Overdentures are dentures which derive their support from one or abutment teeth by completely enveloping them beneath the fitting surface. The abutment teeth are usually root filled and either simply reduced or used in conjunction with stud or bar precision attachments. For both of these situations two techniques of impression taking are used. When the teeth are already root filled and will later be reduced it is termed an immediate overdenture. An impression of the root filled teeth, prior to reduction, is taken in alginate or elastomer in a special tray. The impression is cast and the abutment teeth on the model are reduced in height by 4mm and the dentures are constructed around this. When the processed denture is fitted, it is relined with cold cured acrylic in the areas around the abutment teeth to make it fit as well as possible.
If precision attachments are used the procedure is different. A special tray is used with either impression paste or elastomers depending on the presence of undercuts. The tray has a window over each of the abutments, this ensures any excess material flows out, without displacing any of the tissues. An impression of the abutments is taken, being careful to record postholes. If a stud attachment is used, a plastic stud is placed on the abutment teeth corresponding to the positions where the final attachments will be placed. The impression is taken and poured giving a fairly accurate impression of the ridge with the studs in their intended position. The fitting surface and rest of the denture are carefully constructed around the plastic studs on the model. If a bar attachment is used plastic clips, into which the bar will slot, are placed in the abutment teeth at the correct alignment. An impression is then taken and cast. The bar and rest of denture are then constructed on the model.
