Form and Function of the Dentition
Because teeth develop according to the law of form and function, the rows of teeth are defined in terms of their form and position in relation to each other as a functional dentition. Therefore, the forms should be interpreted as indications of their particular functions; ie, the form is the material expression of function.
A few geometric lines and figures can be used to describe the rows of teeth. They help to define the shape of the dental arches so that, in an edentulous jaw, the original tooth positions can be rediscovered. They are also useful in measuring the mandible and the maxilla. "Joint machines" (articulators) can be constructed with the data obtained to imitate the movement of the mandible.
The rows of teeth are described in the three anatomical planes of reference of the skull: the horizontal, sagittal, and frontal planes. The descriptive models can be taken from orthodontic model analysis and articulation theory.
Dental arch forms in the horizontal plane develop as a result of heredity, the number and form of the teeth, the influence of the muscles of the tongue and cheeks, and the structure of the facial part of the skull. In the primary dentition, the rows of teeth are semicircular. In the permanent dentition, the vestibular edges of the maxillary teeth form a semi-ellipse, whereas they follow the course of a parabola in the mandible. The maxillary and mandibular rows of teeth therefore do not have the same form; the maxillary arch is larger in the anterior area and extends over the mandibular arch, whereas this difference is evened out in the posterior region in favor of the mandibular arch. This difference in arch size arises because the mandibular anterior teeth are much narrower than the maxillary anterior teeth.
The following are the average dimensions for the maxillary and mandibular dental arches: In the maxilla, the distance between the mesiobuccal cusps of the third molars is 57 to 62 mm; the distance from the contact point of the central incisors to the line marking the distance between the mesiobuccal cusps of the third molars is 50 to 55 mm (Fig 5-1).Transferred to an ellipse, the first distance line would be the short axis and the second line the half long axis. In the mandible, the distance between the mesiobuccal cusps of the third molars is 55 to 60 mm and hence is virtually the same as in the maxilla. However, the distance of the contact point of the central incisors to this distance line is only 48 to 52 mm (Fig 5-2).
As for the dental arch width, the ratio of tooth size to width of the dental arch is measured at two points in orthodontic model analysis. What is known as Pont's index (Figs 5-3 and 5-4) gives the average ratio between the width of all four incisors in the maxilla (sum of incisors = SI) and the width of the dental arch at two points, ie, the transverse distance between the first premolars (anterior arch width) and between the first molars (posterior arch width). To obtain actual figures, the numerator is multiplied by 100.
The symmetry of the dental arches can be determined through the midlines of the jaws. In the maxilla, the palatine raphe forms the axis of symmetry. The raphe-papillary transversal or cross line is another orientation line that runs at right angles to the median palatine raphe through the posterior area of the incisal papilla and through the canines (Fig 5-5). In complete denture prosthetics, another cross line is constructed through the middle of the incisal papilla, also at right angles to the middle of the palate. It should run along the palatal surfaces of the central incisors to the tips of the canines; it is used for reconstructing the position of the anterior teeth (Fig 5-6).
The axis of symmetry of the dental arch has to be constructed in the mandible. It runs through the mandibular incisal point, which is the contact point between the two central incisors, and through the center-to-center distance of the two molar triangles.