| SECTION VI | INTERESTING VARIATIONS AND ETHNIC DIFFERENCES IN INCISORS |
There is great morphologic variation in the maxillary lateral incisor. It may be missing altogether; it may resemble a small slender version of a maxillary central incisor; it may be quite asymmetrical; or it may be peg shaped (as seen later in the chapter on anomalies).
Racial differences in the maxillary incisor teeth have been reported in dental literature. Shovel shape is the term commonly used to designate incisor teeth that have prominent marginal ridges and a deep fossa on their lingual surfaces (Fig. 2-28). A high incidence of shovel-shaped incisors has been observed in Mongoloid people, including many groups of American Indians.4,6,7,9 (Mongoloid pertains to a major racial division marked by a fold from the eyelid over the inner canthus, prominent cheekbones, straight black hair, small nose, broad face, and yellowish complexion. Included are Mongols, Manchus, Chinese, Koreans, Arctic coastal populations, Japanese, Siamese, Burmese, Tibetans, and American Indians.) White and black people are reported to have less frequent occurrences of this characteristic.
FIGURE 2-28. Shovel-shaped permanent incisors from a young Native American dentition (incisal views). Note the prominent marginal ridges on the lingual surface.
A study of the skulls of American Indians who lived in Arizona about 1100 AD has disclosed the occurrence of incisor teeth that have a mesial marginal ridge on the labial surface and a depression, or concavity, on the mesial part of the labial surface just distal to this ridge.10 In these teeth, the distal part of the labial surface is rounded in an unusual manner. Such teeth have been referred to as “three-quarter double shovel shaped,” a descriptive, if ponderous, term. Labial “shoveling” has also been reported in some Arctic coastal populations.
There is more uniformity of shape in the mandibular incisor teeth than in other teeth. In some Mongoloid people, the cingulum of mandibular incisors is characteristically marked by a short deep groove running cervicoincisally. This groove is often a site of dental caries.
Later, in the chapter on anomalies, you will read about more variations: peg-shaped incisors, fused mandibular incisors, congenitally missing central incisors, and even a lateral incisor merged distally to the canine.
LEARNING EXERCISE
Assign a Universal number to a handheld incisor:
Suppose a patient just had all of his or her permanent teeth extracted. Imagine being asked to find tooth 8 from among a pile of 32 extracted teeth on the oral surgeon’s tray because you wanted to evaluate a lesion seen on the radiograph on the root of that incisor. How might you go about it? Only after you have written down your own ideas, then consider the following steps:
- From a selection of all permanent teeth (extracted teeth or tooth models), select only the incisors (based on class traits).
- Determine whether each incisor is maxillary or mandibular using arch traits. Review Table 2-2 if needed. You should never rely on only one characteristic difference between teeth to name them; rather, make a list of many traits that apply to a maxillary incisor, as opposed to only one trait that makes you think it belongs in the maxilla. This way, you can act as a detective and become an expert at recognition at the same time.
- Once you determine that the tooth is maxillary, position the root up; if it is mandibular, position the root down.
- Use appropriate traits in order to identify the facial surface. This will permit you to view the tooth as though you were looking into a patient’s mouth.
- Next, using type traits, determine the type of incisor you are holding (central or lateral). Refer to the tables and teeth in the figures throughout this chapter as needed.
- Next, determine which surface is the mesial. Refer to figures throughout this chapter as needed. While viewing the incisor from the facial and picturing it within the appropriate arch (upper or lower), the mesial surface can be positioned toward the midline in only one quadrant, the right or left.
- Once you have determined the quadrant, assign the appropriate Universal number for the incisor in that quadrant. For example, the central incisor in the upper right quadrant is tooth 8.
REVIEW Questions about Incisors
For each of the traits listed below, select the letter(s) of the permanent incisor(s) that normally exhibit(s) that trait. More than one answer may apply.
- Maxillary central incisor
- Maxillary lateral incisor
- Mandibular central incisor
- Mandibular lateral incisor
|
a b c d a b c d a b c d a b c d a b c d a b c d a b c d a b c d a b c d a b c d |
ANSWERS: 1–a, b; 2–a, d; 3–c, d; 4–c, d; 5–a, b, d; 6–a; 7–a; 8–c; 9–a; 10–c
CRITICAL Thinking
1. The only way to master the many traits of the incisors presented in this chapter is to be able to picture each trait in your mind for each type of incisor and for each side of the mouth. Therefore, even though you have probably already looked carefully at each illustration in this chapter, at this time, reread the legends and study each figure in this chapter. If any facts are unclear, review the portion of the chapter that referred to that figure. Also, use the front and back of Appendix pages 1 and 2 to review all identified traits of incisors.
2. While you are recording which teeth are present in the mouth of Mrs. Jenny James, you notice that she has only three mandibular incisors. How might you determine which specific mandibular incisors are still present? Think of things you have learned about incisors, and try to recall facts you may already know about landmarks in the mouth.
3. Look at your mouth in the mirror while you place your anterior maxillary and mandibular teeth edge to edge, and align the arch midlines (the proximal contacts between central incisors) over one another. If your teeth are of average shape and alignment, notice that the distal outline of each maxillary central incisor extends distal to its opposing mandibular central incisor because the maxillary central is wider. Also, notice that the maxillary central incisors are wider and larger than the maxillary lateral incisors and that the maxillary central incisors and maxillary lateral incisors are wider than either the mandibular central or lateral incisors. Finally, note that the mandibular central incisors appear slightly narrower and smaller than the adjacent mandibular lateral incisors.
4. Using a good light source (like a small flashlight), a large mirror (magnifying if possible), and a small, clean disposable dental mirror, carefully compare the maxillary and mandibular incisors in your own mouth while referring to the traits in Table 2-2 from the labial view and lingual view that can be used to differentiate maxillary from mandibular incisors. Write down each trait that can be useful to differentiate the maxillary from the mandibular incisors in your own mouth, and also make note of any of the traits in the text book that do not apply in your mouth.
5. Perform a computer search for images of “close-ups of celebrity teeth,” and answer the following questions for at least five of the images. Remember that many of these celebrities have had major cosmetic dental work, so you may not be viewing their original teeth.
a. Are their maxillary central incisors wider than their maxillary lateral incisors?
b. Can you see evidence of formation from three facial lobes on any of their anterior teeth?
c. Try to find at least one picture where you can see the mandibular incisors, and answer these questions. Are the mandibular incisors narrower than the maxillary incisors? Are their mandibular centrals narrower than the laterals? Do you see any spaces (diastemas) between teeth? If not, try looking for images of Madonna, the early years, for the diastema between her central incisors (which she had closed during her later years).
6. Search the Internet for images of “orthodontics before and after,” and answer the same questions as in exercise #4 above for the “before” pictures and for the “after” pictures.
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