| SECTION III | ARCH TRAITS THAT DIFFERENTIATE MANDIBULAR FROM MAXILLARY INCISORS |
Refer to page 2 of the Appendix while reading about these arch traits that can be used to differentiate mandibular from maxillary incisors.
A. MANDIBULAR INCISORS ARE SMALLER AND LOOK MORE ALIKE
Both types of mandibular incisors are generally smaller than either of the maxillary incisors (Appendix 2p). Mandibular central and lateral incisors look more alike and are more nearly the same size in the same mouth, compared to the larger sizes and greater differences between maxillary central and lateral incisors (best seen in the mouth in Fig. 2-2).
B. CONTACTS POSITIONED MORE INCISALLY ON MANDIBULAR INCISORS
From the facial view, the mesial and distal outlines are flatter on mandibular incisor crowns than on maxillary incisor crowns (compare maxillary to mandibular incisors in Appendix 2q) and have contact areas located closer to the incisal ridge than on maxillary incisors (Appendix 2i and 2r).
C. MANDIBULAR INCISOR CROWNS ARE WIDER FACIOLINGUALLY
From the incisal view, mandibular incisor crowns are relatively wider faciolingually than mesiodistally compared to maxillary central incisors, which are wider mesiodistallyO (compare incisal views in Appendix 2-h to 2-s).
D. MAXILLARY INCISORS HAVE PROMINENT LINGUAL ANATOMY
Mandibular incisor crowns have smoother lingual surfaces with less prominent anatomy than do maxillary crowns, which have deeper fossae and more pronounced marginal ridges (Appendix 2m and Fig. 2-5A and B).
FIGURE 2-5. A. The lingual anatomy of a right maxillary central incisor is typical for maxillary incisors, with prominent marginal ridges and cingulum, and a deep fossa. B. The lingual anatomy of this mandibular central incisor is typical of mandibular incisors since it has very faint marginal ridges and cingulum and a shallow fossa.
!E. MANDIBULAR INCISOR ROOTS ARE RELATIVELY LONGER
Mandibular incisor roots are longer in proportion to their crowns than are maxillary incisor roots, so mandibular incisors have larger root-to-crown ratios.
!F. MANDIBULAR INCISAL RIDGES ARE MORE LINGUAL
From the proximal view, incisal ridges of mandibular incisors are usually positioned lingual to the midroot axis line, whereas the incisal ridges of maxillary incisors are more often on or labial to the root axis line (best seen from the proximal views on Appendix 2o).
!G. INCISAL EDGE WEARS LABIALLY ON MANDIBULAR INCISORS
Tooth wear (attrition) on the incisal ridges of incisors that occurs when shearing or incising food results in tooth wear that is in a different location on maxillary incisors compared to mandibular incisors (Fig. 2-6). This wear occurs when the labial part of the incisal edges of mandibular incisors slides forward and downward while contacting the lingual surface and part of the incisal edge of opposing maxillary incisors. The wear results in a shiny, flat, polished surface of enamel on the incisal edge called a facet [FAS it]. Assuming a normal tooth relationship, wear facets on mandibular incisors form on the labial surface next to the incisal edge. In contrast, wear facets on maxillary incisors form on the lingual surface next to the incisal edge, possibly extending to the lingual marginal ridges.
FIGURE 2-6. A. Proximal view of the ideal relationship between maxillary and mandibular incisors when posterior teeth are biting tightly together. B. The arrow indicates the direction of movement of mandibular incisors when the mandible moves forward (protrudes) with the incisors touching until they align edge to edge. The resultant wear pattern or facets on the incisal edges of maxillary incisors occur more on the lingual surface on the incisal edge, whereas wear occurs more on the facial surface of the incisal edge on mandibular incisors.
LEARNING EXERCISE
Refer to Table 2-2 for a summary of the noticeable arch traits that distinguish maxillary from mandibular incisors, and see how many of them can be used to differentiate the rows of maxillary and mandibular incisors from various views in Figures 2-7, 2-9, 2-13, 2-17, 2-19, 2-22, 2-24, and 2-26.
| TABLE 2-2 | Major Arch Traits That Distinguish Maxillary from Mandibular Incisors |
|
|
Maxillary Incisors |
Mandibular Incisors |
|
LABIAL VIEW |
Wider crowns mesiodistally |
Narrower crowns mesiodistally |
|
Less symmetrical crown |
More symmetrical crowns |
|
|
More rounded mesial and distal incisal angles |
More square mesial and distal incisal angles |
|
|
Contact areas more cervical |
Contact areas very incisal |
|
|
LINGUAL VIEW |
Lingual anatomy more distinct: |
Lingual surface smoother: |
|
Pronounced marginal ridges |
Almost no marginal ridges |
|
|
Deeper lingual fossa |
Shallower lingual fossa |
|
|
Sometimes lingual pits |
No pits |
|
|
Larger cingulum |
Smaller cingulum |
|
|
Incisal edge on or labial to midroot axis line Facets on lingual slope of incisal edge |
Incisal edge on or lingual to midroot axis line Facets on labial slope of incisal edge |
|
|
Crowns wider mesiodistally than faciolingually |
Crowns wider faciolingually than mesiodistally |
|
|
Plus traits seen from lingual view can also be seen from the incisal view. |
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