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Genetic and environmental factors in dentofacial morphology
Genetic and environmental factors in dentofacial morphologjy DR.
FRAN8
Nymegen,
P.
G.
M.
VAN
TlER
LTNUF:N’
The Netherlands
THE importance of genetic and environmental factors in orthodont.iics has been discussed mainly with reference to the etiology of malocclusions. Little thought has been given to the effect of genetic and nongenetic factors on orthodontic therapy and stability of treatment results. The same holds true for the changes in facial appearance and dental conditions taking place between adolescence and old age. However, the same laws which govern the growth and development of the dentofacial complex are applicable to the individual response to orthodontic treatment and the alterations that occur thereafter. The interaction between genetic and environmental factors starts at conception and continues until the end of life. During fetal life the contact of the genetic composition with the environment is rather limited. On the other hand, all components outside the genes (the protoplasm of the ovum, for example) are considered environmental. Until adulthood, the permanent interac,tion between genetic and environmental factors guides and controls the process of growth a.nd development and determines the morphologic and physiologic traits of the individual. There is adequate evidence that many developmental conditions are primarily under genetic control. This has been shown for the sequence of eruption of the deciduous and permanent teeth as well as for the onset of puberty. On the other hand, environmental factors also have a bearing on both phenomena, but these factors seem to be of secondary significance. In this process the prevention and correction of orthodontic deformities can be considered an environmental factor which is artificially added to those already present. Clinical experience supports the theoretical concept that the possibilities of orthodontic treatment are considerably greater in a developing child than in a fully grown person. The effect of such an additional-and often comparatively powerful-environmental factor depends partly on the situation of interaction between genetic and nongenetic factors at a. given moment. That “Professor
576
of Orthodontics,
University
of Nymegen.
Volume Number
52 8
Genetic and environ.mental
factors
577
the moment of interference is an important factor is simply illustrated by the difference in the effects when the mandibular condyle is destroyed at 4 and at 18 years of age. Not much is known about the effect of finger-sucking on the later dental conditions in relation to the habit’s duration and the developmental stage at which it has been practiced. However, it is reasonable to assume that in this respect the individual response also depends partly on the developmental stage at which the specific environmental factor was acting. The degree of stability of orthodontic treatment results is often rather difficult to predict. Retention appliances can maintain a certain situation by suppressing the tendencies toward relapse. After termination of the retention period, however, the interaction between genetic and environmental components is no longer influenced by the extra-environmental factor. The relapse tendencies, which can be genetic or environmental in nature, act until a new state of balance is reached. During the retention period the tissues have a chance to establish more normal anatomic and physiologic conditions, creating a more favora.ble environment for a stable end result. In this respect, the ideal situation at the end of the retention period can be described as one in which genetic and environmental factors are in a new harmonious stage of interaction. It is always difficult to foresee how much the newly obtained situation will be tiected by later changes (for example, eruption of the third molars or anteroposterior growth of the mandible that is relatively greater than that of the maxilla). Very little is known about the interaction of genetic and environmental components in later life. The effect of certain genes may be limited to a specific period of life, as is the case in puberty. It is known from embryologic experiments that some genes do act at a specific stage of fetal development. Genes not only control the growth and development of an individual, but they have a large influence on all aspects of life during the developmental period and later on. How the body reacts depends on the momentary interaction between genetic and environmental factors. The latter are without doubt subjected to continuous alteration. The influence of the genetic components cannot be considered stable, for it, too, changes with time. The field of periodontics presents many examples of both aspects. During the last two decades essential information has been presented concerning the effects of genetic and nongenetic factors on dentofacial morphology. Most of it is baaed on the study of monozygotic and dizygotic twins. Pedigree and population investigations have not been so frequent in this field, for it is much more difficult to collect the material. Since the twin-study method does not present information concerning the manner of inheritance, little is known about the genetic transmission of dentofacial traits. Many dental dimensions, such as tooth size and form and arch width and length, are under genetic control. It also has been found that the pattern of the facial skeleton is primarily genetically determined. The soft tissues and their behavior have not been studied thoroughly enough to warrant a statement on this aspect. However, it seems reasonable to assume that the morphology of the soft tissues, as well as their behavior, is genetically controlled also. The whole picture is complicated by the fact that all of the structures influence each other
to a larger or smaller cxtcnt. The muscles of the tongne do have ;I dcfinitc influence on arch form, and it is questionable how far the UWI’SC holds trr~l. The effect of the environmental and genetic influence is sometimes dificult to distinguish. The finger-sucking tcnt1enc.y may be gclnet ical ly dcterminecl, although bad habits usuallJ7 are considered of environmental nature. on the other hand, a genetically determined large overjet may favor the development of an undesirable habit, such as lip-sucking. In the past the elects o-f genctic and cnrironmental factors on the struct,ure of the facial complex havcb been oversimplified by many authors-partly because of the limited information available on hereditary influences and partly because of the typical method of approach, classified by Graberl as “retroactive reasoning.” In t,he past, many conditions of an environmental nature have been mentioned as causing orthodontic malocclusions. Lundstr6m,s one of the greatcbst authorities on etiology, prescntcd an extensive critical review of the literature on the factors influencing dentofacial morphology. He came to the conclusion that environmental factors in utero might affect the occlusion, but, this possibility has not been examined. Certain slight disturbances may OCCIW during delivery, but they probably are of a temporary nature. Act,ually, later environmental factors have been the main subject of discussion in the literature. Finger-sucking has hecln t,he object of several studies, which indicate that this factor is mostly temporary and of minor significance as far as malocclusion in adults is concerned. Another muchdiscussed factor is the extraction of teeth; investigations mostly show that t,he extraction of deciduous teeth is of minor importance. Impeded nasal breathing also has been given much attention, and studies seem to justify the conclusion that impeded nasal breathing is not the direct cause 01’certain definite malocclusions. The connection between mouth breathing and a rclati\cly long and narrow dental arch in the upper jaw is most simply explained by the fact that, these characteristic are different expressions of a generally narrow type of fact. The studies published after Lundstrijm’s excellent thesis give no reason to change the review presented above. Also, sufficient evidence has never been presented to show that bottle feeding instead of breast feeding is an cbtiologic factor in the development of malocclusion. Too often, and for too long, retroactive reasoning has guided the concepts of etiology. For example, many conditions have been blamed for causing protrusion of the upper incisors merely because they were associated with it. It is true that this kind of malocclusion is often accompanied by or involves a history of bottle feeding, finger-sucking, enlarged tonsils and adenoids, a short upper lip, or a tongue habit, but this does not mean that these may be marked as the causative factors. The fact that certain conditions are associated with a certain malocclusion does not warrant a conclusion regarding the etiology of that malocclusion, but this is very often done. Even classifications of malocclusion are based on this concept, and they are still widely in use. Blue eyes do not cause fair hair, and short, fingers do not cause broad fingernails. In orthodontics, as in most other clinical dental and medical fields, unsound reasoning is not always avoided. The same holds true for overestimating the value of unsystematically made, repeated observations of the same kind. Often this has led to unjustified conclusions.
Volume 52 Number 8
Genetic
and environmental
factors
579
For many years consideration of the importance of genetic factors has been limited mainly to discrepancies between the size of the jaws and the tooth material present. Children with large teeth and small jaws would have inherited the large teeth of one parent (probably the father) and the small jaws of the ot,her. This concept gained importance with the cross-breeding experiments on dogs reported by Johnson.3 Small jaws and large teeth were considered to be caused by more or less independent genes of a dominant nature. However, the intluence of genetic factors is much more complicated. Recent investigations have shown that the composition of the genetic part-of importance for the structure of the dentofacial complex-is much broader than originally was thought. Kraus and Furr,4 in 1953, made a study of the lower first premolar, a tooth with an extremely wide range of morphologic variability, and were able to indicate seventeen traits of common occurrence in the human population. Their sample consisted of Papago Indians, Caucasoids, Yaqui Indians, Mexicans, Chinese, and Negroes. Kraus and Furr stated that each of the seventeen traits is inherited as an independent variable. Their recordings pertain only to tooth form; dimensions were not included. When we realize that such a large genetic spectrum could be indicated for the morphology of one single tooth, the assumption of one gene for tooth size and one for jaw size is by no means tenable. The asymmetry in tooth form and size between the right and left sides can be carried back to genetic variations also. Quite often a difference in dimensions and shape of corresponding teeth can be observed.5Another phenomenon that can be considered an expression of genetic variation is the so-called “tooth size discrepancy” seen when the teeth of the lower jaw cannot occlude adequately with those of the upper jaw becausethe mesiodistal dimensions do not correlate.6 Experiments with animals, and particularly with the Drosophila melanogaster, have yielded information that can clarify the phenomena discussed before. Of the forty-six chromosomes present in most cells of the human body, twenty-three come from the father and twenty-three from the mother. The selection of the pairs of chromosomeswhich go to one sex cell and those which go to the other in meiosis is supposed to happen at random. This is the basis for the large variations in nature and human population. The number of genes present in one set of twenty-three chromosomesis estimated at about 10,000.7 It may be assumed, on the basis of investigations in other fields, that the genes of importance for the structure of the dentition and the facial skeleton are distributed over a certain number of chromosomes. The larger the number of chromosomesinvolved, the larger the potential variation for this area in genetic composition of the offspring. On the other hand, closely related traits may be localized on the same chromosome and even close together. In that case, they will be inherited together except when crossing-over takes place. In crossingover, homologous segments of paired maternal and paternal chromosomes are exchanged in meiosis, As long as we do not know which genes are involved and where they are located, we can only guess about the causes of the phenomena described here. A tooth size discrepancy can be causedby a nonconcordant combination of genes placed in different chromosomes or by a crossing-over that took place recently
580
Tan der Linden
or many generations ago. The relative low frequency of tooth size discrcpanc! favors the assumption that crossing-over is probabl>- of llliljor importanc~c~ 11~11. The whole picture is complicated by the fart that one g,rcnc can ho IW~OIIsible for more traits (pleiotropism j and more than one1 gene can dcternlinc on(’ single character, such as hair color. l’olygenic systems givck raise to c~ontinnous variability, which is often described as normal variat,ion. Some investigations have been made on the influcllc*e of gclnctic* and PIIvironmental factors regarding the morphology of the facial skeleton. In t,hc few that will be mentioned here, cephalometric roentgenograms wwc used fol* ctvaluation. Lundstr6mg studied fift,y monozygotic and fift,y dizygotic twins and coneluded that genetic factors seem to have between once and twice the significance of nongenetic factors wit,h respect to the variability of the characteristics investigated. He studied the angles commonly used in roentgenocephalometrics. Kraus, Wise, a,nd Freil” approached their study of six sets of triplets in another way. The craniofacial complex was divided into its component, parts, and the profile lines of the individual. bones were studied as they could he ohserved in lateral and frontal headfilms. Almost, perfect concordance was the rule in monovular pairs, whereas or117 a low degree of concordance (hc~low 50 per cent) was found in diovular pairs. Sevclntecn bony traits were tlrfined in terms of lateral and frontal headfilm tracings, on which they based the speculation that t,he structure of all t,hc bones of the craniafacial complex is under the rather rigid control of hereditary fortes. Thcv also used the gtnctrally acccptecl cephalometric polygon procedures and found no significant findings in contrast with Lundstrb;m’s observations. This could be due to the kind of analysis used and the considerably smaller sample. Kraus, Wise, arid Frci did not. find :i greater similarity in length of lines or degree of angles in monovular twins than in diovular twins. (The six set.s of triplets could b(k differentiated in eighteen pairings of six monovular and twelve diovular twins.) The dXercnce bet,wcen findings for the individual bony structures and for the craniofacial complt~x ax seem that a whole caused them to make the following statement: *‘It. woulcl heredity governs morphology, but environment in its multitudinous facets has much to say about how these bony elements shall combine to achieve what interests us most-the harmonious (or unharmonious) heatl and f’aee.” This conclusion agrees very well with the findings of \Vieslandcr,ll who studied the effect of occipital anchorage (an environmental factor) upon the dentofaeial area in t,he mixed-dentition period. He regist,ered a, slight clockwise rotation of the sphenoid bone, with an alteration of the facial growth pattern. From his findings, he concluded that there is strong evidence to support the contention that headgear trea.tment durin, w the age period studied (*an alter the interrelationship of the sphenoid bone to the ot,hrr hones in thp craniofacial complex. In a recent publication by Ross and Coupe,12 the morphologic characteristics of six pairs of monozygotic twins, discordant for cleft lip and for cleft palate, were analyzed. A principal finding was that a unilateral cleft of the lip and palate is characterized by an alteration in the relationship of the facial skeleton to the cranial base, rather than by an intrinsic defect of the facial components
Genetic and environmental
factors
58 1
(other than the cleft itself). This also indicates the important role of nongenetic factors in the combination of the bony parts. It is well known that mixing of genetically different populations increases considerably the genetic variability of later generations. This applies to all forms of characters for which more alleles are present. The increased range of variability in morphology does not lead to notable abnormalities and functional insufficiencies in most areas of the body. If the lower arm is relatively short compared to the upper one, or the left arm is a little bit shorter than the right one, no inconveniences will result. Broad and short hands are not a sign of beauty for a slender person, but this is only a matter of esthetics. For the dentofacial complex, however, an increased range of variability in the individual components has not only esthetic but very important functional consequences as well. Nowhere in the rest of the body can nonconcordant combinations so easily be recognized. The occlusion of the teeth is an extremely suitable registration tool. On a roentgenocephalogram a very accurate analysis of the facial skeleton and associated soft tissues can be carried out. On the other hand, the dentofacial morphology is made up of an extremely large number of components which, in combination with each other, are responsible for the end result. The form and dimensions of the cranial base and both jaws are the foundations on which other components are built. Certain variances can cancel each other out in such a way that no malformations will result. On the other hand, they can work in the same direction and cumulate to a marked, unacceptable deviation from the normal standards. An overdeveloped maxilla accompanied by a large lower jaw presents a good basis for a normal occlusion and may lead to a classic profile. An overdeveloped maxilla accompanied by a small lower jaw usually results in an Angle Class II malocclusion and a less favorable facial outline. The position, size, and form of the alveolar processes and the position and inclination of the teeth are variables which are superimposed on the basal bones, and they do influence the occlusion and facial appearance. Many other factors, such as the form and dimensions of the soft Gssue, the position of the medial cranial fossa, and the correlated position of the articular fossae, etc., also influence bhe morphology of the dentofacial complex. It has been shown that the position, form, and dimensions of the mandible depend on a larger number of variances than those of the maxilla. The place of the condyle is largely determined by the location of the occipital and temporal bones in the skull. The height of the ramus, the length of the horizontal part, the size of the gonion angle, and the form of the chin button determine the facial outline to a great extent. The height of the mandible in the incisor and molar region further contributes to it. We do not yet know how many genes and alleles are involved. Studies made on other parts of the body, such as the hand, suggest that there will be many. Their mode of action is largely unknown too. Perhaps some genes control the basic form and shape of the main structures and others act on top of these, causing minor variations. Recent investigations using the multifactor analysis support this hypothesis. I3 It seems that certain genes each govern several parts of the dentofacial complex and, on the other hand, more genes are responsible for one single character.
A number of studies have been undert,aken to determine the essential deviations of certain structures in different dcntofacial malformations as compared with normal ones. Very seldom could marked abnormalities in individual cornponents adequately be shown. In most instances the component parts, in themselves, could bc considered as being within the range of normal variation. ‘I’hc>just did not fit together into a harmonious combination. The fact that only in certain isolated areas, such as some of the Philippine Islands, can a population with normal occlusion be found’ supports the hypothesis that many orthodontic malformations are caused by a disharmonious combination of variables which, considered individually, are within normal limits. The genetically large variation of the individual components probably has reduced considerably the occurrence of ideal occlusions and profiles. The influence of genes that can he made responsible for specific deviations, such as typically rotated teeth, impacbed upper canines, micrognathia, etc., seems to be of minor importance. As stated before, the effect of genetic and environmental factors in dentofacial morphology is by no means limited to the etiology of normal and abnormal orthodontic conditions. Orthodontic treatment can be considered a.s an additional environmental factor. With the type of appliance and the period of application, the results vary. In general, multiband techniques result. in more extensive alterations than those that can be accomplished with removable appliances. In comparison with the other (natural) environmental fact,ors, orthodontic appliances probably exert relatively powerful forces. Very little is known about the intensity of natural environmental components. The same is true of the relationship between the effect of finger-sucking and the time of beginning, duration, frequency, and intensity of this habit. How far the effect of orthodontic therapy can be favored or counteracted by the genetic composition is still an open question. There are some indications-theoretical and clinical, as well as experimental-that this is a factor of importance. The st.ability of the end result will depend on the fact t,hat the genetic and environmental influences are in harmony with each ot,her. If this is not the case, a new state of balance will be established, mostly resulting in partial relapse. In the permanent interaction between genetic and environmental factors, orthodontics has its specific place. Successes and failures probably depend to a large extent on the fact, that the genetic composition allows and favors the planned orthodontic therapy and the end result aimed for. Surveying our knowledge of genetic and environmental factors in dentofacial morphology, we realize that we are just beginning to acquire a better understanding of the problems involved. It may be expected that the present intensive research in genetics will supply us with essential information concerning our field too, but much research in orthodontics and the basic sciences is also needed. In the past the influence of genetic and environmental factors has been frequently oversimplified, and in most cases one was overestimated in an unjustified way, generally at the cost of the other. Seldom was balanced attention given to the two factors involved. Now it is difficult to adopt a position between the two. If some statement should be made, however, perhaps it would be that the permanent interaction between genetic and environmental factors directs the
Volume Number
52 8
Genetic and ewvironmental
factors
583
growth of the craniofacial complex (including the development of malformations). Genetic factors seem to have the greatest influence, whereas environmental factors appear to be of minor importance. Environmental factors, including orthodontic therapy, probably primarily affect the dental alveolar region and the interrelationship of the individual bony element,s. The morphology of the individual bones seems to be under rather rigid genetic control. The stability of the result of orthodontic treatment depends mainly on the fact that a new state of balance in the interaction between genetic and environmental factors is obtained. The changes in dentofacial structure that take place with aging are the result of the same continuous alterations in this interact,ion. SUMMARY
The importance of genetic and environmental factors with regard to growth and development of the dentofacial area has been discussed on the basis of recent information. Their role in orthodontic therapy (quite often a powerful environmental factor), in the stability of the results obtained, and in the alterations in later life is underlined. A permanent interaction between genetic and environmental factors, both of a continually altering nature, determines the dentofacial morphology in every moment of life. Genetic factors seem to have the greatest influence, and environmental factors appea,r to be of minor importance. The stability of the result of orthodontic treatment depends mainly on a new state of balance in the interaction between genetic and environmental factors. REFERENCES
1. Graber, T. M.: Orthodontics: Principles and Practice, Philadelphia, 1961, W. B. Saunders Company. 2. Lundstram, A.: Tooth Size and Occlusion in Twins, Thesis, Upsala, 1948. 3. Johnson, A. L.: The Constitutional Factor in Skull Form and Dental Occlusion, AM. J. ORTHODONTICS & ORAL SURG.~~: 627-663,194O. 4. Kraus, B. S., and Furr, M. L.: Lower First Premolars, J. D. Res. 32: 554-564, 1953. Diagnosis and 5. Ballard, M. L.: Asymmetry in Tooth Size: A Factor in the Etiology, Treatment of Malocclusion, Angle Orthodontist 14: 67-71, 1944. 6. Bolton, W. A.: Disharmony in Tooth Size and Its Relation to the Analysis and Treatment of Malocclusion, Angle Orthodontist 28: 113-130, 1958. 7. Stern, C.: Principles of Human Genetics, San Francisco, 1960, W. H. Freeman & Company. 8. Goodman, H. 0.: Genetic Parameters of Dentofacial Development, J. D. Res. 44: 174184, 1965. of Genetic and Non-genetic Factors in the Facial 9. LundstrSm, A.: The Importance Skeleton Studied in 100 Pairs of Twins, Tr. European Orthodont. Sot., pp. 92-107, 1954. and the Craniofaeial Complex, 10. Kraus, B. S., Wise, W. J., and Frei, R. H.: Heredity AM. J. ORTHOWNTICS45: 172-217,1959. 11. Wieslander, L.: The Effect of Orthodontic Treatment on the Concurrent Development 49: 15-27, 1963. of the Craniofacial Complex, AM. J. ORTHODONTICS 12. Ross, R. B., and Coupe, T. B.: Craniofacial Morphology in Six Pairs of Monozygotic Twins Discordant for Cleft Lip and Palate, J. Canad. Dent. A. 3: 149-158, 1965. Factors in Two Ethnic 13. Brown, T., Barrett, M. J., and Darroch, J. N.: Craniofacial Groups, Growth 29: 109-123, 1965. Sint
Annastraat
318
FRAN8
Nymegen,
P.
G.
M.
VAN
TlER
LTNUF:N’
The Netherlands
THE importance of genetic and environmental factors in orthodont.iics has been discussed mainly with reference to the etiology of malocclusions. Little thought has been given to the effect of genetic and nongenetic factors on orthodontic therapy and stability of treatment results. The same holds true for the changes in facial appearance and dental conditions taking place between adolescence and old age. However, the same laws which govern the growth and development of the dentofacial complex are applicable to the individual response to orthodontic treatment and the alterations that occur thereafter. The interaction between genetic and environmental factors starts at conception and continues until the end of life. During fetal life the contact of the genetic composition with the environment is rather limited. On the other hand, all components outside the genes (the protoplasm of the ovum, for example) are considered environmental. Until adulthood, the permanent interac,tion between genetic and environmental factors guides and controls the process of growth a.nd development and determines the morphologic and physiologic traits of the individual. There is adequate evidence that many developmental conditions are primarily under genetic control. This has been shown for the sequence of eruption of the deciduous and permanent teeth as well as for the onset of puberty. On the other hand, environmental factors also have a bearing on both phenomena, but these factors seem to be of secondary significance. In this process the prevention and correction of orthodontic deformities can be considered an environmental factor which is artificially added to those already present. Clinical experience supports the theoretical concept that the possibilities of orthodontic treatment are considerably greater in a developing child than in a fully grown person. The effect of such an additional-and often comparatively powerful-environmental factor depends partly on the situation of interaction between genetic and nongenetic factors at a. given moment. That “Professor
576
of Orthodontics,
University
of Nymegen.
Volume Number
52 8
Genetic and environ.mental
factors
577
the moment of interference is an important factor is simply illustrated by the difference in the effects when the mandibular condyle is destroyed at 4 and at 18 years of age. Not much is known about the effect of finger-sucking on the later dental conditions in relation to the habit’s duration and the developmental stage at which it has been practiced. However, it is reasonable to assume that in this respect the individual response also depends partly on the developmental stage at which the specific environmental factor was acting. The degree of stability of orthodontic treatment results is often rather difficult to predict. Retention appliances can maintain a certain situation by suppressing the tendencies toward relapse. After termination of the retention period, however, the interaction between genetic and environmental components is no longer influenced by the extra-environmental factor. The relapse tendencies, which can be genetic or environmental in nature, act until a new state of balance is reached. During the retention period the tissues have a chance to establish more normal anatomic and physiologic conditions, creating a more favora.ble environment for a stable end result. In this respect, the ideal situation at the end of the retention period can be described as one in which genetic and environmental factors are in a new harmonious stage of interaction. It is always difficult to foresee how much the newly obtained situation will be tiected by later changes (for example, eruption of the third molars or anteroposterior growth of the mandible that is relatively greater than that of the maxilla). Very little is known about the interaction of genetic and environmental components in later life. The effect of certain genes may be limited to a specific period of life, as is the case in puberty. It is known from embryologic experiments that some genes do act at a specific stage of fetal development. Genes not only control the growth and development of an individual, but they have a large influence on all aspects of life during the developmental period and later on. How the body reacts depends on the momentary interaction between genetic and environmental factors. The latter are without doubt subjected to continuous alteration. The influence of the genetic components cannot be considered stable, for it, too, changes with time. The field of periodontics presents many examples of both aspects. During the last two decades essential information has been presented concerning the effects of genetic and nongenetic factors on dentofacial morphology. Most of it is baaed on the study of monozygotic and dizygotic twins. Pedigree and population investigations have not been so frequent in this field, for it is much more difficult to collect the material. Since the twin-study method does not present information concerning the manner of inheritance, little is known about the genetic transmission of dentofacial traits. Many dental dimensions, such as tooth size and form and arch width and length, are under genetic control. It also has been found that the pattern of the facial skeleton is primarily genetically determined. The soft tissues and their behavior have not been studied thoroughly enough to warrant a statement on this aspect. However, it seems reasonable to assume that the morphology of the soft tissues, as well as their behavior, is genetically controlled also. The whole picture is complicated by the fact that all of the structures influence each other
to a larger or smaller cxtcnt. The muscles of the tongne do have ;I dcfinitc influence on arch form, and it is questionable how far the UWI’SC holds trr~l. The effect of the environmental and genetic influence is sometimes dificult to distinguish. The finger-sucking tcnt1enc.y may be gclnet ical ly dcterminecl, although bad habits usuallJ7 are considered of environmental nature. on the other hand, a genetically determined large overjet may favor the development of an undesirable habit, such as lip-sucking. In the past the elects o-f genctic and cnrironmental factors on the struct,ure of the facial complex havcb been oversimplified by many authors-partly because of the limited information available on hereditary influences and partly because of the typical method of approach, classified by Graberl as “retroactive reasoning.” In t,he past, many conditions of an environmental nature have been mentioned as causing orthodontic malocclusions. Lundstr6m,s one of the greatcbst authorities on etiology, prescntcd an extensive critical review of the literature on the factors influencing dentofacial morphology. He came to the conclusion that environmental factors in utero might affect the occlusion, but, this possibility has not been examined. Certain slight disturbances may OCCIW during delivery, but they probably are of a temporary nature. Act,ually, later environmental factors have been the main subject of discussion in the literature. Finger-sucking has hecln t,he object of several studies, which indicate that this factor is mostly temporary and of minor significance as far as malocclusion in adults is concerned. Another muchdiscussed factor is the extraction of teeth; investigations mostly show that t,he extraction of deciduous teeth is of minor importance. Impeded nasal breathing also has been given much attention, and studies seem to justify the conclusion that impeded nasal breathing is not the direct cause 01’certain definite malocclusions. The connection between mouth breathing and a rclati\cly long and narrow dental arch in the upper jaw is most simply explained by the fact that, these characteristic are different expressions of a generally narrow type of fact. The studies published after Lundstrijm’s excellent thesis give no reason to change the review presented above. Also, sufficient evidence has never been presented to show that bottle feeding instead of breast feeding is an cbtiologic factor in the development of malocclusion. Too often, and for too long, retroactive reasoning has guided the concepts of etiology. For example, many conditions have been blamed for causing protrusion of the upper incisors merely because they were associated with it. It is true that this kind of malocclusion is often accompanied by or involves a history of bottle feeding, finger-sucking, enlarged tonsils and adenoids, a short upper lip, or a tongue habit, but this does not mean that these may be marked as the causative factors. The fact that certain conditions are associated with a certain malocclusion does not warrant a conclusion regarding the etiology of that malocclusion, but this is very often done. Even classifications of malocclusion are based on this concept, and they are still widely in use. Blue eyes do not cause fair hair, and short, fingers do not cause broad fingernails. In orthodontics, as in most other clinical dental and medical fields, unsound reasoning is not always avoided. The same holds true for overestimating the value of unsystematically made, repeated observations of the same kind. Often this has led to unjustified conclusions.
Volume 52 Number 8
Genetic
and environmental
factors
579
For many years consideration of the importance of genetic factors has been limited mainly to discrepancies between the size of the jaws and the tooth material present. Children with large teeth and small jaws would have inherited the large teeth of one parent (probably the father) and the small jaws of the ot,her. This concept gained importance with the cross-breeding experiments on dogs reported by Johnson.3 Small jaws and large teeth were considered to be caused by more or less independent genes of a dominant nature. However, the intluence of genetic factors is much more complicated. Recent investigations have shown that the composition of the genetic part-of importance for the structure of the dentofacial complex-is much broader than originally was thought. Kraus and Furr,4 in 1953, made a study of the lower first premolar, a tooth with an extremely wide range of morphologic variability, and were able to indicate seventeen traits of common occurrence in the human population. Their sample consisted of Papago Indians, Caucasoids, Yaqui Indians, Mexicans, Chinese, and Negroes. Kraus and Furr stated that each of the seventeen traits is inherited as an independent variable. Their recordings pertain only to tooth form; dimensions were not included. When we realize that such a large genetic spectrum could be indicated for the morphology of one single tooth, the assumption of one gene for tooth size and one for jaw size is by no means tenable. The asymmetry in tooth form and size between the right and left sides can be carried back to genetic variations also. Quite often a difference in dimensions and shape of corresponding teeth can be observed.5Another phenomenon that can be considered an expression of genetic variation is the so-called “tooth size discrepancy” seen when the teeth of the lower jaw cannot occlude adequately with those of the upper jaw becausethe mesiodistal dimensions do not correlate.6 Experiments with animals, and particularly with the Drosophila melanogaster, have yielded information that can clarify the phenomena discussed before. Of the forty-six chromosomes present in most cells of the human body, twenty-three come from the father and twenty-three from the mother. The selection of the pairs of chromosomeswhich go to one sex cell and those which go to the other in meiosis is supposed to happen at random. This is the basis for the large variations in nature and human population. The number of genes present in one set of twenty-three chromosomesis estimated at about 10,000.7 It may be assumed, on the basis of investigations in other fields, that the genes of importance for the structure of the dentition and the facial skeleton are distributed over a certain number of chromosomes. The larger the number of chromosomesinvolved, the larger the potential variation for this area in genetic composition of the offspring. On the other hand, closely related traits may be localized on the same chromosome and even close together. In that case, they will be inherited together except when crossing-over takes place. In crossingover, homologous segments of paired maternal and paternal chromosomes are exchanged in meiosis, As long as we do not know which genes are involved and where they are located, we can only guess about the causes of the phenomena described here. A tooth size discrepancy can be causedby a nonconcordant combination of genes placed in different chromosomes or by a crossing-over that took place recently
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or many generations ago. The relative low frequency of tooth size discrcpanc! favors the assumption that crossing-over is probabl>- of llliljor importanc~c~ 11~11. The whole picture is complicated by the fart that one g,rcnc can ho IW~OIIsible for more traits (pleiotropism j and more than one1 gene can dcternlinc on(’ single character, such as hair color. l’olygenic systems givck raise to c~ontinnous variability, which is often described as normal variat,ion. Some investigations have been made on the influcllc*e of gclnctic* and PIIvironmental factors regarding the morphology of the facial skeleton. In t,hc few that will be mentioned here, cephalometric roentgenograms wwc used fol* ctvaluation. Lundstr6mg studied fift,y monozygotic and fift,y dizygotic twins and coneluded that genetic factors seem to have between once and twice the significance of nongenetic factors wit,h respect to the variability of the characteristics investigated. He studied the angles commonly used in roentgenocephalometrics. Kraus, Wise, a,nd Freil” approached their study of six sets of triplets in another way. The craniofacial complex was divided into its component, parts, and the profile lines of the individual. bones were studied as they could he ohserved in lateral and frontal headfilms. Almost, perfect concordance was the rule in monovular pairs, whereas or117 a low degree of concordance (hc~low 50 per cent) was found in diovular pairs. Sevclntecn bony traits were tlrfined in terms of lateral and frontal headfilm tracings, on which they based the speculation that t,he structure of all t,hc bones of the craniafacial complex is under the rather rigid control of hereditary fortes. Thcv also used the gtnctrally acccptecl cephalometric polygon procedures and found no significant findings in contrast with Lundstrb;m’s observations. This could be due to the kind of analysis used and the considerably smaller sample. Kraus, Wise, arid Frci did not. find :i greater similarity in length of lines or degree of angles in monovular twins than in diovular twins. (The six set.s of triplets could b(k differentiated in eighteen pairings of six monovular and twelve diovular twins.) The dXercnce bet,wcen findings for the individual bony structures and for the craniofacial complt~x ax seem that a whole caused them to make the following statement: *‘It. woulcl heredity governs morphology, but environment in its multitudinous facets has much to say about how these bony elements shall combine to achieve what interests us most-the harmonious (or unharmonious) heatl and f’aee.” This conclusion agrees very well with the findings of \Vieslandcr,ll who studied the effect of occipital anchorage (an environmental factor) upon the dentofaeial area in t,he mixed-dentition period. He regist,ered a, slight clockwise rotation of the sphenoid bone, with an alteration of the facial growth pattern. From his findings, he concluded that there is strong evidence to support the contention that headgear trea.tment durin, w the age period studied (*an alter the interrelationship of the sphenoid bone to the ot,hrr hones in thp craniofacial complex. In a recent publication by Ross and Coupe,12 the morphologic characteristics of six pairs of monozygotic twins, discordant for cleft lip and for cleft palate, were analyzed. A principal finding was that a unilateral cleft of the lip and palate is characterized by an alteration in the relationship of the facial skeleton to the cranial base, rather than by an intrinsic defect of the facial components
Genetic and environmental
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58 1
(other than the cleft itself). This also indicates the important role of nongenetic factors in the combination of the bony parts. It is well known that mixing of genetically different populations increases considerably the genetic variability of later generations. This applies to all forms of characters for which more alleles are present. The increased range of variability in morphology does not lead to notable abnormalities and functional insufficiencies in most areas of the body. If the lower arm is relatively short compared to the upper one, or the left arm is a little bit shorter than the right one, no inconveniences will result. Broad and short hands are not a sign of beauty for a slender person, but this is only a matter of esthetics. For the dentofacial complex, however, an increased range of variability in the individual components has not only esthetic but very important functional consequences as well. Nowhere in the rest of the body can nonconcordant combinations so easily be recognized. The occlusion of the teeth is an extremely suitable registration tool. On a roentgenocephalogram a very accurate analysis of the facial skeleton and associated soft tissues can be carried out. On the other hand, the dentofacial morphology is made up of an extremely large number of components which, in combination with each other, are responsible for the end result. The form and dimensions of the cranial base and both jaws are the foundations on which other components are built. Certain variances can cancel each other out in such a way that no malformations will result. On the other hand, they can work in the same direction and cumulate to a marked, unacceptable deviation from the normal standards. An overdeveloped maxilla accompanied by a large lower jaw presents a good basis for a normal occlusion and may lead to a classic profile. An overdeveloped maxilla accompanied by a small lower jaw usually results in an Angle Class II malocclusion and a less favorable facial outline. The position, size, and form of the alveolar processes and the position and inclination of the teeth are variables which are superimposed on the basal bones, and they do influence the occlusion and facial appearance. Many other factors, such as the form and dimensions of the soft Gssue, the position of the medial cranial fossa, and the correlated position of the articular fossae, etc., also influence bhe morphology of the dentofacial complex. It has been shown that the position, form, and dimensions of the mandible depend on a larger number of variances than those of the maxilla. The place of the condyle is largely determined by the location of the occipital and temporal bones in the skull. The height of the ramus, the length of the horizontal part, the size of the gonion angle, and the form of the chin button determine the facial outline to a great extent. The height of the mandible in the incisor and molar region further contributes to it. We do not yet know how many genes and alleles are involved. Studies made on other parts of the body, such as the hand, suggest that there will be many. Their mode of action is largely unknown too. Perhaps some genes control the basic form and shape of the main structures and others act on top of these, causing minor variations. Recent investigations using the multifactor analysis support this hypothesis. I3 It seems that certain genes each govern several parts of the dentofacial complex and, on the other hand, more genes are responsible for one single character.
A number of studies have been undert,aken to determine the essential deviations of certain structures in different dcntofacial malformations as compared with normal ones. Very seldom could marked abnormalities in individual cornponents adequately be shown. In most instances the component parts, in themselves, could bc considered as being within the range of normal variation. ‘I’hc>just did not fit together into a harmonious combination. The fact that only in certain isolated areas, such as some of the Philippine Islands, can a population with normal occlusion be found’ supports the hypothesis that many orthodontic malformations are caused by a disharmonious combination of variables which, considered individually, are within normal limits. The genetically large variation of the individual components probably has reduced considerably the occurrence of ideal occlusions and profiles. The influence of genes that can he made responsible for specific deviations, such as typically rotated teeth, impacbed upper canines, micrognathia, etc., seems to be of minor importance. As stated before, the effect of genetic and environmental factors in dentofacial morphology is by no means limited to the etiology of normal and abnormal orthodontic conditions. Orthodontic treatment can be considered a.s an additional environmental factor. With the type of appliance and the period of application, the results vary. In general, multiband techniques result. in more extensive alterations than those that can be accomplished with removable appliances. In comparison with the other (natural) environmental fact,ors, orthodontic appliances probably exert relatively powerful forces. Very little is known about the intensity of natural environmental components. The same is true of the relationship between the effect of finger-sucking and the time of beginning, duration, frequency, and intensity of this habit. How far the effect of orthodontic therapy can be favored or counteracted by the genetic composition is still an open question. There are some indications-theoretical and clinical, as well as experimental-that this is a factor of importance. The st.ability of the end result will depend on the fact t,hat the genetic and environmental influences are in harmony with each ot,her. If this is not the case, a new state of balance will be established, mostly resulting in partial relapse. In the permanent interaction between genetic and environmental factors, orthodontics has its specific place. Successes and failures probably depend to a large extent on the fact, that the genetic composition allows and favors the planned orthodontic therapy and the end result aimed for. Surveying our knowledge of genetic and environmental factors in dentofacial morphology, we realize that we are just beginning to acquire a better understanding of the problems involved. It may be expected that the present intensive research in genetics will supply us with essential information concerning our field too, but much research in orthodontics and the basic sciences is also needed. In the past the influence of genetic and environmental factors has been frequently oversimplified, and in most cases one was overestimated in an unjustified way, generally at the cost of the other. Seldom was balanced attention given to the two factors involved. Now it is difficult to adopt a position between the two. If some statement should be made, however, perhaps it would be that the permanent interaction between genetic and environmental factors directs the
Volume Number
52 8
Genetic and ewvironmental
factors
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growth of the craniofacial complex (including the development of malformations). Genetic factors seem to have the greatest influence, whereas environmental factors appear to be of minor importance. Environmental factors, including orthodontic therapy, probably primarily affect the dental alveolar region and the interrelationship of the individual bony element,s. The morphology of the individual bones seems to be under rather rigid genetic control. The stability of the result of orthodontic treatment depends mainly on the fact that a new state of balance in the interaction between genetic and environmental factors is obtained. The changes in dentofacial structure that take place with aging are the result of the same continuous alterations in this interact,ion. SUMMARY
The importance of genetic and environmental factors with regard to growth and development of the dentofacial area has been discussed on the basis of recent information. Their role in orthodontic therapy (quite often a powerful environmental factor), in the stability of the results obtained, and in the alterations in later life is underlined. A permanent interaction between genetic and environmental factors, both of a continually altering nature, determines the dentofacial morphology in every moment of life. Genetic factors seem to have the greatest influence, and environmental factors appea,r to be of minor importance. The stability of the result of orthodontic treatment depends mainly on a new state of balance in the interaction between genetic and environmental factors. REFERENCES
1. Graber, T. M.: Orthodontics: Principles and Practice, Philadelphia, 1961, W. B. Saunders Company. 2. Lundstram, A.: Tooth Size and Occlusion in Twins, Thesis, Upsala, 1948. 3. Johnson, A. L.: The Constitutional Factor in Skull Form and Dental Occlusion, AM. J. ORTHODONTICS & ORAL SURG.~~: 627-663,194O. 4. Kraus, B. S., and Furr, M. L.: Lower First Premolars, J. D. Res. 32: 554-564, 1953. Diagnosis and 5. Ballard, M. L.: Asymmetry in Tooth Size: A Factor in the Etiology, Treatment of Malocclusion, Angle Orthodontist 14: 67-71, 1944. 6. Bolton, W. A.: Disharmony in Tooth Size and Its Relation to the Analysis and Treatment of Malocclusion, Angle Orthodontist 28: 113-130, 1958. 7. Stern, C.: Principles of Human Genetics, San Francisco, 1960, W. H. Freeman & Company. 8. Goodman, H. 0.: Genetic Parameters of Dentofacial Development, J. D. Res. 44: 174184, 1965. of Genetic and Non-genetic Factors in the Facial 9. LundstrSm, A.: The Importance Skeleton Studied in 100 Pairs of Twins, Tr. European Orthodont. Sot., pp. 92-107, 1954. and the Craniofaeial Complex, 10. Kraus, B. S., Wise, W. J., and Frei, R. H.: Heredity AM. J. ORTHOWNTICS45: 172-217,1959. 11. Wieslander, L.: The Effect of Orthodontic Treatment on the Concurrent Development 49: 15-27, 1963. of the Craniofacial Complex, AM. J. ORTHODONTICS 12. Ross, R. B., and Coupe, T. B.: Craniofacial Morphology in Six Pairs of Monozygotic Twins Discordant for Cleft Lip and Palate, J. Canad. Dent. A. 3: 149-158, 1965. Factors in Two Ethnic 13. Brown, T., Barrett, M. J., and Darroch, J. N.: Craniofacial Groups, Growth 29: 109-123, 1965. Sint
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