Evidence, reason and occlusion?

We live in a world of evidence-based medicine and the internet now enables us to access the latest evidence from every scientific institution. But, despite this, our beliefs still differ and the pattern of clinical practice is one of great variety, both in relation to time and geographical location.

The changes are often cyclical with succeeding generations opposing each other. The evidence is out there but we believe different bits of it. Evidence is a tricky commodity and, on the subject of occlusion, has frequently been contradictory.

We should also remember that while evidence can confirm or deny, it can never explain – only logic can do that. It can be most unwise to base an idea on one bit of evidence. Karl Popper the philosopher taught us that however strong the evidence might be, you can never prove a hypothesis for certain: one piece of contrary evidence could still destroy it. He suggested that you should compare all the contending theories to see which fits the evidence best. It is not enough for the theory to fit some, or even most, of the evidence. All of it has to fit and it is rare that anything other than the truth can withstand that test.

Too many dentists look for cures and not enough look at causes. It was George Eversall and Harold Gelb who introduced me to the idea that poor occlusal contacts were linked to cranio-sacral problems. I succumbed to arm levitation and experienced the weak and strong responses that related to variations in occlusal contacts. I had no reason to doubt the many anecdotes I was being given, but I was concerned by the lack of measurements and solid evidence. Above all, I was concerned that the explanations were very complex and varied from clinician to clinician. It is easy to be persuaded by a powerful speaker, but we should all recognise the signs of dubious logic. If all the evidence does not fit, it is probably untrue.

I have seen convincing demonstrations of the change that can be achieved by adjusting the bite by fractions of a millimetre or by 20 or more millimetres. However, simple explanations were never forthcoming and this troubled me. It is clear that the occlusions of most ‘industrialised’ humans represent a massive distortion from that of our direct ancestors. For instance, the maxilla of many children with occlusal problems is placed between 10-30mm back and down when compared to the norms of their forbearers.

Natural occlusion

This displaces the mandible by even more, so how can opening it further by a small or large amount help the individual? We are shown examples of grateful patients, but I am struggling to find logic here, let alone a simple truth and where is the evidence? There should be room for 32 straight teeth with 10mm of horizontal bone behind the wisdoms. That was our natural occlusion some 40,000 years ago and the nearest I have seen to that was during my research in the Kenyan bush. We talk about the importance of occlusal guidance, but we should remember that these manmade rules are entirely artificial, because primitive cusps were worn flat within two years of eruption. We need some better theories with sound evidence to support them.

Egil Harvold probably did more to stimulate my ideas than anyone. He had set up monkey experiments to prove that what he believed was the cause of malocclusion. He distorted tongue posture and forced monkeys to breathe with their mouth open, showing that changes in oral posture have a huge influence on facial growth.

My conversations with Egil consolidated the theories that were forming in my mind and which I still hold today. Thirty years ago I put forward a very simple hypothesis called The Tropic Premise (Mew 1981) to explain the aetiology of malocclusion. It stated that if ‘the tongue rested against the palate with the lips sealed and the teeth in or near contact’, then the facial development and occlusion would be correct. There is a simple logic to this concept, because if the tongue rests against the palate with the lips sealed, the erupting teeth have little option but to slide down the space between them, until the cusps contact and guide them into occlusion.

If, on the other hand, the tongue rests between the teeth or if the lips are apart, they will have little guidance and are likely to erupt haphazardly. People talk about ‘functional’ influences and, indeed, functional appliances, but the evidence shows that bone is very resistant to short-term active force (function) but very susceptible to long-term very light forces, so it is posture not function that overcomes all – which is the simple logic behind the Tropic Premise. Think about that carefully because a large number of occlusal, TMD and orthodontic theories and treatments either ignore it or are incompatible with it. There may be many opinions but there can be only one truth.

Force of the bite

We do not know if the Tropic Premise is true, because you cannot measure oral posture with any accuracy. However, I know of no other theory for the aetiology of malocclusion, TMD, and OSA that fits the evidence as well (Mew 2004).

My visits to Bill Proffit in Chapel Hill taught me some simple truths about eruption. Teeth that are in contact for less than about four hours out of 24 will continue to erupt. Teeth that are in contact for more than about eight hours will continue to intrude. Those between will stay more or less the same.

It varies a bit with the force of the bite, but essentially, if you can train the patient to keep their teeth in light contact for the right period then those teeth that are too high will intrude and those that are too low will erupt so that all the teeth finish meeting evenly: no splints, no grinding, teeth at the natural occlusal height, problems gone! Proffit’s evidence is hard to challenge and fits the clinical situation perfectly.

For instance, I was snorkelling off the Barrier Reef recently with a mouthpiece that had lost one of its bite-blocks. However, I was able to hold it in position by biting firmly on the remaining block. After a few hours I came ashore for lunch, to find my teeth did not touch at all on that side! I kept my mouth closed gently and half an hour later, all was fine. Last year, my own dentist put in a filling and found it was about 1mm too high. He was about to grind a sharp cusp above when I said ‘Don’t worry, I have taught myself to keep my teeth in contact at rest and it will be level in a day or two’ and so it was, with no discomfort. However if I had had a tooth apart posture with intermittent clenching, I might have had agony from periodontitis; the Tropic Premise is very reliable, but few clinicians seem to base treatment on it.

Over the years the Tropic Premise has become accepted as a basis of treatment in many countries around the world, but not in the UK, possibly because it was first introduced too far ahead of its time.

If it is true, we should be able to cure malocclusion, along with TMD, OSA, and virtually all the other occlusal ailments, just by changing oral posture. I know the recognised experts in these fields bridle at these comments but is there a logical alternative? Tongue-between-tooth postures are endemic in this country and are the antithesis of the Tropic Premise. They mess up occlusion and are very difficult to correct, but that does not mean that we should not try to correct them and that was why I developed the concept of changing oral posture by means of ‘orthotropics’ (correct growth).

In the 1950s, my research with surgical correction of the TMJ made me realise that it is one of the most adaptable and regenerative joints in the body. It has the ability to remodel several millimetres and even re-grow completely like the leg of a newt (Mew 1997). No one disagrees with this evidence, but it is incompatible with many TMD theories and techniques. The Tropic Premise provides a simple and logical reason for TMD and sleep apnoea, as well as most crainial osteopathic and ENT problems; the maxilla is too far back. The difficult bit is repositioning the maxilla and getting it to stay there. That is what orthotropics does.

Expand narrow arches

In my father’s time, it was standard treatment at Guy’s to expand narrow arches between the age of four and six (Chapman 1931) but 25 years later the establishment viewed expansion as absurd (Townend 1955). I was ridiculed for expanding patients and using functional appliances. I was fined heavily by the NHS and had to take the Minister of Health to the High Court to prove my point. My victory paved the way for others to use these appliances, but I was labelled a maverick by an enraged establishment. It has taken nearly 30 years for them to move in the same direction, and only now are they realising that the Tropic Premise provides so many logical answers.

It is one thing to have a good idea, and quite another to apply it. As an avid reader of the literature, I can see that there is increasing evidence to show that orthodontic treatment tends to increase vertical growth (Battagel 1996). There is also clear evidence to show that longer faces look less attractive (Lundstrom & Woodside 1980) and that the vertical growth increases dental crowding (Franchi et al 1997) – all the things we don’t want. Orthodontists recognise this but feel their prime responsibility is to align the teeth.

Facial damage

As far as I can see, this has been the case for the last 100 years and the evidence would suggest that fixed, removable and functional appliances are all guilty. Again, I hear cries of protest but look at the evidence or – even more significantly – look at the results: straight teeth, yes, but not many facial improvements unless they are disguised by a smile.

I stopped using fixed and functional appliances 20 years ago because of my concerns about facial damage and relapse. This may not matter much if the patient has favourable growth beforehand, although lifetime retention is usually necessary and the facial change is marginal.

However, unfavourable growers are at considerable risk of relapse and facial damage – not much of a cure! By changing oral posture, orthotropics converts unfavourable vertical growth to favourable horizontal and this is why the results are so different. Have a look at a few orthotropic results – no extractions (32 teeth, not 28 or 24), no fixed appliances, no retention and no relapse. You can see that it is a different type of facial change. This is supported by the evidence showing that they do, in fact, convert vertical growth to horizontal and I know of no other treatment that can achieve that. Have an honest look at the maxillary changes of your own results viewed laterally.

What are the disadvantages? Orthotropics requires more skill and knowledge than most techniques, the treatment should be started before the age of nine and the patients have to do exactly what they are asked. Active treatment is shorter than most methods but cases do need to be monitored during night time wear, for the subsequent two years. It makes such sense that few parents take the traditional options when orthotropics is available.

One day, cleverer people will come along with better ways of achieving forward facial growth, but I am sure that without that there is no way of permanently curing malocclusion along with TMD and OSA.

Facial aesthetics is the current buzzword, but what treatment can achieve major skeletal change without surgery?

Orthotropics is ideal for paediatric and general dentists and I am happy to lecture or teach any groups who want to learn about it, free of charge. Distance is of little significance if I have the time.


Battagel, J.M. 1996. ‘The use of tensor analysis to investigate facial changes in treated Class II division 1 malocclusions’. European Journal of Orthodontics. 18: 41-54.

Chapman, H. 1931. ‘Abnormalities of position; treatment’. In Science and practice of dental surgery, Sir Norman Bennett. P 425. Oxford Med Pub

Franchi, L., Baccetti, T, Sacerdoti, R. and Tollaro, I. 1997 ‘Dentofacial features associated with crowding of the lower incisors’. European Journal of Orthodontics. 19: 570.

Lundstrom,A. & Woodside,D.G. 1980. ‘Individual variation in Growth Direction Expressed at the Chin and Midface’. European Journal of Orthodontics. 2:65-79

Mew, JRC. 1997. ‘The aetiology of temporo-mandibular disorders: a philosophical overview’. European Journal of Orthodontics. 19:249-258. 1997.

Mew, JRC. 2004. ‘The Postural Basis of Malocclusion: A philosophical overview’. The American Journal of Orthodontics and Dento-facial Orthopedics. 126:729-738.

Townend, B.R. 1955. ‘The comedy of expansion and the tragedy of relapse’. Dent. Mag. Oral Topics 72: 153-166. 1955.

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