Gemination or dental fusion? Diagnostic dilemma. Case study

Pages: 213-225

Elena Stepco*, Elizabet Sapoznikov**

* PhD., assoc. prof. at Ion Lupan Department of pediatric OMF surgery and pedodontics, Nicolae Testemițanu SUMPh** student 5th year Faculty of Stomatology, Nicolae Testemițanu SUMPh


Congenital dental anomalies that include abnormalities of shape, number, volume, structure, and position occur in both the primary and permanent dentition. Their prevalence is higher in the primary dentition and varies from 0.5% to 7%. “Primary double teeth”, which refers to both fusion and gemination, are described as one of the most common developmental anomalies in the primary dentition. The presence of a dental anomaly in the primary dentition is a major risk factor for the development of a dental anomaly in the permanent dentition. Dental abnormalities of primary teeth can lead to functional and aesthetic disturbances in both primary and permanent dentition, and their early detection contributes to early diagnosis and long-term treatment planning. This study describes some clinical cases of dental  gemination and fusion encountered in our professional practice.

Keywords: gemination, fusion, lateral permanent incisor, lateral primary incisor, supernumerary tooth, etiology, diagnosis.


Congenital dental abnormalities are a large group of hard tissue disorders and include abnormalities of shape, number, volume, structure, and position. They can occur both in isolation and in combination with structural abnormalities and developmental defects of the organs and systems of the child’s body [5].

Dental abnormalities in the primary dentition are usually detected during usually dental examinations. They involve an eminent caries risk, aesthetic discomfort, and in the future can lead to orthodontic problems, including spacing or crowding of the teeth, loss of the length of the dental arch, deviation of the midline, etc [10].

Primary double teeth, which refer to both fusion and gemination, are described as one of the most common developmental abnormalities in primary dentition. Fusion is often confused with gemination, especially if the union took place with a supernumerary tooth. However, dental fusion is the joining by means of crowns of two or more tooth buds at the level of enamel and dentin or by means of roots at the level of cement, or by means of crowns and roots. Gemination is known as the attempt of a single tooth bud to divide [3].

Although both anomalies clinically appear similar, by the formation of an unusually wide tooth, the distinction between the two anomalies is always a challenge for practitioners.

To differentiate dental fusion from gemination, Mader’s “two teeth” rule can be used, which is a practical way to differentiate between fusion and gemination. If the fused teeth are counted as one and the number of teeth in the dental arch is smaller, then the anomaly is called “fusion”. However, when the atypical tooth is counted as one and the number of teeth in the dental arch is normal, then it is called “gemination” or it is a case of fusion between ordinary and supernumerary teeth. A diagnostic consideration would be that the supernumerary teeth are often slightly aberrant or cone-shaped, so the fusion between the anomaly of the supernumerary teeth will show differences in the shape of the two halves of the united crown. However, in gemination, the two halves of the united crown are mirror images and there is a vestibulo-lingual groove, which extends to the incisal edge [4]. Gemination may be associated with syndromes such as achondrodysplasia and chondroectodermal dysplasia or may be found in non-syndromic patients [12].

Primary double teeth account for about 75% of dental abnormalities in the primary dentition, fusion accounting for 94% of cases, and gemination  accounting for 6% of cases [2].

The classification Aguilo et al. is used to describe the type of union between bonded teeth [1], who classified fused teeth based on morphology and degree of fusion as follows (Figure 1):

1. Type I: bifid crown, single root;

2. Type II: large crown, large root;

3. Type III: two fused crowns, double conical root;

4. Type IV: two fused crowns, two glued roots.

Figure 1. Schematic representation of double teeth: a-Type 1, b- Type II, c- Type III, d- Type IV.

“Double teeth” are usually found in the anterior region, and the lower lateral incisors and canines are the teeth most commonly involved. The anomaly is usually unilateral, although the literature describes cases of bilateral fusion or gemination [11].

Although environmental factors such as trauma, vitamin deficiencies, systemic diseases and certain genetic predispositions have been suggested as possible causes, the exact cause of gemination is unknown [7].

Grover and Lorton argue that local metabolic interference, which occurs during tooth germ morpho-differentiation, may be the cause of “double teeth” [6].

The idea has been suggested that this condition has a family tendency [4].

Due to the irregular morphology, the teeth with this anomaly are unsightly. It also poses an increased risk of carious lesions, periodontal disease and space deficit. One of the main periodontal complications in gemination or fusion occurs due to the presence of grooves instead of joining the teeth involved. If these defects are very deep and extend subgingivally, the possibility of bacterial plaque accumulating in this area is quite high. A good oral hygiene is imperative for maintaining periodontal health. Composite sealants  for deep grooves and fissures reduce the risk of tooth decay in these teeth. In addition, gemination can affect the alignment of the teeth, the delayed eruption of other teeth and the deviation of the midline [12].

Primary dental abnormalities in primary dentition can have a direct impact on permanent dentition. Thus, dental abnormalities of successive permanent teeth were diagnosed in 69.4% of children with primary dentition affected. Permanent lateral incisor aplasia was most commonly observed in association with “temporary double teeth” type I (52.7%). Carious lesions were most commonly seen in “temporary double teeth” of type III (56.2%) [2].

Materials and methods

The purpose of this study was to analyze the specialized literature with reference to dental shape anomalies and the description of clinical cases of dental gemination and dental fusion. In the first case, a 5-year-old girl presented herself with her parents to the dental clinic for a routine dental check-up. Some accusations at the time of addressing did not present. Medical history is unremarkable. He is the 3rd child in the family out of a total of 3. The parents and her brothers have no history of dental abnormalities. He had no history of trauma to his teeth or jaws. The extraoral clinical examination is without pathology. During the intraoral clinical examination, the primary dentition was established, the number of teeth is 20. Carious cavities were detected, especially of the proximal surfaces of the upper incisors and molars. The right upper lateral incisor has an unusual, unsightly appearance – a wide crown, consisting of two halves, with a groove that extends over the entire vestibular and lingual surface, to the incisal edge, where it forms a notch. Radiological examination revealed a bifid upper lateral incisor crown and a distinct, broad root. At the level of the vestibulo-lingual groove, carious cavities are present on each half of the bifid crown, without involvement of the dental pulp. The distal incisal angle of the upper right central incisor is flattened at an angle of 15⁰ to the gingival margin (Figure 2, Figure 3).

Figure 2. Clinical appearance of the upper dental arch. The upper lateral incisor has an unusually wide tooth crown. The vestibular groove is observed along the entire length of the surface, as well as the carious cavities on both halves of the bifid crown.

Figure 3. Intraoral radiography demonstrates the presence of a bifid crown and a single root.

The following clinical case is the case of a 6-year-old girl, who was examined by us in the national study on the morbidity of dental diseases among children and adolescents in the Republic of Moldova. Clinically, the upper left primary central incisor has a bifid tooth crown with a characteristic incision that passes through the center of the crown, over the entire vestibular and lingual surface, from the incisal edge of the tooth to the gingival border (Figure 4). It is also noted that there is already a carious cavity in the region of the incision and there was even an attempt to restore this cavity (Figure 5).

Figure 4. Appearance of primary teeth, clinical case no. 2

Figure 5. Appearance of the palatal surface of the examined tooth

We also notice that characteristic notch of the incisal edge of the front group of primary upper teeth on the right (central incisor, lateral incisor, canine), more pronounced on the lateral incisor, which does inspire to believe that it was an attempt to twin of this group of teeth during the morphogenesis period, but which was limited only to the level of the dental crown, especially the incisal edge (Figure 4, Figure 5).

The literature describes several treatment modalities for such abnormalities, but the morphology of the double teeth varies so much that treatment options should be selected on a case-by-case basis. If selective grinding, surgical separation, followed by pulp therapy of the remaining tooth and orthodontic correction can be used for abnormalities in the category of “double teeth” in permanent dentition, for those in primary dentition all efforts will be focused on maintaining the functionality and vitality of the tooth involved to his physiological exchange of the successor tooth, dynamic monitoring of the successor teeth, maintenance of good oral hygiene and periodontal health. If the only treatment option is tooth extraction, steps will be taken to maintain the orthodontic space.

The following case is a case of unilateral fusion between a mandibular lateral incisor and a supernumerary tooth. A 9-year-old boy presented with his parents complaining of insignificant dental pain in the right side of the lower arch lasting several days. There are no other charges at the moment. Medical history is unremarkable. Clinically, multiple caries of primary and permanent teeth were detected. An unusual clinical phenomenon was observed in quadrant four: a lower lateral incisor fused with a supernumerary tooth. Clinically it presented as a large bifid tooth crown with a deep labiolingual groove. In quadrant 1, a supernumerary tooth located distal to the lateral incisor was observed. The supernumerary tooth is smaller in size of both the dental crown and the dental root (Figure 6, Figure 7). Orthopantomography was recommended, which confirmed the diagnosis of dental fusion of the right lower lateral incisor with a supernumerary tooth, type IV. The presence of a supernumerary tooth on the upper dental arch, but not fused, was also confirmed by radiological examination (Figure 8).

Figure 6. Clinical appearance of dental arches. Fusion of the right lower lateral incisor with a supernumerary tooth through a deep groove. Upper right supernumerary tooth located between the central incisor and the ordinary lateral incisor. Multiple caries.

Figure 7. Clinical appearance of the lower dental arch. The groove between the two fused teeth extends over the entire vestibular and lingual surface.

Figure 8. Orthopantomography of the patient confirming the diagnosis of dental fusion and supernumerary teeth.

Results and discussions

Although twinning and dental fusion are the most prevalent congenital dental anomalies, they are all rarely encountered in dental practice. “Double teeth” is the most common tooth-shaped abnormality, characterized by the formation of a tooth, which has an unusually wide crown. Gemination and dental fusion, which refers to dental abnormalities of shape, can be clinically similar, and differentiating between them is a real challenge for the doctor. Fusion is the attempt of two or more tooth buds to fuse, while gemination is the attempt of a single tooth bud to split and form two completely or partially separated crowns and a single root with a single root canal. In the present study we presented a clinical case of fusion of permanent teeth and two cases of twinning of primary teeth. Due to the complicated morphology of the dental crown, conditions are created for the development of dental caries in the groove that separates the two halves of the bifid crown and periodontal diseases, due to bacterial accumulation. Furthermore, due to the low prevalence, the importance of these abnormalities may be underestimated. In the absence of timely treatment, anomalies in the primary dentition can influence the development of dento-maxillary anomalies in the permanent dentition. Often, for successful treatment, an interdisciplinary approach is necessary.


The diagnosis and management of “double teeth” is always a challenge for clinicians. Although clinically very similar, dental fusion and gemination are dental abnormalities based on different processes, and treatment approaches are individual. Careful examination by clinical and radiographic methods ensures early diagnosis and intervention.


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