3-Dimensional Changes in Position and Inclination of Ectopically Erupting Maxillary Canines in Response to Slow Maxillary Expansion: A Retrospective Study

Abstract

At an incidence of 0.8-2.8%, the impacted maxillary canine constitutes one of the more challenging orthodontic problems to manage, often involving interdisciplinary coordination of surgical exposure of the impacted tooth followed by orthodontic traction to align the canine within the dental arch. In addition to the increased orthodontic treatment time, cost, and complexity, other sequelae of impacted canines include root resorption of adjacent teeth, cyst formation, and malocclusion. Interceptive treatment with maxillary expansion has been shown to reduce the risk of ectopically erupting canines (EECs) developing into impacted canines. Although effects of maxillary expansion on ectopic canine eruption have been studied, most have focused on rapid, rather than slow, maxillary expansion protocols and either clinical outcomes or canine localization on 2-dimensional imaging. Therefore, little is known about positional effects in the buccopalatal direction. The novel approach of investigating changes in eruption path in response to slow maxillary expansion based on cone beam computed tomography (CBCT) measurements was used in this study to facilitate precise localization of erupting maxillary canines in all 3 dimensions. In this retrospective study, CBCT records of 24 EECs treated with quad helix slow maxillary expansion (EEC 2x4 + QH group) were compared against those of 15 EECs treated with 2x4 fixed appliance (EEC 2x4 group, control treatment), 39 normally erupting canines (NECs) treated with quad helix appliance (NEC 2x4 + QH group, control diagnosis), and 24 NECs treated with 2x4 appliance (NEC 2x4 group). Linear and angular measurements pertaining to canine position and angulation were made in all 3 dimensions to determine differences at pre-treatment and post-treatment timepoints, as well as change across timepoints. Results showed that 67% of EECs corrected to a normal position, as defined by sector location, in both 2x4 appliance and quad helix appliance treatment groups. Significantly greater uprighting of the erupting canine angulation from a coronal view was found in EECs treated with 2x4 appliance, compared to EECs undergoing the same treatment. On the other hand, the quad helix appliance was found to significantly normalize medial displacement of EECs at cusp tip and apical levels, cause greater uprighting of the EEC inclination from a sagittal view compared to NEC controls, and maintain a normal vertical eruption rate in EECs. Therefore, the improvement in canine eruption path effected by the 2x4 appliance and quad helix appliance are brought about by mechanisms in differing planes of space.