Overdentures are defined as any removable tooth replacement device that is inserted over existing teeth or their remnants, replacing these teeth with false teeth. Prior to modern dentistry, overdentures were very nearly the universal tooth replacement device since surgical removal of teeth was painful, dangerous, and frequently impossible without modern anesthetics. In those days, dentures were made to fit over the rotting stumps of decayed or broken teeth.
Today, non restorable teeth are generally removed prior to the placement of a removable prosthesis, however, there are still instances where these teeth can be maintained to the patient's advantage. The most frequently seen overdenture today involves teeth that have had root canal
therapy. If the roots of these teeth are still serviceable, the crown may be cut off at gum line and a removable appliance may be placed over the stumps. Sometimes, the stumps are themselves covered with filling material or cast metal copings in order to protect them from decay.
The advantage to this is that the roots of these teeth can maintain the bone that supports them. This bone would otherwise resorb away leaving less tissue to support the denture. In addition, the root itself can serve as a "rest", or a vertical support for the denture allowing for more stability than would otherwise be available.