1.0 Malocclusion – Is it genetic?

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The subject has been debated since the times of the ancient Greeks and often generates more emotion than logic. I am constantly amazed that so many orthodontists are prepared to provide treatment without a clear understanding of what has caused the particular problem they are facing. To fail to do so, risks treating the symptoms of malocclusion rather than the cause. Is it genetic, environmental or both and if the last, in what ratio? It would seem essential to answer this question before contemplating how any malocclusion should best be treated.

Gross et al (1994) found 63% of five year olds left their mouth open for long periods and that those with “High levels of open mouth posture manifested significantly smaller growth of the upper jaw (maxilla)”, this in turn restricts the growth the lower jaw resulting in too little room for the teeth.

When I ask clinicians what has caused a particular malocclusion they will often answer like this “this malocclusion is due to a combination of an undersized maxilla and a retruded mandible”, but they will not explain why this discrepancy has occurred. The inference being that it has a genetic cause. Most clinicians would agree that congenital defects and Trauma are responsible for less than 5% of all malocclusions so what causes the rest?

In 1938 Brodie measured a number of X-rays and announced “The most startling find was the apparent inability to alter anything beyond the alveolar process”. This led to a generalised belief amongst orthodontists that the facial bones are inherited and cannot be changed more than a millimetre or two. However it is known that Bones remodel considerably during growth, for instance, as the maxilla enlarges the front of the sinus moves forward while the posterior wall moves back relative to the general forward movement of the bone. As a result of this and other growth changes, the landmarks that are used to overlay consecutive X-rays move, making it impossible to make accurate superimpositions.

Modern computer enhanced X-rays show very sharp 2 dimensional images but we must remember that these are based on rather fuzzy shadows of a three dimensional skull with inevitable slight tilt, differential enlargement and rotation distorting the superimposition often by several millimetres. This strongly affects the landmarks not on the midline which is why X-rays alone must be a less than certain means of determining the movements that actually take place. Hopefully the situation will become clearer as modern scanning techniques improve.

Overall the evidence is not as conclusive as some would suggest and there are clear differences of opinion amongst accepted leaders in the field. Above all, we are left with no explanation for the fact that many mono-zygote twins have substantially different facial form. Clearly it is impossible for these to be genetic if both individuals have the same genes. See the illustration below, by permission of Tom Graber.

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