Options for determining the centric relation
The centric relation position is a static mandibular position in which the condyles take up a symmetrical, resting, central position in the articular fossae, without the influence of any external forcing. Centric relation is therefore independent of tooth contact. When making a restoration, this resting position is what we should take as our basis and mount in the articulator.
If we carry out the registration in such a way that the patient has to make some kind of jaw movement in the meantime — generally to bite together from an open position — and moreover with a registration material of an unfamiliar hardness and consistency in the mouth, then it is very difficult for them to find their resting position. In practice this can lead to a great deal of trouble later on.
The patient bit off to the side, we are asking for a new try-in
Everything is fine, we just need to raise/lower the bite height by 2 mm
I corrected the veneering a little in places, even the framework was already high
Many of us have seen a message like this on the job sheet. Correcting such a problem will predictably influence the quality of the end result unfavourably, however conscientiously it is carried out, sparing neither time, energy nor material. In the end everyone loses: we make corrections in the laboratory, a new appointment has to be freed up in the practice, and for the patient it means one more visit and a poorer end result than would have been possible.
What tools can we have to avoid this?
Registration always consists of two phases: determining the vertical relation and the horizontal relation.
Although centric relation and maximum intercuspation (the mandibular position in which the greatest possible number of contact points is found on the occlusal surfaces of the teeth) most often do not coincide, with a smaller restoration we can generally achieve a satisfactory result starting from the latter position.
With a larger restoration, where the intact teeth still provide occlusal stability but the prepared teeth no longer do, we have a harder task. The most frequently heard pieces of advice relating to this are, for example:
get your bearings before preparation, take study impressions
when registering the bite, there should still be intact, stable occlusal pairs
prepare and register one side first, then the other side, and with the registration replaced, record the bite there as well
during the framework try-in, record/check the bite again — for this a verification island can already be made on the occlusal surface of the framework in the laboratory
Even in the case of more extensive tooth loss or complete edentulism, registration always consists of two phases: determining the vertical relation and the horizontal relation.
The most widespread method for this is the use of bite rims. The laboratory can already play a large part in its success. First we have to ensure that the base plate stays in place as stably as possible during registration, and second that the wax rim is made in the right place and the right dimensions. If this is not the case and, say, the base plate is unstable, then we have not even given a correct registration a chance. It often happens, however, that both the laboratory and the practice do everything for the sake of success, yet at the next try-in we are faced with an error in the vertical or the horizontal relation. During registration the patient generally has to exert force until they reach the resting position. This often leads to inaccuracy. In addition, verifiability is very limited in the case of a wax rim.
Another method, which we generally only use in the case of edentulous jaw(s) although it is not necessarily limited to this, is intraoral central bearing pin registration. Here a metal pin adjustable by a screw thread is fixed to one base plate, and a metal plate to the other. Placing both elements in the mouth, we set the metal pin to the appropriate height; the only contact point is the tip of the pin with the metal plate (vertical relation). The patient makes protrusive–retrusive and lateral jaw movements in such a way that the two metal surfaces remain in contact throughout (horizontal relation). This gives an arrow-point tracing on the surface of the metal plate, the apex of which is the centric relation position. By marking this, the correctness of the registration can easily be verified and then recorded. If we do it correctly, the joints can move without load throughout the whole process.
Naturally, with appropriate practice and care any method can lead to a good result. It is important, however, to emphasise the importance of a correct first registration for the sake of the final quality of the restoration, not to mention the laboratory and chair time required by any large-scale corrections!
At the same time we have not yet talked about possible pathological changes of the temporomandibular joint, registering the spatial position of the maxilla, individual anatomical characteristics, or the limited ability of articulators to imitate the movements of dynamic occlusion
