Abstract
Investigations concerning the aetiology of malocclusion have, in recent years, taken up still greater space in the orthodontic literature. Much of this work has been devoted to the effect of different environmental factors and an endeavour has been made to analyse the effect of such factors as birth injuries, finger sucking and other habits, extractions of deciduous and permanent teeth, insufficient function of the jaws, nasal and throat disturbances and lack of vitamins. These investigations show clearly that none of these factors inevitably produce malocclusion. On the contrary, more exact determinations on larger quantities of material have shown rather slight connections between the occurrence of these factors and malocclusion. This seems to imply that other factors must be of importance, among which the genetic factors take first place. The significance of heredity can be studied mainly along three lines, namely by experiments on animals, by family investigations and by twin investigations. All these methods have been used in the study of malocclusion of the teeth. Crossing-experiments on animals can give us certain indications as to the fundamental principles concerning the inheritance of malocclusion but it is, of course, not possible to know if the results of such experiments are significant for man also regarding similar malocclusions. Family investigations based on fairly large numbers are very difficult to make as it is so difficult to obtain reliable information of the occlusion, even in only two generations, on account of the high frequency of extractions in our time. It is much easier to obtain a fairly large number of twins for investigation and this method has also been used by several authors in the study of the aetiology of malocclusion. As is well known, one can, through observation of twins, get a conception of the influence of genetic factors as compared with …
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