Dentition in the Frontal Plane
The symmetric arrangement of the teeth in relation to the middle of the skull or face can be seen in the frontal view of the dental arches. All the teeth in the maxillary and mandibular dentitions have a characteristic inclination (Figs 5-36 and 5-37). The maxillary teeth are inclined in a vestibular direction, while the mandibular teeth and their masticatory surfaces are inclined lingually (except the mandibular incisors). The alveolar ridges also have this inclination. The inclination of the teeth and alveolar ridges is a definite requirement because it means the masticatory forces are mainly transmitted axially to the periodontium: When food is being ground by the posterior teeth, the mandible glides from outward (out of a lateral position) into the terminal occlusal position. Therefore, food tends to be crushed by the mandible gliding from buccal to lingual, the direction of forces corresponding to the inclination of the alveolar ridges.
The transverse curve of occlusion is the continuous connecting line along the occlusal surfaces of the posterior teeth of the jaws in the frontal plane, which arises from the lingual inclination of the mandibular posterior teeth (see Fig 5-36). A transverse curve of occlusion can be produced by drawing a line that would connect the idealized occlusal surfaces; this gives a curve that bends caudally and runs vertically to the sagittal curve.
The bending of the two curves of occlusion is almost identical. Both curves therefore span an area that forms a section of the surface of a sphere, a calotte (Fig 5-38). The calotte is interrupted in the area where the maxillary anterior teeth occlude over the mandibular anterior teeth.
The function of the curves of occlusion is to separate the rows of teeth selectively in respect of masticatory function: The mandible is pushed forward to bite food, and the incisors come into
shearing contact. If the food is broken up in the posterior region, this always takes place on one side, while no load is placed on the other side. When chewing with the posterior teeth, the mandible glides out of a lateral position into the terminal occlusal position, and the food is crushed. If the opposite side also had cusp contact during this chewing impact, the teeth would be under transverse and hence nonphysiologic stress at those points.