The practice of orthodontics probably began with the Etruscans in the 8th to fourth century BC. In their practice, wires were used for closing the space left by a tooth lost in life. Today the practice of orthodontics flourishes around the world, as witnessed by travelers to any large city, even in the Third World. It is estimated that at least one-half of American youth could benefit from orthodontic therapy. In the U.S., it has become the largest single precollege line item expense for families with children.
This article discuses occlusal variation, malocclusion, and a proposed explanation of its high prevalence in modern commercial societies.
The term occlusal variation is roughly synonymous with malocclusion. The definition of malocclusion is a clinically unacceptable arrangement of the teeth. Occlusal variation includes malocclusion and the many lesser deviations from a rarely attained ideal arangement of teeth that do not cause functional problems.
The first comprehensive inquiry into malocclusion and its causes was by Weston Price. Inn the 1930s, he travelled the world to document the nutritional habits and physical degeneration of people living on contemporary 'civilized' diets. He found a significant incrase in malocclusion in societies living on contemporary diets of prepared foods from domesticated crops.
His work is ignored today, dismissed for its association with alternative medicine and lack of academic rigor demanded now in scientific literature. His evidence was largely anectodal. Yet, read carefully, Price reveals himself as a perceptive. thoughtful, and penetrating thinker.
What he observed about dental degradation is not in dispute. The core of his observations is this: The incidence of malocclusion amongst aboriginal peoples increased after contact with commercial societies. He examined both living populations and many collections of archaeological material.
Relatively little study has gone into probing the underlying causes of occlusal variation as compared with research into correction of orthodontic problems. The emphasis on clinical correction is understandable: in the United States, some 40-60% of youth have malocclusions, with orthodontic treatment either rated highly desirable, mandatory, or already under way. Such prevalence of occlusion that varies from the ideal is considered normal in our Western population. It is the the only epidemiological pattern familiar to American orthodontists.
An important consideration in understanding occlusal variation is this: there is a tendency amongst non-technologic human societies for virtually all individuals to show a nearly ideal occlusion. This observation gives rise to a theory widely favored among anthropologists which may be stated as follows:
Malocclusion arises from the lack of chewing stress with the modern processed diet. This disuse has reduced jaw growth and increased the incidence of occlusal variation.
Price (1989) in the 1930s vividly documented dental degradation across the globe with photography and description in his remarkable text, first published in 1939. He correlated dental problems with the adoption of modern processed foods.
Numerous studies are documented by Corrucinni (1991) which confirm and extend the findings by Price, but much of the cause is now attributed to functional disuse of the masticatory system. The reader is referred to the Corruccinni article cited and to his extensive literature on the subject.
Recall our unit on What is Science? This is an opportunity to look at a hypothesis to see how it ties together existing evidence and how it can suggest additional inquiry.
Numerous examples of increased incidence of malocclusion with modern dietary habits are cited in Price (1989) and Coruccini (1991), which lend support to the theory. Some will be cited here. This article is largely based on Corrucinni. The reader is referred to this excellent article for additional reading. Corrucinni articles are interesting and well written.
I. COMPARISON IN AN ISOLATED RURAL COMMUNITY IN TRANSITION
A rural community in the Mammoth Cave region of central Kentucky was surveyed over a 25 year period as it made the transition to industry and mechanized farming. The diet at the outset was home-produced foods (especially dried pork and fried cornbread) which provided consistently stressful chewing. The transition was from this to a diet of purchased supermarket foods. This study was of special interest since diet changed but residence did not. Furthermore, these are people in the same society--it is not a cross-cultural study with its inherent problems of different genetic constitutions.
Arch breadth was smaller and significantly more variable in younger individuals. Bigonial breadth, measured from an area affected by the medial pterygoid and masseter muscle action was considerably smaller in the younger sample. This study tends to suggest that there is a genetic predisposition or susceptibility to be diverted from programmed oral growth pathway by environmental factors. Lack of function, therefore, led to a different phenotypic expression--one with more occlusal variation. (My note: physical anthropologists who have compared pre-contact Eskimo lower jaws with living peoples today comment are the robustisity of the ancient peoples who used their teeth for many paramasticatory purposes such as softening animal skins.
II. OCCLUSAL VARIATION IN NORTHWEST INDIA
Children of similar genetic heritage, but differing lifestyles have been studied side-by-side in India. The Punjabi communities studied included children born into a higher socioeconomic urban class and children from rural communities with its traditional dietary and residence habits. Neither group had access to significant amounts of orthodontic care.
Significant differences were found in the samples. The lowest socioeconomic group had less variation from ideal occlusal relations and had wider (broader) maxillary arches.
Lack of functional stimulation could explain occlusal changes that were observed: small jaws with normal-size teeth (not normal arches burdened with excessively large teeth), resulting in crowding plus maxillary narrowing.
III. THE BEGG HYPOTHESIS AND THE EPIDEMIOLOGY OF MALOCCLUSION
An Australian, P. R. Begg has studied both living and deceased Australian Aborigines and has used them as a model for Stone Age man.
In 1954, Begg reasoned that the relatively low incidence of malocclusion in Stone Age Man is due to the reduction by more than half an inch in the total length of each of his dental
arches. This was caused by tooth attrition so that the smaller crowns (in mesiodistal diameter) could be more easily accommodated into the jaws. The theory has been widely cited since its publication.
Begg also argued that retention of unworn occlusal-cusp was not a feature of non-modern societies. Since the cusps wear flat relatively early in life, it is argued that the function of cusps is to guide erupting teeth into proper position and occlusion.
What he is saying is this: the function of cusps is transitory: in their brief functional life, they establish mandibular and condylar position. The argument might continue like this: a flat plane occlusion allows protrusive and lateral movments without cuspal interferences and is therefore less susceptible to TMJ disturbances (especially with shallow condylar fossae).
Our orthodox viewpoint, so deeply entrenched in dental education, holds that cusped teeth in adulthood is the proper functional form of teeth. This contrasting view argues that a flat-plane occlusion is the true normal state of adult occlusion.
Begg's specific figure of 10.54 mm loss of tooth substance due to proximal wear has been widely quoted. Is that figure valid? Begg's first attrition assessment were based on 98 worn teeth from nine mandibles at the development state just prior to eruption of third permanent molars and total arch loss amounted to 7.36 mm. These are from his 1935 doctoral thesis.
Calculations reported in 1954 were based on 126 worn teeth. Although Begg did not advance any explanation, it is evident that some arches of less perimeter length were selected for the 1954
calculations. It seems that Begg, unconsciously and over a long time period, selected cases of both unusually large unworn teeth and unusually small worn teeth. This would have inflated his estimate of loss of mesidistal tooth material. Had Begg used earlier mean measurements for unworn teeth, the 1935 result would have been reduced from 7.26 to 4.04 mm.
IV. ANIMAL EXPERIMENTS AND THE DISUSE THEORY
Various experimental attempts to induce developmental change in the mammalian masticatory have been made. These have included mouth breathing induced by nasal passage blockage, stress to the masticatory apparatus or altered occlusal relationships, and calorie-deficient diets. Various studies have examined the influence of hardness or softness of the diet on the masticatory system experimentally.
Rats were fed either pelleted rat chow as hard diet or crushed water-softened chow as a soft diet. The soft-diet animals had smaller mandibles, with condyles smaller and radiographically less dense, they had less width of the maxillary arch, and had smaller masseter and temporalis muscles.
A small population of rhesus macaques which spent a short period during adolescence on a soft instead of a hard diet revealed a significant narrowing of the maxillary arch in the soft-diet monkeys.
In the squirrel monkey, occlusal and craniofacal development on a soft diet was analogous to common human malocclusions--mesially narrow and disproportionately long maxillary arches leading to incisor overjet and occasional overbite. There were impacted malerupted premolars and second molars, malaligned premolar rows, and crowded and rotated incisors. In contrast, mediolateral arch breadths were significantly larger in hard diet animals.
Baboons fed a very soft atherogenic diet consisting of cholesterol-rich foods, lard, butter, egg yolks, and powdered chow were compared with hard diet controls. The various occlusal
anamolies were restricted to the soft diet experimental animals, some being quite typical of human malocclusions.
These studies suggest that a diet of foods requiring chewing is important in coordinating occlusofacial growth and reducing occlusal variation. Should our children eat beef jerky instead of Twinkies?
V. THE ETIOLOGY OF THIRD MOLAR IMPACTIONS
Humans evolved in a high dental attrition environment. At the same time, excessively large teeth may have been a selective disadvantage to an individual. Mastication of tough foods not only involved wear of the occlusal surfaces, but also movement of each tooth within its alveolus, constrained by the periodontal ligament. This movement of teeth within the dental arch also resulted in wear on the interproximal surfaces. This resulted in reduced tooth diameters in the mesio-distal dimension.
Combined with physiological mesial drift, humans would effectively achieve an increasing retromolar space as they age. The delayed eruption of the third molar seems to be an evolutionary adaptation to interproximal wear of the cheek teeth. It seems that the size of teeth were selected in anticipation of the wear and migration of the dental arcade to create sufficient room for the third molars.
The recent secular trend in increasing impactions does not seem to be a genetic change in humans. It is, instead, merely a response to a soft food diet. Without interproximal wear of the teeth, there simply is not enough room for third molar. Other factors at work with modern soft diets is dental arch width: narrower dental arches that result from disuse also contribute to shorter dental arches with less space available distal to second molars. (My note: for further discussion on these issues, see Mucci below.)
The disuse theory for occlusal deterioration says that we no longer use our masticatory apparatus vigorously and therefore it does not grow properly. This notion is quite old. Disuse theory was always included in the original etiological lists in Angle's and other early textbooks. It disappeared from them in the 1920s.
Now, there is much additional evidence to suggest a relationship between occlusal anomalies and softness of diet. Some evidence comes from comparison of older and younger generations where the younger generation has taken on a more Westernized lifestyle involving a softer diet. Another approach has been to compare living groups with younger and older generations of recent aboriginal Australians with museum specimens of pre-European contact aboriginal peoples. The pre-contact people show the fewest malocclusions and the younger generation the most. The implication is that functional stimulation of the developing jaws by a harder diet produced a larger dental arch which in turn would accommodate the teeth more readily. (My note: the comments by Mucci are especially significant: he notes that modern people on processed diets have less jaw robusticity and less proximal wear. Both factors contribute to third molar impactions.)
Let us here conclude with a summary of the critical issues discussed in this article. Polygenic traits (such as teeth and jaw features) are polygenic and thus are vulnerable to environmental influences. Price (1989) and Corrucinni (1991) correlate increased occlusal variation with a change from primitive to modern diets. Corrucinni (1991) suggests that the increased occlusal variaton is associated with a lack of functional use. Mucci (1981) relates diet, jaw robustisity, and proximal wear to the third molar impaction problem.
Should young children chew more?
..... CJ '98 (My note: Be sure to examine the figures at the end of this page.)
Begg, P. Begg Orthodontic Theory and Technique. Philadelphia: W. B. Saunders, 1965.
Corruccini, R. "Anthropological Aspects of Orofacial and Occlusal Variations and Anomalies" in Advances in Dental Anthropology New York: Wiley-Liss, Inc., 1991 pp 295-323.
Hilson, S. Dental Anthropology. New York: Cambridge University Press, 1996.
Mucci, R. The Role of Attrition in the Etiology of Third Molar Impactions: Confirming the Begg Hypothesis. Master's Thesis, University of Illinois at Chicago, 1982.
Price, W. Nutrition and Physical Degeneration. New Canaan: Keats Publishing, Inc., 1989.
Ring, M. Dentistry, an Illustrated History. St. Louis: C. V. Mosby Company, 1985.
Weinberger, B. History of Dentistry. St. Louis: C. V. Mosby Company, 1948.
Comment on the sources shown above: There is an extensive literature on this subject. Corrucini has quite a number of articles and they are intriguing. Begg seems to have been influenced by T. D. Campbell, another Australian with an extensive literature on Australian Aborigine teeth. Sim Wallace in 1904 influenced Campbell when he wrote that a lack of functional tooth use contributes to modern tooth irregularities unknown generations ago.
Figure Two Left: Lateral view of occlusion described as a normal occlusion with good facial relationships of maxillary and mandibular teeth from a current textbook of dental anatomy.* Note the anterior overjet and the acute angle formed by the incisors.
Figure Two Right: Lateral view of a dentition considered "normal" and a proper orthodontic treatment goal for modern man by disciples of the Begg technique. Observe the reduced overjet and the more upright inclination of the incisor teeth, from Graber.**
Adult Australian aboriginal skull with what is described by Begg as an anatomically correct attritional occlusion, including the edge-to-edge bite of the incisor teeth, adapted from Begg***
Adult Australian aboriginal skull with marked attrition of the teeth that has extended well into the dentin, adapted from Begg***
* Ash, M. Wheeler's Dental Anatomy, Physiology, and Occlusion. 7th ed., Philadelphia: W. B. Saunders Company, 1993.
** Graber, T. Orthodontics Principles and Practice. 2nd ed., Philadelphia: W. B. Saunders Company, 1966.
*** Begg, P. Begg Orthodontic Theory and Technique. Philadelphia: W. B. Saunders Company, 1965.