Occlusion .. mode 1

 Occlusion

'Occlusion' = Contacts between teeth

Occlusion can be defined very simply : it means the contacts between teeth.

Before describing the significance of the different ways in which occlusal contacts are made occlusion needs to be put into context.

The masticatory (or stomatognathic) system is generally considered to be made up of three parts: the Teeth, the Periodontal Tissues, and the Articulatory System . The masticatory system

The importance of 'occlusion' in dental practice is based primarily upon the relationships that it has within these interconnected biomechanical systems. When one considers how almost all forms of dental treatment have a potential for causing occlusal change, the need to establish what constitutes good occlusal practice is overwhelming and obvious.

Analysis of occlusion

Having stated that occlusion simply means the contact between teeth, the concept can be further refined by defining those contacts between the teeth when the mandible is closed and stationary as the static occlusion, and those contacts between teeth when the mandible is moving relative to the maxilla as the dynamic occlusion.

development of occlusion

 As the primary (baby) teeth begin to erupt at 6 months of age, the maxillary and mandibular teeth aim to occlude with one another. The erupting teeth are moulded into position by the tongue, the cheeks and lips during development. Upper and lower primary teeth should be correctly occluding and aligned after 2 years whilst they are continuing to develop, with full root development complete at 3 years of age.

Around a year after development of the teeth is complete, the jaws continue to grow which results in spacing between some of the teeth (diastema). This effect is greatest in the anterior (front) teeth and can be seen from around age 4 – 5 years.This spacing is important as it allows space for the permanent (adult) teeth to erupt into the correct occlusion, and without this spacing there is likely to be crowding of the permanent dentition.

In order to fully understand the development of occlusion and malocclusion, it is important to understand the premolar dynamics in the mixed dentition stage. The mixed dentition stage is when both primary and permanent teeth are present. The permanent premolars erupt ~9–12 years of age, replacing the primary molars. The erupting premolars are smaller than the teeth they are replacing and this difference in space between the primary molars and their successors (1.5mm for maxillary, 2.5mm for mandibular), termed Leeway Space. This allows the permanent molars to drift mesially into these spaces and develop a Class I occlusion .

types of occlusion

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, 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 

Functional anatomy of the temporomandibular joint

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, centres 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.

s 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 guidances 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.