Modern dental fillings use a type of material known as composite. Composite is a resin material that is placed into the tooth in a doughy or thick liquid state. After placement, the doctor or assistant applies a blue light, called a curing light, to harden or cure the material.
Composite forms a physical bond to tooth structure, which older metal mercury based materials did not do. This allows the fillings to be stronger at smaller sizes. Dentists can place fillings that are more conservative of tooth structure when using composite.
Obviously, composite represents a huge cosmetic improvement over metal fillings, as they can be shade matched to the natural color of the patient’s teeth. Composite fillings can be almost invisible. Also, corrosion products that build up around metal fillings cause a permanent blue-gray discoloration of the tooth itself. This discoloration remains even after removal of the metal filling. Composite filling does not itself cause staining of a tooth.