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Root dilaceration in maxillary impacted canines and adjacent teeth: A retrospective analysis of the difference between buccal and palatal impaction
ORIGINAL ARTICLE
Root dilaceration in maxillary impacted canines and adjacent teeth: A retrospective analysis of the difference between buccal and palatal impaction Dan Cao,a Bingting Shao,a Iman Izadikhah,a Lizhe Xie,b Bin Wu,c Hu Li,a and Bin Yana Nanjing, China
Introduction: This research aimed to analyze the prevalence of root dilaceration in buccally impacted canines (BICs) and palatally impacted canines (PICs) with their adjacent teeth based on a retrospective cone-beam computed tomography (CBCT) investigation. Methods: Pretreatment CBCT images of 145 subjects with unilateral maxillary canine impaction and 145 age- and sex-matched subjects without impaction were used. Prevalence of dilaceration (subclassified to root curvature and apical hook based on severity) in canines and adjacent teeth was determined in CBCT records. The root length of maxillary impacted canines was measured for further morphologic evaluations. Results: Impacted canines had a significantly higher prevalence of root dilaceration than the control group and compared with the erupted contralateral canines in the experimental group (P \ 0.001 for both). A significantly higher prevalence of root dilaceration was found in adjacent lateral incisors of the PICs subgroup than that of the control group (P \ 0.001). Adjacent premolars had a higher prevalence of dilacerated roots in the PICs subgroup (P \ 0.001) than the control group, but not for the BICs subgroup. Significantly higher prevalence of curvature (P \ 0.001 for both) and hook (P 5 0.008 and P \ 0.001, respectively) were found in BICs and PICs roots compared with the control group. Both types of impacted canines had significantly shorter roots than the control group (P \ 0.001 for both). Conclusions: BICs and PICs have a higher tendency to present root dilaceration and shorter roots. Unlike BICs, adjacent teeth to PICs were more frequently observed to have root dilaceration. (Am J Orthod Dentofacial Orthop 2020;-:---)
A a
bnormal angulation or curvature formed in the root or crown of a tooth is defined as dilaceration.1,2 Dilaceration is of great importance to
Jiangsu Key Laboratory of Oral Diseases, and Department of Orthodontics, Affiliated Hospital of Stomatology, Nanjing Medical University, Nanjing, China. b Jiangsu Key Laboratory of Oral Diseases, Engineering Center for Digital Medical Technology of Stomatology, Nanjing Medical University, Nanjing, China. c College of Mechanical and Electronic Engineering, Nanjing Forestry University, Nanjing, China. Dan Cao and Bingting Shao are joint first authors and contributed equally to this work. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. This study was supported by the National Natural Science Foundation of China (82071143), Key R and D program of Jiangsu province (BE2018723), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, 2018-87). Address correspondence to: Bin Yan, Jiangsu Key Laboratory of Oral Diseases, and Department of Orthodontics, Affiliated Hospital of Stomatology, Nanjing Medical University, 136 Hanzhong St, Gulou District, Nanjing 210029, China; e-mail, [email protected]. Submitted, July 2019; revised, November 2019; accepted, December 2019. 0889-5406/$36.00 Ó 2020 by the American Association of Orthodontists. All rights reserved. https://doi.org/10.1016/j.ajodo.2019.12.019
orthodontics because dilacerated roots are harder to move orthodontically, have a higher risk of impaction3 or external resorption,4 and impede favorable insertion of miniscrews. Orthodontic management of dilacerated roots is complex and lengthy and requires indefinite maintenance.5 The prevalences of root dilaceration have been generally investigated in several ethnic groups, which ranges from 0.3% to 17% without gender predilection.3,6 To date, several potential etiologic factors for root dilaceration have been proposed. When dilaceration occurs in permanent incisors, it is often supposed as a result of trauma to the primary predecessors whose apices lie close to the permanent tooth germ. An intrusion injury places the apex of the primary tooth in close approximation to the tooth bud of permanent successors, increasing the likelihood of damage to the tooth bud and the Hertwig epithelial root sheath.7 However, it has been found that the prevalence of dilacerated permanent successor teeth is generally disproportionate to 1
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the incidence of injuries in the corresponding deciduous teeth.8 This implicates that trauma may not be the major etiologic factor of dilaceration because most dilacerated teeth are found in the posterior area, which is less likely to have a history of trauma.3 Other probable etiologic factors include insufficient space for development, the effect of anatomic structures (such as the cortical bone of the maxillary sinus or the nasal fossa), supernumerary tooth, mechanical interference with an eruption in cases such as an ankylosed or retentive primary tooth, and hereditary factors.9,10 Previously, a relationship between maxillary canine impaction and root dilaceration has been proposed. Hettiarachchi et al11 reported that palatally impacted maxillary canines have a greater tendency to develop apical hooks. He also concluded that PICs have shorter root length than nonimpacted maxillary canines. However, this study covered only PIC in Caucasians. It is wellknown that BIC and PIC are different from various standpoints such as ethnical prevalence and etiologic factors.12,13 Briefly, buccally impacted canines (BICs) are mostly linked to dental arch and skeletal size deficiencies, whereas PICs are theorized to be associated with genetic origins or lack of guidance from the adjacent lateral incisors.13 Moreover, previous studies concentrating on root dilaceration of impacted canines in Asian populations are scarce. Hence, the association of root dilaceration and impacted canines and whether the prevalence of root dilaceration differs between the 2 impaction types in Asians have yet to be determined. Numerous studies on patients with normally erupted maxillary canine reported that maxillary premolars and lateral incisors are relatively more frequently dilacerated.2,14,15 In addition, few case reports on the concurrent incidence of maxillary canine impaction and adjacent premolar root dilaceration suggested a relationship between them.16-18 Therefore, to clarify this issue, a large sample study is necessary to investigate
the prevalence pattern of root dilaceration in adjacent teeth of maxillary impacted canines. The purpose of this study was to investigate the association between root dilaceration and BICs or PICs as well as their adjacent teeth in population with the cone-beam computed tomography (CBCT) technique. The null hypothesis was that either the maxillary impacted canines or their adjacent teeth would not present a difference in their dilaceration prevalence. MATERIAL AND METHODS
Pretreatment CBCT images of 145 Chinese subjects with unilateral maxillary canine impaction (experimental group) and 145 age- and sex-matched subjects without impaction (control group) were obtained from the Affiliated Hospital of Stomatology, Nanjing Medical University (Nanjing, China) (Table I). The inclusion criteria for the experimental group were clinically and radiographically diagnosed patients of unilateral maxillary canine impaction. The experimental group was subclassified as BICs and PICs. The exclusion criteria were patients having canines with open apices, missing or root resorption of adjacent lateral incisors and first premolars, history of systemic disorders, and undergoing orthodontic treatment. All subjects in the control group were patients with normally erupted canines who met the same exclusion criteria and prescribed CBCT for another diagnostic reason. Before the imaging process, all patients signed the consent form that their radiological results (demanding no personal identification disclosure) could be used for future research on maxillary canine impaction. Ethics approval was granted by the Ethical Committee Department of Affiliated Hospital of Stomatology, Nanjing Medical University (PJ2018031-001). The present study was carried out after approval from the International Clinical Trial Registry (Protocol no. ChiCTR1800020036).
Table I. Characteristics of the sample Experimental group Characteristics Sex Female Male Age, y Angle classifications Class I Class II Class III
Buccal (n 5 74)
Palatal (n 5 71)
Control group, n 5 145
P value
49 (66.2) 25 (33.8) 17.08 6 4.88
39 (54.9) 32 (45.1) 18.62 6 5.18
88 (60.7) 57 (39.3) 17.73 6 5.04
0.394*
33 (44.6) 28 (37.8) 13 (17.6)
30 (42.3) 23 (32.4) 18 (25.3)
70 (48.3) 48 (33.1) 27 (18.6)
0.705*
0.183y
Note. Values are n (%) or mean 6 standard deviation. *Comparison with chi-square tests among the 3 groups; yComparison with 1-way analysis of variance tests among the 3 groups.
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All CBCT data were acquired from the patients with a standard acquisition protocol using the same CBCT machine (NewTom VG; QR, Verona, Italy) and the same parameters: 16-cm diameter field of view, 110 kV, 1-20 mA (pulsed mode), and 0.3-mm voxel size. After removing patient identification information from the CBCT data (digital imaging and communications in medicine files), they were entitled to randomly generated codes and subsequently analyzed by a rater on the basis of the following standardized protocol. Digital imaging and communications in medicine files were first imported into SimPlant O&O (version 13.0; Materialise NV, Leuven, Belgium) and segmented to display the teeth and skull for 3-dimensional surface rendering at the threshold value of teeth (1250-3070) and skull (2503070) in the software. The images were reoriented according to a protocol described by Molen.19 Afterwards, qualitative and quantitative variables (Table II) were assessed and confirmed in the volumetric and 3 orthogonal (sagittal, coronal, and axial) 2-dimensional views. In both study groups, maxillary impacted canines and their adjacent teeth were classified as no dilaceration, with a root curvature, and with an apical hook according to the angular degree of root dilaceration (Fig).11 The landmarks for measuring quantitative variables (Table II) were identified in the volumetric view using the digitizing landmarks tool of the software. After verifying the sagittal, coronal, and axial views, these landmarks were used to measure quantitative image variables using a digital linear measurement tool (to 0.01 mm precision) in the software. Before the measurement process, each to-be-evaluated tooth in the control group was selected according to its own maxillary left or right coinciding canine or adjacent tooth present in the matched subject from the experimental group. To assess intraexaminer reliability, we repeated measurements of 100 randomly selected subjects from the whole sample by the same rater (B.S.) after a 1-month
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interval. With assumptions of 30% and 10% prevalence rates of curvature in our impacted and control samples, respectively, a minimum sample size of 56 was required to achieve 80% power (calculated in G*Power [version 3; Heinrich Heine University, D€ usseldorf, Germany]) to detect differences of root curvature prevalence and even less than that for the hook. Our samples with buccal (n 5 74), palatal (n 5 71), and combined (n 5 145) impactions had power values of 96.5%, 96.0%, and 99.5%, respectively. Statistical analysis
Cohen weighted kappa (for qualitative variables) and intraclass correlation tests (for quantitative variables) were used to assess intrarater reliability. The prevalence of dilaceration in canines and adjacent teeth were comparatively analyzed between the experimental and control groups and between the BICs and PICs subgroups by chi-square tests or Fisher exact tests (if required). McNemar test was used to compare the prevalence of dilaceration between impacted and erupted contralateral canines in the experimental group. Chisquare test also helped us to evaluate the prevalence of root curvature and apical hook in canines. One-way analysis of variance was used to compare mean crown and root length measurements among the 2 canine impaction subgroups and the control group. The angular positioning of all the impacted canines was also analyzed by 1-way analysis of variance. In case of any found significance, the least significant difference post-hoc test was used. All statistical analyses were performed using SPSS (version 24.0; IBM, Chicago, Ill), and P \ 0.05 was considered statistically significant. RESULTS
Characteristics of sex, age, Angle classifications, and canine eruption site of the subjects are shown in Table I.
Table II. Measurements from the CBCT images Measurements Qualitative variables Root curvature
Apical hook Quantitative variables Crown length Root length Angulation of the canine to the midsagittal plane
Definition Presence of flexion of the apical third of the root between 10 and 50 from the long axis of the tooth (a line joining the cusp tip and the midpoint of the root) Presence of angulation in the apical third of the root 50 or more to the long axis of the root (Fig) Distance between the cusp tip and the midpoint of the labial cementoenamel junction Distance between the labial cementoenamel junction and the root apex The acute angle between the long axis of the impacted canine and the midsagittal plane
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Fig. A maxillary impacted canine with apical hook: A, 3-dimensional volumetric reconstruction of a maxillary canine with an angulation of more than 50 which was considered as hook; B, x-ray constructed image of the same impacted root from CBCT coronal view.
Table III. Prevalence of root dilaceration in canines
and adjacent teeth: Difference of the experimental and control groups Evaluated teeth Canine Lateral incisor First premolar
Experimental group (n 5 145) 77 (53.1%) 35 (24.1%) 44 (30.3%)
Control group (n 5 145) 11 (7.59%) 12 (8.28%) 21 (14.5%)
P value* \0.001 \0.001 0.001
Table IV. Prevalence of root dilaceration in canines and adjacent teeth in experimental group: Difference of the impacted and contralaterally erupted sides Evaluated teeth Canine Lateral incisor First premolar
Impacted side (n 5 145) 77 (53.1%) 35 (24.1%) 44 (30.3%)
Erupted side (n 5 145) 35 (24.1%) 46 (31.7%) 36 (24.8%)
P value* \0.001 0.052 0.332
Note. Values are n (%). *P values based on chi-square tests for each target tooth; P \ 0.05 was considered statistically significant.
Note. Values are n (%). *P values based on McNemar tests for each target tooth; P \ 0.05 was considered statistically significant.
Among all the unilateral maxillary canine impaction subjects, 74 were buccally positioned, and 71 were palatally positioned. In general, intrarater reliability for image assessment was excellent for both qualitative (Cohen weighted kappa tests, k . 0.9) and quantitative variables (intraclass correlations, r . 0.9). As shown in Table III, the prevalence of root dilaceration in maxillary impacted canines of the experimental group was significantly higher than the coinciding erupted canines in the control group (P \ 0.001). Significant differences in root dilaceration prevalence were also found in adjacent lateral incisors (P \ 0.001) and premolars (P 5 0.001) of the experimental group, compared with those of the control group (Table III). In a comparison between the impacted and normally erupted side of experimental subjects, a significantly higher prevalence of root dilaceration in canines was found on the impacted side (P \ 0.001; Table IV). Although in the same comparison, the prevalence differences of root dilaceration in both lateral incisors and first premolars were insignificant (P 5 0.052 and P 5 0.332, respectively). Table V displays comparisons specifically focused on the different impaction types regarding root
dilaceration. Compared with the control group, a significantly higher prevalence of root dilaceration was found in canines of the BICs subgroup (P \ 0.001), unlike the adjacent lateral incisors and premolars (P 5 0.075 and P 5 0.072, respectively). PICs subgroup had a significantly higher prevalence of root dilaceration in either canines or adjacent teeth than control subjects (all P values \0.001). However, no significant differences were detected in the root dilaceration prevalence of all the evaluated teeth in a direct comparison of BICs and PICs subgroup. Root curvature prevalence of maxillary impacted canines in either BICs or PICs subgroups yielded a significant difference compared with the control group (P \ 0.001 for both; Table VI). The observations of an apical hook in canine roots for both BICs and PICs subgroups were significantly higher (P 5 0.008 and P \ 0.001, respectively) compared with the control group (Table VI). Furthermore, comparisons between BICs and PICs subgroups revealed no significant difference regarding the root curvature or the apical hook in impacted canines (P 5 0.477 and P 5 0.122, respectively). Because of the negligible finding of hooks in adjacent premolars and lateral incisors in our samples,
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Table V. Prevalence of root dilaceration in canines and adjacent teeth: Comparisons between subgroups and control
group Evaluated teeth Canine Lateral incisor First premolar
BICs subgroup (n 5 74) 34 (45.9%) 12 (16.2%) 18 (24.3%)
PICs subgroup (n 5 71) 43 (60.6%) 23 (32.4%) 26 (36.6%)
CG (n 5 145) 11 (7.6%) 12 (8.3%) 21 (14.5%)
P value* BICs/CG \0.001 0.075 0.072
P value* PICs/CG \0.001 \0.001 \0.001
P value* BICs/PICs 0.078 0.023 0.107
Note. Values are n (%). CG, Control group. *P values based on chi-square tests for each target tooth; the significance level for multiple comparisons were adjusted to 0.017 based on Bonferroni correction.
Table VI. Prevalence of curvature and hook in impacted canines: Comparisons between subgroups and control group Variables Root curvature in canines Apical hook in canines
BICs (n 5 74) 27 (36.5%) 7 (9.5%)
PICs (n 5 71) 30 (42.3%) 13 (18.3%)
CG (n 5 145) 9 (6.2%) 2 (1.4%)
P Value BICs/CG \0.001 0.008*
P Value PICs/CG \0.001 \0.001*
P Value BICs/PICs 0.477 0.122
Note. Values are n (%). CG, Control group. *P values based on Fisher exact test; unlabeled P values were obtained from chi-square tests; the significance level for multiple comparisons was adjusted to 0.017 based on Bonferroni correction.
the adjacent teeth analysis in terms of root curvature and the apical hook is not presented. The mean root lengths of BICs and PICs were 12.42 6 1.68 mm and 13.54 6 2.24 mm, which were significantly shorter than that of coinciding canines in the control group (P \ 0.001 for both; Table VII). Further comparison showed that BICs’ roots were significantly shorter than that of PICs (P 5 0.001). The comparisons between the crown length of canines in these 3 groups did not reveal any significance (P . 0.05). Another comparison with angular positioning was carried out among the maxillary impacted canines with root curvature, apical hook, and those without either hook or curvature. In these 3 categories, mean angulations of the impacted canine long axis to the midsagittal plane (36.72 6 18.33 , 25.16 6 17.68 , and 23.83 6 19.69 , respectively) were significantly different (P 5 0.016). With this regard, maxillary impacted canines with apical hook had a significantly higher degree of angulation to the midsagittal plane than those without root dilaceration (P 5 0.012). DISCUSSION
This retrospective study aimed to differentiate BICs and PICs regarding the prevalence of dilaceration in their own and adjacent roots based on the retrieved CBCT diagnostic data. Overall, we observed that maxillary impacted canines had a statistically elevated tendency
to develop dilaceration on their roots. In addition, adjacent teeth to the BICs and PICs subgroups exhibited different prevalence patterns in root dilaceration. Normally, during the eruption, the apical area remains at the same location, whereas the crown moves occlusally, but in case of mechanical blockage in eruption, the proliferating apical area will move in the opposite direction, causing root dilaceration.20 Expectedly, the comparison between the experimental and control group enabled us to imply that maxillary impacted canines are more prone to develop root dilaceration (Table III). This result was further supported by observing higher root dilaceration prevalence of canines in the impacted side than the normally erupted side merely in our experimental subjects. The observed inclination of impacted canines toward presenting a root dilaceration in our study emphasizes how crucial a timely orthodontic treatment can be to provide an unrestricted environment for the root to elongate. This finding has been examined successfully in previous case reports, when the root with incomplete formation continued to grow and reorient in the proper direction relative to the crown, ensuring a more favorable prognosis.21,22 Regarding the lateral incisors, the PICs subgroup had a higher prevalence of dilaceration, but not so for the BICs subgroup (Table V). Moreover, adjacent lateral incisors in PIC had a significantly higher prevalence of dilaceration than BIC subjects, which is in concordance with
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Table VII. Crown and root length of the maxillary canines: Comparisons between subgroups and control group Measurements Crown length of canines (mm) Root length of canines (mm)
BICs (n 5 74) 10.94 6 1.12 12.42 6 1.68
PICs (n 5 71) 10.85 6 1.06 13.54 6 2.24
CG (n 5 145) 10.81 6 1.01 15.45 6 1.89
P value* BICs/CG 0.370 \0.001
P value* PICs/CG 0.750 \0.001
P value* BICs/PICs 0.621 0.001
Note. Values are mean 6 SD. SD, Standard deviation; CG, control group. *P, analysis of variance, least significant difference post-hoc analysis.
the guidance theory. As proposed by this theory, in case of missing, late-developing, or peg-shaped lateral incisors, the canine will not find the guidance to descend correctly along its normal eruption path.23 It is plausible that a curved lateral incisor root may disturb the eruption of a canine and lead to a palatal impaction. As for the other adjacent teeth to maxillary impacted canines, generally, premolars in the experimental group had higher prevalence of dilaceration (Table III). Furthermore, we found a significantly higher prevalence of root dilaceration in adjacent first premolars was present in the PICs than the BICs subgroup. Pedulla et al24 reported that a deviated premolar apex might contribute to the palatal impaction of the canine because the passive eruption of the impacted tooth occurred on surgical removal of the mechanical obstruction. Besides, the apices of PICs are also prone to be distributed toward the first premolar's root apex.25 Together with the previous findings and our study with the substantial sample size, mechanical interference is presumed between the maxillary canine impaction and root dilaceration of adjacent premolars because of spatial immediacy. Thus, in the clinical approach to patients with PIC, the root morphology of adjacent teeth deserves close attention throughout CBCT because it is directly related to the treatment planning and prognosis. As presented in a previous case with severe crowding, the total extraction of adjacent premolar with a deviated root was necessary to eliminate the obstruction.26 But in mild or noncrowded patients, surgically resection of dilacerated adjacent root or orthodontically rotation out of the way for impacted canine alignment should be considered.18,24 Regarding the severity of dilaceration in impacted canines, we found that the prevalences of root curvature and apical hook in impacted canines were significantly higher in PICs than in the control group. Collectively, our data were synchronized with the findings of Hettiarachchi et al,11 but did provide an important addition to their proposal by taking the buccally impacted maxillary canines into consideration. Notably, direct comparison between BICs and PICs in terms of root curvature or apical hook revealed no significant difference (Table VI).
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This perception is more validated because there was no significant difference between the BICs and PICs subgroup regarding the prevalence of root dilaceration in canines (Table V). When it comes to root development, it is reported that tooth root undergoes a series of interactions between the mesenchymal and adjacent epithelium cells.27 Hence, it is possible that for impacted canines buried in the maxilla, an approximation to nearby anatomic structures such as maxillary sinus floor or nasal fossa might cause a deflection in the epithelial diaphragm and initiate the formation of severe curvature or hook in impacted roots.28 Moreover, complications such as root resorption induced by torque control and apical exposure out of the labial plate have been reported in dilacerated impacted teeth.4 Therefore, failure in orthodontic traction of both BICs and PICs with severe root curvature or apical hook is highly probable, which necessitates prior patient advisory. In the current study, the mean length of canine roots in the BIC and PIC subgroup was significantly shorter than that of the control group; this echoed the findings of Hettiarachchi et al11 about palatal maxillary canine impaction in Caucasians. This result suggests a relation between tooth impaction and short root length. In a genetic study in mice, Ono found that cells in the dental follicle and root surface express parathyroid hormonerelated peptide, and the deletion of its receptor leads concurrently to failure of eruption and significantly truncated roots.29 But the analysis in mice sparks the question of whether the relationship is also applicable to humans.29,30 Our study did provide important evidence by observing the shorter root length of maxillary impacted canines radiographically. In addition, BICs had significantly shorter roots than PICs in this study. Because buccally impacted maxillary canines have been previously linked to deficient growth of maxillary arch and lack of adequate space to emerge, we assume their shorter root might be related to insufficient space for the normal growth in roots. Our analysis indicates a significant relationship between their angulation and dilaceration in terms of angular positioning of the maxillary impacted canines. These data suggest that when the canine is more
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angulated in reference to the midsagittal plane, the root might develop a more severe form of dilaceration, resulting in a questionable prognosis.22 A larger angle between the canine and the midsagittal plane is indicative of more horizontal positioning in impactions. This alteration will put the displaced tooth in a closer approximation to other important anatomic structures and result in a greater dilaceration degree, consistent with our justifications described previously on hook formation. As with other literature, this study also was confronted by different limitations. First, the concurrence of dilaceration and dental developmental alterations suggested they are closely related to genetic circumstances.1,31,32 However, because of the limited knowledge on root formation, precise inferences to the pathogenesis of dysplasias, including dilaceration, are still hampered and demanding more investigations.33 Findings in our study were laid on an expansive definition of dilaceration. Therefore, further studies focusing on the severity of dilaceration are required with more distinctive classifications in accordance with the curving radius or angle under different circumstances. Ultimately, in this study, although the sample size had a power value of over 95% and represented our main population well, we cannot completely rule out the possibility that there may be differences between our sample and the general population. CONCLUSIONS
1.
2.
3.
Buccally and palatally impacted maxillary canines both had a higher tendency to present curved or hooked root configuration. Adjacent teeth to palatally impacted maxillary canines were more frequently observed to have root dilaceration. Buccally maxillary impacted canines had shorter roots compared with palatally impacted and normally erupted canines.
AUTHOR CREDIT STATEMENT
Dan Cao and Bingting Shao have designed the study, performed the literature search, acquired the data, assessed the results, and drafted the manuscript. Iman Izadikhah participated in designing the study and revised the manuscript critically for important intellectual content. Lizhe Xie and Bin Wu participated in image processing and statistical analysis of the study. Hu Li performed the retraction and collection of the required diagnostic data. Bin Yan has developed the theoretical
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framework, supervised and corrected the study, and finally approved the publication. REFERENCES 1. Regezi JA, Sciubba JJ, Jordan RCK. Abnormalities of teeth. In: Oral Pathology: Clinical Pathologic Correlations. 6th ed. St Louis: Saunders; 2017. p. 374. 2. Neville BW, Damm DD, Allen CM, Chi AC. Oral and Maxillofacial Pathology. 4th ed. St Louis: Elsevier; 2016. 3. Topouzelis N, Tsaousoglou P, Pisoka V, Zouloumis L. Dilaceration of maxillary central incisor: a literature review. Dent Traumatol 2010;26:427-33. 4. Chang NY, Park JH, Kim SC, Kang KH, Cho JH, Cho JW, et al. Forced eruption of impacted maxillary central incisors with severely dilacerated roots. Am J Orthod Dentofacial Orthop 2016;150:692-702. 5. Campbell CM, DiBiase A, Fleming PS. Concomitant dilaceration, transposition, and intraosseous migration: report of a patient treated with maxillary canine-central incisor substitution. Am J Orthod Dentofacial Orthop 2014;146:514-21. 6. Sultan N. Incidental finding of two rare developmental anomalies: fusion and dilaceration: A case report and literature review. J Nat Sci Biol Med 2015;6(Suppl 1):S163-6. 7. Soxman JA, Wunsch PB, Haberland CM, Haberland CM. Anomalies of tooth formation, Anomalies of the Developing Dentition. Cham, Germany: Springer; 2018. p. 75-107. 8. de Amorim CS, Americano GCA, Moliterno LFM, de Marsillac MWS de, Andrade MRTC, Campos V. Frequency of crown and root dilaceration of permanent incisors after dental trauma to their predecessor teeth. Dent Traumatol 2018;34:401-5. 9. Bodrumlu E, Gunduz K, Avsever H, Cicek E. A retrospective study of the prevalence and characteristics of root dilaceration in a sample of the Turkish population. Oral Radiol 2013;29:27-32. 10. Walia PS, Rohilla AK, Choudhary S, Kaur R. Review of dilaceration of maxillary central incisor: a mutidisciplinary challenge. Int J Clin Pediatr Dent 2016;9:90-8. 11. Hettiarachchi PVKS, Olive RJ, Monsour P. Morphology of palatally impacted canines: a case-controlled cone-beam volumetric tomography study. Am J Orthod Dentofacial Orthop 2017;151: 357-62. 12. Yan B, Sun Z, Fields H, Wang L. Maxillary canine impaction increases root resorption risk of adjacent teeth: a problem of physical proximity. Am J Orthod Dentofacial Orthop 2012;142:750-7. 13. Yan B, Sun Z, Fields H, Wang L, Luo L. Etiologic factors for buccal and palatal maxillary canine impaction: a perspective based on cone-beam computed tomography analyses. Am J Orthod Dentofacial Orthop 2013;143:527-34. 14. Jafarzadeh H, Abbott PV. Dilaceration: review of an endodontic challenge. J Endod 2007;33:1025-30. 15. Thongudomporn U, Freer TJ. Prevalence of dental anomalies in orthodontic patients. Aust Dent J 1998;43:395-8. 16. Chate RAC. Maxillary canine displacement; further twists in the tale. Eur J Orthod 2003;25:43-7. 17. Chate RAC. Maxillary canine impaction; a final twist in the tale? J Orthod 2004;31:13-4. 18. Turk T, Elekdag-Turk S. Case report: management of an impacted maxillary canine in association with a deviated palatal premolar root. J Contemp Dent Pract 2008;9:108-14. 19. Molen AD. Protocols for the use of cone beam computed tomography in orthodontic practice. In: Kapila SD, editor. Cone Beam Computed Tomography in Orthodontics: Indications, Insights,
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and Innovations. 1st ed. Oxford, United Kingdom: John Wiley & Sons, Inc; 2014. p. 153-4. Proffit WR. Concepts of growth and development. In: Proffit WR, Fields H, Larson B, Sarver DM, editors. Contemporary Orthodontics. 6th ed. Philadelphia: Mosby; 2019. p. 69. Topouzelis N, Tsaousoglou P, Gofa A. Management of root dilaceration of an impacted maxillary central incisor following orthodontic treatment: an unusual therapeutic outcome. Dent Traumatol 2010;26:521-6. Wei YJ, Lin YC, Kaung SS, Yang SF, Lee SY, Lai YL. Esthetic periodontal surgery for impacted dilacerated maxillary central incisors. Am J Orthod Dentofacial Orthop 2012;142:546-51. Becker A, Chaushu S. Etiology of maxillary canine impaction: a review. Am J Orthod Dentofacial Orthop 2015;148:557-67. Pedulla E, Valentino J, Rapisarda S. Endodontic surgery of a deviated premolar root in the surgical orthodontic management of an impacted maxillary canine. J Endod 2015;41:1730-4. Kim SH, Kim YM, Oh S, Kim SS, Park SB, Son WS, et al. How far is the root apex of a unilateral impacted canine from the root apices’ arch form? Am J Orthod Dentofacial Orthop 2017;151:351-6. Kerrigan J, Sandy JR. Displacement of maxillary canines: a twist in the root. Br J Orthod 1995;22:275-8.
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27. Zhang R, Li T. Modulation of microRNAs in tooth root and periodontal tissue development. Curr Stem Cell Res Ther 2018;13: 118-24. 28. Walton RE, Herbranson EJ. Internal anatomy. In: Torabinejad M, Walton RE, Fouad AF, editors. Endodontics: Principles and Practice. 4th ed. St Louis: Elsevier; 2009. p. 239. 29. Ono W, Sakagami N, Nishimori S, Ono N, Kronenberg HM. Parathyroid hormone receptor signalling in osterix-expressing mesenchymal progenitors is essential for tooth root formation. Nat Commun 2016;7:11277. 30. Frazier-Bowers SA, Vora SR. Genetic disorders of dental development: tales from the bony crypt. Curr Osteoporos Rep 2017;15: 9-17. 31. Bertl K, Benk€o G, Bertl MH, Breu M, Gahleitner A, Ulm C. A retrospective study on the influence of maxillary canine impaction on premolar root morphology. Clin Oral Investig 2013;17: 943-8. 32. Ledesma-Montes C, Jimenez-Farfan MD, Hernandez-Guerrero JC. Dental developmental alterations in patients with dilacerated teeth. Med Oral Patol Oral Cir Bucal 2019;24:e8-11. 33. Luder HU. Malformations of the tooth root in humans. Front Physiol 2015;6:307.
American Journal of Orthodontics and Dentofacial Orthopedics
Root dilaceration in maxillary impacted canines and adjacent teeth: A retrospective analysis of the difference between buccal and palatal impaction Dan Cao,a Bingting Shao,a Iman Izadikhah,a Lizhe Xie,b Bin Wu,c Hu Li,a and Bin Yana Nanjing, China
Introduction: This research aimed to analyze the prevalence of root dilaceration in buccally impacted canines (BICs) and palatally impacted canines (PICs) with their adjacent teeth based on a retrospective cone-beam computed tomography (CBCT) investigation. Methods: Pretreatment CBCT images of 145 subjects with unilateral maxillary canine impaction and 145 age- and sex-matched subjects without impaction were used. Prevalence of dilaceration (subclassified to root curvature and apical hook based on severity) in canines and adjacent teeth was determined in CBCT records. The root length of maxillary impacted canines was measured for further morphologic evaluations. Results: Impacted canines had a significantly higher prevalence of root dilaceration than the control group and compared with the erupted contralateral canines in the experimental group (P \ 0.001 for both). A significantly higher prevalence of root dilaceration was found in adjacent lateral incisors of the PICs subgroup than that of the control group (P \ 0.001). Adjacent premolars had a higher prevalence of dilacerated roots in the PICs subgroup (P \ 0.001) than the control group, but not for the BICs subgroup. Significantly higher prevalence of curvature (P \ 0.001 for both) and hook (P 5 0.008 and P \ 0.001, respectively) were found in BICs and PICs roots compared with the control group. Both types of impacted canines had significantly shorter roots than the control group (P \ 0.001 for both). Conclusions: BICs and PICs have a higher tendency to present root dilaceration and shorter roots. Unlike BICs, adjacent teeth to PICs were more frequently observed to have root dilaceration. (Am J Orthod Dentofacial Orthop 2020;-:---)
A a
bnormal angulation or curvature formed in the root or crown of a tooth is defined as dilaceration.1,2 Dilaceration is of great importance to
Jiangsu Key Laboratory of Oral Diseases, and Department of Orthodontics, Affiliated Hospital of Stomatology, Nanjing Medical University, Nanjing, China. b Jiangsu Key Laboratory of Oral Diseases, Engineering Center for Digital Medical Technology of Stomatology, Nanjing Medical University, Nanjing, China. c College of Mechanical and Electronic Engineering, Nanjing Forestry University, Nanjing, China. Dan Cao and Bingting Shao are joint first authors and contributed equally to this work. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. This study was supported by the National Natural Science Foundation of China (82071143), Key R and D program of Jiangsu province (BE2018723), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, 2018-87). Address correspondence to: Bin Yan, Jiangsu Key Laboratory of Oral Diseases, and Department of Orthodontics, Affiliated Hospital of Stomatology, Nanjing Medical University, 136 Hanzhong St, Gulou District, Nanjing 210029, China; e-mail, [email protected]. Submitted, July 2019; revised, November 2019; accepted, December 2019. 0889-5406/$36.00 Ó 2020 by the American Association of Orthodontists. All rights reserved. https://doi.org/10.1016/j.ajodo.2019.12.019
orthodontics because dilacerated roots are harder to move orthodontically, have a higher risk of impaction3 or external resorption,4 and impede favorable insertion of miniscrews. Orthodontic management of dilacerated roots is complex and lengthy and requires indefinite maintenance.5 The prevalences of root dilaceration have been generally investigated in several ethnic groups, which ranges from 0.3% to 17% without gender predilection.3,6 To date, several potential etiologic factors for root dilaceration have been proposed. When dilaceration occurs in permanent incisors, it is often supposed as a result of trauma to the primary predecessors whose apices lie close to the permanent tooth germ. An intrusion injury places the apex of the primary tooth in close approximation to the tooth bud of permanent successors, increasing the likelihood of damage to the tooth bud and the Hertwig epithelial root sheath.7 However, it has been found that the prevalence of dilacerated permanent successor teeth is generally disproportionate to 1
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the incidence of injuries in the corresponding deciduous teeth.8 This implicates that trauma may not be the major etiologic factor of dilaceration because most dilacerated teeth are found in the posterior area, which is less likely to have a history of trauma.3 Other probable etiologic factors include insufficient space for development, the effect of anatomic structures (such as the cortical bone of the maxillary sinus or the nasal fossa), supernumerary tooth, mechanical interference with an eruption in cases such as an ankylosed or retentive primary tooth, and hereditary factors.9,10 Previously, a relationship between maxillary canine impaction and root dilaceration has been proposed. Hettiarachchi et al11 reported that palatally impacted maxillary canines have a greater tendency to develop apical hooks. He also concluded that PICs have shorter root length than nonimpacted maxillary canines. However, this study covered only PIC in Caucasians. It is wellknown that BIC and PIC are different from various standpoints such as ethnical prevalence and etiologic factors.12,13 Briefly, buccally impacted canines (BICs) are mostly linked to dental arch and skeletal size deficiencies, whereas PICs are theorized to be associated with genetic origins or lack of guidance from the adjacent lateral incisors.13 Moreover, previous studies concentrating on root dilaceration of impacted canines in Asian populations are scarce. Hence, the association of root dilaceration and impacted canines and whether the prevalence of root dilaceration differs between the 2 impaction types in Asians have yet to be determined. Numerous studies on patients with normally erupted maxillary canine reported that maxillary premolars and lateral incisors are relatively more frequently dilacerated.2,14,15 In addition, few case reports on the concurrent incidence of maxillary canine impaction and adjacent premolar root dilaceration suggested a relationship between them.16-18 Therefore, to clarify this issue, a large sample study is necessary to investigate
the prevalence pattern of root dilaceration in adjacent teeth of maxillary impacted canines. The purpose of this study was to investigate the association between root dilaceration and BICs or PICs as well as their adjacent teeth in population with the cone-beam computed tomography (CBCT) technique. The null hypothesis was that either the maxillary impacted canines or their adjacent teeth would not present a difference in their dilaceration prevalence. MATERIAL AND METHODS
Pretreatment CBCT images of 145 Chinese subjects with unilateral maxillary canine impaction (experimental group) and 145 age- and sex-matched subjects without impaction (control group) were obtained from the Affiliated Hospital of Stomatology, Nanjing Medical University (Nanjing, China) (Table I). The inclusion criteria for the experimental group were clinically and radiographically diagnosed patients of unilateral maxillary canine impaction. The experimental group was subclassified as BICs and PICs. The exclusion criteria were patients having canines with open apices, missing or root resorption of adjacent lateral incisors and first premolars, history of systemic disorders, and undergoing orthodontic treatment. All subjects in the control group were patients with normally erupted canines who met the same exclusion criteria and prescribed CBCT for another diagnostic reason. Before the imaging process, all patients signed the consent form that their radiological results (demanding no personal identification disclosure) could be used for future research on maxillary canine impaction. Ethics approval was granted by the Ethical Committee Department of Affiliated Hospital of Stomatology, Nanjing Medical University (PJ2018031-001). The present study was carried out after approval from the International Clinical Trial Registry (Protocol no. ChiCTR1800020036).
Table I. Characteristics of the sample Experimental group Characteristics Sex Female Male Age, y Angle classifications Class I Class II Class III
Buccal (n 5 74)
Palatal (n 5 71)
Control group, n 5 145
P value
49 (66.2) 25 (33.8) 17.08 6 4.88
39 (54.9) 32 (45.1) 18.62 6 5.18
88 (60.7) 57 (39.3) 17.73 6 5.04
0.394*
33 (44.6) 28 (37.8) 13 (17.6)
30 (42.3) 23 (32.4) 18 (25.3)
70 (48.3) 48 (33.1) 27 (18.6)
0.705*
0.183y
Note. Values are n (%) or mean 6 standard deviation. *Comparison with chi-square tests among the 3 groups; yComparison with 1-way analysis of variance tests among the 3 groups.
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All CBCT data were acquired from the patients with a standard acquisition protocol using the same CBCT machine (NewTom VG; QR, Verona, Italy) and the same parameters: 16-cm diameter field of view, 110 kV, 1-20 mA (pulsed mode), and 0.3-mm voxel size. After removing patient identification information from the CBCT data (digital imaging and communications in medicine files), they were entitled to randomly generated codes and subsequently analyzed by a rater on the basis of the following standardized protocol. Digital imaging and communications in medicine files were first imported into SimPlant O&O (version 13.0; Materialise NV, Leuven, Belgium) and segmented to display the teeth and skull for 3-dimensional surface rendering at the threshold value of teeth (1250-3070) and skull (2503070) in the software. The images were reoriented according to a protocol described by Molen.19 Afterwards, qualitative and quantitative variables (Table II) were assessed and confirmed in the volumetric and 3 orthogonal (sagittal, coronal, and axial) 2-dimensional views. In both study groups, maxillary impacted canines and their adjacent teeth were classified as no dilaceration, with a root curvature, and with an apical hook according to the angular degree of root dilaceration (Fig).11 The landmarks for measuring quantitative variables (Table II) were identified in the volumetric view using the digitizing landmarks tool of the software. After verifying the sagittal, coronal, and axial views, these landmarks were used to measure quantitative image variables using a digital linear measurement tool (to 0.01 mm precision) in the software. Before the measurement process, each to-be-evaluated tooth in the control group was selected according to its own maxillary left or right coinciding canine or adjacent tooth present in the matched subject from the experimental group. To assess intraexaminer reliability, we repeated measurements of 100 randomly selected subjects from the whole sample by the same rater (B.S.) after a 1-month
3
interval. With assumptions of 30% and 10% prevalence rates of curvature in our impacted and control samples, respectively, a minimum sample size of 56 was required to achieve 80% power (calculated in G*Power [version 3; Heinrich Heine University, D€ usseldorf, Germany]) to detect differences of root curvature prevalence and even less than that for the hook. Our samples with buccal (n 5 74), palatal (n 5 71), and combined (n 5 145) impactions had power values of 96.5%, 96.0%, and 99.5%, respectively. Statistical analysis
Cohen weighted kappa (for qualitative variables) and intraclass correlation tests (for quantitative variables) were used to assess intrarater reliability. The prevalence of dilaceration in canines and adjacent teeth were comparatively analyzed between the experimental and control groups and between the BICs and PICs subgroups by chi-square tests or Fisher exact tests (if required). McNemar test was used to compare the prevalence of dilaceration between impacted and erupted contralateral canines in the experimental group. Chisquare test also helped us to evaluate the prevalence of root curvature and apical hook in canines. One-way analysis of variance was used to compare mean crown and root length measurements among the 2 canine impaction subgroups and the control group. The angular positioning of all the impacted canines was also analyzed by 1-way analysis of variance. In case of any found significance, the least significant difference post-hoc test was used. All statistical analyses were performed using SPSS (version 24.0; IBM, Chicago, Ill), and P \ 0.05 was considered statistically significant. RESULTS
Characteristics of sex, age, Angle classifications, and canine eruption site of the subjects are shown in Table I.
Table II. Measurements from the CBCT images Measurements Qualitative variables Root curvature
Apical hook Quantitative variables Crown length Root length Angulation of the canine to the midsagittal plane
Definition Presence of flexion of the apical third of the root between 10 and 50 from the long axis of the tooth (a line joining the cusp tip and the midpoint of the root) Presence of angulation in the apical third of the root 50 or more to the long axis of the root (Fig) Distance between the cusp tip and the midpoint of the labial cementoenamel junction Distance between the labial cementoenamel junction and the root apex The acute angle between the long axis of the impacted canine and the midsagittal plane
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Fig. A maxillary impacted canine with apical hook: A, 3-dimensional volumetric reconstruction of a maxillary canine with an angulation of more than 50 which was considered as hook; B, x-ray constructed image of the same impacted root from CBCT coronal view.
Table III. Prevalence of root dilaceration in canines
and adjacent teeth: Difference of the experimental and control groups Evaluated teeth Canine Lateral incisor First premolar
Experimental group (n 5 145) 77 (53.1%) 35 (24.1%) 44 (30.3%)
Control group (n 5 145) 11 (7.59%) 12 (8.28%) 21 (14.5%)
P value* \0.001 \0.001 0.001
Table IV. Prevalence of root dilaceration in canines and adjacent teeth in experimental group: Difference of the impacted and contralaterally erupted sides Evaluated teeth Canine Lateral incisor First premolar
Impacted side (n 5 145) 77 (53.1%) 35 (24.1%) 44 (30.3%)
Erupted side (n 5 145) 35 (24.1%) 46 (31.7%) 36 (24.8%)
P value* \0.001 0.052 0.332
Note. Values are n (%). *P values based on chi-square tests for each target tooth; P \ 0.05 was considered statistically significant.
Note. Values are n (%). *P values based on McNemar tests for each target tooth; P \ 0.05 was considered statistically significant.
Among all the unilateral maxillary canine impaction subjects, 74 were buccally positioned, and 71 were palatally positioned. In general, intrarater reliability for image assessment was excellent for both qualitative (Cohen weighted kappa tests, k . 0.9) and quantitative variables (intraclass correlations, r . 0.9). As shown in Table III, the prevalence of root dilaceration in maxillary impacted canines of the experimental group was significantly higher than the coinciding erupted canines in the control group (P \ 0.001). Significant differences in root dilaceration prevalence were also found in adjacent lateral incisors (P \ 0.001) and premolars (P 5 0.001) of the experimental group, compared with those of the control group (Table III). In a comparison between the impacted and normally erupted side of experimental subjects, a significantly higher prevalence of root dilaceration in canines was found on the impacted side (P \ 0.001; Table IV). Although in the same comparison, the prevalence differences of root dilaceration in both lateral incisors and first premolars were insignificant (P 5 0.052 and P 5 0.332, respectively). Table V displays comparisons specifically focused on the different impaction types regarding root
dilaceration. Compared with the control group, a significantly higher prevalence of root dilaceration was found in canines of the BICs subgroup (P \ 0.001), unlike the adjacent lateral incisors and premolars (P 5 0.075 and P 5 0.072, respectively). PICs subgroup had a significantly higher prevalence of root dilaceration in either canines or adjacent teeth than control subjects (all P values \0.001). However, no significant differences were detected in the root dilaceration prevalence of all the evaluated teeth in a direct comparison of BICs and PICs subgroup. Root curvature prevalence of maxillary impacted canines in either BICs or PICs subgroups yielded a significant difference compared with the control group (P \ 0.001 for both; Table VI). The observations of an apical hook in canine roots for both BICs and PICs subgroups were significantly higher (P 5 0.008 and P \ 0.001, respectively) compared with the control group (Table VI). Furthermore, comparisons between BICs and PICs subgroups revealed no significant difference regarding the root curvature or the apical hook in impacted canines (P 5 0.477 and P 5 0.122, respectively). Because of the negligible finding of hooks in adjacent premolars and lateral incisors in our samples,
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Table V. Prevalence of root dilaceration in canines and adjacent teeth: Comparisons between subgroups and control
group Evaluated teeth Canine Lateral incisor First premolar
BICs subgroup (n 5 74) 34 (45.9%) 12 (16.2%) 18 (24.3%)
PICs subgroup (n 5 71) 43 (60.6%) 23 (32.4%) 26 (36.6%)
CG (n 5 145) 11 (7.6%) 12 (8.3%) 21 (14.5%)
P value* BICs/CG \0.001 0.075 0.072
P value* PICs/CG \0.001 \0.001 \0.001
P value* BICs/PICs 0.078 0.023 0.107
Note. Values are n (%). CG, Control group. *P values based on chi-square tests for each target tooth; the significance level for multiple comparisons were adjusted to 0.017 based on Bonferroni correction.
Table VI. Prevalence of curvature and hook in impacted canines: Comparisons between subgroups and control group Variables Root curvature in canines Apical hook in canines
BICs (n 5 74) 27 (36.5%) 7 (9.5%)
PICs (n 5 71) 30 (42.3%) 13 (18.3%)
CG (n 5 145) 9 (6.2%) 2 (1.4%)
P Value BICs/CG \0.001 0.008*
P Value PICs/CG \0.001 \0.001*
P Value BICs/PICs 0.477 0.122
Note. Values are n (%). CG, Control group. *P values based on Fisher exact test; unlabeled P values were obtained from chi-square tests; the significance level for multiple comparisons was adjusted to 0.017 based on Bonferroni correction.
the adjacent teeth analysis in terms of root curvature and the apical hook is not presented. The mean root lengths of BICs and PICs were 12.42 6 1.68 mm and 13.54 6 2.24 mm, which were significantly shorter than that of coinciding canines in the control group (P \ 0.001 for both; Table VII). Further comparison showed that BICs’ roots were significantly shorter than that of PICs (P 5 0.001). The comparisons between the crown length of canines in these 3 groups did not reveal any significance (P . 0.05). Another comparison with angular positioning was carried out among the maxillary impacted canines with root curvature, apical hook, and those without either hook or curvature. In these 3 categories, mean angulations of the impacted canine long axis to the midsagittal plane (36.72 6 18.33 , 25.16 6 17.68 , and 23.83 6 19.69 , respectively) were significantly different (P 5 0.016). With this regard, maxillary impacted canines with apical hook had a significantly higher degree of angulation to the midsagittal plane than those without root dilaceration (P 5 0.012). DISCUSSION
This retrospective study aimed to differentiate BICs and PICs regarding the prevalence of dilaceration in their own and adjacent roots based on the retrieved CBCT diagnostic data. Overall, we observed that maxillary impacted canines had a statistically elevated tendency
to develop dilaceration on their roots. In addition, adjacent teeth to the BICs and PICs subgroups exhibited different prevalence patterns in root dilaceration. Normally, during the eruption, the apical area remains at the same location, whereas the crown moves occlusally, but in case of mechanical blockage in eruption, the proliferating apical area will move in the opposite direction, causing root dilaceration.20 Expectedly, the comparison between the experimental and control group enabled us to imply that maxillary impacted canines are more prone to develop root dilaceration (Table III). This result was further supported by observing higher root dilaceration prevalence of canines in the impacted side than the normally erupted side merely in our experimental subjects. The observed inclination of impacted canines toward presenting a root dilaceration in our study emphasizes how crucial a timely orthodontic treatment can be to provide an unrestricted environment for the root to elongate. This finding has been examined successfully in previous case reports, when the root with incomplete formation continued to grow and reorient in the proper direction relative to the crown, ensuring a more favorable prognosis.21,22 Regarding the lateral incisors, the PICs subgroup had a higher prevalence of dilaceration, but not so for the BICs subgroup (Table V). Moreover, adjacent lateral incisors in PIC had a significantly higher prevalence of dilaceration than BIC subjects, which is in concordance with
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Table VII. Crown and root length of the maxillary canines: Comparisons between subgroups and control group Measurements Crown length of canines (mm) Root length of canines (mm)
BICs (n 5 74) 10.94 6 1.12 12.42 6 1.68
PICs (n 5 71) 10.85 6 1.06 13.54 6 2.24
CG (n 5 145) 10.81 6 1.01 15.45 6 1.89
P value* BICs/CG 0.370 \0.001
P value* PICs/CG 0.750 \0.001
P value* BICs/PICs 0.621 0.001
Note. Values are mean 6 SD. SD, Standard deviation; CG, control group. *P, analysis of variance, least significant difference post-hoc analysis.
the guidance theory. As proposed by this theory, in case of missing, late-developing, or peg-shaped lateral incisors, the canine will not find the guidance to descend correctly along its normal eruption path.23 It is plausible that a curved lateral incisor root may disturb the eruption of a canine and lead to a palatal impaction. As for the other adjacent teeth to maxillary impacted canines, generally, premolars in the experimental group had higher prevalence of dilaceration (Table III). Furthermore, we found a significantly higher prevalence of root dilaceration in adjacent first premolars was present in the PICs than the BICs subgroup. Pedulla et al24 reported that a deviated premolar apex might contribute to the palatal impaction of the canine because the passive eruption of the impacted tooth occurred on surgical removal of the mechanical obstruction. Besides, the apices of PICs are also prone to be distributed toward the first premolar's root apex.25 Together with the previous findings and our study with the substantial sample size, mechanical interference is presumed between the maxillary canine impaction and root dilaceration of adjacent premolars because of spatial immediacy. Thus, in the clinical approach to patients with PIC, the root morphology of adjacent teeth deserves close attention throughout CBCT because it is directly related to the treatment planning and prognosis. As presented in a previous case with severe crowding, the total extraction of adjacent premolar with a deviated root was necessary to eliminate the obstruction.26 But in mild or noncrowded patients, surgically resection of dilacerated adjacent root or orthodontically rotation out of the way for impacted canine alignment should be considered.18,24 Regarding the severity of dilaceration in impacted canines, we found that the prevalences of root curvature and apical hook in impacted canines were significantly higher in PICs than in the control group. Collectively, our data were synchronized with the findings of Hettiarachchi et al,11 but did provide an important addition to their proposal by taking the buccally impacted maxillary canines into consideration. Notably, direct comparison between BICs and PICs in terms of root curvature or apical hook revealed no significant difference (Table VI).
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This perception is more validated because there was no significant difference between the BICs and PICs subgroup regarding the prevalence of root dilaceration in canines (Table V). When it comes to root development, it is reported that tooth root undergoes a series of interactions between the mesenchymal and adjacent epithelium cells.27 Hence, it is possible that for impacted canines buried in the maxilla, an approximation to nearby anatomic structures such as maxillary sinus floor or nasal fossa might cause a deflection in the epithelial diaphragm and initiate the formation of severe curvature or hook in impacted roots.28 Moreover, complications such as root resorption induced by torque control and apical exposure out of the labial plate have been reported in dilacerated impacted teeth.4 Therefore, failure in orthodontic traction of both BICs and PICs with severe root curvature or apical hook is highly probable, which necessitates prior patient advisory. In the current study, the mean length of canine roots in the BIC and PIC subgroup was significantly shorter than that of the control group; this echoed the findings of Hettiarachchi et al11 about palatal maxillary canine impaction in Caucasians. This result suggests a relation between tooth impaction and short root length. In a genetic study in mice, Ono found that cells in the dental follicle and root surface express parathyroid hormonerelated peptide, and the deletion of its receptor leads concurrently to failure of eruption and significantly truncated roots.29 But the analysis in mice sparks the question of whether the relationship is also applicable to humans.29,30 Our study did provide important evidence by observing the shorter root length of maxillary impacted canines radiographically. In addition, BICs had significantly shorter roots than PICs in this study. Because buccally impacted maxillary canines have been previously linked to deficient growth of maxillary arch and lack of adequate space to emerge, we assume their shorter root might be related to insufficient space for the normal growth in roots. Our analysis indicates a significant relationship between their angulation and dilaceration in terms of angular positioning of the maxillary impacted canines. These data suggest that when the canine is more
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angulated in reference to the midsagittal plane, the root might develop a more severe form of dilaceration, resulting in a questionable prognosis.22 A larger angle between the canine and the midsagittal plane is indicative of more horizontal positioning in impactions. This alteration will put the displaced tooth in a closer approximation to other important anatomic structures and result in a greater dilaceration degree, consistent with our justifications described previously on hook formation. As with other literature, this study also was confronted by different limitations. First, the concurrence of dilaceration and dental developmental alterations suggested they are closely related to genetic circumstances.1,31,32 However, because of the limited knowledge on root formation, precise inferences to the pathogenesis of dysplasias, including dilaceration, are still hampered and demanding more investigations.33 Findings in our study were laid on an expansive definition of dilaceration. Therefore, further studies focusing on the severity of dilaceration are required with more distinctive classifications in accordance with the curving radius or angle under different circumstances. Ultimately, in this study, although the sample size had a power value of over 95% and represented our main population well, we cannot completely rule out the possibility that there may be differences between our sample and the general population. CONCLUSIONS
1.
2.
3.
Buccally and palatally impacted maxillary canines both had a higher tendency to present curved or hooked root configuration. Adjacent teeth to palatally impacted maxillary canines were more frequently observed to have root dilaceration. Buccally maxillary impacted canines had shorter roots compared with palatally impacted and normally erupted canines.
AUTHOR CREDIT STATEMENT
Dan Cao and Bingting Shao have designed the study, performed the literature search, acquired the data, assessed the results, and drafted the manuscript. Iman Izadikhah participated in designing the study and revised the manuscript critically for important intellectual content. Lizhe Xie and Bin Wu participated in image processing and statistical analysis of the study. Hu Li performed the retraction and collection of the required diagnostic data. Bin Yan has developed the theoretical
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