EVOLUTION OF CERAMIC PROCESSING TECHNIQUES - A REVIEW

Author :

Dr. Shalini Joshi, Dr. Chandrashekar Sajjan,
Dr. Kalwa Pavankumar

ABSTRACT

Objectives: To review the history of processing techniques available for ceramic dental restorations, as dental ceramic is considered the most esthetic and biocompatible material available for dental restorations.

Discussion: Ceramic objects have been constructed for thousands of years. In the last few decades there has been a tremendous advance in the mechanical properties and methods of fabrication of these materials. Whilst porcelain based materials are still a major component of the market there have been moves to replace metal cored systems with all ceramic systems.

Conclusion: Although other ceramic products and processing methods will be introduced in future, the principles for selection and use of ceramics based on the properties and micro structural characteristics of these ceramics and fundamental concepts of prosthesis design will endure.

INTRODUCTION
One of the greatest assets a person can have is a smile that shows beautiful, natural teeth. Children and teenagers are especially sensitive about unattractive teeth equally not to exclude aged individuals. The creation of an esthetic restoration is a demanding task. Esthetic dentistry faces new challenges in adopting emerging technologies related to ceramics and in meeting patient’s demands for esthetic restoration.1 Dental ceramics are usually processed by sintering, but in the last few years, processing techniques used for high-technology ceramic materials have been applied to dental ceramics, leading to the development of glass ceramics, slip-cast ceramics, heat-pressed ceramics and machinable ceramics. Ceramic material because of biocompatibility, long term color stability, wear resistance, and their ability to be formed into precise shapes, although they require processing equipment and specialized training are fast becoming the primary restoration of choice for anterior teeth.

HISTORY OF PROCESSING TECHNIQUES

Archeologists have uncovered human-made ceramics that date back to at least 24,000 BC. These ceramics were found in Czechoslovakia and were in the form of animal and human figurines, slabs, and balls. These ceramics were made of animal fat and bone mixed with bone ash and a fine claylike material. After forming, the ceramics were fired at temperatures between 500-800°C in domed and horseshoe shaped kilns partially dug into the ground with loess walls. While it is not clear what these ceramics were used for, it is not thought to have been a utilitarian one. The first use of functional pottery vessels is thought to be in 9,000 BC. These vessels were most likely used to hold and store grain and other foods.

It is thought that ancient glass manufacture is closely related to pottery making, which flourished in Upper Egypt about 8,000 BC. While firing pottery, the presence of calcium oxide (CaO) containing sand combined with soda and the overheating of the pottery kiln may have resulted in a colored glaze on the ceramic pot. Experts believe that it was not until 1,500 BC that glass was produced independently of ceramics and fashioned into separate items. 2

In approximately 700BC the Etruscans made teeth of ivory and bone that were held in place by gold framework. Animal bone and ivory from hippopotamus or elephant were used for many years thereafter.  Animal teeth were unstable toward the “corrosive agents” in saliva, and elephant ivory and bone contained pores that easily stained. Hippopotamus ivory appears to have been more desirable than other esthetic dental substitutes. John Greenwood carved teeth from hippopotamus ivory for at least one of the four sets of complete dentures he fabricated for George Washington. 3

Mineral teeth or porcelain dentures greatly accelerated an end to the practice of transplanting freshly extracted human teeth and supplanted the use of animal products. Feldspathic dental porcelains were adapted from European triaxial Whiteware formulations (clay-quartz-feldspar), nearly coincident with their development. After decades of efforts, Europeans mastered the manufactured of fine translucent porcelains, comparable to porcelains of the Chinese, by the 1720’s. the use of feldspar, to replace lime (calcium oxide) as a flux, and high firing temperatures were both critical developments in fine European porcelain. Approximately 1774, a Parisian apothecary Alexis Duchateau, with assistance of a Parisian dentist Nicholas Dubois de Chemant, made  the first porcelain dentures at the Guerhard porcelain factory, replacing the stained and malodorous ivory prostheses of  Duchateau.4 In 1791 the first British patent was granted to Nicholas Dubois De Chemant, previously assistant to Duchateau, for De Chemant's Specification, "a composition for the purpose of making of artificial teeth either single double or in rows or in complete sets and also springs for " He began selling his wares in 1792 with most of his porcelain paste supplied by Wedgwood. However, this baked compound was not used to produce individual teeth because there was no effective way at that time to attach the teeth to a denture base. 

As early as the second half of the eighteenth century, Pierre Fauchard and others attempted to use porcelain in dentistry. Their efforts were largely unsuccessful. However, porcelain was successfully used for dental prostheses by the end of the 1800s. In 1808 Fonzi, an Italian dentist invented a "terrometallic" porcelain tooth that was held in place by a platinum pin or frame. Planteau, a French dentist, introduced porcelain teeth to the United States in 1817, and Peale, an artist, developed a baking process in Philadelphia for these teeth in 1822. Commercial production of these teeth began in 1825 by Stockton. In England, Ash developed an improved version of the platinum tooth in 1837. In 1844, the nephew of Stockton founded the S.S. White Company, and this led to further refinement of design and the mass production of porcelain denture teeth.

Dr. Charles Land patented the first Ceramic crowns in 1903. Land, who was grandfather of aviator Charles Lindbreg, described a technique for fabricating ceramic crowns using a platinium foil matrix and high-fusing feldspathic porcelain.

The metal-ceramic restoration first became available commercially during the later 1950s.  Porcelain compositions suitable for metal-ceramic restorations were introduced in 1962 by Weinstein and Weinstein and led to the success of this technology.  Among their two Patents, one described the formulations of feldspathic porcelain that allowed systematic control of the sintering temperature and thermal expansion coefficient.5 The other patent described the components that could be used to produce alloys that bonded chemically to and were thermally compatible with feldspathic porcelains.6 In 1963, Vita Zahnfabrik introduced the first commercial porcelain products which were known for their aesthetic properties.
The development of aluminous core and veneer porcelains was first described by McLean and Hughes in 1965.7 Aluminous core porcelain is a typical example of strengthening by dispersion of a crystalline phase (McLean and Kedge, 1987).8

Hi-Ceram (Vident, Baldwin Park, CA) is a technique, where Aluminous core porcelain is  baked directly onto a refractory die was introduced by McLean et alin 19949 and Optec HSP material was introduced by leneric / Pentron, Inc. is a feldspathic porcelain containing up to 45 vol% tetragonal leucite by Schmid et al10, Piche et al11  Denry & Rosenstiel12 where as Magnesia core ceramic was developed as an experimental material in 1985 by O'Brien13 then Mirage II (Myron International, Kansas City, KS)  a conventional feldspathic porcelain with tetragonal zirconia fibers was introduced for improved properties.14,15
Glass ceramics was first discovered by McCullock in 1968. Cast glass ceramic was first described by Grossman in 1970 (Nicor glass) and applied in a fixed prosthetic by Stookey in 1974.16 Dicor (Dentsply Inc., York, PA) is a mica-based machinable glass-ceramic. After being cast, the Dicor glass is converted into a glass-ceramic by means of a single-step heat treatment with a six-hour dwell at 1070°C.17 The machinability of Dicor glass-ceramic is made possible by the presence of a tetrasilicic fluormica (KMg25Si4O10F2) as the major crystalline phase by Grossman and Johnson in 1987.18

Cerapearl (Kyocera, San Diego, CA) is a castable glassceramic in which the main crystalline phase is oxyapatite, transformable into hydroxyapatite when exposed to moisture by Hobo and Iwata  1985.17  

Alceram (Innotek Dental Corp, Lakewood, CO) is a material for injection-molded technology and contains a magnesium spinel (MgAl2O4) as the major crystalline phase by McLean and Kedge in 1987 initially introduced as the "shrink-free" Cerestore system, which relied on the conversion of alumina and magnesium oxide to a magnesium aluminate spinel.19

Pressed glass ceramics was introduced almost at the same time as the casting method by Wohwend. This was also accepted, which in cooperation with the company “Ivoclar”, has been developed up to clinical application. This process relies on the application of external pressure at elevated temperatures to obtain sintering of the ceramic body. The hot-press furnaces EP500 and EP600 have been specially designed to press the IPS Empress ingots for the staining and layering technique as well as the IPS Empress 2 ingots for the layering technique, using the lost wax method. IPS Empress 2 (E2) has a lithium disilicate core (Li2Si2O5) was introduced in late 1990s.3, 16 

CURRENT PROCESSING TECHINQUES

Though idea of using CAD/CAM (Computer Aided Design / Computer Aided Manufacture)  techniques for the fabrication of tooth restorations originated with Duret in the 1970s. It first appeared in dental medicine in 1989 with the device CEREC (Ceramic REConstruction) for the fabrication of inlays, onlays and labial veneers during one appointment in the dental surgery. The schematic CEREC system functions in the following way: the oral video camera transfers the image onto the screen of the monitor where it is memorised and the edges of the veneer/inlay determined. The information is then processed by the computer and sent to the milling devise which fabricates the veneer/inlay from the ceramic block. The ceramic block is positioned on a metal stub which enables its fixation in the milling chamber. Cutting is carried out by diamond disks and drills, with simultaneous cooling by a water spray. While the ceramic block rotates on its axis the diamond disk and drill also rotate and translate up and down over the porcelain block, during which the block is cut. A series of approximately 200-400 cuts is necessary to cut a veneer/inlay from one ceramic block. The motion of the diamond disk is driven by an electric drive. An optical “impression” is used instead of the classical impression procedure, and the dentist’s own evaluation of the contour and size of the inlay, onlay or veneer. The first CAD/CAM apparatus for the fabrication of veneers and inlays was developed by Mormann and Brandestini in 1985, although earlier attempts had been made, the system experienced significant progress in 1986 when it was taken over by Siemens.20 

The Procera Crown Alumina was introduced in 1991. It is composed of densely sintered, high-purity aluminium oxide core combined with compatible AllCeram veneering porcelain and 1992 Duceram LFC (low fusing ceramic) was marketed as ultralow-fusing ceramic.3
ln-Ceram (Vident, Baldwin Park, CA) is a slip-cast aluminous porcelain. The alumina-based slip is applied to a gypsum refractory die designed to shrink during firing. The alumina content of the slip is more than 90%, with a particle size between 0.5 and 3.5 micrometers. After being fired for four hours at 1100°C, the porous alumina coping is shaped and infiltrated with a lanthanum-containing glass during a second firing at 1150°C for four hours. After removal of the excess glass, the restoration is veneered with matched expansion veneer porcelain [Probster and Diehl, 1992].21

The Celay system (Mikrona Technologie, Spreitenbach,Switzerland) uses a copy-milling technique to manufacture ceramic inlays or onlays from resin analogs. The Celay system is a mechanical device based on pantographic tracing of a resin inlay or onlay fabricated directly onto the prepared tooth or onto the master die was introduced by Eidenbenze et al., in 1994.22

In-Ceram pre-sintered slip-cast alumina blocks (Vident, Baldwin Park, CA) have been machined with the Celay copy-milling system used to generate copings for crowns and fixed partial dentures (McLaren and Sorensen, 1995).23

The CAD/CIM (Computer Aided Design / Computer Integrated Manufacturing) was first introduced in 1996 with advantage of the method being simple.20 The Cercon and Lava Zirconia All-Ceramic System comprise a CAD/CAM procedure for the fabrication of all ceramic Crowns and Bridges for anterior and posterior applications. Procedure for production of zirconia-based  prosthesis involves, scanning  the modelled wax object, digitisation of the shape, conversion of the data for milling a blank using complex software, initially with rough milling and then with fine milling, sintering of the framework in the heat sintering furnace, finally veneering.3

Lava Chairside Oral Scanner C.O.S. is a digital impression system that provides a powerful new connection and improved productivity for a doctor and their dental laboratory. The efficiency begins with the very first step: capturing a digital impression with the Lava Chairside Oral Scanner C.O.S. thus eliminating steps at both the dental office and the laboratory. Later dentist can decide to use a traditional process, like PFM, or sophisticated CAD/CAM processes, including Lava Restorations using Lava Digital Veneering System.24

DISCUSSION

Dr. Charles Land‘s crowns exhibited excellent aesthetics, but the low flexural strength of porcelain resulted in a high incidence of failures.3 Ceramco porcelain was developed with a coefficient of thermal expansion similar to that of dental casting alloys and this porcelain could be used safely with a wider variety of alloys. Since the alloys could form naturally integrated oxide coatings on their surfaces, the feldspathic porcelains formulated to veneer these frameworks could bond intimately with their surfaces. Their major drawback is the opacity and color of the metal substructure.  Aluminous core porcelain has relatively high fracture rate in posterior site; the principal indication for use of aluminous porcelain crowns is the restoration of maxillary anterior crowns when aesthetics is of paramount importance and when no other ceramic product is available.8 To produce substantial improvements in fracture toughness, strength, and thermal shock resistance tetragonal zirconia fibers was introduced conventional feldspathic porcelain.14, 15

The Glass ceramics is that the dental restorations can be cast by means of the lost-wax technique, thus increasing the homogeneity of the final product compared with conventional sintered feldspathic porcelains and one of the recognized advantages of Alceram material for injection-molded technology was the excellent marginal fit of the restoration.19 Pressed glass ceramics using the lost wax method permits the fabrication of dental restorations that demonstrate excellent accuracy of fit and aesthetic properties.  IPS Empress 1 (E1) - leucite (KA1Si2O6) strengthened glass-ceramics, share of crystals (1.7 μm in size) is around 35% of the volume provided strength and marginal adaptation similar to those of Dicor glass-ceramic, neither of these materials where indicated  for producing FPDs.  IPS Empress 2 (E2) has periphery of the main crystal phase (crystals of lithium phosphate). The total share of crystals is around 60% of the volume. Apatite glass ceramics is used for layering. This product could be used for 3-unit FPDs up to second premolar. Fracture toughness of IPS Empress 2 glass-ceramic (3.3 MPa.m1/2) is 2.5 times greater than that of  IPS Empress glass ceramic (1.3 MPa.m1/2).3,16

An advantage of CAD-CAM ceramics is that one can select a core ceramic either for strength and fracture resistance, for low abrasiveness, or for translucency3 but the main disadvantages of the CAD/CAM system are the high costs of the CEREC apparatus and mastering the technique. Materials suitable for the Cerec system -Vita CEREC Mark 1 (Vita Zahnfabrik, Bad Sackingen, Germany), DICOR MGC (Dicor, De Trey Dentsply Int., Wiesbaden, Germany) in the form of porcelain blocks which, according to their physical properties (hardness) most resemble enamel. Siemens improved the computation of the edges of the cavity surface and the marginal edge by introducing the COS 2.0 programme for the computer (CEREC Operating System 2.0 software, Siemens, Bensheim, Germany) 20 and Procera Crown Alumina has good prognosis even on posterior teeth and precision of fit.3

Duceram LFC are claimed as “self healing” through a process of forming a 1µm thick hydrothermal layer along the ceramic surface and the extremely small size (400-500nm)  enhances opalescence of ceramic also some  of low-fusing ceramics  are kinder to opposing tooth enamel3and ln-Ceram processing technique is unique in dentistry and leads to a high-strength material due to the presence of densely packed alumina particles and the reduction of porosity.21 In-Ceram Spinell contains a magnesium spinel (MgAl2O4) as the major crystalline phase with traces of alpha-alumina, which seems to improve the translucency of the final restoration. The second material contains tetragonal zirconia and alumina.23

In CAD/CIM (Computer Aided Design / Computer Integrated Manufacturing) process the laboratory is not required; there is no classical impression procedure (the method can be repeated as necessary), rapid fabrication (it is possible to fabricate several veneers during one appointment), acceptable cost (no laboratory costs, time saving), and the fabricated restoration is of the same or higher quality than the laboratory fabricated restoration.20
Lava Digital Veneering System has been designed to offer high esthetics, strong clinical performance and a system that is flexible, easy to use and improves productivity. Supported by full contour Lava Design Software and the Lava CNC 500 milling machine, this system will enable to produce esthetically pleasing, digitally precise porcelain work that is designed for Lava Zirconia copings.24

CONCLUSION

Dental ceramic technology is one of dental materials research and development. The future of dental ceramics is bright because the increased demand for tooth-colored restorations will lead to an increased demand for ceramic-based restorations.

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