The crown of the tooth is that portion of the tooth that is covered with enamel. In most people, the crown lies nearly entirely exposed above the gum line. In children, the gingiva may partially cover the cervical (lower) part of the enamel.
Enamel is the substance that covers the crown of the tooth. It is very hard and quite resistant to mechanical and chemical attack. Its purpose, of course, is to protect the tooth from the dangers posed to the teeth by the oral environment. In general, it is vulnerable only to acid attack from excess sugar (decay), generalized trauma such as a blow from a hard object, and serious bruxing with associated attrition. It is white, but somewhat translucent and allows the color of the underlying dentin to shine through to a certain extent which is why teeth look yellow. In the diagram to the left, the enamel is represented by the top layer on the tooth. Here it looks a bit like a neat haircut. The reason it is drawn that way is because the enamel is made up of microscopic enamel rods all of which run about parallel to each other and which project perpendicularly from the surface of the underlying dentin. When you are looking at a tooth in the mouth, you are seeing millions of these little enamel rods packed side by side, but you are seeing them end-on, as in the illustration on the right which is a reasonable representation of their cross section. They are packed together a bit like an Escher drawing.
The micrograph above on the left shows the rods in sagittal section, which means you are seeing them as if the tooth were cut like the tooth in the "haircut" image above. The image on the right shows the enamel rods end-on after etching the surface with acid. The acid treatment dissolves the internal parts of the enamel structure faster than the outer parts, so you are seeing only the outline of the rods. The actual rods are solid structures, but do not show up well in micrographs.
Each enamel rod is attached to the dentin underneath it. For this reason, cracks in the enamel (crazes) penetrate only as far as the dentin. This method of attachment makes it impossible for the enamel to separate from the tooth no matter how many crazes develop in its structure.
Dentin is the hard, yellow bone-like material that underlies the enamel and surrounds the entire nerve. It composes the bulk of the tooth, and is sensitive to touch and other stimuli. In the image at the top of this page, the illustration shows thousands of tiny little lines that run approximately parallel to each other and perpendicular to the surface of the nerve space. These lines represent tiny tubes that run parallel to one another throughout the structure of the dentin. These are called dentinal tubules, and they originate from the inner surface of the nerve space and travel perpendicularly from their point of origin to the surface of the tooth terminating at the undersurface of the enamel, or the surface of the root itself in areas where it is not covered with enamel.
The tubules contain tiny projections of cells that line the inside of the nerve space. These cells are called odontoblasts, and they are actually the covering layer of the nerve itself. The projections of the odontoblasts into the dentinal tubules are not nerves. However, the odontoblasts connect with nerve axons in the dental pulp (nerve). Exposed dentin is sensitive to touch, air and other stimuli because these stimuli cause movement of the fluid in the odontoblast projections inside the tubules. This movement of fluid can be sensed by nerve endings in the dental pulp which anastomose (connect) with the odontoblasts. The image on the right above is an electron micrograph of actual dentinal tubules seen end-on.
What the lay public calls the nerve of a tooth is called the dental pulp by dentists. It is a complex organ composed of connective tissue, blood vessels, and nerve axons. It is pink and soft, and looks just like the lining of the mouth when it is removed during root canal procedures. Its original purpose during development is the formation the teeth themselves. In other words, the nerve of a tooth is a "generative" organ. The nerve starts out as a clump of specialized cells, and as we begin to grow, it slowly takes the shape of a tooth. The cells on the outside of the pulp begin to form the various hard structures , enamel and dentin, that we associate with the tooth itself. The tooth is formed from the outside toward the inside, with the dental pulp slowly replacing itself with tooth structure. While we are still young, the nerves in our teeth are relatively large, but they slowly shrink becoming more and more narrow throughout our lives. Once the tooth is fully formed, the nerve slows its formative functions, but it keeps building dentin in a process called dentinogenesis. During this slow growth phase of its life, the nerve serves mostly to keep the teeth hydrated and allows the dentin to retain a certain amount of elasticity. Thus, living dentin acts something like a shock absorber, preventing the teeth from fracturing. Whenever a nerve in a tooth dies, the tooth looses this shock absorber effect and is more prone to fracture. This is the reason that a tooth that has been endodontically treated needs to be protected with a crown.
A "root canal" is actually only a part of the dental pulp. It has all the same characteristics and functions as the rest of the dental pulp, except that it is located inside the root portion of the tooth and is thus rather thin and spindly. When we tell a patient that they need a "root canal", we are not talking about the anatomical structure itself. We are talking about a procedure. We really mean that the nerve is sick and must be removed in its entirety from the tooth, the empty space where it used to live cleaned and sterilized and finally sealed with a form of rubber called gutta percha or one of the newer materials designed for this purpose. The technical name for this procedure is "endodontic treatment". The root canal(s) in any given tooth start out just like the rest of the nerve, as a solid piece of soft tissue. Blood vessels and nerves enter through a hole at the tip of each root. The tip of the root is called the "apex", and the hole that allows the nerve tissue, with its accompanying blood vessels to enter the tooth is called the apical foramen. Of course, blood must traverse through the root canals in order to infuse the nerve. As we age, the root canals too replace themselves with more and more dentin until they become less tube like and more like a network of blood vessels and nerves running down approximately the center of the root. The image to the right shows some of the complex anatomy that the dentist is presented with when he must perform a root canal procedure to relieve pain and infection. In fact, the nerve anatomy can become even more complex as we age. As the canal becomes thinner and thinner, we say that it has become sclerosed. One can see that it could be quite difficult to remove ALL the dead tissue in the root canals if its internal anatomy has become more and more sclerosed and difficult to negotiate as the tooth ages or becomes sick. While it is important for the endodontist to remove as much dead nerve tissue as possible from the pulp chamber and root canals, the final line of defense against endodontic failure is to make sure that any remaining dead nerve tissue inside the tooth is properly sealed off at the apical foramen and any other openings in the root by properly fitted and placed sealing materials.
Cementum is to the root of a tooth as enamel is to the crown. Cementum is a relatively soft bony tissue that covers the root surface in a thin layer. Its main function is to act as an attachment layer for the periodontal ligament which is a soft tissue sheath that acts as a cushion between the bony socket and the tooth itself. It is relatively soft and does not wear well against environmental assaults, so it abrades away rapidly whenever it is exposed to the oral environment because of recession. The image to the right shows the relationship of the enamel that covers the crown of a tooth, to the cementum that covers the root. Unless there is wear of the cementum due to recession, or attrition of the enamel due to bruxing or mechanical abrasion, the dentin is never exposed. The cementum meets the enamel in a line that surrounds the tooth. This line is called the cementoenamel junction.
The apical foramen is simply the hole in the tip of the root where the nerve and all its accompanying blood vessels must enter the tooth. Each root has a foramen at its tip and blood must both enter and exit the dental pulp from this point. The foramen is often not located at the very tip of the root, but may be offset one to three millimeters toward the crown of the tooth.
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