Background
This study aimed to evaluate the impact of maxillary sinus-root relationship (SRR) on orthodontic tooth movement and root resorption measured by using cone-beam computed tomography (CBCT) during first premolar extraction space closure in adults receiving traditional treatment (TT) and clear aligner therapy (CAT).
Methods
This retrospective study analyzed pre and posttreatment CBCT scans of 80 Chinese adults (type I vs type V SRR; TT vs CAT), with 40 patients in each SRR type and equal allocation to TT and CAT groups. All measurements were performed on CBCT images at T1 and T2 using InVivo 6.0.3 software (Anatomage, San Jose, Calif).
Results
Greater mesial molar movement was observed in type I SRR than in type V SRR. In the TT group, mesial root movement was greater in type I SRR (between-group mean difference [ΔΔT] =–1.24 mm; confidence interval =–2.35 to–0.13; P = 0.029). In the CAT group, type I SRR showed greater mesial root movement (ΔΔT =–1.12 mm; confidence interval =–1.84 to–0.41; P = 0.003) and larger cusp displacement at mesial and distal cusps (ΔΔT =–2.12 mm; ΔΔT =–1.54 mm). Within type I SRR, the CAT group produced greater cusp movement than TT (ΔΔT = 1.29 mm; ΔΔT = 0.90 mm). No significant TT vs CAT group differences were observed in type V SRR. All groups had significant root resorption; TT group was associated with greater resorption than CAT group, particularly in the second premolar (ΔΔT = 0.36 mm; P = 0.010), palatal root of the first molar in type I SRR (ΔΔT 0.37 mm; P = 0.006), and distobuccal root in type V SRR (ΔΔT = 0.30 mm; P = 0.013).
Conclusions
The findings suggest that molar movement may be greater when the sinus floor is farther from the roots, and CAT may induce more tipping, whereas TT may cause more root resorption. Sinus proximity did not appear to increase resorption risk. However, the imbalance in treatment duration between TT and CAT remains a potentially biasing limitation.
Highlights
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This study examined how sinus position affects tooth movement and root changes.
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Eighty adults treated with traditional treatment or clear aligners after extraction were studied.
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Greater mesial molar movement occurred when the sinus floor was farther from the tooth roots.
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Traditional treatment showed more root resorption, whereas sinus proximity posed no added risk.
Orthodontic tooth movement (OTM) occurs when mechanical forces applied to the teeth stimulate remodeling of the surrounding alveolar bone. After the first premolar extraction, the maxillary first molars commonly undergo substantial movement, including translation and tipping. Among anatomic factors influencing this process, the proximity of the maxillary sinus floor (MSF) to molar roots, referred to as the sinus-root relationship (SRR), is particularly relevant. , The MSF often pneumatizes into the alveolar process, bringing it close to posterior root apices, which may influence biomechanics and treatment outcomes. Kwak et al described an SRR classification with 5 spatial relationships between the MSF and molar roots, from no contact (type I) to bilateral root protrusion into the sinus (type V). Extreme forms, such as type V, may restrict tooth movement and increase apical stress, potentially raising the risk of orthodontically-induced root resorption (OIRR). ,
Despite the clinical importance of SRR, 3-dimensional (3D) cone-beam computed tomography (CBCT) studies comparing tooth movement and root resorption between traditional treatment (TT) and clear aligner therapy (CAT) across distinct SRR patterns remain scarce. , CAT has become a popular alternative to TT, offering improved oral hygiene, esthetics, and patient comfort, but its biomechanical behavior in anatomically challenging cases, such as sinus-involved SRRs, is still under investigation. , A previous 2-dimensional study reported that the MSF did not significantly influence molar movement under TT. Research on the association between MSF proximity and external apical root resorption also remains inconclusive. Recent CAT-based findings suggest that root position relative to the MSF may not substantially affect resorption, with OIRR generally <1 mm and clinically insignificant. However, most existing work focuses on molar distalization rather than premolar extraction space closure, and heterogeneity in methods and samples limits interpretation.
To address this gap, the present study focused on SRR types I and V, representing 2 anatomically contrasting conditions in which molar roots are either distant from or protrude into the sinus. This selection allows evaluation of the maximum variation in biomechanical behavior and potential differences in treatment outcomes between TT and CAT.
This study aimed to evaluate how the type of SRR and treatment modality affect maxillary first molar movement and root resorption in second premolars and first molars during space closure after first premolar extraction. This was achieved by analyzing pre and posttreatment CBCT data using InVivo 6.0.3 software (Anatomage, San Jose, Calif) in a sample of adult patients. We hypothesized that the type of SRR influences both the magnitude and pattern of molar movement and the degree of OIRR, and that CAT would produce less resorption than TT.
Material and methods
In this retrospective study, 80 consecutive Chinese adult patients who underwent orthodontic treatment at the Lanzhou University Department of Orthodontics, School of Stomatology, between 2013 and 2024, were included. All procedures were performed in accordance with the Declaration of Helsinki. Ethical approval was obtained from the ethics committee of the School of Stomatology, Lanzhou University, Gansu Province, China (Approval No. LZUKQ-2025-065).
The following were the requirements for inclusion: (1) patients who presented with skeletal Class I or II malocclusion; (2) patients who had CBCT scans taken before and after treatment; and (3) patients whose maxillary first premolars were extracted during orthodontic treatment primarily to correct bimaxillary dentoalveolar protrusion accompanied by mild to moderate dental crowding. Those who had dentofacial trauma, ≥1 congenital or acquired missing tooth, had undergone orthodontic treatment, severe crowding in the maxillary anteriors, craniofacial anomalies, had a treatment plan other than first premolar extraction, any extraoral or intraoral anchorage device, an SRR type other than types I and V, or had distorted CBCT images were excluded. Accordingly, posterior tooth movement evaluated in this study predominantly reflects anchorage response during anterior retraction after premolar extraction, rather than posterior space usage for relief of severe crowding.
Each subgroup consisted of 20 patients, resulting in a total sample of 80 patients (40 patients for the primary comparison). Based on the observed pooled standard deviation (SD) of the ΔT measurements (SD = 1.93 mm) and a clinically meaningful difference of 2 mm, a 2-sample power analysis indicated 89% power for this comparison (power t test: n = 20, delta = 2, SD = 1.93, and significance level = 0.05). Therefore, a sample size of 20 patients per subgroup was considered sufficient to detect statistically significant differences.
Patients were evaluated using pretreatment CBCT and were categorized into the treatment groups according to type I and V of SRR classifications ( Fig 1 ). Type I SRR, defined as the sinus floor above the line connecting buccal and palatal root apices, was assigned to the control group of 40 patients; 20 patients underwent TT, and the remaining 20 underwent CAT. In contrast, in the sinus group, the buccal and palatal roots exhibited apical extension to the sinus floor, corresponding to type V SRR. This group also comprised 40 patients, with 20 patients treated using TT and the remaining 20 using CAT. SRR types II-IV were excluded because they represent intermediate or mixed anatomic patterns that would introduce greater variability and reduce group homogeneity. The difference between dental width and dental arch length was used to determine the degree of crowding: mild (0-3 mm), moderate (4-8 mm), and severe (>8 mm). Patients under TT had full-mouth orthodontic treatment using standard fixed straight wire appliances with slots measuring 0.022 × 0.028-in, from their right to their left second molar. Arch wires made of 0.014-in nickel-titanium and 0.016 × 0.022-in stainless steel were used. Subsequent stainless steel wires were placed after the initial leveling and alignment with nickel-titanium wires. Canine retraction using a power chain and sliding mechanism, followed by anterior retraction using nonfriction closing loops, allowed for 2 stages of tooth retraction. During space closure, the estimated force magnitude applied was approximately 75-150 g per side, which is widely regarded as an optimal range for controlled bodily tooth movement. No additional anchorage preparations were made except for banding the maxillary first molar with the transpalatal arch on each side. The average treatment duration was 36.85 ± 7.68 months in the control group TT type I SRR and 39.45 ± 9.74 months in the sinus group TT type V SRR, after which all the extraction spaces were closed. For patients with CAT, the treatment protocol included using attachments that were placed after the attachment protocol of Align Technology (Santa Clara, Calif) to achieve predictable tooth movement. The aligners were designed to move the teeth in small increments, typically 0.1-0.2 mm per aligner. Sequential tooth movement was employed to close the extraction spaces: canines were first retracted into the spaces, followed by collective retraction of the incisors, and this alternating sequence continued until full space closure. Power ridges on the incisors helped control root torque, and attachments on canines and posterior teeth guided rotation and root positioning. During treatment, patients were instructed to wear their aligners for a minimum of 22 hd for 7-14 days before progressing to the next stage. In addition, no adjunctive skeletal anchorage was used. Elastics were used while retracting the premolars, canines, and incisors to prevent the uncontrolled proclination of the anterior teeth and reinforce the anchorage. The average treatment duration was 52.17 ± 7.99 months for the control group (CAT type I SRR) and 48.60 ± 4.61 months for the sinus group (CAT type V SRR), after which all the extraction spaces were closed ( Table I ).
Type I SRR was identified when the MSF was located above the line connecting the apices of the buccal and palatal roots of the maxillary first molar, indicating no contact. Type V SRR was identified when the apices of both the buccal and palatal roots protruded beyond the sinus floor, indicating full penetration into the sinus cavity.
Table I
Summary of key orthodontic treatment characteristics
| Characteristic | TT | CAT |
|---|---|---|
| Appliance type | Fixed straight-wire appliance (0.022 × 0.028-in slot) | Clear aligners with attachments (Align Technology, Santa Clara, Calif) |
| General force strategy | Continuous light forces using sliding mechanics and nonfriction closing loops; estimated force magnitude approximately 75-150 g per side | Intermittent light forces via incremental aligner activation (0.1-0.2 mm per aligner) |
| Average treatment duration (type I SRR) | 36.85 ± 7.68 mo | 52.17 ± 7.99 mo |
| Average treatment duration (type V SRR) | 39.45 ± 9.74 mo | 48.60 ± 4.61 mo |
Note. Summary of key treatment characteristics for TT and CAT, including appliance type, overall force application strategy, and average treatment duration stratified by SRR type. Detailed appliance protocols and biomechanical procedures are provided in the Methods section.
CBCT was performed using the I-CAT Imaging System (Imaging Sciences International Inc, Hatfield, Pa). The standard protocol was followed for scanning all patients: image acquisition at 0.4 mm voxel size, exposure parameter setting of 18.54 mAs, 8.9 seconds, 120 kV, and a field of view of 16.0 × 13.0 cm. In addition, with maximal occlusal intercuspation, the Frankfort horizontal plane is parallel to the floor and standardizes the head posture. The patients were instructed to refrain from eating or moving during the scanning procedure in accordance with the imaging protocol.
This study used CBCT imaging to obtain detailed and accurate 3D measurements of both OTM and root resorption. These 3D assessments enable more personalized orthodontic treatment planning based on individual variations in sinus anatomy. All measurements were performed using the InVivo 6.0.3 software (Anatomage). For OTM analysis, the reference coordinate system was established by defining the true horizontal plane as 7° to the sella-nasion (SN) plane and the true vertical as perpendicular to it, passing through the sella. A total of 8 variables were measured: 6 linear and 2 angular measurements ( Fig 2 , A ). These landmarks and measurements were adapted from a validated 3D study conducted on Chinese patients ( Fig 2 , B ). CBCT scans were evaluated at 2 time points: the earliest pretreatment scan (T1) and the most recent posttreatment scan (T2). Root resorption analysis was also performed at T1 and T2 using 3D CBCT reconstructions in the InVivo 6.0.3 software (Anatomage) ( Fig 3 ), employing the ENDO module for detailed root length evaluation. Measurements were obtained for the maxillary second premolar (P2) and the 3 roots of the maxillary first molar: mesiobuccal (6-MB), distobuccal (6-DB), and palatal (6-P). To standardize the methodology, a horizontal reference line was drawn at the widest mesiodistal dimension of the crown. Root lengths were measured from the midpoint of this line to the apex of each root.
A, True vertical reference line was constructed as a line through sella perpendicular to the true horizontal, which was defined as 7° to the SN plane. Eight measurements were obtained: 6 linear and 2 angular from the maxillary landmarks to the true vertical (a, IRA; b, IEE; c, MMR; d, MDR; e, MMC; f, MDC; g, U1-SN; and h, U6-SN); B, Slice locator illustrating how 3D landmarks were identified across CBCT planes. IRA , maxillary incisor root apex; IEE , maxillary incisor incisal edge; MDR , maxillary first molar distal root apex; MMC , maxillary first molar mesial cusp tip; MDC , maxillary first molar distal cusp tip; U1-SN , angular measurement from the SN plane to the long axis of the maxillary central incisor; U6-SN , angular measurement from the SN plane to the long axis of the maxillary first molar.
A standardized horizontal reference line was constructed at the level of the widest mesiodistal dimension of the second premolar and first molar crowns. Root lengths were measured from the midpoint of this reference line to the apex of each root using 3D reconstructed models.
For reliability assessment, a random subset of 20 CBCT scans was remeasured by the same examiner 2 weeks later and by a second examiner independently. All measures were overseen by experts with ≥5 years of experience who used the same software Intraexaminer reliability was evaluated using a 2-way mixed-effects model with absolute agreement for single measures (intraclass correlation coefficient = 3,1). The analysis showed excellent repeatability, with an intraclass correlation coefficient of 0.925 and a 95% confidence interval (CI) of 0.820-0.970, confirming that landmark identification and measurements were highly consistent across repeated assessments.
Statistical analysis
All analyses were performed using the R software (version 4.4.3; R Foundation for Statistical Computing, Vienna, Austria). For each patient, T1 and T2 scans were used. Age at T1 and the time interval between scans were recorded. Treatment duration was considered descriptively when interpreting differences in tooth movement and root resorption; no analytical adjustment for treatment duration was applied. Change values for each measurement were calculated as Δ = T2 − T1. The primary outcomes were changes in maxillary first molar movement and root resorption comparing SRR type I and type V within each treatment modality. Comparisons between TT and CAT within the same SRR type were considered exploratory. The Shapiro-Wilk test was applied to assess the normality of the paired differences. When normality was satisfied ( P >0.05), a paired-samples t test was used to compare T1 and T2 within groups; otherwise, the Wilcoxon signed-rank test was applied. Between-group comparisons were performed on the change scores (Δ). If both groups met the normality assumption, an independent 2-sample t test assuming equal variances was used; otherwise, the Mann-Whitney U test was performed. Results are reported as means ± SD, mean ΔT for each group, the between-group mean difference (ΔΔT), and 95% CIs for ΔΔT, along with 2-sided P values, with statistical significance defined as α = 0.05. Statistically significant findings were interpreted alongside both effect size and absolute differences to ensure that results were evaluated for clinical as well as statistical relevance. For maxillary molar movement, changes of ≥1 mm were considered potentially clinically relevant. For root resorption, reductions of ≥1-2 mm were considered potentially clinically relevant, based on Sharpe, who defined 1-2 mm as mild (1° severity); smaller changes were deemed clinically insignificant. Within-group comparisons were summarized in tables, whereas between-group differences were illustrated using forest plots. Standardized effect sizes (Cohen d ) were additionally calculated for the between-group comparison of ΔT. Statistical significance was denoted by stars (∗ P <0.05; ∗∗ P <0.01; ∗∗∗ P <0.001; ns, not significant).
Results
This retrospective study analyzed a total of 80 patients using pre and posttreatment CBCT scans. Patients were divided into 2 groups based on the SRR of the maxillary first molar. The control group (type I SRR, in which roots are distant from the sinus floor) included 40 patients. Twenty were treated with TT (5 men and 15 women, mean age = 22.87 ± 4.67 years) and 20 with CAT (2 men and 18 women, mean age = 23.16 ± 2.96 years). The sinus group (type V SRR, in which roots protrude into the sinus floor) also included 40 patients. Twenty were treated with TT (7 men and 13 women, mean age = 20.08 ± 2.67 years) and 20 with CAT (2 men and 18 women, mean age = 21.54 ± 3.17 years). The mean duration of active orthodontic treatment was 36.85 ± 7.68 months in TT-type I SRR and 39.45 ± 9.74 months in TT-type V SRR, and 52.17 ± 7.99 months in CAT-type I SRR and 48.60 ± 4.61 months in CAT-type V SRR ( Table II ).
Table II
Demographic data of the participants
| Comparison | Variable | N | Group 1 | Group 2 | P value |
|---|---|---|---|---|---|
| TT I vs TT V | Age (y) | 20 vs 20 | 22.87 ± 4.67 | 20.08 ± 2.67 | 0.208 |
| Duration (mo) | 20 vs 20 | 36.85 ± 7.68 | 39.45 ± 9.74 | 0.353 | |
| Sex | 20 vs 20 | 15 F (75%)/5 M (25%) | 13 F (65%)/7 M (35%) | 0.730 | |
| CAT I vs CAT V | Age (y) | 20 vs 20 | 23.16 ± 2.96 | 21.54 ± 3.17 | 0.079 |
| Duration (mo) | 20 vs 20 | 52.17 ± 7.99 | 48.60 ± 4.61 | 0.092 | |
| Sex | 20 vs 20 | 18 F (90%)/2 M (10%) | 18 F (90%)/2 M (10%) | 1.000 | |
| TT I vs CAT I | Age (y) | 20 vs 20 | 22.87 ± 4.67 | 23.16 ± 2.96 | 0.473 |
| Duration (mo) | 20 vs 20 | 36.85 ± 7.68 | 52.17 ± 7.99 | <0.001 | |
| Sex | 20 vs 20 | 15 F (75%)/5 M (25%) | 18 F (90%)/2 M (10%) | 0.407 | |
| TT V vs CAT V | Age (y) | 20 vs 20 | 20.08 ± 2.67 | 21.54 ± 3.17 | 0.310 |
| Duration (mo) | 20 vs 20 | 39.45 ± 9.74 | 48.60 ± 4.61 | 0.001 | |
| Sex | 20 vs 20 | 13 F (65%)/7 M (35%) | 18 F (90%)/2 M (10%) | 0.127 |
Note. Values are presented as mean ± SD.
F , females; M , males.
TT I , control group receiving TT included patients with type I SRR; TT V , sinus group receiving TT included patients with type V SRR; CAT I , control group receiving CAT included patients with type I SRR; CAT V , sinus group receiving CAT included patients with type V SRR.
In TT groups, type I SRR showed greater root and cusp displacement, with minimal angular change in the angular measurement from the SN plane to the long axis of the maxillary first molar (U6-SN) (T1 = 72.69° ± 5.48°; T2 = 72.95° ± 6.89°). In contrast, type V SRR showed reduced root displacement and slightly greater U6-SN change (T1 = 75.72° ± 5.19°; T2 = 76.96° ± 5.58°; P = 0.063), suggesting a tendency toward tipping ( Table III ). In CAT groups, type I SRR showed significant root and cusp displacement, with a notable angular increase in U6-SN (T1 = 72.34° ± 7.14°; T2 = 75.77° ± 4.15°; P = 0.003), indicating more tipping than TT. In contrast, type V SRR showed slightly reduced but still significant displacement and minimal angular change in U6-SN (T1 = 73.47° ± 6.07°; T2 = 74.76° ± 6.37°; P = 0.145) ( Table IV ). Across all groups, incisor inclination (U1-SN) decreased significantly after treatment, and both mesial and distal molar cusps and roots showed greater mesial movement in type I SRR than in type V SRR. In comparison between groups, patients with type I SRR showed greater mesial molar movement than those with type V SRR. Patients with TT type I SRR exhibited significantly greater mesial displacement of the molar mesial root (MMR) than patients with TT type V (ΔΔT =–1.24 mm; CI =–2.35 to–0.13; P = 0.029), exceeding the ≥1 mm threshold and considered potentially clinically relevant ( Fig 4 , A ). In contrast, patients with CAT type I SRR showed significantly greater mesial movement of the MMR (ΔΔT =–1.12 mm; CI =–1.84 to–0.41; P = 0.003) and greater mesial cusp displacement of the molar mesial cusp (MMC; ΔΔT =–2.12 mm; CI =–2.66 to–1.57) and molar distal cusp (MDC; ΔΔT =–1.54 mm; CI =–2.10 to–0.99), both P <0.001, all exceeding the predefined threshold for potential clinical relevance ( Fig 4 , B ). Within type I SRR, TT and CAT had similar root apex displacement. However, CAT produced significantly greater mesial movement of cusp tips MMC (ΔΔT = 1.29 mm; CI = 0.40-2.18; P = 0.006), MDC (ΔΔT = 0.90 mm; CI = 0.05-1.74; P = 0.038), and a greater angular increase in U6-SN (ΔΔT = 3.18°; CI =–0.06 o 6.41; nearly significant at P = 0.054), indicating potentially clinically relevant crown movement ( Fig 4 , C ). In type V SRR, no significant differences were observed between TT and CAT ( Fig 4 , D ). Cohen d effect sizes for the primary between-group comparisons in OTM are presented in Table V .
Table III
Comparison of T1 and T2 in OTM
| Control group TT I | Sinus group TT V | |||||||
|---|---|---|---|---|---|---|---|---|
| Variable | T1 | T2 | ΔT | P value | T1 | T2 | ΔT | P value |
| IRA (mm) | 56.58 ± 4.04 | 55.96 ± 4.61 | –0.62 ± 1.88 | 0.157 | 57.76 ± 3.11 | 57.38 ± 3.33 | –0.39 ± 0.93 | 0.079 |
| IEE (mm) | 65.02 ± 5.81 | 61.31 ± 4.51 | –3.71 ± 2.62 | <0.001 | 67.28 ± 6.52 | 62.84 ± 4.78 | –4.43 ± 3.74 | <0.001 |
| MMR (mm) | 39.25 ± 3.65 | 42.12 ± 3.63 | 2.87 ± 1.87 | <0.001 | 38.99 ± 2.64 | 40.62 ± 2.19 | 1.63 ± 1.59 | <0.001 |
| MDR (mm) | 36.14 ± 3.35 | 38.49 ± 3.46 | 2.35 ± 1.53 | <0.001 | 36.11 ± 2.32 | 37.53 ± 2.01 | 1.42 ± 1.57 | <0.001 |
| MMC (mm) | 37.30 ± 4.94 | 40.02 ± 4.60 | 2.72 ± 1.86 | <0.001 | 38.00 ± 3.35 | 39.99 ± 3.20 | 1.99 ± 1.82 | <0.001 |
| MDC (mm) | 32.74 ± 4.76 | 35.59 ± 4.43 | 2.84 ± 1.70 | <0.001 | 33.27 ± 3.05 | 35.28 ± 3.09 | 2.00 ± 1.74 | <0.001 |
| U1-SN (°) | 107.29 ± 7.86 | 99.59 ± 5.04 | –7.70 ± 8.67 | <0.001 | 110.53 ± 11.08 | 102.15 ± 6.60 | –8.39 ± 8.97 | <0.001 |
| U6-SN (°) | 72.69 ± 5.48 | 72.95 ± 6.89 | 0.26 ± 5.56 | 0.838 | 75.72 ± 5.19 | 76.96 ± 5.58 | 1.24 ± 2.80 | 0.063 |
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