Abstract
Objectives
To compare the collum angle (CA) and labial surface angle (LSA) of the maxillary central incisors across different skeletal malocclusion classes and between genders.
Methods
This cross-sectional study screened lateral cephalogram of 504 patients above 18 years seeking for orthodontic consultation. Skeletal classification was based on ANB angle, Wits appraisal and Beta angle; Class II cases were further divided by maxillary incisor inclination to the SN plane. Landmarks were digitised using tpsDig, and measurements were obtained via a custom Python script. CA and LSA were compared across skeletal classes using ANOVA and between genders using t-tests.
Results
The data of 432 patients (213 males, 219 females) were analysed after eligibility assessment. The mean CA were found to be 1.45°, -1.44°, 4.80°, and 1.20° for Class I, Class II with proclined incisors, Class II with retroclined incisors and Class III respectively, with a statistically significant difference in ANOVA (f = 7.9, p < 0.001). Similarly, mean LSA were 20.55°, 21.35°, 22.11°, and 20.83° for Class I, Class II with proclined incisors, Class II with retroclined incisors and Class III respectively, with no significant intergroup difference in ANOVA (f = 1.135, p = 0.335). No significant gender difference was found with t-test for the CA (t = 0.577, p = 0.564). However, the LSA was significantly greater in females compared to males (t = − 2.158, p = 0.031).
Conclusions
Skeletal Class II patients with retroclined incisors exhibited a greater CA compared to other skeletal classes. The LSA was slightly greater in females than in males.
Keywords: Collum Angle, Labial Surface Angle, Maxillary Central Incisor, Skeletal Malocclusion
Introduction
The aim of orthodontic treatment is to establish harmonious tooth relationship with optimal aesthetics. The maxillary central incisors are particularly important in smile aesthetics and their inclination also plays an important role in aesthetic perception [1, 2]. The labio-lingual crown inclination is mainly determined by the inclination of the root and to some extent with collum angle (CA), which is the angle formed between the long axis of the crown and the long axis of root.
The CA differs among individuals, and influences torque expression and final position after orthodontic treatment [3]. The deviation of the CA in either direction implies poor alignment of the crown and root that may lead to unintended root positioning while attempting to ideal crown positioning during orthodontic treatment. Lack of consideration to this angle may pose risk of root encroaching the cortical plate leading to root exposure or gingival recession [4]. The CA has been reported by several authors and most of them found it to be larger in Class II division 2 malocclusion when compared to Class II division 1 or Class I and Class III malocclusion [5–9].
Labial surface angle (LSA), formed between the crown long axis and the tangent to the labial surface at the bracket site, is another important feature whose variation also affects the expression of torque of the preadjusted brackets and the final labio-lingual inclination of the tooth. A wide variation of the LSA angle among individuals has been reported by several authors [4, 7, 10, 11], warranting the need to evaluate the labial surface morphology of incisors for better treatment outcomes [12].
The assessment of these angles forms an important aspect of treatment planning in orthodontics, aesthetic dentistry and in planning dental implants in the anterior region [13]. Prior information about the CA and LSA provides additional information about the crown and root of the incisor that is important for selecting appliance prescription and planning biomechanics during orthodontic treatment. LSA reflects the torque expression at the level of crown whereas the CA determines transmission of this torque to the root.
Most of the studies in the literature had several limitations, such as smaller sample sizes [14]; focusing only on CA, without LSA [8, 9, 15–19]; including only Class II Division 1 and Division 2 cases [6]; failing to differentiate between Class II Division 1 and Division 2 [7, 18]; lacking comparison with class I and Class III cases [16, 19]; and evaluating dental classifications alone while ignoring the skeletal component [6, 20]. Hence, this study was designed to assess both the CA and LSA of maxillary central incisor in all kinds of skeletal malocclusion classes and compare the gender differences.
Main text
Materials and methods
This cross-sectional study was conducted at the Department of Orthodontics, B. P. Koirala Institute of Health Sciences, Dharan, Nepal after ethical clearance (IRC-407/076/077). This study included patients who reported for an orthodontic consultation. Clinical records and digital lateral cephalograms of patients (above 18 years and below 35 years) who attended for orthodontic consultation between October 2024 and September 2025 were screened. Patients with a history of dental trauma, orthodontic treatment, restoration of the maxillary central incisors, attrition of the labial surface or the incisal edge of the central incisors, any apparent craniofacial or dental anomalies, and poor-quality radiographs were excluded.
The sample size was calculated using G*Power (3.1.9.7) [21] considering the following input parameters: effect size f = 0.25, alpha = 0.05, power = 0.8, number of groups = 4. This resulted in a sample size of 32 per group. Hence, we aimed to include at least 32 samples in each our study groups.
The patients were classified as skeletal Class I, II, or III based on the ANB angle, Wit’s appraisal, and Beta angle (Table 1) [22–24]. A class was assigned when at least two criteria agreed; cases with discordant results were excluded. Skeletal Class II cases were further subdivided proclined or retroclined incisors based on the incisor- SN plane angle, using a cut off at 103.97° [25].
Table 1.
Criteria for different skeletal Classes
| Parameters | Class I | Class II | Class III |
|---|---|---|---|
| ANB angle | 0° to 4° | > 4° | < 0° |
| Wits appraisal |
Male − 3 mm to + 1 mm Female − 2 mm to + 2 mm |
Male > + 1 mm Female > + 2 mm |
Male < − 3 mm Female < − 2 mm |
| Beta angle | 27° to 35° | < 27° | > 35° |
For CA measurement, the root apex (R), crown tip (C), labial (La), and lingual cementoenamel junction (Li) were identified. The midpoint of La and Li (O) was joined to R and C to define the long axes of the root and crown, respectively. The angle between the extension of the crown’s long axis and the root’s long axis was measured as the CA. The angle was recorded as positive when the root apex lay lingual to the extension of the crown axis, and negative if otherwise (Fig. 1).
Fig. 1.
Schematic representation of landmarks for the measurement of CA and LSA
The LSA is the angle between the crown axis and the tangent to the labial crown surface. As the clinical crown is usually 1 mm shorter than the anatomic crown [26], a point 1 mm coronal to the cemento-enamel junction (CEJ) is marked to define the clinical crown. The midpoint of the clinical crown (T) was judged by the investigator and a tangent was constructed by joining the two points on the labial surface corresponding to the intersections of a 1 mm-diameter circle centred at point T.
All measurements were computed using a custom Python script after landmark digitisation in TPSdig2 (v2.32) by a single investigator (RG), and the data were analysed in SPSS. Intra-observer reliability was assessed by re-measuring 30 randomly selected cephalograms after 2 weeks using the ICC, while skeletal class agreement was evaluated with Cohen’s kappa. CA and LSA were compared across skeletal classes and between genders.
Results
The records of 504 patients were screened for eligibility. Excluding poor quality radiographs, patients with restored central incisors and not well aligned maxillary central incisors, 451 patients were considered further. Their records were examined, and the three parameters for assessing skeletal class showed different results in 19 cases, therefore, they were excluded from further analysis (Fig. 2). A total of 432 patients were included in the final analysis, among whom 213 were male and 219 were female. Similarly, on assessing skeletal class, 163 were found to be Class I, 107 were Class II, and 162 were Class III (Table 2). The test for normality with Kolmogorov–Smirnov and Shapiro Wilk tests showed values above 0.05 suggesting the normal distribution of the data for both CA and LSA.
Fig. 2.
STROBE flow diagram
Table 2.
Gender and skeletal class distribution of malocclusion
| Skeletal Malocclusion | Gender | Total | |
|---|---|---|---|
| Male | Female | ||
| Class I | 72 | 91 | 163 |
| Class II with proclined incisors | 35 | 38 | 73 |
| Class II with retroclined incisors | 19 | 15 | 34 |
| Class III | 87 | 75 | 162 |
| Total | 213 | 219 | 432 |
Mean difference is significant at 0.05 level.
The ICC with absolute agreement was found to be 0.933 (95% CI 0.860–0.968) for CA, 0.940 (95% CI 0.874–0.972) for LSA and 0.988 (95% CI 0.975–0.994) for maxillary incisor inclination reflecting high reliability of the measurement. Similarly, the value of Cohen’s Kappa was found to be 0.80 reflecting substantial agreement in the identification of skeletal class.
The mean values of CA were found to be 1.45°, − 1.44°, 4.80°, and 1.20° for Class I, Class II with proclined incisors, Class II with retroclined incisors and Class III respectively. These values were statistically different when compared using ANOVA [F (3428) = 7.9, p < 0.001]. Similarly, the mean LSA was found to be 20.55°, 21.35°, 22.11°, and 20.83° for Class I, Class with proclined incisors, Class II with retroclined incisors and Class III respectively. There was no significant difference when compared with ANOVA [F (3, 428) = 1.135, p = 0.335] (Table 3). A post-hoc Tukey HSD test demonstrated that the mean CA of Class II with retroclined incisors was significantly different from that of other classes of malocclusion (Table 4).
Table 3.
Comparison of CA and LSA across different malocclusion types
| Measure | Class I (n = 163) | Class II with proclined incisors (n = 73) | Class II with retroclined incisors (n = 34) | Class III (n = 162) | F (3428) | p-value | ||||
|---|---|---|---|---|---|---|---|---|---|---|
| Mean | SD | Mean | SD | Mean | SD | Mean | SD | |||
| Collum angle (°) | 1.45 | 6.13 | − 1.44 | 5.63 | 4.80 | 7.04 | 1.20 | 6.73 | 7.9 | < 0.001 |
| Labial surface angle (°) | 20.55 | 4.91 | 21.35 | 4.73 | 22.11 | 4.95 | 20.83 | 5.21 | 1.135 | 0.335 |
Table 4.
Post-hoc Tukey test to compare CA between different skeletal classes
| (I) Skeletal Class | (J) Skeletal Class | Mean Difference (I-J) | 95% Confidence Interval | p-value | |
|---|---|---|---|---|---|
| Lower Bound | Upper Bound | ||||
| Class I | Class II with proclined incisors | 2.89* | 0.58 | 5.20 | 0.007 |
| Class III | 0.25 | − 1.57 | 2.07 | 0.985 | |
| Class II with retroclined incisors | − 3.34* | − 6.43 | − 0.25 | 0.028 | |
| Class II with proclined incisors | Class III | − 2.64* | − 4.95 | − 0.33 | 0.018 |
| Class II with retroclined incisors | − 6.24* | − 9.64 | − 2.83 | < 0.001 | |
| Class III | Class II with retroclined incisors | − 3.59* | − 6.69 | − 0.50 | 0.015 |
*Mean difference is significant at 0.05 level
There appears to be no significant gender difference in the CA (t = 0.577, df = 430, p = 0.564). However, the LSA was found to be significantly greater in females compared to males (t = − 2.158, df = 430, p = 0.031) (Table 5).
Table 5.
Gender difference in CA and LSA
| Parameter | Male (n = 213) | Female (n = 219) | Mean Difference | 95% CI | t | p-value | |||
|---|---|---|---|---|---|---|---|---|---|
| Mean | SD | Mean | SD | Lower | Upper | ||||
| Collum angle (°) | 1.32 | 6.55 | 0.95 | 6.47 | 0.36 | − 0.87 | 1.59 | 0.577 | 0.564 |
| Labial Surface Angle (°) | 20.39 | 5.08 | 21.42 | 4.88 | − 1.03 | − 1.98 | − 0.09 | − 2.158 | 0.031 |
There was no significant linear correlation between CA and LSA (r = 0.01, p = 0.987) (Fig. 3).
Fig. 3.
Scatterplot showing the relationship between LSA and CA
Discussion
This cross-sectional study evaluated the maxillary central incisor morphology, as these teeth dominate smile aesthetics and contribute nearly half of the total visible mesiodistal dimension of anterior teeth [1, 27]. As the variations in crown morphology affects torque expression by prescription brackets, this research evaluated the CA and LSA of maxillary central incisors in patients with different skeletal patterns. This study found CA to be greatest in skeletal Class II patients with retroclined incisors and showed no gender difference. LSA did not vary with skeletal pattern but was slightly greater in females, and no linear relationship was found between CA and LSA.
The CA was highest in skeletal Class II cases with retroclined incisors (mean 4.80°), and differed significantly from other classes of malocclusion, which is in agreement with previous studies [4–6, 8, 9, 20, 28, 29]. A large collum angle in Class II Division 2 cases is attributed to both genetic and environmental factors. Logan suggested purely genetical determination [30], whereas Backlund emphasised the influence of lower lip pressure and increased muscle tone during incisor eruption [31]. Other authors have proposed that forces acting during root formation or alterations in perioral muscle balance may influence crown–root morphology, although some argue such forces primarily affect the apical root rather than the crown–root junction [7, 32, 33].
The LSA represents the convexity of the labial surface of the clinical crown. The present study did not find any significant difference in the LSA of maxillary central incisors among different skeletal classes, although Class II subjects demonstrated slightly higher LSA values than the other groups. In the population examined, females exhibited an approximately 1° greater LSA than males. A study by Wang et al., in Chinese subjects, reported significantly larger mean LSA values of the maxillary central incisors in Class II patients compared with Class I and Class III patients, however, the mean angles reported were slightly lower than those observed in the present study [7]. In contrast, Carlson and Ronnerman reported a mean LSA of 21°, which is comparable to the findings of the present study [10].
The variation observed in the values of CA and LSA across different studies conducted around the globe most likely reflects multifactorial causes. Ethnic and racial differences account for significant differences in tooth morphology and the craniofacial skeleton [34]. Methodological differences among studies may also contribute to the observed variation; some investigations utilized two-dimensional cephalograms, whereas others employed three-dimensional CBCT imaging, which may account for differences in the accuracy and reliability of landmark identification [35].
The degree of convexity of the labial surface of the incisor influences the torque expression from the preadjusted brackets; therefore, accurate assessment of the labial surface in the bracket bonding area is essential. Andrew suggested that the bracket should be positioned at the middle of the clinical crown [36], and the clinical crown is usually 1 mm shorter than the anatomic crown. Accordingly, this study used the centre of the clinical crown as a reference point to draw a tangent to the labial surface for the assessment of the LSA. Some studies [7] used the point at which the labial surface meets the perpendicular bisector of long axis of the anatomic crown as a reference for LSA measurement, however, this may not represent the true centre of the clinical crown because of the convexity of the labial surface. To better approximate the actual bracket placement site, other studies identified the point at a fixed distance from the incisal edge [12].
This study did not identify any linear association between CA and LSA, indicating that the curvature of the labial surface is independent of the crown-root angle. Hence, the labial surface curvature should be carefully evaluated in all cases, irrespective of the type of skeletal malocclusion. In contrast, Wang et al. reported a strong positive correlation between these two angles [7]. The correlation between these two angles has not been extensively investigated in other studies; therefore, no clear conclusions could be drawn. However, the observed difference could be attributed to the ethnic origin of the samples, which may account for variations in tooth morphology, as well as methodological differences (CBCT vs. lateral cephalogram).
Routine cephalometry often uses the root apex and incisal edge to define the long axis of the incisor. However, the significant crown–root angulation observed in skeletal Class II cases with retroclined incisors, as consistently reported in Class II Division 2 patients, suggests this approach may be inappropriate for this group [4–6, 8, 9, 20, 28, 29]. Because such angulation can position the root close to the palatal cortical bone, intrusion or correction of incisor inclination in such cases require special caution to minimise the risk of adverse effects [37]. Proclination of maxillary incisors with high torque brackets or application of torque to the anterior segment of the wire should thus be planned with caution in Class II Div 2 cases.
The absence of significant difference in LSA among different skeletal malocclusion classes suggest that the labial crown morphology of the maxillary central incisors is independent of the underlying skeletal pattern. Clinically, this finding underscores the importance of individual assessment of labial surface curvature during bracket placement and torque prescription, rather than relying solely on skeletal classification. Even in the absence of intergroup differences, the observed inter-individual variability in LSA remains clinically relevant for achieving optimal torque expression. In particular, the increased collum angle observed in skeletal Class II patients with retroclined incisors highlights the need for cautious torque application to avoid undesirable root positioning. Additionally, the independence of LSA from skeletal pattern emphasizes the importance of evaluating labial surface morphology during bracket placement and appliance selection.
Conclusion
Skeletal Class II patients with retroclined incisors exhibited a greater collum angle compared to other skeletal classes. The labial surface angle was slightly greater in females than in males. No linear correlation was found between the two angles.
Limitations
This study used lateral cephalograms due to their routine availability in orthodontic practice and suitability for large samples. CBCT, on the other hand could offer greater accuracy, but at the cost of higher radiation exposure and expense. Besides, the vertical skeletal component was not assessed in this study.
Acknowledgements
The authors would like to thank all the participants of this study.
Abbreviations
- LSA
Labial Surface Angle
- CA
Collum Angle
Author contributions
RG, PRP and JG contributed to the design of the study; RG collected and analysed the data; RG, PRP and JG verified the analysis and interpretation of the data; RG prepared the manuscript; RG, PRP and JG read and approved the final manuscript.
Funding
No source of funding.
Data availability
The datasets generated for this study can be available from the corresponding author on reasonable request.
Declarations
Ethics approval and consent to participate
Ethical clearance was obtained from the Institutional Review Board of B.P. Koirala Institute of Health Sciences, Dharan, Nepal (IRC-407/076/077). Informed consent was obtained from all participants for enrolment in this study. All procedures were conducted in compliance with the ethical standards of the institutional and in accordance with the Declaration of Helsinki.
Consent for publication
No personal identifying data are presented in the manuscript.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets generated for this study can be available from the corresponding author on reasonable request.