Congenital and
developmental anomalies of the teeth
Dental lamina
Teeth develop from the dental lamina, a strand of epithelium originating
from the oral mucosa, which migrates backwards from the incisor region of both
mandible and maxilla beginning in the sixth week of intrauterine life. As it
migrates backwards tooth buds are formed which eventually develop into the
teeth and roots, each with its characteristic shape, according to the position
in the jaw. Abnormalities in its migration may lead to abnormalities that are
laid out below. The concept of the dental lamina, however, has changed in recent
years, as it would seem that the epithelium responsible for developing the teeth
is not the dental lamina as such but modifications of the oral mucosa. This
invaginates beneath the alveolus progressively posteriorly from the incisor
region under the influence of genes and activators, which are specific for each
part of the mouth, in a time-window of 2—3 days.
The
total number of the secondary dentition is 32. The third molar or wisdom tooth
may fail to develop in any of the four quadrants. Lack of space may cause them
to impact into the second molars or backwards into the ramus. These may lead to
pericoronal infection later in life and the third molars may require removal.
The upper lateral incisors may fail to develop or develop as small, peg shaped
teeth. This is normally associated with an impaction of the canines and is
thought to be associated with faults in the dental lamina or gene deficiencies
during development.
Failure
to develop a large number of primary and secondary teeth is known as
hypodontia. It may be associated with other forms of ectodermal dysplasia where
there is a lack of sebaceous and sweat glands. Considerable problems are
encountered in providing a functional dentition, particularly during the growth
period.
Supernumerary
teeth and odontomes
Supernumerary teeth tend to occur in the same
areas where teeth may be absent — the lateral incisor, second premolar and the
third molar areas. Generally they are smaller than the normal teeth in the same
area and seldom develop little more than a crown and a vestigial root. They may
be multiple. In the incisor region they may be responsible for the failure of
eruption of adjacent permanent teeth, particularly the central and lateral
incisors and canine. They generally lie palatal to their normal counterparts.
They have to be removed to allow eruption of the permanent teeth to occur.
Rarely, in the third molar region, dentigerous cysts may develop around the
crown. These are characterised by a radiographic
Odontomes
are aggregations of tooth-like material and may take the form of multiple small
teeth (denticles, a compound complex odontome) or irregular masses of dentine,
cementum and enamel (complex composite odontome). They lie within the alveolus,
frequently inhibiting the eruption of the adjacent teeth and are encapsulated.
The most common site is the premolar region of the mandible. Odontomes are, in
effect, hamartomas. Once formed they do not increase in size. Occasionally they
become infected.
Developmental disturbances in the structure of the
teeth
The enamel of primary and secondary dentition
may be affected by a number of genetic or congenital conditions. There may be
effects due to local problems such as trauma during birth or infection. The most
notable change affecting all teeth is the result of amelogenesis imperfecta
which is genetic and may be dominant or recessive. It is an ectodermal
disturbance and the anomalies can range from pits in the teeth to failure of
enamel development. It may be associated with osteogenesis imperfecta. By
contrast, dentinogenesis imperfecta affects the mesodermal portion of the
odontogenic
apparatus. The appearance of the teeth is variable with the poorly supported
enamel taking on a opalescent tinge. The enamel is readily damaged. Both
amelogenesis and dentinogenesis imperfecta result in rapid early tooth loss.
Odontogenic
cysts and tumours
Abnormalities in the dental lamina may form
benign cysts and odontogenic rumours (keratocyst, odontogenic myxoma). A
malignant form, ameloblastoma, is variable between cystic and solid, and is
locally invasive. The most common site for these to occur is in the third molar
region and ramus of the mandible.