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2-5    TOOTH MORPHOLOGY

This section describes the external features of the teeth. A tooth is defined as "one of the hard, bony appendages that are borne on the jaws. ..and serve for the seizing and mastication of food, as weapons of offense and defense, etc." In man and the lower animals, the design of the teeth are a reflection of eating habits. Animals, classified according to their eating habits, are carnivorous (flesh eating), herbivorous (plant eating), or omnivorous (eating everything; both flesh and plant).

TYPES OF TEETH

Man is omnivorous, so his teeth are formed for cutting, tearing, and grinding food. The human permanent dentition is divided into four classes of teeth based on appearance and function or position. Figure 2-11 illustrates the types and working surfaces of the four classes of teeth.

Figure 2-11.—Working surfaces of human teeth.

Incisors

Incisors are named because they are used to incise food. They are located in the front of the mouth and have sharp, thin edges for cutting. The lingual surface can have a shovel-shaped appearance.

• Cuspids--cuspids, also referred to as canines, are at the angles of the mouth. Each has a single cusp in stead of an incisal edge and are designed for cutting and tearing.
• Bicuspids--Bicuspids, also referred to as premolars, are similar to the cuspids. They have two cusps used for cutting and tearing, and an occlusal surface that is wider to crush food.
• Molars--Molars are located in the back of the mouth; their size gradually gets smaller from the first to third molar. Each molar has four or five cusps, is shorter and more blunt in shape than other teeth and provides a broad surface for grinding and chewing solid masses of food.

DENTAL ARCHES

The teeth of the upper arch are called maxillary teeth, (fig. 2-12) because their roots are embedded within the alveolar process of the maxilla. Those of the lower arch are called mandibular teeth because their roots are embedded within the alveolar process of the mandible. Each arch contains 16 teeth. The teeth in an arch are composed of 6 anteriors (cuspid to cuspid) and 10 posterior (all teeth distal to the cuspids). In a quadrant, there are 3 anterior and 5 posterior teeth.

Figure 2-12.—Maxillary and mandibular arches showing relationship of the bones and teeth.

DENTAL QUADRANTS

Each dental arch is divided into a right and a left quadrant. The quadrants are formed by an imaginary line called the midline that passes between the central incisors in each arch and divides the arch in half (fig. 2-13). There are four quadrants in the mouth (two per arch) that divide the mouth into four equal parts. Quadrant means one fourth, and each quadrant is one fourth of the entire mouth. Teeth are described as being located in one of the four quadrants: right maxillary quadrant, left maxillary quadrant, right mandibular quadrant, or the left mandibular quadrant.

Figure 2-13.—Maxillary and mandibular arches divided into quadrants.

LOCATION OF THE TEETH

Normally, a human receives two sets of teeth during a lifetime. The first (deciduous or primary) set consists of 20 teeth ("baby" teeth). The second (permanent) set usually consists of 32 teeth. In each quadrant, there are eight permanent teeth: two incisors, one cuspid, two bicuspids, and three molars (fig. 2-14). The tooth positioned immediately to the side of the midline is the central incisor, so called because it occupies a central location in the arch. To the side of the central incisor is the lateral incisor. Next is the cuspid, then the two bicuspids (the first bicuspid, followed by the second bicuspid). The last teeth are three molars. After the second bicuspid comes the first molar, followed by the second molar, followed by the third molar or more commonly called the "wisdom tooth."

Figure 2-14.—Names of the teeth in the right maxillary and mandibular quadrants; anterior and posterior teeth.

Another method of describing the location of teeth is to refer to them as anterior or posterior teeth (fig. 2-15). Anterior teeth are those located in the front of the mouth, the incisors, and the cuspids. Normally, these are the teeth that are visible when a person smiles. The posterior teeth are those located in the back of the mouth-the bicuspids and molars.

IDENTIFICATION OF TEETH

To avoid confusion, you must identify a tooth as completely as possible. Give its full name: Central incisor (not incisor), second molar (not molar), etc. But even the full name of a tooth does not provide adequate identification because several teeth have the same name. Complete tooth identification requires that you identify:

• the quadrant in which the tooth appears, and
• the full name of the tooth.

Therefore, you would identify a specific second molar in the following manner: right mandibular second molar. Although there are four second molars in the mouth, naming the quadrant (right mandibular) narrows the field down to one specific second molar.

UNIVERSAL NUMBERING SYSTEM

The Universal Numbering System is a simplified method of identifying teeth that is approved by the American Dental Association and used by the armed services. This method employs numbers with each tooth designated by a separate number from 1 to 32. Figure 2-15 illustrates the numbering system used on a Standard Dental Chart. When charting, you would refer to a tooth by number rather than the name. Instead of referring to the right maxillary third molar, you would refer to tooth No. 1. Each permanent tooth has its own number.

Figure 2-15.— Standard dental chart; names and numbers of teeth.

The 20 primary teeth are identified on the dental chart by the use of capital letters A to T. Lettering starts with upper right second primary molar (tooth A, located above the root of the maxillary second premolar); goes across to the upper left second primary molar (tooth J); down to the lower left second primary molar (tooth K), and across to the lower right second primary molar. Please note that the letters of the primary second and first molars appear above the roots of the permanent teeth of the second and first premolars.

When using aa dental form, remember that the right and left sides are reversed. The right side of the patient’s mouth appears on the left side of the dental chart; the left side of the patient’s mouth appears on the right side. This arrangement is necessary because the dental officer and the assistant see the sides reversed when they look into a patient’s mouth.

SURFACES OF THE TEETH

Not only must the assistant be able to name and locate a tooth, but must also be able to identify the different types of tooth surfaces. Figure 2-16 shows a number of different surfaces of the teeth.

Figure 2-1 6.—Surfaces of the teeth.

Facial, Mesial, Distal, Lingual, and Incisal Surfaces

The facial is the surface of a tooth that "faces" toward the lips or cheeks. When there is a requirement to be more specific, terms like labial and buccal are used. The labial is the surface of an anterior tooth that faces toward the lips. The buccal is the surface of a posterior tooth that faces toward the cheek.

The mesial is the proximal surface closest to the midline of the arch. The distal is the opposite of mesial. The distal is the proximal surface oriented away from the midline of the arch.

The lingual is the surface of an anterior or posterior tooth that faces toward the tongue. Incisal edges are narrow cutting edges found only in the anterior teeth (incisors). Incisors have one incisal edge.

Proximal Surfaces

A tooth has two proximal surfaces (fig 2-17), one that is oriented toward the midline of the dental arch (mesial) and another that is oriented away from the midline of the arch (distal). Other important surfaces of the proximal area are discussed in the following paragraphs.

Figure 2-17.—Proximal tooth surfaces and spaces.

• CONTACT POINT.—The point on the proximal surface where two adjacent teeth actually touch each other is called a contact point. An example of a contact point is when you pass dental floss in between two teeth. You should feel some resistance of the contact point while the floss is being passed through.
• INTERPROXIMAL SPACE.—The inter-proximal space is the area between the teeth. Part of the interproximal space is occupied by the interdental papilla. The interdental papilla is a triangular fold of gingival tissue. The part of the interproximal space not occupied is called the embrasure.
• EMBRASURE. —The embrasure occupies an area bordered by interdental papilla, the proximal surfaces of the two adjacent teeth, and the contact point (fig 2-18). If there is no contact point between the teeth, then the area between them is called a diastema instead of an embrasure.

Figure 2-1 8.—Embrasure.

Occlusal

The occlusal surface is the broad chewing surface found on posterior teeth (bicuspids and molars).

To get a clearer picture of the various tooth surfaces, refer to figure 2-15, which has previously been discussed. The Dental Chart shows each of the teeth "unfolded" so that the facial, occlusal, incisal, or lingual surfaces of the teeth can be shown. For posterior teeth, the facial surfaces are shown adjacent to the roots, followed by the occlusal surfaces, and then by the lingual surfaces (which are located next to the numbers on the chart). For the anterior teeth, the facial surfaces are shown as a line between the facial and lingual surfaces. The lingual surfaces are located next to the numbers on the chart.

• OCCLUSION.—Occlusion is the relationship between the occlusal surfaces of maxillary and mandibular teeth when they are in contact. Many patterns of tooth contact are possible. Part of the reason for the variety is the mandibular condyle’s substantial range of movement within the temporal mandibular joint. Malocclusion occurs when any abnormality in occlusal relationships exist in the dentition. Centric occlusion, figure 2-19, is the centered contact position of the chewing surfaces of mandibular teeth on the chewing surface (occlusal) of the maxillary teeth.

Figure 2-1 9.—Centric occlusion.

• OCCLUSAL PLANE.—Maxillary and mandibular teeth come into centric occlusion and meet along anteroposterior and lateral curves. The anteroposterior curve is called the Curve of Spee (fig. 2-20) in which the mandibular arch forms a concave (a bowl-like upward curve). The lateral curve is called the Curve of Wilson (fig. 2-21). The composite (combination) of these curves form a line called the occlusal plane, and is created by the contact of the upper and lower teeth as shown in figure 2-22.

Figure 2-22.—Occlusal plane.

• VERTICAL AND HORIZONTAL OVERLAP.—Vertical overlap is the extension of the maxillary teeth over the mandibular counterparts in a vertical direction when the dentition is in centric occlusion (fig. 2-23). Horizontal overlap is the projection of maxillary teeth over antagonists (something that opposes another) in a horizontal direction.

Figure 2-23.—Vertical and horizontal overlap.

• ANGLES CLASSIFICATION.—Angle was a dentist who developed a classification of normal and abnormal ways teeth meet into centric occlusion. Angle came up with three classes, Class I, II and III, as illustrated by figure 2-24.

Class I—patient’s profile is characterized as normal.

Class II—patient’s profile is deficient in chin length and characterized as a retruded (retrognathic) profile.

Class III—patient’s profile is excessive in chin length and characterized as protruded (prognathic) profile.

Figure 2-24.—Angle’s classification.

• KEY TO OCCLUSION.—The occlusal surfaces of opposing teeth bear a definite relationship to each other (fig. 2-25). In normal jaw relations and when teeth are of normal size and in the correct position, the mesiofacial cusp of the maxillary first molar occludes in the facial groove of the mandibular first molar. This normal relationship (fig. 2-26) of these two teeth is called the key to occlusion.

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