There are two main reasons for the aesthetically unpleasing gray or purple shadow seen at the gum line with long-term use of metal-supported porcelain veneers. The mechanical reason is that the nickel-chromium or chromium-cobalt alloy metal substructure on the inside of the veneer prevents light from passing through the gum and reaching the tooth root (shadow effect). The histological reason is that over time, metal ions (especially nickel ions) released from the metal alloy by oral fluids and brushing mechanics seep into the surrounding gum tissue, creating a kind of tattoo effect and staining the tissue. This leads to chronic irritation and discoloration at the cellular level in the gum. Nowadays, zirconium or all-ceramic (E-max) substructures, which transmit light like natural teeth, are used to prevent this problem.
