ORIGINAL ARTICLE
New clues for early management of
maxillary impacted central incisors based
on 3-dimensional reconstructed models
Jiahong Lyu,a Yi Lin,b Han Lin,b Ping Zhu,b and Yue Xua
Guangzhou, China
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 di-
lacerations 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 dilacera-
tions. Analyses of crown direction and K-value may aid in evaluating root dilacerations at early dental ages
and facilitating early intervention of impacted incisors. (Am J Orthod Dentofacial Orthop 2018;154:390-6)
I
mpacted maxillary central incisors pose a challenge Orthodontic treatment entails guiding the impacted
for dental treatment due to their influence on facial tooth into proper alignment in addition to establishing
esthetics and phonetics.1 Among the various treat- the normal contour of the tooth. The key to a successful
ment alternatives, which include incision sprouts, ortho- treatment outcome depends on the method and timing
dontic extrusion, denture replacement, autologous of traction.2 Therefore, it is important to understand the
transplantation, extraction, and biologic induction, eruption mechanisms and that root dilacerations of
appropriate orthodontic traction is the optimal choice impacted maxillary incisors occur in different locations
for the maintenance of bone volume to achieve func- and at different development stages so that appropriate
tional and esthetic outcomes.2-7 treatment regimens can be designed.
Conventional theories associate tooth eruption with
From the Guanghua School of Stomatology, Hospital of Stomatology, Sun root extension, periodontal ligament, and dental follicle,
Yat-sen University, Guangdong Provincial Key Laboratory of Stomatology, whereas recent studies have challenged these theories in
Guangzhou, China.
a
Department of Orthodontics.
several ways.8 It has been advocated that the tooth is
b
Department of Oral and Maxillofacial Surgery. pushed upward toward the oral cavity because of growth
All authors have completed and submitted the ICMJE Form for Disclosure of of the lamina dura rather than growth of its own root,
Potential Conflicts of Interest, and none were reported.
Supported by the National Natural Science Foundation of China (81571020) and
confirming the independence between tooth eruption
the Natural Science Foundation of Guangdong Province (2015A030313179). and root elongation.9 Artificial teeth with intact dental
Address correspondence to: Yue Xu, Department of Orthodontics, Guanghua follicles and metal or silicone “tooth germs” were found
School of Stomatology, Hospital of Stomatology, Sun Yat-sen University,
Guangdong Provincial Key Laboratory of Stomatology, Guangzhou 510055,
to erupt on schedule.10 Tooth eruption is also influenced
PR China; e-mail, kou9315@hotmail.com. by cortical plate, mucosa, retained deciduous incisors,
Submitted, December 2016; revised and accepted, November 2017. odontomas, and supernumerary teeth.11 The peri-
0889-5406/$36.00
Ó 2018 by the American Association of Orthodontists. All rights reserved.
odontal ligament was confirmed to provide an eruptive
https://doi.org/10.1016/j.ajodo.2017.11.034 impetus by the shrinking and cross-linking of collagen
390
Lyu et al 391
Fig 1. A and B, 3D models of the skull and the impacted incisor, color-coded to identify and inspect the
relationship between the impacted incisor and the maxilla; C-F, anterior view, posterior view, and right
and left lateral views of the 3D models of the impacted incisor, respectively.
fibers and the contraction of periodontal ligament fibro- Although there are many case reports of impacted di-
blasts.12 lacerated incisors, a quantitative study on impacted in-
Previous studies related to root dilacerations of cisors and root dilacerations is lacking. We conducted
impacted teeth were based on either 2-dimensional a retrospective analysis of a large sample of CBCT images
images or certain views of cone-beam computed tomog- and proposed some parameters to evaluate root dilacer-
raphy (CBCT) images, which cannot accurately show the ations of impacted central incisors in reconstructed 3D
3-dimensional (3D) root morphology due to labiolingual models, and also compared different groups based on
or mesiodistal inclination and rotation of impacted the direction of the crown. We aimed to provide
teeth. To evaluate tooth impactions and root dilacera- new clues for early management of impacted maxillary
tions accurately, it is necessary to interpret CBCT images central incisors based on the direction of the crown
along the long axis of the impacted incisor on the and the extent of the dilacerations to prevent severe
sagittal plane in a 3D model. root dilacerations from affecting esthetics and oral
American Journal of Orthodontics and Dentofacial Orthopedics September 2018 Vol 154 Issue 3
392 Lyu et al
Fig 2. Measurements and evaluations of nasal, labial, and palatal impactions: A, D, and G, schematics
of measurements in nasally, labially, and palatally impacted teeth, respectively; B, E, and H, sagittal
views of nasally, labially, and palatally impacted incisors, respectively; C, F, and I, reconstructed
models of dental arch in nasal, labial, and palatal impactions, respectively. In total impactions (A
and G), aLR was calculated by the formula: aLR 5 ① – ②. In labial submucosal impactions (D),
aLR was calculated by the formula: aLR 5 ① – (② – ③). ①, the red line segment refers to BT; ②,
the purple line segment refers to HC; ③, the blue line segment refers to L; and ④, the green arc refers
to angle a.
functions and to decrease the loss of maxillary central arch within the expected developmental period were
incisors. included in the study, whereas those with combined im-
pactions of central incisors and canines, retained decid-
MATERIAL AND METHODS uous incisors, odontomas, and supernumerary teeth
This retrospective study was approved by the institu- were excluded.
tional review board (ERC-2014-1) of the Hospital of Sto- This study included 108 subjects (60 girls, 48 boys)
matology, Sun Yat-sen University, Guangzhou, China. who were referred to our hospital between January
Informed consent was obtained from the guardians of 2010 and February 2015. They were between 8 and
all children. The participants were orthodontic patients 16 years of age (mean, 11.8 6 2.6 years) with unilateral
who had been diagnosed with unilateral impaction of impacted maxillary central incisors.
a maxillary central incisor. The patients underwent pre- All CBCT images were obtained using a DCT Pro
operative CBCT examinations to evaluate the location CBCT device (VATECH, Hwaeong, Gyeonggi-do, Korea)
and developmental stage of the impacted incisors. Pa- operating at 90 kV(p), 6 mA, 0.4 mm voxel size, exposure
tients with teeth that had failed to erupt into the dental time of 24 seconds, and field of view of 16 3 10 cm. The
September 2018 Vol 154 Issue 3 American Journal of Orthodontics and Dentofacial Orthopedics
Lyu et al 393
when part of the crown has broken through the cortex of
Table I. Root formation stage of impacted and
the alveolar bone (Fig 2).
contralateral incisors (n 5 108)
Crown height and root length were measured in the
Impacted incisors contralateral normally erupted incisors with complete
Stage Dilacerated Nondilacerated Contralateral incisors
root formation; thus, the crown height/root length ratio
Cervical third 7 5 0 was calculated. In this large-sample study, iLR was esti-
Middle third 38 28 0 mated according to the measured HC value and the
Apical third 21 11 16 crown height/root length ratio.
Completed 5 3 92 Determination of the developmental stages of the
P 0.001* 0.001*
root was based on the length of the developing root
*Compared with contralateral incisors, chi-square test, P \0.05. and the patency of the apical foramen (open or closed).13
The process of root formation can be divided into 4
stages: cervical third, middle third, apical third, and
data were exported into DICOM format and then recon- completed. The stages were distinguished in the CBCT
structed into a 3D model after being imported into an images and recorded accordingly. In addition, 2 other
interactive image analysis system (Mimics, version parameters including the position and the angle were
14.0; Materialise, Leuven, Belgium). A pilot experiment measured on the 3D reconstructed model to determine
was conducted with 15 patients to explore the initial the dilacerated conditions of the roots. The position
threshold value for the maxilla and the tooth structure was determined by measuring the distance between
segmentation. The thresholding ranges were set at the point of intersection of the long axis of the crown
226-3071 and 1200-3071 to show the 3D structures and the long axis of the dilacerated root and the center
of the anterior maxilla and maxillary anterior teeth. of the tooth cervix. The angle was measured as the sup-
With profile lines measured in Hounsfield units, region plementary angle between the longitudinal axis of the
growing was used to separate the impacted incisors crown and that of the dilacerated portion of the root
from the peripheral bony structures, and 3D models of (Fig 2).
the incisors were reconstructed (Fig 1).
The longitudinal axis of the crown of the impacted Statistical analysis
central incisor was defined by connecting a line between
All measurements were repeated by the same
the midpoint of the incisor edge and the center of the
researcher (J.L.) at 2-week intervals; The intraclass corre-
cervix in the axial view, using the Mimics software. Sub-
lation coefficient was calculated to assess the intraexa-
sequently, a sagittal view along the long axis was
miner error. The average of the 3 measurements was
created. We evaluated and recorded different aspects
used as the final result. Descriptive statistics were ex-
of eruption at the workstation from the sagittal plane
pressed as means and standard deviations for metric var-
of every subject.
iables, and as frequencies and percentages for nominal
Coronal hindrance of eruption varied with the direc-
variables.
tion of eruption of the impacted incisor. The bony walls
All statistical analyses were conducted using software
facing the involved tooth during eruption were recorded
(version 16.0; SPSS, Chicago, Ill). The chi-square test was
as labial cortical plate, nasal floor, or palatal plate in
used to compare root formation stages between the
every subject (Fig 2).
impacted incisor and the contralateral normal central
K-value was defined as the ratio between the avail-
incisor. For comparison of the data between the different
able length of the direct root (aLR) and the ideal length
groups, the 1-way analysis of variance test for multiple
of the direct root (iLR) in the long axis of the crown.
comparisons with the post hoc Bonferroni adjustment
K 5 aLR/iLR. aLR can be calculated with the following
was used. The independent t test was used to compare
formulas (Fig 2).
measurement variables between dilacerated and nondi-
aLR 5 BT HCðtotal impactionÞ lacerated groups in labial impaction. All P values
aLR 5 BT ðHC LÞðsubmucosal impactionÞ (2-tailed) less than 0.05 were defined as significant.
In the formulas, BT represents the bone thickness in
the long axis of the crown. HC refers to the anatomic RESULTS
crown height, the distance from the incisal edge to the The intraclass correction coefficient was 0.916.
intersection of the long axis and cementoenamel junc- Root resorption of adjacent teeth was barely
tion. L is measured as the length from the incisor edge observed except for 2 lateral incisors with slight root
to the crown-heading bone cortex, which must be added resorption. In the teeth with complete root formation,
American Journal of Orthodontics and Dentofacial Orthopedics September 2018 Vol 154 Issue 3
394 Lyu et al
Table II. Impacted teeth (nasal, labial, and palatal) with dilacerated and nondilacerated roots
Palatal Labial Nasal
Total
n K-value n K-value n K-value n
Dilacerated 18 (16.7%) 0.16-0.19 46 (42.6%) 0.25-0.53 7 (6.4%) 0.69-0.75 71 (65.7%)
Nondilacerated 5 (4.6%) 0.18-0.20 23 (21.3%) 0.32-0.72 9 (8.3%) 1.38-1.52 37 (34.3%)
Total 23 0.16-0.20 69 0.25-0.72 16 0.69-1.52 108
K-value, K 5 aLR/iLR.
curvature of the apical third of the root was observed
Table III. Position (D) and angle (a) of root dilacera- in 7 subjects. For labial impactions, a majority of incisors
tions of impacted incisors with root dilacerations had middle third (25; 23.1%) and
D a apical third (14; 13.0%) root formation, with position
values calculated as 4.33 6 0.45, and angle values
Type of impaction Mean (mm) SD Mean (mm) SD
ranging from 87.21 to 154.38 ; the dilacerated por-
Palatal 2.13 0.32 88.47 6.28
Labial 4.33 0.45 112.46 9.67
tions of the roots were found to be curved along with
Nasal 9.80 1.29 59.83 7.27 the palatal plate (Fig 3). The dilacerated roots of the
F 493.33 135.65 palatally impacted incisors were distinct with sharply
P 0.003* 0.008* curved angles (88.47 6 6.28 ) at the cervical thirds of
P (palatal/labial) 0.005* 0.001* the roots.
P (labial/nasal) 0.006* 0.001*
P (palatal/nasal) 0.002* 0.006*
Table IV shows that L and L/HC values were greater
for dilacerated impactions than for nondilacerated
*One-way analysis of variance with the Bonferroni adjustment, ones (P \0.01). The cingulum was embedded in the
F0.01 (2, 75) 5 99.49, P \0.01.
alveolar bone in all submucosal impacted incisors. Dila-
cerations were only seen in subjects where L/HC was
crown height and root length values were 10.51 6 0.23 greater than 0.44, thus implying that the cingulum
and 12.79 6 0.27 mm, respectively. The crown height/ may have a role in eruption resistance when nearly half
root length ratio was 0.81 6 0.02 mm; 65.7% of the of the crown was perforated through the labial cortical
impacted incisors in this sample had dilacerations plate.
greater that 20 .
Table I reports the root formation. All contralateral DISCUSSION
teeth were normally erupted, including 92 teeth In this study, we reconstructed 3D models of CBCT
(85.2%) with complete root formation and the remain- images and evaluated coronal hindrance and root dila-
ing 16 (14.8%) with apical-third root formation. A cerations as well as the developmental state of impacted
distinct retardation in root growth was noted in both maxillary incisors. The results may provide new clues to
the dilacerated and nondilacerated impacted incisors analyze impaction and root dilacerations, thus facili-
compared with the normally erupted incisors tating selecting appropriate therapeutic timing and pro-
(P \0.001). tocol of impacted maxillary central incisors.
Labial impaction was the most common, followed by Impacted teeth have varying responses to different
palatal and nasal impactions. The corresponding eruptive resistances. According to our observations,
numbers of subjects with or without root dilacerations eruption of an impacted tooth through the nasal floor
are given in Table II. The K-value of the nasal impactions is extremely rare. The special anatomic structure of the
was the highest in all embedded teeth. However, no nasal floor may result in the deflection of the epithelial
eruption occurred through the nasal floor, implying diaphragm, acting as a barrier during tooth eruption.14
that the floor of the nasal cavity provides absolute resis- Failure of eruption delays or impedes root growth
tance against tooth eruption. The K-value of palatal im- because the relatively immobile crown fails to generate
pactions was the lowest of the 3, with barely any more space for root elongation, thus resulting in dilacer-
eruption through the palatal plate, except for 1 subject, ations when root extension is restricted by cortical bone.
suggesting strong resistance in tooth eruption. These results indicate that the palatal plate is thicker
Table III shows that the position and angle of root di- than its labial counterpart, making it a relatively effec-
lacerations were different among nasal, labial, palatal tive barrier against tooth eruption. In brief, the nasal
impactions (P \0.01). For nasal impactions, an acute floor, palatal cortical plate, and labial cortical plate serve
September 2018 Vol 154 Issue 3 American Journal of Orthodontics and Dentofacial Orthopedics
Lyu et al 395
Fig 3. Dilacerated root curved along with the palatal plate in a labial impaction: A, sagittal view of a
labially impacted incisor with a dilacerated root in the alveolar bone; B, the profile of the impacted
incisor in A was delineated to show the axis of the dilacerated root.
cingulum, and also root elongation is restricted by the
Table IV. Dilacerated and nondilacerated groups of
palatal lamina dura. The smaller the K-value, the nearer
labially impacted incisors
the dilacerated position is to the cervix; this may make
L (mm) HC (mm) L/HC the prognosis worse for orthodontic traction.
Mean Mean Mean
For palatal impaction, since the original tooth germ is
(mm) SD (mm) SD (mm) SD usually at a lower position, the K-value is small, and the
Dilacerated 5.43 0.02 10.48 0.38 0.52 0.02 space is exceedingly limited for root elongation. This is
Nondilacerated 2.83 0.01 10.49 0.42 0.27 0.06 why palatally impacted incisors have distinct dilacerated
P 0.002* NS 0.003* roots (18; 78.3%) with sharply curved angles at their cer-
NS, not significant. vical thirds. It can be extremely difficult to guide
*Independent t test, P \0.05. impacted incisors with such abnormal root dilacerations
into the dental arch. The treatment option may be
extraction and subsequent restoration.
as eruption hindrances, with eruption resistance in The prophylaxis of root dilacerations is undoubtedly
reverse order. an important issue for determining the therapeutic
For nasal impaction, the floor of the nasal cavity pro- timing. The treatment and prognosis of teeth vary with
vides absolute resistance against tooth eruption. the crown direction, root formation stage, and position
Although root dilacerations (7; 43.8%) occurred at the and angulation of root dilacerations. The study shows
apical third of root formation, intervention should begin that root growth of impacted incisors was retarded
as early as possible so that the direction of eruption can compared with normally erupted incisors, indicating
be altered and normal root development can continue in that treatment may start after root elongation in nor-
the new position with enough alveolar bone during trac- mally erupted incisors and no later than closure of the
tion. The Hertwig sheath has the potential to grow in apex foramen in impacted ones. Orthodontic traction
abnormal upward and lingual directions irrespective of at the appropriate time may allow for redirection of
its crown direction as previous studies have suggested.15 the Hertwig epithelial root sheath, with the development
For the most common labial impaction, dilacerated of the root in the normal spatial position and the crown
incisors (46; 66.7%) mainly have obtuse angles and mid- in alignment.15 More specifically, the analytic methods
dle and apical third root formations. Root dilacerations used in this study may serve as a reference in the early
with an obtuse angle and incomplete root formation diagnosis of root dilacerations, thus enabling early inter-
of the tooth would have a better prognosis for orthodon- vention for normal root development.
tic traction.16 The finding that root dilacerations occur However, there are some limitations in this study.
when half of the crown has emerged through the labial First, the measurements may not apply to other ethnic
cortical plate implies that the cingulum may increase groups in different regions because the anatomy of teeth
eruption resistance. It can be speculated that roots can vary among different populations. In addition,
tend to dilacerate when eruption is stuck with the further longitudinal studies are needed.
American Journal of Orthodontics and Dentofacial Orthopedics September 2018 Vol 154 Issue 3
396 Lyu et al
CONCLUSIONS 7. Uematsu S, Uematsu T, Furusawa K, Deguchi T, Kurihara S. Ortho-
dontic treatment of an impacted dilacerated maxillary central
Analysis of the direction of the crown, K-values, and incisor combined with surgical exposure and apicoectomy. Angle
eruption resistance of the cingulum in 3D reconstructed Orthod 2004;74:132-6.
models may help to evaluate the occurrence and position 8. Kjær I. Mechanism of human tooth eruption: review article
of root dilacerations of impacted incisors. Accordingly, the including a new theory for future studies on the eruption process,
2014. Cairo: Scientifica; 2014;2014:341905.
pattern of root dilacerations in different impactions can
9. Marks SC Jr, Schroeder HE. Tooth eruption: theories and facts.
provide new clues for prevention and early management Anat Rec 1996;245:374-93.
of root dilacerations in impacted maxillary central incisors. 10. Marks SC Jr, Cahill DR. Experimental study in the dog of the non-
active role of the tooth in the eruptive process. Arch Oral Biol 1984;
29:311-22.
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