|SECTION IV||TYPE TRAITS THAT DIFFERENTIATE MAXILLARY CENTRAL FROM LATERAL INCISORS (FROM ALL VIEWS)|
OBJECTIVES FOR SECTION IV
This section is designed to prepare the learner to perform the following:
- Describe the type traits that can be used to distinguish the permanent maxillary central from lateral incisor.
- Describe and identify the labial, lingual, mesial, distal, and incisal surfaces for all maxillary incisors.
- Assign a Universal number to maxillary incisors present in a mouth (or on a model) with complete dentition. If possible, repeat this on a model with one or more maxillary incisors missing.
- Select and separate maxillary incisors from a selection of all teeth on a bench top.
- Holding a maxillary incisor, determine whether it is a central or a lateral and right or left. Then assign a Universal number to it.
Within this section, type traits are presented that distinguish maxillary central from lateral incisors. Other traits will be helpful when distinguishing right maxillary incisors from left maxillary incisors. These traits are presented for each view of the tooth: facial, lingual, proximal (includes mesial and distal traits), and incisal.
Ideally, examine several extracted maxillary central and lateral incisors or tooth models as you read the following. Hold these teeth root up and crown down, as they are positioned in the mouth. Also, refer to Appendix page 2 and refer to Figure 2-7 as you study the labial traits of incisors.
1. Crown Shape of Maxillary Incisors (Labial View)
Based on Woelfel’s studies, the crown of the maxillary central incisor is the longest of all human tooth crowns (although at least two other authors describe the mandibular canine crown as the longest crown overall2,3). The maxillary central also has the widest crown of all incisors. The crown is usually longer (incisogingivally) than wide (mesiodistally)A (Appendix 2a).
The crown of the maxillary lateral incisor is also longer incisocervically, but it is considerably narrower mesiodistally than the crown of the maxillary central incisor. Since its root is longer, the entire lateral has a longer, more slender lookB (Appendix 2a). The crown outline is less symmetrical than on the central incisor. Normally, the contour of the labial surface of the maxillary lateral incisor is more convex mesiodistally compared to the maxillary central. This may be best appreciated from the incisal view. Mamelons, and particularly labial depressions, are less prominent and less common on the maxillary lateral incisor than on the central incisor.
2. Proximoincisal Line Angles of the Maxillary Incisor (Labial View)
On the outline of maxillary central incisors, the angle formed by the mesial and incisal surfaces (called the mesioincisal angle) is close to a right angle. The distoincisal angle is more rounded, and the angle is slightly obtuse or greater than a right angle (Appendix 2b).
On maxillary lateral incisors, both the mesioincisal and distoincisal angles are more rounded than on the central incisor (Appendix 2b). The mesioincisal angle is more acute (sharper) than the distoincisal angle, accentuated by the incisal edge sloping cervically toward the distal, more so than on the maxillary central incisor (Appendix 2c).
3. Proximal Contact Most Cervical on Distal of Laterals (Labial View)
The mesial contacts of both the maxillary central and lateral incisors are in the incisal third, very near the incisal edge for the central and slightly more cervical on the lateral. The distal contacts of both incisors are more cervical than the mesial. For a maxillary central incisor, the distal contact is near the junction of the incisal and the middle thirds; for the maxillary lateral incisor, it is even more cervical, often in the middle third (making this distal contact the most cervical for any incisor, seen clearly on a lingual view in Fig. 2-8).
4. Maxillary Central Incisors Have Relatively Shorter Roots (Labial View)
On a maxillary central incisor, the root is only slightly longer than the crown resulting in a root-to-crown ratio that is the smallest of any permanent tooth (Appendix 2d). The maxillary lateral incisor root is longer than on the central.C This results in a root that appears longer in proportion to the crown than on the maxillary central incisor.
5. Root Thinner and Longer on Lateral Incisors (Labial View Compared with the Proximal View)
The root of the maxillary central incisor is thick in the cervical third and narrows through the middle to a blunt apex. Its outline and shape is similar to an ice cream cone. An apical bend is not common in the maxillary central incisor. The central incisor root is the only maxillary tooth that is as thick at the cervix mesiodistally as faciolingually. Compare the root width seen on the mesial view to the root width seen on the labial view in Appendix 2n. All other types of maxillary teeth have roots that are thicker faciolingually than mesiodistally.D Because of its shortness (a small root-to-crown ration) and conical shape, the maxillary central incisor root may be a poor choice to support a replacement tooth as part of a dental bridge, that is, a replacement tooth crown attached to, and supported by, two adjacent teeth.
The root of a maxillary lateral incisor is wider faciolingually than mesiodistally. It tapers evenly toward the rounded apex, and the apical end is commonly bent distally (seen in 12 of the 14 maxillary lateral incisors in Fig. 2-7, lower row). This potential for distal (or mesial) bend on most roots must be confirmed on radiographs and considered when extracting an incisor.
Refer to Figure 2-9 while studying the lingual traits of maxillary incisors.
1. Lingual Fossae Deeper on Maxillary Lateral Incisors (Lingual View)
On maxillary incisors, large lingual fossae are located immediately incisal to the cingulum and bounded by two marginal ridges. The lingual fossa of the maxillary lateral incisor, although smaller in area, is often even more pronounced than on the central incisor, and it is more likely to have a pit in the deep fossa that is prone to decay (Fig. 2-10A). Note the deeper lingual fossae on many maxillary lateral incisors compared to central incisors in Figure 2-9. Although not common, accessory grooves may extend from the lingual fossa around the cingulum and onto the root surface resulting in an increased risk of decay and gingival disease (Fig. 2-10B).
2. Cingulum of Maxillary Incisors (Lingual View)
The cingulum on the maxillary central incisor is usually well developed and is located off-center, distal to the root axis line that bisects the root longitudinally. (This can also be seen from the incisal view.) The cingulum on the maxillary lateral incisor is narrower than on the central, and it is almost centered on the root axis line. This difference in location of the cingula, more distal on the central and more centered over the root on the lateral, is best seen incisally in Appendix 2e and is highlighted on many teeth in Figure 2-9.
3. Marginal Ridges of Maxillary Incisors (Lingual View)
The mesial and distal marginal ridges vary in prominence on all maxillary incisors from one person to another. They may be prominent or inconspicuous, possibly worn smooth from attrition. One type of maxillary incisor with a deep lingual fossa and prominent mesial and distal marginal ridges is called “shovel-shaped incisors”E (Fig. 2-11).4–8
On the both types of maxillary incisors, mesial marginal ridges (from the incisal edge to the cingulum) are longer than the distal marginal ridges due in part to the taper of the incisal edge from mesial to distal (Appendix 2f and Fig. 2-11). This is also accentuated on the maxillary central incisor because its cingulum is off center toward the distal. Further, the shorter distal marginal ridges are more curved compared to the mesial marginal ridges that are straighter incisocervically (seen in Fig. 2-11 and on most incisors in Fig. 2-9).
4. Pits on Maxillary Incisors (Lingual View)
On both types of maxillary incisors, but more frequently in lateral incisors, a lingual pit may be detectable at the incisal border of the cingulum where the mesial and distal marginal ridges converge. This pit may need to be restored or sealed in order to prevent or arrest decay. (Notice the deep lingual pits in Fig. 2-10A and on several maxillary lateral incisors in Fig. 2-9.)
5. Accessory Ridges and Grooves on Maxillary Incisors (Lingual View)
Small, narrow accessory lingual ridges separated by subtle grooves may extend vertically from the cingulum toward the center of the lingual fossa in both types of maxillary incisors (fewer in maxillary laterals). There could be from one to four accessory ridges. Tooth 9 in Figure 2-12 shows these accessory ridges most clearly. Tiny grooves separate these ridges.F,G
6. Root Contour of Maxillary Incisors (Lingual View)
The root contour of all maxillary incisors, like all anterior teeth, is convex and tapers, becoming narrower toward the lingual surface so that some of the proximal surfaces are visible from the lingual view (Fig. 2-9).
Refer to Figure 2-13 while studying the proximal traits of maxillary incisors.
1. Incisal Edges Labial to Root Axis: Maxillary Central Has Slight Distolingual Twist (Proximal Views)
On both maxillary incisors, the incisal edge is commonly just labial to the root axis line or may be on the root axis line (Appendix 2o). From the mesial side, the slight distolingual twist of the incisal ridge of the maxillary central incisor places the distal portion at the ridge even somewhat more lingual than on the mesial (best appreciated from the incisal view in Appendix 2g and Fig. 2-14).
2. Cervical Line of Maxillary Incisors (Proximal Views)
As on all anterior teeth, the cervical line of both types of maxillary incisors curves incisally on the mesial and distal tooth surfaces, and this curvature is greater on the mesial surface than on the distal surface, as can be seen on the lingual surface of a central incisor in Figure 2-15. To confirm this difference in curvature, it is often necessary to turn the tooth slightly to view the amount of curvature on the mesial and then turn it the other way to confirm the amount of curvature on the distal. This difference is most pronounced on anterior teeth. The amount of curvature of the cervical line on the mesial of the maxillary central incisor is larger than for any other tooth, extending incisally one fourth or more of the crown length (Fig. 2-16), whereas the distal cervical line curves less. The curvature of the mesial cervical line of the maxillary lateral incisor is also considerable but slightly less than on the central.H
3. Root Shape and Root Depressions of Maxillary Incisors (Proximal Views)
The root of the maxillary central incisor is wide faciolingually at the cervix and tapers to a rounded apex. The lingual root outline is nearly straight in the cervical third and then curves labially toward the tip in the middle and apical thirds. The entire labial root outline is even straighter. From the proximal view, this flatter facial root outline and more convex lingual root outline are evident in many central incisors in Figure 2-13. In contrast, the root of the maxillary lateral incisor tapers more evenly throughout the root toward the blunt apex.
The mesial root surfaces of both types of maxillary incisors could have a slight depression in the middle third cervicoapically, slightly lingual to the center faciolingually, but the distal root surfaces are likely to be convex. A slight mesial root depression is discernible in the shaded line drawings in Figure 2-13.
Refer to Figure 2-17 when studying the incisal view. To follow this description, a maxillary incisor should be held in such a position that the incisal edge is toward you, the labial surface is at the top, and you are looking exactly along the root axis line. You should see slightly more lingual surface than labial surface if the incisal ridge is located somewhat labial to the root axis line (as in many teeth, especially the lateral incisors, in Fig. 2-17).
1. Crown Proportion Faciolingually versus Mesiodistally for Maxillary Incisors (Incisal View)
The incisal outline of the maxillary central incisor is noticeably wider mesiodistally than faciolingually (Appendix 2h). The mesiodistal measurement of the lateral incisor crown is also most often greater than the labiolingual measurement but less so than on the central incisor.I On some lateral incisors, the two dimensions of the crown are almost the same size faciolingually as mesiodistally. Notice this difference in the proportion of maxillary central incisors (relatively wider mesiodistally) compared to lateral incisors in Figure 2-17.
2. Crown Outline Shape and Cingulum Location of Maxillary Incisors (Incisal View)
The incisal outline of the maxillary central incisor is somewhat triangular. The labial outline is broadly curved (on some teeth, the middle third may be nearly flat) forming the base of the triangle, and the other two sides of the triangle converge toward the cingulum (Fig. 2-18). As was seen from the lingual view, the cingulum of the maxillary central incisor is slightly off-center to the distal, resulting in the mesial marginal ridge measuring longer than the distal marginal ridge (seen best from the lingual view in Appendix 2f).
The crown of the lateral incisor resembles the central incisor from this aspect, but its outline is more round or oval than triangular since the labial outline is noticeably more convex than on the central incisor. The cingulum of the lateral incisor is nearly centered mesiodistally. Compare the triangular shape of the maxillary central incisor to the more round or slightly oval shape of the maxillary lateral incisor in Figure 2-18. These differences in outline shapes are also evident in all teeth in Figure 2-17.
3. Maxillary Central Incisor May Have a Very Slight Distolingual Twist (Incisal View)
The incisal ridge or edge of the maxillary central incisor is thick faciolingually (1.5 to 2 mm) and is slightly curved from mesial to distal, the convexity being on the labial side. It terminates mesially and distally at the widest portion of the crown (Appendix 1q). The position of the distoincisal angle may be slightly more lingual than the position of the mesioincisal angle, which then gives the incisal edge its slight distolingual twist as though someone took the distal half of the incisal edge and twisted it to the lingual (Appendix 2g and Fig. 2-14). Be aware that for maxillary central incisors, the two traits just discussed (the cingulum displaced to the distal and the distolingual twist of the incisal edge) are dependent on how the tooth is held. When viewed from the incisal, the distolingual twist of the incisal edge is more obvious when the cingulum is aligned vertically (Appendix 2g), whereas the displacement of the cingulum to the distal is more obvious when the incisal edge is aligned horizontally (Appendix 2e). This is why these two traits are shown on page 2 of the Appendix, showing two views of the same tooth, each having a slightly different alignment to accentuate the trait being discussed.
The incisal ridges on lateral incisors are straighter mesiodistally than on the central incisors.
In determining a right from a left maxillary central incisor when it is not in the mouth (such as on the operatory table with other incisors after multiple extractions), you need to distinguish the mesial from the distal surface. If you look at the facial surface of a tooth with its root aligned correctly for the correct arch and are able to identify the mesial or distal surface, you can place the tooth in its correct quadrant (right or left) and assign its Universal number. Evaluate the rows of maxillary incisors in the figures in this chapter, and, using the chart in each figure, see how many “right from left” traits can be used to differentiate the mesial from the distal surfaces and therefore right from left incisors. For example, in Figure 2-7, look at the labial surfaces for the shape of the incisal angles (more rounded on distal) and the position of the contact areas (more cervical on distal), or look at the proximal surfaces in Figure 2-13 for the amount of cervical line curvature on the mesial and distal sides (more curved on mesial). From the lingual view, look at Figure 2-9 for the length of the marginal ridges (mesial is longer), and from the incisal view on the maxillary central (Fig. 2-17), look for the distal location of the cingulum on many maxillary central incisors.