Is postural asymmetry a criterion of good occlusion?

Pierre-Marie GAGEY
Institut de Posturologie, Paris

(That text is published with the formal authorisation of Professors Bolender, Maïnetti, Nahmani and of the Board of the Alsacian group of the Collège National d'Occlusodontie; with the implicit authorisation of the management of the Collège National d'Occlusodontie)

     Occlusodontists and posturologists know that occlusion and posture are related, but how can they improve together their knowledge on that relation?

     On one hand, posturologists have understood the importance of mandibular afferences in postural control. It is true that they cannot explain it, but they observe regularly and by a series of different tests that, in certain patients, a modification of the intercuspidation by interposition of a bite modifies the symmetry of postural tonus in a way they find favourable. Without asking themselves questions on those patients' occlusion and unable to say anything about it, they have taken the habit of telling their colleagues the dentists to equip them with a cradle. The results being generally favourable and the patient satisfied, posturologists do not ask themselves any further questions, in particular no question on those patients' occlusion as they let dentists take care of it if by chance a definitive treatment in the mouth appears necessary. That silent dialogue, through the patients' intermediary, does not help improve the debate on the relations between occlusion and posture! But how could a posturologist speak on the subject of occlusion?

     On the other hand, occlusodontists - as seen by posturologists - are well informed on the possible postural consequences of anomalies in the bucco-dental sphere. We observe that they agree to undertake provisional or definitive treatments in the mouth at the posturologists' request, whereas the latter do not always clearly understand why the former should accept to do such treatments. Indeed, it is hardly believable that posturologic arguments can manage to obtain the dentists' consent, as they cannot have our experience of pathology and postural clinic. For instance, what is the meaning, for a non-specialist, of a difference of a few millimetres on the X-mean position in of a statokinesigram? The posturologist's letter of introduction cannot be more, to the dentist, than an invitation to examine a patient's mouth, and if he undertakes something, it means he must have found, in his field, something to do...

     Anomaly for anomaly, we merely exchange patients, without really «exchanging». Postural anomaly on one side, dental on the other - the dentist treats the dental anomaly and cures the postural anomaly. Until now we merely welcome the new evidence that posture and intercuspidation are closely related, because in order to go further and try to understand what those relations mean, many efforts will be needed to understand the other's thinking.

     To understand the posturologic thinking, we observe that hours of theoretical lectures are just not enough. It is necessary for the dentist to confront personally, in some way or another, to postural clinic in order to have some experience of it. Professor Bolender understood it as he equipped his service in Strasbourg with a stabilometry platform so that his students can work on posture by themselves. Professor Nahmani, Chairman of the Société Française d'Occlusodontie, also understood it as he demanded that his students check their work in mouth with a stepping test. And in Nantes, as part of a postgraduate course leading to the university diploma of occlusodontology and treatments of the temporo-mandiblar dysfunctions, Professor Maïnetti teaches his students Posturology tests.
As for myself, it is only very recently that I became aware of the necessity of a personal experience of Posturology for the occlusodontist, and here is how.
The Alsacian group of the Collège National d'Occlusodontologie had organised a seminary on posturology. After a four-hour, patiently attended, lecture on the subject, the time had come for a practical demonstration of a few tests of the postural check-up on a participant that would agree to act as a «guinea pig». Now, his check-up was abnormal. On the stabilometry platform, the mean position in X of his centre of pressure, that is to say the mean position of his vertical of gravity, was outside the limits of normality at 95%. In the absence of orthopaedic anomalies, that position reveals an abnormal asymmetry of postural tonus. During the clinical examination, the test of the thumbs was abnormal: on the full height of the rachis the right thumb went systematically higher than the left - it was a systematised tonic asymmetry, not limited to any stage of the rachis. In Fukuda's stepping test, we observed a prevailing of the gain of the right nucal reflex by 70°, with inversion of the left reflex. That systematic and abnormal postural asymmetry was already characterised enough and its origin had to be searched thanks to the test of the rotators. Now the simple fact of interposing a piece of bristol board between the cusped teeth was enough to transform the tonus of the external rotators of the hip. That first clue of a modification of the tonus by a modification of the intercuspidation had to be confirmed - indeed, the test of the thumbs was normalised by the bite
, and the prevailing of the gains of the nucal reflexes in the Fukuda test was inverted by the bite
, it became left by 30° with normalisation of the gain of the left nucal reflex.

     A posturologist gets convinced by such a series of observations that it is necessary to look closely at what happens in the mouth. Now that patient, himself a dentist and an occlusodontist, had just been treated by an occlusodontist that was present in the assembly, so they both had the conviction that there was nothing abnormal from that point of view...

     It was a lucky coincidence that led, that day, to an open and loyal opposition between two convictions each born of a different approach. The occlusodontists had done their work according to the rules of the art they master, how could they have admitted to criticise their results on the basis of the data of the postural examination they are not used to handle? Inversely, when I observed the "patient"'s mouth, I noticed that the labial frena were not aligned, the mandible was deported a few millimetres away, on the left I think. But what value can a posturologist give to a minor variation in a field he does not master?

     In Posturology, we all agree to observe that 95% of humans, at least, are asymmetric. Therefore, asymmetry is statistically "normal". But we also know that this asymmetry can be more or less important and that some stronger asymmetries are outside the limits of statistical normality. For instance, in Fukuda's stepping test, a movement of the spin superior to 50°, in a neutral head position, is outside the limit of normality at 95%. So there are normal asymmetries and abnormal asymmetries on a statistical point of view, we know it and we observe that the patients do not complain anymore about difficulties in standing up when their abnormal asymmetry has been reduced.

     That statistical criterion of the abnormal asymmetry is simple, easy to understand and to use. And if one doubts of it - which is always allowed - one just has to study the research on that subject: a few hundreds stepping tests executed by apparently normal subjects help build a serious opinion.

     The criteria of the normal postural tonus are beginning to be accessible, yet we still find hard to understand the nature of the consensus among dentists on the normality criteria of occlusion. Do we lack information? Or is that difficult subject still under study? In that last case, we would like to suggest searchers to add to the list of the possible criteria of a good occlusion the symmetry of postural tonus. Is the relative postural symmetry a criterion of good occlusion? Why not?
Even if that symmetry appears as a bad criterion of good occlusion, the work will not be useless because it is obvious that dentists and posturologists have to start not only discussing but also working together if we want to improve our knowledge on the relation that exists, we are sure of it, between occlusion and posture.