Traditional fillings can reduce the strength of a natural tooth by up to 50 percent. Alternatively, inlays and onlays, which are bonded directly onto the tooth, using special high-strength resins, actually increase the strength of a tooth by up to 75 percent. As a result, they typically last from 10 to 30 years. In some cases, where the damage to the tooth is not extensive enough to merit an entire dental crown, onlays provide a very good alternative.