Conjoined Teeth

Conjoined or double teeth can occur through different processes. Traditionally, conjoined teeth fall into three categories: gemination, fusion, and concrescence.⁵ In some cases it is quite difficult to discern the true pathogenesis of the particular anomaly.⁵ 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 32) with a shared pulp canal and root.⁹ Complete division, while rare, produces a normal tooth along with a supernumerary tooth (Figure 33).⁹ Gemination usually affects the primary teeth but the permanent dentition can be involved as well.^5,9 The incisor region is the most commonly affected area with no apparent gender predilection.⁵ In gemination, the number of teeth is usually normal.¹⁰

Figure 32.

Figure 32.

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

Figure 33.

Figure 33.

Periapical radiograph of maxillary lateral incisor tooth #10 with 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 34).^5,9,10 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.^5,9 Like gemination, fusion is more common in the primary dentition but can occur in the permanent dentition as well.^5,9,10 It also leans toward involvement in the anterior region of the dental arch.^5,9,10 While there is no gender predilection, incidence is higher among Native American, Asian, and other indigenous peoples.^5,9

Figure 34.

Figure 34.

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 (Figures 35, 36).^5,9 It favors posterior teeth and the maxillary arch, often involving a second molar tooth closely approximating the roots of an impacted third molar.⁵ 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 35.

Figure 35.

Periapical radiograph demonstrating molar concrescence.

Figure 36

Figure 36

Periapical radiograph showing concrescence of premolar supernumerary teeth.