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, 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
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
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
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