There are a number of developmental alterations that may occur which influence the number of teeth that are present. Several different terms are used to describe the absence or presence of teeth.7-9 Anodontia is the proper term for the total lack of tooth development involving the entire dentition.7 This is uncommon, particularly in the primary dentition. Several other terms are used to define missing teeth such as hypodontia, one or more missing teeth, and oligodontia, six or more missing teeth excluding the third molars.7 In contrast, hyperdontia is the development of more than the usual complement of teeth.7 These extra teeth are sometimes referred to as supernumerary teeth.7-9
While rare in the primary dentition, hypodontia is not uncommon in the permanent dentition. In fact, it is one of the most common developmental abnormalities affecting nearly 20% of individuals with a slight female predominance.7 The most frequently missing teeth are the third molars, followed by the second premolars (Figure 19), the maxillary lateral incisors, and mandibular central incisors (Figure 20).7,9 The maxillary central incisors, the mandibular first molars, and canines are the least likely teeth to be absent.7
In instances in which multiple teeth are missing, especially key teeth like maxillary central incisors or the first molars, a hereditary condition known as ectodermal dysplasia should be considered.7,9 In this entity, the teeth that are present have an abnormal crown shape (Figure 21).9 Clinically, dermal adnexal structures like hair, eyebrows and eyelashes are also affected.9,10
Extra or supernumerary teeth usually involve a single tooth in the permanent dentition within the maxilla, particularly in the anterior segment.7 Other common sites for supernumerary teeth include the maxillary molar area, and the mandibular molar, premolar, canine, and lateral incisor areas.7-9 A supernumerary tooth in the anterior maxilla is a mesiodens (Figure 22), while a fourth molar tooth is a distodens or distomolar (Figure 23), and an extra tooth in the premolar area is a peridens (Figure 24).7-10 The term paramolar is used for a posterior extra tooth positioned buccally or lingually to a molar tooth.7,10 Supernumerary teeth can be of normal size and shape or abnormally shaped and diminished in size.7 The later type can display various shapes including conical, barrel-like or a petite version of a molar or premolar tooth.7 These teeth can be erupted or unerupted.
It is possible that supernumerary teeth will present in a multiple manner. When multiple extra teeth occur, the most common location is in the mandibular premolar region (Figure 24), but molar and anterior areas also can be involved (Figure 25).7