| SECTION VII | TERMINOLOGY RELATED TO THE IDEAL TOOTH ALIGNMENT OF TEETH IN DENTAL ARCHES |
When viewed from the occlusal aspect, the alignment of teeth within each dental arch is somewhat U shaped or parabolic like the famous landmark in Missouri, the St. Louis Arch (Fig. 1-45). The incisal edges and the buccal cusp tips follow a curved line around the outer edge of the dental arch; the lingual cusp tips of the posterior teeth follow a curved line nearly parallel to the buccal cusp tips. Between the buccal and lingual cusps of posterior teeth is the sulcular groove (occlusal sulcus), a V-shaped depression that extends anteroposteriorly through all of the posterior teeth in each quadrant. This sulcular groove is made up of the occlusal sulcuses of adjacent posterior teeth in each quadrant (Fig. 1-45).
FIGURE 1-45. This model of the maxillary dentition with ideal alignment form an arch shape and the rope of blue wax on the left half falls within the occlusal sulcus.
A. MIDROOT AXIS LINE AND TOOTH ALIGNMENT
The midroot axis line (or root axis line) is an imaginary line through the center of the tooth root. When viewing the facial or lingual surface, it can be visualized as a line that divides the bulk of the root into mesial and distal halves (Fig. 1-46A). When viewing the mesial or distal surface, it divides the bulk of the root into facial and lingual halves (Fig. 1-46B). It is an important reference line for describing the location of tooth landmarks. For example, you will learn that the incisal ridge (or cusp tip) of a maxillary canine is more likely to be labial to the midroot axis line (as seen in Fig. 1-46B).
FIGURE 1-46. The midroot axis line is drawn on two views of a canine. A. When viewed from the facial (or lingual), the greatest bulges on the mesial and distal surfaces (crests of curvature) on this canine are the widest points on the crown that touch lines parallel to the midroot axis line. These crests of curvature are essentially the same as the contact areas of teeth when they are aligned ideally in the mouth. Notice that these contact areas are positioned more incisally on the mesial surface than on the distal. B. When viewed from the proximal, the greatest bulges on the facial and lingual surfaces (crests of curvature) are the points on the facial and lingual crown outline that touch lines that are parallel to the midroot axis line. They are located in the cervical third on both the facial surface and on the lingual surface (on the cingulum shaded green) for all anterior teeth.
When the posterior teeth in each arch are viewed from the buccal aspect as in Figure 1-47, notice that posterior teeth are not aligned exactly parallel to one another and all cusp tips in a quadrant do not normally fall along a ruler-straight line. The axial alignment changes gradually from posterior to anterior teeth, which is evident when you compare the alignment of the mandibular third molar (tipped noticeably more to the mesial) than on the first premolar. Subsequently, if you were to connect the buccal cusp tips with a line, a gradual anteroposterior curve (curve of Spee) is evident (see Fig. 1-47). This curve is convex in the maxillary arch, while the curve is concave in the mandibular arch.
FIGURE 1-47. A wax strip placed between stone models of the maxillary and mandibular teeth demonstrates the anteroposterior curve (curve of Spee), which is concave in the mandibular arch but convex in the maxillary arch. Note the difference in the axial alignment of the teeth within each arch demonstrated by the axial lines placed on the third molars and on the first premolars.
When viewed from the distal, maxillary posterior teeth are axially tilted facially within the maxillary arch, whereas mandibular posterior teeth are tilted lingually within the mandibular arch (Fig. 1-48). Therefore, in the mouth, lingual cusps of maxillary posterior teeth appear longer than the buccal cusps, while the lingual cusps of mandibular posterior teeth appear shorter than the buccal cusps due to the lingual tilting within the mandible. However, to avoid confusion, you need to realize that later in the book, you will learn that when you hold a mandibular molar in your hand with the midroot axis aligned vertically, the lingual cusps of mandibular molars are actually slightly longer than buccal cusps (Fig. 1-49). When a line connects the buccal and lingual cusps of the same type of molars and premolars on opposite sides of the arch, this side-to-side curve is the mediolateral curve (of Wilson). The mediolateral curve of the maxillary arch is convex, whereas that of the mandibular arch is concave (Fig. 1-48).
FIGURE 1-48. Dental stone casts viewed from the distal with a wax strip used to demonstrate the mediolateral curve (of Wilson). It is convex in the maxillary arch, but concave in the mandibular arch. Note the red lines that denote posterior tooth alignment within each arch: maxillary molar crowns tilt toward the facial, and mandibular molar crowns tilt toward the lingual.
FIGURE 1-49. This mandibular second molar with its mid-root axis aligned vertically (left drawing) has lingual roots that appear longer, whereas the same tooth with the midroot axis tipped lingually, as it would be in the mandible, has lingual cusps that appear shorter.
B. CREST OF CURVATURE (HEIGHT OF CONTOUR) ON THE FACIAL AND LINGUAL SURFACE
The shape and extent of the greatest bulge on the facial and lingual crown surfaces help determine the direction that food particles are deflected as they are crushed between tooth surfaces when chewing. When we chew food, these natural tooth convexities divert food away from the thin free gingiva and gingiva sulcus surrounding the cervix of the tooth and toward the firmer tissues of the mouth, thus minimizing trauma to the gingiva. If teeth were flat facially and lingually, food could more likely damage the gingiva (Fig. 1-50). Needless to say, it is best for the dentist, dental hygienist, and/or dental technician to reproduce and maintain these natural convexities when restoring a tooth, when finishing and polishing fillings near the gum line, or when contouring a replacement tooth crown.
FIGURE 1-50. A. Normal facial and lingual crests of curvature help divert food away from the gingival sulcus. B. When crests of curvature are not adequate, food can more readily damage the gingival sulcus.
The facial or lingual crest of curvature (height of contour) is the point on a crown outline where a line drawn parallel to the midroot axis line touches the greatest bulge on the crown (Fig. 1-51). It is usually located in either the cervical third or the middle third, normally not in the occlusal or incisal third. When viewed from the proximal, the location of the crest of curvature on the facial surface on both anterior and posterior tooth crowns is normally located in the cervical third. The location of the lingual crest of curvature depends on whether the tooth is anterior or posterior. The lingual crest of curvature on anterior teeth is in the cervical third, on the cingulum (Fig. 1-46B). The lingual crest of curvature on posterior teeth is most often located in the middle third (Fig. 1-51). Refer to Table 1-4 for a summary of the location of the facial and lingual heights of contour for anterior teeth compared to posterior teeth.
FIGURE 1-51. On the proximal view of this premolar, the buccal crest of curvature (height of contour) is located in the cervical third, while the lingual height of contour is located more occlusally, in the middle third. This is typical of most posterior teeth.
| TABLE 1-4 | Summary of the Location of Facial and Lingual Heights of Contour (Greatest Bulge) of the Crown (Best Seen from the Proximal View) |
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FACIAL (HEIGHT OF CONTOUR) |
LINGUAL (HEIGHT OF CONTOUR) |
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Anterior teeth (incisors and canines) |
Cervical third |
Cervical third (on cingulum) |
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Posterior teeth (premolars and molars) |
Cervical third |
In or near middle third |
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General learning guidelines: 1. Facial crest of curvature for all teeth is in cervical third. 2. Lingual crest of curvature for all anterior teeth is in the cervical third (on the cingulum). 3. Lingual crest of curvature for posterior teeth is in the middle third (slightly more occlusal in mandibular teeth due to the lingual tilt of the crown). |
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