[The transmigration of the permanent lower canine]

A rare dental anomaly--the transmigration--is analysed. This condition, which is sparsely recorded in the literature, typically affects the mandibular canine. Twelve cases in which an unerupted mandibular canine has crossed the midline, are described and discussed. Reference is made to considerations put forward in the past by other Authors on this subject, after which personal conclusions are drawn on the diagnosis, etiology and therapy of this rare but serious behaviour of the mandibular canine. The anomaly can be properly diagnosed only by an attentive radiographic evaluation which is primarily based on the orthopantomogram. A series of successive panoramic radiographs taken over years documents the progressive ectopic movement of an unerupted mandibular canine from its developmental site, through the symphysis of the mandible. Lateral, postero-anterior cephalometric films and periapical radiographs, provide additional important information. After having analysed the most frequent etiological factors of mandibular canine impaction, the authors present their considerations concerning the probable pathogenetic mechanism of transmigration. In addition to the proinclination of the lower incisors and the increasing axial inclination of the unerupted canine, already pointed out by other authors, an enlargement of the symphyseal cross-sectional area has been observed and documented. In conclusion it is important, also from a therapeutical point of view, to keep under control, by periodical panoramic radiographs, all those clinical situations considered at risk due to their possible evolution in transmigration and which are characterized by: 1) an unerupted canine which can be palpated in a mesial position in the labial sulcus; 2) an enlarged symphyseal cross-sectional area; 3) an obstacle to normal eruption (cysts, odontomas, ecc.); 4) a proinclination of the lower incisors. A progressive worsening of the position of the unerupted canine should induce to consider the possibility of an early interception of this anomalous condition, which, once established, results in the definitive impaction of the tooth.