Average-Value Articulators
Average-value articulators are fixed-value devices whose overall size is defined by the average Bonwill triangle (approximately 11-cm side length) and an average Balkwill angle (approximately 22 degrees). The Camper plane is the reference plane. The horizontal condylar inclinations are generally at an angle of 30 to 34 degrees to the occlusal plane. A Bennett angle of 0 to 20 degrees is also firmly set (Figs 8-11 to 8-13).
The relationship of the occlusal plane to the joints corresponds to average values in fixed-value articulators; it is determined by the Bonwill triangle and the average Balkwill angle and identified with markings. The distances of the joints from each other and from the mandibular incisal point are measured according to the average values of the Bonwill triangle.The overall height and hence the Balkwill angle can be changed in some devices simply by altering the distance from the joints to the occlusal plane (eg, with spacer discs).
Average-value articulators enable the basic mandibular movements to be performed with tooth contact: protrusive excursions and lateral movements as well as opening and closing in a narrowly limited opening range. The movements are guided by straight condylar paths and the incisal guide table with average-value inclination, as the upper arm of the device is supported against the lower arm on the incisal guide table via the vertical guide pin. Anterior guidance and the two articular points give rise to three-point support of the upper arm of the articulator.These three guide areas (condylar paths and incisal guide table) establish the movement paths of the models or of the rows of teeth against each other.
System errors occur as a result of the shape, position, and inclinations of the condylar paths and the position of the models in relation to the joint axis. The setting of the angles of inclination of the condylar path, the Bennett angle, and incisal guidance is not based on precise measurement of the patient's TMJs but rather lies within a rough range of values. Occlusal guidance that only roughly approximates the real TMJ guidance is generated based on average values.
In contrast to the shape of the anatomical path, the condylar paths in average-value articulators are mainly depicted as straight. The natural joint, however, has a curved path. Therefore, the curved movement path of the condyle is reproduced inadequately in the mechanical device. If the Bennett movement is not simulated in an articulator, noticeable deviations from real movement will arise in the occlusal movement paths. In average-value articulators, the position of the orthodontic models at the articular points can only be set as an average, possibly with a calibration key. The position of the occlusal plane is set as a fixed value. The incisal point and the occlusal plane of the models are aligned with the occlusal plane markings on the average-value articulator by visual judgment or with a calibration key.