The dentigerous cyst (DTC), one of the types of odontogenic cyst, develops from the remnants of the enamel organs of impacted teeth or reduced enamel epithelium (15,18). DTCs are the most common types of odontogenic cyst rarely found in dogs, and have been found associated with unerupted canine and first premolar tooth (15,17). These cysts have been found ranging in age from 6 months to 10 years in dogs, but are most commonly diagnosed about 2-3 year (6,20). DTCs are more commonly in some brachycephalic or small breeds, including Boxers, Pugs, Shih Tzus, Chihuahua and Maltese (3,17,18,20). In general, patient with DTC may show no clinical sign except for missing one or more teeth (2,7). However, pain can occur if the cyst becomes infected, and pathological fractures of the jawbone may occur if the cyst changes too large (2,7,10). Also DTC can invade the nasal cavities (7).

‘Double teeth’ is a common term used to describe any form where two teeth are conjoined by dental hard tissue (5,9,17). Double teeth are classified into gemination, concrescence and fusion based on whether they are originated from one or two tooth germs and the extent of shared dental hard tissue (5,12). In gemination, two teeth are budded from a single germ, and in the other two types, teeth are originated from two germs respectively (5). Concrescence differs to fusion due to the two teeth are joined only by the cementum (5,19). The purpose of this report is to describe a case with double teeth existed in the DTC.

An 11-year-old spayed female Maltese dog was referred to Veterinary Teaching Hospital in Jeju National University in complaint of mass in oral cavity. The owner noticed the lesion for 4 days before in the regular examination.

The patient was bright, alert, and responsive and had a body condition score 6/9 and normal vital sign on physical examination. On conscious oral examination, the right maxillary canine tooth (104) was not visualized and only a little part of the crown of the right maxillary first premolar tooth (105) was visible due to buried in the lesion (Fig. 1). A lesion that had been presumed to mass was in area of the canine tooth (104) and has dark mucous membrane color. Radiographically, the right maxillary canine tooth (104) placed in the lesion horizontally, and most of the right maxillary first premolar tooth (105) was belonged to the lesion (Fig. 2). An overlap was observed between the root parts of two teeth. On pre-anesthetic laboratory tests including a hematology, blood chemistry, and blood coagulation, the abnormalities were not found.

Figure 1. Oral photograph of the right maxillary teeth. The right maxillary canine tooth is not visualized and the lesion has dark membrane color. The only a little part of the right maxillary first premolar tooth can be seen.

Figure 2. Dorsoventral (A) and right lateral (B) views on radiograph of the skull. The right maxillary canine tooth, that is placed in the lesion horizontally, and the maxillary first premolar tooth overlap each other at roots (white arrow).

The patient was premedicated and induced with medetomidine (Domitor^®, Pfizer, USA, 12 µg/kg), tiletamine and zolazepam (Zoletil50^®, Virbac, France, 1.5 mg/kg), tramadol (Trodon^®, Aju pharm, Korea, 0.9 mg/kg) intravenously and maintained with isoflurane (Ifran^®, Hana pharm, Korea) and oxygen. For extracting the teeth, the softest part of the cyst was incised and a small amount of serosanguinous liquid was discharged from the cyst. In the cyst, the right maxillary canine tooth (104) was buried horizontally and combined with the right maxillary first premolar tooth (105) to form double teeth (Fig. 3). The double teeth were extracted and the cystic epithelium was curetted. The incision was closed with simple interrupted sutures. Cephalexin (Phalexin^®, Dongwha, Korea, 20 mg/kg PO BID, 5 days) and meloxicam (Metacam^®, Boehringer Ingelheim, Germany, 0.1 mg/kg PO BID, 2 days) were administered for postoperative medication. No abnormality of the patient was confirmed in the two weeks and one year follow-up through the telephone call respectively.

Figure 3. Intraoperative photograph of teeth in the cyst incised (A) and extracted double teeth (B). The maxillary canine tooth is aligned horizontally in the alveolus with the root facing distally and the concave surface facing palatally. The maxillary canine tooth and the maxillary first premolar tooth are united to form double teeth.

Unerupted tooth include impacted tooth and embedded tooth (1). Uneruption is most common in the first premolar in the mandible and the canine tooth (7,18,20). Impaction occurs due to physical obstruction during tooth eruption process, and embedded tooth caused by lack of eruption force (17). Misdirection of tooth aligned horizontally is one of these physical barriers, thus it causes tooth impaction (4). Infections or trauma that can alter the eruption process of the deciduous teeth, cause ectopic positioning or prevent permanent teeth from erupting during the normal physiological eruption period (8). The enamel-forming organ of the unerupted tooth can develop the odontogenic cyst (15,17). The incidence of cystic formation is 29% in impacted teeth in dogs (1). And DTCs made up 71% of odontogenic cysts in the dog (20). In this case, the maxillary canine tooth was horizontally aligned in the gingiva with the root facing distally and the concave surface facing palatally (Fig. 3). This ectopic positioning of the tooth due to congenital or acquired factors is thought to have caused the impacted tooth, which developed into a DTC.

Treatment of DTC is includes surgical extraction of unerupted tooth and curettage of cystic lining which, if remain, can cause recurrence (4,6,19). Also, the defective area can be filled with a cancellous bone graft (4,19). The prognosis for most DTCs is good with early noticing and extraction before the DTC damages to surrounding structure (7,11). In this case, surgical extraction of unerupted teeth and removal of the cystic epithelium were performed for treatment, and bone grafting did not performed as the remained defect was not large. And the progress in this case was good. The little degree of invasion of DTC into the surrounding tissues was considered to have contributed to this prognosis.

Gemination is the division of a single tooth during development, with a single root on the radiograph and more teeth in the arch than normal clinically (17). Fusion of teeth due to the union of adjacent tooth germs of developing teeth, results in combination at the crowns and/or roots (5,9,12). The lower number of crown of teeth can be seen with fusion of teeth (5,16). Concrescence differs from other types of double teeth due to the fact that it describes union of two or more fully formed teeth by only their cementum with no involvement of other structures (14,16). In this case the double teeth combined the right maxillary canine tooth (104) and the right maxillary first premolar tooth (105) are considered either one of fusion or concrescence. Since the patient was 11 years old, her teeth were already missing, so it would be difficult to judge by the number of teeth. On the surface, both of the combined teeth were fully formed respectively, and the combination occurred only near the root. So the double teeth were presumed to concrescence, but it couldn't be accurately distinguished based on the external form alone. If radiographs of the extracted double teeth had been taken, the type could be distinguished by whether the root canal is shared or not. But in this case, we had not known about the types of double teeth before, so we didn't take a radiograph of the extracted teeth. For this reason, a limitation of this report is thought to failure to make a definite diagnosis. However, both fusion and concrescence are a rare dental abnormality in human (9,12,13). Also, according to one report, these anomalies are considered to be rare, as few cases were reported in a total of over 18,000 cases during 20 years in dogs (5). Therefore, this case with this dental abnormality in DTC is thought to be meaningful as a report itself.

This case report describes diagnosis and treatment process of an 11-year-old Maltese dog with double teeth in DTC. Although this case not confirmed whether concrescence or fusion among the types of double teeth, it is considered a rare case because of the double teeth with DTC.

This work was supported by the 2021 education, research and student guidance grant funded by Jeju National University.

The authors have no conflicting interests.