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Evolution of Artificial Tooth Replacements From an Aesthetic Point of View

Even though people have been engaged in replacing missing teeth since a relatively early stage in history, the use of artificial teeth to make people more beautiful and for chewing food was for a long time a neglected and unsuccessful undertaking. We know from Etruscan grave findings in the region of modern-day Tuscany that tooth replacements were worn as early as 700 BC. The Greeks and Phoenicians fastened loose and artificial teeth to neighboring teeth by means of gold wire (Woodforde 1869).

However, it was only in the 20th century that the development of artificial tooth replacement reached such a stage of perfection that permitted the user to openly laugh and to chew food without any problems. Approximately 100 years ago, artificial teeth were still so unreliable that they generally had to be taken out before meals. Unlike today, it was considered inappropriate in those days to talk about dental problems.

1 Ladies with fans

In his painting Ladies with fans Edgar Degas(1834–1917) illustrated the typical gestures of noble ladies, with which they tried elegantly to conceal their dental problems. Missing teeth were freguently hidden behind a fan, not only when smiling. Furthermore, a lady never ate in company, resulting in a saying that she lived only on love and air. In reality, the reason was missing teeth.

The Long Road to Individual, Functional Tooth Replacements

Today we can barely imagine the pain our ancestors must have suffered or how suffering from toothache must have influenced decision-making by leading historical figures. In those days, teeth were lost at a young age. Today, attempts are made to restore damaged teeth for as long as possible, while in the past dental treatment consisted of pulling the damaged, painful tooth. Since very few academically trained dentists were available, barbers (hairdressers) carried out tooth extractions as a side line. The average citizen was not aware of tooth replacements. However, certain possibilities for substituting teeth were available to the prosperous. The aesthetic value of tooth replacements was more important than was the value of improving the ability to bite and chew food.

Up until the mid-18th century, the material used for artificial teeth and the base plate to which they were attached consisted of cattle bone, horse teeth, and walrus teeth, or ivory. Extracted human teeth were used in the fabrication of expensive artificial dentures. The teeth were cut off at the neck and fixed in prepared holes on the base plate (Wood-forde 1869). Such teeth often came from poor people, who sold their own healthy teeth for money, or from dead people found on battlefields, in graveyards, or at execution sites.

2 Tooth extractors in history

Left: Wilhelm Busch's (1832–1908) idea of a tooth being extracted.

Right: This painting by Francisco de Goya (1746–1828) shows a young woman pulling teeth from a hanged delinquent in order to sell them for money, a procedure then considered as the customary practice to replace extracted teeth.

These dentures did not always improve the face aesthetically, as is best illustrated in the case of George Washington's portrait on the one-dollar bill. His face is clearly disfigured by the lower denture made of hippopotamus bone, onto which eight human teeth were attached, making him appear to have a dumpling in his mouth. However, this aesthetic disadvantage had to be accepted at the time because a toothless mouth would have appeared even more disfiguring. Washington's denture was not suitable for biting and chewing.

Fortunately, over the course of time, advances were also made in artificial tooth replacement. After porcelain was invented in 1710 by Böttgers in Saxony, the material was offered to the manufacturers of artificial tooth replacement. Porcelain was white, resistant to wear, and, in an unsintered state, could easily be molded into teeth.

In 1774, the two Frenchmen Duchateau and de Chemant were the first to use ceramic masses to manufacture a tooth replacement. This first attempt heralded continuous developments in artificial teeth. Over the subsequent decades, the initial uniform blocks of teeth evolved into transparent, tooth-colored single teeth with a functional shape and equipped with retentive pins made of noble metal that had been fused with porcelain on the back of the tooth. Consequently, the porcelain tooth can be seen as the beginning of the development of ceramic dental reconstructions (Krumbholz 1992).

The industrial production of porcelain teeth began around 1900. In 1893, the Wienand tooth factory was built in Germany; this was followed by the Hoddes factory (Bad Nauheim) in 1900, the Hutschenreuther factory in 1921, and Dr. Hildebrandt's tooth factory in 1922. Hildebrandt developed the first porcelain tooth based on dentin enamel layering principles. He also set another milestone in successfully reinforcing the artificial tooth structure by building it around a hard kernel. This approach led to ceramic teeth attaining their functional ability. A significant aesthetic improvement occurred when Gatzka introduced vacuum firing in 1949 (Claus 1980). This approach meant that the pore volume of the teeth decreased from 5.0% to 0.5%, resulting in superior translucency.

Today, industrially produced nonmetallic, inorganic teeth are mineral or feldspathic teeth. Apart from the few chemicals added to the ceramic mass to influence expansion, fracture strength, transparency, and color, the most important raw material is feldspar. The most frequently used feldspars are potassium feldspar orthoclase (K₂O · Al₂O₃ · 6 SiO₂), sodium feldspar albite (Na₂O · Al₂O₃ · 6SiO₂), and nepheline syenite (K₂O · Na₂O · 4Al₂O₃ · SiO₂). These crystalline feldspars are mixed with another raw material, crystalline quartz (SiO₂), and made into a frit by melting the mixture. This destroys the crystal structure and a largely amorphous, glassy material (Claus 1985; Claus 1990) is obtained.

Since the beginning of World War II, artificial teeth have also been produced from plastics. The original qualitative deficiencies have been overcome, and today they have replaced porcelain teeth because they are lighter in weight.

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 systems for crowns and partly for bridges

(Products marked with^* are, to our knowledge, no longer available on the market).

Conclusion and Outlook

Functioning artificial tooth constructions that are also aesthetically acceptable are nowadays taken for granted. In the past 30 years, a revolution has taken place in artificial tooth reconstruction, the end of which is not yet foreseeable. Dentists are increasingly interested in all-ceramic systems with improved aesthetics and biocompatibility; inlay, onlay, and veneer processing methods with all-ceramic systems such as Cerec and Celay are increasingly being taken into consideration.

In the future, new methods will evolve and it is also to be expected that crowns and bridges will be computer-processed or mechanically produced. Indications for using all-ceramic will continue to increase. It will be used in the front tooth area with even better aesthetic results. McLean (1993) and Lauer (1996), for example, have clearly shown that it is possible to improve the strength of all-ceramic systems by observing basic laws of structure and design.

It is to be expected, however, that in the future metal-ceramics will continue to cover a large part of the prosthetic field, since all-ceramic reconstructions will be overtaxed due to clinical realities. For this reason, it makes sense to continue the development of metal-ceramic systems with improved biocompatibility and aesthetics, and based on gold-colored alloys.