Summary of Physiological Contours of Tooth Crowns, Facially and Lingually
All tooth crowns exhibit some curvature above the cervical line. Again, this slight bulge at the cervical third is sometimes called the cervical ridge. Although the extent of curvature will vary in different individuals, apparently it is not normal for the curvatures on permanent teeth to extend out as much as 1 mm beyond the cervical line; usually the curvature will be less.
The curvatures on the labial, buccal, and lingual surfaces of all maxillary teeth and on the buccal surfaces of mandibular posterior teeth are rather uniform; the average curvature, as mentioned before, is about 0.5 mm.
Mandibular posterior teeth have a lingual curvature of approximately 1 mm, with the crest of curvature at the middle third of the crown instead of at the cervical third. Occasionally maxillary posterior teeth have similar curvatures on the lingual aspect. (Compare the lingual curvatures in Figures 5-20 and 5-21.)
Mandibular anterior teeth show less curvature on the crown above the cervical line than any of the other teeth. Usually it is less than 0.5 mm, and occasionally it is so slight that it is hardly distinguishable. The mandibular canines may show a little more curvature than central and lateral incisors.
Regardless of whether theories explaining the functional significance of these contours are correct, there is little doubt that they are important. If this were not so, there would be more variance in the presence or the extent of curvature. The curvatures as described seem to be as standard and as constant as any anatomical detail can be.
Figure 5-24 Curvatures of the cervical line (cementoenamel junction) mesially and distally on the maxillary central incisor, demonstrating the points of measurement in determining the relation between the curvatures of the cervical line mesially and distally. Other points of measurement of the crown and root, when one observes the mesial and distal aspects, are outlined and are considered average measurements for a mandibular central incisor. The shaded area in the form of a band on the enamel follows the cervical curvature and represents the epithelial attachment of gingival tissue to the enamel of the crown. The unit of measurement in the figure is millimeters.
The Height of Epithelial Attachment: Curvatures of the Cervical Lines (Cementoenamel Junction [CEJ]) Mesially and Distally
The epithelial attachment seals the soft tissue to the tooth. It is a remarkable system capable of adjustment to local physiological changes, but it is vulnerable to physical injury. Careless treatment can cause breaks in the attachment, making the tooth liable to further physical or pathological injury. The teeth can be injured by careless probing during clinical examination, by improper scaling during prophylactic treatment, by tooth preparation techniques in operative procedures, and so forth.
The height of normal gingival tissue, mesially and distally, on approximating teeth is directly dependent on the heights of the epithelial attachment on these teeth. Normal attachment follows the curvature of the CEJ if the teeth are in normal alignment and contact. This does not mean that the CEJ and the epithelial attachment are at the same level, but it does mean that they tend to follow the same curvature, even though the epithelial attachment may be higher on the crown on its enamel surface (Figure 5-24). A comparison of the curvatures of the CEJ mesially and distally on the teeth is therefore in order. Measurements and comparisons are shown in Figures 5-25 and 5-26; see also Table 1-1.
The extent of curvature seems to depend on the height of the contact area above the crown cervix and also on the diameter of the crown labiolingually or buccolingually. The crowns of anterior teeth, which are narrower and longer from these aspects, show the greatest curvature (see Figure 5-25). In the use of the words height and above, the supposition is made that in either the maxillary or the mandibular arch the occlusal surfaces of the teeth are above the cervices.
Any point approaching the incisal edge or occlusal surface of a crown is considered above the cervix, and the height is increased as it approaches occlusal levels.
Periodontal attachment that follows the curvature of the CEJ mesially and distally seems to be about as high on mandibular anterior teeth as on their counterparts that are larger in the maxillary arch. Although the crowns of the mandibular anterior teeth average 1 mm less in labio-lingual diameter (the lateral incisors excepted), the contact areas are higher accordingly, being near the incisal edges on centrals and laterals. Consequently, measurements usually show less than 1-mm variation in mesial and distal curvatures between maxillary and mandibular anterior teeth.
Figure 5-25 Schematic drawing of the crowns of maxillary and mandibular teeth with associated graphs of the average extent of curvatures of the cervical line mesially and distally. A, Maxillary teeth. B, Mandibular teeth. Compare the graph of cervical curvatures with a line drawn through the center of contact areas. Note that the graph tends to run somewhat parallel to this line. (Grid = 1 sq mm.)
Posterior teeth show little variation in either arch. Figure 5-25 is a diagrammatic drawing of the outlines of the teeth on one side of the arch when viewed from the labial and buccal aspects. These outlines have been placed so that a direct comparison can be made with the graphs below them. The graphs demonstrate the relative height of individual attachments in the average normal case. They are based on cases having upper central incisors with crowns 10.5 to 11 mm in length. Unless the teeth were very large or very small, the graphs would not vary from those illustrated by more than 0.5 mm.
The curvature of the cervical line of most teeth (CEJ) is approximately 1 mm less distally than mesially. The greater curvature is found at the median line on central incisors, maxillary and mandibular. The height of attachment is dependent on the height of the contact areas of the two teeth creating the interproximal space. If the mesial curvature of the central incisor is calibrated at 3.5 mm, the distal curvature is about 1 mm less, or 2.5 mm.
Figure 5-26 Graphs of typical forms of maxillary teeth accenting the outlines of cervical lines mesially and distally. (Grid = 1 sq mm.)
Figure 5-27 Graphs of typical forms of mandibular teeth accenting the outlines of cervical lines mesially and distally. (Grid = 1 sq mm.)
The six anterior teeth, both maxillary and mandibular, when compared with posteriors, exhibit the greatest curvature. Because the canine crowns function distally as posterior teeth, their curvatures distally represented by the CEJ are slight, being about 1 to 1.5 mm on the average. Premo-lars and molars have rather uniform but slight curvatures. The contact levels are low in relation to total crown length; consequently, these teeth do not have high periodontal attachments interproximally. The average premolar or molar has a curvature mesially of only 1 mm or less, with no curvature or even a minus curvature distally (Figure 5-27).
From the operator’s point of view, anterior teeth especially must be carefully approached (when attachment is normal) as, for example, in preparing them for restoration. Posterior teeth have curvatures that are less critical.
A summary of the height of periodontal attachment interproxi-mally indicates the attachment to be highest at the median line on central incisors. In distal progression the height of attachment decreases along with the decrease in curvature of CEJs until the mesial surface of the first premolar is reached. From this point distally through third molars, curvatures are slight.
The possibility of injury to mesial and distal periodontal attachment during tooth preparation should be considered. The height of attachment must be ascertained by careful probing and by the continuous observation of landmarks during the operation. Improper usage of impression materials must also be avoided.
To secure scientific data regarding comparative curvatures, it was necessary to examine many tooth specimens. It was found that, usually, a graph of the cementoenamel curvatures from the median line distally could be staggered, as in Figure 5-25. It is noticed that in the posterior teeth, the variation in curvature is slight and the amount of curvature is minor, with the variance ranging from 1 mm mesially toward the occlusal surface to a slight curvature in the opposite direction. (Note the distal aspect of the mandibular first molar in Figure 5-27.)
Figures 5-26 and 5-27 are graphs of maxillary and mandibular teeth demonstrating typical cervical curvatures mesially and distally.
