What are teeth? |

Structure and Functions

Teeth are functional portions of the mouths of animals that assist them in processing food prior to swallowing. This process is called chewing, or mastication. Teeth are also primary offensive and defensive weapons for most animals. Many animals have no hands to grasp or capture food, and their teeth become the principal means of grabbing and killing prey. In humans, teeth not only process food but also have a sociological significance in displaying anger, friendliness, and desirability.

A tooth is composed of three basic parts: the crown, the dental pulp, and the root. The crown of the tooth is that portion exposed above the gingiva, commonly called the gums. The outer surface of the crown is covered by a hard, crystalline substance called enamel. Enamel is an almost completely inorganic material, calcium hydroxyapatite, and it is the hardest tissue in the human body. Underneath the enamel, the bulk of the crown is made up of a substance known as dentin. It, too, is quite hard, but it has more organic material, ground substance and nerve fibers, within it. The dentin is honeycombed by small tubules radiating from the dental pulp chamber at the center of the tooth. These tubules carry nerve fibers from the central nerve within the pulp to the junction of the enamel and dentin.

In the center of the crown is the pulp chamber. The pulp contains nerves and blood vessels that give sensations to and nourish the tooth. These nerves and blood vessels enter at the tip of the root, the apex, and arise from nerve trunks and blood vessels that run through the jawbone.

The root is the portion of the tooth joining the crown to the jawbone. The outer surface of the root is covered by a thin layer of bonelike substance called cementum, and it runs from the junction of the enamel of the crown to the apex of the root. Under the cementum, dentin composes the bulk of the root, continuing from the crown to the apex. The root is attached from the cementum to the bony socket in the jaw by the periodontal ligament. This ligament is composed of elastic connective tissue fibers that act not only as part of the attachment apparatus for the tooth but also as a shock absorber from chewing forces. The pulp chamber narrows into a thin, constricted channel from the pulp chamber to the apex; this is known as the root canal. In some animals, such as rodents, the root canal of incisors remains relatively large throughout life and is called “evergrowing.” In most animals, including humans, the root canal constricts to prevent further growth while still providing sufficient blood flow for proper tooth nourishment. These teeth are termed “rooted”
(although all teeth are actually rooted). The tooth is surrounded by the tissues of the attachment apparatus called the periodontium, consisting of the gingiva, the periodontal ligament, and the alveolar bone of the jaws.

In humans, there are normally thirty-two adult teeth and twenty deciduous (or baby) teeth. Deciduous teeth start calcifying in the embryo at about five to six weeks of development. Teeth start to form from two types of cells at an interface in the tooth bud, which becomes the dentoenamel junction. The enamel is formed by a cell called an ameloblast, and the dentin is formed by a cell known as an odontoblast. The enamel grows outward and the dentin inward from the interface. The pulp is formed from nerves and blood vessels in the developing jawbone. While the crown is growing, the root starts to form, lengthening as the tooth develops. After the crown is formed, the ameloblast cells rest on the outer surface of the enamel, while the odontoblasts line the internal cavity of the pulp chamber.

When the tooth erupts through the gum tissue, the ameloblasts are compressed and destroyed. Enamel is one of the two tissues in the human body that cannot repair itself (the other being the cornea of the eye). The odontoblasts can be activated inside the pulp chamber to form a secondary or reparative dentin. This formation of insulating dentin is in response to aging, advancing tooth decay, or trauma to the tooth or in reaction to the placement of restorative materials into a tooth.

The deciduous teeth begin to erupt through the gums in infancy and continue to do so until the full complement of primary dentition has erupted. Most of the adult teeth develop below the baby teeth, and as they push on the roots of the primary teeth, these roots are resorbed from the pressure of erupting permanent crowns. The primary teeth are shed throughout childhood and into adolescence. Other animals have both primary and permanent teeth. Sharks continually develop full sets of teeth, sometimes as many as seven or eight at a time; as their teeth loosen or break off, new teeth replace them. Humans, however, possess only one set of permanent teeth, and they are not naturally replaced if they are lost through trauma or disease.

Mammals differ from all other animals in having teeth set in sockets (thecodont) and in having several different kinds of teeth (heterodont), compared to the homodont teeth of animals such as sharks and reptiles.

The shape and size of teeth are closely related to their functions. The four front incisor teeth, both upper and lower, are used to shear and cut food. The movement of the mobile lower jaw, the mandible, against the static upper jaw, the maxilla, causes these teeth to work as scissors on food. Horses crop grasses with their large incisors. Cusps are conical projections of the cuspids and posterior teeth. They act as crushing and grinding segments of the posterior teeth, reducing the food into smaller portions that may be swallowed and digested efficiently. The four cuspids, or canines, are conical and pointed. Their primary function is to grasp and tear food. The fangs of a tiger are cuspids, as are those of lions. The eight premolars have two conical cusps and are called bicuspid teeth. They too are used for tearing, but working against the opposing teeth, they also function as a mill, crushing and grinding the food.

The twelve molars are multicusped teeth that grind food into smaller portions; mixed with saliva, the food is then readied for swallowing. The grooves of the occlusal (chewing) surfaces act as sluiceways to channel the food in the oral cavity. The tongue folds the food back onto the surfaces of the grinding teeth until it is chewed sufficiently.

The third molars are commonly called wisdom teeth. These teeth are the last to develop and are more prone to irregular calcification and morphology. In the course of human evolution, the jaws have shortened, and the third molars often do not have enough room in the jaw to erupt in a normal, perpendicular mode. The result is an irregular angle of eruption. When the third molars lock and push against the second molars and are unable to erupt normally, the condition is called impaction. Thus, in many cases, the wisdom teeth must be removed.

Teeth and their supporting tissues are susceptible to disease. The primary disease of the tooth is dental caries, or tooth decay. Caries (cavities) begin with the decalcification of the enamel crystals by acids. These acids are products of bacteria caught in a sticky film that forms on the teeth called plaque. These bacteria use refined hydrocarbons, principally sugars, for their food. If the plaque is not removed from the surface of the teeth, the acids are kept close to the enamel. Over a period of time, the decalcification of the enamel reaches the internal dentin, and the acid begins to decay the less calcified enamel at a more rapid rate. The acids also can touch the nerve fibers within the dental tubules, causing sensitivity and pain. If this process is not stopped, the decay can penetrate into the pulp and cause infection.

When the pulp becomes infected, it invariably dies. The dead tissues gradually seep through the apex, and bacteria feed on the necrotic material. Inflammatory cells from the blood vessels in the bone try to fight the infection, which can cause swelling, pain, and pus. The result is a periapical abscess.

The periodontium is also susceptible to disease, and again, the bacterial plaque is the chief cause. Surrounding the neck of the tooth is a cuff of gum tissue. The bacteria in the plaque release their waste products into the cuff and irritate the lining. Inflammatory white cells, known as lymphocytes or chronic inflammatory cells, are brought from the blood vessels in the gum tissue to the point of irritation in order to attack the bacteria and their by-products. This disease is called gingivitis, which can be reversed by removing the dental plaque.

If the plaque is allowed to remain on the surface of the tooth, it may calcify into a hard, rough, and porous substance called dental calculus or, more commonly, tartar. This material attracts and entraps more bacteria, and it is abrasive to the soft lining of the gingiva. The gums become further inflamed, bringing more lymphocytes into the area. These cells try to attack the bacteria and dissolve the dead and dying cells of the diseased gingiva.

Over a period of time, the inflammatory cells move deeper into the periodontium, dissolving and detaching the elastic fibers of the periodontal ligament and eroding the crest of the bony socket. This inflammation of the periodontium is called periodontitis. With the destruction of the fibers attaching the gingiva and bone to the root, the cuff of gum tissue deepens into a pocket around the neck of the tooth. The depth of the pocket facilitates the further entrapment of bacteria and makes it more difficult to clean.

If the condition is not corrected, eventually there is enough destruction of the periodontium that the tooth becomes loose. Often, the chronic inflammatory cells at the bottom of a deep pocket are joined by acute inflammatory cells, producing painful swelling and pus. This condition is known as a periodontal abscess. Most extractions of adult teeth are the result of periodontitis.

Disorders and Diseases

The principal scientific professions requiring knowledge of teeth and their surrounding structures are those of dentistry and dental hygiene. Dentists need to have thorough knowledge of the anatomy and physiology of the teeth and their surrounding structures in the mouth. They must be able to detect and treat diseases of the mouth and all its tissues. Dental caries, infected pulp, diseases of the periodontium, and tooth loss are treated by dentists. Dental hygienists aid dentists by treating and identifying diseases of the mouth. Their principal duty is to remove harmful deposits on the teeth, but hygienists also identify diseases of the teeth and periodontium. Using their anatomical and physiological knowledge of the teeth and surrounding structures, hygienists teach patients preventative techniques that can help prevent or halt the spread of dental disease.

Since the teeth and oral tissues are only a part of the body, knowledge of general human anatomy, physiology, and pathology is a must for dentists. Oral symptomatology discovered by dentists is often the first sign of a serious systemic disease. For example, a certain fruity odor on the breath is a sign of ketosis, which is a symptom of diabetes. Kaposi’s sarcoma, a rare type of skin cancer, is often manifested as lesions of the oral tissues; the presence of such lesions may be a strong indication that the patient has acquired immunodeficiency syndrome (AIDS).

The successful treatment of diseases of the teeth and periodontium must be based on a thorough understanding of the anatomy and physiology of these tissues. To restore a decayed tooth, the dentist must know how deep to cut into the tooth, the probable location of the pulp, the irritating factors of the restorative material, the possible traumatic chewing forces on the new restoration, and the compatibility between the restoration and the tissues of the periodontium.

The treatment of tooth loss is literally as old as the pharaohs. An X ray of the skull of an Egyptian mummy displayed an attempt to construct a dental bridge using gold wire to secure a tooth between two natural teeth. At present, there are several ways to restore lost teeth. Cemented fixed bridges constructed of metal and porcelain are often the treatment of choice when there are sound teeth to support them. In the case of a partial or total loss of the teeth, removable dentures constructed of plastic or porcelain teeth fixed in an acrylic plastic base are used.

Introduction of newer materials and techniques for the restoration of teeth is a constant challenge for the dental scientist. While the theory of implanting restorations into the jaw to replace teeth is not new, some of the materials are. The recent use of titanium implants into the jaw reinforces the dentist’s need to know the anatomy and physiology of the surrounding tissues. It is known that the bone of the jaw attaches to the surface of titanium, a process called osseointegration. Great care must be used, however, in placing correct chewing forces on the supporting bone, and nonirritating restorations must be placed near the periodontal tissues for the implants to be successful.

Special acrylic plastics called composites are sometimes used in restoring lost tooth structure by chemically bonding to the enamel and dentin. Thin films of these materials are placed in the grooves of the newly erupted posterior teeth. This treatment has been shown to prevent decay in the chewing surfaces of the teeth. Laser technology is being explored by scientists to see if the enamel surface might be fused to withstand decay.

While the bone lost as a result of periodontal disease cannot be regenerated, new techniques of grafting the patient’s own bone, freeze-dried sterile bone, and other materials show some promise in strengthening the weakened tooth.

A number of recent studies clearly demonstrate a close connection between oral health and disease of other body systems. Patients with serious gum disease or tooth decay are at increased risk of cardiac arrhythmias, stroke, and kidney disease. Several studies reveal that older patients are especially susceptible. The American Geriatric Society, for example, published a study showing that patients over the age of eighty with three or more active root caries were twice as susceptible to cardiac arrhythmias when compared to individuals with disease-free teeth and gums.

Increased risk of heart attack and stroke may come from acute and or chronic inflammation caused by bacterial infection originating from bacterial pockets in gums and rotting teeth. Such inflammation leads to platelet activation and elevated levels of clotting factors in the blood, which increases the risk of cryptogenic stroke and cardioembolism.

Periodontal disease has been linked to another heart health problem called subacute bacterial endocarditis, a severe bacterial infection of the lining of the heart that can cause heart irregularities and heart attacks. This disease is also caused by bacterial infections that originate and reside in periodontal pockets known as gingival crevices. These bacteria eventually spread via the blood to all parts of the body and lodge in the tissues that line the heart, causing subacute bacterial endocarditis. Links between heart disease, stroke, and gum diseases are elevated in individuals who have lost teeth because the bacteria colonize the residual periodontal pockets, from which they leach into the bloodstream. Both periodontal disease and the secondary but important inflammations of the heart, brain, and other body systems that result are treatable and preventable conditions for many patients. Periodic cleaning and prompt attention to dental caries, along with complete removal of old amalgam fillings and other metals, have proven very helpful.

Perspective and Prospects

Recorded history and archaeological findings show that humans have tried to treat the teeth and their related diseases probably since the Stone Age. There has been speculation among archaeologists that the practice of trepanning, the surgical opening of the skull, could have been in response to severe toothaches as well as other pain in the head. Mutilation of the teeth by the Incas and Mayans was common in noble families; skulls have been discovered in burial sites of both nations showing the insertion of jade disks in slots filed into the front teeth.

In ancient Greece, Hippocrates wrote of treating a severe tumor of the jaw of a young man. After lancing the lesion, he wrote that the condition was morbid and that the young man would surely die. The Greeks also supposed that tooth decay was caused by small worms that bored into the tooth and ate it from within.

In medieval Japan, dentists were trained to extract teeth with their thumb and forefingers. They practiced on tapered wooden pegs pounded into a board. A soft wood was used at first for easier removal, then successively harder boards and pegs were introduced until the dentist could then remove a tooth from the jaw.

Most of the dentistry in the past was surgical removal of painful teeth. From the Middle Ages to the mid-nineteenth century, barbers performed extractions. Without the benefit of anesthetic, this practice was quite painful. Horace Welles found in 1894 that a patient could be put to sleep with ethyl ether, allowing painless tooth extraction. With the introduction of local anesthetics in the 1920’s and later of intravenous drugs, pain during the extraction of a tooth has been virtually eliminated.

In modern practice, the study of the teeth has found new applications. For example, dental forensics contributes to the identification of abusers and criminals who bite their victims. The dental arch and the relationship of the teeth within it are unique, and bite marks are like fingerprints: No two are alike. The forensic scientist can take impressions of the bite marks on the victim’s body with an accurate impression material. Plaster casts are formed in the impressions and compared to a cast of the suspect’s dental arch. The evidence may either confirm or rule out the suspect’s participation in the crime. In addition, dental forensics is used to identify the remains of people who are burned beyond recognition or whose bodies are badly decomposed. The teeth and their dental restorations are often the only way that a deceased person may be identified, especially in a major disaster such as an airplane crash.

The restoration and replacement of diseased teeth is an ongoing challenge, but with the new materials and technology available, humans should have healthier teeth in the future.

Bibliography

Cook, Allan R., ed. Oral Health Sourcebook: Basic Information About Diseases and Conditions Affecting Oral Health. Detroit, Mich.: Omnigraphics, 1998. This handy reference source, which covers all aspects of dental health, includes helpful statistics on dental disease.

Ferracane, Jack L. “Using Posterior Composites Appropriately.” Journal of the American Dental Association 123 (July, 1992): 53-58. A discussion of the mechanical properties of acrylic composites, including resistance to wear and the use of bonded seals with this restorative material.

Foster, Malcolm S. Protecting Our Children’s Teeth: A Guide to Quality Dental Care from Infancy Through Age Twelve. New York: Insight Books, 1992. This book, meant for parents, is clear and easy to understand. A good starting point.

Langlais, Robert P., and Craig S. Miller. Color Atlas of Common Oral Diseases. 4th ed. Philadelphia: Lippincott Williams & Wilkins, 2009. Provides six hundred color photographs of the most commonly seen oral problems accompanied by descriptive text for each condition.

Parker, James N., and Philip M. Parker, eds. The Official Patient Sourcebook on Tooth Decay. San Diego, Calif.: Icon Health, 2002. Self-described as a reference manual for self-directed patient research, this book describes in clear detail relationships among types of tooth decay and relationships between dental problems and overall health. Includes lists of Web links for many topics treated within.

Standring, Susan, et al., eds. Gray’s Anatomy. 40th ed. New York: Churchill Livingstone/Elsevier, 2008. The definitive book on human anatomy. With 780 illustrations in the text, the interrelationship of nerves, blood vessels, bones, and other anatomical structures of the human body is displayed in a practical manner as a fascinating biological machine.

Zablotsky, Mark H. “The Periodontal Approach to Implant Dentistry.” Journal of the California Dental Association 19, no. 12 (1991): 39. An excellent article outlining the important relationships between dental implants and the restorations attached to them and the periodontal tissues.