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Dept. of Dental Health, Dundee Dental Hospital, Park Place, Dundee DD1 4HR, UK
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Abstract |
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 Top Abstract IntroductionThe Nature of MalocclusionFamily Studies of Heritability...Heritability of Local Occlusal...Genetic Influence on Tooth...Practical and Clinical...The FutureReferences |
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Key words: Genetics, Malocclusion
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Introduction |
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TopAbstractIntroductionThe Nature of MalocclusionFamily Studies of Heritability...Heritability of Local Occlusal...Genetic Influence on Tooth...Practical and Clinical...The FutureReferences |
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) studied fraternal and identical adult
twin
pairs using only linear cephalometric measurements, and he demonstrated highly significant
hereditary
variations in the anterior cranial base, mandibular body length, lower face height, and total face
height.
Hunter (1965) also used linear measurements on lateral cephalograms
and
concluded that there is a stronger genetic component of variability for vertical measurements,
rather
than for measurements in the anteroposterior dimension.
Fernex et al. (1967) found boys to show more similarities to
their
parents than girls. Facial skeletal structures were more frequently transmitted from mothers to
sons than
from mothers to daughters. Female twins showed greater concordance in facial features than
male
twins. While the profile outline coincided most frequently, this was not true of the cranial base
and
differences increased with age. Hunter et al. (1970) found
genetic
correlation to be strongest between fathers and children, especially in mandibular dimensions.
There
was a significant relation in facial height between mothers and their offspring. However, the
regression
equations of parental facial dimensions were of questionable clinical value in predicting adult
facial
dimensions in offspring. Litton et al. (1970) concluded that
siblings
usually show similar types of malocclusion and examination of older siblings can provide a clue
to the
need for interception and early treatment of malocclusion.
Harris (1963) recommended that any study of genetic variation using
lines
and angles requires the
use of multivariate analysis in order to identify significant relationships, while Kraus et al.
(1959) criticized the use of lines and angles to study heredity, and
preferred
superimposition of bony
profiles to illustrate genetic control of craniofacial morphology. Their study involved
superimposition of
lateral cephalograms of a sample of identical twins and showed that many bony contours are in
almost
perfect concordance. This applied equally to contours across sutures and to individual bony
contours
such as the mandible. The latter method had first been described by Curtner (1953) who superimposed
lateral cephalograms of children on those of their parents. The superimposed profiles were scored
visually and subjectively for concordance or discordance, and close parent–sibling
similarities for
many craniofacial structures were found. Margolis et al. (1968)
came
to a similar conclusion in
a cephalometric study of the parents and siblings of 68 families.
Since there is evidence that these orofacial structures are under genetic control and are significant in craniofacial development they must be considered in the aetiology of malocclusion. It is also well established that many craniofacial abnormalities are not monogenic disorders and are produced by a combination of many genes interacting with the environment, i.e. they are multifactorial. The same is true of malocclusion and the best evidence in establishing the relative contribution of genes and environment in determining certain craniofacial parameters is from familial and twin studies. The following outlines some of the evidence.
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The Nature of Malocclusion |
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TopAbstractIntroductionThe Nature of MalocclusionFamily Studies of Heritability...Heritability of Local Occlusal...Genetic Influence on Tooth...Practical and Clinical...The FutureReferences |
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). Many components are
involved in
normal occlusion. The
most important are: (a) the size of the maxilla; (b) the size of the mandible, both ramus and body;
(c) the
factors which determine the relationship between the two skeletal bases, such as cranial base and
environmental factors; (d) the arch form; (e) the size and morphology of the teeth; (f) the number
of
teeth present; and (g) soft tissue morphology and behaviour, lips, tongue, and peri-oral
musculature.
The term ` normal occlusion's is arbitrary, but is generally accepted to be Class I
molar
relationship with good alignment of all teeth and represents a situation that occurs in only
30–40
per cent of the population.
There is dental anthropological evidence that population groups that are genetically
homogeneous
tend to have normal occlusion. In pure racial stocks, such as the Melanesians of the Philippine
islands,
malocclusion is almost non-existent. However, in heterogeneous populations, the incidence of
jaw
discrepancies and occlusal disharmonies is significantly greater. Stockard (1941) carried out breeding
experiments with dogs, and produced gross orofacial deformities and associated malocclusions.
He
concluded that individual features of the craniofacial complex could be inherited according to
Mendelian
principles independently of other portions of the skull, and that jaw size and tooth size could be
inherited independently, and as genetically dominant traits. These experiments have been
severely
criticized on the basis that the gene for achondroplasia is likely to have contributed and in the
context of
inheritance of malocclusions in humans, extrapolation to conclude that racial cross-breeding may
explain
tooth size/arch size discrepancies is entirely unjustified. Evidence from human family studies and
twin
studies is considered much more credible. A study by Suarez (1974)
examining aetiology of the
variation in crown form of the mandibular first premolar indicates that this is determined by a
myriad of
genes with at least seven different genetic traits had to be taken into account. This points to the
much
more credible polygenic theory for craniofacial and dental morphogenesis.
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Family Studies of Heritability of Dentofacial Phenotypes |
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TopAbstractIntroductionThe Nature of MalocclusionFamily Studies of Heritability...Heritability of Local Occlusal...Genetic Influence on Tooth...Practical and Clinical...The FutureReferences |
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The study of craniofacial relationships in twins has provided much useful information concerning the role of heredity in malocclusion. The procedure is based on the underlying principle that observed differences within a pair of monozygotic twins (whose genotype is identical) are due to environment and that differences within a pair of dizygotic twins (who share 50 per cent of their total gene complement) are due to both environment and genotype. A comparison of the observed within-pair differences for twins in the two categories should provide a measure of the degree to which monozygotic twins are more alike than dizygotic twins. The larger this difference between the two twin categories, the greater the genetic effect on variability of the trait. This model implies that zygosity is accurately determined and that environmental effects are equal in the two twin categories. At the present time, accurate zygosity classification is seldom a problem due to the ability to identify the large number of available polymorphic blood group and enzyme markers.
The bulk of the evidence for the heritability of various types of malocclusion arises from family and twin studies.
Class II Division 1 Malocclusion
Extensive cephalometric studies have been carried out to determine the heritability of certain
craniofacial parameters in Class II division 1 malocclusions (Harris, 1963, 1975). These investigations
have shown that, in the Class II patient, the mandible is significantly more retruded than in Class
I
patients, with the body of the mandible smaller and overall mandibular length reduced. These
studies
also showed a higher correlation between the patient and his immediate family than data from
random
pairings of unrelated siblings, thus supporting the concept of polygenic inheritance for Class II
division 1
malocclusions.
Environmental factors can also contribute to the aetiology of Class II division 1 malocclusions. Soft tissues can exert an influence on the position or inclination of upper and lower incisors and the need to achieve lip/tongue contact for an anterior oral seal during swallowing can encourage the lower lip to retrocline the lower incisors and the protruding tongue to procline the uppers, influencing the severity of the overjet. Likewise, digit sucking habits can produce a Class II division 1 incisal relationship, even if the underlying skeletal base relationship is Class I. Lip incompetence also encourages upper incisor proclination by virtue of the imbalance in labial and lingual pressures on the teeth.
Class II Division 2 Malocclusion
Class II division 2 malocclusion is a distinct clinical entity and is a more consistent
collection of
definable morphometric features occurring simultaneously, i.e. a syndrome than the other
malocclusion
types put forward by Angle in the early 1900s. Class II division 2 malocclusion comprises the
unique
combination of deep overbite, retroclined incisors, Class II skeletal discrepancy, high lip line
with
strap-like activity of the lower lip, and active mentalis muscle. This is often accompanied by
particular
morphometric dental features also, such as a poorly developed cingulum on the upper incisors
and a
characteristic crown root angulation. Peck et al. (1998) also
describes characteristic smaller
than average teeth when measured mesio-distally, reinforcing a similar observation made by
Beresford
(1969) and a study by Roberston and Hilton (1965),
which found these teeth to be significantly `
thinner's in the labial/lingual dimension. A further feature of the Class II division 2
`
syndrome's is a tendency to a forwardly rotating mandibular development, which
contributes to
the deep bite, chin prominence, and reduced lower face height. This last feature, in turn, has an
influence in the position of the lower lip relative to the upper incisors, and an increase in
masticatory
muscle forces has been reported by Quinn and Yoshikawa (1985).
Familial
occurrence of Class II
division 2 has been documented in several published reports including twin and triplet studies
(e.g.
Kloeppel, 1953; Markovic, 1992) and in family
pedigrees from Korkhaus (1930), Rubbrecht (1930),
Trauner (1968) and Peck et al. (1998).
Markovic (1992) carried out a clinical and
cephalometric study of 114 Class II division 2 malocclusions, 48 twin pairs and six sets of
triplets.
Intra- and inter-pair comparisons were made to determine concordance/ discordance rates for
monozygotic and dizygotic twins. Of the monozygotic twin pairs, 100 per cent demonstrated
concordance for the Class II division 2 malocclusion, whilst almost 90 per cent of the dizygotic
twin
pairs were discordant. This is strong evidence for genetics as the main aetiological factor in the
development of Class II division 2 malocclusions.
These studies point to incontestable genetic influence, probably autosomal dominant with
incomplete penetrance and variable expressivity. It could also possibly be explained by a
polygenic
model with a simultaneous expression of a number of genetically determined morphological
traits (acting
additively), rather than being the effect of a single controlling gene for the entire occlusal
malformation.
The controversy regarding the aetiology of the Class II division 2 malocclusion arises from a
failure to
appreciate the synergistic effects of genetics and environment on facial morphology. Ballard (1963),
Houston (1975), Mills (1982), and others
considered
that a high lip line, and a particular lip
morphology and behaviour were the main aetiological factors. Graber (1972),
Hotz (1974), Meskov
(1988), and Markovic (1992) stressed the
predominant role of genetic factors in the aetiology of Class
II division 2 malocclusions. These views are of course not incompatible if the lower lip
morphology,
behaviour, and position relative to the upper incisors is considered to be genetically determined
or
influenced. Aspects of skeletal and muscle morphology are genetically determined and there is
some
recent experimental evidence from a twin study (Lauweryns et al. 1995) indicating strong
genetic factors in certain aspects of masticatory muscle behaviour.
Class III Malocclusion
Probably the most famous example of a genetic trait in humans passing through several
generations
is the pedigree of the so-called Hapsburg jaw. This was the famous mandibular prognathism
demonstrated by several generations of the Hungarian/Austrian dual monarchy. Strohmayer (1937)
concluded from his detailed pedigree analysis of the Hapsburg family line that the mandibular
prognathism was transmitted as an autosomal dominant trait. This could be regarded as an
exception
and, in itself, does not provide sufficient information to predict the mode of inheritance of
mandibular
prognathism. Suzuki (1961) studied 1362 persons from 243 Japanese
families
and noted that, while the
index cases had mandibular prognathism, there was a significantly higher incidence of this trait
in other
members of his family (34.3 per cent) in comparison to families of individuals with normal
occlusion (7.5 per cent). Schulze and Weise (1965) also studied
mandibular prognathism in
monozygotic and dizygotic twins. They reported that concordance in monozygotic twins was six
times
higher than among dizygotic twins. Both of the above studies report a polygenic hypothesis as the
primary cause for mandibular prognathism (Litton et al., 1970).
The relative contribution of genetic and environmental factors to Class III has been the
subject of a
number of previous studies. A Class III malocclusion resulting from a skeletal imbalance
between the
maxillary and mandibular bases may result from deficiency in maxillary growth, excessive
mandibular
growth, or a combination of both. Various studies have also highlighted the influence of a
distinctive
cranial base morphology with a more acute cranial base angle and shortened posterior cranial
base
resulting in a more anterior position of the glenoid fossa, thus contributing to the mandibular
prognathism
(Ellis and McNamara, 1984; Singh et al., 1997).
Familial studies of mandibular prognathism are suggestive of heredity in the aetiology of this
condition (Castro, 1928; Downs, 1928; Keeler, 1935; Moore and Hughes, 1942; Gottlieb
and
Gottlieb, 1954). Various models have been suggested, such as autosomal dominant
with
incomplete
penetrance (Stiles and Luke, 1953), simple recessive (Downs,
1928), variable both in expressivity and
penetrance with differences in different racial populations (Kraus et al.,
1959).
A wide range of environmental factors have also been suggested as contributory to the
development of mandibular prognathism. Among these are enlarged tonsils (Angle,
1907), nasal
blockage (Davidov et al., 1961), congenital anatomic defects (Monteleone and Davigneaud,
1963), hormonal disturbances (Pascoe et al., 1960),
endocrine imbalances (Downs, 1928),
posture (Gold, 1949) and trauma/disease including premature loss of the
first
permanent molars (Gold, 1949). Litton et al. (1970) carried out an analysis of the literature to that date and also
analysed a group of probands, siblings and parents with Class III malocclusion, and analysed the
results
in an effort to determine a possible mode of transmission. Both autosomal dominant and
autosomal
recessive transmission were ruled out and there was no association with gender since there were
equal
numbers of males and females. The polygenic multifactorial threshold model put forward by
Edwards
(1960), however, did fit the data that these authors presented and,
accordingly, they proposed a
polygenic model with a threshold for expression to explain familial distribution, and the
prevalence both
within the general population and in siblings of affected persons. They also made the sensible
suggestion
that different modes of transmission might be operating in different families or different
populations.
Soft tissues do not generally play a part in the aetiology of Class III malocclusion, and in fact there is a tendency for lip and tongue pressure to compensate for a skeletal Class III discrepancy by retroclining lower incisors and proclining uppers.
Summary
Polygenic inheritance, by definition, implies that there is scope for environmental
modification and
many familial and twin studies bear this out. Horowitz et al. (1960)
studied adult monozygotic
and dizygotic twins using lateral skull cephalograms. The statistics derived from their data
indicated that
there was a highly significant hereditary variation in the anterior cranial base, mandibular body
length,
and lower face height. In a similar study, Watnick (1972) studied 35 pairs
of
monozygotic and 35 pairs
of dizygotic like-sexed twins using lateral cephalometry. He concluded that the analysis of unit
areas
within the craniofacial complex represent local growth sites and revealed different modes of
control
within the same bone. Certain areas, such as the lingual symphysis, lateral surface of the ramus,
and
frontal curvature of the mandible are predominantly under genetic control. Other areas, such as
the
antegonial notch, are predominantly affected by environmental factors. This is in line with the
conclusions from numerous other studies such as the famous Lundstrom study (1948) and the study by
Kraus et al. (1959) on six sets of like-sexed triplets. In the latter
study
17 skeletal traits from
lateral and frontal cephalograms were studied. Both these studies concluded that although genetic
factors appear to govern the basic skeletal form and size, environmental factors in their
multitudinous
facets have much influence on the bony elements, and both factors combine to achieve the
harmonious
or disharmonious head and face. The foregoing also provides support for the view of Hughes and
Moore (1941) that the mandible and maxilla are under separate genetic
control, and that certain
portions of individual bones, such as the ramus, body, and symphysis of the mandible are under
different genetic and environmental influences.
The simultaneous and synergistic influence of genetics and environment on the development
of
malocclusion is well illustrated by the Class II division 2 scenario described above. Some
workers,
such as Graber (1972), Hotz (1974), Meskov (1988) and Markovic (1992) stressed the role of
genetic factors in the aetiology of Class II division 2 malocclusions, while others such as Ballard
(1963)
and Mills (1982) preferred to emphasize the importance of an
environmental
influence. The truth lies in
the interaction between genetics and the environment in the determination of facial and dental
morphology.
The literature also provides evidence of a secular trend towards increasing prevalence of
malocclusion (Price, 1945; Dixon, 1970; Weiland et al., 1996). It is argued that this is proof of
an environmental determination of certain types of malocclusion (e.g. Mew, 1981), but this is
undoubtedly an over-simplification. The trend towards narrower maxillary arches and greater
crowding
is compatible with a polygenic multifactorial determination or gene/environment interaction,
where
certain genetically-determined craniofacial phenotypes will show a greater susceptibility to
certain
environmental factors. This tendency could be explained, at least in part by the simultaneous
increase in
interracial mixing which is also a feature of ` westernization's .
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Heritability of Local Occlusal Variables |
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TopAbstractIntroductionThe Nature of MalocclusionFamily Studies of Heritability...Heritability of Local Occlusal...Genetic Influence on Tooth...Practical and Clinical...The FutureReferences |
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). In
an analysis of nature
versus nurture in malocclusion Lundstrom (1984) concluded that the
genetic
contribution to anomalies
of tooth position and jaw relationship overall is only 40 per cent, with a greater genetic influence
on the
skeletal pattern than on the dental features.
Evidence from other studies, however, would challenge this view. Lundstrom (1948) studied 50
pairs of monozygotic and 50 pairs of dizygotic twins and concluded that heredity played a
significant
role in determining, among other factors, width and length of the dental arch, crowding and
spacing of
the teeth, and degree of overbite. A study by Hu et al. (1992)
also
reported familial similarity
in dental archform and tooth position. In a more recent study by King et al. (1993) initial
treatment records of 104 adolescent sibling pairs, all of whom subsequently received orthodontic
treatment, were examined. Heritability estimates for occlusal variations such as rotations,
crossbites and
displacements, were significantly higher than in a comparable series of adolescents with naturally
good
occurring occlusions. The explanation offered was that, given genetically-influenced facial types
and
growth patterns, siblings are likely to respond to environmental factors, e.g. chronic mouth
breathing
and reduced masticatory stress in similar fashions. The similarity of the sibling pair tooth
malpositions
and malocclusions may well be because of fundamentally similar craniofacial form, which is
genetically
determined. They will be diverted to comparable physiological responses leading to the
development of
similar malocclusions. It is also important to remember that soft tissue morphology and
behaviour have
a genetic component and they have a significant influence on the dentoalveolar morphology. This
concept is described by van der Linden (1966) as the balance between the
internal and external
functional matrices. For example, in a Class II division 1 malocclusion a short upper lip and low
lip level
with flaccid lip tone will reduce the external influence and the balance will favour proclination of
the
upper incisors. On the other hand, a high lip level and more expressive lip behaviour will tend to
produce a Class II division 2 incisor relationship. This external matrix is thought to be strongly
genetically determined. The internal matrix is determined mainly by tongue posture and
behaviour which
can be influenced by environmental, as well as a genetic factors.
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Genetic Influence on Tooth Number, Size, Morphology, Position, and Eruption |
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TopAbstractIntroductionThe Nature of MalocclusionFamily Studies of Heritability...Heritability of Local Occlusal...Genetic Influence on Tooth...Practical and Clinical...The FutureReferences |
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). The molecular genetics of tooth
morphogenesis with the homeostatic
Hox 7 and Hox 8 (now referred to as MSX1 and MSX2) genes being responsible for stability in
dental
patterning (Mackenzie et al., 1992), is confirmation of
Butler's
field theory (1963).
This refers to primate tooth development in evolution with the stability of morphology, eruption
pattern
and tooth number in the incisor, canine, premolar, and molar domains. As dietary habits in
humans
adapt from a hunter/gatherer to a defined food culture evolutionary selection pressures are
tending to
reduce tooth volume, which is manifest in the third molar, second premolar and lateral incisor
`
fields' . Hypodontia involving the aforementioned teeth shows a familial tendency and
fits the
polygenic model (Grahnen, 1956; Gravely and Johnston, 1971), but this evolutionary theory suggests
an environmental influence also.
Clinical evidence suggests that congenital absence of teeth and reduction in tooth size are
associated, e.g. hypodontia and hypoplasia of maxillary lateral incisors frequently present
simultaneously. Numerous pedigrees have been published linking the two characteristics and
implying
that they are different expressions of the same disorder. Gruneberg (1965)
suggested that a tooth germ
must reach a critical size during a particular stage of development or the structure will regress,
and
Suaraz and Spence (1974) showed that hypodontia and reduction in tooth
size
are in fact controlled by
the same or related gene loci. It is apparent from all the evidence in this respect that tooth size
fits the
polygenic multi-factorial threshold model.
Supernumerary teeth most frequently seen in the pre-maxillary region and with a
male sex predilection also appears to be genetically determined. Niswander and Sugaku (1963)
analysed the data from family studies and have suggested that, like hypodontia, the genetics of
the less
prevalent condition of supernumerary teeth is under the control of a number of different loci.
The hereditary nature of hypodontia is revealed in familial and twin studies. A study of
children with
missing teeth found that up to half of their siblings or parents also had missing teeth, while the
population
prevalence is about 5 per cent (Grahnen, 1956). Markovic (1982) found a high rate of concordance
for hypodontia in monozygous twin pairs, while dizygous twin pairs he observed were
discordant.
These and other previous studies concluded that the mode of transmission could be explained by
a
single autosomal dominant gene with incomplete penetrance.
Supernumerary Teeth
Brook (1974) reported that the prevalence of supernumerary teeth in
British school children is
2.1 per cent in the permanent dentition with a male:female ratio of 2:1. In Hong Kong, however,
the prevalence is around 3 per cent with a male:female ratio of 6.5:1 (Davis, 1987). The most
common type of supernumerary is a premaxillary conical midline tooth (mesiodens). These are
more
commonly present in parents and siblings of patients who present, although inheritance does not
follow
a simple mendalian pattern (Brook, 1984; Mercuri and
O'Neill, 1980; Mason and Rule,
1995). Evidence from twins with supernumeraries also supports this theory (Jasmin et al.,
1993).
Abnormal Tooth Shape
Alvesalo and Portin (1969) provided substantial evidence supporting
the
view that missing and
malformed lateral incisors may well be the result of a common gene defect. Abnormalities in the
lateral
incisor region varies from peg shaped to microdont to missing teeth, all of which have familial
trends,
female preponderance, and association with other dental anomalies, such as other missing teeth,
ectopic canines, and transposition, suggesting a polygenic aetiology. Aspects of tooth
morphology such
as the Carabelli trait also seem to be strongly influenced by genes as evidenced by an Australian
twin
study (Townsend and Martin, 1992).
Ectopic Maxillary Canines
Various studies in the past have indicated a genetic tendency for ectopic maxillary canines
(Zilberman et al., 1990). Peck et al. (1994) concluded that palatally ectopic canines
were an inherited trait, being one of the anomalies in a complex of genetically related dental
disturbances, often occurring in combination with missing teeth, tooth size reduction,
supernumerary
teeth, and other ectopically positioned teeth. Previous studies have also shown an association
between
ectopic maxillary canines and Class II division 2 malocclusion, a genetically-inherited trait (Mossey et al., 1994). Peck et al. (1997)
classified a number of different types of tooth
transposition in both maxillary and mandibular arches, with maxillary canine/first premolar class
position
being the most common. They also provided strong evidence of a significant genetic component
in the
cause of this most common type of transposition in that there was familial occurrence, bilateral
occurrence in a high percentage of cases, female predominance and a difference in different
ethnic
groups. An increased frequency of associated dental anomalies, tooth agenesis and peg-shaped
maxillary lateral incisors were also reported.
Submerged Primary Molars
Primary molar submergence occurs most often in the mandibular arch with a wide variation
in the
reported general population prevalence, but this would be expected to be less than 10 per cent (Kurol,
1981). The siblings of affected children are likely to also be affected in about 18 per
cent of cases, and
in monozygous twins there is a high rate of concordance (Helpin and Duncan, 1986) indicating a
significant genetic component in the aetiology. A number of other studies provide evidence for
genetically determined primary failure of eruption such as those by Kurol (1981), Koyoumdjisky-Kaye
and Steigman (1982 a,b), and Brady (1990). It is also of interest that a variety of
abnormalities are also
associated with tooth submergence with a suggestion that this may encompass different
manifestations
of one syndrome, each manifestation having incomplete penetrance and variable expressivity.
Bjerklin et al. (1992) and Winter et al. (1997) suggested that taurodontism may form part of
this syndrome.
Summary
There is considerable evidence suggesting that genes play a significant role in the aetiology
of many
dental anomalies. Furthermore, a frequency of association of one or more of these dental
anomalies
coincidentally in the same pedigree suggests some kind of genetically controlled
inter-relationship. This
may add further support to Butler's classic field theory of tooth bud differentiation (Butler,
1939, 1982) and Sperber (1967)
speculated that transposition is indicative of faulty field gene function,
which would explain why there is an increased occurrence of variations in teeth on either side of
transposed teeth. This may also explain the fact that teeth in the critical marginal areas of the
dental
lamina, lateral incisors, second premolars and third molars are the most vulnerable. The clinical
significance of the inheritance of certain dental anomalies is that clinicians should be vigilant in
the
expectation that the clinical or radiographic detection of one anomaly should alert them to the
possibility
of other defects in the same individual or other family members. Early diagnosis would enable
interceptive paediatric and orthodontic opportunities in relation to ectopic, missing or malformed
teeth,
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Practical and Clinical Implication |
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TopAbstractIntroductionThe Nature of MalocclusionFamily Studies of Heritability...Heritability of Local Occlusal...Genetic Influence on Tooth...Practical and Clinical...The FutureReferences |
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; Harvold et al., 1981; Moss
and Salentijn, 1997). Orthodontic correction of a malocclusion is in effect altering
the
phenotypic
expression of a particular morphogenetic pattern. The degree to which this can be successfully
achieved
depends on (a) the relative contribution of each factor to the existing problem, and (b) the extent
to
which skeletal pattern can be influenced by orthodontic and orthopaedic appliances. In clinical orthodontics it must be appreciated that each malocclusion occupies its own distinctive slot in the genetic/ environmental spectrum and, therefore, the diagnostic goal is to determine the relative contribution of genetics and the environment. The greater the genetic component, the worse the prognosis for a successful outcome by means of orthodontic intervention. The difficulty, of course, is that it is seldom possible to determine the precise contribution from hereditary and environmental factors in a particular case. For example, the simultaneous appearance of proclined maxillary incisors and digit sucking may lead to the assumption that the digit was the sole causative factor, but the effect of the digit may very well be either potentiated or mitigated by other morphological or behavioural features in that particular individual. A similar argument may apply in cases of mouth breathing where the influence of the habit and associated posture is very much dependant on the genetically determined craniofacial morphology on which it is superimposed, and the reason for the habit developing may well be dependant on the morphology in the first place. These senarios are classical examples of the interaction of genotype and environment, and ultimately success of treatment will depend on the ability to ascertain the relative contribution of each.
Every orthodontist believes that it is possible to influence the dento-alveolar regions of the
jaws
within certain parameters using environmental forces—otherwise orthodontic therapy
would be
futile. The division in orthodontic opinion arises from the doubt as to whether the skeletal bases
can be
influenced to any significant effect beyond their genetically-predetermined potential. There is
still
considerable debate about this as conclusive evidence is lacking in both camps, but what
evidence is
available from human studies to date tends to support the genetic determination of craniofacial
form
with a lack of evidence to show any significant long term influence on mandibular or maxillary
skeletal
bases using orthopaedic appliances. The search for evidence to support the environmental
influence on
craniofacial growth is not easy and will require randomized clinical trials on longitudinal cohorts
of
patients treated with various types of appliance using longitudinal growth studies as controls. It is
also
possible to determine the relative contribution of genes and environment applying genetic
modelling and
statistical techniques to family and twin data (e.g. Van Cawenberge et al.,
1996).
If dentofacial structure and malocclusion are primarily genetic, e.g. severe mandibular prognathism or endogenous tongue thrust, then treatment will either be palliative or surgical. The search for a solution would ultimately focus on delineating the responsible genes. Conversely, if components of dentofacial structure and malocclusion have trivial heritabilities, then the search needs to be directed at environmental factors inducing malocclusion during growth and development. The goal would be to identify causes and formulate means of intercepting their negative influences. Such is the case with much of the interceptive orthodontic treatment presently carried out, in which the long-range goal is to permit the face to grow according to its fundamental genetic pattern with minimal obstruction from environmental influences, habit and adverse functional factors. An appropriate dental analogy in environmental manipulation is the reduced caries incidence over the past few decades by introduction of fluoride supplements and public water fluoridation programmes.
|
The Future |
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TopAbstractIntroductionThe Nature of MalocclusionFamily Studies of Heritability...Heritability of Local Occlusal...Genetic Influence on Tooth...Practical and Clinical...The FutureReferences |
|---|
What scientific morphometric evidence can we provide to back up the hypothesis that
malocclusion
is genetically determined, or to quantify the effect of environmental influences? Since subtle
morphological changes are occurring very sensitive techniques and three dimensional models are
required to identify these. The limitations of conventional cephalometric analysis are well
recognised and
more discriminating techniques for craniofacial morphometric analysis have now become
available.
New techniques such as Procrustes analysis, finite element morphometry, thin plate spline
transformations, and Euclidean distance matrix analysis allow computer based morphological
analysis of
craniofacial configurations that will enable the longitudinal mapping of spatial changes during
craniofacial
morphogenesis, and from these techniques predictive biomodelling will be possible. Such
morphometric
computer programmes are being applied to internal craniofacial data obtained derived from
lateral and
postero-anterior cephalograms (Singh et al., 1996), and similar
programmes have been or are
in the process of being developed for surface data obtained by linear laser scanning (Moss et al., 1987) and stereophotogrammetry (Ayoub et al., 1996).
On the genetic side the advent of diagnostic techniques in the field of molecular genetics make it possible to identify relevant morphogenes or genetic markers such as those for mandibular prognathism, or to influence the development of malocclusion, e.g. could crowding be eliminated by selective manipulation of the homeobox gene responsible for initiation of tooth formation and patterning of the dentition? The latter is more of a theoretical concept than a practical proposition, but aspects of orthodontic diagnosis and treatment planning may well take on a completely new meaning as we move into the twenty-first century. Molecular therapeutics is being employed in the field of maxillofacial surgery where knowledge of bone morphogenetic proteins (BMPs) is exploited in therapeutic regeneration in cases of congenital or acquired bone deficiency. It is therefore incumbent on the orthodontic speciality to keep abreast of developments in molecular genetics.
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Acknowledgments |
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References |
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