Errors of Technique with Average-Value Articulators
The quality of a mechanical articulator must be measured by what system errors occur and the seriousness of the individual errors. The best and most expensive equipment will not achieve the desired results if it is operated incorrectly or carelessly. Any system errors will be compounded by operating errors.
The geometric arrangement of the occlusal pattern at the articular surfaces is an important measure with which the movement paths of the centric stops in their contact areas are determined. If the anatomical system is transferred to a mechanical system, special attention must be paid to accurate measurement and transfer of these geometric values. If the opposing occlusal pattern with different geometric values is mounted in an articulator, the centric stops are unable to follow the same movement paths as in the anatomical system.
The positional relationship between occlusal patterns and articular surfaces is established by an average-value method with statistical averages, dispensing with accurate measuring (Fig 8-15). The mandibular incisal point, based on the average Bonwill triangle, is shown as a marking in the articulator by the incisal indicator, which is moved by the vertical support pin. The dorsal markings are found on the joint columns of the lower arm of the device; as a result, the occlusal plane is fixed. If transfer errors are made when models are being mounted in an average-value articulator, it is no longer possible to fabricate a denture fit for function.
The following errors can occur when aligning the models in an average-value articulator (Fig 8-16):
Articulator marks are not placed in the neutral position:
- Vertical support pin is not firmly seated on the upper arm.
- Incisal guide table is not in the neutral position.
- Support pin does not rest midway on the guide table.
- Joints are not in the starting position.
- Model holder is not firmly screwed in place.
- The occlusal plane of the models does not run parallel to the occlusal plane marking on the articulator.
- The incisal point of the models (interocclusal record) is not lined up with the appropriate marking on the device.
- The middle of the models and the middle of the articulator are not lined up (the models are twisted). It makes sense to mount the models in two phases; first the mandibular model is aligned, and only after a thorough check is the maxillary model placed in the correct position in relation to the other.
Calibration keys are accessories for average-value mounting in average-value articulators (Fig 8-17). These accessories can be used to position the mandibular model correctly in relation to the articulator joints according to average values. To do this, defined fixed points on the models are lined up with markings on the calibration key. In most cases, the mandibular incisal point and the retromolar triangle are used as fixed points (Figs 8-18 and 8-19). A vertically movable incisal guide pin is placed into the vestibular fornix at the mandibular incisal point (mainly labial frenum), while a horizontal slide should be placed onto the occlusal plane marking of the retromolar triangle (usually upper third of the triangle). Errors arise if these fixed points are not adhered to.
Setting the occlusal height too high is a basic error. With every articulator there is a possibility of pushing the vertical support pin against the upper arm of the device to enlarge or reduce the working space. Before the models are mounted, it is important to check whether the support pin is fixed at the neutral height. If the working space is too small, this can be remedied by moving the support pin and the incisal guide table before mounting. Changing the occlusal height after mounting, eg, increasing it for a complete denture, is a gross error (Figs 8-20 and 8-21). Changing the occlusal height in the articulator changes the vertical distance between the jaws while the position of the occlusal plane in the articulator remains unchanged. In the anatomical system, however, the occlusal plane is shifted by exactly half the amount of the occlusal height increase, and therefore the distance of the occlusal plane to the joints is also shifted.
A similar error is to ignore the occlusal plane (eg, when setting up teeth). This will change the positional relationship of the occlusal plane to the joints in the anatomical system, which also occurs if the height of the occlusal plane was not maintained on one side and the occlusal plane was tilted.