, are considered signs for definitive diagnosis of CBD1. The incubation periods can last up to three decades13. Due to the available evidence of carcinogenicity in humans and the risk of developing lung cancer by occupational exposure, Be and Be compounds have also been classified as category 1 carcinogens by the International Agency for Research on Cancer14.

As a consequence of the increased occupational exposure to Be in dental laboratories, dental technicians appear at a higher risk of primarily developing CBD8,15,16,17,18. Therefore, to protect workers, the Occupational Safety and Health Administration recently established a new limit of 0.2 µg of Be per cubic meter of air for an exposure duration of eight hours or of < 2 µg of Be per cubic meter of air for more than 15 min18. According to the current ISO standard for fixed and removable restorations (ISO 22674:2016), the limit value for Be in metallic materials is 0.02% (mass fraction)19. Exposure to Be is considered the causal agent for CBD development, and it remains unclear why dental technicians might be more affected. Therefore, the present study aimed to determine the elemental composition of commonly used dental materials and assess the exact amount of Be. Both, non-precious and precious metal alloys used to fabricate prosthetic reconstructions were included. Furthermore, different types of dental ceramics, titanium alloys, polymethyl methacrylate (PMMA), polyether ether ketone (PEEK), and polycarbonate were analyzed. Inductively coupled plasma-optical emission spectrometry (ICP-OES) represents a highly sensitive analytical technique with wide elemental coverage and was applied in the present study20,21 The null hypothesis at study conceptualization assumed that evaluated materials contain traces of Be.

Material and methods

Study design

The analytical work was performed by the Institute of Applied Materials–Applied Material Physics of the Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen, Germany. The study proposal for cooperation between the Department of Prosthetic Dentistry and Clinic of Respiratory Medicine of the University of Freiburg, Freiburg, Germany, and the KIT has been approved and accepted by the Karlsruhe Nano Micro Facility in 2020.

Investigated materials and interdisciplinary cooperation

Four non-precious (Co–Cr) and five precious (Au) alloys were evaluated in the present investigation. In addition, seven ZrO2-based ceramics, two feldspathic ceramics, one lithium disilicate glass-ceramic (Li2Si2O5), one nano-fluorapatite glass-ceramic, and one nano-hybrid composite for veneering were included. Furthermore, five implant-supported abutments made of titanium or ZrO2, three PMMA-based materials, one polycarbonate, and one PEEK were examined. The evaluated samples represented a selection of the most frequently used materials for each category in two German dental laboratories, which provided the samples for elemental analysis. An overview of the investigated materials and their commercial name and article number is given in Table 1.

Table 1 Investigated materials.
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Preanalytical sample procession

The samples were divided into seven different groups depending on their different chemical composition. In Table 2 the preanalytical preparations and the chemical digestion are described for each group.

Table 2 Preanalytical preparations and chemical digestion for each included group.
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Inductively coupled plasma-optical emission spectrometr

Each sample solution was diluted several times depending on the concentration of the various elements. Instead of using volumetric dilution methods, the sample solution and ultrapure water were weighed (XP 205, Mettler-Toledo, Gießen, Germany). Analysis of the elements was accomplished with four different calibration solutions and an internal standard (Sc) by ICP-OES (iCAP 7600 ICP-OES Duo, Thermo Fisher Scientific Inc., Waltham, MA, USA) (Table 3). For Be the solution was, if necessary, matrix adapted (Ti, Co, Cu, Zr, Mo, Pd, In, W, Pt, Au). The range of the calibration solutions extended from 0.0005 to 0.01 mg/l. One to three wavelengths of Be were used for the calculation.

Table 3 Instrument settings for ICP-OES.
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X-ray fluorescence spectroscopy

All samples were analyzed semiquantitative via X-ray fluorescence spectroscopy (XRF) (Pioneer S4, Bruker AXS, Karlsruhe, Germany) against different universal calibrations depending on the material of the samples (metal, oxide, etc.).

Quality control

The certified ICP calibration solutions (Aesar, Thermo Fisher (Kandel) GmbH, Karlsruhe, Germany, CPAChem, Bogomilovo, Bulgaria) were controlled with another certified ICP solution from a different producer (Agilent, Waldbronn, Germany; Merck, Darmstadt, Germany). The recovery of these standards in matrix-adapted solutions was between 95 and 105%.

Descriptive statistics

Results of the elemental analysis are described in Tables 2, 3, 4, 5 and 6 as the mean outcome, standard deviation (SD) and measurement uncertainty ( ±). Data regarding the oxides are semiquantitative results determined with XRF against a universal calibration. The results were normalized to 100.

Table 4 ICP-OES results of non-precious metal alloys.
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Table 5 ICP-OES results of precious metal alloys.
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Table 6 ICP-OES results of oxide ceramics.
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Results

Detailed results of the ICP-OES elemental analysis are shown in Tables 4, 5, 6, 7 and 8.

Table 7 ICP-OES results of further investigated ceramic materials.
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Table 8 ICP-OES results of metallic and non-metallic implant abutment materials.
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Non-precious metal alloys

The limit of quantitation of Be in ICP-OES analysis is 0.1 mg/kg (Table 4). This Be level could not be measured in any non-precious dental alloy sample. Co was the main component in all the samples studied, followed by Cr. Heraenium PW (15.9 mass %) and Remanium Star (9.6 mass %) are the only ones containing W; Heranium PW showed the highest amount of Fe (4.2 mass %).

Precious metal alloys

The main component of all precious alloys was Au (50.8–84.6 mass-%; Table 5). Maingold EH and Bio Maingold SG presented also Cu, Ag and Pt, whereas in Bio Herador GG the second main element besides Au was Pt. Heraloy G showed the highest amount of Pd (35.9 mass-%) and Ecobest the highest amount of Ag (29.1 mass-%). Be was not determined in any of the precious metal alloy samples tested (limit of quantitation of 0.04 mg/kg).

Oxide ceramics

Data of the three most present elements are semiquantitative and determined with XRF against a universal calibration. The concentration data can vary from more than 25 to < 5%. The results were normalized to 100. Be concentration of the tested oxide ceramics resulted below the measuring limit of 0.5 mg/kg (Table 6). All the oxide ceramics investigated had a Zr content between 66.1 and 68 mass%. Y was contained in all samples and DD cubeX2® ML (7.6 mass-%) showed the highest content. Furthermore, Hf, which belongs to the group of heavy metals, was detected in all samples ranging between 1.29 and 1.55 mass %.

Other ceramics

As for the oxide ceramics, semiquantitative results were obtained with XRF against a universal calibration. The concentration data can vary by more than 100% at concentrations < 5% and the results were normalized to 100. None of the evaluated samples contained Be (limit of quantitation < 0.2 mg/kg) (Table 7). IPS e.max Press contained 76 mass-% SiO2, while IPS e.max Ceram contained 49.1 mass-% SiO2 with a higher content of ZrO2 (9.49 mass-%) compared to the other groups. Gradia™ Plus is the only investigated nano-hybrid composite containing BaO (41.3 mass-%), the second-largest component in this material after SiO2 (49.8 mass-%).

Implant abutments

The elemental analysis of the implant abutments showed that the SIC standard abutment and the cara i-abutment ® titanium had a similar elemental composition (Table 8). Both consisted of approximately 90 mass-% Ti, 4 mass-% V and 6 mass-% Al. The RN Variobase abutment was measured to be 100 mass-% Ti (with a standard deviation of 0.5 mass-% and measurement uncertainty of 2.4 mass-%). The values for Al, Ti and V were below the respective detection limit. Therefore, a possible Be content was below the detection limit for all the evaluated samples.

PMMA, PEEK, polycarbonate

The analysis of the three PMMA, one PEEK, and one polycarbonate material revealed a Be content < 0.08 mg/kg for all the evaluated samples (Table 9).

Table 9 ICP-OES results of included PEEK-, PMMA- and polycarbonate-based materials.
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Discussion

The objective of this study was to investigate multiple currently used dental materials concerning their possible Be content. To our best knowledge, no studies have conducted an elemental analysis for the detection of Be using a comparable broad spectrum of different dental materials, which includes precious and non-precious alloys, ceramics, PMMA, PEEK, and polycarbonate. Importantly, due to the low occupational exposure limits (0.2 mg Be/m3 air), a very sensitive methodology is necessary to determine the concentration of Be at ultra-trace levels.

Be has been widely used in the past decades to manufacture dental appliances22,23. To date, Be-exposure is considered "a modern industrial hazard"24 which can lead to sensitization and CBD, chronic lung disease2. A key factor for the management of CBD is the prevention of workplace-related and environmental Be exposure25. Frye et al. described a cluster of workers in an industry not directly related to Be processing and suffering from BeS caused by the high levels of Be contained in the concrete dust25. Appropriate protective equipment and preventive measures are mandatory to reduce the risk of respiratory diseases. In addition, routine medical examinations should be provided as for other high-exposure worker categories. Although exposure to Be in working places is being strictly regulated by the Occupation Health and Safety Administration, controlling is difficult26. Dental technicians are at higher risk of developing occupational respiratory disorders such as pneumoconiosis, caused by exposure to dust while handling dental materials27,28,29,30. They still seem to represent a population at higher risk of Be-associated disorders as compared to non-exposed workers despite the increasingly use of Be-free materials17,31. Furthermore, while the term "beryllium free" is used by several manufacturers to name their dental products, the concentration threshold for defining a material "free" from Be is still not defined. Further research groups aimed to assess the amount of Be contained in dental materials. Alkmin et al. investigated the microstructural characteristics of eight Ni–Cr alloys in the commerce6. The samples were analyzed using an inductively coupled plasma spectrometer (ICP-OES). Of the eight investigated alloys, five presented Be traces up to 2.05 mass-% and in two of these cases, the Be amount was not reported by the manufacturer.

There are different methods for ICP elemental analysis. On one side, inductively coupled plasma mass spectrometry relies on a high-temperature ionization source paired with a mass spectrometer. After nebulization, the samples are atomized, and ions are generated for the mass analysis20,21. On the other side, ICP-OES technology is based on the light transmission at specific wavelengths by atoms that move to a lower energy level. Element type and concentration are calculated based on the position and the intensity of the photon rays. All the analytical investigations of this study were performed with ICP-OES, which allows for precise multi-element tracing with high sensitivity and low detection limits.

Within the limit of quantification of the adapted methodic, Be traces ranged from below 0.000004 to 0.00005 mass-% depending on the group of materials analyzed. Based on these analyses, conducted at the ultra-trace level, it can be assessed that traces of Be are not of clinical significance in the evaluated samples. Therefore, the null hypothesis of the present investigation, assuming traces of Be are contained in the investigated dental materials, has to be rejected. Be was not found in the investigated materials, but further independent studies should address the elemental composition of used dental materials, focusing on heavy metals. A thorough understanding of health risks and the development of strategies to minimize occupational exposure to hazards should be continually pursued.

These results, however, raise further questions regarding the increased prevalence of Be-associated disorders in dental technicians and an evidence-based explanation. Firstly, some of the studies were conducted several years ago8,15,32, and the identification of health hazards, as well as the consequent restrictions adopted, might have caused the modification of the material compositions by the manufacturers. Secondly, despite the large-scale screening, the analyzed samples represent only a minimal fraction of the materials currently used in dental laboratories. Finally, it should also be considered that this study included only materials used in German dental laboratories, while most recent articles describing the prevalence of Be-associated diseases in dental technicians were assessed in other countries17,31,33. Despite the analyses of a large amount of samples by several sensitive methods, this study has a few limitations, including the restriction of the geographic area to Germany and to certain types of material. Analogue evaluations should be considered in future investigations involving a broader group of materials and different countries.

Conclusions

Based on the described elemental analysis, the following conclusions can be drawn:

  • The applied ICP-OES method allowed for a highly sensitive elemental analysis at ultra-trace levels.

  • Be concentration was below the respective limit of quantification (< 0.00005 mass-%) for all the evaluated samples.

  • Further studies are needed to assess the Be amount in currently commercialized dental materials.