Function of the Closed Dentition
Why the dentition is made up of individual teeth needs to be explored. A permanently fused ridge of bone, for example, would be a feasible alternative. However, the individual teeth are anchored in the bone with ligaments, and the teeth are able to tip. Why the teeth are arranged in a curved arch also needs to be explained; an angular dental arch could indeed be conceivable.
The first consideration is the teeth under stress. The predominant force component acting on each tooth is directed apically, but each tooth can be tipped in sagittal and transverse directions by the sloping surfaces of the cusps (Fig 5-43). To a certain extent, this is a good thing because this redirection of forces transfers a large proportion of the energy to the neighboring teeth.
Tipping in a sagittal direction within the dental arch pushes the tooth against the adjacent teeth. As a result, the vertical component of masticatory forces is redirected horizontally and, similar to an elastic impact, is passed on from crown to crown until the energy is absorbed by all the periodontal tissues of the teeth. This is only possible if there are no gaps between teeth and all the teeth are connected by approximal contact points. In an angular dental arch, a sagittally directed elastic impact could not be smoothly transferred without interruption. Within a round dental arch, this transfer of forces is far easier. This is why restoration of the gap-free pattern of the dental arch ought to be clearly identified as a primary aim of prosthetic measures.
Lingual tipping of the tooth is only possible if the neighboring teeth are pressed mesiodistally, because a tooth is basically broader in its vestibular aspect than lingually. The tooth is wedged in the dental arch, or the transverse impact can be passed on to the neighboring teeth via the contact points as a sagittal force component (Fig 5-44). Once again, the rounded arch form of the dentition is therefore advantageous. Transverse thrusts in a lingual direction can be well compensated for by a multirooted tooth when the roots are flared. The roots of the maxillary molars are flared in just such a functionally advantageous manner (Fig 5-45).
Vestibular tipping of the tooth is absorbed by the marginal fibers. The teeth are linked to each other by the fibers of the marginal periodontium encircling the teeth, so that any movement of a tooth pulls the subsequent teeth in the same direction and the teeth support one another. This can be referred to as tissue linkage, and it also operates in the sagittal direction (Fig 5-46). A tooth that is tipped inside the row of teeth passes that tipping on to its neighboring teeth, partly in the form of an elastic impact via the contact points and partly via the ligamentous apparatus. The ligamentous apparatus is most effective when the teeth are tipped outward: In this situation, tissue linkage is usually the only form of support. The complex structure of the ligamentous apparatus can be interpreted as the ligaments running around the teeth in loops. If a tooth is tipped, the ligaments tense up and the tooth is supported.
However, this linkage alone is not enough. During the chewing process, the mandible slides out of a lateral position into terminal occlusion, in which the buccal cusps of the maxillary teeth enclose the mandibular teeth like a reinforcing ring, so that support here is provided by the antagonists. At the same time, the mandibular teeth offer the maxillary teeth support in the same way because the lingual cusps of the maxillary teeth fit into the central developmental grooves of the mandibular teeth and are thus "wedged."
These observations primarily apply to the terminal occlusion or a position directly before it. The masticatory force is generally greatest in terminal occlusion because beforehand the most masticatory force is used for reshaping the food. Furthermore, as the mandible slides into terminal occlusion, the direction of the force acting on the maxillary dental arch is directed lingually in its transversally acting portion.
Another effective protection against excessive transverse stresses is provided by the nerve supply to the periodontium in the form of a pain warning system. The nerves of the periodontium with the conduction nerves of the masticatory muscles form a reflex path that works as follows: When the critical stress limit for the periodontium is reached as a result of masticatory pressure, the masticatory muscles are "short-circuited" and the masticatory force immediately diminishes. This is a familiar feeling to anyone who has ever had the misfortune to bite onto a cherry stone. Before any awareness of pain, the mouth has already reopened as a reflex action.