Dentition in the Horizontal Plane
Geometric constructions can be used to describe the mandibular dental arch form. These include the Bonwill circle with its tangents, the Pound line, and the centers of the ridges.
The Bonwill circle (after William Gibson Bonwill) describes a circular arch that runs over the incisal edges of the mandibular anterior teeth and the buccal cusp tips of the mandibular first premolars (Fig 5-7). Tangents to this circle can be placed in the tips of the premolar cusps.These tangents run over the buccal cusp tips of the posterior teeth, over the buccal contour of the retromolar triangle (or mandibular alveolar tubercle), to the mid-
points of the condyles of the mandible (Fig 5-8). The Bonwill circle and its tangents provide an initial approximation of the arch form of the mandibular teeth.
The Pound line is a connecting line between the mesial contact point of the mandibular canine and the lingual contour of the retromolar triangle. The line runs through the tips of the lingual cusps of the mandibular posterior teeth (see Fig 5-8).
The ridge or crest line is the midline on the (edentulous) alveolar crests. In the mandible, it runs from the canine points to the middle of the mandibular alveolar tubercle; the central developmental grooves follow the course of this line. When setting up artificial posterior teeth, this line is essential for producing statically reliable tooth positions.
The position of the mandibular dental arch in the skull or facial area is determined by the occlusal plane, the Bonwill triangle, and the Balkwill angle.The Bonwill triangle defines the position of the occlusal plane in relation to the joints via the mandibular incisal point and the Balkwill angle. This establishes the average position of the mandibular teeth or the mandibular model at the joint points.
The mandibular incisal point is the contact point between the mandibular central incisors. On the articulator, a mark is made that denotes the mandibular incisal point, either directly by means of an incisal pointer or indirectly with an adjustment tool for fixing the mandibular model.
The occlusal plane (or bite plane) is defined by three points in the mandibular dentition. It runs through the mandibular incisal point as far as the distobuccal cusps of the mandibular second molars (Fig 5-9). This forms a triangular shape. If the occlusal plane is extended forward and backward, the plane bisects the lip closure line at the front and the superior third of the retromolar triangle at the rear (Fig 5-10). The occlusal plane lies roughly parallel to the Camper plane (approximately 2 cm inferiorly) and parallel to the interpupillary line (Fig 5-11). Once the lip closure line on the occlusion rims and the half-height of the retromolar triangles on mandibular models are established, the course of the occlusal plane can be determined in edentulous jaws. The occlusal plane is a mean value for construction of articulators; it is permanently drawn onto average-value articulators and is hence an orientation guide when mounting the models in the articulator and when setting up the teeth.
The Bonwill triangle is an equilateral triangle in which the 11.5-cm-long sides (formerly 10 cm) are statistically calculated and can stretch from the mandibular incisal point to the midpoints of the mandibular condyles (Figs 5-12 and 5-13).The size of the Bonwill triangle is a statistical average that is constantly being reviewed and corrected. It enables the size of an articulator to be fixed in its key dimensions (distance between the joints and the incisal point).
The Balkwill angle denotes the angle between the Bonwill triangle and the occlusal plane (the Camper plane); the angle at the incisal point is 22 degrees on average (Fig 5-14). This statistical mean is integrated into an average-value articulator and thus establishes the position of the mandibular incisal point.
The lines and figures described above refer to the mandibular dental arch, but they also define the ellipsoid course of the maxillary teeth because the teeth have a definite positional relationship to each other as a result of their occlusal pattern. Once the mandibular tooth position has been clearly found with the aid of these lines, the dentition in the maxilla can be reconstructed on the basis of regular intercuspation.
The premolar tangent in the maxilla is a form characteristic of the maxillary arch that is seen as an esthetic requirement: So that the mouth does not appear too full, the premolars stand within a line that runs from the labial medial ridge of the canine to the mesiobuccal cusp ridge of the first molar (Fig 5-15).