Figure 1: It has been claimed without evidence that occlusion causes:

Figure 3: The articulatory system

Figure 4: (a) Interconnections of the articulatory system (b) Interconnections of the masticatory system

Static occlusion

The first essential question when considering a patient’s static occlusion is: ‘Does centric occlusion occur in centric relation?’

This question will be clarified after defining terminology, which has been a ‘red herring’ and has been the cause of enormous and sometimes acrimonious debate. We, also, have preferred terms, but do not feel that they are important.

Centric Occlusion (CO) can be described as the occlusion the patient makes when they fit their teeth together in maximum intercuspation. Common synonyms for this are Intercuspation Position (ICP), Bite of Convenience or Habitual Bite. It is the occlusion that the patient nearly always makes when asked to close their teeth together, it is the ‘bite’ that is most easily recorded. It is how unarticulated models fit together. Finally, it should be remembered that it is the occlusion to which the patient is accustomed ie the habitual bite.

The word
‘Centric’
is an adjective.
It should only be used to qualify a noun.
Centric what?

Centric Relation (CR) is not an occlusion at all. CR has nothing to do with teeth because it is the only ‘centric’ that is reproducible with or without teeth present. Centric Relation is a jaw relationship: it describes a conceptual relationship between the maxilla and mandible. All attempts to lay down rigid definitions of centric relation are plagued by the fundamental difficulty that there is no sure or easy way of proving that the locating criteria have been achieved.

Centric Relation has been described in three different ways: anatomically, conceptionally,² and geometrically.

Anatomical

Centric Relation can be described as the position of the mandible to the maxilla, with the intra-articular disc in place, when the head of the condyle is against the most superior part of the distal facing incline of the glenoid fossa. This can be paraphrased as uppermost and foremost (Fig. 5).

This is subject to debate. Some clinicians prefer the idea that centric relation occurs in an ‘uppermost and midmost’ position within the glenoid fossa; whereas very few people now support the idea that it is in an ‘uppermost and rearmost’ position. There is support for the uppermost and foremost hypothesis from a study of anatomy: the bone and fibrous articulatory surfaces are thickest in the anterior aspect of the head of the condyle and the most superior aspect of the articular eminence of the glenoid fossa. This is, however, of only academic interest and not of clinical significance as there is no reliable simple means of determining the exact position of the head of the condyle within the glenoid fossa.

Conceptual

Centric relation can be described as that position of the mandible relative to the maxilla, with the articular disc in place, when the muscles that support the mandible are at their most relaxed and least strained position. This description is pertinent to an understanding of ‘ideal occlusion’. This definition supports the concept of a ‘qualitative’ relationship between a jaw position and another element of the articulatory system.

Geometrical

Centric Relation can be described ‘as the position of the mandible relative to the maxilla, with the intra-articular disc in place, when the head of the condyle is in terminal hinge axis’.

In order to understand what this frequently used definition means it is easier, initially, to think about one side of the mandible only. The mandible opens by firstly a rotation of the condyle and then a translation which is downwards and forwards. Therefore, when the mandible closes the the terminal closure is purely rotational. At this phase of closure the mandible is describing a simple arc, because the centre of its rotation is stationary. This provides the ‘terminal hinge point’ (of rotation) of one side of the mandible; but because the mandible is one bone with two connected sides these two terminal hinge points are connected by an imaginary line: the terminal hinge axis. This axis is, therefore, envisaged by imagining the stationary, centers of rotation of each condyle whilst the mandible is moving only in the rotational phase of movement. It is the fact that the mandible is describing this simple arc, when the heads of condyle are in the terminal hinge axis which is of the most clinical significance. This will be discussed later, when the techniques for finding centric relation are presented.

Significance of Centric Relation

There may be arguments about the exact position of centric relation and on how that position is clinically best found. There is, however, a broad agreement between dentists who have studied this subject that there exists a reproducible position of the mandible relative to the maxilla, and that this position is reproducible irrespective of the guidance that the occlusal surfaces of the teeth may provide. Patients with no teeth still have a centric relation. Furthermore there is inter- and intra-operator reliability in finding it.

‘Freedom in centric’

‘Does Centric Occlusion Occur in Centric Relation?’
This is an essential question.

Another aspect of the static occlusion is the presence or absence of ‘freedom in centric’, this is also known as ‘long centric’.

As previously stated the word centric is an adjective and so strictly it should never be used without a defining noun. So this long established term would better read: Freedom in centric occlusion or long centric occlusion. Freedom in centric occlusion occurs when the mandible is able to move anteriorly for a short distance in the same horizontal and sagittal plane while maintaining tooth contact (Fig. 6b). Alternatively there will be no freedom in centric occlusion if either the front teeth or the posterior occlusion do not allow this horizontal movement (Fig. 6a).

An easier way of imagining Freedom in Centric Occlusion is to state that if the front teeth hit together as hard or harder than the back teeth, then there is no freedom in that centric occlusion. Two common examples of occlusions that may not have this freedom are firstly those which have an Angles Cl II div (ii) incisor relationship and secondly when anterior crowns have been provided with palatal surfaces which are too thick.

In Figure 6a, there is no freedom in centric occlusion as the occlusal contacts ‘lock in’ the mandible to the maxilla. Whereas in Figure b the mandible can move anteriorly, for a short distance, in the same signal and horizontal plane.

Other aspects of the static occlusion that can be described are the extent of the posterior support, the Angle’s classification of the incisor relationship together with measurement of the overbite and overjet, and the existence of any cross bites.

The answer to the question: ‘Does Centric Occlusion occur in Centric Relation?’ is therefore essential, because it describes the relationship of the mandible to the maxilla when the teeth fit together.

Dynamic occlusion

Ideal Occlusion
Q. Who or what is it ideal for?

Posterior Interference
Q. Who or what is it interfering with?

The dynamic occlusion refers to the occlusal contacts that are made whilst the mandible is moving relative to the maxilla. The mandible is moved by the muscles of mastication and the pathways along which it moves are determined not only by these muscles but also by two guidance systems.

The posterior guidance system of the mandible is provided by the temporomandibular joints. As the head of the condyle moves downwards and forwards the mandible is moving along a guidance pathway which is determined by the intra-articular disc and the articulatory surfaces of the glenoid fossa, all of which is enclosed in the joint capsule.

If teeth are touching during a protrusive or lateral movement of the mandible then those (touching) teeth are also providing guidance to mandibular movement. This is the anterior guidance and this is provided by whichever teeth touch during excentric movements of the mandible.

No matter how far back these teeth are they are anterior to the temporomandibular joints and so a patient with a severe anterior open bite would still always have anterior guidance of their mandible, it could, for instance be on the second molars. Therefore, despite the ambiguity of the word ‘anterior’ in the term anterior guidance, it does not mean that the anterior guidance of the mandible is always on the front teeth. This definition differs from that given in some restorative textbooks, when the term anterior guidance is used to describe only those anterior guidences which involve front teeth.

Anterior guidance may be further classified. ‘Canine guidance’ refers to a dynamic occlusion that occurs on the canines during a lateral excursion of the mandible. A canine protected occlusion refers to the fact that the canine guidance is the only dynamic occlusal contact during this excursive movement.

Group function. In this type of anterior guidance the contacts are shared between several teeth on the working side during a lateral excursion. To qualify for the term ‘group function’, the contacts would be towards the front of the mouth and the most anterior of the group would be the earliest and hardest contacts. This would contrast with a ‘working side interference’, which infers a heavy or early occlusal contact towards the back of the mouth during an excursive movement. A ‘non working side interference’ is an anterior guidance on the back teeth on the non working side during lateral excursion. The working side is the side of the mandible towards which the mandible is moving during a lateral excursion. The non working side is the side of the mandible away from which the mandible is moving. These terms can be confusing when considering the temporomandibular joints, because it is the TMJ on the non working side which is moving the most.

One reason why restorative textbooks define anterior guidance as being solely the dynamic occlusal contacts between the front teeth is that it is generally considered to be more ideal if the anterior guidance is on those front teeth. Furthermore, the fact that the word ‘interference’ is used to describe an occlusal contact between back teeth infers that this anterior guidance is less ideal than others. This introduces the concept of ‘ideal occlusion’ and this raises two important considerations:

1. If some occlusions are ideal, for what or for whom are they ideal?
2. If occlusal contact between back teeth is deemed a posterior interference with what is it interfering?

Ideal occlusion

Let us examine this concept and question whether it has any useful function in routine clinical dentistry.

If two molars on the side from which the mandible is moving during an excursive movement can be deemed to provide a non working side interference, then what are they interfering with?

The posterior guidance of the mandible is provided by the temporomandibular joints. As the head of the condyle translates down the articular eminence of the joint on the non working side (which, paradoxically is the side that is moving the furthest) the mandible is being guided by this joint. If, as this is happening, a posterior maxillary and mandibular tooth hit against each other and because these two posterior teeth are close to the joint, then there is potential for the contact between these two teeth to influence or ‘interfere’ with the movement of the condyle within that joint. Contrast this with the situation, where the anterior guidance is provided not by posterior teeth which are close to the joint, but by front teeth which are further away; then the likelihood of ‘interference’ of condylar movement within the non-working side temporomandibular joint is less.

Anterior guidence, therefore, on back teeth, whilst still providing anterior guidence to the mandible, is described as a posterior interference because it may interfere with the posterior guidance system of the mandible, namely the temporomandibular joints. Posterior interferences are, therefore, considered to be a less ideal type of dynamic occlusion; and the term ideal relates to whether or not it is ideal for another part of the articulatory system: the temporomandibular joints (Fig. 7a). The anterior guidance provided by front teeth is potentially more ideal for the temporomandibular joints for the simple reason that it is further away from the temporomandibular joints. Consequently it can be argued that one aspect of the occlusion (anterior guidance) may or may not be ideal for another part of the articulatory system.

It is, also, potentially more ideal if the teeth fit together (centric occlusion), in a position of the mandible relative to the maxilla, with the disc in place, where the muscles supporting the mandible are at their most relaxed and least strained (conceptual description of centric relation). This establishes another criterion of the occlusion that can be considered ideal or not ideal for the other part of the articulatory system, namely the muscles of mastication (Fig. 7b).

1. Pretreatment examination and records

The first and most important reason for defining ideal occlusion is that it gives a benchmark against which patients’ occlusion can be measured. This needs to be done before, during and after dental treatment; especially in this increasingly litigious environment.

It is of paramount importance that dentists examine and record the pre-existing occlusion before providing treatment which involves changes to that occlusion. Study models would be a good way of doing this as long as they are mounted on an articulator in centric relation. At least the condylar guidance angles would also need to be correctly set so that the dynamic occlusion was recorded. This is not practicable for the vast majority of dentists. Alternatively and infinitely more easily notes can be made, which describe the patient’s occlusion. These notes use criteria of ideal occlusion as a benchmark. To record an occlusion using only the criteria of Angle’s classification is of very limited value, whereas to use the benchmark of ideal occlusion is considerable more informative Fig. 8.

2. Treatment of Pain Dysfunction Syndrome (PDS)

Force > Resistance

• Reduces tissue resistance
• Causes failure
• Promotes pain/dysfunction

The second reason why ideal occlusion is an important concept is found in the long held view that the most important factor in the development of Pain Dysfunction Syndrome (PDS) is ‘the individual patient’s lack of adaptation to a less than ideal occlusion’. ⁴ This is not describing a causal relationship between a less than ideal occlusion and PDS, as different patients will have different thresholds of tolerances to occlusion, in fact the same patients have a different tolerance to their occlusions at different times. It is, however, the case that some patients, at some times, do react adversely to their less than ideal occlusions; and this can lead to pain and dysfunction.

For these patients the provision of an ideal occlusion is, therefore, one but by no means the only way of treating the condition. When an ideal occlusion is provided this should always be initially in a temporary and reversible way: that is a stabilization splint. The indications for the provision of an ideal occlusion in the patient’s natural dentition, for the treatment of PDS are very rare.

3. Conformative versus reorganized approach

In providing treatment with an occlusal element, one of the first questions to be decided in the treatment planning stage is whether the aim is to maintain the same occlusion during treatment. If the pretreatment occlusion is to be preserved, then the is described as ‘conformative approach’.⁵

Some dental treatment, such as most major restorative and all orthodontic treatment, will however, inevitably change the patient’s occlusion: this is known as the ‘reorganized approach’. It is, therefore, prudent to design an occlusion that is more ideal and so potentially better tolerated by the patient’s articulatory system.

Guidelines of good occlusal practice