Teeth – Number, Size, Form

Disorders of development of teeth (number, size, form)

Number

Hypodontia; the congenital absence of teeth, where one or several teeth are missing

• May or may not be symmetrical
• Likely caused by mutations in regulatory genes involved in tooth development
• Hypodontia may be associated with other craniofacial anomalies and developmental syndromes
• If deciduous teeth are 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
• More common in permanent dentition. 2-10% in diff populations (excluding absent third molars). More common in females
• Primary dentition (less than 1 percent)
• Japanese & Swedish: prevalence of missing mandibular permanent central incisors
• Anodontia; complete absence of one or both dentitions
• The most frequent associated defect is hereditary hypohidrotic ectodermal dysplasia which is characterized by the congenital absence of ectodermal structures
• The disorder is rare and is usually inherited as an X-linked recessive trait
• Smooth, dry skin with fine, scanty hairs and partial or total absence of sweat glands which leads to hypothermia

Hyperdontia; supernumerary teeth

• Teeth additional to those of the normal series
• Occur most frequently in the anterior and molar regions of the maxilla followed by the premolar region of the mandible
• Occasionally associated with other defects such as cleft palate or cleidocranial dysplasia
• May prevent eruption or cause malposition or resorption of adjacent teeth, and may develop dentigerous cysts if unerupted
• More common in females, usually single and occur in about 1-3% of the population in the permanent dentition

Size

The size of both the teeth and the jaws is influenced by genetic and environmental factors and considerable variation occurs. Macrodontia – larger than normal. Microdontia – smaller than normal

• 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, e.g. Down syndrome and congenital heart disease
• Abnormalites in number and size often occur together
• More common examples are seen in patients with one missing permanent maxillary lateral incisor, in which case the contra-lateral tooth is frequently peg-shaped

Form

Disturbances in form may involve the crown, root or both

Dilaceration; a deformity in which the crown of the tooth is displaced from its nomral alignment with the root, so that the tooth is severly bent along its long axis

• Usually the result of acute mechanical trauma and most frequently involves the maxillary incisors

Taurodontism; “bull-like tooth”, where the pulp chamber has a greater apico-occlusal height than in normal teeth, with no constriction at the level of the amelocemental junction. The result is that the chamber extends apically, well beyond the neck of the tooth

• Affects multi-rooted teeth and is thought to be caused by the failure of Hertwig’s sheath to invaginate at the proper horizontal level
• Rare in the primary dentition
• An association with abnormalites in the number of the sex chromosmes such as Klinefelter and poly-x syndromes

Double teeth; developmental anomaly where two teeth appear joined together

• The degree of union is variable and may involve the crown, the roots, or both
• It is very unusual for teeth to be united by enamel only, joining of the dentine and also the pulp chamber being more frequent
• Fusion – the union between dentine and/or enamel of two or more separate developing teeth
• Gemination – the partial development of two teeth from a single tooth bud following incomplete division
• Unknown cause, genetic basis suggested; for this reason the general term ‘double teeth’ which describes the appearance with no implication regarding aetiology is preferred
• More common in the primary than in the permanent dentition
• Incisors are most frequently  affected and the condition may be bilaterally symmetrical
• In the primary dentition the majority of cases involve the anterior mandibular teeth

Concrescence; an acquired disorder in which the roots of one or more teeth are united by cementum alone after formation of the crowns

• Most frequently seen in the permanent dentition where the roots of teeth develop close together
• May also occur following hypercementosis associated with chronic inflammation