Introduction
The objective of this study was to provide new clues for the prevention and early management of root dilacerations in impacted maxillary central incisors.
Methods
Cone-beam computed tomography images of 108 patients with unilateral impacted maxillary central incisors were obtained and reconstructed into 3-dimensional models. Crown direction, crown height, root length, bone thickness, and position and angle of root dilaceration were measured in the sagittal-view sections. K-value, defined as the ratio between the available length of the direct root and the ideal length of the direct root, was proposed, and the relationships between K-values with root dilacerations were studied. Root development of the contralateral erupted maxillary incisor was also assessed. Independent t test, chi-square test, and 1-way analysis of variance were used for data analysis.
Results
Root dilacerations occurred when the K-values were 0.16 to 0.19 (palatal impaction), 0.25 to 0.53 (labial impaction), and 0.69 to 0.75 (nasal impaction). The position and angle of root dilacerations were different among nasal, labial, and palatal impactions ( P <0.01). K-values and positions of root dilacerations among nasally, labially, and palatally impacted incisors were in descending order, respectively. Retarded root formation was noted in the impacted incisors compared with the contralateral incisors ( P <0.001).
Conclusions
Nasal, labial, and palatal impacted incisors had different patterns of root dilacerations. Analyses of crown direction and K-value may aid in evaluating root dilacerations at early dental ages and facilitating early intervention of impacted incisors.
Highlights
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The study included 108 patients with unilateral impacted maxillary central incisors.
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Patients were divided into nasal, labial, and palatal groups according to the direction of the crown.
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Nasally, labially, and palatally impacted incisors had different patterns of root dilaceration.
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Analysis of crown direction and K-value may aid in evaluating root dilacerations at early dental age and facilitate early intervention of impacted incisors.
Impacted maxillary central incisors pose a challenge for dental treatment due to their influence on facial esthetics and phonetics. Among the various treatment alternatives, which include incision sprouts, orthodontic extrusion, denture replacement, autologous transplantation, extraction, and biologic induction, appropriate orthodontic traction is the optimal choice for the maintenance of bone volume to achieve functional and esthetic outcomes.
Orthodontic treatment entails guiding the impacted tooth into proper alignment in addition to establishing the normal contour of the tooth. The key to a successful treatment outcome depends on the method and timing of traction. Therefore, it is important to understand the eruption mechanisms and that root dilacerations of impacted maxillary incisors occur in different locations and at different development stages so that appropriate treatment regimens can be designed.
Conventional theories associate tooth eruption with root extension, periodontal ligament, and dental follicle, whereas recent studies have challenged these theories in several ways. It has been advocated that the tooth is pushed upward toward the oral cavity because of growth of the lamina dura rather than growth of its own root, confirming the independence between tooth eruption and root elongation. Artificial teeth with intact dental follicles and metal or silicone “tooth germs” were found to erupt on schedule. Tooth eruption is also influenced by cortical plate, mucosa, retained deciduous incisors, odontomas, and supernumerary teeth. The periodontal ligament was confirmed to provide an eruptive impetus by the shrinking and cross-linking of collagen fibers and the contraction of periodontal ligament fibroblasts.
Previous studies related to root dilacerations of impacted teeth were based on either 2-dimensional images or certain views of cone-beam computed tomography (CBCT) images, which cannot accurately show the 3-dimensional (3D) root morphology due to labiolingual or mesiodistal inclination and rotation of impacted teeth. To evaluate tooth impactions and root dilacerations accurately, it is necessary to interpret CBCT images along the long axis of the impacted incisor on the sagittal plane in a 3D model.
Although there are many case reports of impacted dilacerated incisors, a quantitative study on impacted incisors and root dilacerations is lacking. We conducted a retrospective analysis of a large sample of CBCT images and proposed some parameters to evaluate root dilacerations of impacted central incisors in reconstructed 3D models, and also compared different groups based on the direction of the crown. We aimed to provide new clues for early management of impacted maxillary central incisors based on the direction of the crown and the extent of the dilacerations to prevent severe root dilacerations from affecting esthetics and oral functions and to decrease the loss of maxillary central incisors.
Material and methods
This retrospective study was approved by the institutional review board (ERC-2014-1) of the Hospital of Stomatology, Sun Yat-sen University, Guangzhou, China. Informed consent was obtained from the guardians of all children. The participants were orthodontic patients who had been diagnosed with unilateral impaction of a maxillary central incisor. The patients underwent preoperative CBCT examinations to evaluate the location and developmental stage of the impacted incisors. Patients with teeth that had failed to erupt into the dental arch within the expected developmental period were included in the study, whereas those with combined impactions of central incisors and canines, retained deciduous incisors, odontomas, and supernumerary teeth were excluded.
This study included 108 subjects (60 girls, 48 boys) who were referred to our hospital between January 2010 and February 2015. They were between 8 and 16 years of age (mean, 11.8 ± 2.6 years) with unilateral impacted maxillary central incisors.
All CBCT images were obtained using a DCT Pro CBCT device (VATECH, Hwaeong, Gyeonggi-do, Korea) operating at 90 kV(p), 6 mA, 0.4 mm voxel size, exposure time of 24 seconds, and field of view of 16 × 10 cm. The data were exported into DICOM format and then reconstructed into a 3D model after being imported into an interactive image analysis system (Mimics, version 14.0; Materialise, Leuven, Belgium). A pilot experiment was conducted with 15 patients to explore the initial threshold value for the maxilla and the tooth structure segmentation. The thresholding ranges were set at 226-3071 and 1200-3071 to show the 3D structures of the anterior maxilla and maxillary anterior teeth. With profile lines measured in Hounsfield units, region growing was used to separate the impacted incisors from the peripheral bony structures, and 3D models of the incisors were reconstructed ( Fig 1 ).
The longitudinal axis of the crown of the impacted central incisor was defined by connecting a line between the midpoint of the incisor edge and the center of the cervix in the axial view, using the Mimics software. Subsequently, a sagittal view along the long axis was created. We evaluated and recorded different aspects of eruption at the workstation from the sagittal plane of every subject.
Coronal hindrance of eruption varied with the direction of eruption of the impacted incisor. The bony walls facing the involved tooth during eruption were recorded as labial cortical plate, nasal floor, or palatal plate in every subject ( Fig 2 ).
K-value was defined as the ratio between the available length of the direct root (aLR) and the ideal length of the direct root (iLR) in the long axis of the crown. K = aLR/iLR. aLR can be calculated with the following formulas ( Fig 2 ).
aLR = BT − HC ( total impaction ) aLR = BT − ( HC − L ) ( submucosal impaction )
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