One of the most difficult challenges that dentists must face is the task of retrofitting fillings and crowns to an existing removable partial denture. For those who do not wear partials, a partial denture grips natural teeth with cast metal clasps. Sometimes, these teeth break or develop tooth decay before the partial needs to be remade. However, it is extremely difficult to sculpt restorations accurately so that the partial fits as intimately and accurately as it did when it was first made.
Recreating proper supporting tooth contours with direct fillings like amalgam or resin is next to impossible to do predictably. Usually, the technique involves removing the decay and old filling materials and then placing a malleable filling in the cavity. Incidentally, this is the same process as is done to treat almost any filling. The problem is that usually a band is used to control the filling material while it sets up. In the retrofit procedure, the partial denture itself serves as the mold for the filling. Essentially, the soft filling material is first placed in the tooth and then the partial is quickly seated to place before the filling sets up. Unfortunately, this technique may result in weak fillings because amalgam cannot be packed as tightly as when it is packed against a tension-held band, and the light used to set up resin fillings is prevented from reaching all aspects of the resin fillings. The result is usually a restoration that provides little functional support to the partial and that may create food traps.
The problems associated with retrofitting fillings often justify the decision to retrofit with a crown, which is sculpted to fit the existing partial in a dental laboratory and out of the mouth. Although many accepted techniques are utilized for providing the necessary information to the lab, probably the most common involves taking an impression with the partial in place after the tooth is prepared for the crown. This is called a pick-up impression. The advantage of this type of impression is that a skilled lab technician can sculpt the crown to intimately fit the partial. The disadvantage is that the patient may have to be without his/her partial denture for about two weeks until the crown is finished and cemented.