MASTICATORY FUNCTION AND MALOCCLUSION:

A CLINICAL PERSPECTIVEJUHA VARRELA

SEMIN ORTHOD 2006

INTRODUCTION Correct orthodontic treatment plan can be

made only through the understanding of how the genetic and epigenetic factors interact during growth and development.

This article emphazises the importance of mastication as an epigenetic factor in controlling growth and development of jaws.

The human dentition and occlusion have undergone significant changes during the recent history of man.

Orthodontic textbooks often discuss various genetic mechanisms that might explain the high frequency of malocclusions in contemporary humans.

GENETIC THEORIES OF MALOCCLUSION A popular genetic model was based on the

hypothesis that immigration, and admixture of genetically unrelated populations and ethnic groups, is the cause of increased occlusal variation in contemporary humans.

However, this hypothesis can be refuted as stable populations showed similar increases in occlusal variation as those who have undergone rapid immigration and admixture.

Another genetic model that was frequently mentioned in the literature assumed that the trends toward smaller jaw size and tooth size during human evolution have not proceeded in concordance. This hypothesis fails to recognize that neither the timing nor the speed of the change is compatible with evolutionary processes.

A closer look at the epidemiological data shows that none of the evolutionary or genetic explanations can offer a model to which the evidence satisfactorily fits.

A far better fit can be obtained with a model where the major causative factors are environmental rather than genetic.

ATTRITIONAL OCCLUSION The first systematic attempt to explain the role of

masticatory activity in the development of malocclusions was made by Begg.

After studying the dentitions of Australian aborigines, who lived on highly attritive diet, he concluded that dental wear, interproximal wear in particular, is the key factor adjusting the size of the dental arch to that of the jaw bones.

According to his calculations, extra space corresponding to the mesiodistal width of one molar is gained through interproximal wear.

Begg’s material was remeasured by Corruccini, who discovered that Begg had significantly overestimated the degree of dental reduction and he claimed that even in extreme conditions, the amount of tooth material lost in each quadrant is not more than 2 to 3 mm.

Extensive wear, whether occlusal or interproximal, was present only in adults but the fact that the skulls of children invariably show well-aligned dental arches,but only a moderate degree of wear.This indicates that iterproximal wear, as such, has very little to do with the development of good occlusion.

According to Corruccini hypothesis,hard diet requires vigorous mastication that in turn stimulates the growth of the facial bones,especially the transversal growth of the maxilla and mandible.

Tooth size/arch size discrepancies are solved by increasing arch size, not by dental reduction.

Tooth wear is merely a by product, brought about by the attritive diet and high masticatory activity, and has only a minor effect on the alignment of the teeth.

Begg formulated his model when the prevailing theory of craniofacial growth was based on the view that the size and shape of the facial bones were genetically predetermined and not affected by external influences.

Only after introduction of the functional matrix hypothesis, was it understood that bone growth can be secondary and adaptive to functional stimuli, including those originating from mastication.

Another analysis of a Finnish skeletal sample representing people who lived around the year 1600 AD, shows that the features of attritional occlusion were evident as recently as 400 years ago.

The Finnish samples showed a harmonious skeletal relationship and an acceptable overjet and overbite.

Seventeenth century Finns show a larger ramus, increased posterior facial height, a smaller gonial angle, a more horizontally oriented mandibular base, more palatally inclined upper incisors, a smaller interincisal angle.

Comparison of cephalometric features of Finns born about the year 1600 and about 1930 .

1930 Born finns1600 Born finns

Observations concerning the frequency of distal occlusion among Finns indicate that its frequency almost doubled during the first half of the 20th century.

It can therefore be speculated that a major increase in the occurrence of malocclusions may have taken place during the last 100 to 150 years owing to the diet of the modern man.

Moreover studies on experimental animals, including rodents, carnivores, ungulates, and nonhuman primates, which have been raised on soft showed similiar growth and occlusal discrepancies.

EPIGENETIC CONTROL A Class I type occlusion with well-aligned dental

arches has been the biological norm of the human species for most of its history.

But All the previously mentioned studies points the epigenetic control of growth also.

Epigenetic factors turn on and off the genomic processes in the same way as the genome itself.

This has been termed tertiary induction of the genome.

Malocclusions can be considered as developmental disorders or malformations of the occlusion that evolve postnatally because the inductive stimuli that is mastigatory function necessary for normal development are missing.

Malocclusions have become common with the decrease in masticatory functional demands. This indicates that all humans possess a genetic makeup that is sufficient for the development of a normal occlusion given the correct environmental conditions.

The ability to react to external influences, referred to as developmental or phenotypic plasticity, is a fundamental characteristic that enables organisms to adapt to a particular environment during ontogeny, thereby increasing their chances to survive.

CLINICAL IMPLICATIONS During orthodontic treatment it is this

environmental component of development, not the genetic, that can readily be influenced by therapeutic actions.

No growth modification would be possible if the soft and hard tissues of the jaws and face did not show developmental plasticity.

The occlusions of premodern populations indicate that malocclusions could be eliminated by changing the environmental conditions.

If the major environmental components controlling development are related to orofacial function, treatment with functional appliances would seem to offer an obvious solution.

A logical conclusion arising from Beggs hypothesis was to recommend extraction therapy as the treatment modality of choice because it would apply nature’s own strategy to clinical orthodontics.

REFERENCE ARTICLES Masseter muscle thickness and maxillary dental

arch width- European Journal of Orthodontics(2003) The purpose of the present investigation was to

study the relationship between the ultrasonographic thickness of the masseter muscle and the width of the maxillary dental arch.

The sample comprised 60 consecutive orthodontic patients (37 females, 23 males), 7–18 years of age with a Class I relationship and minor malocclusion. The thickness of the masseter muscle was measured ultrasonographically.

In the female group, maxillary intermolar width showed a direct, significant association with masseter thickness. i.e. females with thicker masseter muscles had a wider maxillary dental arch.

the male group, showed no significant relationship.

CONCLUSION The challenge for us being a specialist is to

learn how to mimic the external control of occlusal development by orthodontic treatment procedures. This could open up possibilities to actively promote a harmonious development of the jaws, facial soft tissues, and occlusion, instead of the present situation where disturbances are allowed to develop before treatment is initiated.