Hyperdontia

Hyperdontia and supernumerary teeth are terms describing an excess in tooth number that can occur in both the primary and permanent dentitions. Reports on the incidence of hyperdontia include values as high as 3%, with males being affected twice as frequently as females.² Ninety percent to 98% of supernumerary teeth occur in the maxilla, with the permanent dentition being more frequently affected than the primary dentition. The most common supernumerary tooth is the mesiodens, which occurs in the palatal midline and can assume a number of shapes and positions relative to the adjacent teeth. The majority tend to be located palatal to the central incisors.³

As reported by Primosch in 1981, supernumerary teeth are morphologically classified as either supplemental or rudimentary.² Supplemental supernumerary teeth (Figure 3-1, A) duplicate the typical anatomy of posterior and anterior teeth. Rudimentary supernumerary teeth are dysmorphic and can assume conical forms, tuberculate forms (Figure 3-1, B), or shapes that duplicate molar anatomy. From a clinical standpoint, the tuberculate, or barrel-shaped, supernumeraries generate the most severe complications with respect to difficulty of removal and adverse effects on adjacent teeth, such as impaction or ectopic eruption. Additional complications associated with supernumeraries include dentigerous cyst formation, pericoronal space ossification, and crown resorption.³ It is important in supernumerary tooth detection to rule out the presence of odontoma in light of the fact that the morphologic characteristics of a compound odontoma are similar to those of supernumerary teeth.

Cleft lip and palate (CLP) commonly demonstrates an excess or deficiency in the normal complement of teeth and provides a clear example of physical disruption of the dental lamina as an etiologic factor. Patients with CLP have been reported to have an increased incidence of hyperdontia, with reports indicating an incidence of up to 5%.⁴ Classic syndromes involving supernumerary teeth are summarized in Table 3-1; cleidocranial dysplasia has the highest association with this dental anomaly.

Hypodontia

Hypodontia, or congenital tooth absence, is a deficiency in tooth number. Familial heredity patterns account for the largest etiologic correlation with patterns of hypodontia. Incidence reports identify a range of 1.5% to 10% excluding third molars in U.S. populations.⁵ The most frequently occurring congenitally absent permanent tooth, excluding the third molar, tends to be the mandibular second bicuspid (3.4%), followed by the maxillary lateral incisor (2.2%).⁶

There is a high correlation between primary tooth absence and permanent tooth absence.7–9 Ectodermal dysplasia (ED) represents a group of classic syndromes characterized by oligodontia or multiple congenitally missing teeth. The most common is hypohidrotic ED, followed in descending frequency of occurrence by hidrotic ED, ectrodactyly ED plus cleft lip and palate, Rapp-Hodgkin ED, and Robinson-type ED (Figure 3-2). Other conditions involving hypodontia are summarized in Table 3-2 and those involving both hyperdontia and hypodontia in Box 3-1.

Microdontia and Macrodontia

Abnormalities in tooth size are epitomized in microdontia and macrodontia. Hemifacial microsomia is thought to result from a hematoma of the stapedial artery during embryologic development giving rise to a deficient nutrient supply to the affected side of the face. In addition to unilateral mandibular hypoplasia and ear malformation, less growth occurs in this less vascularized area, with smaller teeth forming as a result. Peg-shaped lateral incisors, an example of microdontia, is common in Down syndrome. Other conditions demonstrating microdontia include ectodermal dysplasia and chondroectodermal dysplasia. These size abnormalities are thought to originate during the morphodifferentiation stage of tooth development.

Facial hemihypertrophy can demonstrate comparatively larger teeth on the affected side (Figure 3-3). Of the many factors thought to cause this condition, vascular and neurogenic abnormalities are considered the most likely. In addition to an increase in crown and root size, affected teeth develop more rapidly and erupt earlier than on the uninvolved side. The otodental syndrome (also known as otodental dysplasia), consisting of high-frequency hearing loss and globe-shaped fused molar teeth, is another condition that involves macrodontia. Isolated teeth with macrodontia can also result from twinning abnormalities that originate during the proliferation stage of tooth development. Fusion and gemination are the most common twinning abnormalities, and both include enlarged crowns.

Fusion

Fusion has an incidence of 0.5% and is more common in the primary dentition.⁷ The classic definition of fusion is the dentinal union of two embryologically developing teeth. Although fused teeth can contain two separate pulp chambers, many appear as large bifid crowns with one chamber, which makes them difficult to distinguish from geminated teeth.

Gemination

Gemination has a frequency of 0.5%, which is similar to that of fusion and is more common in the primary dentition. Conceptually, a geminated tooth represents an incomplete division of a single tooth bud resulting in a bifid crown with a single pulp chamber. Gemination tends to occur in a familial pattern, and its significance is similar to that of fusion in that both conditions may result in retarded eruption of the permanent successor. Clinically, fusion and gemination are usually distinguished by counting the teeth in the arch. If there is a deficiency in the normal complement including the bifid crown, the condition is fusion. Fusion with a supernumerary tooth must also be considered and ruled out because this would not affect the normal number of teeth.

Concrescence is a twinning anomaly involving the union of two teeth by cementum only. Its cause is thought to be trauma or adjacent tooth malposition. Because it can occur after root development, concrescence is technically not a developmental anomaly.

Dens Evaginatus

Dens evaginatus is an extra cusp, usually in the central groove or ridge of a posterior tooth and in the cingulum area of the central and lateral incisors (Figure 3-4). In incisors, these cusps appear talon-shaped and can approach the level of the incisal edge. This extra portion contains not only enamel but dentin and pulp tissue; therefore a pulp exposure can result from radical equilibration. It occurs with a frequency of 1% to 4% and results from the invagination of inner enamel epithelium cells, which are the precursors of ameloblasts.¹

Dens in Dente

Dens in dente is a condition resulting from invagination of the inner enamel epithelium producing the a/>