Gray's Anatomy
Except in mankind, teeth are necessary for survival in most mammals and other vertebrates, and longevity is related to the endurance of the dentition under the abrasive process of mastication. In non-mammalian vertebrates, teeth are constantly replaced, a condition known as polyphyodonty, related to the need for successi 18518j97s vely larger teeth in animals which grow throughout life. In mammals, where skeletal growth is typically limited to an early period of life, there are generally two dentitions, the first deciduous and the other permanent, the condition of diphyodonty; in some mammalian species, e.g. the rat, there is only one set (monophyodonty), there being continuous growth of individual teeth. Emergence and success of the phyodonty was probably related to the evolution of occlusion during mastication in non-mammalian vertebrates, the jaw joint is formed between the quadrate bone of the upper jaw and the articular bone of the lower one, structures homologous respectively with the incus and malleus of mammalian skulls.
The evolution of mammals was associated with the posterosuperior growth of the dentary bone (one of several lower jaw elements existing in all non-mammalian vertebrates towards the squamosal, a bone homologous with the squamous temporal in most mammals. In accord with this skeletal changes, the jaw muscles were also rearranged to move the mandible (now formed entirely by the dentary) transversely. Together with these trends, there was a change in the shape of the teeth; from the simple conical structures of reptilian ancestors, mammals involved teeth with complex sharing plans. Lateral movements of the mandible were now possible, allowing the lower teeth to grind across the upper to produce a more effective trituration.
After a few months of active use, the newly erupted teeth in mammals are worn to produce precisely matching upper and lower shearing edges. A continued eruption of new teeth would constantly disrupt this relationship and therefore be disadvantageous. However, because of the need to accommodate teeth in small, young jaws, a deciduous dentition is an almost universal requirement in mammals. With this reduction of replacement, dental tissues evolved to minimize the effects of wear. Thus a harder, thicker enamel emerged in mammals, with a prismatic structure which evolved and diversified into a variety of complex and distinctive patterns to resist wear and breakage through fracture, and produce sharp cutting edges.
The introduction of refined carbohydrates in the human diet has made human teeth susceptible to caries and periodontal disease. Outside human culture such dental impairment would probably have led to extinction, but this problem has to some extent been overcome by breaking down, softening or in a sense predigesting food by cooking and other types of culinary preparation. Nevertheless, chewing does facilitate the digestion of most foods, including cooked meat and vegetables and the natural dentition comminutes food much more efficiently than an artificial replacement. However, teeth are no longer vital to survival and therefore selective pressure leading to further evolutionary change in the human dentition will probably be limited.
General arrangement of dental tissues
A tooth consists of a crown, covered by very hard translucent enamel and a root covered by yellowish bone-like cement. These meet at the neck or cervical margin. A longitudinal section reveals that a tooth is mostly dentine (ivory) with enamel covering about 1.5 mm thick, while the cement is usually much thinner. The dentine contains a central pulp cavity, expanded at its coronal end into a pulp chamber and narrowed in the root as a pulp canal, opening at or near its tip by an apical foramen, occasionally multiple.
The root is surrounded by alveolar bone, its cement separated from the osseous socket (alveolus) by the soft periodontal ligament, about 0.2 mm thick. Coarse bundles of collagen fibres, embedded at one end in cement, cross the periodontal ligament to enter the osseous alveolar wall. In most non-mammalian vertebrates teeth are rigidly connected directly to bone, a rather brittle attachment. Only in mammals does a periodontal ligament provide an independent, tough suspension for each tooth. Near the cervical margin, the tooth, periodontal ligament and adjacent bone are covered by the gingiva (gum), clearly recognizable in health by its pale pink, stippled appearance.
This is continuous at the mucogingival junction with the red, smooth oral mucosa lining much of the oral cavity and is adherent to the tooth near the cervical margin by an epithelial attachment. The pulp is a connective tissue, continuous with the periodontal ligament via the apical foramen. It contains vessels for the support of the dentine and sensory nerves.
The central incisior
The central incisors play a leading part in showing the physical character of a person. They are the first to be seen, and, as they can raise a lot of profound and complex problems, they represent the touchstone of all dentists. They support the lips while smiling and through size, shape and colour concentrate an important assessment of the entire arch.
Besides the general characteristics, there are some situations when a smaller or a greater number of morphological features can be different from normal.
Some authors established different connections between the shape of the front teeth and the facial shape, or the general configuration of the body.
Restoring a Tooth
The first step in restoring a tooth is to determine the extent of decay. We do this with an x-ray. To the left, you see a large area of decay in the bottom tooth, second from the right. The bright areas in the teeth are all old amalgam fillings. The decay is seen as the dark area to the left of the amalgam in that tooth. The dark area in the center of the teeth are where the nerves are located. You can see that the decay has gotten very close to the nerve.
After the patient has been made numb, the tooth is prepared with a high speed hand piece removing all the old fillings, and all the decayed areas in the tooth. After the tooth is completely clean, all the holes are filled with light cured composite, and the tooth is carved to resemble the form it had before the decay occurred. This tooth looks wonderful now, but it is important to remember that the original decay was very close to the nerve, and it is not possible to guarantee that the nerve will not become inflamed and start to swell causing a toothache later.
The white covering on the tooth is called enamel. It really is white in color, but somewhat translucent and allows the color of the underlying structures to shine through. It is also very hard and quite resistant to acid attack. The brownish yellow material underlying the enamel is called dentin, and it too is hard, but it is much less hard than enamel.
The dentin has a density like that of hard bone. It is much less resistant to acid attack. Underlying the dentin is the nerve of the tooth. The nerve is actually a complex organ. In a healthy state, it is pink and soft, like the lining of your mouth, and is composed of blood vessels, connective tissue and, of course, nerve fibers. The dentin is permeated with thousands of tiny tubules which run perpendicularly from the nerve to the enamel/dentin interface, and also to the outer surface of the root in areas which are not covered by enamel. These tubules are filled with fluid. The fluid is actually contained within tiny projections from cells that line the inside of the nerve space. These cells are part of the nerve complex and are called odontoblasts.
Touching the living dentin produces movement of the fluid in the tubules which transmits impulses back to the nerve making the dentin sensitive to any type of direct stimulus. Because of the presence of these tubules, the dentin is actually quite permeable to fluids.
Note that while the hole in the enamel is relatively small, the decay has rapidly progressed within the dentin to a much larger extent due to the relative softness and permeability of the dentin as compared with the enamel. The decay has a tendency to spread along the dentinal tubules from the enamel surface toward the nerve from which the tubules arise. There are a number of treatment options for any damaged tooth. The choice of restoration depends on the extent of the damage. Your dentist will discuss with you which type of restoration is most suitable.
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