Use of Cobalt in Dental Alloys “Justified”

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European manufacturers find the use of cobalt in dental alloys under MDR to be justified

Cobalt is registered as a substance in the EU under the European Chemicals Regulation REACH (Registration, Evaluation, Authorisation of Chemicals). Under the CLP Regulation (Classification, Labelling and Packaging of Substances and Mixtures) cobalt is assessed as a “CMR substance”, in specific terms cobalt is subject to harmonised classification as carcinogenic category 1B, mutagenic category 2, toxic to reproduction category 1B.

MDR requirements

Cobalt is a component of medical devices and is used in dental products including alloys (cobalt-chromium). Since 26th May 2021 the European Medical Devices Regulation (MDR) has set out limitations for the use of CMR substances. According to Annex I Section 10.4.1 of the MDR, category 1A & 1B CMR substances may only be contained in a concentration of more than 0.1% by mass fraction if there is a specific justification for the presence of this substance.

According to Annex I Section 10.4.2 of the MDR this justification must, in particular, be based on an analysis of the potential exposure of patients or users to the substance, an analysis of possible alternative substances taking into account independent scientific studies, and a reason why possible alternative substitutes are inappropriate in relation to functionality, performance and benefit/risk ratio of the product.

Several manufacturers including BEGO Bremer Goldschlägerei Wilh. Herbst GmbH & Co. KG; Dentaurum GmbH & Co. KG; Dentsply Sirona; Kulzer GmbH; and Institut Straumann AG; have provided rationales for the continued use of cobalt in dental alloys. These manufacturers have jointly concluded that cobalt may continue to be used in dental alloys.

These grounds for justification are as follows:

High-corrosion-resistant cobalt-chromium alloys are used for fabricating dental restorations such as all-metal crowns or bridges, denture frameworks or implant abutments. Cobalt is used thanks to its strength and ductility to withstand any masticatory forces that occur, and achieves a high degree of elasticity.

Alternative substances

Possible alternative materials to cobalt offer significant disadvantages. Nickel-chromium alloys or beryllium-containing alloys have a much higher allergenic potential. Precious metal alloys have inferior mechanical properties.

Titanium and titanium alloys are greatly inferior with regard to their modulus of elasticity. The properties of other metals such as rhodium or iridium are not comparable with those of cobalt-chromium, while iron and copper would suffer from greater corrosion.

Substitute materials also offer disadvantages in clinical indications. In the case of metal denture bases and clasps there are no comparable alternatives to cobalt-chromium alloys thanks to their exceptional mechanical properties (spring-hardness, flexibility, corrosion-resistance, bending strength).

In principle, high-gold-content alloys, titanium or more specifically titanium alloys, or zirconium dioxide-based ceramics, can be considered for use with bridges. However, high-gold-content alloys do not exhibit the same strength as the corresponding cobalt-chromium alloys. If they are used, wall thicknesses and connector dimensions must be strengthened. The same applies for titanium and titanium alloys.

Cobalt ingestion

Cobalt can be naturally ingested by humans through food. Data on the average daily cobalt intake varies greatly, but currently the estimated values provided by scientific literature are between 5 µg and 82 µg per day.

While it’s true that cobalt-chromium alloys can release cobalt ions into the body due to corrosion, and ISO 22674 requires a static immersion test to prove the material’s corrosion resistance, the highest recommended value of ion release is 200 µg/cm2 over seven days. Cobalt-chromium alloys fall well below this value with a total ion release of between 0.5 and 20 µg/cm2 across seven days.

Cobalt can also be released into the human body due to abrasion during mastication, however research has found that the average value of cobalt-chromium alloy released by abrasion is just 0.034 µg/d.

Dental technicians can be exposed to an increased volume of cobalt in dust, aerosols and gases created by milling or grinding. Exposure can, however, be prevented by following standard health and safety measures such as the use of extraction systems and wearing face masks; and by following the guidelines for the use of the material.

Calculations have demonstrated that the total exposure due to abrasion and corrosion based on a worst-case scenario (a custom-made dental laboratory restoration with a surface area of 45 cm2) is a value of between 0.59 and 4.97 µg per kg of body weight per day. When crowning a single tooth, exposure value was only 0.00053 to 0.136 µg per kg of body weight per day.

In contrast the values, which the EFSA and AFSSA (European and French Food Safety Authorities) specify as the acceptable daily intake amount, are considerably higher. The EFSA and the German Federal Institute for Risk Assessment (BfR) specify this value with 10 and 1.6 µg per kg of body weight per day.

Conclusion

The use of cobalt-chromium alloys is a valuable and currently irreplaceable mode of treatment in dentistry. The amounts of cobalt released by these alloys are very low and therefore acceptable. In comparison to daily acceptable intake amounts they are harmless.

The following companies collaborated in the preparation of this statement:
BEGO Bremer Goldschlägerei Wilh. Herbst GmbH & Co. KG
Dentaurum GmbH & Co. KG
Dentsply Sirona
Kulzer GmbH
Institut Straumann AG

The joint statement is available, upon request, from VDDI (Association of German Dental Manufacturers). Visit www.vddi.de, or email Gregor Stock, This email address is being protected from spambots. You need JavaScript enabled to view it.