The ultimate goal of orthodontic treatment is to create “white” and “pink” esthetics in the front smiling zones. The interdental papilla is of great importance for achieving a pleasant smile. Despite an invisible appearance during orthodontic treatment, our present study clearly showed that aligner treatment creates a new dilemma in the esthetic zone, the higher incidence of OGEs.

One explanation for the higher presence of OGEs in the aligner group is the better periodontal health. The removable nature of clear aligners may facilitate better oral hygiene and less plaque accumulation. Indeed, patients treated with clear aligners developed fewer gingival diseases than those with fixed appliances in 12 months [11]. Similar discoveries have also been reported in the systemic reviews [12, 13]. However, Madariaga and Chhibber discovered no significant difference in oral hygiene levels among different orthodontic appliance groups with frequent hygiene instructions after 3 and 18 months, respectively [14, 15]. Since the appraisal of OGEs is completed in the photograph at the removal of braces, follow-up photographs with a receded periodontal inflammation after fixed appliance treatment may show a differed result. And further studies with detailed periodontal parameters may help clarify whether the occurrence of OGEs in the aligner group is a result of less periodontal inflammation.

The mechanic nature of correcting irregularity in the aligner technology may also contribute to the occurrence of OGEs. The clear aligners are designed based on the final 3-dimensional models by automated software with a mean accuracy of only 41%-50% in achieving the predicted tooth movement [16, 17]. Furthermore, correction of rotation and vertical issues is more difficult with clear aligners [18, 19]. Therefore, over-correction may be applied to fulfill the final results. Consequently, part of the space ought to be filled with papilla is occupied by aligners and remain open after orthodontic treatment.

In addition, the SmartTrack® material with greater elastic recovery and better adaptability makes for the close-fitting of the aligners to the dentition [20]. Furthermore, the aligners cover all the teeth and partly the keratinized gingiva 22–24 h a day and should be worn totally 400 h for efficacy [21]. The extension of the aligner tray into the interproximal area for retention may fill the occlusal part of the embrasure. This can lead to inadequate space for the gingiva filling, especially in adult patients with crowding because anatomical and physiological features of interdental papilla house are closely related to gingival papilla contour [22].

Since the clear aligners contact with the gingival margin directly, the biocompatibility of aligner materials has also been considered correlated with periodontal health and tested in several in-vitro studies. Martina et al. noted slight cytotoxicity of clear aligner materials on human gingival fibroblasts, pointing out that the thermoforming process increased the cytotoxicity [23]. The expression of proteins related to the inflammatory response in human oral epithelial cells was also observed to be affected by Invisalign appliances [24]. The inactive or dead cells as well as periodontal tissues in inflammatory conditions can possibly lead to gingival recession and the failure of papilla filling.

Some investigators [25, 26] reported that the risk of OGEs increased with the aging of periodontal tissues and the papilla height decreased 0.012 mm/year of age. The susceptibility to gingival recession were also noted in females compared with males [27]. In the present study, the age and sex ratio in the two groups were not significantly different, which helps improve the comparability.

Patients with crowded central incisors were reported to exhibit OGEs after orthodontic treatment more likely [2]. As clear aligner treatment is applied more often in cases with mild to moderate malocclusion [28], the pretreatment incisor rotation and overlap were measured in the two groups. No difference was found. Therefore, the occurrence of OGEs is not a result of differed case difficulty.

Tarnow has reported that the incidence of OGEs was 2% when the distance from ICP to ABC was within 5 mm, and the incidence rate increased as the distance increased [29]. No difference was found between the two groups, which may be a result from patient inclusion and periodontal measures. However, the determination of ICP-ABC distance was usually measured on periapical radiographs in the study of the periodontic field for its good accordance with the actual distance [30]. Periapical parallel radiograph should be included in further study to explore whether ICP-ABC distance accounts for the difference in OGEs incidence.

Divergent roots and triangular-shaped crown increased distance from the alveolar bone crest to the interproximal contact [4, 31, 32]. A triangular-shaped crown may also be related to “scalloped-thin” gingiva that experiences higher risks of deficient papilla [31, 33]. No relationship between IPR and the incidence of OGEs was observed in a recent research [5]. Similarly, no significant difference was found on crown ratio, root angulation or the amount and distribution of IPR between the two groups in our present study.

Because tooth morphology at the cervical third are vital for gingival filling, we defined CW by the width at the borderline of the gingival and middle third [9]. No difference in CW/CL between two groups was found in the present study. However, determining the most appropriate reference points is rather difficult. For crown length, it is affected by attachment loss, gingival inflammation and incisal attrition. For crown width, it is influenced by height of gingival papilla, level of gingival margin and the morphology of the interproximal area, and the width of contact area [34]. For example, gingival swelling may reduce crown length, while gingival recession may increase crown length. In addition, a high gingival papilla will conceal the crown width line.

The nature of retrospective study made the randomization and standardization difficult. Several key issues, such as cost difference between the expensive clear aligners and the relative cheap fixed appliances, patient preference for appliances, and case selection, make randomized controlled tests difficult. For example, socio-economic factors may affect the patient's motivation for dental health and periodontal treatment; in addition, orthodontists may recommend clear aligners for cases who need arch expansion and prefer IPR in treatment with aligners [5, 35]. Therefore, multicenter randomized controlled trials are needed to explore the difference in the incidence of open gingival embrasures in future studies.

Complete orthodontic records were needed for all the included patients in our present study. The incidence of OGEs may be underestimated due to survivorship bias and patient recall bias [36]. A long-term recall examination should also be included in the further study to verify the influence of clear aligners on the OGEs.

Oral hygiene compliance, medication and lifestyles may significantly affect periodontal health during orthodontic treatment. Although the distance from ICP to ABC was measured in the present study, the measurement was not enough to reflect the periodontal health status. Periodontal parameters, such as probing depth, bleed on probing, clinical attachment level and gingival index should be included in further studies. Moreover, self-perception of smile esthetics should be further investigated to obtain an overall treatment outcome from the patient’s perspective, since the OGEs greatly compromise an esthetic smile [37].

Although more studies with expanded sizes of samples and sites are required to confirm the discovery, it is necessary for practitioners to have a discussion with patients about the occurrence of OGEs before treatment and take the type of orthodontic appliances into account to prevent or reduce esthetic problems. Moreover, further investigations are expected to explore the association between Invisalign system aligners and the incidence of OGEs among different populations, for example, patients with teeth extracted or periodontal diseases. In this study, we also attempted to evaluate the severity of OGEs by calculating their areas. However, an OGE is actually a three-dimensional structure. Thus, a 3D analysis of the OGE volume merits further research for better accuracy in grading the severity of OGEs.