Occlusion

Explaining Occlusion

There are complex inter-relationships between the central nervous system, muscles, the periodontium, the TMJ and the occlusion to consider (Fig 1) and one of which may be the weakest link in the chain.

As dentists we often look at models of patient’s teeth to make diagnoses, but this is static, and if we want to understand each patient then we need to look at how their occlusion is working in their mouths, a dynamic diagnosis.

I would like to start by considering occlusion for complete dentures as paradoxically we can learn a great deal from this. In our training we spent significant periods of clinical time trimming bite blocks (occlusion rims) to the correct vertical dimensions (OVD), then getting patients to relax, put the tip of their tongue to the back of the mouth etc as we attempt to move the condyles back into centric relation, the physiological reproducible position, before joining the blocks together. These blocks are then articulated and secondarily the teeth set into maximum intercuspation, that is centric occlusion. We know that, in order to achieve stable dentures, those that are not displaced during function, that we must develop centric occlusion, maximum intercuspation, at centric relation. This is simply because the muscles of mastication want to move the head of the condyles after each chewing or biting mandibular stroke back into this reproducible, physiological position. Our bodies are programmed to do this!

This physiological position is controlled by alpha motor neurones as prime movers of muscles and is under proprioceptive control from both Golgi bodies and the gamma motor neurone system in muscles and nerve endings in the joint capsules and ligaments.

It is standard clinical practice when examining patients wearing complete dentures or who require making new dentures, to determine centric relation and what is the occlusion of the teeth, before making a diagnosis about any faults that may be present in those prostheses. The illustrations (Fig 2 & 3) show loss of occlusal vertical dimension and forward positioning of the condyles to achieve tooth contact in this patient with worn complete dentures.

Contrast this with her successful new complete dentures result from the development of centric occlusion at centric (jaw) relation at the correct occlusal vertical dimension. The compensating curves of Spee and Monson are also built in. This is the basis of occlusion. Determining centric (jaw) relation is fundamental to understanding occlusion.

In marked contrast, when the occlusion of dentate patients is examined, the usual instruction is to ‘close together’ or ‘bite together’. In so doing all you have found is that patient’s centric occlusion, and this may well be an adaptive position, acquired reflex or engram. What you have singularly failed to find is the occlusion at centric relation, if there are any premature contacts and if there are any slides from the occlusion at centric relation having carried out a Dawson manoeuvre to find that reproducible condylar position. Refer back to TMJ examination. (Figs 4 & 5) .

  

Centric occlusion – Centric relation

This man in his mid fifties had been advised by an Oral Surgeon that he needed osteotomies to correct his occlusion, which figure 4 shows is his centric occlusion, the bite together position he has been asked to show. In contrast Figure 5 shows his occlusion at centric jaw relation following a Dawson manoevre. Restoration to this occlusion using conventional crown and bridgework is now relatively easy. Osteotomies were not required, but an incorrect clinical examination had led to that erroneous conclusion.

Unlike the occlusion for successful complete dentures with the coincidence of centric occlusion at centric relation this may not occur in our dentate patients because:

1. Teeth may have been lost through caries, periodontal disease or trauma
2. There may have been orthodontic extractions and/or orthodontic tooth movements
3. Deciduous teeth may have been lost early, allowing first molars to move mesially
4. Teeth may have overerupted and there may be plunger cusps.

In all these instances teeth may have moved out of a balanced occlusion under the influence of mesial drift. In the mandible teeth move mesially and tip mesially. In the maxilla there is a further movement with rotation around the axis of the palatal root of molars. Once this has occurred at a last standing molar, then this is an indication for a ring clasp in partial denture design.

Overerupted teeth will encroach on the freeway space and disrupt the occlusion. Rarely oral soft tissues as in this case of an enlarged maxillary tuberosity will do the same (Fig 6)

I have been at lengths to describe that the real cause of TMJDS pain is muscle spasm. So what is the pathophysiological mechanism by which the masticatory muscles go into spasm?

As mentioned above, the masticatory muscles are moved by impulses down alpha motor neurones in the trigeminal nerve. Mandibular opening is achieved by contraction of the upper and lower heads of lateral pterygoid. The upper head is inserted into the anterior aspect of the articular disc, the lower onto the anterior border of the ascending ramus, just below the joint capsule. Masseter, medial pterygoid and temporalis relax. There are complex interactions between the muscles during border mandibular movements. These movements are controlled by proprioceptors in the lateral ligament of the TMJ, the periodontal membrane and Golgi bodies in the muscles which are associated with the gamma efferent loop.

For Homo sapiens when teeth are lost, others move as previously described. Any such movement will disrupt the compensating curves of Monson and Spee and this disruption, which varies from slight to severe, will prevent the muscles of mastication returning the condyles into their reproducible centric relation within the glenoid fossa. By the way, this happens at least 1500 times daily, the reported number of times we swallow.

I believe the explanation of the pain due to muscle spasm in temporo-mandibular joint dysfunction syndrome (TMJDS) is simply this. The derived tooth position of centric occlusion prevents the muscles of mastication moving the condyles back into centric relation. This state of affairs will be monitored unconsciously by the proprioceptor network. As the condyles have not been returned to their physiological position, signals will instruct muscles to continue to contract. But the condyles cannot move to centric relation due to the unbalanced position of the teeth. Thus a vicious circle is created. The muscles are instructed to contract continually and eventually this will lead to oxygen debt, anaerobic respiration, and greater or lesser muscle pain.

However, only some patients whose centric occlusion does not occur at centric relation will develop pain, the compliant occlusion. The reason is not entirely clear. What is clear is that if restorations are planned, it is incumbent on the treating dentist to have a detailed record. In these cases it could be safe to restore to this compliant occlusion provided that

1. The occlusion has been fully examined and recorded ideally with mounted study casts
2. You have explained to the patient what you intend to do and why, the duties of care and candour, and have obtained informed consent, preferably in writing.
3. You have the technical ability and support to restore to this exactly.

If you fail, and your patient develops symptoms of TMJDS you could be held liable. Careful and detailed records will help your Protection Society!