In addition to esthetics problems, OGE is also associated with various functional implications, such as gingival inflammation, poor gingival health and the loss of integrity . In our study, OGE occurred in 25.7% of maxillary and 40.3% of mandibular central incisors after clear aligner therapy. IPR was not associated with the occurrence of OGE but was associated with severity. The number of attachments in the anterior teeth or central incisors was significantly related to the incidence of OGE but this was not associated with severity. Furthermore, the incidence and severity of OGE in the mandibular was higher and more severe than in the maxillary. The risk of aesthetic loss caused by clear aligners calls for clinicians’ attention.
The incidence of OGE between the incisors in non-extraction patients after clear therapy ranged from 25.7–40.3%, while other researchers previously reported that the incidence of OGE in both non-extraction and extraction patients was 22–36% after the fixed appliance treatment . The incidence of OGE seems to be higher in extraction patients than in non-extraction patients because of the greater difficulty and more retraction of teeth [21,22,23]. Consequently, we conjectured that the incidence of OGE in patients with clear aligners is higher than that of patients with fixed appliances. However, more precise clinical studies are needed to confirm this conjecture.
Clear aligners appear to be associated with periodontal risk for several reasons. Firstly, the initial stress from clear aligners is approximately 50 to 500 times more than that of fixed orthodontic loading , and the instantaneous higher-stress may have a certain impact on the periodontal tissues. The excessive initial stress of the clear aligners was demonstrated in rats that received a clear plastic appliance; hyalinization of the PDL was evident in the areas of compression as early as day 1 . In the meanwhile, the labial movement of the anterior teeth in non-extraction cases induced alveolar bone loss and jeopardized periodontal health, especially in adults [23, 25], which suggest a risk for non-extraction patients. Due to the advantages of clear aligners, such as molar distalization, non-extractive orthodontics may be pursued excessively in borderline extraction cases. Additionally, high active forces and levels of compression, which could cause net bone loss and extensive gingival recession, were identified in the marginal tissues during tipping movements and intrusion . Unfortunately, torque control in clear aligners is not as effective as in braces . Therefore, orthodontists need to reconsider the biomechanics of clear aligners and determine the applicable population of clear aligners cautiously.
Researchers have also found that OGE may appear when the distance from the contact point to the alveolar crest is > 5 mm ; thus, IPR is an effective means with which to relocate the contact point [14, 15], although there is no clinical evidence that IPR can prevent the occurrence of OGE. Interestingly, in the present study, we did not find a relationship between IPR and the occurrence of OGE, but we conjectured that IPR could reduce the severity of OGE. Further studies with larger sample size are required to confirm this conjecture through multinominal regression analysis. Perhaps preventive IPR is lack of accuracy, and the amount of preventive IPR is inadequate. The contact points at the beginning of treatment are usually different from the contact points after treatment because the root angle of the untreated anterior teeth is abnormal and teeth may overlap due to crowding . Preventive IPR cannot directly target the position of contact points after treatment which are really related to OGE. Also, the amount of IPR of no more than half the enamel coating’s original thickness was recommended to avoid proximity to the dentin, thus indicating that the amount of IPR at an interproximal gap is finite and may not be sufficient to prevent the occurrence of OGE . In contrast, as a means of treatment, IPR can be accurately located and the amount of IPR can be adjusted based on comprehensive consideration. IPR is generally proposed in treatment programs to provide space to solve crowding or repair abnormal tooth morphology . In addition to being used as a therapeutic method, IPR is associated with the release of anterior teeth adjacent points, and incomplete release of anterior teeth will hinder tooth movement, thus leading to adverse stress transfer among teeth. The categorical and measurement data of IPR in this paper reflects not only the amount at a specific interdental gap, but also the extensiveness of its distribution. Unfortunately, we did not identify a relationship between IPR in anterior teeth and OGE in the central incisors. The relationship between the release of the interproximal space by IPR and force conduction among teeth requires further in-depth studies. Above all, for inexperienced clinicians, additional preventive IPR may be unnecessary but regular interdental release is necessary, clinicians only need to carry out IPR according to specific clinical needs. When OGE occurs, a small amount of IPR (but applied frequently) as a treatment method could be carried out, and combined with other methods like the intrusion to improve OGE.
There are two types of force transmitted from the attachments to the periodontium: active force  and removal stress . The odds ratio of subgroups in which the patients had attachments on both central incisors was 9.501, indicating that the occurrence of OGE was 9.501-fold higher when compared to the subgroup of patients who did not have attachments in the central incisors. For patients in which both central incisors had attachments, the two types of forces from attachments are greater than those in patients without attachments in the incisors. Furthermore, the number of attachments reflects the complexity of tooth movement . In order to exclude other interfering factors as accurately as possible, the thickness and material of the diaphragm remained the same , and non-extraction patients were selected for this study so as to unify and simplify tooth movement types as far as possible. In addition, our results showed that attachments on the canines had a greater effect on OGE between the central incisor than attachments on the lateral incisor. Because our study did not distinguish between optimal attachments and the traditional attachments more commonly used on canine teeth, the removal forces may be greater for the retainer attachments in canine teeth. Besides, in two-step intrusion, attachments in the canines can also act as anchorage to the intrusion of incisors; this may be related more closely to the movement of central incisors. Nevertheless, further biomechanical investigation of the relationship between the distribution of attachments and OGE is needed. The features of attachments not only help control tooth movement but also promote optimal biological responses. Consequently, in addition to tooth movement needs, the design of the attachment should also consider the periodontal conditions of the anterior teeth.
We found that the incidence of OGE at the end of the first version of treatment was similar to that after complete treatment, thus suggesting that the data on IPR and attachments collected from subgroups was representative. Age, the duration of treatment and gender were not significantly related to the occurrence and severity of OGE. As patients with periodontitis were excluded from this study, the effect of age may have been underestimated. Since we only selected non-extraction patients, the treatment difficulty was similar, and the treatment time was not statistically different. Moreover, due to the aesthetic outcomes associated with clear aligners, most patients are female; there was no significant difference between the groupings with respect to gender.
To the best of our knowledge, this study is the first to assess the prevalence of OGE after clear aligner therapy. We attempted to objectively investigate the relationship between OGE and contributing factors that are unique to clear aligners, such as attachments. Furthermore, this article was an initial clinical study to explore that whether IPR can prevent OGE. With the increasing popularity of clear aligners, more studies focusing on clear aligners are necessary in the future.
There are some limitations in this study that need to be considered. Additional variables, such as crowding and alveolar bone height, could not be included in the regression model due to numerical insufficiency; because the number of variables entered into a logistic regression model increases, the stability of the model decreases. For this reason, we utilized stricter inclusion and exclusion criteria to minimize the risk of bias as much as possible. Because there may have been more than one version of Clincheck software for each patient, we could only evaluate the number of attachments according to the first version, select the first version of treatment that was longer than 24 months, and match corresponding intraoral photographs. Due to the limitations of the retrospective study, we could only assess the prevalence of OGE from frontal intraoral photographs, the periodontal index could not be obtained. In addition, the condition of dentition gathered from the overlap of the intraoral scan model before and after treatment would be more accurate and instructive for the research in invisible orthodontics. Our study only evaluated the central incisors because of distortion and poor reproducibility of the lateral or canine areas in intraoral photographs. Additional studies that evaluate the incidence of different patients following tooth extraction after clear therapy and the biotype of the gingiva would be more meaningful and more instructive for clinical practice.