|SECTION V||TYPE TRAITS THAT DISTINGUISH MANDIBULAR CENTRAL FROM LATERAL INCISORS (FROM ALL VIEWS)|
This section is designed to prepare the learner to perform the following:
- Describe the type traits that can be used to differentiate the permanent mandibular central from the lateral incisor.
- Describe and identify the labial, lingual, mesial, distal, and incisal surfaces for mandibular lateral incisors and the labial, lingual, and incisal surfaces for the symmetrical mandibular central incisor (where the mesial may be difficult to distinguish from the distal).
- Assign a Universal number to mandibular incisors present in a mouth (or on a model) with complete dentition. If possible, repeat this on a model with one or more mandibular incisors missing.
- Select and separate mandibular incisors from a selection of all teeth on a bench top.
- Holding a mandibular incisor, determine whether it is a central or a lateral and right or left. Then, assign a Universal number to it (which may not be possible for the symmetrical mandibular central incisor, which you could identify as tooth 24 or 25).
Within this section, type traits are presented that distinguish mandibular central incisors from lateral incisors. Other traits will be helpful when distinguishing right from left mandibular incisors. These traits are presented for each view of the tooth: facial, lingual, proximal (including mesial and distal), and incisal.
Examine several extracted teeth and/or models as you read. Also, refer to page 2 of the Appendix and Figure 2-19 while you study the labial surface of mandibular incisors. Hold mandibular teeth with the root down and crown up, the position of the teeth in the mouth.
1. Crown Shape of Mandibular Incisors (Labial View)
Three mamelons are often present on newly emerged mandibular incisors and reflect the formation of the facial surface from three labial lobes (Fig. 2-20). In most persons, they are soon worn off by functional contacts against the maxillary incisors (a process called attrition).
All mandibular incisor crowns are quite narrow relative to their crown length, but the mandibular central incisor crown is the narrowest crown in the mouth and is considerably narrower than the maxillary central incisor.J Unlike maxillary incisor crowns in the same mouth where the central is larger than the lateral, the mandibular lateral incisor crown is a little larger in all dimensions than the mandibular central incisor in the same mouth, as seen in Figure 2-21. Further, the mandibular central incisor is so symmetrical that it is difficult to tell lefts from rights unless in place in the mouth or on full arch models. About the only notable difference to be found is the greater mesial than distal curvature of the cervical line, but this is not a trait that can be seen in the mouth.
The crown of the mandibular lateral incisor resembles that of the mandibular central incisor, but it is slightly wider and is not as bilaterally symmetrical. Its crown appears to tilt slightly distally on its root, giving the impression that the tooth has been bent at the cervix (Appendix 2l). This tilt, plus the sloping of the incisal edge shorter on the distal (Appendix 1s), makes the curved distal outline of the crown from proximal contact area to cervical line shorter than the straighter mesial crown outline. Look at the incisors in Figure 2-19, and notice the lack of symmetry of the outline of most mandibular lateral incisors compared to the symmetry of the central incisors. When comparing these teeth, be aware that the incisal edges may have worn unevenly.
The labial surfaces of both types of mandibular incisors are most often smooth, but could have two shallow developmental depressions in the incisal third if you examine the surface closely.K
2. Incisal Proximal Angles of Mandibular Incisors (Labial View)
The crown of the mandibular central incisor is nearly bilaterally symmetrical, so the mesioincisal and distoincisal angles are very similar, forming nearly right angles (Appendix 2j). The distoincisal angle may barely be more rounded than the mesioincisal angle. The distoincisal angle of the mandibular lateral incisor, however, is noticeably more rounded than the mesioincisal angle (Appendix 2j). This helps to distinguish right mandibular lateral incisors from lefts prior to incisal tooth wear.
3. Proximal Contact Areas of Mandibular Incisors (Labial View)
The mesial and distal contact areas of the mandibular central incisor are at the same level: in the incisal third (Appendix 2i) almost level with the incisal edge. The mesial and distal contact areas of the lateral incisor are not at the same level (Appendix 2i). Although both the mesial and distal contacts are in the incisal third fairly close to the incisal edge, the distal contact is noticeably more cervical than the mesial contact on lateral incisors. Refer to Table 2-3 for a summary of the location of proximal contacts for all incisors.
|TABLE 2-3||Location of Proximal Contacts (Proximal Height of Contour) on Incisors (Best Seen from Facial View)|
General learning guidelines for incisors:
1. On the same incisor, the distal proximal contact is more cervical than the mesial contact EXCEPT on mandibular central incisors, where the mesial and distal contacts are at the same height.
2. All contacts for both types of mandibular incisors are in the incisal third, as are the mesial contacts on maxillary incisors. Distal contacts of maxillary central incisors are near the incisal/middle junction, and the distal contacts on maxillary lateral incisors are most cervical: in the middle third.