Fabrication

Traditionally, the impression was cast and a die of the preparation poured using die-stone. A platinum foil matrix was then swaged over the die and an aqueous slurry of aluminous porcelain powder placed over the foil to form the coping. The foil supports the aluminous porcelain slurry in the furnace and may be removed once the crown has been fired and completed. The space left by the platinum foil effectively acts as a die spacer for the luting cement. In order to reduce porosity and shrinkage, firing takes place in a porcelain furnace under vacuum. The process in which the ceramic particles are fused together under heat in this way is called sintering. The aluminous coping is now ready for veneering with feldspathic porcelain which in turn is sintered.

Instead of a stone die model, refractory material may be used which maintains its dimensional stability when subjected to the heat of the porcelain furnace (see Chapter 12 on ceramic veneers). It has been argued that the use of a refractory die results in a more accurate fit of the final restoration to the prepared tooth.

The tooth preparation for the PJC has become a template for modern all-ceramic crowns with one notable exception: the margin has now become a large chamfer or a rounded shoulder. Furthermore, some of the latest all-ceramic crowns now have sufficient strength with an axial tooth reduction of as little as 0.6 mm.

The PJC remained a very popular and widely used restoration for many years. However, there was concern over the bulk of tooth tissue reduction and the inability to withstand occlusal loading. To address these concerns, research concentrated on:

There is now scope to use all-ceramic crowns on any tooth and within increasingly challenging occlusal environments.

The dentine-bonded crown

With the advent of adhesively retained porcelain laminate veneers in the 1980s and 1990s, it was a natural progression to extend the preparation to cover the whole surface of the crown, and thus the dentine-bonded crown concept was developed. The strength of this restoration is developed once it is bonded, using a composite resin luting cement, to the underlying tooth structure or composite core. Prior to bonding, the dentine-bonded crown is very fragile and should be treated in the same way as a porcelain laminate veneer.

The use of dentine-bonded crowns is mainly for anterior teeth where occlusal loading is relatively low (Figure 11.4). Such restorations are contraindicated for those patients with an obvious bruxing habit. Application of this type of restoration for premolar and molar teeth, which are normally subject to higher occlusal loading, should be made with caution and after careful examination of the patient’s occlusion. Generally, such applications should be avoided. The patient seen in Figure 11.5 has four posterior dentine-bonded crowns chosen for optimum aesthetics as the patient was young. No signs or symptoms of bruxism were noted and canine guidance was achieved; however, despite this, the lower crown fractured and had to be replaced with a metal–ceramic crown.

Figure 11.4 Dentine-bonded crowns made from feldspathic porcelain on all six upper anterior teeth.

Figure 11.5 Dentine-bonded crowns made from feldspathic porcelain on the upper right premolar and first molar and lower right first molar teeth.

Tooth preparation for dentine-bonded crowns is kept as minimal as possible and less than that required for metal–ceramic crowns or a traditional PJC. The preparation, in some instances, can be confined to enamel. However, the ceramic should be sufficiently thick to mask discoloured teeth prior to cementation. A minimal shoulder or, more often, a minimal chamfer is the restoration margin of choice. All margins should be supragingival wherever possible to avoid the problems of moisture control at cementation. The axial reduction is in the order of 0.5 mm while the occlusal reduction is between 1.0 and 1.5 mm, with at least 1.0 mm reduction in all excursive movements associated with the preparation (Figure 11.6).

Figure 11.6 Schematic diagram of a dentine-bonded crown: preparation and anatomy.

In common with other all-ceramic crown preparations, line and point angles should be rounded to avoid stress concentrations within the porcelain. Figures 11.7–11.9 show the tooth preparation that was carried out for the patients seen in Figures 11.4 and 11.5. Marked palatal erosion (Figure 11.7) has led to exposure of the tertiary (reactionary) dentine that has formed. The only preparation carried out palatally is the cervical chamfer; the tooth wear has removed the rest of the palatal tooth tissue(Figure 11.7). Interocclusal clearance has been created following anincrease in the patient’s occlusal vertical dimension by placing gold onlays on some of the posterior teeth (Figure 11.8).

Figure 11.7 Occlusal view of dentine-bonded crown preparations. The majority of palatal tooth removal has been a result of erosion. The completed result is show/>

Only gold members can continue reading. Log In or Register to continue