3 Portrait of George Washington on a one-dollar bill
It is clear to what extent the lower denture disfigured the face of the President of the United States of America. He looks as if he has a dumpling in his mouth.
4 George Washington's lower denture
The denture, fabricated in 1789, is carved from hippopotamus tusk and originally contained eight human teeth.
Courtesy of The Academy of Medicine, New York
Individual Tooth Replacements
Metal Crowns
Up until the 1960s, no dental ceramic system was available that could be generally accepted for individual tooth reconstructions. The capping of prepared abutments with gray or gold-colored metal crowns, which had begun after the turn of the last century, was a first step toward individual tooth restoration. Repeated attempts were thus made to cover the metal with a tooth-colored glaze similar to enamel.
Metal-Ceramics
Glazing was also explored for dental reconstructions by melting several layers of glaze on top of each other to cover the metal surface. It was believed that with this system the superior tensile strength of the metal could be combined with the advantages of nonmetal, inorganic materials, such as tooth-like color, hardness, chemical resistance, and bio-compatability.
After the successful production of a metal alloy with low melting point and increased hardness, the era of metal-ceramics began in the United States after World War II with the Permadent method (Weinstein, New York). In Europe, this method was not successful because of the high production and license costs (Claus 1980). Here, the first ceramics fused to metal alloy system became generally accepted during the early 1960s, as a result of a cooperation between the two corporations Degussa and Vita. The past 30 years have seen dramatic developments in metal-ceramics. The technique was applied to produce a lasting, aesthetic crown which could be used to restore teeth and to bridge gaps produced by missing teeth (Caesar and Hermann 1986; Caesar and Steger 1986).
5 Portraits by old masters
Our ancestors had themselves portrayed in dignified pose exhibiting a stern face. The lips always remained shut. One reason for this was that a session with the portraitist lasted many hours and it would have been too exhausting to keep smiling during this time; a further reason, however, was that the subject often had missing teeth!
6 Modern portraits
In contrast, cover pages of present-day magazines show beautiful people with smiling faces. In private we also usually smile at the camera. The reason for this is that, for the first time in history, both young and elderly people have teeth that can be displayed because both groups have no teeth missing.
Today, a large number of different metal-ceramic materials are available to the dental technician. The materials include metal-ceramics that melt at a relatively low temperature (800°C). These ceramics enable the development of further alloys with advantages for aesthetics and biocompatibility.
Since titanium has served as a framework material, dental ceramic masses to cover this metal have also been available. The advantages of titanium include its good biocompatibility and light weight.
Biocompatibility and Aesthetics
The advantages of dental ceramics as coatings of metal-ceramic frameworks prevail, but problems due to biocompatibility and aesthetics cannot always be avoided. The weak point in the system is the metal alloy. With more than 1000 metal alloys available, more and more complaints are being voiced about their bioincompatibility. Patients are becoming increasingly more critical of this problem (Gall 1983; Hermann 1985). An aesthetic disadvantage of metal-ceramic restorations is that they are not translucent because of the metal layer. In contrast to natural teeth, light cannot penetrate the metal-ceramic interface. Metal edges may be visible through the ceramic and gray areas may appear. There is an increasing desire for metal-free tooth replacements. The development of transparent dental ceramic shoulder masses enables aesthetic improvements in the neck area of the tooth.
Today's informed patients request improved biocompatibility and aesthetics. The prerequisites for an alternative nonmetallic, inorganic material are increased strength and hardness as well as optimized chemical stability and resistance to corrosion.
All Ceramic
The goal of research has always been to develop a suitable all-ceramic tooth substitute. This goal was never achieved because of the brittleness of the material. This is the “Achilles heel” of all nonmetallic, inorganic materials. In contrast to metals, ceramic materials are flexible and elastic, which means that their application has mainly been restricted to single crowns, inlays, and veneers.
It was the American Land who, in 1896, developed a procedure for fabricating the metal-free jacket crown. He fired the ceramic on shaped platinum foil. The shaped platinum cap was coated with the porcelain mass and then fired. Later, the porcelain masses were replaced with kaolin-free feldspar frit. In 1925, Brill improved the procedure, resulting in a breakthrough for the jacket crown in Germany (Krumbholz 1992; Strub 1992).
McLean and Huges achieved the crucial breakthrough in 1965. Further cooperation with McLean and the company Vita resulted in the development of the Vitadur (1968) and Vitadur N system (1976), which came to dominate the aesthetic treatment of front teeth.
7 VMK-68 crown
One of the central incisors was restored using a VMK-68 crown.
Courtesy of B. Scherer
8 Posterior crowns
Left: Section of an opaque metal-ceramic posterior crown.
Right: Section of a transparent all-ceramic posterior crown.
Additional All Ceramic Systems
The desire for more biocompatible and aesthetic materials, combined with the rise in the price of gold, necessitated the development of usable, all-ceramic systems. These began to appear during the early 1980s when European dentists enthusiastically adopted the Cerestore and Dicor systems developed in the United States. As a result, other systems, such as Hi-Ceram, Optec HSP, Mirage II, Empress, and In-Ceram, were developed (Strub 1992; Pöbster 1993). Most systems use completely different processes. Layering, casting, infiltration, and press techniques are used as well as different glass ceramic systems (Bolten and Mönkmeyer 1987; Bolz 1987; Geller et al. 1987). Crystals or other stable particles were incorporated as strengthening units.
Unfortunately, because of the rather low fracture resistance of most systems, the failure rate was high, particularly in the posterior tooth regions. Systems such as Empress and In-Ceram became generally accepted. While Empress, inferior in strength, has mainly been used for inlays, onlays, veneers, and anterior crowns, In-Ceram, far superior in strength, has also been used successfully for premolars and molars and for smaller anterior bridges.
9 All-ceramic