Thank you Mr. Dimitris

Your reply is of great help 

Cobalt, combined with chromium and other base metals, has been used for denture construction since the 1930s, for orthopedic implants since the late 1930s, and for subperiosteal dental implants since the late 1940s.  They are very strong and easy to fabricate by casting.  Cobalt allergies are very rare, far less than allergies to nickel, which are also relatively rare (most Ni allergies come from body piercing with Ni alloys or Ni plating under gold plating, not from dental exposure).

Thank you Mr. Lawrence