CBCT Analysis of Maxillary Incisor Root Resorption and Incisive Canal Changes After Clear Aligner Therapy Considering Demographic and Skeletal Patterns

1. INTRODUCTION

The maxillary central incisors (U1) play a pivotal role not only in dental esthetics but also in functions, such as articulation and mastication [1, 2]. Precise three-dimensional (3D) positioning of these teeth is therefore fundamental for accurate orthodontic diagnosis and treatment planning. In many cases involving dental protrusion or skeletal imbalances, retraction of the upper central incisors is a common therapeutic goal aimed at improving facial profile and occlusal relationships [3, 4].

However, the extent to which these teeth can be repositioned is constrained by anatomical and biomechanical limitations. These include the periodontium, the surrounding alveolar bone, and anchorage-dependent mechanics [3, 5, 6]. The widely referenced “envelope of discrepancy” outlines the physiological boundaries for orthodontic tooth movement, typically permitting retraction of up to 7 mm, protraction by 2 mm, and extrusion or intrusion by approximately 2–4 mm [3, 7]. In addition, a previous study of treated orthognathic cases has indicated the boundaries of overjet and sagittal skeletal discrepancy for these cases [8]. The mean overjet and reverse overjet for Class II and III malocclusions were 6.96 mm (95% CI, 6.40–7.53 mm) and 3.26 mm (95% CI, 2.87–3.65 mm), respectively [8]. These values, which are not markedly different from the “envelope of discrepancy” limits, further highlight the anatomical constraints in sagittal correction [8]. Notably, an often-overlooked constraint is the incisive canal (IC), located directly posterior to the central incisor roots, and enveloped by a dense cortical bone layer [9-11]. This anatomical proximity can restrict incisor movement and potentially pose clinical risks.

Of particular concern is the potential contact between the incisor roots and the IC during retraction, which has been associated with increased risk of apical root resorption, a serious complication that can undermine the long-term success of treatment [12-14]. Despite its relevance, the spatial relationship between the IC and U1 remains underexplored in the orthodontic literature, particularly through high-resolution 3D imaging, such as cone beam computed tomography (CBCT). Evidence from earlier studies suggests that excessive retraction may cause the incisor roots to impinge upon the IC, resulting in varying degrees of root damage and resorption [15, 16].

As clear aligner systems become increasingly popular, understanding how this treatment modality influences IC-root dynamics is critical. Unlike fixed appliances, aligners apply forces in a different manner, potentially resulting in distinct patterns of tooth movement and bone remodeling. However, there remains a paucity of data on how clear aligners affect the IC-U1 relationship.

This study seeks to bridge that gap by investigating changes in IC morphology, U1 spatial positioning relative to the IC, and associated root resorption following extraction-based retraction using clear aligners. Additionally, it examines how these outcomes are modulated by gender, age, and skeletal characteristics. By focusing on a demographically diverse patient sample, the study aims to enhance current clinical understanding and inform more individualized treatment planning in clear aligner therapy.

2. MATERIALS AND METHODS

2.2. Radiographic Assessment and Measurements

All CBCT scans were obtained using a standardized imaging protocol (120 kVp, 5 mAs, voxel size: 0.3 mm) on a KaVo CBCT unit (KaVo® Dental GmbH, Germany), with images oriented parallel to the Frankfort horizontal plane. Analysis of the IC and U1 was performed on axial and sagittal views using Invivo software (version 6, Anatomage, San Jose, CA).

Three axial levels-H1 (2 mm), H2 (4 mm), and H3 (6 mm) above the labial cementoenamel junction of U1-were used to evaluate spatial relationships (Fig. 1A) [17, 19-21]. At each level, IC width (ICW) was measured as the maximum horizontal diameter through the radiolucent center (IC-Cen). The root-IC distance was defined as the shortest linear distance from the incisor root surface to the outer cortical border of the IC. Cortical bone width (CBW) was measured along the same axis from IC-Cen outward, and CBW-mid was assessed in the midsagittal slice (Fig. 1B).

The root of U1 was measured in mesiodistal and labiopalatal dimensions at the most medial (U1W-m), posterior (U1W-p), and labial-palatal (U1W-lp) aspects. Inter-root distances were recorded as Rm-Rm (most medial points) and Rp-Rp (posterior points) between the two central incisors (Fig. 1B). U1 root length was measured from the incisal tip to the apex in the sagittal plane. IC height was calculated as the vertical distance from the incisive foramen to the palatal CEJ.

Based on CBCT evaluation, post-treatment root-to-IC relationships were categorized as follows: separation, increased distance between root and IC cortical boundary; approximation, root nearing the IC without contact; contact, root abutting the IC cortical margin (distance = 0, Fig. 2A-B); and invasion, root partially or fully penetrating the IC cortical plate or lumen (distance < 0), with remaining CBW noted as zero if the lumen was involved (Fig. 2C-D).

Root resorption was assessed through changes in U1 root length and width, comparing T1 and T2 (Fig. 3). Incisor movement was calculated via CBCT-based cranial base superimposition. Other cephalometric variables recorded included SNA angle, U1-SN angle, and overall treatment duration.

Table 1.

Item	Gender		Age Groups		Skeletal Sagittal Groups		Skeletal Facial Groups
	Male, (15)	Female, (25)	Teenagers (15)	Adults (25)	Class I (15)	Class II (25)	Low (10)	Average (13)	High (17)
Age, y	21.00±7.21	23.46±7.27	14.90±1.14	27.12±5.14	19.57±6.46	24.32±7.23	23.60±7.62	25.54±7.34	19.62±6.17
Treatment duration, y	3.23±1.46	2.75±1.27	2.73±1.43	3.05±1.31	2.59±1.09	3.14±1.46	2.85±1.08	2.38±1.33	3.40±1.40
U1 move, mm	-1.76±0.72	-2.43±2.14*	-1.64±1.46	-2.50±1.87	-1.04±1.33	-2.87±1.65*	-3.15±0.83*	-2.78±2.13	-1.15±1.34
ABO score
Overjet	1.00±1.20	2.08±1.55*	1.47±1.73	1.80±1.38	0.87±0.92	2.16±1.60*	2.60±0.84*	2.46±1.51	0.53±1.01
Overbite	1.27±1.44	1.88±1.54	0.93±1.03	2.08±1.61*	0.67±0.98	2.24±1.48*	2.80±2.04*	1.85±0.90	0.82±1.01
Anterior open bite	2.00±2.85*	0±0	1.53±2.53*	0.28±1.40	2.00±2.85*	0±0	0±0	0±0	1.76±2.75*
Posterior open bite	0±0	0±0	0±0	0±0	0±0	0±0	0±0	0±0	0±0
Crowding	0.60±0.83	0.48±0.77	0.87±0.92*	0.32±0.63	0.87±0.92*	0.32±0.63	0.40±0.84	0.46±0.78	0.65±0.79
Occlusion	4.27±2.81	4.16±1.62	3.87±2.97	4.40±1.41	4.27±3.10	4.16±1.28	4.40±2.07	3.69±0.75	4.47±2.79
Posterior crossbite	0±0	0±0	0±0	0±0	0±0	0±0	0±0	0±0	0±0
ANB	1.63±2.41	2.90±2.73	1.17±2.01	3.17±2.75*	0±0	3.87±2.37	3.40±3.10	2.31±2.66	1.93±2.38
SN-GoGn	0.44±0.76	0.47±0.97	0.94±1.08*	0.17±0.61	0.99±1.14*	0.15±0.51	0.87±1.14	0±0	0.58±0.95
IMPA	2.23±3.51	2.90±3.39	2.31±3.67	2.84±3.29	0.89±2.19	3.70±3.60*	5.74±3.53*	1.94±3.40	1.36±2.10
Total ABO score	16.06±6.07	15.27±6.29	14.81±4.53	16.02±6.98	12.36±5.64	17.49±5.70*	20.34±6.06*	13.41±5.10	14.41±5.69

Note: * p-values Significant at 0.05 level; Values for overjet, overbite, ANB, and other occlusal characteristics in this table are ABO discrepancy index scores. Corresponding raw cephalometric measurements (in mm or degrees) are provided in Supplementary Table 1 for reference.

3. RESULTS

Participant characteristics and baseline ABO discrepancy index scores were categorized by gender, age group, sagittal skeletal classification, and vertical skeletal pattern (Table 1 and Supplementary Table 1). Across subgroups, most baseline parameters showed comparable distributions.

4. DISCUSSION

This investigation explored how anterior retraction using clear aligners influences the morphology of the incisive canal, its spatial association with the maxillary central incisors, and the degree of root resorption in cases involving premolar extraction. Unlike earlier studies centered on fixed appliance therapy, this research offers novel insight into the biomechanical and anatomical effects of clear aligners, particularly when stratified by gender, age, and skeletal pattern.

Post-treatment, changes in IC width were evident, especially at the H1 and H2 reference levels. Among male patients, expansion was noted at H2, whereas female participants exhibited IC narrowing at both H1 and H2. These findings echo the concept that orthodontic forces can induce bone remodeling within the anterior maxilla, although responses appear to differ by sex, potentially due to hormonal factors and underlying bone density variation [22]. Interestingly, while teenagers and adults both showed reduced IC width at H1, their IC dimensions remained relatively stable otherwise, contrasting with earlier findings from fixed appliance studies that reported broader IC changes. This may reflect the more targeted and controlled forces exerted by clear aligners, which could result in localized rather than extensive skeletal remodeling.

Cortical bone measurements consistently decreased across genders and age groups, indicating the potential for retraction forces to alter cortical architecture, even in aligner-based mechanics. Notably, men demonstrated greater thinning at H2 and H3, whereas women had significant reductions at H1, reinforcing the idea that gender-specific physiological differences influence how bone responds to mechanical stress during orthodontic treatment [22].

A pronounced decline in U1 root-to-IC distance post-treatment was found in all subgroups, but this change was more marked in females and adults. This narrowing of spatial separation was accompanied by a greater reduction in U1 root length and width in these groups. Such trends align with prior research indicating that adult patients and females are more susceptible to orthodontic root resorption, likely due to variations in bone turnover rates and mechanical sensitivity [15, 23].

The sagittal skeletal pattern also influenced the extent of root displacement and resorption. Class II cases, characterized by more protrusive maxillary anatomy, experienced greater reductions in U1–IC distance and a higher incidence of root contact or invasion-particularly at H1 and H3. These results support the notion that anatomical predisposition in Class II patients may amplify the risk of root proximity to the IC when anterior teeth are retracted.

Vertical facial classification offered additional insights. High-angle patients showed the greatest drop in root-IC spacing, particularly at H1 and H2. These individuals, often exhibiting thinner alveolar bone, appear more vulnerable to root encroachment on the IC during retraction. This underscores the need for clinicians to consider vertical skeletal characteristics when planning incisor movement, especially in extraction protocols.

Overall, root resorption was more pronounced in females and Class II patients, with average resorption of 1.13±0.90 mm in females compared to 0.53±0.38 mm in males, and 1.06±0.79 mm in Class II versus 0.64±0.76 mm in Class I subjects. These results align with those of earlier studies, indicating that specific demographic and skeletal profiles are associated with a higher risk of adverse root changes. The combination of reduced root-IC spacing and increased IC contact likely contributes to elevated resorption rates in these groups.

When examining proximity patterns at three measurement levels (H1–H3), contact and invasion were most frequent at H1, while approximation dominated at H2 and H3. This aligns with findings linking increased resorption severity to closer IC-root relationships [15, 16, 19]. Gender-specific trends persisted, with males primarily showing approximation, and females experiencing more frequent contact and invasion. Similar patterns were observed across age and skeletal classifications: adults and Class II/high-angle patients exhibited more IC-root invasions, whereas teenagers and Class I/low-angle patients showed greater approximation without contact.

The clinical relevance of the current findings is substantial. Recognizing that specific groups, such as females, adults, and individuals with Class II or high-angle profiles, are more prone to root resorption and IC proximity helps clinicians tailor interventions accordingly. These patients may benefit from lighter retraction forces, reduced displacement targets, or more frequent radiographic monitoring to limit risks.

Our findings align with prior CBCT-based research on fixed appliances, which reported that maximal anterior retraction can reduce root–IC distance and increase resorption risk [15, 16, 24]. However, the present study demonstrates that clear aligners, despite their staged force application, can produce similar proximity changes in susceptible subgroups. In contrast to reports where fixed appliances caused more generalized IC remodeling, our results suggest that aligners appear to produce more localized changes, potentially due to differences in force vectors and torque control.

To mitigate the risk of root–IC contact, especially in females, Class II patients, and those with high-angle skeletal patterns, aligner treatment plans should incorporate strategies, such as reducing the retraction amount per stage, using optimized torque and root control attachments in the anterior segment, and employing mini-screw anchorage to facilitate bodily movement rather than tipping. Techniques such as pre-adjusted edgewise brackets with customized torque values or segmented arch mechanics may offer improved root positioning control, though these methods require careful biomechanical planning to avoid overloading or imprecision [25, 26]. In addition, increasing the frequency of radiographic monitoring in high-risk profiles and staging extraction space closure to allow adequate bone remodeling before final root positioning can further reduce the likelihood of adverse root–IC interactions.

The IC exhibits considerable variation in shape, ranging from Y-shaped and cylindrical to funnel-shaped morphologies, which may influence root proximity outcomes. Future studies should evaluate whether these morphological subtypes correlate with different resorption risks and adapt force protocols accordingly.

While the present findings provide meaningful insight into IC–root interactions during clear aligner therapy, they should be interpreted with caution. The retrospective design, limited subgroup sizes, and reliance on linear CBCT measurements mean that the results are preliminary and require confirmation in larger, prospective cohorts. Thus, clinical recommendations, such as routine CBCT screening or adjustments in retraction staging, should be viewed as provisional guidelines rather than definitive protocols. Compared with fixed appliances, where continuous forces often produce broader patterns of root resorption and more generalized IC remodeling, clear aligners appear to induce more localized changes, likely due to their segmented force delivery and torque expression. Nevertheless, the overall biomechanical risks, particularly in high-risk profiles, such as females, adults, and Class II or high-angle patients, remain similar between treatment modalities. Future work should integrate volumetric or morphometric assessments, account for potential confounders, such as treatment duration, compliance, and adjunctive anchorage use, and explore patient-specific factors like bone density and genetic susceptibility to further refine risk prediction and clinical decision-making.

5. STUDY LIMITATIONS

Several limitations warrant acknowledgment. First, the retrospective design introduces potential selection bias and restricts control over confounding variables such as treatment variability and patient compliance. Second, although the overall sample size was statistically justified, subdivision into gender, age, and skeletal subgroups reduced statistical power for certain analyses, increasing the risk of type II errors. Third, all outcomes were assessed using linear CBCT measurements; volumetric or surface-based morphometric approaches, as well as finite element modeling, would provide a more comprehensive 3D evaluation of IC and root changes. Fourth, while inter- and intra-rater reliability was high, potential observer bias cannot be entirely excluded. Fifth, the CBCT voxel size of 0.3 mm may underestimate subtle root resorption or cortical remodeling; future studies employing smaller voxel sizes may improve detection. Sixth, root morphology variation (e.g., conical, dilacerated forms), a known factor in resorption susceptibility, was not classified in this study and warrants future investigation. Finally, additional confounders such as treatment duration, aligner wear compliance, and use of adjunctive anchorage (e.g., mini-screws) were not systematically accounted for, which may have influenced outcomes.

Future prospective and randomized studies with larger, more balanced samples are needed to validate these findings. Incorporating volumetric analyses, standardized root morphology classifications, and direct comparisons with fixed appliance mechanics will provide deeper biomechanical insights. Moreover, emerging approaches, such as bone density assessment, genetic predisposition testing, and computational biomechanical simulations, may enhance the prediction of resorption risk and guide individualized treatment planning.

CONCLUSION

Clear aligner retraction of maxillary central incisors in extraction cases significantly influences incisor root resorption and proximity to the incisive canal, with variation by gender and skeletal pattern. Greater incisor retraction, particularly in females, Class II, and high-angle patients, increases the risk of root resorption and canal contact. Treatment should incorporate CBCT-based evaluation, reduced forces, and close monitoring in high-risk cases to minimize anatomical compromise.

REFERENCES