The ectopic canines have started tooth formation seemingly in a tilted position either within the canine axis (Group 1), posteriorly to the canine axis (Group 2) or anterior to the canine axis (Group 3). The last case shown in Fig. 5 is an example of this uncertainty. The mandibular canine in this case seems to have moved a long distance during eruption and could as well have been originally positioned posteriorly as anteriorly to the canine axis.

Ectopic eruption of the permanent mandibular canine appears as a mystery, which over the years has been discussed regularly (Mapparapu 2002; Delessandri et al. 2017; Batwa and Alzain 2018). The main problem has been a lack of longitudinal studies, answering the questions why is the canine ectopic? Can it be prevented, predicted and treated?

The ectopic mandibular canine has often been studied in cross-sectional studies and the focus has often been: where did the permanent canine come from? Is the primary mandibular canine present, resorbed or not in these cases? Few cases are dealing with surgical treatment (Cavoti et al. 2016). A recent radiographic study on mandibular canine eruption demonstrated that the canine crown morphology appeared different on different orthopantomograms, with canines located in different positions (Svanholt et al. 2020). The canine morphology could either be designated “facial morphology” as the morphology observed in the facial view or “lateral morphology” as observed in the lateral view. Facial morphology of the crown seemed to appear in cases where eruption was close to normal, while lateral morphology appeared in more complicated, often horizontally located, canine cases (Svanholt et al. 2020). This former observation seems to be confirmed in the present study, where facial morphology is registered in Figs. 2 and 3 (upper and middle cases), while lateral view is predominantly seen in Figs. 4 and 5 (lower case). Insecurity and errors might be noticed in orthopantomographic studies.

Another aspect in studies of eruptive movements is the simultaneous jaw growth. By comparing radiographs of the same mandibular canine under eruption, it may appear that the canine moves downwards during the eruption, but this could be an optical illusion, caused by alveolar bone growth.

The mandibular growth has been studied in prenatal specimens and in children. It was demonstrated in prenatal life, that the mandibular bone formation started in the canine region (Kjær 1975, 1989, 1990). In these studies, also formation of the primary canine appeared. In the study of Kjær and Bagheri (1999), specific focus was on the alveolar bone, surrounding the mandibular canine. In this study, it was demonstrated that the alveolus of the canine was not ossified in the facial aspect, which was not the case in the neighbouring incisors. This gave the impression that the canine was a stabile tooth, not moving mesially or distally during development. Also, the mental foramen seems to have a stable position, only dependent on changes as a result of facial bone apposition (Kjær 1989). These prenatal studies may indicate canine and bone stability in the actual region, but it is difficult to prove this and impossible to follow up in longitudinal studies.

Former studies on ectopic mandibular canines have also focused on the resorption aspect of the roots of the primary canines. It has been demonstrated that resorption occurs in the primary canines without pressure from erupting permanent canines. This is also proved in the present study. Cases where the primary canines were extracted or totally resorbed and exfoliated were not included in this study.

The present study raises the question whether the crown follicle has an inborn “mechanism” steering the eruption direction. It is supposed that the tilting of the early tooth bud could result in abnormal function of the crown follicle. This is a hypothesis, which cannot be proved in the present study.

The usability of an orthopantomogram in the visual analysis of bone morphology and root morphology is also dubious because the orientation of the patient is a main factor for obtaining valuable and comparable results. Another aspect of importance is that the exact time for onset of eruptive movements of a tooth is unknown. Is it when the root has gained half length or possibly before? In this connection it is also problematic that the root is estimated as “half-sized”, when the final root length is not known. Furthermore, it is questioned if the teeth always erupt at the same stage of root maturity in different individuals.

Developmental stages of permanent canines have been studied by Svanholt and Kjær (2008), and eruption times of permanent canines have been investigated by Parner et al. (2002). The agreement in maturation and eruption times in these two studies is convincing for at study like the present.

The present cross-sectional study, with all the questions raised, gives a new insight in different locations from where the ectopic mandibular canines have started tooth formation. Instead of many cross-sectional studies from prenatal and postnatal periods, it would be extremely valuable to obtain results from postnatal longitudinal studies of ectopically erupting mandibular canines.