Abstract
Introduction:
Orthodontic traction of impacted maxillary canines remains a clinical challenge, particularly when the tooth is severely angulated. Conventional force application to the crown may increase the risk of damage to adjacent roots due to insufficient control of root positioning. This case demonstrates the use of a boot loop in combination with a micro-screw to upright the canine root within the alveolar bone before potential root contact. Treatment alternatives and biomechanics are discussed to illustrate the effectiveness of this approach.
Diagnosis and treatment plan:
A boy aged 14 years 1 month presented with a severely angulated impacted maxillary left canine, with the root apex located above the maxillary left second premolar. The primary treatment objective was to align the impacted canine while preventing injury to adjacent roots. The treatment plan involved uprighting the canine root using a boot loop and micro-screw designed to apply light, controlled forces through loop activation.
Results:
During active treatment, the root angulation of the impacted canine was corrected from 60° distal to 13° mesial, achieving a maximum angular change of 73° through controlled uprighting. The canine was then successfully repositioned into its ideal position. No root resorption, gingival recession, or loss of supporting tissue was observed.
Conclusion:
The combination of a boot loop and micro-screw offers effective three-dimensional control of crown and root movement, providing a reliable and effective solution for managing severely angulated impacted canines.
Plain language summary
Maxillary permanent canines are the second most frequently impacted teeth after third molars (De Ridder et al., 2022). Several treatment options are available, including surgical repositioning, orthodontic traction, and extraction of the impacted teeth followed by prosthetic treatment (Counihan et al., 2013). Among these, orthodontic traction remains the only approach that achieves satisfactory functional and aesthetic outcomes, although it requires a longer treatment duration and more complex mechanics.
In the present case, the maxillary left canine was severely impacted and mesially tipped, with the root apex located above the root of maxillary left second premolar. Previous studies have emphasized the value of panoramic radiographs in assessing the difficulty of canine impaction cases (Kumar et al., 2015; Stivaros and Mandall, 2000; Yamamoto et al., 2003). Key indicators include the angulation to the midline, vertical height relative to adjacent teeth, and the anteroposterior position of the root apex. A similar case was reported by Ferreira et al. (2017), who successfully applied orthodontic traction to severely impacted canines; however, root resorption of the adjacent first premolars was observed due to their close proximity.
To minimize the risk of iatrogenic damage, especially to adjacent roots, careful control of root movement is essential during canine traction. Conventional techniques often apply force directly to the crown, which may fail to control root positioning and increase the likelihood of root collisions. In this case, a boot loop combined with a micro-screw was used to upright the impacted canine root within the alveolar bone before potential contact. By aligning the long axis of the tooth throughout the movement, the canine was repositioned without root resorption or loss of supporting tissue.
Although panoramic radiographs are traditionally used, Cone-beam computed tomography (CBCT) has been shown to provide more accurate three-dimensional (3D) information on root position and angulation, which can aid treatment planning when indicated (Al-Homsi and Hajeer, 2015; Hajeer et al., 2022). In the present report, CBCT was utilized to determine the 3D position of the impacted canine, to guide micro-screw placement planning, and to evaluate treatment outcomes.
This case report aims to present this biomechanical strategy and discuss its application in managing severely angulated impacted maxillary canines.
Diagnosis
A boy aged 14 years 1 month presented to our department with the chief complaint of unerupted teeth in the upper arch. The patient was in good general health, with no significant medical or family history related to the dental anomaly. The patient had not received any previous orthodontic or surgical interventions related to the impacted canine.
Clinical examination
Facial evaluation revealed a slightly convex profile, with good facial symmetry and no apparent asymmetry or deviation (Figure 1). Intraoral examination showed the patient was in the permanent dentition stage, with unerupted maxillary right and left canines, as well as the maxillary left lateral incisor. A bilateral Class I molar relationship was observed, with normal overjet and overbite. Both maxillary and mandibular arches exhibited generalized spacing (Figure 1).
Pre-treatment facial photographs, intraoral photographs, and dental casts.
Radiographic and cephalometric analysis
CBCT scans were performed using a KaVo 3D eXam unit under the following parameters: field of view (FOV) = 16 × 13 cm, voxel size = 0.2 mm, 120 kV, 20.27 mAs, and scan time = 14.7 s. Based on manufacturer exposure reference values and published dosimetry data for the KaVo 3D eXam system, the effective dose for a single scan was estimated to be approximately 100–120 μSv (Schilling and Geibel, 2013). Although this is higher than that of a panoramic radiograph, CBCT was justified under the as low as reasonably achievable (ALARA) principle due to the need for precise 3D evaluation of the severely angulated impacted canine and its relation to adjacent teeth.
The patient was positioned upright with the Frankfort horizontal plane parallel to the floor and the mid-sagittal plane centered and perpendicular to the floor. The head was stabilized using the unit’s chin rest and temple supports, and scans were obtained with teeth in light intercuspation and lips relaxed.
Cephalometric analysis indicated a skeletal Class I relationship with brachyfacial growth pattern (Table 1). Skeletal maturation was assessed as cervical vertebral maturation stage 3 (CVM 3), indicating that the patient was in the pubertal growth phase. CBCT revealed the following findings:
Maxillary left canine: the tooth was impacted in an anteroposterior position, with its crown impinging on the buccal surface of the root of the maxillary left lateral incisor. The root apex was located above the root of the maxillary left second premolar (Figure 2).
Cephalometric analysis.
Values are given as mean ± SD unless otherwise indicated. Normative values were based on Chinese population standards as described in an authoritative textbook (Zhao, 2020).
Pre-treatment radiographic images: (a) panoramic reconstruction from CBCT; (b) magnified view of the impacted maxillary left canine; (c) lateral cephalogram; (d) CBCT cross-sectional view showing the crown of the impacted canine exerting pressure on the buccal surface of the maxillary left lateral incisor; (e–g) CBCT cross-sectional views showing the root of the impacted canine centered buccolingually within the alveolar bone; (g) CBCT cross-sectional view showing the root apex penetrating into the maxillary sinus. CBCT, cone-beam computed tomography.
Cross-sectional CBCT imaging showed the crown and root of the impacted canine were positioned centrally within the buccolingual dimension (Figure 2), with the root apex extending into the maxillary sinus.
The angulation of the impacted canine’s long axis relative to the midline was measured at 60°.
Maxillary left lateral incisor: impacted and displaced mesially and lingually, likely due to a dentigerous cyst associated with the impacted left canine. The crown of the lateral incisor was distally angulated within the alveolar bone. The tooth also exhibited microdontic morphology.
Maxillary right canine: unerupted but positioned favorably for spontaneous eruption.
Both maxillary canines exhibited root curvature.
Based on clinical and CBCT findings, the case was diagnosed as a severely angulated impacted maxillary left canine associated with displacement of the adjacent lateral incisor, which exhibited microdontic morphology. No associated pathology was detected.
Treatment alternatives
Two treatment options were presented to the patient and his family:
Option 1: Extraction of the maxillary left impacted canine
Given the patient’s slightly convex profile and generalized spacing in both arches, extracting three premolars along with the impacted canine was considered likely to worsen the facial appearance. Therefore, this option proposed extracting only the maxillary left impacted canine, followed by closure of the residual spaces, retraction of the upper and lower anterior teeth, alignment of the maxillary right canine and left lateral incisor, and preservation of the left canine space for future prosthodontic rehabilitation.
The primary advantage of this approach was a shorter overall treatment time. However, it required subsequent prosthodontic replacement of the left canine and prolonged retention to maintain the space until definitive restoration could be performed.
Option 2: Closed-eruption surgery with orthodontic traction
This approach involved performing a closed-eruption surgical exposure followed by orthodontic traction to align the impacted maxillary left canine into the arch.
The advantage was the preservation of the natural canine, eliminating the need for future prosthetic treatment. However, this option required a longer treatment duration and carried potential risks, including traction failure and root resorption of the impacted canine or adjacent teeth.
After discussing the risks and benefits, the patient and his family elected to proceed with option 2. Written informed consent for this treatment was obtained from the patient’s legal guardian before initiating therapy. Ethical approval (DLKQLL20240110), covering both the clinical technique and the case documentation, was subsequently granted by the institutional review board. Additional written consent for publication of clinical photographs and CBCT images was obtained from the patient’s legal guardian before manuscript submission.
Treatment objectives
Based on the treatment option selected by the patient’s family, the objectives were as follows:
To align the maxillary left impacted canine into the dental arch using controlled orthodontic traction.
To improve facial aesthetics by closing generalized spacing in both the upper and lower arches, except around the microdontic maxillary left lateral incisor. Adequate space mesial and distal to the lateral incisor was planned to be preserved for future restoration enhancement (e.g. composite build-up or veneer), to be carried out once the patient reaches adulthood and when the prosthodontist considers it the optimal time, in consultation with the patient and parents.
To maintain bilateral Class I molar and canine relationships.
Treatment progress
Based on panoramic CBCT reconstructions, which indicated that the maxillary right impacted canine and the left lateral incisor were likely to erupt spontaneously (Figure 2), full-arch bonding was deferred at the beginning of treatment.
Instead, a closed-eruption surgical technique was performed to expose the crown of the maxillary left impacted canine, and a low-profile button attachment (lingual-type button) was bonded to the buccal surface. A titanium micro-screw (1.6 mm diameter × 8 mm length; Dentaurum, Germany) with a cross-shaped groove on the top cap was inserted between the maxillary left second premolar and first molar, perpendicular to the mucosal surface (Figures 3 and 8). The mini-screw was inserted using a self-drilling technique without predrilling or flap elevation. Insertion torque was not quantitatively recorded at placement; however, good primary stability was achieved clinically, and no loosening occurred throughout the entire treatment period.
(a–c) Intraoral photographs during the first stage of treatment: (a) right-side view showing the unerupted maxillary right canine; (b, c) closed-eruption technique applied to the maxillary left canine, with a button bonded to the crown. A micro-screw was vertically placed in the alveolar bone between the maxillary left second premolar and first molar, and a short 0.016 × 0.022-inch rectangular stainless-steel segmental archwire connected the impacted canine to the micro-screw. (d–f) Intraoral photographs 10 months into treatment. Upper fixed appliances were bonded, and a 0.012-inch nickel–titanium wire was placed for alignment. A spring was positioned between the maxillary left lateral incisor and first premolar to maintain space and upright the lateral incisor. (g–i) Transition to root uprighting mechanics. The button was replaced with a bracket on the impacted canine, and the straight segmental archwire was substituted with one featuring a boot loop.
No special soft-tissue management was required beyond routine topical disinfection, as the mini-screw was placed transgingivally without incision. No prophylactic antibiotics or postoperative analgesics were prescribed at the time of micro-screw insertion, and standard postoperative oral hygiene instructions were given. And healing was uneventful without soft-tissue inflammation or infection.
A short segment of 0.016 × 0.022-inch rectangular stainless-steel archwire was used to connect the button to the micro-screw. One end of the wire was bent into a cross shape to fit into the micro-screw slot and was secured with composite resin, which sealed and reinforced the engagement between the slot of the micro-screw head and the cross-shaped stainless-steel wire, ensuring stable force transmission. The other end was ligated to the button. To minimize gingival irritation, the wire was wrapped in a rubber tube when necessary.
To guide the canine buccally and occlusally, the archwire was bent in that directions, with the bend positioned mesial to the micro-screw to avoid mucosal compression during ligation.
The maxillary left impacted canine, positioned centrally in the buccolingual dimension of the alveolar bone, took approximately 10 months to erupt through the mucosa. During this period, the maxillary right canine and left lateral incisor erupted sufficiently to allow bonding. Upper arch brackets (0.018-inch slot system) were then bonded, and a 0.012-inch nickel–titanium (NiTi) archwire was placed for initial alignment. A coil spring was inserted between the maxillary left lateral incisor and first premolar to maintain space and assist in uprighting the lateral incisor (Figure 3).
Once the impacted canine erupted buccally with sufficient crown exposure, the button was replaced with a bracket. A new segmental archwire with a distally bent boot loop was then fabricated and placed (Figure 3). The posterior end of the archwire remained engaged in the micro-screw using the cross-shaped groove secured with composite resin, while the anterior end was ligated into the bracket slot.
Loop design and activation protocol
A boot-loop was fabricated from 0.016 × 0.022-inch stainless-steel wire in full-hard temper, with a loop approximately 6 mm in height and 10 mm in width (Figure 8). Uprighting force was applied using biomechanics similar to the tip-back bend in the multiloop edgewise archwire (MEAW) system, with the horizontal arm of the boot-loop bent to approximately 15° relative to the bracket slot. Owing to the long distance between the impacted canine and the mini-screw—substantially greater than a typical interbracket span—the resulting uprighting force was light and continuous in magnitude. Because this activation generates a couple rather than a single point force, direct in vivo force measurement was not feasible.
At the wire–screw interface, the primary load transmitted to the mini-screw was extrusive rather than rotational (Figure 8); therefore, no special anti-rotational wire–screw engagement design was required. The loop was reactivated when needed rather than replaced at each visit, typically at intervals of 4–6 weeks when regular follow-up was possible. In this case, the boot-loop plus temporary anchorage devices (TADs) produced progressive uprighting of the root over 25 months without radiographic signs of resorption. At 40 months of treatment, a panoramic radiograph was obtained, and canine angulation was assessed using the same method as at pretreatment—defined as the angle between the long axis of the impacted canine and the maxillary dental midline. Using this definition, the angulation was corrected from 60° distal to 13° mesial, representing a total angular change of 73° (Figure 4).
Intraoral photographs and panoramic radiograph showing treatment progress. (a) As the upper dentition alignment progressed, brackets were bonded to the lower arch. (b) Both arches were leveled and aligned, with closure of generalized spaces. The impacted canine root was uprighted by the boot loop. (c) Panoramic radiograph showing the change in the long-axis angulation of the impacted canine relative to a vertical reference line, decreasing from +60° to −13° (total correction 73°). (d, e) Final alignment of the canine to the arch using nickel–titanium wire after it reached the occlusal level.
Clinical how-to box: Boot loop–micro-screw technique for root uprighting of an impacted maxillary canine
Materials and appliances:
0.016 × 0.022-in stainless-steel segmental archwire
Boot loop bent with approximately 15° tip-back angle at the free horizontal end
Steps:
Initial traction: bond button on the surgically exposed canine crown. Place micro-screw vertically between upper second premolar and first molar. Engage a short straight 0.016 × 0.022-inch stainless-steel segmental wire from the micro-screw to the button to begin gentle traction.
Transition to boot loop uprighting: After partial eruption and exposure of the labial surface, replace button with a bracket on the canine. Replace straight segmental wire with one containing a boot loop. Insert the cross-shaped end of the segmental wire into the micro-screw head slot.
Activation: Activate the boot loop by adjusting the free horizontal arm to create a ~15° tip-back bend relative to the bracket slot. Reactivate every 4–6 weeks based on panoramic monitoring to ensure progressive root uprighting.
Finishing: Once the canine root was uprighted and crown approached occlusal plane, remove micro-screw and continue final alignment with NiTi archwire.
Safety tips for boot-loop and micro-screw mechanics
Mucosal impingement
The free arm of the boot loop should be checked at each visit to ensure it does not impinge on the movable mucosa. Any signs of irritation, ulceration, or patient discomfort require immediate trimming or recontouring of the wire.
Micro-screw hygiene
Patients should be instructed to clean around the micro-screw head daily using a soft toothbrush and chlorhexidine or saline rinse. Accumulation of plaque or soft-tissue inflammation should prompt professional cleaning and reinforcement of oral hygiene instructions.
Prevention of over-uprighting
Over-uprighting was primarily monitored clinically by observing changes in crown angulation and eruption trajectory during follow-up visits. As the root becomes upright, progressive improvement in crown position provides a reliable indirect indicator of root correction. Radiographic reassessment was performed only when clinically indicated and not on a routine basis, to minimize unnecessary radiation exposure. If over-uprighting is suspected, the boot loop should be immediately released from the bracket slot to allow spontaneous correction. Counter-bending should be reserved only for severe overcorrection.
Timing of loop removal
Once root uprighting is achieved, the boot loop should be reconfigured for vertical traction rather than removed, to continue guiding the canine toward the occlusal level.
Although the boot-loop mechanics were intended to be reactivated every 4–6 weeks, the patient’s treatment was unfortunately significantly disrupted by the COVID-19 pandemic and related quarantine measures, resulting in only 10 in-person visits between January 2020 and December 2022. During the periods of interrupted in-person follow-up, no unsupervised activations of the boot loop were performed by the patient or family. Remote communication was limited to oral-hygiene guidance and emergency advice only. All biomechanical reactivations were carried out exclusively during in-office visits, ensuring controlled force delivery and clinical safety. Despite the limited clinical supervision, the impacted canine’s root uprighted more than anticipated. After sufficient uprighting, the canine was successfully aligned into the dental arch.
Meanwhile, brackets were bonded to the lower arch to close generalized spacing and retract the anterior teeth. The micro-screw was removed once the impacted canine had been brought near the occlusal level and no longer required additional traction. The surrounding soft tissue was managed with routine oral hygiene instructions and regular inspections at each visit; no inflammation, soft-tissue irritation, or screw-related complications were observed throughout treatment. The total treatment duration, including delays caused by the pandemic, was approximately 4.5 years. The treatment timeline and major appliance activations are summarized in Table 2.
Treatment timeline.
NiTi, nickel–titanium.
After active treatment, a vacuum-formed retainer was prescribed for both arches to control the pre-existing generalized spacing and to maintain alignment; the patient was instructed to wear the retainer full-time for the first 12 months and nightly thereafter as part of a standard retention protocol.
Treatment results
The maxillary left impacted canine was successfully aligned into its proper position within the dental arch, exhibiting stable periodontal health and harmonious gingival contours. Class I canine and molar relationships were achieved bilaterally, along with normal overbite and overjet. Generalized spacing in both arches was fully closed, except for the maxillary left lateral incisor, where approximately 1 mm of space was maintained mesially and distally to accommodate future restorative treatment (Figure 5).
Post-treatment facial photographs, intraoral photographs, and dental casts.
Post-treatment CBCT, including panoramic reconstructions, showed well-aligned roots of the impacted teeth without any sign of root resorption—despite the unique ‘L’-shaped curvature of the maxillary canine root (Figure 6). Root resorption was assessed qualitatively based on CBCT root morphology, including evaluation for apical blunting, external surface irregularity, and loss of normal root contour. Quantitative root length measurements were not performed because the maxillary lateral incisor root on the affected side was incompletely developed at pretreatment, and the impacted canine root exhibited physiological curvature, making linear length comparison unreliable. CBCT slices were examined in serial axial, sagittal, and coronal views at the native reconstruction thickness provided by the imaging software. No radiographic signs of external root resorption or alveolar bone loss were observed on the impacted canine or adjacent teeth.
Post-treatment radiographs. (a) Panoramic reconstruction from CBCT images; (b) magnified view of the maxillary left canine; (c) lateral cephalogram; (d) frontal CBCT 3D reconstructions; (e) left-side CBCT 3D reconstructions; (f) magnified view of the maxillary left canine showing sufficient root length and distinctive ‘L’-shaped curvature in both canines. CBCT, cone-beam computed tomography.
3D linear displacement was measured directly on the CBCT dataset by comparing pre- and post-treatment coordinates of the canine crown and root apex. The total linear displacement was 22.20 mm for the canine crown and 18.64 mm for the root apex. The maxillary left canine exhibited intact surrounding alveolar bone and no signs of gingival recession. Periodontal probing depths and standardized gingival indices were not recorded during treatment; periodontal health was assessed clinically and radiographically, with no evidence of pathological attachment loss or inflammatory periodontal changes.
Cephalometric analysis confirmed the maintenance of a skeletal Class I relationship (ANB 2.4°), attributed primarily to mandibular growth. Retraction of the maxillary and mandibular anterior teeth during space closure contributed to the improvement of the patient’s facial profile, resulting in a more straightened appearance (Table 1, Figure 7).
Superimpositions of pre-treatment (red) and post-treatment (blue) cephalometric tracings.
All image analyses and measurements were performed using Dolphin Imaging software (version 11.95; Dolphin Imaging & Management Solutions, USA). Linear displacement of the impacted canine crown and root apex was measured directly in three dimensions on CBCT images to reflect true spatial movement. Cephalometric evaluation of hard-tissue profile changes was performed on two-dimensional reconstructed lateral cephalograms generated from the CBCT data. Images were reoriented to standardized reference planes before measurement to ensure consistency.
Discussion
The maxillary canine plays a crucial role in both function and aesthetics, making its preservation and proper alignment a key objective in orthodontic treatment. In the present case, the patient exhibited a slightly convex profile and generalized spacing in both arches, rendering the extraction of three premolars and the impacted canine an unsuitable option. Even in the event of failed traction, prosthetic restoration after canine extraction would have been the preferred alternative.
Orthodontic traction of impacted teeth is among the most challenging procedures, requiring careful biomechanical planning and control. The degree of treatment difficulty may be informed by the position of the impacted canine on the panoramic radiograph (McSherry, 1998; Pitt et al., 2006). Counihan et al. (2013) reported that a canine angulation to the midline exceeding 31° is associated with a poor prognosis. In addition, if the root apex is located above the second premolar, the likelihood of successful traction diminishes further. In this case, the angulation of the impacted canine was 60° to the midline—nearly double the threshold—while the root apex was located above the second premolar, indicating a high degree of difficulty. CBCT was used for 3D localization and angulation assessment of the impacted canine, as it provides significantly higher diagnostic accuracy than conventional two-dimensional radiography (Hajeer et al., 2022).
Regarding vertical positioning, the crown of the impacted canine extended more than halfway up the root of the adjacent lateral incisor but did not exceed the full root length. According to Counihan et al. ( 2013), this corresponds to a moderate prognosis.
In summary, the overall position of the impacted canine posed significant biomechanical challenges, particularly due to the considerable distance between the root apex and its ideal position within the alveolar bone. Controlling the angulation of the root during traction was especially difficult. Moreover, the proximity of the second premolar root presented a risk of obstructing canine root movement or suffering root resorption from unintended contact during traction. These concerns necessitated meticulous treatment planning and precise biomechanical execution.
At the initial stage of treatment, the crown of the maxillary left impacted canine was surgically exposed using the closed-eruption technique. A button was bonded to the crown, and a micro-screw was vertically inserted into the alveolar bone between the maxillary left second premolar and first molar. A segmental archwire was used to connect the micro-screw to the button, and directional force was applied by bending the wire buccally and occlusally to guide the eruption pathway of the canine.
Alternative methods, such as cantilevers or long-arm wires inserted into the auxiliary tube of a molar band, are sometimes employed for canine traction. These are typically activated by tying the wire to the canine attachment, generating a second-order couple in the molar tube that tends to tip the molar crown mesially. In addition, cantilever activation produces extrusive force on the impacted canine and intrusive force on the molar, potentially resulting in undesirable side effects (Lindauer and Isaacson, 1995). Similarly, some orthodontists use a Nance appliance (Mannathoko‑Molefhe and Hu, 2015; Schroeder et al., 2019) to deliver traction forces, although this method can also contribute to molar intrusion.
In the present case, a mini-screw was selected to provide absolute anchorage for the application of buccal and occlusal traction forces to the impacted canine, thereby completely eliminating reciprocal side effects on the posterior teeth. Compared with conventional cantilever-based systems, the boot-loop plus mini-screw configuration allowed the delivery of a localized and well-controlled force system that was independent of molar anchorage. The long distance between the impacted canine and the mini-screw also resulted in a low load–deflection rate, enabling light and continuous force delivery during the uprighting process.
Another reason for not bonding the full dentition at the initial stage was the unerupted status of the maxillary right canine and left lateral incisor. The root of the left lateral incisor was inclined palatally due to pressure from the buccal surface of the impacted canine’s crown. Early relief of this buccal pressure was essential, as it enabled the left lateral incisor to erupt spontaneously. By using a stainless-steel segmental archwire, the impacted canine was simultaneously moved occlusally and buccally. As a result, the left lateral incisor erupted naturally without any additional intervention.
Once the buccal surface of the impacted canine emerged from the mucosa, the next challenge was to upright the root within the alveolar bone while avoiding interference with adjacent roots. Initially, the button provided only a single extrusive force, which resulted in tipping movements rather than controlled root uprighting. To overcome this, the button was replaced with a bracket, and the straight segmental archwire was substituted with one incorporating a boot loop. The horizontal free end of the boot loop was ligated into the bracket slot and a tip-back bend was applied to generate an uprighting force (Figure 8).
Illustration of the biomechanical system for canine uprighting using a boot loop and micro-screw. (a) Schematic representation of the boot-loop–micro-screw biomechanics: (1) micro-screw with cross-shaped slot on the top cap. (2) Cross-shaped end of the segment wire fitting into the micro-screw slot, made of 0.016 × 0.022-inch stainless steel. (3) Boot loop. (4) Horizontal free end of the boot loop. Uprighting was activated by adjusting a tip-back bend to create an angle of approximately 15° between the free end and the bracket slot. (5) Mechanical couple at the bracket, producing an anticlockwise rotation of the impacted canine. (6) Clockwise internal reaction acting on the free arm of the archwire. (7) Extrusive load transmitted to the micro-screw via the archwire as a result of the clockwise reaction. (8) Center of resistance of the impacted canine. (b) As uprighting forces were applied, the canine root was uprighted. (c) Enlarged clinical photograph of the micro-screw head (cap) used in this case, showing its detailed morphology. With occlusal traction from the micro-screw, the canine was pulled toward the occlusal level and subsequently aligned to the maxillary arch using nickel–titanium wire. (d) Schematic model illustrating the cap and the full body of the micro-screw employed for anchorage.
The uprighting activation was delivered as a mechanical couple at the bracket, consisting of two equal and opposite forces separated by a small lever arm and producing a pure moment that rotates the tooth around the bracket (center of rotation), independent of the tooth’s center of resistance. According to Newton’s third law, the reaction forces exerted by the wire on its anchorage are equal in magnitude and opposite in direction to the forces applied to the tooth. In the configuration used here, the active couple produces an anticlockwise rotation of the canine (as illustrated in Figure 8a); consequently, the internal reaction acting on the free arm of the archwire is clockwise. This clockwise reaction, transmitted along the wire to the mini-screw, results in a downward extrusive force acting on the screw head. In short, because the mechanic is a pure couple applied at the bracket, the mini-screw experiences an extrusive reaction rather than a net rotational torque about the screw head. Clinically, this loading pattern proved stable and did not result in screw loosening during the treatment course.
The boot loop design used in this case was biomechanically analogous to a single loop in the MEAW system. Because the distance between the impacted canine and the micro-screw was substantially greater than a typical interbracket span, this configuration likely produced a lower load–deflection rate and a relatively gentle, continuous uprighting force compared with conventional MEAW mechanics.
Prolonged intervals between activations—at times up to 7 months—resulted in consistent light uprighting force without frequent adjustments. This uninterrupted light force facilitated significant root uprighting, improving the canine’s angulation by 73° (from 60° distal to 13° mesial). However, the lack of regular monitoring also contributed to over-uprighting, highlighting the importance of the gentle, consistent force generated by the boot loop, which provides favorable biomechanics.
In this case, the canine exhibited approximately 13° of mesial angulation after the root was uprighted within the alveolar bone. This angulation did not interfere with adjacent teeth or occlusal alignment during eruption; therefore, no active correction was required. The slight over-uprighting likely resulted from the prolonged interval between activations during the COVID-19 quarantine period.
Under normal circumstances, regular follow-up visits and panoramic radiographic monitoring help maintain appropriate root angulation, and insufficient uprighting is usually of greater concern than excessive uprighting.
If over-uprighting is detected early, the boot loop can simply be released from the tooth, allowing spontaneous relapse toward the desired axis. Counter-bends would be reserved only for severe cases in which the crown risks collapsing against adjacent teeth.
Once the root of the impacted canine was successfully uprighted, the subsequent steps became more straightforward. The canine needed to be brought to the occlusal level and aligned into the dental arch. The boot loop continued to serve as the primary force delivery system, now adjusted to generate vertical traction while conserving vertical anchorage. The horizontal free end of the loop was bent occlusally and ligated into the bracket slot, allowing the micro-screw to deliver the required force to guide the canine into position. After the tooth reached the occlusal level, final alignment was completed using NiTi wires.
Conventional methods for impacted tooth traction—such as the transpalatal arch, Nance button (Schroeder et al., 2019), or modified Nance appliance (Mannathoko‑Molefhe and Hu, 2015)—typically employ traction hooks attached to molar bands or palatal acrylic buttons to apply force from various directions. However, these techniques primarily apply force to the crown, with limited ability to control root positioning, often leading to unintended root resorption of adjacent teeth (Ferreira et al., 2017).
By contrast, TADs, such as micro-screws, provide absolute anchorage and can deliver force vectors closer to the tooth’s center of resistance, thereby improving root control. During the initial traction phase using the short segmental wire, the intended vector on the canine was downward (occlusal) and buccal; the reaction forces acting on the micro-screw were therefore in the opposite direction and were absorbed as compressive and intrusive loads. When the boot loop was engaged to apply a tip-back moment for root uprighting, the micro-screw functioned as the sole anchorage unit, resisting the reciprocal forces and moments generated by the loop—primarily in the direction of extrusion. No auxiliary dental anchorage (e.g. transpalatal arch or headgear) was required, as all anchorage was provided by the micro-screw. This TAD-supported strategy minimizes unwanted tooth movements and anchorage loss compared with conventional dental anchorage, consistent with evidence showing superior anchorage preservation with TADs (Khlef et al., 2018).
In the present case, combining a boot loop and micro-screw in this case enabled precise, multi-directional control of both crown and root movements. This biomechanical approach allowed the impacted canine to be positioned successfully without causing root resorption, gingival recession, or loss of periodontal support. In addition, the micro-screw–based system provided maximum anchorage control throughout treatment, further contributing to its favorable outcome. Furthermore, patient-reported outcomes regarding TAD-based anchorage have been shown to be generally favorable, with only mild and transient discomfort after insertion (Mousa et al., 2023).
Limitations
This report describes a single clinical case; therefore, the findings should be interpreted with caution. The overall treatment period was prolonged due to delayed eruption and extended intervals between follow-up visits, which may have influenced the timing of outcomes. In addition, the success of this approach may be partly operator-dependent, as precise design and activation of the boot-loop mechanics require clinical experience.
Conclusion
In this case, the combination of a boot loop and micro-screw provided a feasible means of generating controlled, multidirectional force for the management of a severely angulated impacted canine. Although the outcome in this patient was favorable, further controlled clinical studies are required to determine the predictability, safety, and broader applicability of this technique.
Patient’s perspective
‘I live quite far from the hospital where I received my orthodontic treatment. The local clinic could not treat my impacted canine and suggested it might need to be extracted, so my family decided to travel to the specialist hospital. Every visit required us to come a day earlier, which was sometimes inconvenient, but we were determined to save the tooth. The whole treatment took about four and a half years, but when I finally saw all my teeth aligned, I felt it was worth the effort. I am especially grateful to my orthodontist and the team for their patient care and persistence throughout this long journey. I’m happy that we chose this treatment because I was able to keep my natural tooth, and I’m very satisfied with the result.’
Footnotes
Ethical considerations
Ethical approval for this case treatment was obtained from the Stomatological Hospital of Dalian University (Dalian Stomatological Hospital) Ethics Committee (Approval No. DLKQLL20240110).
Informed Consent
Written informed consent for treatment and for publication of clinical details and images was obtained from the patient and the patient’s legal guardian.
Author contributions
RX performed orthodontic treatment, collected clinical data, and drafted the manuscript. RD performed orthodontic treatment, prepared cephalometric tracings, and edited clinical photographs.
XZ performed orthodontic treatment and prepared illustrative figures. XQ performed orthodontic treatment, biomechanical interpretation, figure revision, and edited clinical photographs. JW performed orthodontic treatment, contributed to treatment planning discussions, and assisted in data interpretation. YL performed endodontic treatment. MW designed the treatment plan, oversaw clinical execution, analyzed treatment outcomes, critically revised the manuscript, and approved the final version.
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Data availability statement
Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.