Over millions of years of evolution, the teeth have gradually developed a specific shape, with fundamental curvatures at certain areas on each tooth, representing successful adaptation toward the maintenance of the teeth within the dental arch. In other words, the curvatures aid the teeth in preventing disease, damage, bacterial invasion, and calculus buildup; dispersing excessive occlusal trauma and biting forces; and protecting the gingiva and periodontium. The periodontium comprises the supporting structures surrounding the teeth. If these tissues are damaged, then the vascularity, interdental gum tissue, gingiva, and finally the bone between and surrounding the teeth will be jeopardized, decreasing the life expectancy of the tooth within the dental arch.
Throughout evolution, the curvatures, by preserving the teeth, also increased the life expectancy and productivity of the possessor. As the life expectancy of the animal or person increased, so did the number of potential offspring. Thus through the process of evolution and through successful traits outnumbering, outlasting, and outproducing the less successful traits, the teeth of modern humans possess certain successful characteristics of shape and alignment (their position in the jaw). Some of these successful adaptations and characteristics are as follows:
The proximal (mesial or distal) contact areas of the teeth are the areas on the surfaces of the teeth where the proximal surfaces touch one another. The contact area between two teeth prevents food from packing between them. In a healthy mouth the contact surface formed is small enough to prevent a buildup of excessive amounts of bacteria, food, or proximal debris, but large enough to be an effective barrier and to prevent food from packing between the teeth. This affords protection to the underlying gum tissue between teeth. Finally, because the teeth do slightly touch, they offer support and anchorage to one another and resistance to displacement from traumatic forces.
Finally, we must remember that two adjacent teeth share the same interproximal bone. The same bone that supports the distal root portion of the first tooth also supports the mesial root portion of the second tooth. If something happens to cause the loss of this bone, it affects both teeth. Sometimes a periodontally involved tooth is removed to protect a neighboring tooth. By removing the tooth with the most bone loss, the periodontium of the adjacent tooth has a chance to heal.
The proximal contact areas are located on the mesial and distal surfaces of each tooth at the widest portion and the greatest curvature. The distal contact area of one tooth touches the mesial contact area of the tooth posterior to it. For example, the distal contact area of the lower first premolar touches its neighbor, the lower second premolar.
Fig. 3-1 shows the contact areas of two premolars. The contact area on the distal surface of the first premolar is called the distal contact area. What would the contact area on the mesial surface of the second premolar be called? The contact area is not just a point but rather a flattened portion of the tooth where it actually touches the tooth next to it in the same dental arch.
A contact point differs from a contact area. A contact point is where the occlusal cusp of one tooth touches the occlusal portion of another tooth in the opposing arch (Fig. 3-2). The contact point of an upper tooth hits (occludes and makes contact with) the contact point of a lower tooth.
Looking at a buccal view of a tooth (Fig. 3-3), notice that the contact area occurs at the portion of the tooth that has the greatest curvature. In other words, the distal contact area occurs at the part of the distal portion of the tooth that bulges or curves out the most.
Even from an occlusal view it is apparent that, although the proximal contacts do not take up the entire surface, at least a considerable portion of the proximal surface does touch the adjacent tooth.
Interproximal spaces are triangular-shaped spaces between the teeth formed by the bone on one side and the proximal surfaces and their contact area on the other side (Fig. 3-4). The contact area forms the apex of the triangle, the proximal surfaces form the sides, and the alveolar bone makes up the base. These spaces are normally filled with gingival tissue called papillary gingiva or interdental papilla. By its presence the interdental papilla keeps food from collecting cervically to the contact areas between the teeth. The interdental space (space between teeth) provides a place for a bulk of bone, thus affording better anchorage and support. This space is wider cervically than occlusally to provide more access for the vascular support to nourish the interdental bone and papillary tissue. This also affords a stronger bony base. When gingival recession occurs between the teeth, the interdental papilla and bone no longer fill the entire interproximal space; a void exists cervically to the contact area. This void is called a cervical embrasure. The more bone that is missing, then the larger is the interproximal space because bone forms the base of the latter. The cervical embrasure could become larger because gum tissue receded while the bone level stayed the same. In this case the interproximal space would remain the same. Cervical embrasures occur often as a pathologic consequence of periodontal or orthodontic causes, and these embrasures offer a place in which bacteria, calculus, and food debris can accumulate (see Fig. 3-4).
Embrasures (spillways) are the spaces between the teeth that are occlusal to the contact areas (Fig. 3-5). They allow for the passage of food around the teeth so that food is not forced into the contact area between the teeth. These embrasures are named for their location in relation to the contact area. For instance, the space buccal to the contact area is the buccal embrasure; the lingual embrasure is lingual to the contact area. The names of the embrasures are facial (buccal or labial), lingual, incisal, or occlusal. Gingival embrasures are also evident, but only if the interproximal space is not occupied by any gingiva or bone. The gingival embrasure is gingival to the contact area and not usually present; it is the same as the cervical embrasure discussed in the last section. The embrasures have the following purposes: