Full Crowns: Are They Still Necessary?
Most of the time, the indication for a full non-metal crown will be replacing an existing full crown (Figure 11.1a–e). On rare occasions, caries literally envelopes every surface of the tooth, and a full coverage crown is indicated. Even in such cases, however, the additional removal of healthy tooth to create the axial wall and any other form of mechanical retention is unnecessary. Crowns may also be used as abutments for a fix prosthesis, although adhesive techniques and improved materials offer new options, such as bonded bridges (Figures 11.2a–e, 11.3a–d). Full crowns are used as a final restoration covering the abutments of an implant, and even in these cases supragingival margins are desirable and make it easier to remove cement on cemented crowns (Figures 11.4a–d, 11.5a,b). Regardless of the indication, thanks to better restorative materials, full crowns today should be very different from those used in the past. More translucent restorative materials for crowns allow for a more supragingival margin placement whenever possible. The choice of restorative material for a full crown is very important. The correct choice of restorative material for the specific needs of full crowns will dictate the amount of tooth reduction required, margin placement and choice of cement to be used.
There are several choices for restorative materials specifically for full crowns, each with different characteristics, and it would be a mistake to use one for all cases. The choice of the restorative material depends on the condition of the tooth after the removal of the old crown, the margin location, the cement to be used, and where the tooth is located in the mouth (anterior or posterior area). Understanding that anterior and posterior teeth have very different thresholds for esthetic requirements, as well as occlusal loads and forces, means that material choices will vary based on those requirements.
The indications and space requirements for each material differ, with specific consideration for full crowns. This chapter considers the current most popular restorative materials for crowns, discussed from the weakest and most translucent to the strongest and least translucent . An important additional consideration is that the thickness of the material alters its level of translucency , so its blending ability will contribute to its strength as well as the thickness of the material .
Leucite-reinforced feldspathic porcelain, layered or pressed, is an option in the anterior region when margins are supra-gingival and there is a good amount of remaining enamel for adhesive luting, the adjacent teeth are being treated with feldspathic veneers and the esthetic demands are great (Figure 11.6a–d). Using leucite-reinforced feldspathic porcelain for full crowns in the posterior area is questionable, although it has been popular with computer-aided designed and manufactured (CAD-CAM) crowns. Because of its intrinsic weakness, this material depend on adhesion for its strength and a strong enamel substrate is very desirable. Subgingival margins are undesirable because they complicate adhesion. Generally speaking, this material is better used for bonded partial coverage veneers and, in rare situations, onlays [4,5]. The mode of failure of crowns made from this material is fracture of the restorative material, and for this reason they are not a primary choice (Figure 11.7a,b) [6,7].
Lithium disilicate is unique because it can be used in two different modalities. First, bonded lithium disilicate may be used when margins are supragingival, to allow for predictable adhesive luting, because lithium disilicate is much stronger when bonded to the tooth, space requirements are less, 1 mm axial, 1.5–2 mm occlusal (Figures 11.8, 11.9). Lithium disilicate (e-Max, Ivoclar) is much stronger when bonded than when cemented. When it is bonded to mostly rigid tooth structures such as enamel, its strength can be close to that of zirconia, even when it is thinly applied, but when bonded to a less rigid structure like dentin, it is considerably less strong . Lithium disilicate can be pressed or milled in a CAD-CAM machine. For many reasons, this material has quickly become the most ideal for bonded onlays [8,9]. The pressed version of e-Max appears to be a little stronger, with a better fit . It comes in high-translucency and low-translucency versions. High translucency is preferable for most cases of partial coverage and supragingival margins where the cavosurface margin is considerably coronal to the gingival line and good blending is very important. Low translucency may be preferred if the color of the tooth needs to be hidden, but the disadvantage of opacious materials is that the margin line of the tooth and restoration will be very visible and will often need to be hidden, subgingivally. E-Max can also be layered, but its strength decreases considerably .
Second, cemented (not-bonded) lithium disilicate can be used when margins are subgingival and isolation will be difficult, requiring the use of more contamination resistant traditional crown and bridge cement, such as RMGI Meron Plus AC (VOCO; Figures 11.10a,b, 11.11).