Download Supernumerary teeth (hyperdontia)

INTRODUCTION
The development of teeth and of the face is regulated by genes, but the genetic programme is very
sensitive to disturbances in the environment such as exposure to infection or toxic chemicals,
including drugs. The specific genetic abnormalities underlying some developmental disorders are
now known, and for several others a strong genetic association has been established, even though
the genes have yet to be identified. However, there remain many where the causes appear complex
and multifactorial, involving the interaction of genetic and environmental factors.
DISORDERS OF DEVELOPMENT OF
TEETH
Disorders of development of teeth may be prenatal or postnatal in origin and may be inherited or
acquired. Their recognition and evaluation require a thorough knowledge of the normal chronology
of the human dentition and of the normal development and structure of the teeth.
Disorders of development of teeth may be due to abnormalities in the differentiation of the dental
lamina and the tooth germs, causing anomalies in the number, size, and form of teeth (abnormalities
of morphodifferentiation) or to abnormalities in the formation of the dental hard tissues resulting in
disturbances in tooth structure (abnormalities of histodifferentiation). Abnormalities of
histodifferentiation occur at a later stage in development than abnormalities of
morphodifferentiation; in some disorders both stages of differentiation are abnormal.
Disturbances in number of teeth
The congenital absence of teeth may be referred to as hypodontia, when one or several teeth are
missing, or anodontia when there is a complete absence of one or both dentitions.
Hypodontia is more common in the permanent dentition, occurring in about 2-10 per cent in
different populations (excluding absent third molars) compared to the primary dentition where the
prevalence is less than 1 per cent. It is more common in females and there are also racial
differences. For example, the prevalence of missing mandibular permanent central incisors is much
more common in Japanese and Swedish populations than in other groups studied. Hypodontia may
be symmetrical when particular teeth or groups of teeth are involved, or haphazard when no pattern
is discernible. Although it is very unusual for deciduous teeth to be congenitally absent, it is likely
that in such cases the permanent successional tooth will also fail to form. Third molars, permanent
maxillary lateral incisors, and mandibular second premolars are the teeth most frequently involved
in symmetrical forms and a hereditary trait can sometimes be shown with missing maxillary lateral
incisors.
Although the genetic basis of hypodontia is not yet understood, several regulatory genes involved in
tooth development have been identified and it is likely that mutations in these result in tooth
agenesis .These control or regulatory genes are not unique to tooth development but are the same
genes that control the development of the face and of many other tissues and organs in the embryo.
Thus, hypodontia may be associated with other craniofacial anomalies and developmental
syndromes .
Severe hypodontia and anodontia are rare and in most cases are associated with other defects, the
most frequent being hereditary hypohidrotic ectodermal dysplasia which is characterized by the
congenital absence of ectodermal structures. The disorder is rare and is usually inherited as an Xlinked recessive trait, although a very rare autosomal recessive form has been described and
sporadic cases have been reported. Affected patients have smooth, dry skin with fine, scanty hairs
and partial or total absence of sweat glands which leads to hyperthermia. Some have a few teeth
present, but these are often retarded in eruption, deformed, and frequently have conical crowns.
Female carriers usually show only mild manifestations that may be restricted to minimal hypodontia,
such as absent maxillary lateral incisors, but carriers may be detected by a reduced sweat pore
count.
These are teeth additional to those of the normal series. They may develop in any tooth-bearing
area but occur most frequently in the anterior and molar regions of the maxilla followed by the
premolar region of the mandible. Occasionally, they are associated with other defects, such as cleft
palate or cleidocranial dysplasia. They may prevent the eruption, or cause malposition or resorption
of adjacent teeth, and may develop dentigerous cysts if unerupted. Supernumerary teeth are more
common in females, are usually single and occur in about 1-3 per cent of the population in the
permanent dentition. They are unusual in the deciduous dentition.
Supernumerary teeth occurring at certain sites may be referred to by special terms. A mesiodens is a
supernumerary tooth developing between the maxillary central incisors and is the most common of
all supernumerary teeth. The majority have conical crowns and short roots. A paramolar arises
alongside the maxillary molars and is usually buccally placed, and a distomolar develops distal to a
third molar. Supernumerary teeth which morphologically resemble those of the normal series are
called supplemental teeth but most are reduced in size.
Disturbances in size of teeth
The size of both the teeth and the jaws is influenced by genetic and environmental factors and
considerable variation occurs. Studies of twins have shown that for the teeth, at least, genetic
factors account for a large part of this variation. The terms 'macrodontia' and 'microdontia' are used
to describe teeth which are larger or smaller than normal, respectively, but the limits of normal
variation have never been adequately defined. Both macrodontia and microdontia may involve the
entire dentition, or only one or two teeth symmetrically distributed in the jaws. Microdontia of the
whole dentition may be associated with other defects, for example Down syndrome and congenital
heart disease.
Whilst it is convenient to consider abnormalities in the number and the size of teeth separately the
anomalies often occur together. For example, hypodontia and microdontia may occur together in
several of the conditions listed in. More common examples are seen in patients with one missing
permanent maxillary lateral incisor, in which case the contralateral tooth is frequently peg-shaped.