T. Tsurumachi & T. Kuno
Departments of Endodontics, Division of Advanced Dental Treatment, and Oral and Maxillofacial Surgery, Nihon University School of Dentistry, Tokyo, Japan.

Introduction.
Fusion is defined as a union of two separate tooth buds at some stage in their development. The pulp chamber and root canal may be joined or separated, depending on the stage of development at the time of union (Duncan & Helpin 1987). The aetiology of this phenomenon is unknown. Shafer et al. (1983) suggested that physical force or pressure leading to prolonged contact of the adjacent tooth follicles causes fusion of buds. Gemination, sometimes called ‘twinning’ is a similar dental anomaly and is defined as an attempt of the tooth bud to divide (Braham 1995). In some instances, it is difficult to differentiate between fusion of a permanent tooth with a supernumerary tooth and gemination of a single tooth. Clinically, both types of tooth anomaly may result in functional and aesthetic problems and thus may require some kind of endodontic, prosthetic, surgical and/or orthodontic treatment. Variations in the fused tooth morphology present a clinical challenge when endodontic treatment is required (Budd et al. 1992).
This case report describes the endodontic and orthodontic management of a maxillary lateral incisor fused with a supernumerary.

Case report.
A 10-year-old boy was referred for endodontic treatment of his maxillary right lateral incisor (tooth 12). The medical history was noncontributory. Clinical examination revealed that the  maxillary lateral incisor was fused with a supernumerary and in cross-bite, and that the maxillary canine was missing from the dental arch (Figs 1 and 2). There was an extra crown of normal appearance adjacent to the right lateral incisor. No caries could be detected. Both teeth displayed physiological mobility, and responded within normal limits to electric pulp sensitivity testing. Radiographic examination demonstrated a fused tooth with two separate pulp chambers and two separate root canals connecting via a large fin in mid-root. The maxillary canine was impacted (Fig. 3). A decision was made to divide tooth 12 from the supernumerary to improve aesthetics and move tooth 12 out of cross-bite. Endodontic and orthodontic treatment was therefore initiated. After local anaesthesia and rubber dam isolation, working lengths were established (Fig. 4) and chemomechanical preparation performed with 2.6% sodium hypochlorite solution as irrigant. After drying the root canals with paper points, a calcium hydroxide paste was applied and the access cavities temporarily sealed with Cavit (ESPE, Seefeld, Germany). The patient returned after 2 months; the calcium hydroxide paste was removed and the root canals were obturated with injection-moulded thermoplasticized gutta-percha (Obtura II, Obtura Corp., Fenton, MO, USA) and zinc oxide–eugenol sealer (Canals, Showa Yakuhin, Tokyo, Japan). A postoperative radiograph was taken (Fig. 5). Three weeks later, the patient reported that he had been completely without symptoms. The fused tooth was anaesthetized and buccal and palatal flaps were raised. The fused tooth was extracted (Fig. 6) and kept in a moist condition with normal saline solution. The tooth was held gently by the crown with wet gauze during the procedure of dividing with a diamond bur. The distal part of the fused tooth was removed. At this time the communication between root canals was exposed. Inspection of the exposed pulp fin area indicated well-adapted gutta-percha and sealer, requiring no additional preparation or seal. The mesial part of the tooth was then replanted into its original site in the socket. Before replantation, blood clot was aspirated from the socket, and the replanted tooth was rinsed with saline to remove all debris. The entire time from extraction to replantation was 20 min. The replanted tooth was splinted to adjacent teeth for 3 weeks (Figs 7 and 8). Three months later, the orthodontic appliances were put in place (Fig. 9). Three months after orthodontic treatment, the impacted canine erupted between the remaining tooth and the first premolar. The patient was recalled for periodic checkups and healing was uneventful (Fig. 10). The recall examination after 3 years revealed asymptomatic and healthy periodontal conditions (Figs 11 and 12).

Figure 1. Preoperative view of the fused maxillary lateral incisor and the supernumerary tooth. Note the palatally dislocated teeth.



Figure 2. Preoperative palatal view of the fused maxillary lateral incisor.



Figure 3. Preoperative radiograph of the fused maxillary lateral incisor showing two completely separated pulp chambers and roots. The communication of the pulp systems can be detected radiographically.



Figure 4. Periapical radiograph for determination of both working lengths.



Figure 5. Radiograph immediately after obturation of root canals. The communication of the pulp systems is evident.



Figure 6. The extracted fused maxillary lateral incisor.



Figure 7. Palatal view of the mesial part of the tooth following suturing.



Figure 8. Radiograph immediately after the fixation.



Figure 9. Orthodontic extrusion of the impacted canine.



Figure 10. Radiograph taken after 2 years of active orthodontic therapy.



Figure 11. Clinical side view of maxillary lateral incisor after 3 years treatment. The alignment of the teeth is satisfactory.



Figure 12. Three-year follow-up radiograph ofmaxillary lateral incisor, showing external root resorption.