When dental implants cannot be employed to replace a missing or extracted tooth, then a fixed or permanently cemented dental bridge is the next best solution. Porcelain bridges are not removable and can be constructed out of many materials. It is easier to achieve a high level of esthetics when the restorations are entirely constructed of porcelain. The design and overall strength of these restorations are evolving quickly. At this time, the most common porcelain varieties (e.g. Zirconium, Procera, E Max) used as substructures or frameworks still have lower flexural strength relative to metallic versions but their popularity is growing rapidly.
When the supporting substructure is placed under unusually heavy loads, veneering porcelain can chip or fracture off of any bridge or crown. Porcelain is like glass; it has a low tensile strength. If there is significant flexure, it will crack, chip, or break. Adding a metallic substructure under the porcelain will reduce flexing under trauma or excessive loads. If the patient is hard on their dentition with clenching or grinding, it is advisable to support the superficial veneering layer of ceramic with some sort of metal substructure. There are many types of metallic substructures which can be added when maximum strength is needed. Metallic substructures are cast in the lab with various quality levels of metal alloys.