Retention of permanent teeth is a common problem in everyday dental practice. Retained teeth are completely formed teeth with a fully developed root, yet they fail to erupt in the period relevant for a given group of teeth and remain either in the maxilla or the mandible. In the upper jaw the retention of permanent teeth involves the central incisors and canine teeth, while in the lower jaw this happens with third molars or second premolars. According to different reports, the retention of front teeth is observed in 0.9-4.3% of population (with 70% of retained teeth being the canine teeth, and 11.5% – the incisors). In case of deciduous teeth, retention is observed on very rare occasions (1). The subject literature does not provide any numerical data with this respect.

Teeth may be retained for a number of reasons, one of them being dental developmental abnormalities related to an increased number of teeth and their abnormal shape. Double teeth are exemplary of such anomalies. The subject literature differentiates three forms of this defect, namely concrescent, fused and geminated teeth (2). Double teeth are a far more frequent occurrence in deciduous teeth. This abnormality occurs in 0.15-1.2% of population, whereas for the permanent teeth – 0.1%. The lateral form of this defect affects 0.02% of population (3, 4).

Double teeth are caused by disorders in the odontogenesis process within primary dental lamina. The contributing local etiological factors include the aggregation of adjacent tooth buds and traumas (4, 5). The general factors include avitaminoses, hypervitaminosis A, systemic and infectious diseases, endocrine disorders and syndromes such as achondrodysplasia, chondroectodermal dysplasia, Cornelia de Lange syndrome and Gorlin-Goltz syndrome (4, 6-8).

The most common form of double teeth is concrescence. For deciduous teeth this occurs in 1% of population, whereas for permanent teeth – in 0.1% of population (3). Concrescent teeth (lat. dentes concreti) develop when two tooth buds join together at a late stage of odontogenesis. The union of both teeth occurs in the cementum overlying the roots during the development of roots, but the crowns develop separately. Concrescent teeth may be joined by roots either fully or partially. Two separate pulp cavities and root canals are seen on x-ray photos. Concrescence may occur between two normal, adjacent teeth or between a normal tooth and the supernumerary one (7). Concrescent teeth are considered typical symptoms of some syndromes, namely disostosis (3). If two teeth, despite their initial separate development, eventually join together by buds at an early stage of odontogenesis, namely prior to the calcification of dental crowns, this result is fused teeth (lat. dentes confusi). As a result, the teeth are joined by the enamel and the dentin of two or more adjacent tooth buds. This applies mostly to deciduous teeth (9). The fusion may occur between two normal teeth, a supernumerary tooth and a normal one as well as between a supplemental tooth and a normal one. Their crowns are vast, they feature a vertical furrow on their vestibular surface that ends with a cavity on the incisal edge. A single extended pulp cavity, sometimes with the dentin bulged towards the lumen of the cavity where buds are fused as well as a single and vast root canal may be seen on an x-ray photo. The third type of double teeth are geminated teeth (lat. dentes geminati). They develop as a result of division of a single tooth bud or a union between a normal bud and a supernumerary one. They feature two crowns and a single root, in the dental arch they exist as a single tooth with a vast crown divided by a vertical furrow on the labial surface. It is a malformation consisting in the split of a tooth bud at an early stage of odontogenesis during the development of the crown. A common dental crown may be seen on the x-ray photo. The presence of geminated teeth in deciduous teeth suggests a greater possibility of appearance of this anomaly also in the permanent teeth. Double teeth create aesthetic and functional problems and require a thorough diagnosis and specialized treatment. It is difficult to tell geminated teeth from clinically fused teeth, particularly when the fusion occurs between a normal and a supernumerary tooth (10, 11).