Abstract
Introduction
The relationship between maxillary posterior tooth roots and the maxillary sinus floor is a key risk factor in orthodontic treatment. This study systematically evaluated this relationship for the maxillary second premolar, first molar, and second molar in adults with different growing patterns.
Methods
This retrospective cross-sectional study analyzed CBCT data from 502 adults (20–45 years). Patients were grouped by sagittal pattern (ANB angle: Class I, II, III) and vertical skeletal pattern (MP-SN angle: hyperdivergent, normodivergent, hypodivergent). A modified classification (scores 1–5) assessed the vertical relationship between each tooth apex and the sinus floor. The effects of age, sex, and skeletal pattern on scores were analyzed.
Results
After adjusting for age and sex, the vertical pattern showed no significant association with root-sinus scores (P > 0.05). Sagittally, Class II patients had significantly lower first molar scores than Class I patients (P = 0.032), indicating roots positioned farther from the sinus floor. Age was significantly negatively correlated with scores (P < 0.001), and females had lower scores than males (P < 0.001). While scores among tooth positions were correlated at the population level, high individual heterogeneity was observed, with 43.4% of patients showing irregular fluctuations.
Conclusions
Vertical skeletal pattern is not significantly associated with the maxillary posterior root-sinus relationship in adults. Skeletal Class II patients exhibit a more distant maxillary first molar apex-to-sinus adjacency, indicating a static anatomical association. Increasing age is associated with more distant root-to-sinus adjacencies.
Keywords: Maxillary sinus-root relationship, Facial biotype, 3-dimensional, Adult orthodontics
Introduction
The anatomical relationship between the maxillary sinus floor (MSF) and the apices of maxillary posterior teeth is a critical consideration in orthodontic and orthognathic treatment planning [1]. Root protrusion into the sinus can increase the risk of root resorption, mucosal irritation, and infection during tooth movement [2, 3]. When extensive posterior movement is required, accurate diagnosis of this sinus-root relationship (SRR) is crucial for predicting treatment feasibility, duration, and potential complications [4–7]. Thus, preoperative evaluation of this proximity is of significant clinical importance.
Some studies suggest a link between facial skeletal morphology and SRR. Research indicates that in hypodivergent patients, maxillary second molars may be located farther from the MSF [8], while skeletal Class III patients may have a lower probability of root protrusion into the sinus [9]. However, these studies are often limited by specific age groups or single tooth types, and a comprehensive evaluation of SRR across different skeletal patterns in a large adult sample is lacking.
This study aims to systematically assess the anatomical relationship of the maxillary second premolars, first molars, and second molars with the MSF in different vertical and sagittal skeletal patterns using a large CBCT dataset. The influence of age and sex will also be explored to provide a more precise anatomical basis for clinical risk assessment.
Method
This retrospective cross-sectional study reviewed CBCT records from 10,937 patients at a tertiary stomatology hospital in Jiangsu, China, between April 2024 and August 2025. Inclusion criteria: adults (20–45 years) with permanent dentition and no prior orthodontic treatment. Exclusion criteria: missing target teeth or antagonists; periapical pathology or root canal treatment on target teeth; sinusitis, sinus defects, or hypoplasia; and temporomandibular joint disorders. The sample size was determined by consecutive sampling of all eligible CBCT records meeting the inclusion criteria during the study period (April 2024 to August 2025). No a priori power calculation was performed, as this was an exploratory anatomical study; however, the final sample of 502 adults is among the largest reported in CBCT-based analyses of maxillary sinus–root relationships. Ethical approval was granted by the Ethics Committee of the Medical University.
All CBCT images were acquired using the same i-CAT CBCT unit (96 kV, 9 mA, 0.4 mm voxel). Lateral cephalograms were reconstructed from CBCT data and analyzed with Dolphin Imaging software. Patients were categorized by sagittal skeletal pattern based on ANB angle: Class I (0.7°–4.7°), Class II (> 4.7°), and Class III (< 0.7°). Vertical skeletal pattern was based on MP-SN angle: hyperdivergent (> 36°), normodivergent (24.8°–36°), and hypodivergent (< 24.8°) (Fig. 1). An orthodontist (Dr. P.W.J.) performed cephalometric measurements in a blinded manner. Intra-examiner reliability was assessed by re-measuring 50 randomly selected images after three weeks.
Fig. 1.
Cephalometric measurements were used for sample grouping in this study. Sagittal skeletal pattern classification: Patients were classified into skeletal Class I (ANB 0.7°–4.7°), Class II (ANB > 4.7°), and Class III (ANB < 0.7°) based on the ANB angle (A point–Nasion–B point angle). Vertical skeletal pattern classification: Based on the mandibular plane angle (MPSN, the angle between the mandibular plane and the anterior cranial base), patients were divided into hyperdivergent (MPSN > 36°), normodivergent (MPSN 24.8°–36°), and hypodivergent (MPSN < 24.8°) groups
The vertical relationship between each posterior tooth apex (second premolar, first molar, second molar) and the MSF was scored on CBCT images by another blinded orthodontist (Dr. T.N.) using a modified 5-point classification system based on Jung and Cho’s method [10] (Fig. 2): 1, no contact; 2, point contact; 3, lateral contact; 4, protrusion ≤ 1/2 root length; 5, protrusion > 1/2 root length. Intra-examiner reliability for scoring was assessed by re-evaluating 30 randomly selected teeth after three weeks. A patient score was calculated as the average of all six tooth scores (left and right for each of the three tooth types). A tooth score for each tooth type was calculated as the average of the left and right scores.
Fig. 2.
Schematic representation of the classification criteria for the root–maxillary sinus relationship score based on Jung and Cho’s method [10]: 1, no contact; 2, vertical point contact; 3, lateral contact; 4, intrasinus extension ≤ 1/2 root length; 5, intrasinus extension > 1/2 root length. This modified classification was used to assess the proximity relationship between maxillary posterior teeth and the sinus floor
Statistical analyses were performed using R (version 4.5.0). Normality was assessed with the Shapiro-Wilk test. Due to non-normal distributions, differences among groups were analyzed using Kruskal-Wallis tests with Bonferroni-corrected post-hoc comparisons. Ordinal logistic regression models were constructed to analyze the effects of skeletal pattern (vertical or sagittal) on patient and tooth-specific scores, adjusting for age and sex as covariates. Spearman’s correlation was used to assess relationships between tooth scores. A P-value < 0.05 was considered statistically significant.
Result
The final sample included 502 patients (Fig. 3). Intra-examiner reliability was excellent for facial biotype determination (κ = 0.82, P < 0.001) and root-sinus scoring (κ = 0.89, P < 0.001). Sample characteristics are in Table 1. Age was similar across vertical and sagittal groups. Sex distribution differed significantly in vertical groups (P < 0.001), with more females in hyper- and normodivergent groups, but was similar across sagittal groups.
Fig. 3.
Flowchart illustrating the complete process of screening the final study sample from the initial patient pool. The initial sample was derived from the cone-beam computed tomography (CBCT) database of 10,937 patients at a tertiary stomatology hospital in Jiangsu Province, China. Following the sequential application of inclusion and exclusion criteria, a total of 502 adult patients (aged 20–45 years) were ultimately included in the analysis
Table 1.
Baseline characteristics according to skeletal facial pattern
| Group | Subgroup | Cell content |
|---|---|---|
| Vertical Biotype | Hyperdivergent | 131 (28.7 ± 6.4, 25 (19.1%), 786) |
| Normodivergent | 294 (29.6 ± 6.7, 81 (27.6%), 1764) | |
| Hypodivergent | 77 (28.4 ± 6.2, 40 (51.9%), 462) | |
| P Value | Age: P = 0.255; Gender: P<0.001 | |
| Sagittal Biotype | Class I | 261 (29.2 ± 6.6, 75 (28.7%), 1566) |
| Class II | 203 (29.2 ± 6.7, 61 (30%), 1218) | |
| Class III | 38 (29.3 ± 6.5, 10 (26.3%), 228) | |
| P Value | Age: P = 0.985; Gender: P = 0.887 |
a, Data are presented as: sample size (mean age ± standard deviation, number of males (percentage %), total number of teeth)
b, P-values were derived from the Kruskal–Wallis test (for age) and the chi-squared test (for sex) to assess differences in baseline characteristics among groups (within vertical facial pattern groups or within sagittal classification groups)
c, P > 0.05 indicates no statistically significant difference between groups
The distribution of SRR scores (Tables 2 and 3) showed that Score 4 (partial protrusion) was most prevalent for the second premolar and second molar. Score 3 (lateral contact) was the least common and often unilateral. Score 1 (no contact) was the most frequent category for the maxillary first molar.
Table 2.
Distribution of scores by tooth type in vertical dimension (based on MPSN)
| Tooth | Score value | hyperdivergent | normodivergent | hypodivergent | Total |
|---|---|---|---|---|---|
| Second Premolar | 1 | 23 (62) | 56 (148) | 10 (28) | 89 (238) |
| Second Premolar | 2 | 10 (33) | 23 (102) | 12 (38) | 45 (173) |
| Second Premolar | 3 | 8 (7) | 34 (13) | 10 (4) | 52 (24) |
| Second Premolar | 4 | 48 (125) | 105 (282) | 31 (78) | 184 (485) |
| Second Premolar | 5 | 12 (35) | 12 (43) | 2 (6) | 26 (84) |
| First Molar | 1 | 64 (142) | 131 (302) | 27 (66) | 222 (510) |
| First Molar | 2 | 16 (42) | 30 (104) | 10 (34) | 56 (180) |
| First Molar | 3 | 5 (13) | 24 (16) | 6 (7) | 35 (36) |
| First Molar | 4 | 21 (58) | 56 (156) | 16 (42) | 93 (256) |
| First Molar | 5 | 3 (7) | 3 (10) | 2 (5) | 8 (22) |
| Second Molar | 1 | 21 (59) | 54 (142) | 12 (35) | 87 (236) |
| Second Molar | 2 | 11 (49) | 23 (95) | 12 (34) | 46 (178) |
| Second Molar | 3 | 15 (9) | 32 (21) | 7 (3) | 54 (33) |
| Second Molar | 4 | 44 (121) | 104 (287) | 26 (72) | 174 (480) |
| Second Molar | 5 | 8 (24) | 11 (43) | 2 (10) | 21 (77) |
a, Root–maxillary sinus relationship score: 1 = no contact; 2 = vertical point contact; 3 = lateral contact; 4 = intrasinus extension ≤ 1/2 root length; 5 = intrasinus extension > 1/2 root length
b, Cell format: The number of patients corresponding to this score (the total number of teeth corresponding to the score)
c, Patient score = mean of bilateral scores for the same tooth type, standardized on a 1–5 scale
d, Total number of teeth includes all assessed teeth, and thus is often more than twice the number of patients. Verified all figures remain consistent with the tooth-level analytical approach
e, Vertical facial pattern was classified based on the MPSN angle: hyperdivergent (> 36°), normodivergent (24.8–32°), and hypodivergent (< 24.8°)
Table 3.
Distribution of scores by tooth type in sagittal dimension (based on the ANB angle)
| Tooth | Score value | ClassⅠ | ClassⅡ | ClassⅢ | Total |
|---|---|---|---|---|---|
| Second Premolar | 1 | 47 (123) | 32 (93) | 10 (22) | 89 (238) |
| Second Premolar | 2 | 23 (92) | 17 (66) | 5 (15) | 45 (173) |
| Second Premolar | 3 | 27 (16) | 20 (6) | 5 (2) | 52 (24) |
| Second Premolar | 4 | 93 (245) | 79 (208) | 12 (32) | 184 (485) |
| Second Premolar | 5 | 16 (46) | 9 (33) | 1 (5) | 26 (84) |
| First Molar | 1 | 100 (239) | 103 (228) | 19 (43) | 222 (510) |
| First Molar | 2 | 31 (97) | 21 (69) | 4 (14) | 56 (180) |
| First Molar | 3 | 15 (21) | 17 (14) | 3 (1) | 35 (36) |
| First Molar | 4 | 57 (154) | 30 (84) | 6 (18) | 93 (256) |
| First Molar | 5 | 3 (11) | 5 (11) | 0 | 8 (22) |
| Second Molar | 1 | 49 (131) | 30 (85) | 8 (20) | 87 (236) |
| Second Molar | 2 | 24 (90) | 18 (75) | 4 (13) | 46 (178) |
| Second Molar | 3 | 25 (20) | 26 (12) | 3 (1) | 54 (33) |
| Second Molar | 4 | 88 (243) | 74 (202) | 12 (35) | 174 (480) |
| Second Molar | 5 | 10 (38) | 10 (32) | 1 (7) | 21 (77) |
a, Root–maxillary sinus relationship score: 1 = no contact; 2 = vertical point contact; 3 = lateral contact; 4 = intrasinus extension ≤ 1/2 root length; 5 = intrasinus extension > 1/2 root length
b, Cell format: The number of patients corresponding to this score (the total number of teeth corresponding to the score)
c, Patient score = mean of bilateral scores for the same tooth type, standardized on a 1–5 scale
d, Total number of teeth includes all assessed teeth, and thus is often more than twice the number of patients. Verified all figures remain consistent with the tooth-level analytical approach
e, Sagittal facial pattern was classified based on the ANB angle: ClassⅡ (ANB > 4.7°), ClassⅠ (ANB 0.7–4.7°), and ClassⅢ (ANB < 0.7°)
Ordinal logistic regression analysis (Table 4) revealed no significant association between vertical facial pattern and root-sinus scores after adjusting for age and sex (hyperdivergent vs. normodivergent: OR = 1.059, 95% CI: 0.738–1.519, P = 0.757; hypodivergent vs. normodivergent: OR = 0.759, 95% CI: 0.485–1.188, P = 0.228). Regarding sagittal patterns, neither Class II nor Class III showed significant differences compared to Class I (Class II: OR = 0.939, 95% CI: 0.684–1.289, P = 0.697; Class III: OR = 0.783, 95% CI: 0.421–1.455, P = 0.439). However, covariates showed significant effects: age was negatively associated with scores (OR = 0.913, 95% CI: 0.891–0.936, P < 0.001), and females had significantly lower scores than males (OR = 0.472, 95% CI: 0.331–0.673, P < 0.001). Tooth-specific analyses (Table 5; Fig. 4) confirmed no significant association between vertical pattern and scores for any individual tooth.
Table 4.
Patient score
| Variable | Group | n | Median Range | OR (95% CI) | P value |
|---|---|---|---|---|---|
| Vertical Biotype | Normodivergent | 294 | 2.86 (1.00-4.86) | (Reference Group) | — |
| Hyperdivergent | 131 | 3.00 (1.00–5.00) | 1.059 (0.738, 1.519) | 0.757 | |
| Hypodivergent | 77 | 2.86 (1.00-4.86) | 0.759 (0.485, 1.188) | 0.228 | |
| Sagittal Biotype | ClassⅠ | 261 | 3.00 (1.00–5.00) | (Reference Group) | — |
| ClassⅡ | 203 | 2.86 (1.00–5.00) | 0.939 (0.684, 1.289) | 0.697 | |
| ClassⅢ | 38 | 2.79 (1.00-4.43) | 0.783 (0.421, 1.455) | 0.439 |
a, Data are presented as: sample size (n), median (range)
b, The normodivergent group served as the reference for vertical facial pattern, and ClassⅠ served as the reference for sagittal classification
c, OR (95% CI) and P-values were derived from ordinal logistic regression models. The vertical facial pattern model was adjusted for age and sex (reference level: Male). Sex effect: OR = 0.472, 95% CI: 0.331–0.672, P < 0.001. Age effect (in the vertical facial pattern model): OR = 0.913, 95% CI: 0.891–0.935, P < 0.001. The sagittal classification model was adjusted only for age, as no significant difference in sex distribution was observed within this grouping (P = 0.887).
d, All P-values for between-group comparisons were > 0.05, indicating no statistically significant differences in patient scores among the various vertical facial pattern and sagittal classification groups after adjusting for age and sex (where applicable)
Table 5.
Comparison of maxillary posterior tooth scores among patients with different vertical facial patterns
| Tooth | Group | n | Median Range | OR (95% CI) | P value |
|---|---|---|---|---|---|
| Second Premolar | Average | 294 | 3.00 (1.00–5.00) | (Reference Group) | — |
| Second Premolar | High | 131 | 4.00 (1.00–5.00) | 1.371 (0.863, 2.179) | 0.181 |
| Second Premolar | Low | 77 | 3.00 (1.00–5.00) | 0.791 (0.473, 1.323) | 0.372 |
| First Molar | Average | 294 | 1.50 (1.00–5.00) | (Reference Group) | — |
| First Molar | High | 131 | 1.50 (1.00–5.00) | 0.784 (0.497, 1.237) | 0.296 |
| First Molar | Low | 77 | 2.00 (1.00–5.00) | 0.986 (0.566, 1.717) | 0.96 |
| Second Molar | Average | 294 | 3.00 (1.00–5.00) | (Reference Group) | — |
| Second Molar | High | 131 | 3.00 (1.00–5.00) | 1.150 (0.734, 1.803) | 0.542 |
| Second Molar | Low | 77 | 3.00 (1.00–5.00) | 0.631 (0.367, 1.084) | 0.095 |
a, Data are presented as: sample size (n), median (range)
b, Vertical facial pattern was classified into three groups: hyperdivergent (High), normodivergent (Average), and hypodivergent (Low), with the normodivergent group serving as the reference
c, Scoring criteria: scores ranged from 1 to 5 for second premolar, first molar, and second molar
d, OR (95% CI) and P-values were derived from ordinal logistic regression models, all adjusted for age and sex (reference level for sex: Male)
e, All P-values for between-group comparisons were > 0.05, indicating no statistically significant differences in scores for any tooth type among different vertical facial patterns after adjusting for age and sex
Fig. 4.
Distribution of scores for the relationship between the maxillary second premolar, first molar, and second molar and the maxillary sinus floor among the three vertical skeletal pattern groups (hyperdivergent, normodivergent, and hypodivergent), after adjusting for age and sex. Scores were assigned using the modified classification (1–5 scale, with higher scores indicating deeper root penetration into the sinus cavity). Boxes represent the interquartile range, the central line indicates the median, and whiskers show the data range
In contrast, sagittal pattern analysis (Table 6; Fig. 5) revealed that Class II patients had significantly lower first molar scores than Class I patients (OR = 0.647, P = 0.032), indicating roots farther from the sinus floor. No significant differences were found for the second premolar or second molar among sagittal groups.
Table 6.
Comparison of maxillary posterior tooth scores among patients with different sagittal facial patterns
| Tooth | Group | n | Median Range | OR (95% CI) | P value |
|---|---|---|---|---|---|
| Second Premolar | ClassⅠ | 261 | 3.00 (1.00–5.00) | (Reference Group) | — |
| Second Premolar | ClassⅡ | 203 | 4.00 (1.00–5.00) | 1.105 (0.747, 1.636) | 0.617 |
| Second Premolar | ClassⅢ | 38 | 3.00 (1.00–5.00) | 0.610 (0.306, 1.219) | 0.162 |
| First Molar | ClassⅠ | 261 | 2.00 (1.00–5.00) | (Reference Group) | — |
| First Molar | ClassⅡ | 203 | 1.00 (1.00–5.00) | 0.647 (0.435, 0.963) | 0.032 |
| First Molar | ClassⅢ | 38 | 1.25 (1.00–4.00) | 0.599 (0.283, 1.266) | 0.18 |
| Second Molar | ClassⅠ | 261 | 3.00 (1.00–5.00) | (Reference Group) | — |
| Second Molar | ClassⅡ | 203 | 3.00 (1.00–5.00) | 1.223 (0.828, 1.807) | 0.312 |
| Second Molar | ClassⅢ | 38 | 3.25 (1.00–5.00) | 0.735 (0.346, 1.562) | 0.423 |
a. Data are presented as: sample size (n), median (range)
b. Root-maxillary sinus relationship score: 1–5 scale (higher scores indicate closer root proximity to the sinus floor)
c OR (95% CI) and P-values were derived from ordinal logistic regression models adjusted for age
d ClassⅠ served as the reference group for sagittal classification
e. OR < 1 indicates more distant root-to-sinus adjacencies (lower scores)
Fig. 5.
Distribution of scores for the relationship between maxillary posterior teeth (second premolar, first molar, and second molar) and the maxillary sinus floor among patients with three sagittal skeletal patterns (skeletal Class I, II, and III), after adjusting for age and sex. Scores were assigned using the modified classification (1–5 scale, with higher scores indicating deeper root penetration into the sinus cavity). Boxes represent the interquartile range, the central line indicates the median, and whiskers show the data range
Correlation analysis (Fig. 6) showed significant positive correlations among all three tooth scores at the population level (strongest between second premolar and second molar, ρ = 0.79). However, substantial individual heterogeneity (Fig. 7) was observed: 43.4% of patients showed irregular fluctuations in scores across the three teeth, while only 24.8% had identical scores. This heterogeneity was most pronounced in hyperdivergent patients (47.3% fluctuation).
Fig. 6.
Correlation heatmap of maxillary posterior tooth root–sinus scores. This heatmap displays the Spearman’s rank correlation coefficients for pairwise comparisons of the maxillary sinus floor relationship scores among the maxillary second premolar (U5), first molar (U6), and second molar (U7) in 502 patients. Color intensity represents the strength of the correlation (see color scale on the right)
Fig. 7.
Distribution of intra-individual trends in root–sinus scores for three maxillary posterior teeth. This bar chart illustrates the distribution of trends in the root–sinus relationship scores for the maxillary second premolar, first molar, and second molar within 502 individual patients. Trends were classified into five categories: fluctuating (no systematic trend across the three tooth types), stable (score differences between adjacent tooth types ≤ 1), identical (all three scores equal), decreasing (U5 > U6 > U7), and increasing (U5 < U6 < U7)
Discussion
This CBCT study of 502 adults found that after adjusting for age and sex, the vertical skeletal pattern showed no significant association with maxillary posterior root-sinus relationship (P > 0.05), contrasting with findings from adolescent populations. This suggests that in adults, vertical facial morphology may not be the primary determinant of sinus-tooth proximity.
The root-sinus relationship between the maxillary sinus and maxillary posterior teeth has consistently been a key focus of risk assessment for orthodontists. This relationship is particularly critical when extensive posterior tooth movement is required. Current studies evaluating the root-sinus relationship mostly rely on three‑dimensional CT assessment and employ inconsistent criteria. If the root‑sinus relationship could be inferred from routinely used cephalometric measurements, it would not only save clinicians’ time but also provide more comprehensive treatment considerations. Evidence from existing case reports and case series indicates that when adult teeth have roots adjacent to the maxillary sinus, continuous light forces and slow tooth movement are often necessary to prevent root penetration into the sinus. Although bodily tooth movement can be achieved, there is a higher probability of tipping movement and an increased risk of root resorption [2, 7, 11, 12]. Recent evidence indicates that sinus contact during molar distalization with aligners correlates with significant root resorption in adults [13, 14].
Previous studies have suggested a potential association between specific facial biotypes and the relationship between tooth roots and the maxillary sinus. Research has found that although there are no significant differences in the size and position of the maxillary sinus among different vertical skeletal patterns in skeletal Class I populations [15], in skeletal Class II populations, hyperdivergent patients exhibit statistically significantly larger maxillary sinuses compared to hypodivergent and normodivergent patients, regardless of sex [16]. A retrospective cross-sectional study from Romania reported that in a young population (aged 7–24 years), hypodivergent patients had maxillary second molars located farther from the maxillary sinus floor compared to normodivergent and hyperdivergent patients [8]. In contrast, a study from China found that in an adult population (aged 20–45 years), skeletal Class I patients had the highest probability of maxillary posterior tooth roots protruding into the maxillary sinus, while skeletal Class III patients had the lowest probability [9].
Based on the above research, we conducted a comprehensive evaluation of the maxillary root‑sinus relationship from both vertical and sagittal dimensions, yielding the following results: In the adult population, no statistically significant differences were found in the relationship between the maxillary posterior teeth and the maxillary sinus among different vertical skeletal patterns (P < 0.05).Regarding the sagittal skeletal pattern, this study identified for the first time that skeletal Class II adults exhibit a more distant maxillary first molar root apex-to-maxillary sinus floor adjacency on CBCT images compared to Class I individuals. This finding may be related to maxillary pneumatization and alveolar process morphological characteristics in Class II malocclusion patients. A recent study on Caucasian adults aged 18–25 years showed that three-dimensional analysis of the maxillary sinus indicated smaller maxillary sinus volumes in skeletal Class II individuals compared to skeletal Class I individuals [17]. However, a CBCT study of 1600 roots found that palatal roots of the first molar and the palatal, mesio-buccal and disto-buccal roots of the second molars had significantly less distance from MSF in skeletal class II than in class III (P < 0.05) [18]. It can thus be inferred that the relationship between the maxillary sinus and tooth roots may also be influenced by maxillary sinus morphology, with the maxillary sinus—being the largest paranasal sinus—exhibiting a pyramidal rather than regular geometric shape [1, 19]. Because few studies specifically investigate maxillary sinus morphology concerning sagittal facial biotype, whether and how sinus shape influences root-sinus proximity remains unknown and warrants future investigation. However, because this study is retrospective and cross-sectional, this finding represents a static anatomical association rather than direct evidence of treatment-related safety. Whether this anatomical difference confers greater safety during orthodontic tooth movement—such as a lower risk of sinus penetration or root resorption—remains unknown and requires future prospective studies that actually evaluate tooth movement and sinus responses.
Furthermore, age showed a significant negative correlation with scores (P < 0.001), indicating an increasing root-sinus separation with age. This may be attributed to physiological alveolar bone resorption with aging or pneumatization expansion of the maxillary sinus [20]. Classical theory suggests that the maxillary sinus reaches maturity at approximately 20 years of age and subsequently enters a relatively stable state [20, 21]. However, as research has progressed, an increasing number of investigators have discovered that even after adulthood, the separation of the maxillary sinus and posterior tooth roots gradually increases. Multiple studies on Chinese adults over 18 years of age have reported an increased distance between the maxillary sinus and maxillary posterior tooth roots with advancing age [22, 23], and a recent study on Romanian adults has reached similar conclusions [24].
Besides, sex differences were observed, with females showing a more distant adjacency in hyperdivergent patients. This finding is consistent with studies in China and Romania [24, 25]. However, a certain number of studies have suggested that the distance between the female maxillary sinus and posterior tooth roots is closer compared to males [23], or that the relationship between roots and the maxillary sinus is not influenced by sex [23, 26], warranting further investigation.
Importantly, despite population-level correlations among tooth positions, 43.4% of patients showed marked individual heterogeneity, underscoring the necessity for CBCT-based individualized assessment of each tooth rather than extrapolating from skeletal patterns or adjacent teeth. However, this cross-sectional, single-institution study with a limited Class III sample size precludes causal inference. Future multi-center longitudinal research is needed. Due to our use of a categorical scoring system rather than continuous distance measurements and the absence of mixed-effects modeling, the tooth-level findings should be interpreted as exploratory. Still, even with the median, a patient-level summary cannot fully capture the most severe single-tooth condition. Future studies using linear distance measurements and multilevel models are needed to confirm these associations.
Conclusion
(1) Vertical skeletal pattern is not significantly associated with SRR after adjusting for age and sex. (2) Skeletal Class II patients have a significantly more distant adjacency between the maxillary first molar apices and the MSF. (3) Age and sex are significant covariates, with older age and female sex associated with greater root-to-sinus distance. (4) Despite population-level correlations, substantial individual heterogeneity in SRR exists among tooth positions.
Acknowledgements
We are very grateful to Dr. Yaoyao Jiang from the Imaging Department of Xuzhou Stomatological Hospital and Engineer Pu Wang from the Information Department for their data support for this study.
Authors’ contributions
W.j.P .: Conceptualization, methodology, formal analysis, writing original draft, Manuscript submission, peer review coordination, and serves as the central point of contact for the editorial and scientific community regarding the published work. N. T.: Data curation, investigation, and software validation, funding acquisition, and serves as the collaborative point for the editorial and scientific community regarding the published work. H. L.: Data curation, investigation, and software validation, funding acquisition, and serves as the collaborative point for the editorial and scientific community regarding the published work. L. Y.: Visualization, writing, review and editing, and resources. Y. F.: Supervision, project administration, and funding acquisition. All authors reviewed the manuscript.
Funding
This work was supported by the Xuzhou Municipal Health Commission Science and Technology Project (Grant No. XWKYHT20250060).
Data availability
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.
Declarations
Ethics approval and consent to participate
The retrospective study was approved by the Ethics Committee of Affiliated Stomatology Hospital of Xuzhou Medical University (Approval No. 2023-KY-003-01), and its requirement for informed consent was waived by this Committee. All procedures performed in this study involving human participants were in accordance with the ethical standards of the institutional research committee and with the 1964 Helsinki Declaration and its later amendments.
Consent for publication
Not applicable. This manuscript does not contain any person’s data, images, or clinical details requiring consent for publication.
Competing interests
The authors declare no competing interests.
Clinical Study Registration Number
Not applicable.
Footnotes
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Wenjing Peng, Na Tang and Hao Liu are co-corresponding authors.
Contributor Information
Wenjing Peng, Email: 209062968@qq.com.
Na Tang, Email: tn1205@163.com.
Hao Liu, Email: haoliu1023@163.com.
References
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Associated Data
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Data Availability Statement
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.