Anatomical Values Used
The possibilities and limitations of movement replication with mechanical articulators depend on the aforementioned joint replication and furthermore on the use of anatomical geometric values of the masticatory system. Accordingly, all articulators can be classified as fixed-value, semi-adjustable, or fully adjustable devices. The values that can be individually measured and variably applied are the following:
- Horizontal condylar inclination
- Bennett angle (Bennett paths)
- Bonwill triangle, Balkwill angle, and incisal point
- Relationship between the joints and the occlusal field
Fixed-value articulators are all devices in which the geometric values used are incorporated in a nonadjustable way. These devices can be differentiated according to the range of values they use.
A fixator (occlusion holder) is a device or plaster block on which two models are mounted in a fixed occlusal situation (Fig 8-7). The models can be separated to carry out dental laboratory work. Fixators are mainly used to fix orthodontic models according to a construction bite position (Fig 8-8). The maxillary and mandibular models are fixed in the sagittal, vertical, and horizontal planes at a distance determined by the construction bite so that maxillomandibular appliances can be fabricated. In a fixator or a plaster base, occlusal surfaces or a denture can be reconstructed according to the terminal occlusal position.The mandibular movements are not taken into account initially with this technique; the occlusal surfaces have to be more or less roughly reground in the patient's mouth. Such fabrication methods are outdated and are only rarely used today. A variety of fixators are used for preparing indirect relinings to hold the dental arch of the denture being worked against the model.
An occluder is the simplest device with which centric occlusion can be reproduced (Fig 8-9). Occluders are devices in which maxillary and mandibular models can be moved against each other via a simple hinge joint. Sliding movements cannot be replicated.
A distinction is made between occluders with and without occlusal height fixation. Occluders with a locking screw for occlusal height can fix the vertical distance between the jaws. Occluders without a locking screw for occlusal height can only be used for dentate models in which the occlusal height is established by opposing groups of teeth. Occluders are entirely unsuitable for individual shaping of occlusal surfaces.
Average-value cranial devices are occluders with mechanisms for incorporating so-called average-value calottes (a device for holding a plate to which teeth can be formed). This average-value calotte is arched in the same way as the curves of occlusion would be if TMJs had average inclinations and angles. These devices cannot be used to simulate any of the articulation movements (lateral or protrusive) performed byTMJs. However, once the occlusal surfaces have been reconstructed, the models can be moved back and forth on these occlusal surfaces while the upper base holder is loosened. Nevertheless, these are not genuine jaw movements because an essential guidance element, namely the TMJ, is absent.
It is assumed that the movements of the mandible follow specific paths, guided by the TMJ, so that very specific occlusion curves and shapes of occlusal surfaces develop or are ground in. In this state, the mandible is guided no longer by the TMJ alone but primarily by the occlusal pattern of the teeth. Changes to the occlusal surfaces, particularly in elderly people, will involve a change to the TMJ, which will display signs of wear and no longer guarantee accurate guidance of the mandible. That is why occlusion curves are set up on the basis of calottes, which present the average values for the condylar path inclinations (Fig 8-10). Thus, a "chewing path" is created that directs the mandibular movements and hence the TMJ.
This method, in which curves of occlusion are preset, may be described as chewing path-based generation of the occlusal field. With these devices, a chewing path based on average values is generated that enforces a mandibular movement with tooth contact but does not necessarily coincide with TMJ guidance. The natural TMJs have to be adjusted to this occlusal guidance (ie, altered).