Abstract
Cobalt-chromium (CoCr) alloys have been used in dentistry for dental bridges, crowns and implants for decades. When using CoCr alloys, a number of fractures have occurred in the Dental Laboratory, both when handling the castings and after they have been placed in the patient’s mouth. It is assumed that the key cause of the resulting fractures of CoCr dental bridges is the casting process, which includes the preparation and mixing of the basic components of the CoCr dental alloy, unstable solidification and the final treatment of the tooth casting surface. The aim of this study was, therefore, to examine three castings differently prepared from the CoCr alloy. For the initial CoCr alloy, we selected the one supplied directly from the manufacturer; three test samples were CoCr alloy remelted four times in the same crucible, while the fourth sample was the remaining solidified alloy from the crucible, taken at the last remelting. Characterisation of the microstructure of all four samples was performed by optical and scanning electron microscopy equipped with an energy dispersive X-ray spectroscope and X-ray diffractometry. Microhardness measurements were also performed. The investigation revealed that the microstructure of the castings is composed of a CoCr alloy matrix with a eutectic interdendritic composition and interdendritic precipitates, which were rich in W and Mo. The two oxides were identified as chromium oxide with silicon content and chromium oxide, which originated from the CoCr alloy as casting residue. The high content of silicon in the chromium oxide can be attributed to the silicon oxide from the ceramic melting crucible, mixed in with the remains from the CoCr alloy melting. The second oxide showed a more regular elemental content for chromium oxide, mixed with a small quantity of impurities and the casting CoCr alloy. Based on this research, some recommendations were made for working with CoCr alloys in the Dental Laboratory, with the aim of reducing the risk of dental bridge fractures in the future.
Highlights
Cobalt-chrome (Co-Cr) alloys are well known for their biocompatibility in addition to their good mechanical properties, high melting points and excellent corrosion resistance [1,2]
This study revealed that recasting up to four times seems acceptable only if at least 50% of new alloy is added during each recasting procedure [14]
The revealed microstructures, determined elemental composition and microhardness results suggested that the CoCr alloy samples had different properties, which indicated that they were in different microstructures, which are not characteristic of the equilibrium state
Summary
Cobalt-chrome (Co-Cr) alloys are well known for their biocompatibility in addition to their good mechanical properties, high melting points and excellent corrosion resistance [1,2]. They have been used in dentistry for decades, for dental bridges, crowns and implants [3]. Biocompatibility is the most important property for such applications, which comes from the formation of a hard, passive, oxide layer on the surface of these alloys [4] This prevents further corrosion of the alloy and the release of metal ions from the alloy into the mouth, which could cause systemic and local toxicity, allergies, or carcinogenicity [5]. Studies investigating substance release from dental alloys in the mouth or saliva show different releases of metallic ions, depending on the alloy used [4,7,8,9,10]
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