A digitalized analysis of incisal changes among orthodontically treated patients: A retrospective comparative study

Introduction

The scope for orthodontic treatment in creating beautiful smiles appears to be seamless, as people increasingly desire to be more conscious about their socially desirable appearance. Several parameters are considered while planning orthodontic treatment to recreate smiles, such as the smile arc, smile line, smile symmetry, and buccal corridors.¹ The development of digital imaging techniques and technologies has drastically improved over time, leading to the development of significantly superior aids in diagnostic and treatment planning, particularly in the field of orthodontics.

Among these, the incisal position in the arch is key to diagnosis. The angulation and inclination of crowns of teeth are said to affect the arch perimeter during orthodontic treatment planning. Therefore, incisal angulation plays an important role in supporting dentition and obtaining normal occlusion with facial harmony to the surrounding soft tissue.²

Dental crowding, also known as swarming, especially in the anterior region, is one of the most common findings in orthodontic patients.³ Dental crowding can be defined as a disparity in the relationship between tooth and jaw sizes, which results in imbrication and rotation of teeth. Dental crowding may occur because of excessively large teeth, excessively small bony bases of the jaws, and a combination of large teeth and small jaws.⁴ The clinical forms of dental crowding may be classified based on the degree of manifestation: mild, moderate, or severe; location, anterior, intermediary, lateral, or posterior position; and cause/etiology: primary, secondary, tertiary, combined, or transient.⁵

Transverse arch expansion, tooth extraction, and interproximal enamel reduction (IPR) are among the several methods suggested to relieve dental crowding.³ When arch widths are less in the case of crowding when compared to spacing, this is an indication of arch expansion to accommodate all teeth.⁶ Extraction of all first premolars is usually indicated in cases of moderate to severe crowding in the labial segment to create space and relieve crowding.⁷ Finally, IPR is a clinical procedure that involves reduction, anatomic re-contouring, and protection of the interproximal enamel surfaces of permanent teeth, and it is indicated for patients with mild to moderate crowding (4-8 mm), which is an alternative procedure to extraction.⁸

Orthodontic treatment may require tooth extraction to create space for teeth to move into their newer, straighter positions. Owing to the expected effect on the dental arch length, there has been a continuous debate between extraction and non-extraction planning.⁹ A study conducted by Konstantonis¹⁰ compared the difference between both protocols, and the results showed a significant increase in lip retraction alongside the development of the morea obtuse nasolabial angle among the extraction subjects.¹¹ However, incisor display was insignificant. Orthodontic diagnosis and treatment planning on accurately analyzed lateral cephalometric radiographs.¹⁰ Although cephalometric studies are subject to errors and are mostly affected by bias, randomization of measurements is one of the most effective methods for avoiding bias.¹² However, metal brackets have been the conventional and effective orthodontic appliance for over a hundred years. Owing to the increase in patients’ demand for a more esthetic and comfortable orthodontic treatment, the technique has fueled the concerns of clear aligners. With the development of orthodontics, clear aligner therapy was introduced in the early 1900s, and the initial cases involved minor crowding or spacing.¹³

Although both clear aligners alongside the metal brackets are effective in treating malocclusion, clear aligners have an advantage in the segmented movement of teeth and shorten the treatment duration. Moreover, metal brackets are more effective than aligners in achieving great improvement, producing adequate occlusal contacts, controlling teeth torque, and increasing transverse width and retention. Therefore, clinicians should consider the characteristics of these orthodontic appliances when making treatment decisions.¹³ Therefore, the rationale of the current study was to assess the efficiency of clear aligners’ expansion mechanism of action and determine if it would influence the need for IPR in overcoming incisal flaring and/or crowding compared with conventional fixed appliances. Hence, this study aimed to compare incisors’ angulation and/or position changes among orthodontically treated subjects with metal brackets versus clear aligners.

Methods

Instruments

Digital tracing of the enrolled lateral cephalometric radiographs was conducted at T0 and T1. Innovative artificial intelligence software (WEBCEPH Medical Image Analysis, Version 1.5.0) designed and coded by an orthodontist was used. The software was used to fabricate analysis of certain angles alongside linear interpretations in reference to the measurements as described in Figure 1 and Table 2.¹⁵ Double confirmation of repeated cephalometric tracing was performed to reduce the potential number of errors within the analysis.

Figure 1. WEBCEPH analysis of linear and angular measurements.

Table 2. Lateral cephalometric measurements and interpretation description.

Measurement	Type	Description
SNA	Angular	Angle between lines SN and NA representing the position of Max in relation to the cranial base
SNB	Angular	Angle between lines SN and NB representing the position of Mand in relation to the cranial base
ANB	Angular	Angle between lines AN and NB representing the skeletal classification
U1-SN	Angular	Angle between a line through the long axis of the upper central incisor and SN line represents the UI angulation to the cranial base
UI-FH	Angular	Upper incisor to Frankfurt horizontal plane represents the UI angulation
UI-PP	Angular	Upper incisor to palatal plane represents the UI angulation
UI-NA	Angular	Angular Angle between a line through the long axis of upper central incisor and NA line
LI-MPA	Angular	Formed by the mandibular plane and a line drawn down the long axis of the mandibular incisor representing the LI angulation
LI-NB	Angular	Angular Angle between a line through the long axis of lower incisor and NB line
UI-LI	Angular	Angle between a line through the long axis of upper and lower incisors representing the inter-incisal angle
Overjet	Linear	Horizontal distance between tips of upper incisor and the labial surface of lower incisor
Wits Value	Linear	Measurements of perpendicular projection of points A and B to occlusal plane to confirm the classification

For both groups, the required variables were gender, age, whether conventional or clear aligners were used, and whether expansion or IPR was performed. For the clear aligners group, the required information was collected from private practices in Riyadh, alongside the 3-dimensional virtual simulation (Clin-check) of an orthodontist prescribed that shows the stages of the treatment plan and makes the required changes observed for each case, respectively.

Clin-check enables clinicians to make changes to the treatment plan themselves, such as changing the size and rotation of the attachments. The arch shape can also be adjusted as needed by expanding or narrowing the anterior or posterior segments of the aligners. It has specific tables that prescribe the required amount of IPR and its position, whether anterior or posterior, in the upper or lower arches. In addition, it also provides expansion details that affect the intercanine or intermolar width, reflecting a glance over expected changes throughout the patient’s treatment journey. On the other hand, for conventional metal cases, the site of IPR and/or type of expansion using an appliance or an archwire were collected and exported from the included patient’s files (progress notes) written by his/her orthodontists.

Descriptive statistics of the frequency distribution, percentages, means, and standard deviations were calculated for the study variables. The study data followed a normal distribution. Pre and post changes in cephalometric variables after conventional orthodontic treatment and clear aligner therapy were compared using paired t-tests. At the same time, an independent samples t-test was applied to compare the mean differences in cephalometric variables between conventional and clear aligner orthodontic treatments. Differences were considered statistically significant at P <0.05. Data analyses were performed using SPSS version 25 (IBM-SPSS, Armonk, NY, USA).

Results

Table 3 shows the distribution of the study variables. A total of 31 patients who underwent fixed orthodontic therapy using the conventional method and 31 who used clear aligners were included in this study. Most study participants were females in both treatment groups. Irrespective of whether the patients were treated using the conventional method or clear aligners, expansion, IPR, or both were performed on these patients. However, more than half (54.8%) of the clear aligner patients received IPR.

The mean age of the patients treated by the conventional method was 28.97 ± 4.35 years, while for the patients treated by clear aligners was found to be 18.81 ± 2.57 years. The mean and standard deviation values of each cephalometric variable before and after the conventional orthodontic treatment are shown in Figure 1. All variables considered in this study showed slight changes in mean values pre and after treatment.

Figure 2 shows the pre- and post-treatment changes in cephalometric variables following clear aligner orthodontic treatment. Cephalometric variables of U1-SN (106.19 ± 7.57 vs.101.83 ± 5.97, p = 0.001), UI-FH (115.89 ± 6.74 vs. 111.59 ± 5.40, p < 0.001), U1-PP (54.70 ± 5.01 vs. 58.41 ± 4.66, p = 0.002), U1-NA (24.12 ± 6.64 vs. 19.98 ± 6.38, p = 0.002), UI-LI (122.71 ± 10.93 vs. 128.3 ± 79.95, p = 0.005) showed statistically significant difference before and after treatment using Clear aligners. In contrast, no such significance was observed with conventional orthodontic treatment using paired t-test (Table 4 & Figure 3).

Figure 2. Conventional orthodontic treatment.

Figure 3. Clear aligner treatment.

A comparison of the mean differences in cephalometric variables between clear aligner therapy and conventional orthodontic treatment using an independent t-test is shown in Table 5. Cephalometric variables UI-SN (-4.361 ± 6.770 vs. -0.670 ± 5.348, p = 0.020), UI-FH (-4.301 ± 5.727 vs. -0.574 ± 5.461, p = 0.011), and UI-NA (-4.135 ± 6.612 vs. -0.141 ± 5.183, p = 0.010) were found to be significantly decreased in torque after clear aligner therapy compared with conventional orthodontic treatment. However, UI-LI (5.660 ± 10.279 vs.-0.369 ± 9.398, p = 0.019) demonstrated a significant increase with clear aligner therapy compared with conventional treatment. The intercanine and intermolar widths were significantly higher in the expansion cases than in the IPR cases (p < 0.05) (Table 6). The amount of orthodontic space gained was not significantly higher in the upper jaws than in the lower jaws after IPR (Table 7).

Discussion

The Assessment of incisor inclination is a major part of orthodontic treatment. The main objective of this study was to compare the effects of metal brackets and clear aligners on changes in incisor position. By choosing cephalometric landmarks and based on their predictability, the results were predictable, and the use of the WEBCEPH for the calculations reduced measurement errors.^14,16,17

Clear aligner patients were significantly younger than those in the fixed appliance group (18.81 and 28.97 years old, respectively), which was indicative of younger patients preferring clear aligners over the conventional method for esthetic benefit, which upholds their level of confidence. However, the findings of our study contrasted with those of a study conducted in Ohio by Gu et. al., in which older patients underwent orthodontic correction by Invisalign. The contrasting result could be suggestive of the consciousness regarding physical appearance, easier approach to treatment by placing the trays in the mouth as directed by orthodontists, and shorter treatment time with clear aligners.¹⁸ Since all patients included in the current study were older than 16 years, it was unlikely that growth played a significant role in the treatment outcome.

Our study included more female patients than male patients in both groups, which holds true because females are more concerned about their aesthetics and appearance.¹⁹ Clements et al. conducted a study at the University of Washington. al. supported clear aligners as being more successful in improving anterior alignment, transverse relationships, and overbite than fixed appliances, indicating a significant reduction in UI-SN, UI-FH, and UI-NA, because clear aligners improved the anterior segment by lowering upper incisor flaring, which is consistent with our study findings. The study also suggested that clear aligners have no adverse effects on gingival health during treatment.²⁰

A study by Ke et al. compared the treatment effectiveness between fixed orthodontic treatment and clear aligners and found that tooth torque was controlled more effectively when fixed appliances were used¹³; however, our study showed that clear aligners were more effective in controlling incisor inclination and showed a statistically significant difference in the inter-incisal angle (UI-LI) before and after treatment. Hence, we reject the null hypothesis and state the alternative hypothesis that clear aligners are more effective than conventional fixed appliances in improving incisor inclination.

A study by Kassas et al. at the State University of New York evaluated 31 Invisalign cases before and after treatment and the results of their study showed a significant improvement between the pre- and post-treatment models of the patients which supports our hypothesis that clear aligners provide a useful orthodontic correction in cases of mild to moderate malocclusions.²¹ However, the retrospective study by Djue et al. compared the outcomes of Invisalign treatment to those of fixed appliances using the objective grading system of the American Board of Orthodontics (ABO) in a sample of adult patients and showed that treatment with fixed appliances was significantly more effective than treatment with Invisalign, which was the least successful in correcting occlusal contacts.²²

In relation to existing literature on incisal inclination, Ghaleb et al. evaluated the impact of maxillary incisor inclination on the aesthetics of the profile view of a smile and found that smile aesthetics in the profile view were dramatically affected by incisor inclination.²³ Cao et al. reported that maxillary incisor protrusion and lingual inclination were preferable to retruded or flared incisors, although increased maxillary incisor labial inclination might flatten the smile arc and decrease the incisal display, which showed the importance of aesthetics and appearance in the current study.²⁴

The use of artificial intelligence in the WEBCEPH software for analyzing cephalometric radiographs leads to absolute measurements. Katyal et al. in their study, WEBCHEPH was more reliable than digital tracing using FACAD and manual tracing for analyzing lateral cephalometric radiographs. Moreover, multiple other studies conducted over various other imaging software to evaluate the incisor inclination in lateral cephalometries such as Aldrees,²⁵ used Dolphin Imaging^® 10.0 software.²⁶ Shahakbari et al. who used Onyx Ceph version 3.6 software examined its reliability compared to manual measurements.²⁷

IPR and expansion are frequently used methods in conjunction with orthodontic treatments, and this has gained popularity in view of the increasing trend towards non-extraction-based treatment modalities.²⁸ In the current study, the amount of expansion was compared with the IPR performed. It was found that the inter-canine and inter-molar distances of both upper and lower arches were significantly higher in expansion in comparison to the IPR cases, thus providing more space to facilitate treatment and improve the final results, which was in line with a study conducted by Chung et al. in 2015, which suggested that expansion is the treatment of choice in the presence of a normal-sized dentition and decreased arch width because it enables widening of the dental arches, which predictably increases the arch perimeter, which could be a reason for the expansion method to create more space than IPR.²⁹

Our study has certain limitations. Since the design of our study was retrospective, some difficulties were encountered in tracing the exact amount of IPR and expansion values among the fixed appliance group due to insufficient documentation. The data that support the findings of this study are openly available in public repository.³⁰

Conclusions

Within the limitation of the current study, it can be concluded that:

Consent

Written informed consent for publication of their clinical details and clinical images was obtained from the patients.