Impacted maxillary canine–linked severe lateral incisor root resorption (SIRRc) is rare, but it greatly influences the survival of the affected teeth. Our study was designed to investigate the risk factors for SIRRc.
Eighty-two patients with SIRRc and 81 patients with impacted maxillary canines but without SIRRc were included and evaluated by cone-beam computed tomography in software programs by 1 examiner (H.W.). Age, sex, positions, and dental follicles and angular inclinations of impacted canines were measured in this study. Binary logistic regression was used to analyze the risk factors for SIRRc.
SIRRc was highly related to sex, vertical and mesiodistal position, dental follicles sizes of canines, and intersection angles in 3 dimensions. The regression analysis showed female sex, dental follicles between 1 mm and 3 mm, mesial third and apical third position, vertical angle smaller than 30°, and the relative angle between 30° and 60° were significant risk factors for SIRRc.
Early diagnosis and treatment for SIRRc are imperative, especially in Asian patients that are female with apically and mesially positioned canines as well as wider dental follicles. Vertical angles and relative angles of impacted canines should also be noticed.
This study focused on the predictors for severe lateral incisor root resorption (SIRRc) by CBCT.
Asian females with mesially and apically impacted canines were at risk for SIRRc.
A vertical angle smaller than 30° might cause a higher occurrence of SIRRc.
The relative angle between 30° and 60° might be a risk factor for SIRRc.
Dental follicles between 1 mm and 3 mm might promote the incidence of SIRRc.
Canine impaction is a common problem in orthodontic clinical practice. The maxillary canine is the second most frequently impacted tooth in the permanent dentition, , the reported prevalence of which ranged from 0.92% to 6.81%. With advanced diagnostic techniques, an increasing number of patients with impacted canines are diagnosed early and referred for treatment, satisfying their increasing demands for oral health care.
Untreated maxillary canine impaction causes serious clinical complications, including malocclusion, shorter dental arches, worse facial profiles, and a higher risk of follicular cyst formation. , Among these, root resorption of the lateral incisors is 1 of the most extensive complications, which may threaten the long-term survival of lateral incisors or even lead to tooth loss. , Thus, early diagnosis and intervention of canine impaction are extremely important, especially in terms of preventing maxillary impacted canine–associated lateral incisor root resorption (SIRRc).
Traditionally, clinicians used mainly 2-dimensional radiograph techniques as routine methods to diagnose canine impaction in virtue of their relatively lower radiation doses and costs. However, these methods have several limitations, such as distortion, an overlap of structures, and enlarged images, which may produce an inaccurate diagnosis. , In recent years, cone-beam computed tomography (CBCT) has been introduced to the clinic and been widely adopted because of its excellent sensitivity and high reproducibility. As a result, image quality and diagnosis accuracy improved significantly. One study reported that 37% more of lateral incisors root resorption could be detected by CBCT than with 2-dimensional radiographs. Therefore, CBCT is a more effective method for the diagnosis and evaluation of lateral incisors root resorption.
According to the classification reported by Ericson and Kurol, the degree of root resorption was graded in 4 levels: no resorption, slight resorption (resorption less than half of the dentine thickness), mild resorption (half or more but pulp was unbroken), and severe resorption (reaching pulp). The life of the affected incisors is more likely to be shortened by severe resorption. However, only limited studies concerning severe root resorption have been reported until now, and the included sample size was comparatively small (ie, 11-69). , Chaushu et al reported some predisposing risk factors for severe incisor root resorption caused by maxillary canine impaction in white people. It should be noted that the angle analysis in Chaushu’s study was obtained from a panoramic radiograph, which neglected different angular inclinations in 3 dimensions. Furthermore, it is still unknown whether those predisposing factors are shared by an Asian population.
Our study aimed to identify the potential risk factors for maxillary impacted SIRRc in an Asian population using CBCT in a 3-dimensional view. Compared with other studies, , , , a larger sample size was included, and intersection angles in 3 dimensions were analyzed to evaluate the potential risk factors for SIRRc more comprehensively.
Material and methods
This retrospective longitudinal study was reviewed and approved by the Ethics Committee of State Key Laboratory of Oral Diseases, National Clinical Research Center for Oral Diseases, West China School and Hospital of Stomatology, Sichuan University, Chengdu, China and Hospital of Stomatology, Lanzhou University, Gansu, China. Files of subjects were retrospectively selected from the Departments of Orthodontics of 2 hospitals. CBCT was used to localize impacted maxillary canines and identified SIRRc. Eighty-two patients with SIRRc were included in the study group, and the control group comprised 81 patients with impacted maxillary canines but without SIRRc.
The sample size was determined on the basis of the regression analysis used in this study, in which a minimum of 10 subjects per variable was recommended. Sample inclusion criteria for the study group included patients with (1) permanent dentitions, (2) at least 1 severely resorbed incisor root caused by an adjacent impacted maxillary canine, and (3) no systemic diseases and previous orthodontic treatment histories. , , For the control group, patients with maxillary canines impactions but without lateral incisors root resorption were included as subjects. The exclusion criteria for both groups were (1) edentulous patients, and (2) root resorption secondary to periapical inflammation, trauma, dentigerous cyst, odontoma, or other pathologies. , Subjects from either group who congenitally missed lateral incisors were also excluded from the study. , , , All CBCT images were randomly coded with no information on patient identification and retrospectively analyzed by 1 examiner (H.W.).
All CBCT records were obtained by a CBCT machine (i-CAT KaVo 3D eXam; KaVoDental GmbH, Biberach, Germany) with the same settings: 5 mA, 120 kV, exposure time of 14 seconds, voxel size of 0.2 mm, and scanning area of 13 × 16 cm. Patients were scanned in an upright position and examined by 1 author (C.L.).
The reconstruction processes and evaluation methods of SIRRc have been reported in earlier studies. , , Briefly, all CBCT data (Digital Imaging and Communications in Medicine format) were imported into Dolphin Imaging 11.7 Premium software (Dolphin Imaging and Management Solutions, Chatsworth, Calif). SIRRc was identified after confirmation of 3 perpendicular planes (sagittal plane, coronal plane, and transversal plane) and volumetric views in the Dolphin Imaging program, according to Yan et al ( Fig 1 ). CBCT data were screened and analyzed using a 0.2-mm voxel size. The experimental design for sample screening and data analysis was presented in Figure 2 .
Angular inclinations of impacted canines were analyzed from 3 perspectives: transversal angle, vertical angle, and relative angle. Evaluation of transversal angle was performed by measuring the angle between the axis of impacted canine and mid-palatal suture on transversal plane , ( Fig 3 ). Vertical angle was measured as the angle between the axis of impacted canine and the occlusal plane , , ( Fig 4 ). Evaluation of relative angle was performed by measuring the angle between the axis of impacted canine and the axis of the lateral incisor, and the supplementary angle of relative angle (<90°) was taken into analysis ( Fig 5 ).
Mesiodistal positions of impacted canines were assessed by taking lateral incisors as references. On the basis of a previous study, the most mesial contours of both canines and lateral incisors crowns were taken. The mesial position was defined as the impacted canine located mesially at the mesial surface of the lateral incisor. The middle position was identified as an impacted canine located between the mesial surface and the long axis of the lateral incisor. The distal position was evaluated as an impacted canine positioned distally from the long axis of the lateral incisor ( Fig 6 ). The vertical position of the impacted canine was assessed referring to the apical third, middle third, and the cervical third of the lateral incisor root ( Fig 7 ). Buccolingual position of impacted canine was divided into 3 areas (buccal, palatal, and the line of the arch) ( Fig 8 ). Dental follicle size of the impacted canine was measured at the widest area from the crown to the periphery of the follicle ( Fig 9 ). ,