The excellent nature of teeth comes from the microstructure of the material they are made of. They are not homogenous like cement used in construction work. It is composed of various different layers such as enamel, dentine, and cementum which are formed by enamel prisms and dentine tubules. This structure of the teeth is the source of inspiration for the produced restorations. Since an exact imitation is aimed, in addition to other properties, the microstructure of the teeth also needs to be imitated.
When the general structure of the teeth is investigated, it is observed that there are two major components: organic and inorganic structures. The filling material, referred to as composite, must be in the color of the tooth and must contain the same type of structure. The filling material in the composite increases the resistance of the restoration and has the same function of the mineral hydroxyapatite in the enamel. In the composite, resin matrix takes the place of the organic matrix in the enamel. Up to this point, everything seems fine but the images of scanning electron microscope (SEM) showing the cross sections portray the trivial difference between the composite and the natural tooth. The homogenous, ordinary appearance of the filling and the astonishingly embellished tooth are very easy to differentiate.
There are prerequisites that make tooth restoration successful. We can summarize these as esthetics, functionality, and phonation. These properties are affected with the loss of the tooth or the structure of the tooth. What follows is an analysis as to how and to what extent these properties can be replaced.
Esthetics in the dentistry means exact imitation of the tooth color. To be able to give a natural color to the filling or to the crown could be considered a miracle. It might sound like a very simple issue. However, this is probably the most challenging thing for dentists. If the color does not match, your restoration will look different and will easily be noticeable. Maybe it is my professional curiosity because while watching television I have a tendency to look at people's teeth before I look anywhere else. From the most popular singers and actors to mighty and scholarly politicians, I can always recognize their prosthesis. Ironically, these people were spending their wealth on professional dentists who could not give their original teeth back to them. The color is not something that could be mixed up together by fulfilling only one of its features. The color comes into existence as a result of common features of many factors. A dentist’s only hope is one day using homogeneous clay typed structures squeezed from a tube.
The structure of the enamel and the dentine are deeply related with opacity and translucency, two important subjects of optics. That is why some parts of the tooth are translucent while the other parts are opaque. It is possible to restore it with a few optical tricks. The tooth is neither completely opaque nor translucent. For example, while the cutting edges of the tooth are translucent (especially the cutting edges of newly erupted milk teeth, they are almost glassy), the cervix of tooth, adjacent to gingiva, is opaque and dusky. Thus, in big restorations, we implant materials that reflect the light differently for every surface of the tooth. We try to compare and contrast to the original teeth by using optical tricks. However, a more important thing is the sustainability of this quality. Especially, for restorations made to anterior teeth, you can observe that the colors have become darker and have separated from adjacent tooth with a dusky-colored borderline. Indeed, it is evident that color harmony is not preserved properly.
We cannot expect to reach a result that is equivalent to the look and feel of natural teeth through esthetic restoration only. The mechanical properties of restoration must be similar to that of the natural teeth. The reaction of the teeth to a certain force is very important. Teeth are subjected to forces of different types and magnitudes throughout the day, and they are created so as to endure those forces. Any restoration must be as strong and durable as the natural teeth. A piece of filling that is placed into a tooth becomes a working part of the entire system. If it is not as resolute as the rest of the system, then it cannot integrate and may fall out. In this regard, the mechanical properties of the material for restoration are crucial. There are dozens of other aspects in which one can compare those properties with a natural tooth. Considering just a couple of the important properties such as hardness and flexibility is enough to show significant differences between natural teeth and restorative materials.
Both hardness and flexibility of a material are determined by certain characteristics and are all formulated in scientific terms. We can see the difference of natural teeth very easily by comparing the hardness and flexibility of the most commonly used composites in the fore teeth and that of hard tissues like enamel and dentine. There is yet another point not to be missed: What makes our teeth perfect is that they are both hard and flexible at the same time. We can see the big gap between real and artificial teeth when we compare the quantitative values of these mechanical properties.
Teeth, in addition to their superior mechanical features, impress us with their exceptional capacity to transmit the force they face. Teeth come in contact with their hard counterpart everyday while eating or otherwise. A casual observer might think that a tooth is directly attached to the chin bone. Let alone being attached, the tooth does not even touch the chin bone. It is tied to the chin bone with flexible strings. A fusion of the bone and teeth is out of the question. It’s as if a bucket is lowered in a well. Whenever we chew, the pressure endured by the teeth is transferred to the bone through these strings in the best manner. These periodontal ligaments serve as shock absorbers. It would be incomplete to explain force transfer in teeth with these strings. It is imperative to remember the structure called lines of forces located on the chin bone that are shaped in such a manner as to guide the force.
It is a noteworthy challenge to make restorations and to replace a tooth in every aspect. Man-made products or works of art cannot be completely imitated; experts at least could tell the differences right away. Could it then ever be possible to imitate an artwork of the Divine to the same degree of perfection in its authentic form?
Mehmet Yildiz is a Professor of operative dentistry at Ataturk University, Erzurum, Turkey.