Anterior complete crowns for vital teeth
In the anterior part of the mouth appearance is of overriding importance, and so the only types of crown to be considered are those with a tooth-coloured labial or buccal surface. These fall into three groups:
• Ceramic crowns
• Metal–ceramic crowns
• Other types of crowns.
Ceramic crowns
In recent years there have been several developments in the ceramics used for crowns. These can be classified as:
• Traditional fused porcelain jacket crowns(PJCs)
• Pressed ceramic crowns
• Milled ceramic crowns
• Cast crowns
• Reinforced crowns
• Ceramic resin bonded crowns.
Traditional porcelain jacket crowns (PJCs)
This is the oldest type of tooth-coloured crown and has been in use for more than a century. It consists of a more or less even layer of porcelain, usually between 1 and 2 mm thick, covering the entire tooth. Following Figures show a selection of traditional feldspathic porcelain jacket crowns in place.
The traditional feldspathic PJC is made by adapting a very thin platinum foil to a die made from an impression of the prepared tooth. Porcelain powder, mixed with water or a special fluid, is built onto the platinum foil and fired in the furnace. All PJCs made in this way are now strengthened by having alumina incorporated into the porcelain powder. A core of high-alumina porcelain is fired onto the platinum foil. This high-alumina core is opaque and needs to be covered by more translucent porcelain that contains less alumina.
Variations on fused ceramic crowns have been developed but most have now been replaced by the systems described later. Examples of these strengthened ceramic systems were Hi-Ceram and In-Ceram.
Strengthened porcelain crowns (Hi-Ceram) on both central incisor teeth
The upper left lateral incisor tooth has been
moved orthodontically into the position of the central incisor and crowned with an infiltrated, reinforced system (Inceram) to resemble the missing central incisor
An alternative approach is to fire an extra-strong core of ceramic material to a refractory die and then add further layers of conventional feldspathic porcelain. Once finished, the refractory die is grit-blasted away, leaving a fitting surface that is slightly rough, aiding retention. Both these systems can also be used to make porcelain veneers.
Conventional dental porcelain is physically more like glass than the porcelain used for domestic purposes. It is relatively brittle, and before a PJC is cemented it can be broken fairly easily. However, once it is cemented and supported by the dentine of the tooth, the force required to fracture it is of the same order of magnitude as the force required to fracture the enamel of a natural tooth.
However, traditional PJCs do fracture and so the other systems have all been developed to produce stronger all-ceramic crowns.
Pressed ceramic crowns
There are a number of systems used to produce a strong ceramic core by pressing a ceramic material at high temperature and pressure onto a die and then building up layers of traditional porcelain to create the right contour and shade. Examples of crowns made by this process are shown bellow.
Equipment for construction of an Empress crown
Pressed crowns on the upper and lower incisor teeth
in a patient with mild amelogenesis imperfecta. This can
be seen on the uncrowned teeth, particularly the lower
right first premolar. The system used was Empress
Laboratory milled cores and crowns
Several computer aided designs/computer aided manufacture (CAD-CAM) have been developed and more are in the pipeline. One example is a technique by which the die is scanned in a computer and the digital record is sent electronically to a central laboratory, often in another country, where a core is produced in a strong ceramic material. This is then returned to the original laboratory where additional porcelain is added to complete the crown. The system can also mill metal, in particular titanium (Procera). The computer equipment is shown in Figure 2.2d and crowns made by this process are shown in Figure 2.2e and f.
The scanner used in the process of making a Procera
crown
A Procera core of dense aluminous porcelain ready.The porcelain will be fired directly onto this.
Procera crowns restoring three incisor teeth. Note how the crowns disguise the gold post and cores so the appearance is the same as the crown on the
natural tooth
Zirconia cores
Porcelain crowns can also be manufactured using complex laboratory techniques with a zirconia core onto which conventional porcelain is built. Zirconia is an extremely strong material comparable to metal and is dense white in appearance. The conventional porcelain therefore needs to be relatively thick for the translucent porcelain to mask the opaque core underneath. Zirconia-based crowns are proposed by the manufacturers for use on posterior as well as anterior teeth due to their great potential strength, but there is not yet sufficient evidence that they will survive for the long term in high stress situations.
Chair-side milled restorations
Chair-side milling systems have been in development for some years. Although these systems do not yet produce reliable crowns it is anticipated that they will do so in the foreseeable future.
Cast ceramic systems
One of the earliest developments in the 1970s was of a system to produce strong ceramic crowns by the Dicor process in which a wax pattern of the crown was made on a conventional die, invested and cast in a glass/ceramic material. The casting was then placed in a ceramming oven for several hours, during which it went through a crystallization conversion and became much stronger. At this stage the casting had a cloudy-clear appearance (similar to frosted glass). It was then stained and characterized using conventional feldspathic porcelains in a porcelain furnace. Although the commercially available Dicor system was developed by the same company that devel-oped domestic Pyrex glassware, the manufacturers state that the material is not the same as Pyrex. A number of other castable ceramic or hot transfer-moulded glass ceramic systems have been developed.
A cast ceramic crown (Dicor) on the upper right central incisor
Reinforced porcelains
One technique is to form an alumina substructure on a special plaster die and following sintering in a furnace the porous substructure is coated with glass fired powder and further fired for several hours. This infiltrates the pores and eliminates them as a source of weakness. Filling the pores also improves translucency and the final appearance when additional porcelain is added.
Thin ceramic resin bonded crowns
Peg-shaped lateral incisors.
The upper right lateral incisor prepared for a resin bonded ceramic crown. The preparation is entirely within enamel. The neck of the tooth has been prepared all round with a chamfer finishing line, similar to the preparation for a veneer, and a small amount has been removed from the incisal edge. Nothing has been removed from the labial or proximal surfaces other than blending them into the gingival finishing line.
Palatal view of the finished crowns.
The labial appearance.
All the previous crowns are usually cemented by conventional means. However, a conservative technique is to prepare the tooth within enamel exposing the minimum amount of dentine and then making thin ceramic crowns, no thicker than a porcelain veneer but covering the whole tooth. This is then bonded by etching the enamel surface, grit-blasting the fit surface of the crown and bonding with an adhesive resin material. These restorations are becoming more popular with some dentists, who believe that their prognosis may be better than ceramic veneers because they go right round the tooth, thereby reducing the effects of the difference in thermal expansion between porcelain and enamel, and also the risk of marginal leakage and staining is reduced.
