Discussion.
Clinically, it may be difficult to differentiate between fusion and gemination when a supernumerary tooth is joined with a permanent tooth. However, there is no clinical value in differential diagnosis of fusion and gemination. Mader (1979) suggested that all permanent successors that are joined or fused together by dentine be referred to as fused teeth because of the frequent difficulty in differentiating fusion from gemination in the permanent dentition. A preoperative radiograph showed two almost completely separated pulp chambers and roots. Separation of the fused tooth was required to correct the palatally dislocated incisors and to establish an aesthetic and functional occlusion. Hemisection is defined as the division of a tooth in half and removal of the unwanted, diseased portion, together with its root. In this case, the fused tooth was extracted and the mesial part of the fused tooth was replanted. The separation and hemisection of the fused tooth into a single incisor would have been a possible treatment method as described in previous reports (Itkin & Barr 1975, Blank et al. 1985, Kayalibay et al. 1996). The point of crown and root separation is as important in determining the prognosis of a fused tooth as it is in determining the prognosis of a molar with furcation involvement. Teeth that are fused too far apically cannot be sectioned without disturbing a major part of the attachment apparatus on the root to be retained. Extraoral separation and hemisection procedures were performed to achieve smooth contours of the replanted tooth. Such contouring and inspection of the interconnecting fin may have been difficult if division had taken place in situ. In the present case, care was taken in the sectioning process to cut at the expense of the tooth to be discarded, thus preserving a normal shape on the retained tooth.
It has been advocated that if endodontic treatment is to be performed, orthodontic treatment should be postponed until completion of endodontic treatment and clinical and radiographic evidence of healing is seen. In this case, endodontic treatment was completed before the active orthodontic treatment was applied. Successful root canal treatment depends on thorough cleaning, shaping and complete obturation of the root canal system. Mechanical debridement of the root canals connecting via a large fin in mid-root was difficult, but the combination of chemomechanical instrumentation and the use of calcium hydroxide were sufficient. As calcium hydroxide has been reported to successfully dissolve pulp tissue remaining on the root canal wall (Wadachi et al. 1998), it was decided to treat the root canals with this medicament before obturating the root canal with gutta-percha. The use of a warm gutta-percha technique helped to obturate the root canal system, as it was possible to compact the softened material into the major irregularities within the root canal system (Budd et al. 1991, Gutmann 1993).
According to previous investigations (Andreasen & Kristerson 1981), ankylosis was expected on the denuded root surface because of the lack of periodontal membrane at the separated root surface. No treatment was considered helpful to encourage periodontal regeneration of the site. Most ankylosis is diagnosed within the first 2 years after replantation of human avulsed teeth (Andreasen et al. 1995). In the present case, 3 years after the replantation, ankylotic symptoms such as loss of periodontal space and possible root resorption were observed. However, there is no treatment for ankylosis at present. On the other hand, there is potential for an eventual long-term periodontal problem because there was no covering bone and epithelial attachment over the separated root surface. Periodontal care will be an important issue for predicting long-term prognosis.

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