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Abnormal Tooth Numbers

The number of teeth changes depending on the course of exfoliation in that individual. Normally the primary dentition consists of 20 teeth, and the permanent dentition contains 32 teeth. However, the number of teeth present at any particular moment during exfoliation will not always correspond to the standard number. Part of an orthodontic examination is to establish whether any anomaly exists and its severity.

Supernumerary teeth (hyperdontia) can result from splitting of the tooth bud or the formation of a third tooth germ. Supernumerary structures are mainly found in the posterior region and at the end of the row of teeth, often combined with a complete cleft (lip, jaw, and palate). Frequently, they do not have regular tooth forms but have small, abnormally shaped crowns (mesiodens = conical form of crown and root). Supernumerary teeth in the posterior region are generally removed. Atypical supernumerary formations in the anterior region always have to be removed, and extraction must be followed by a jaw-regulating measure to close the spaces.

A distinction can be made between true and false hyperdontia as well as between true and false hypodontia. True hyperdontia refers to supernumerary formations, mainly as normal or abnormal tooth shapes in the permanent dentition (Fig 9-7). False hyperdontia means localized abnormalities of exfoliation when primary teeth have not yet been shed (a primary tooth persists) but the permanent teeth are already erupting (Fig 9-8); the permanent tooth usually erupts paraxially (not straight).

True hypodontia denotes the loss of one or more teeth as a result of accident or disease; this term is also used to describe the state of dental aplasia. Whether or not a specific tooth is formed is decided during the embryonic period.

Dental aplasia refers to the failure of teeth to develop. This aplasia can be partial or complete. In complete aplasia, there are neither primary teeth nor permanent teeth present in one or both jaws; it is also possible that only the permanent teeth are absent. This condition is extremely rare and congenital. Partial aplasia is also genetically controlled and usually occurs symmetrically. It is most likely to affect the maxillary lateral incisors, the second premolars, and the third molars. Aplasia of the primary teeth is very rare.

In cases of true hypodontia, the decision must be made whether to achieve space closure by orthodontic measures or to leave the gap for later prosthetic treatment. Space closure is preferred if natural narrowing of the spaces has already started in a small, underdeveloped jaw. However, the space is kept open if the jaws are very large and other gaps exist, or if the outlook for tooth displacement does not appear very promising because the distance involved is too large.

False hypodontia can exist in the short term and temporarily during physiologic exfoliation. There can be a considerable difference between a patient's age and the stage of his or her occlusal development, which is why analysis of the absence of teeth needs to be confirmed with radiographs of the tooth germs. Premature loss of a primary tooth prior to eruption of its corresponding successional tooth can occur quite often. However, false hypodontia can also occur when primary teeth are shed at the normal time but there is a delay in eruption. False hypodontia may require treatment if space narrowing occurs or if jaw growth is inhibited, although jaw growth takes place independently of occlusal development. However, a space maintainer will become necessary because extreme space narrowing and hence displacement or disturbed tooth eruption can arise.

The systematic extraction of primary teeth for orthodontic planning requires precise analysis of the number of teeth. If premature loss of primary teeth is only unilateral, it may be worthwhile to extract the corresponding tooth on the opposite side to maintain the symmetry of the dental arch. Bilateral extraction of individual primary teeth may also be considered if eruption of succession-al teeth is abnormal.

Inhibited development of the jaws and dental arches will occur if there is premature loss of primary teeth. This is because the tongue muscles lack the attachment surfaces for widening the dental arch. Unilateral primary tooth loss is therefore considered a localized abnormality of exfoliation from the orthodontic point of view.

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