Tooth Development, Permanent

Definition

Permanent teeth, which are also known as adult teeth, are the second and final set of teeth in the human mouth. There are generally 32 permanent teeth in an adult mouth—16 in the upper jaw and 16 in the lower jaw. The permanent teeth replace the 20 primary teeth, which are also known as baby teeth, milk teeth, or deciduous teeth.

Description

In the mouth, a combination of hard and soft tissue areas form the occlusion (bite). The teeth, along with upper and lower jaw bones, are among the hard tissues. The soft tissue includes the gums, tongue, and salivary glands.

Teeth, both primary and permanent, are used to chew and swallow food. Each tooth is divided into a crown and root. The crown is visible. The root grows below the gum and is attached to the jawbone. A pulp chamber located in the center of the crown houses pulp tissue.

The crown is covered with enamel, the hardest substance in the body. It is 95% calcified (mineralized). Cementum, a thinner material, surrounds a portion of the root.

Types of teeth

The shape of the crown determines the purpose of the tooth:

• Incisors have a straight edge to incise or cut food. The two central incisors in each jaw are also known as the front teeth, indicating their location in the mouth. A lateral incisor is located on each side of the front teeth. There is one root in each incisor.
• The canine teeth are located in the corners of the mouth, with two in each jaw. The canine teeth have pointed crowns and are longer than the other teeth. These teeth are used to grip and tear food. They are also known as cuspids or eye teeth. Each canine tooth has a single heavy root.
• On each side of the six front teeth (incisors and canines) are five molars known as the back teeth. The crowns have wider surfaces that are used to chew food. On the surface of the molar are two or more cusps, slight elevations in the crown that are used to grind and pulverize food before it is swallowed.
• The premolars, which are also known as bicuspids, are located behind the cuspids (canine teeth). They help the canine teeth to grip and tear food. There are eight premolars in the adult mouth, with half in the upper jaw and half in the lower jaw. A premolar has one or two roots.
• The remaining molars in each jaw are used to grind food. The first molar, also known as the six-year molar, is adjacent to the second bicuspid. On the other side of the first molar is the second molar, the twelve-year molar. At the back of the mouth are the third molars, which are also known as wisdom teeth. The upper molars generally have three roots, and there are usually two or three roots in the lower molars. These roots help bolster the teeth for the heaviest pressure of chewing and grinding food.

Permanent tooth development

The development of both primary and permanent teeth starts long before these teeth are visible. When a child is born, the primary teeth are partially formed, and development of permanent teeth has started in the jaw bone.

At about the age of six, a child begins losing primary teeth and permanent teeth erupt (appear). The primary teeth fall out (exfoliate) to make room for the permanent teeth to erupt. Generally, girls' teeth develop before boys, and lower teeth grow through the gums before upper teeth. Development of this second set of teeth can sometimes continue into adulthood. A delay in the development process of two years or more could be a symptom of hormonal deficiencies.

TOOTH DEVELOPMENT IN THE UPPER JAW According to the American Dental Association (ADA), permanent teeth in the upper jaw generally erupt in this order:

• Between the ages of 6 and 7, the permanent first molars erupt. These teeth erupt behind the child's primary second molars.
• Between the ages of 7 and 8, central incisors appear.
• Lateral incisors erupt between the ages of 8 and 9.
• Between the ages of 10 and 11, the first premolars (first bicuspids) appear.
• The second premolars appear between ages of 10 and 12.
• Between the ages of 11 and 12, the canine teeth (cuspids) erupt.
• Between the ages of 12 and 13, second molars erupt.
• Between the ages of 17 and 21, the molars known as wisdom teeth appear.

TOOTH DEVELOPMENT IN THE LOWER JAW According to the ADA, permanent teeth in the lower jaw generally erupt in this order:

• Between the ages of 6 and 7, the permanent first molars and central incisors erupt.
• Between the ages of 7 and 8, lateral incisors appear.
• Between the ages of 9 and 10, the canine teeth (cuspids) erupt.
• Between the ages of 10 and 12, the first premolars (bicuspids) appear.
• Between the ages of 11 and 12, second premolars (bicuspids) erupt.
• Between the ages of 11 and 13, second molars erupt.
• Between the ages of 17 and 21, third molars (wisdom teeth) erupt.

CHARACTERISTICS OF PERMANENT TOOTH DEVELOPMENT Permanent teeth tend to have a yellowish color and are generally larger than primary teeth. Since permanent teeth are larger, their development could crowd other teeth. For example, permanent incisors may be more closely spaced together than primary teeth, particularly in the lower jaw.

There could be space between the upper incisors. The eruption of the upper canine teeth will generally push those incisors together.

The premolars are smaller than the primary premolars. After the adult teeth erupt, the permanent first molars move and fill the space left by the exfoliated premolar.

The third molars are the last teeth to erupt, and there may not be room in the mouth for some or all four of the wisdom teeth. These molars have a tendency to be impacted (out of alignment) and may be unable to erupt. Extraction (removal) of unerupted wisdom teeth may be required.

MISSING PERMANENT TEETH. Some people may not develop all permanent teeth. This lack of teeth is believed to be genetic. The teeth most often missing include the lateral incisors, second premolars, and third molars. The absence of wisdom teeth is generally not a problem unless the third molars in the opposite jaw over-erupt.

EXTRA PERMANENT TEETH. Supernumerary teeth are those teeth in excess of the usual 32 permanent teeth. Most frequently, a supernumerary tooth erupts between the two central incisors in the upper jaw. This extra incisor is called a mesiodens (middle tooth). The presence of these extra teeth has been linked to two hereditary conditions, Gardners's syndrome and cleidocranial dysostosis. Because extra teeth can cause orthodontic problems, dentists generally remove them.

Function

Humans are omnivores, which means they eat meat and vegetables. Permanent and primary teeth make this possible. The location and shape of the tooth indicates its role in separating food into smaller pieces that can be swallowed and digested. The incisors incise or cut food; the canine teeth tear the food; premolars crush the food; and permanent molars grind it into pieces that can be swallowed.

Role in human health

Teeth allow a person to bite and chew food. Without them, a person could eat only soft foods. Teeth also contribute to understandable speech. For example, when a person speaks, the sound of a letter such a "t" is conveyed by the tongue striking the back teeth.

Common diseases and disorders

Tooth decay and injury can result in the loss of or damage to permanent teeth. Dentists should advise patients about how to prevent decay and injury, advice that includes cautions about sugar and the use of protective athletic gear.

Dental health

As permanent teeth develop, it is advisable for the dentist to see the patient every six months. The dental appointment includes the application of fluoride because newly erupted teeth are prone to tooth decay. The areas most susceptible to tooth decay are the chewing surface of the back teeth, the area where adjacent teeth meet, and the surface closest to the gumline.

The dentist may use a sealant (plastic coating) on the permanent back teeth (molars and premolars). The sealant protects against plaque, which produces tooth decay. If the dentist finds tooth decay, the patient's cavities should be treated with fillings. Small tooth-colored composites are recommended.

An adolescent patient also may experience gum inflammation known as gingivitis. Most cases are mild. However, the dental staff needs to remind the patient about the importance of a nutritional diet and oral hygiene. In addition, some teenagers may smoke and should be cautioned that tobacco can harm the teeth and gums.

Accidents and injuries

Accidents and injuries can result in the loss of permanent teeth. As with oral hygiene, prevention is the best method of combating injury. Children and teenagers should be advised to wear sports equipment such as a baseball catcher's mask and a football helmet with a mouth guard.

If a tooth is broken or knocked out, the patient and tooth should be taken to the dentist as soon as possible. In some cases, the tooth can be repaired or reinserted.

Irregular development

Teeth may not develop according to the traditional pattern. A difference in the shape or size of teeth can affect the spacing of teeth. In addition to problems with the alignment of adjacent teeth, there may be a misalignment in the meshing of teeth in the upper and lower jaws. Common problems include large central incisors, or the "peg" lateral incisor that is thinner and has a sharper point than the normal incisor.

Missing teeth may also affect the alignment of teeth. Adjacent teeth can drift towards the empty area. This situation can also cause over-eruption of the opposing teeth in the other jaw. If several side teeth are missing, the person may have a collapsed bite.

KEY TERMS

Fluoride— A mineral that helps fight tooth decay.

Gingivitis— The inflammation of the gingiva (gums).

Plaque— A transparent material in the mouth that contains bacteria and causes tooth decay.

Health care team roles

In the case of irregular tooth development or missing teeth, orthodontic treatment could provide adjustments. Regular dental appointments and daily oral hygiene that includes brushing the teeth and flossing can help fight tooth decay. Patients should also be advised to play safely.

Resources

BOOKS

Guerini, Vincenzo. A History of Dentistry From the Most Ancient Times Until the End of the Eighteenth Century. Boston, MA: Longwood Press, 1977.

Leonardi Darby, Michele, ed. Mosby's Comprehensive Review of Dental Hygiene. St. Louis, MO: Mosby, Harcourt Health Sciences, 1998.

Taintor, Jerry, and Mary Jane Taintor. The Complete Guide to Better Dental Care. New York: Facts on File, Inc., 1997.

Teabord, Mark, et al, eds. Development, Function, and Evolution of Teeth. New York: Cambridge University Press, 2000.

ORGANIZATIONS

Academy of General Dentistry. 211 E. Chicago Ave., Chicago, IL 60611. (312) 440-4300. 〈http://www.agd.org〉.

American Dental Association. 211 E. Chicago Ave., Chicago, IL 60611. (312) 440-2500. 〈http://www.ada.org〉.

American Dental Hygienists' Association. 444 N. Michigan Ave., Suite 3400, Chicago, IL 60622. (312) 440-8900. 〈http://www.adha.org〉.

Centers for Disease Control and Prevention. National Center for Chronic Disease Prevention and Health Prevention. Division of Oral Health, MS F-10. 4770 Buford Highway, NE. Atlanta, GA 30341. (888) CDC-2306. 〈http://www.cdc.gov〉.

National Institute of Dental & Craniofacial Research. National Institutes of Health. Building 45, Room 4AS-18. 45 Center Drive MSC 6400, Bethesda, MD 2089-6400. 〈http://www.nidr.nih.gov〉.