Dental Anatomy

Definition

Dental anatomy is the study of the classification and morphology of teeth, as well as the study of the principles of occlusion.

Description

Dental anatomy may be considered the study of teeth at three different levels. First, teeth may be studied in terms of the elements of each individual tooth (for example, dentin and enamel). Second, teeth may be studied in terms of classification and numbering systems. Third, teeth may be studied in the larger context of the oral cavity, along with the principles of occlusion and structures that may display pathology such as the gingiva and the temporomandibular joint.

Teeth

The teeth are derived from the ectoderm and the mesoderm in the embryonic stages of development. Each tooth arises from either the maxilla (the bone that makes up the upper jaw), or the mandible (the bone that makes up the lower jaw). Teeth originating from the maxilla are considered to be in the superior dental arch, or the maxillary dental arch. Teeth arising from the mandible are considered to be in the inferior dental arch, also known as the mandibular dental arch.

Each tooth consists of:

• Crown: The visible portion of a tooth.
• Root: The portion of the tooth embedded in the gum.
• Pulp: Located in the center of the tooth, it contains the arteries, veins, nerves and lymphatic tissue.
• Blood vessels: They carry nutrients to the pulp.
• Root canal: The canal in the root of the tooth is where the nerve and blood vessels travel with nutrients to the tooth from the mandible or the maxilla.
• Ligament: The connective tissue that surrounds the root of a tooth and connects it to the maxilla or mandible.
• Bone: Alveolar bone forms tooth socket and part of the teeth.
• Cementum: The layer of tissue covering the dentin on the root of the tooth. Serves the same role as enamel.
• Dentin: The calcified tissue underlying the enamel (on the crown) and cementum (on the root), making up the main bulk of the tooth.
• Enamel: The calcified outer layer of the crown of the tooth.

Although each tooth has the same basic structure, some variation exists. Different types of teeth have variation in their roots. Incisors and cuspids have only one root. Maxillary (upper) premolar teeth commonly have two roots, whereas the mandibular premolars commonly have one. The premolars may also have two roots fused to look as one. The molars on the maxillary arch have three roots, while, on the other hand, the molars on the mandibular arch have two roots.

Tooth types

Permanent teeth are divided into four groups based on their function and placement in the jaw. Teeth are given the same name whether they are in the upper or lower jaw. From medial (middle of the mouth) to lateral, the four types of teeth are listed below.

• Incisors cut the food. They are divided into central and lateral. They are the two most medial teeth on either side (left and right) in the upper and lower jaw, for a total of eight.

• Cuspids, also known as canines, cut and tear the food. They are the third teeth from the center, and there are a total of four (two upper, two lower).
• Pre-molars seize and shear the food. They are also known as bicuspids because they have two cusps, or projections on the surface of the teeth. There are eight total (four upper, four lower) between the cuspids and the molars.
• Molars grind the food. They are the most lateral teeth. There are a total of twelve, including the four "third molars," also known as wisdom teeth.

Children who still have their temporary teeth (also known as primary, deciduous, or "baby" teeth) have incisors (four upper, four lower), cuspids (two upper, two lower), and molars (four upper and four lower), but lack pre-molars. There are 20 total primary teeth.

Tooth classification and numbering systems

Universal tooth numbering and classification allows health care professionals to discuss and identify teeth with a measure of certainty. Permanent dentition has a universal numbering system that begins by splitting the mouth into four quadrants; the upper right quadrant, the upper left quadrant, the lower right quadrant, and the lower left quadrant. Each quadrant contains eight teeth. The upper right quadrant consists of the eight teeth on the upper right side of the mouth from the third molar (wisdom tooth) to the right central incisor. The upper left quadrant is the same except on the left portion of the mouth from the third molar (wisdom tooth) to the left central incisor. The lower right quadrant consists of the teeth from the third molar on the lower left to the lower right central incisor. The same with the lower left quadrant from the third molar on the lower left to the lower left central incisor. Each quadrant can be named using acronyms; upper right (UR), upper left (UL), lower right (LR), lower left (LL).

General dentists generally number the teeth, beginning with the third molar in the upper right quadrant (number one), crossing to the upper left quadrant (the third molar is number 16), proceeding to the lower left quadrant (the third molar is number 17), and ending in the lower right quadrant with the third molar number 32 (third molars are known more commonly as the wisdom teeth). In contrast, orthodontists and oral surgeons use the acronyms of quadrants to help identify teeth, and number the teeth from medial to lateral (one to eight). Thus the upper right first (or central) incisor is also known as UR1, whereas the upper left third molar is known as UL8. LL5 would be the lower left second premolar and LR6 is the lower right first molar.

Primary, or temporary, teeth are systematically lettered in an alphabetical order beginning with A on the upper right quadrant, ending with J for the second molar in the upper right quadrant, proceeding with K for the second molar in the lower left quadrant, and ending with T for the second molar in the lower right quadrant. Primary teeth consist of twenty teeth total; ten on the maxilla and ten on the mandible.

Occlusion

When discussing occlusion, one is essentially discussing the operation and conjunction of the teeth at rest and in the process of mastication (chewing). Two important elements in mastication are the muscles involved, and the temporomandibular joint.

MUSCLES. The four main mandibular muscles for dental anatomy are the:

• Masseter: The main muscle for the movement of the mandible for chewing.
• Temporalis: Maintains the position of the mandible at rest when a person is upright. During chewing it pulls the mandible back into the rest position.
• Lateral pterygoid: This muscle has a horizontal position and is the prime mover of the mandible into a protrusive position.
• Medial pterygoid: Works with the masseter muscle to aid in the elevation of the mandible. It is almost a mirror image of the masseter muscle—it gives extra aid in chewing.

TEMPOROMANDIBULAR JOINT. Also known as the TMJ, this is the connecting hinge between the lower jaw (the mandible) and the bone at the (lateral) base of the skull (the temporal bone). This joint is very fragile. A meniscus (piece of cartilege) separates the lower jawbone from the socket where it rests. Tendons, ligaments and muscles hold the TMJ in position. TMJ disorders can arise from pain because of muscle dysfunction (myofascial pain), pain because of degenerative bone disease (like arthritis), pain because of a broken bone, pain arising from dislocation of the meniscus, or pain arising from the dislocated joint. These dysfunctions can result in headaches, jaw soreness, neckaches, and pain when chewing (often accompanied with a noise). Bite splints may be used to treat TMJ disorders caused by dislocation. Other therapies, ranging from isometric muscle exercises to oral surgery, may be required, based on the nature and severity of the TMJ disorder.

Gingiva

Normal, healthy gingiva may appear smooth or stippled. Known also as the gum tissue of the mouth, the gingiva shows the first signs of periodontal disease. Daily brushing and flossing help keep the gingiva healthy.

Function

Dental anatomy serves the important function of mechanical disruption and processing of food.

Role in human health

Properly functioning dental anatomy is essential for one of the first stages of food digestion—chewing. Chewing is essential for human health because (most) food is not prepared in a readily digestable form. Chewing increases the total surface area of the food by breaking it into smaller pieces, which allows more food to be digested. Chewing also breaks down any less-digestible barriers that may hinder absorption of food in the gastrointestinal tract. For example, even after an orange is peeled, thin membranous tissue still surrounds the juicy, fruity portion of the orange. Chewing breaks down that barrier, making the nutrients inside much more accessible.

Common diseases and disorders

The most common childhood disease of the dentition is tooth decay. The bacteria demineralizes the enamel and dentin of the teeth, causing pain and swelling of the surrounding tissue if left untreated. Periodontal disease is a common disease among older adults associated with infectious bacteria affecting the tissues that support the teeth. Temporomandibular joint (TMJ) disorders cause pain to patients during talking or chewing. TMJ disorders are commonly found in patients with physical, emotional, and social problems, as well as in individuals who experience work-related stress. Many types of TMJ have an excellent prognosis if treated.

KEY TERMS

Gingiva— The gum tissue surrounding the teeth.

Mandible— The bone that helps make up the lower jaw of the mouth.

Mastication— Chewing and biting of the food in the mouth.

Maxilla— The bone that helps make up the upper jaw of the mouth.

Occlusal— Biting surface of the teeth.

Resources

BOOKS

Karst, N.S., and S.K. Smith. Dental Anatomy: A Self-Instructional Program, 10th ed. Stamford, CT: Appleton and Lange, 1998.

ORGANIZATIONS

Adam.com Health and Medical Association Online. adam.com; 90 Tehama Street, SF, CA 94105. (415) 541-9164. 〈http://www.adam.com/home.htm〉.

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

National Institute of Dental & Craniofacial Research. Building 45, Room 4AN-24, 45 Center Drive, MSC 6402, Bethesda, MD 20892-6402. (301) 594-9898. 〈http://www.nidcr.nih.gov/research/extramural/cranio2.htm〉.

Oregon Health & Science University. 3181 SW Sam Jackson Park Rd., Portland, Oregon 97201. (503) 494-3462. 〈http://www.teleport.com/∼bobh/tongue_anatomy.htm〉.

OTHER

Ladd Dental. Dental Anatomy of a Tooth. 2333 W Lincoln Rd, Kokomo, IN 46902. (765) 455.0085. 〈http://www.ladddental.com/AnatomyOfATooth.htm〉.

Mediplus Online Medical Resources. Adam.com. 〈http://medlineplus.adam.com/imagepage/9445.htm〉.

Nangle, Simon J. Dental Wisdom. 〈http://dentalwisdom.com/training/anatomy/traininganatomy1.html〉.

National Institute of Dental and Craniofacial Research National Institutes of Health. Pamphlet. Brochure for TMJ anatomy. 〈http://www.nidr.nih.gov/news/pubs/tmd/main.htm〉.

The National Women's Health Information Center. TMD/TMJ (Jaw disorders). 〈http://www.4woman.gov/faq/TMDTMJJawDisorders.htm〉.

Nirog.com. Online Magazine for Medical and Dental Professionals. 〈http://nirog.com/dental/anatomy/Dental_Anatomy.htm/〉.

Project Micro Images. 〈http://resolution.umn.edu/MMS/ProjectMicro/Explorations/Jan1998.html〉.

Wyche, Haywood E., DDS. Dental Anatomy. 1145 19th Street, N.W. Suite 316, Washington, DC 20036. 〈http://www.wychedent.com/index.htm〉.