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A partial is a dental prosthetic (artificial tooth or teeth) that can be removed from the mouth somewhat like an orthodontic retainer. A bridge, on the other hand, is a dental prosthetic that's permanently anchored inside the mouth. Partials are sometimes referred to as RPDs (removable partial dentures). They're often selected as an inexpensive option over a bridge or dental implant for replacing lost teeth. Most partials work in a similar way: Artificial teeth are placed on a molded base to which metal clasps are added.
The partial denture that uses a metal framework is the traditional design, due to the rigidity and strength of the metal. Plastic partial dentures have normally been used as emergency or temporary replacements of missing teeth, allowing the gums and bone to heal before a definitive restorative solution is obtained.