|RESTORING LARGE TOOTH DEFECTS AND TOOTH REPLACEMENT
When a tooth is too badly broken down to be restored with a conventional restoration and only a thin shell of enamel remains, it may be necessary to protect the remaining tooth structure with a crown. Before preparing a tooth for a crown, the dentist first removes decay and then restores some or all of the lost tooth structure with amalgam or composite. This restoration that replaces defective and carious tooth structure prior to preparing a tooth for a crown may be called a restoration under crown (RUC) or crown buildup. When there is too little remaining tooth, particularly on an anterior tooth, a custom cast metal core (resembling a tooth prepared to receive a crown) with a metal post can be designed. The post provides the retention by fitting snugly into a previously endodontically treated and prepared root canal. This restoration is called a cast post and core (Fig. 10-44). Prefabricated posts are also available in metal, or esthetic materials (such as fiber posts), which can be cemented into the prepared canal and then be bonded with a composite crown buildup.
On posterior teeth, a crown can be constructed entirely of cast metal and is called a complete cast metal crown (Fig. 10-45B). To prepare a tooth for a complete crown, the previously restored anatomic tooth crown (or prepared crown buildup) is externally reduced with specially shaped burs in a dental drill to make room for the required thickness of the cast metal crown (Fig. 10-45A). Crowns obtain retention from the nearly parallel preparation walls that slightly converge toward the occlusal, an accurate fit, and the cement. The preparation ideally extends gingivally beyond the margins of the cast post and core or crown buildups so that the crown margins end on sound tooth structure. Full cast metal crown preparations end at the gingival cavosurface with a rounded shape called a chamfer (Figs. 10-45A and 10-46A).
When esthetics is important, especially on anterior teeth and maxillary premolars, one option is a metal crown covered facially with porcelain. The preparation for this type of crown requires more reduction of facial tooth structure to make room not only for the thin cast metal but also for an additional thickness of a tooth-colored porcelain veneer, which is fused onto the facial surface of the metal (Fig. 10-46B). This restoration is called a metal ceramic restoration (also called a porcelain fused to metal crown) and is seen on tooth #30 in Figure 10-47. Another esthetic solution for a full coverage restoration is an all-ceramic restoration. Since there is no internal metal support under the porcelain, the resultant increased translucency more closely resembles a natural tooth (seen on the maxillary incisor teeth in Fig. 10-48). Zirconia is a newer esthetic restorative material that is ideal for crowns since it is translucent and biocompatible, less susceptible to fracture than porcelain, and requires less tooth reduction than porcelain.25
Even when little or no caries or breakdown is evident, a crown may be recommended if the tooth is cracked or when needed to support an adjacent false tooth that replaces a missing tooth. The replacement tooth crowns can be attached to other crowns that cover the adjacent supporting teeth, and this is called a fixed dental prosthesis (also called a fixed partial denture [FPD] or a bridge by many people) (Fig. 10-49). The false tooth is called a pontic, and the teeth that support the pontic are called the abutment teeth. Abutment teeth are covered by crowns called retainers that are attached to the pontic. In Figure 10-49B, a fixed dental prosthesis is used to replace tooth #4 by attaching it to an abutment metal ceramic crown on tooth #5 and a complete cast metal crown on tooth #3. The metal pontic replacing #4 is veneered with porcelain. A fixed dental prosthesis is seen before cementation in Figure 10-49C.
Lost teeth can also be replaced with dental implants (Fig. 10-50). A dental implant involves embedding an artificial root into the bone. Three to six months after surgical placement, the embedded implant can be used to provide retention for a crown or provide retention as an abutment for a screw-retained fixed dental prosthesis or to provide support for a removable partial denture. See Chapter 7, Section X, for more discussion about dental implants.
Groups of lost teeth can also be replaced with multiple implants, a longer fixed dental prosthesis, or a removable dental prosthesis, which is also called a removable partial denture. One type of removable dental prosthesis is made with an acrylic saddle that adapts comfortably over the edentulous area, artificial replacement tooth crowns on the acrylic saddle, and a framework (usually metal) with parts that adapts around adjacent teeth to provide stability and retention (Fig. 10-51A). The part of the framework that connects the left and right sides of the prosthesis is called a major connector. The framework has clasps, which surround and contact abutment teeth just cervical to the height of contour facially or lingually to provide retention, and rests that fit into small depressions (rest seats) prepared mostly into occlusal enamel in order to keep the partial denture from seating too firmly against the mucosa.
Without looking at the key for each photograph of restorations in this chapter, identify the material used, the surfaces involved, and the abbreviation that could be used to denote the restoration. Do the same with clean extracted teeth that have existing restorations. Look into your mouth using an excellent light source, and identify the classification of any existing restorations (according to Dr. G.V. Black). Repeat this when looking into the mouth of a friend as they retract their lips and cheeks. Do you suspect any areas of decay? If so, check with a dentist.