Isolated hypodontia or anodontia
It is relatively common and often hereditary to see failure of development of one or two teeth. Most frequently missing teeth include third molars, second premolars, or maxillary second incisors.
(Congenital absence of lateral incisors with spacing of the anterior teeth)
Absence of third molars can be a disadvantage if first or second molars, or both, have been lost. The absence of lower premolars worsens malocclusion if there is already disparity between an underdeveloped mandible and a normal upper arch. Otherwise absence of these teeth may have little or no noticeable effect.
Absence of lateral incisors can sometimes be conspicuous because the large, pointed canines erupt in the front of the mouth beside the central incisors. It is usually impossible to prevent the canine from erupting into this empty space, even if the condition has been noticed early. It is also very difficult to make room between the centrals and canines by orthodontic means to replace the laterals. An attempt has often therefore to be made to disguise the shape of the canines.
Total failure of development of a complete dentition (anodontia) is exceedingly rare. If the permanent dentition fails to form, the deciduous dentition is retained for many years, but when these deciduous teeth become excessively worn or too much damaged by caries then they must be replaced by dentures or implants.
Hypodontia or anodontia with systemic defects
Anhidrotic (hereditary) ectodermal dysplasia
In severe cases no teeth form. More often, most of the deciduous teeth form but there are few or no permanent teeth. The teeth are usually peg-shaped or conical.
(Anhidrotic ectodermal dysplasia showing conical teeth, giving an undesirable, Dracula-like appearance)
When there is anodontia, the alveolar process, without teeth to support, fails to develop and has too little bone to support implants. The profile then resembles that of an elderly person because of the gross loss of vertical dimension. The hair is fine and sparse, particularly in the tonsural region. The skin is smooth, shiny and dry due to absence of sweat glands.
(Another case showing typical fine and scanty hair and loss of support for the facial soft tissue)
Heat is therefore poorly tolerated. The finger nails are usually also defective. All that can be done to improve the patient’s appearance and mastication is to fit dentures, which are usually well tolerated by children.
Other conditions associated with hypodontia
There are many rare syndromes where hypodontia is characteristically seen, but the only common one is Down’s syndrome. One or more third molars are absent in over 90% of these patients, while absence of individual teeth scattered about the arch is also common. Anodontia is rare. Palatal clefts may be associated.
Additional teeth: hyperdontia
Additional teeth are relatively common. They are usually of simple conical shape (supernumerary teeth) but less frequently resemble teeth of the normal series (supplemental teeth). These are the results of excessive but organized growth of the dental lamina of unknown cause.
Conical or more seriously malformed additional teeth most frequently form in the incisor or molar region and very occasionally, in the midline (mesiodens).
(A paramolar, a buccally placed supernumerary molar tooth)
Occasionally an additional maxillary incisor, premolar or, rarely, a fourth molar develops.
(Maleruption of a midline tuberculate supernumerary and two supplemental premolars)
Syndromes associated with hyperdontia
These syndromes are all rare but probably the best known is cleidocranial dysplasia where many additional teeth develop but fail to erupt. In this rare familial disorder there is defective formation of the clavicles, delayed closure of fontanelles and sometimes retrusion of the maxilla. Partial or complete absence of clavicles allows the patient to bring the shoulders together in front of the chest.
(Cleidocranial dysplasia. Defective development of the clavicles allows this abnormal mobility of the shoulders. Other members of the family were also affected)
This disorder is one of the few recognisable causes of delayed eruption of the permanent dentition. Many permanent teeth may remain embedded in the jaw and frequently become enveloped in dentigerous cysts.
(Cleidocranial dysplasia. A radiograph of the jaw of the father of the patient in previous figure. There are many additional teeth but widespread failure of eruption)