Conjoined Teeth

Conjoined or double teeth can occur through different processes. Traditionally, conjoined teeth fall into three categories: gemination, fusion, and concrescence.⁷ In some cases it is quite difficult to discern the true pathogenesis of the particular anomaly.⁷ Some of the traditionally accepted parameters discussed in the following text do not apply to every occurrence of conjoined teeth.

Gemination

Gemination, or twinning, is a rare abnormality in which a single tooth bud tries to divide.⁹ Partial division may produce a bifid crown (Figure 28) with a shared pulp canal and root.⁹ Complete division, while rare, produces a normal tooth along with a supernumerary tooth (Figure 29).⁹ Gemination usually affects the primary teeth but the permanent dentition can be involved as well.^7,9 The incisor region is the most commonly affected area with no apparent gender predilection.⁷ In gemination, the number of teeth is usually normal.¹⁰

Figure 28.

Figure 28.

Periapical radiograph of maxillary lateral incisor #7 with coronal cleft.

Figure 29.

Figure 29.

Periapical radiograph of maxillary lateral incisor #10 showing complete division.

Fusion

Fusion is defined as two adjacent tooth buds that join together to form one normal looking tooth or a much larger tooth (Figure 30).^7,9,10 The degree of union may be total or partial and often presents with a coronal cleft.¹⁰ The normal complement of teeth is reduced by one.^7,9 Like gemination, fusion is more common in the primary dentition but can occur in the permanent dentition as well.^7,9,10 It also leans toward involvement in the anterior region of the dental arch.^7,9,10 While there is no gender predilection, incidence is higher among Native American, Asian, and other indigenous peoples.^7,9

Figure 30.

Figure 30.

Periapical radiograph of fused primary teeth F & G and permanent teeth #9 & #10. Note the larger fused crown size.

Concrescence

Concrescence is the joining of two completely formed teeth by the cementum along the root surface (Figure 31).^7,9 It favors posterior teeth and the maxillary arch, often involving a second molar tooth closely approximating the roots of an impacted third molar.⁷ The affected teeth may fail to erupt or only partially erupt.⁹ The cause of this altered morphology is not known.⁹ In instances in which concrescence causes eruption difficulties, surgical extraction may be necessary and result in loss of both teeth.^9,10

Figure 31.

Figure 31.

Periapical radiograph depicting molar concrescence.