Developmental Anomalies in Individual Teeth
Developmental anomalies and structural defects (tooth dysplasias) are commonly found in individual teeth as a consequence of genetic or infectious factors, vitamin deficiency diseases (eg, rickets), or certain medications (eg, tetracycline or antibiotics). Because the development of the teeth takes place over very extended periods of time (from the fifth week of embryonic life through to the age of 16 years), during which time many events may intervene directly or indirectly in this process, the teeth often show traces of disorders and structural defects. The results are structural flaws, deviations in tooth size and shape, and abnormal numbers of teeth (Fig 9-4).
Dysplasias of dental hard tissue (defective development or underdevelopment) affecting dentin and enamel arise before eruption of the teeth and are relatively common.They are less frequent in the primary teeth than in the permanent dentition. Enamel hypoplasias disturb the tooth shape so that furrows, pits, convolutions, and jagged cusps are formed. These features are accompanied by discoloration of the enamel layers, such as whitish or yellowish spots.
Enamel hypoplasia is a disturbance of the cellular activity of ameloblasts. As the ameloblasts are unable to divide and therefore cannot be replaced, the degree of malformation depends on when the cells were damaged or killed.
Damage to primary teeth, caused by the effects of impact during childhood or inflammation of the roots of the primary teeth, can lead to changes of shape and color in the enamel of the successional teeth. The enamel can be discolored by the effect of medications, or it may display other structural flaws.
Dentin hypoplasia, caused by defective functioning of the odontoblasts, is accompanied by enamel defects, is only visible under the microscope, and is not very noticeable because the odontoblasts can be replaced by other cells that form dental hard substance.
Abnormal root shapes result from the effects of trauma during eruption, which means that stunted or kinked roots may develop (Fig 9-5). Congenital dysplasias of the enamel and dentin are dominant hereditary conditions and also lead to conical or peg-shaped teeth. Such abnormal tooth shapes commonly occur as supernumerary teeth.
Congenital anomalies are more commonly found in the size and shape of the teeth. Distinct abnormalities of the tooth shape are evident on both the crown and the root.
Defects of the dental crown are manifest as conical canines, additional cusps on premolars and molars (Carabelli cusp), or stunted forms of the lateral incisors and third molars (less common on the second premolars). Additional roots on the premolars and molars are relatively common; shape anomalies of this kind are regularly found on third molars.
When tooth germs divide, the result is schizo-dontia of a tooth, where two identical teeth erupt. If division starts but is not completed, the result is doubling (gemination), where a far broader tooth with a distinct occlusal notch and only one pulp cavity is found. As well as abnormalities such as hypoplasia of the dental hard tissue or multiple structures, there can also be abnormalities in the number of teeth such as too many teeth (hyper-dontia) or too few teeth (hypodontia).
If tooth germs fuse, teeth that have grown together will erupt. The fusion may be partial or complete, with separate pulp chambers developing (Fig 9-6). Coalescence occurs in the root area of adjacent teeth as a result of fusion of the ce-mentum.
Distinctive features that are characteristic of individual people can be identified in the structure of dental hard tissue because widened growth lines are formed as a result of the periodic process of enamel and dentin formation.
Forensic identification based on individual teeth is possible because childhood illnesses disrupt the normal variance, leaving lines of varying width and at varying distances in and on the teeth. A corpse or its remains can be identified by the individual, line-type disrupted patterns in the dental hard tissue. By means of forensic identification, it is possible, for instance, to tell whether several teeth-bearing jaw fragments or single teeth come from one or more people; equally, the age of a person can be roughly determined from the dental tissue structures.