Approximation and contact of the maxillary central incisor roots with the incisive canal after maximum retraction with temporary anchorage devices_ Report of 2 patients

CLINICIAN'S CORNER
Approximation and contact of the maxillary
central incisor roots with the incisive canal after
maximum retraction with temporary anchorage
devices: Report of 2 patients
Chooryung J. Chung, a Yoon Jeong Choi, b and Kyung-Ho Kim c
Seoul, Korea
The incisive canal is located on the median plane of the maxilla, posterior to the roots of the central incisor.
Although the incisive canal is not considered an anatomic structure that may limit tooth movement, it has recently
gained attention regarding the possibilities of surgical invasion and associated complications because of its
proximity to the maxillary central incisors. In the 2 illustrated cases, lip protrusion was improved by en-masse
bodily retraction of the anterior teeth (.8 mm) using temporary anchorage devices. Three-dimensional conebeam
computed tomography showed that the maxillary incisor roots were approximated to the incisive canal after
maximum retraction. One central incisor root was in direct contact with the incisive canal with severe root
resorption, but tooth vitality and the overall occlusion were stable in the long term without any sensory dysfunction.
The apparent root resorption may be mainly related to the large amounts of anterior retraction and root
movement in the 2 patients. However, the anatomic location of the incisive canal and the possibilities of its invasion
after tooth movement should be closely monitored when maximum retraction is planned, to prevent potential
complications. (Am J Orthod Dentofacial Orthop 2015;148:493-502)
The determination of the position of the maxillary
incisors is a key issue in orthodontic diagnosis
and treatment planning. In patients with bimaxillary
or bialveolar protrusion, premolar extraction followed
by maximum retraction of the anterior teeth is
required for esthetic improvement. In general, the ideal
position of the maxillary incisor is determined based
on various soft and hard tissue criteria, and orthodontic
tooth movement within the biologic limitations is
From the Department of Orthodontics, Gangnam Severance Hospital, Institute of
Craniofacial Deformity, College of Dentistry, Yonsei University, Seoul, Korea.
a Associate professor.
b Assistant professor.
c Professor and chairman.
All authors have completed and submitted the ICMJE Form for Disclosure of Potential
Conflicts of Interest, and none were reported.
Supported by the Basic Science Research Program, through the National Research
Foundation of Korea funded by the Ministry of Science, ICT, and Future Planning
(NRF-2013R1A1A3011648).
Address correspondence to: Kyung-Ho Kim, Department of Orthodontics, Gangnam
Severance Hospital, Institute of Craniofacial Deformity, College of Dentistry,
Yonsei University, 211 Eonjuro, Gangnam-gu, Seoul 135-720, Korea; e-mail,
khkim@yuhs.ac.
Submitted, August 2014; revised and accepted, April 2015.
0889-5406/$36.00
Copyright Ó 2015 by the American Association of Orthodontists.
http://dx.doi.org/10.1016/j.ajodo.2015.04.033
desirable for a successful treatment outcome with
long-term stability.
According to the well-known concept of the “envelope
of discrepancy,” the clinical guidelines recommend
that the maximum amounts of maxillary incisor retraction
and intrusion by orthodontics alone are 7 and
2 mm, respectively. 1,2 The hard tissue limit for
retraction in the maxilla is reportedly the lingual
cortical plate; however, it is also commonly accepted
that the range of maxillary incisor retraction is greater
because of fewer anatomic and physiological
constraints than in the mandible. 2
The incisive canal is an anatomic structure located on
the median plane of the palatine process of the maxilla,
posterior to the roots of the central incisor, surrounded
by thick cortical bone (Fig 1). 3,4 It transmits nasopalatine
vessels and nerves, branches of the maxillary division of
the trigeminal nerve, and the maxillary artery. 4,5
Although the incisive canal has not been proposed as an
anatomic structure that may limit tooth movement, it
has gained attention because of the possibilities of
surgical invasion and associated complications such as
nonosseointegration or sensory dysfunction owing to
its proximity to the maxillary incisor region. 6-8 Approximation
and contact of the mandibular molars to the
493

494 Chung, Choi, and Kim
Fig 1. CBCT images of the incisive canal. The incisive canal (arrows) is surrounded by cortical bone.
The axial section represents the apical third of the incisor roots. Notice that the width of the incisive canal
is larger than the interroot distance between the central incisors in the axial view.
mandibular canal and the inferior alveolar neurovascular
bundle during orthodontic tooth movement have also
been reported 9-11 and in rare incidences were associated
with temporary paresthesia of the lower lip. 9,10
The following 2 case reports illustrate the contact
and approximation of the maxillary central incisor roots
to the incisive canal after maximum retraction with temporary
anchorage devices.
CASE REPORTS
Patient 1
Patient 1 was a 19-year-old woman with chief complaints
of lip prominence and protrusion. The intraoral
photographs and cephalograms indicated a skeletal
Class II malocclusion with a protrusive profile. A panoramic
radiograph showed no gross abnormalities. She
had no notable medical history (Fig 2).
The overall goals of treatment were to improve the
protrusive profile and to obtain a Class I occlusion with
ideal overjet and overbite. Four first premolars were extracted,
and orthodontic treatment was initiated using
0.022-in slot Roth prescription self-ligating brackets
(Clippy-C; Tomy International, Tokyo, Japan). In addition,
4 miniscrews (Ortholution, Gyeonggi-do, Korea),
one per quadrant, were inserted between the second
premolars and the first molars to reinforce anchorage
during space closure. We used 150-g nickel-titanium
coil springs (Tomy International) and elastomeric modules
for anterior retraction via en-masse sliding mechanics.
Since the patient lived abroad, she was able
to visit the clinic only once every 4 months on average
during her vacations.
The duration of the treatment was 24 months. After
treatment, her profile was improved with a Class I occlusion
and ideal overjet and overbite, and the dental
midline was in accordance to the facial midline. Fixed retainers
were bonded, and removable retainers were provided
for use at night. The patient was fully satisfied with
the overall outcome. The cephalometric superimposition
indicated absolute retraction of the maxillary incisors
through bodily movement and controlled tipping of
the mandibular incisors along with intrusion (Table I).
However, the posttreatment radiographs indicated moderate
to severe apical root resorption, especially in the
maxillary right central incisor, compared with the neighboring
teeth. Interestingly, according to the periapical
radiograph, the incisive canal, which was identified as
a radiolucent tube-like structure surrounded by a thin
radiopaque margin between the central incisor roots,
was curved to the right and not to the left, making contact
with the right central incisor root and not the left
central incisor (Figs 3-5).
Considering the large amounts of anterior retraction
and root movement, and the severity of root resorption
at the right central incisor, we performed a cone-beam
computed tomography (CBCT) scan (Pax-Zenith3D;
Vatech, Seoul, Korea) to further evaluate the
3-dimensional (3D) structural changes, including the
amount of alveolar bone surrounding the maxillary incisors.
In general, the palatal surface of the maxillary teeth
was covered with a thin layer of cortical bone. However,
to our surprise, the mesiopalatal surface of the maxillary
right central incisor root apex was in contact with the
incisive canal (Fig 4, A and B), which was associated
with severe root resorption (Fig 4, C and D), whereas
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Chung, Choi, and Kim 495
Fig 2. Pretreatment intraoral photographs and radiographs.
Table I. Cephalometric analysis of patient 1
Measurement Norm Pretreatment Posttreatment Postretention
Skeletal ( )
SNA 81.6 6 3.1 76.2 74.9 74.8
SNB 79.1 6 3.0 71.1 70.8 70.9
ANB 2.4 6 1.8 5.1 4.1 3.9
FMA 24.0 6 5.0 26.3 23.7 25.9
Dental ( )
IMPA 94.0 6 5.0 113.5 93.8 97.4
U1 to SN 106.0 6 5.0 104.7 92.1 97.4
Interincisal angle 126.0 6 7.0 103.6 136.6 128.5
Soft tissue (mm)
Upper lip E-plane 1.0 6 2.0 2.7 1.4 2.4
Lower lip E-plane 1.0 6 2.0 5.2 0.1 1.4
the left central incisor was in proximity rather than in
contact, showing mild apical root blunting (Fig 4,
E and F). Specific symptoms, such as numbness or
pain of the teeth or periodontium, were not noted,
and the incisors were vital.
After 30 months, the patient revisited the clinic.
Although the maxillary fixed retainer had been detached
for approximately a year after debonding and she had
stopped using the removable retainers, her occlusion
was stable. No detrimental changes in the maxillary incisor's
root were noted. Rather, a periapical radiograph
showed that the apical surface had become smoother
with even periodontal ligament space. Considering the
crown-root ratio of the incisors, new fixed retainers
were bonded on the maxillary incisors (Fig 5). To determine
whether any changes had taken place with regard
to the occlusion and the incisive canal, another CBCT
scan was performed at 36 months of retention with
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496 Chung, Choi, and Kim
Fig 3. Posttreatment intraoral photographs, radiographs, and cephalometric superimposition.
the patient's consent. The 3D CBCT findings obtained
immediately after the treatment and those obtained
36 months after treatment were superimposed by automatic
voxel-by-voxel registration at the cranial base
(OnDemand3D; Cybermed, Seoul, Korea). 12,13 The 3D
superimposition did not indicate any specific changes
in the surrounding alveolar bone, the position of the
central incisors in contact or approximation with the
incisive canal, or the incisive canal itself (Fig 6). No
specific symptoms or pain was reported; in general, the
probing depth was less than 2 mm, except for the mesiopalatal
interproximal gingiva, where the depth was
approximately 3 to 4 mm, but it was not associated
with bleeding.
Patient 2
Patient 2 was a 46-year-old woman with chief complaints
of lip prominence and protrusion. Intraoral photographs,
radiographs, and cephalograms indicated a
skeletal Class II malocclusion with a protrusive profile
(Fig 7). To improve the protrusive profile and to obtain
a Class I occlusion with ideal overjet and overbite, 4 first
premolars were extracted, and orthodontic treatment
was begun with 0.018-in slot Roth prescription
self-ligating brackets (Tomy International). Two miniscrews
(Ortholution) were inserted between the maxillary
second premolars and the first molars to reinforce
anchorage. The anterior teeth were retracted via enmasse
sliding mechanics with elastomeric modules.
The duration of the treatment was 28 months. As a
result, her profile was improved, and she had a Class I
occlusion with ideal overjet and overbite (Table II). The
cephalometric superimposition indicated maximum
retraction through controlled tipping and intrusion of
the maxillary incisors. The total amounts of retraction
at the incisor tips of the maxilla and the mandible
were 8.5 and 8.0 mm, respectively. A slight apical root
resorption of the maxillary incisors was noted after the
treatment (Fig 8). Compared with the CBCT images
before treatment, the maxillary incisor root apex was
approximated to the cortical bone surrounding the incisive
canal, which was nearly in contact after retraction
(Fig 9, A and B). The 3D superimposition of the CBCT
images obtained before and after treatment indicated
retraction of the teeth along with remodeling of the surrounding
alveolar bone. However, the incisive canal did
not show any gross positional changes after tooth movement
(Fig 9, C-E).
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Fig 4. CBCT images of the maxillary arch and the central incisors after maximum retraction: A, axial
and B, coronal sections show that the right central incisor root is in contact with the incisive canal;
C, sagittal sections of the right central incisor slice passing the incisive canal; D, another slice slightly
distal passing the tooth axis; E, sagittal section of the left central incisor slice passing the incisive canal;
F, another slice slightly distal passing the tooth axis. The arrows indicate the incisive canal.
DISCUSSION
The proximity of the maxillary incisor roots to the
incisive canal or the possibility of approximation or invasion
of the tooth roots into the incisive canal after anterior
retraction has not been closely evaluated in the
orthodontic literature. The lack of information is
possibly because the incisive canal is a midsagittal structure
positioned between the roots, and its dimension or
morphology is not clearly defined by conventional
2-dimensional radiographs. However, with the help of
3D imaging, the incisive canal was noted as an anatomic
structure that may be associated with orthodontically
induced inflammatory root resorption of the maxillary
central incisors during maximum retraction.
The maxillary central incisors are most frequently
involved in orthodontically induced inflammatory root
resorption, which is reportedly associated with patientrelated
risk factors such as genetic influence, root
morphology, root proximity to the cortical bone, alveolar
bone density, and type of malocclusion, and with orthodontic
treatment-related risk factors such as treatment
duration, magnitude of force, and amount of apical
root movement. 14,15 Although force levels about 150 g
of force were used for en-masse retraction in our
patients, it is possible that the apparent root resorption
was mainly related to the large amounts of anterior
retraction and root movement to maintain anterior
torque.
However, in patient 1, the differences in the levels of
root resorption between the right and left central incisors
were quite evident with distinct morphologic features,
whereas the patient-related and treatment-related
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498 Chung, Choi, and Kim
Fig 5. Postretention intraoral photographs, radiographs, and cephalometric superimposition.
Fig 6. The 3D CBCT superimposition immediately after and 36 months after treatment. These images
were superimposed by automatic voxel-by-voxel registration at the cranial base. No changes were
noted in the surrounding alveolar bone, the position of the central incisors in contact or approximation
with the incisive canal, or the incisive canal itself. A, Sagittal plane, B, coronal plane, and C, axial plane
indicating the incisive canal. Gray, Immediately after treatment; yellow, 36 months after treatment;
arrows, the incisive canal.
factors were equal between both central incisors, supporting
the involvement of the incisive canal. The incisive
canal is a structure that transmits soft tissues such as vessels
and nerves but is surrounded by a thick layer of
cortical bone. Thus, the mechanism of root resorption
for patient 1 may be similar to the condition when the
roots are in contact with or near the cortical plate. 15,16
Unlike patient 1, in patient 2, the involvement of the
incisive canal in root resorption could not be
determined per se. But patient 2 also illustrates the
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Fig 7. Pretreatment intraoral photographs and radiographs.
Table II. Cephalometric analysis of patient 2
Measurement Norm Pretreatment Posttreatment
Skeletal ( )
SNA 81.6 6 3.1 89.2 88.5
SNB 79.1 6 3.0 81.7 81.9
ANB 2.4 6 1.8 7.5 6.7
FMA 24.0 6 5.0 25.6 25.3
Dental ( )
IMPA 94.0 6 5.0 107.8 94.1
U1 to SN 106.0 6 5.0 113.9 96.3
Interincisal 126.0 6 7.0 105.1 137.7
angle
Soft tissue (mm)
Upper lip 1.0 6 2.0 4.3 1.1
E-plane
Lower lip
E-plane
1.0 6 2.0 5.3 0.7
possibility of maxillary incisor approximation, nearly in
contact with the incisive canal after maximum retraction.
The average width of the incisive canal in the axial
plane at the level of the apical third of the root is
reportedly about 3 to 5 mm, with a large variation
ranging from 1.1 to 6.7 mm. 4,5,17 Since the average
interroot distance between the maxillary central
incisors is about 3 to 4 mm, root touching or
approximation with the incisive canal, especially in
the mesiopalatal surface, can be speculated in certain
cases after maximum amounts of distal root
movement (Fig 1). 18,19 Variations in the morphology
of the incisive canal have been frequently reported
with 3D imaging, including deviation to 1 side,
widening or cystic changes, furcations, and so
on. 4,5,17 The incisive canal, in many circumstances, is
deviated toward the right central incisor, similar to
the situation of patient 1 (Figs 3 and 5, periapical
radiographs). 8 The 3D CBCT images also show the
root touching the incisive canal only in the right central
incisor. During treatment, changes in root parallelism
in cases of improper bracket positioning or addition
of root torque during retraction may also lead to root
convergence of the incisors, decreasing the interroot
distance. 20 Although the sagittal distance between
the incisor roots and the incisive canal is yet to be
determined, 3D evaluations during orthodontic diagnosis
and close monitoring of the incisor roots
throughout treatment would be advantageous in
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500 Chung, Choi, and Kim
Fig 8. Posttreatment intraoral photographs, radiographs, and cephalometric superimposition.
preventing potential complications, especially in
patients requiring maximum retraction.
The application of skeletal anchorage has enabled orthodontists
to effectively achieve 3D control of the anterior
teeth without the risk of losing anchorage. Thus, a
modified “envelope of discrepancy” was recently proposed,
indicating the broadened range of orthodontic
tooth movement with skeletal anchorage compared
with the original inner envelope. 2 The application of skeletal
anchorage allowed us to retract the maxillary incisors
by more than 8 mm in both patients, thus exceeding the
conventional guidelines according to the esthetic needs.
However, the basic biologic structure and the response to
tooth movement may vary among patients, and evidence
is still lacking with regard to the changes or adaptations
of the anatomic structures after the application of
advanced biomechanical techniques. When temporary
anchorage devices are applied as absolute anchorage
during en-masse retraction, greater amounts of retraction
along with anterior root movement and spontaneous
intrusion can be achieved relatively effectively
and easily compared with conventional mechanics. 21,22
Thus, the application of temporary anchorage devices
may provide biomechanical advantages in terms of
increased amounts of retraction along with torque and
vertical control of the incisors, but this may have also
aggravated the severity of the root resorption and
raised the possibilities of root approximation to the
incisive canal.
Orthodontic movement of the mandibular molars
sometimes induces temporary paresthesia when the
molars are in close contact with the mandibular canal,
9,10,23 and excessive amounts of mandibular molar
intrusion also resulted in approximation or touching of
the molar root to the inferior alveolar nerve in dogs. 11
In general, neurovascular bundles in the incisive canal
are covered by cortical bone. Similar to the structures
of the incisive canal, the mandibular canal surrounds
the inferior alveolar neurovascular bundle. But in the
mandibular molar region, cortical bone surrounding
the mandibular canal is deficient in some patients,
possibly causing temporary paresthesia in response to
mandibular molar movement. 9,24,25 Interestingly,
repositioning of the inferior alveolar neurovascular
bundles and remodeling of the canal after tooth
movement were suggested in histologic sections. 11
However, based on our 3D superimpositions, it is
apparent that the remodeling of the incisive canal was
not clinically detected after tooth movement (patient
2) and during retention (patient 1).
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Chung, Choi, and Kim 501
Fig 9. CBCT images of the maxillary arch and 3D superimposition before and after maximum retraction:
axial section A, before and B, after maximum retraction; 3D superimposition before and after
maximum retraction in the sagittal (C), coronal (D), and axial (E) planes. Note the changes in the tooth
positions and the remodeling of the surrounding alveolar bone, whereas the incisive canal is stable with
no distinct changes in position. Gray, Immediately after treatment; yellow, 36 months after treatment;
arrows, the incisive canal.
Fortunately, the occlusion of patient 1 was stable in
the long term, and the central incisor with severe root
resorption did not lead to specific symptoms with
regard to mobility, sensitivity, or vitality issues during
the 3-year follow-up period. However, the probing
depth of the mesiopalatal side was slightly increased
compared with the other sides. En-masse retraction
with maximum anchorage results in significant vertical
alveolar bone loss of the incisors in the palatal side. 26
Thus, anatomically, the contact with the incisive canal
in the mesiopalatal angle region of the central incisor
root may create a state similar to that of a 2-walled
defect, which calls for careful attention in the long
term.
CONCLUSIONS
Approximation of the maxillary central incisors to the
incisive canal was noted after maximum retraction in the
2 illustrated cases. Interestingly, a maxillary central
incisor root was in direct contact with the incisive canal
with apparent root resorption, but tooth vitality and the
overall occlusion were stable in the long term. When
maximum retraction of the maxillary incisors is planned,
customized 3D evaluation of the dimension and location
of the incisive canal would be advantageous in preventing
potential complications.
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