Anodontia refers to a lack of tooth development and usually does not occur in the deciduous dentition. The teeth most often affected are the third molars, the second premolars, and the lateral incisors.
Hypodontia refers to the lack of development of one or more teeth. Hyperdontia refers to an increase in the number of teeth and occurs more frequently in the permanent dentition. Most hyperdontia occurs in the maxilla and specifically the anterior region.
Dental transposition refers to normal teeth erupting in an inappropriate position.
Hyperdontia refers to additional teeth as stated above and mesiodens fall into this definition. The number of those affected with hyperdontia is approximately 0.15% to 3.9% of the population. Most supernumerary teeth occur in the maxillary arch, they occur unilaterally, and over half are found in the anterior region. It is not uncommon to find other supernumerary teeth within the same patient, and there may also be other craniofacial anomalies as well. Russell and Folwarczna (2003) distinguished between the types of mesiodentes such as rudimentary (defined as those occurring in the permanent dentition), primary (those occurring as supplementary), and those according to morphology such as conical, tuberculate or molarform.
Etiology: The etiology of mesiodens is unclear, but males are twice as affected as females in permanent teeth (possibly an autosomal recessive gene), and there is a familial trait. The genetic susceptibility along with environmental factors appears to increase the activity of the dental lamina leading to the mesiodens. The hyperactivity theory appears to be the most accepted possibility, although genetic transmission is also favored. An additional theory is that the tooth bud splits, creating two teeth.
Reported increases of mesiodens within various ethnic groups are documented. For instance, supernumerary teeth in Japanese children are reported to be 0.05%, in Canadian pupils, 0.64%, and in Caucasian general populations between 0.1 and 3.8%. Higher percentages are reported in African and Asian populations as well.
The mesiodens interfere with eruption and cause other alignment problems with the existing teeth. Only a small portion of supernumerary teeth eventually erupts (approximately 25%) and the pattern of eruption is usually the first indication of a problem in the case of mesiodens. Because of the concern in eruption patterns, the dental professional usually orders radiographs to assess any problems that may be unseen clinically. Radiographs provide some assistance, but are often vague because of primary teeth obstruction.
Early intervention is suggested to prevent additional damage such as misalignment and delayed eruption of the permanent central incisors. Meighani and Pakdaman (2010) point out in a review of the literature that some practitioners prefer to wait to remove mesiodens until the root of the central and the lateral teeth are completely formed, so that damage to the permanent tooth is not an issue.