It has been mentioned that the aetiology behind horizontally positioned permanent mandibular canines might be a malposition of the initial crown anlage before root formation early in childhood (Joshi 2001). However, histological or radiologically studies proving this hypothesis have never been carried out. The problem is that the ectopic horizontally positioned permanent canine is usually diagnosed at puberty, but the position of the initial crown anlage can only be diagnosed before age 5—an age where there is no clinical indication of canine ectopia or justification for its radiographic investigation. This explains the severe lack of longitudinal series of radiographs demonstrating development of permanent mandibular canine ectopia.

The purpose of the present study is to focus on where the crowns of the developing mandibular canines are located in early childhood, in relation to the preceding primary canines. Furthermore, the goal is to suggest what might be considered a normal site for location of the permanent mandibular canine and what might be considered a pathological site for location.

A similar study demonstrating initial positions of the permanent crown anlage of the mandibular canine, which is necessary for understanding the aetiology behind canine ectopia, seems to be lacking in the literature.

The primary mandibular canine bordering the incisors and premolars has a stable position in the dental arch (Kjær 2017). The bone in the canine area is the first bony tissue formed in the prenatal mandible (Kjær 1975, 1989, 2017). The prenatal mandibular canine develops in a separate alveolus in the growing alveolar process (Kjær and Bagheri 1999). Space for the developing mandibular incisors, located in a shared alveolus, is gained from early growth in the symphysis menti region and growth of the alveolar process (Kjær 1975, 2017). Space for the mandibular primary molars and premolars is obtained from the posterior elongation of the mandible during growth combined with growth in the alveolar process (Bjørk and Skieller 1983). The stable primary canine position means that the primary canine and its successor, the permanent mandibular canine, do not migrate during development in the mandible, neither in the mesial direction nor in the distal direction. The vertical alveolar bone growth in the mandible creates space for the canines.

The formation of the permanent canine starts from a tooth bud growing out from the primary dental lamina (Kjær1980), demonstrated in Fig. 1. This permanent tooth bud marked by a red circle, is initially located laterally to the primary tooth anlage.

Histological section demonstrating an early tooth bud from a primary tooth and the extended dental lamina forming its permanent successor, marked by a red circle. The red dot underneath the primary tooth bud indicates the normal final position of the permanent tooth

From this lateral position, the permanent tooth bud normally finds its final position underneath the primary tooth anlage. This seems to happen during onset of eruptive movements of the primary tooth (Kjær et al 2008). Accordingly, the primary and permanent canines will share the same vertical axis during normal development. If this normal change in position of the permanent tooth bud does not occur, then the crown of the permanent canine will develop in an ectopic position and, as a consequence, develop and erupt ectopically (Kjær 2021).

In a recent study, abnormal premolar eruption was classified, based on the insight from the same normal histological development illustrated in Fig. 1 (Kjær 2021). This study demonstrated that ectopic premolars could be located both posteriorly and anteriorly to the primary molars. Also abnormal upside-down position of premolars was recorded, with the root of the premolar between the primary molar roots and the crown of the premolar in the direction opposite to the primary crown (upside-down) (Kjær 2021). This study also demonstrated that normal shedding and ankyloses of the primary molars influenced the ectopic positions.

The background for comparing permanent canine position with premolar position is that the mandibular premolars and the mandibular canine belong to the same developmental field with the same innervation (Kjær et al 1994; Kjær 1995, 2014, 2017). It has been described that the postnatal stages of tooth development are closely interrelated within the canine/premolar field (Svanholt and Kjær 2008). Furthermore, the eruption times of the mandibular permanent canine and first and second premolars within the developmental field are closely identical and different from eruption times in other fields (Parner et al. 2002). These circumstances make it obvious to compare the initial positions of the mandibular premolars with the initial positions of the mandibular permanent canines.