As stated in the previous section, the principle function of the concept of ideal occlusion in everyday dentistry is to provide a benchmark against which any patient’s occlusal pattern can compared. This does not infer that the provision of an ideal occlusion is the treatment objective for that particular patient nor for patients in general.
The three question examination
Does Centric Occlusion occur in Centric Relation?
Centric relation describes a relationship between the two jaws, it has nothing to do with teeth; it is not an occlusion. When the head of the condyle is moving purely in the rotational phase of its movement, then the mandible is in a terminal hinge axis. This concept provides one of the three pillars of the definition of centric relation that is given in the previous section. If the head of the condyle is the stationary centre about which the mandible is rotating, then the mandible, during this phase of movement will describe an arc. Whereas if the mandible is not in terminal hinge axis, then the head of the condyle will not be purely rotating, because there will be an element of translation; and as a consequence of the fact that the head of the condyle is, therefore, not stationary (in the antero-posterior plane) the mandible will not describe an arc.
Manipulation of patient to find the Centric Relation
It is feeling that the patient’s mandible is describing a perfect arc during manipulation that gives the experienced operator the confidence that the terminal hinge axis of the mandible has been found.
|Illustration of the relationship between the condyles|
It is certain that in reality the mandible is not describing a perfect arc because unlike on an articulator the condyle is never a perfect sphere and the glenoid fossa is never a hemisphere. There is, however, one important test that provides confidence that Centric Relation has been found. This test is based upon the fact that because the Centric Relation is a jaw relation-ship not guided by teeth nor by the patient’s muscles but by the operator arcing the mandible in its terminal hinge axis towards the maxilla, and that the end point of this arc will be consistent. The end point of this arc occurs when the first teeth touch and this is known as the premature contact in the Centric Relation. Centric Relation is the only ‘Centric’ which is consistent. It is the fact that the Centric Relation has been found to be consistent in any one patient irrespective of time or operator that makes it so significant as a concept. It may be that the end point of the closing arc of the mandible whilst it is in terminal hinge axis (Centric Relation) is not a premature contact but rather an even contact of all of the teeth (Maximum Intercuspation). In this case the Centric Relation and Centric Occlusion coincide. If, however, as is usually the case this ideal situation does not occur then the Centric Occlusion will not occur in Centric Relation. It will then be possible discover where the premature contact in Centric Relation is.
|How to find the Centric Relation|
The positional difference between Centric Relation and Centric Occlusion can further be examined by noting the direction of slide of the mandible when the patient is asked to clench his or her teeth together whilst resting on the Centric Relation premature contact.
If Centric Relation (CR) and Centric Occlusion(CO) do not coincide, in many ways it would make more sense to describe the relationship between the jaws when the teeth are in Centric Occlusion, and not the other way round; but that is impossible to do because there are no landmarks on the jaws that can be examined whilst the patient is holding his or her teeth in Centric Occlusion. So the question has to be: ‘ Does CO occur in CR?’.
Does the patient have Freedom in Centric Occlusion?
This investigation will answer the question: ‘Is the patient’s Centric Occlusion locked in?
This means when the patient is biting together normally, do his or her incisor teeth prevent those teeth moving slightly forward, or are they prevented from doing so (locked), by the fact that the lower incisor teeth heavily contact the palatal surfaces of the upper incisor teeth?
It can be examined by in one of three ways:
1. Marking the occlusal contacts and seeing if the anterior contacts are heavier than the posterior ones
2. Asking the patient to close together slowly and reporting which teeth hit first
3. Feeling for tremors on the upper incisor teeth with our finger nail whilst the patient repeatedly taps up into Centric Occlusion.
This was easier in the days before routine use of gloves.
Where is the patient’s Anterior Guidance?
It has already been discussed that the term ‘anterior guidance’ should not be taken to mean the guidance that is on the front teeth: it is the mandible that is being guided, by the temporo-mandibular joints (posterior guidance) and by the teeth (anterior guidance). Therefore, which ever teeth touch during excursive movements of the mandible provide the anterior guidance or the dynamic occlusion.
However, the benchmarch against which the patient’s occlusion is measured is ideal occlusion, and in an occlusion which is ideal for the rest of the articulatory system the anterior guidance is on the front teeth.
|How to examine anterior guidance|
When the anterior guidance is on the back teeth the terminology used is posterior interference. This may be either on the working or non-working side. If interferences are present then they may extend beyond the crossover position. If there are no posterior interferences, then the anterior guidance will be on the front teeth, and this is described as being either ‘canine guidance’ (where the contact between the upper and lower teeth during an excursive movement of the mandible is against the upper canine and then eventually on the upper central incisors); or it is described as a ‘group function’ where the anterior guidance is on several teeth. In a group function these contacting teeth are usually the canines and first and second pre-molars; the more anterior teeth of the group should provide the earlier and harder contacts, otherwise the contact could be considered as a working side interference.
A Video on Mandibular movement
A Video on Mandibular movement