by Dr M K Vasant MBE MGDS RCS (Eng), MGDS RCS (Edin), FFGDP (UK), FDS RCS (Edin) & Dinesh Jani BDS, LDS RCS, FDS RCPS, RDT Porcelain is often the material of choice for quality restorations. Manny Vasant and Dinesh Jani evaluate a number of porcelain systems available to dentists

Dentists today have a variety of porcelain systems to choose from. Many more systems are being introduced and the discerning practitioner should be able to make an informed choice in conjunction with his/her dental technician.

BACKGROUND

Porcelain, a specific type of ceramic is essentially made from white clay, quartz and feldspar. The ingredients are pulverised, blended, formed into shape and baked. This material is essentially the same as 'whiteware' used in industry for construction of tiles, sanitary ware etc. Historically Pierre Fauchard was credited in recognising the potential of porcelain enamels and initiating research in porcelains to imitate the colour of teeth and gingivae (Kelly J 1996).

In the 1950's with the addition of leucite (crystals of a potash-alumina-silica complex) increased the strength by limiting crack propagation. The increase in coefficient of thermal expansion also enabled the fusion to certain gold alloys to form complete crowns and metal ceramic bridges.

The advent of vacuum-fired porcelains in 1960's led to the improvements in aesthetics due to the reduction in internal porosity. Much of the research and developments for the next 20 years or so were directed to improving strength and marginal integrity.

IMPROVING STRENGTHS OF PORCELAIN RESTORATIONS:
ALUMINOUS PORCELAINS

Great improvements in strength followed the introduction of alumina-reinforced feldspathic porcelain. These were called Aluminous Porcelains. The first work on pre-fitting alumina-glass composites was described by John Maclean. However whilst the addition of aluminium oxide increased the strength, the presence of a second phase reduced the translucency of the feldspathic porcelains. This therefore restricts the use of the alumina-reinforcement for the construction of the internal core to support the more translucent feldspathic porcelain.

1. Bonding porcelain to metal substructure
The most commonly used restorations in clinical practice is feldspathic porcelain bonded onto a metal framework. The mechanism of bonding is facilitated by matched thermal expansions of the two materials, Van der Waal forces and the chemical union between various metalic oxides. Despite the advent of other systems described herewith, the strength, longevity and performance of this material is undisputed.

2. Leucite Reinforcements (Figure 1) A new wave of ceramics has appeared since the 1980s attempting to further improve aesthetics and address one of the most important drawbacks of porcelains- the potential of a catastrophic failure. Sadly, the leucite increases the potential for wear of opposing teeth, another major drawback of porcelains. The proportion of the crystals varies with each system. To counteract the reduced opalescence with increased leucite content, blue pigments are added to the system.

Figure 1: Leucite crystals

3. Bonding to dentine

This is shown to increase the strength of the porcelains. Effectively, natural dentine itself acts as a core. The feldspathic porcelain crown is resin bonded to the natural dentine core. This enables the operator to prepare the tooth more conservatively (Figures 2 to 5). The results are aesthetically very pleasing. In effect, this is an extension of a porcelain veneer. In Vitro testing of natural teeth restored with dentine-bonded crowns has yielded fracture strengths similar of those of intact teeth (Burke et al, 1995). It is also worth noting that in a retrospective study by Isidor and Brondum (1995), a much higher failure rate was noted when light cured, rather than dual cured, cements were used.

Figure 2: Pre-operative  Figure 3: Prepared teeth

Figure 4: Feldspathic porcelain crown.

Figure 5: Finished restoration

4. Chemical surface treatments
Ionic exchange strengthening (ion stuffing) is a process that creates a thin surface layer of high compressive stress by exchange of smaller glass modifying ions with larger ones, for example replacing sodium with potassium ions. The larger ions enter the glass or porcelain by diffusional exchange at elevated temperature, from a molten salt bath. The larger ions produce atomic crowding and thus surface compression. Modification of the surface chemistry also causes a reduction in thermal contraction, hence resulting in surface compression.

Commercially available pastes Ceramicoat and Tufcoat have eliminated the use of hazardous salt baths and simplified the process (Piddock V et al, 1990).

5. Thermal treatments
Thermal tempering has also been studied for strengthening dental porcelains. These extend much deeper than chemical treatment. However, controlling cooling rates for a complex object such as crowns makes it impractical to use.

Despite the newer developments to improve strength, most of the porcelain systems rely on the use of conventional feldspathic porcelains to reproduce the outer surface for good aesthetics.

CHOICE AND PROPERTIES OF PORCELAIN SYSTEMS:
CONVENTIONAL POWDER SLURRIES

These include:

• Optec HSP
  Due to the increased leucite content they have higher strength(146 Mpa) compared to conventional feldspathic porcelains. As would be expected, in vitro studies have shown increased wear of the opposing tooth. A special semi-permeable die material is necessary for the processing. Beyond this no special equipment is necessary in the laboratory.
  
  
• Duceram Low Fusing Ceramic (LFC)
  This is a relatively a new porcelain referred to as 'hydrothermal low fusing ceramic'. It is composed of an amorphous glass containing hydroxyl ions. The manufacturer claims to have greater density, higher flexural strengths, greater fracture resistance and lower hardness than feldspathic porcelain (hence less abrasion). The inner core for the crowns is made from Duceram Metal Ceramic (MC) - a leucite containing porcelain placed on a refractory die and baked at 930 C. This core is then layered with Duceram LFC which is baked at 660 C. This can be surface characterised if required. It has been suggested that LFC does not etch well. Hence it cannot be used by itself for bonded restorations. For this application, a thin coping of porcelain must first be fired, over which the LFC is applied.
  
  There are no clinical studies substantiating these claims, but in theory, the material sounds promising. Furthermore, it has been suggested that the material is "self healing" as the potential cracks self-repair within the material. It is also claimed that the wear rate equals that of the natural tooth. Apparently, the polishing of the surface with rubber wheels (Brasseler polishing wheels), generates enough heat to "heal" the micro-cracks thus reducing the potential for crack propagation (Documentation Ducera Dental, 1993) .
  
  The reader must not confuse this with the third type of Duceragold, which is a ceramic layered onto a yellow gold alloy. Duceragold is a 2-phased hydrothermal bonding ceramic. The homogeneity of Ducergold lies between Ducera LFC and Ducera MC.
  
  
• Finesse (Dentsply UK)
  This is also a low fusing ceramic, which can be used with many high gold alloys as well. Its leucite content is below 10 % making it kinder to opposing tooth. It is now available in shades A0 and B0 to match bleached teeth. Presumably, as Ducera LFC, it does not etch very well and for bonding purposes it is recommended that a thin coping of conventional porcelain be used. Due to its high polishability at chairside, it eliminates the need for reglazing after adjustments.
  
  
• Fortress (Mirage Dental System)
  This leucite reinforced ceramic claims to have refractive index of surrounding glass matrix virtually identical to that of the leucite crystals. Apparently, this results in better aesthetics as it enhances the natural look of the finished restoration.
  

THERMAL SPRAY TECHNIQUE

Techceram, a British development, uses a thin (0.1-1mm) alumina core base produced by thermal spray technique, resulting in a density of 80-90%. The surface is then constructed using conventional technique with specifically developed porcelains. The inner fitting surface has microscopic roughness conducive to bonding and does not require silane coupling or etching.

CASTABLE CERAMICS

Dicor is a tetrasilicic micaglass ceramic. As the name suggests it is a product of a marriage between Dentsply Inertnational (DI) and Corningware New York (COR). The material is translucent and the abrasiveness is same as that of the tooth. The laboratory technique required is similar to that of producing gold crowns (Figures 6 and 7). The surface glaze and colourants are necessary to reproduce the colour. Unfortunately the wear of the glaze poses problems. Hence, it is now recommended that Dicor Plus- a feldspathic porcelain is layered on the surface after cutting back the original cast core. This sadly negates the alleged beneficial wear characteristics. Furthermore, although the flexural strengths yielded values of 240 Mpa compared to 116 Mpa for Alumina reinforced core porcelain, force required to break DICOR crowns is not significantly different to the latter (Dickinson et al, 1989). The core is etchable for bonding to the tooth.

Figure 6: Dicor wax pattern

Figure 7: Dicor casting

PRESSABLE CERAMICS

Empress (Ivoclar Vivadent) and Optec OPC utilises a technique where by a wax pattern of the proposed restoration is invested in a phosphate-bonded material. Following the burnout procedure, this leucite reinforced material is pressed into the mould using a special furnace (Figure 8). Final shading of the restoration is done with surface stains or by cutting back to apply a leucite reinforced porcelain in powder and slurry form, depending on the aesthetic requirements. The core material is shaded, translucent and etchable for bonding to the tooth. This produces a very aesthetic final restoration (Figure 9). Flexural strength of the material ranges from 126-165 Mpa. If this material is used for veneers or thin restorations, one would be restricted to surface staining only. It is expected that the marginal fit of these restorations will be superior due to the technique used.

Figure 8: Empress furnace

Figure 9: Empress crowns .

Figure 10: Empress 2-lithium disilicate core for a three-unit bridge

More recently IPS Empress 2 layering ceramic has been developed with flexural strength greater than 350 Mpa which is suitable for 3 unit bridges up to second premolar as the final retainer. This material is quite different to the original Empress. Empress 2 consists of the lithium disilicate glass ceramic as a framework material which is then coated with sintered glass ceramic (Figure 10). Their crystalline structure consists of only apatite crystals (fluoroapatite) unlike the conventional layering materials whose crystalline phase consists of leucite. The antagonist abrasion is claimed to be superior due to the apatite crystals which matches with that of enamel (Scientific Documentation IPS Empress 1998).

INFILTRATED CERAMICS

In-Ceram core is formed from slurries of fine alumina powder and water ('slip') which is applied to an absorbent refractory matrix, dried and lightly sintered to produce a porous core (Figure 11). The residual pores are then infused with molten glass by capillary action- hence the name In-Ceram (Figure 12). The resultant structure is the most efficient strengthening technique giving a flexural strength value of upto 450Mpa. Unfortunately the high degree of alumina content makes it difficult to etch with hydrofluoric acid for bonding to tooth structure. The restoration can be however be bonded with Panavia 21TC after sandblasting. It is also more opaque than natural tooth.

Figure 11: In-ceram slip .

The In-Ceram Spinel material substitutes magnesium aluminosilicate for the aluminium oxide for improved transluscency avoiding the typical yellow opacity of the In-Ceram. The yellow opacity of In-Ceram may hinder reproduction of grey shades (eg Vita C and D). The abrasiveness is comparable to the conventional feldspathic porcelains.

Figure 12: In-ceram with glass infusion

MACHINEABLE CERAMICS: CAD CAM

Computer assisted design and computer assisted manufacturing systems are available using various techniques.

The longest established system is the Cerec System (Siemens) and uses milling of a ceramic block from a digitised optical scan. Cerec have now introduced finer grained porcelain blocks to reduce opposing tooth wear and a wider range of tooth shades. They have also converted to electric turbine for better cutting control. Despite these improvements, Siervo et al (1994) have shown need for two to three times greater depth at approximal margins and occlusal interfaces compared to Celay or sintered inlays. The inlay can be characterised with surface stains.

1. Celay (Mikrona Switzerland)
The original system used a pre-formed porcelain block similar to above system except that the diamond cutting wheel is steered by a pantographic arm with an attached probe which is guided by a pre-formed acrylic or wax pattern. The technique is similar to the established key cutting principle in industry.

A modification of the technique is to use a In-Ceram block manufactured for the purpose to produce a porous structure. The latter is then infused with glass akin to In-Ceram restorations discussed above.

2. Procera (Nobel Biocare): Procera Allceram
These are all ceramic individual restorations, which comprise of a densely sintered alumina core. A die is first created by a computer controlled milling process at the laboratory in Sweden from an impression scanned at a local laboratory with this facility and sent to Sweden via a modem . Aluminium oxide is compacted onto the die to form the inner surface. The outer coping is then milled before the sintering is carried out. Build up with feldspathic porcelain is carried out to the completed anatomic form and final appearance at any local laboratory.

Figure 13: Preparations for Procera crowns

Due to the translucency of the core, the final restoration is more aesthetic (Figures 13 and 14). The marginal fit is clinically acceptable. Studies have shown that the strength is twice that of In-Ceram and five times that of Empress.

Figure 14: Finished Procera crowns .

Figure 15: Procera core three-unit bridge

However, other studies have conflicted these results. This material is not indicated for inlays and veneers. More recently its use has been advocated for the construction of three unit all ceramic bridges (Figure 15). The two systems discussed below are metal cores with porcelain veneers. Brief details of the metal cores are given as they are different to conventional cast metal cores.

3. Procera Alltitan
The external contours of the individual titanium cores for Procera bridges are milled and graphite rods create the fitting surface by the spark erosion process. Individual components of the bridge are then welded by laser before the addition of special porcelains to layer the surface to the full contour. The deficiency of the system appears to be with processing titanium at elevated temperatures. The use of low fusing ceramic alleviates this problem to a certain extent (Walter, 1994).

CAPTEK SYSTEM (SCHOTTLANDER)

The technique is an alternative to metal ceramic crowns. It involves adaptation of a wax strip impregnated with gold-platinum-palladium powdered alloy, to a refractory die. Firing produces a rigid porous layer, which then is in-filled with gold from a second wax strip by capillary action. This is then layered with porcelain. The proponents claim improved marginal fit, better aesthetics and improved biocompatability compared to ceramo-metal crowns. The improved biocompatability may be due to relatively non-oxidising alloy that is used.

SUMMARY

Many stronger systems have developed over the years. Aesthetically pleasing restorations are now taken for granted. With the advent of LFCs, the problem of antagonistic wear has been largely tackled. However, long term clinical results are awaited to confirm the in vitro results. Further improvements in this context will follow.

Although there is very little reason to use ceramo-metal crowns for single crowns, multiple units are still not totally predictable in these materials. Research in this field continues.

APPLICATIONS

In choosing the material, the clinician should choose the most appropriate material. Table 1 offers some guidance.

TABLE 1

REFERENCES

Burke F et al (1994). Fracture resistance of teeth restored with dentine bonded crowns. Quint Int J 25: 335-340

Documentation Duceram Dental GmbH (1993). Rodheimer Strasse 7. 611191 Rosbach

Kelly R (1996). Ceramics in dentistry: historical roots and current perspectives. J Pros Dent 75: 18-32

McLean JW, Hughes TH (1965). The reinforcement of dental porcelain with ceramic oxides. Br Dent J 119: 251-267

Piddock V, and Qualtrough (1990). Dental Ceramics - an update. J Dent 18(5): 227-235

Rosenblum M (1997). A Review of all-ceramic restorations. J Am Dent Assoc 128(3): 297-307

Scientific Documentation (1998). IPS Empress Meridian South Liecester LE3 2WY

Walter M et al (1994). Clinical performance of machined titanium restorations. J Dent 22: 346-348

FURTHER RECOMMENDED READING

Jones DW (1998). Developments in dental ceramics, J of Can Dental Assoc 64: 648-650

Qualtrough A, and Piddock V (1999). Recent advances in ceramic materials and systems for dental restorations. Dental Update 26(2): 65-70