An inlay or onlay is a much more conservative restoration for the tooth than a metal filling. While traditional fillings can reduce tooth strength up to 50 percent, inlays and onlays made of high-strength porcelain or ceramic can actually increase tooth strength by up to 75 percent, lasting from 10 to 30 years. An onlay restores inside the cusp of the tooth; an onlay is a larger restoration, and restores inside the cusp as well as over one or more sides of the tooth.