An unusual case of gemination in both primary and permanent maxillary right lateral incisors is presented. Potential difficulty in classifying the anomaly and ways of differentiating gemination from fusion as well as management of the anomaly are discussed.

 
Gemination is currently recognized as an attempt by a single tooth germ to divide, resulting in an incomplete formation of two teeth. The structure is usually one with two completely or incompletely separated crowns that have a single root and root canal. The term "twinning" or double tooth has sometimes been used to designate the production of identical structures by division resulting in one normal and one supernumerary tooth.1-3 The reported prevalence of the anomaly is .08%.4's Although the occurrence of these conditions are relatively rare, the anomaly, if present, is more often found in the mandible than in the maxilla and frequently seen in primary than in permanent dentition.6-8 Difficulty in differentiating fusion with gemination led other authors to prefer the term "twinning" or "double tooth" to describe both conditions.2,3,7,9
Relationship between dental twinning defects and corresponding anomalies in permanent dentition have been reported.3,6-10 Presence of the anomaly in the primary dentition could be followed by a congenitally missing tooth, peg incisors or the same anomaly will occur in the permanent dentition. In two studies, gemination defects led to hyperdontia in permanent dentition while hypodontia followed 70% of fused primary teeth.7,8 These studies point out the importance of distinguishing a case of fusion from gemination to be able to predict its effect on the permanent dentition.
This report describes a case in which a geminated primary maxillary right lateral incisor was replaced by another geminated permanent lateral incisor.

Case Report

A 14-year-old Saudi male was referred to the King Saud University College of Dentistry's preventive clinic. Medical history was non-remarkable. A series of intra and extraoral radiographs taken in 1987, 1988 and 1992 were available. Intraoral examination revealed poor oral hygiene, with full complement of permanent teeth except for third molars. All findings were unremarkable with the exception of a dental twinning defect on the per­manent maxillary right lateral incisor.

The geminated incisor was vital, non-carious and almost of the same shade as the adjacent and contralateral teeth. The mesiodistal dimension was almost twice that of the contralateral tooth and ang­led at about 120°. The anomalous tooth looked like two lateral incisors separated by a groove running in the middle cervicoincisally. Periapical radiograph of the tooth showed a single completely developed root. When compared with the radiograph of the contralateral tooth, the involved tooth has a wider and larger root canal. A review of previous radiographs (taken in the year 1987) showed the same condition, a geminated primary right lateral incisor. The anomaly was noticed by the referring orthodontist and was observed for possible space problem.

 
Confusion arose in classifying the anomalous tooth as to whether it is a case of gemination or fusion. In gemination, the two halves of the joined crown are usually mirror images and the total number of teeth in the affected dental arch is normal. Radiographically, there is usually a common root and root canal. In fusion, there is one less tooth than the normal countand, radiographically, there are two separate canals. Difficulty arises in cases where a fusion occurs between a normal tooth and a supernumerary tooth. It is important to note however that supernumerary incisors are usually cone shaped and aberrant such that a case of fusion between a normal and a supernumerary tooth will show difference between the two fused teeth. In the present case, the clinical and radiographic findings fit the description of gemination more than that of fusion.
The problems usually associated with geminated teeth include insufficient space in the dental arch which results in crowding, malocclusion, delayed or ectopic eruption of teeth and/or poor esthetics. Caries could also result due to plaque accumula­tion along the line of demarcation. Endodontic management problems is also possible because of varied root canal morphology.
While most cases of gemination and fusion have resulted into a space problem due to a great mesiodistal dimension of the involved tooth, this case is of special interest because it did not create a space deficiency for the patient, which could be attributed to the angulation of the two halves of the geminated crown. Only a slight midline shift to the left had occurred.
The usual approach to treatment varies from full crown coverage to extraction of the anomalous tooth then replace it. In the case presented, only a slight reduction of labial enamel was made and composite resin was applied after acid etching the groove. The tooth involved is aesthetically accept- able and the only problem seen is its proneness to dental caries due to the patient's oral hygiene and the presence of a deep groove. Oral hygiene instructions and regular recall appointments were also emphasized to the patient.