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Revista Española de Cirugía Oral y Maxilofacial

versão On-line ISSN 2173-9161versão impressa ISSN 1130-0558

Rev Esp Cirug Oral y Maxilofac vol.30 no.5 Madrid Set./Out. 2008




Fusion of mandibular third molar with supernumerary fourth molar

Fusión de un tercer molar mandibular con un cuarto molar supernumerario



C. López Carriches1, I. Leco Berrocal2, R. Baca Pérez-Bryan3

1 Profesora Asociada de Cirugía.
2 Profesora Colaboradora de Cirugía.
3 Profesor Titular de Cirugía.
Facultad de Odontología. Universidad. Complutense de Madrid. España





Dental fusion is the union of two tooth buds that normally are separated, while gemination is defined as an attempt by a single tooth bud to divide.
The fusion and gemination of molars is uncommon in permanent teeth.
We report a clinical case of a right lower third molar fused to a supernumerary fourth molar in a 36-year-old male patient with repeated episodes of inflammation. After the radiologic study, the semi-impacted third molar was extracted under local anesthesia.
The literature was reviewed.

Key words: Dental fusion; Dental germination; Double tooth; Third molar; Fouth molar.


La fusión dental es la unión de dos gérmenes dentales normalmente separados, mientras que la geminación se define como el intento de división de un único germen dental.
La fusión y geminación de molares es poco frecuente en la dentición permanente.
Describimos un caso clínico de un tercer molar inferior derecho fusionado a un cuarto molar supernumerario en un paciente varón de 36 años que ha presentado repetidos episodios de pericoronaritis. Tras el estudio radiológico se realiza la exodoncia del cordal semiincluido bajo anestesia local.
Llevamos a cabo una revisión bibliográfica al respecto.

Palabras clave: Fusión dental; Geminación dental; Doble diente; Tercer molar; Cuarto molar.



Anomalies of the union of teeth have received many names: fusion, gemination, double teeth, concrescence, twinning, and others.1

The definition is based on how the teeth that join together eventually develop. Consequently, fusion is defined as the union of two normally separated tooth germs, whereas gemination is the tentative division of a single dental germ. The differential diagnosis between fusion and gemination is difficult when the union involves a normal tooth and a supernumerary tooth. In the case of fusion, the number of teeth in the arcade diminishes, since two germs of two different teeth join during development. In contrast, gemination results in an apparent increase in the number of teeth. However, when the other tooth in the union is a supernumerary tooth, the number of teeth does not decrease despite the existence of fusion.2,3 In order to avoid terminology controversies, many authors call these anomalies generically "double tooth".4

The fusion and gemination of molars are infrequent in permanent teeth. The prevalence ranges from 0.08% to 0.5%.4-8 The central maxillary incisors are the teeth most frequently involved in fusion or gemination (3.55%), followed by the mandibular third molars (0.91%).4

The etiology of dental fusion is not known. It has been suggested that the pressure of adjacent dental follicles produces their contact and fusion before calcification; heredity and racial differences may predispose to fusion.4,9 Fusion is observed more frequently in the Japanese and in deciduous teeth.1 There is no difference between men and women in incidence.4


Clinical case

We report a clinical case of a third molar fused to a supernumerary fourth molar in a 36-year-old man. He was seen in the Department of Buccofacial Medicine and Surgery for episodes of pain, inflammation, and repeated trismus caused by a partially erupted lower right third molar. In the examination, a third molar of large dimensions was observed in a partially erupted mesioangular position. This third molar was responsible for episodes of pericoronaritis. In orthopantomography, the third molar was joined to a supernumerary tooth by the crown and root (Fig. 1). The patient had another supernumerary molar in the right upper jaw (Fig. 2).

After the patient’s clinical history was obtained (no history of interest), the purpose and risks of the intervention were explained to him and he signed the informed consent form. The patient rejected the possibility of undergoing a scan to determine the relation of the molar with the canal of the inferior dental nerve. The third and fourth molars were extracted surgically under local anesthesia. Tooth section was necessary due to the large size of the fused molar (Figs. 3 and 4).

There were no intraoperative or postoperative complications.

In the three month follow-up visit, periodontal problems were ruled out and a cavity was found in the second molar.



In a review of the literature, there are few studies on the prevalence of double teeth.4 Dental fusion is more frequent in deciduous teeth, particularly in the anterior region.1 Fusion of deciduous teeth usually gives rise to delayed eruption or ectopic eruption of the permanent tooth. Radiographic follow-up should be conducted to evaluate the development of the permanent teeth and remove them if necessary to avoid eruption delays or problems. In the definitive teeth, the central maxillary incisors are the teeth that most frequently present this anomaly, followed by the mandibular third molar.4 Cases of fusion of the maxillary third and fourth molars also have been described in the literature.10 In fact, in the case in question the patient also presented a supernumerary molar in the maxilla, but it did not seem to be fused.

Fused teeth are generally asymptomatic. The condition usually is diagnosed when the patient visits the clinic for a double tooth in the anterior part of the mouth that causes cosmetic concern due to its size or because it produces crowding, pain due to cavities, or pericoronaritis in the case of molars. Although dental anomalies have a low incidence, they are important because awareness of their presence will facilitate treatment. Treatment may be endodontic, for fused canals, curved roots, etc.9,11 orthodontic, because the roots of these teeth are resistant to movement; periodontal, a procedure that may be delicate when the fusion affects the roots; prosthetic because resection may affect pulp vitality; or surgical, as in the case that occupies us here. In every case, it is important to know how to evaluate the radiograph to avoid committing interpretation errors or attributing the images to artifacts. 12

Of course, if the fused tooth does not produce symptoms or affect the patient’s appearance, treatment will not be needed.13

In our case, the fused third and fourth molars were removed for repeated episodes of pericoronaritis and the risk of periodontal problems and tooth decay on the distal face of 47 due to impacted food. We agree with Henández-Guisado et al.13 regarding a case of gemination of 38 and 39 presented. Hernández-Guisado et al. claimed that the fact that the fourth molar has a volume equal to that of the third molar and that the fusion furrow extends from the crown to the apex suggests that two independent germs that were not separated by a bone septum during development fused together. Chen2,3 used the same treatment in similar cases of fusion of the third and fourth molars. If the aim is to conserve the third and fourth molars because the patient is missing the first and/or second molars and requires endodontic treatment, special attention will be paid to determining the number of canals and their form.9



Dra. Carmen López Carriches
Departamento de Medicina y Cirugía Bucofacial
Facultad de Odontología. Universidad Complutense de Madrid. España
28040 Madrid. España

Recibido: 06.11.2007
Aceptado: 29.04.2008



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