Twenty years of clear aligner therapy: a bibliometric analysis (2002-2022)

Citation ‘bursts’ are articles that have seen a significant increase in citations over a short period and may reflect the focus of research at that time. The top 25 studies with the highest burst values are shown in Figure 4. Nine of the articles reported on the comparison between clear aligners and traditional fixed orthodontic appliances (2, 12, 14, 15, 18, 20, 21, 22, 25). Seven studies focused on the accuracy of clear aligners (4, 5, 8, 10, 13, 14, 17), while five studies reported the periodontal health of patients treated with clear aligners (12, 18, 19, 20, 22). Other topics included orthodontic force, attachments, material science and clear aligner generations.

Figure 4.

Keywords can accurately reflect the research focus of a certain period. Therefore, detecting emergent keywords can help summarise the development of research frontiers and explore new topics. The nine most frequently cited keywords are provided in Figure 5. The keywords with the longest use include orthodontic treatment, digital orthodontics, superimposition, and apical root resorption.

Figure 5.

Digital orthodontic technology allows three-dimensional (3D) image manipulation through computer software and 3D printing of custom devices made of different materials. Of the orthodontic applications, a series of custom-made clear aligners that move the teeth throughout the entire treatment period are supported by three pillars: digital image acquisition of the patient’s dental arch, visualisation and processing of the images using specific software, and the 3D printing of files.²⁵

A conventional study model is based on the acquisition of a physical impression and the subsequent casting of plaster models. Compared to a conventional impression, intraoral scanning directly captures optical impressions, offering shorter chair time, a reduction of consumables costs, greater patient comfort and a high digital accuracy. Like other 3D scanners, the intraoral scanner captures images through the projection of a laser light source or structured light without interacting with biological tissue.²⁶ These devices provide specific software for processing data and generating 3D virtual images of the dental arches.

The arch surface morphology data is usually saved on the computer as a Standard Triangular Language (STL) format file. The virtual dental casts allow clinicians to quickly obtain diagnostic information related to arch width and perimeter, model discrepancies, a Bolton discrepancy, overjet, and overbite. Several studies^27-31 (ranks 15, 32, 38, 41, 48) used an electronic digital calliper to make measurement on STL-generated dental casts. Moreover, an optical impression makes patients feel more involved in their treatment and is a powerful tool to establish more effective communication.^32,33

The most common use of technical processes in orthodontics is the digital setup and fabrication of clear aligners. Traditionally, an orthodontic setup from crown separation to repositioning in wax, are performed on plaster models. With the help of digital technology, orthodontic treatment simulation processes have become faster and more practical. The model for the reference treatment phase can be automatically generated by the setup software and used for orthodontic appliance production.³⁴

Applying digital technology in orthodontics aims to reduce the professional’s chair and laboratory time, as well as make treatment faster, predictable, aesthetic, and more comfortable for patients. It may be concluded that the advent of digital orthodontic technology is a unique evolutionary milestone in orthodontic history, as it offers great possibilities for use in clinical practice, with potential benefits for both patients and practitioners.

It is noteworthy that, unlike actual biological dental movements, virtual movements are infinite and often the results may not be realistic.³⁵ The ability to measure the amount of tooth movement is important for assessing orthodontic treatment outcomes. An analysis of tooth movement allows clinicians to better understand orthodontic biomechanics, the speed and type of movement, as well as the efficiency of therapeutic interventions. The use of 3D scanners in dentistry provides a non-invasive method for measuring dental changes because it does not expose the patient to radiation.³⁶ In addition, 3D-model rendering by most scanners is considered accurate and reliable. Of the most-cited publications (ranks 2, 5, 14, 21, 25, 39), there were several studies using superimposition to calculate the mean accuracy of tooth movement. The most cited article (overall rank 2) was by Kravitz,³⁷ who measured the mean movement accuracy of 401 anterior teeth by the overlay of the virtual models.

Digital maxillary study casts are more commonly used to measure tooth movement because the palatal vault is considered stable throughout treatment for the evaluation of tooth positional change.^38,39 Several investigators have suggested using the medial point^38,40 or the medial two-thirds of the third palatal fissure^41,42 as a reference landmark for maxillary cast superimposition.⁴¹ However, the mandibular arch lacks stable landmarks, and requires the use of a combination of cone-beam computed tomography (CBCT) images to digitally align the surface superimposition of the model on the mandibular basal bone structure.⁴³

Root resorption is an unavoidable sequela of orthodontic tooth movement. In addition to genetic influences and trauma, orthodontic treatment is a factor that may lead to partial resorption of the root apices of the teeth,⁴⁴ generally called external apical resorption (EARR). Age, the force applied during treatment, extraction or non-extraction conditions, treatment duration, the distance of tooth movement and the level of pre-treatment root resorption are factors that impact on root resorption.⁴⁵ Because EARR is irreversible and may affect tooth longevity, it is important for the clinician to determine changes in root resorption during orthodontic treatment. Several studies have examined the effect of CA therapy on EARR, but there is disagreement regarding the level of EARR. Gandhi et al.⁴⁶ concluded that the mean root resorption for the permanent maxillary incisors was in the range of 0.25 mm to 1.13 mm (overall: 0.49 mm; 95% confidence interval [CI] = 0.24 to 0.75 mm). Previous studies have reported that root resorption associated with CA therapy for the maxillary permanent incisors was less than for fixed orthodontic appliances, but was not statistically significant, except for tooth 12. This effect may result from the decreased magnitude of force delivered by CA therapy compared to FOA or due to discontinuous force application with CA therapy.^47,48

Pain complaints are a common feature of the orthodontic treatment process⁴⁹ and directly affect patient satisfaction.⁵⁰ It has also become a principal topic in the field and clusters 7, 18, 33 were found to be related to this issue.^51–53 During orthodontic treatment, it is common to feel pain and discomfort, reaching a peak at 24 hrs after force application but imperceptible after 7 days. However, removable appliances produce intermittent forces, which allow the tissues to re-organise before compressive forces are reapplied⁴⁸ and have a reduction in the level of pain and discomfort reported by patients.⁵⁴ Most studies found that patients treated by CA therapy experienced less pain than those treated by FOA during the first week of orthodontic treatment.⁵⁵

Like other bibliometric analyses, the present study had limitations. The first relates to the time delay as recently published high-quality studies and highlights may have been excluded because of insufficient citation. Secondly, although the analysis was conducted objectively by software, there is an inherent subjective bias in the interpretation of the results.

Despite the limitations, the study has relevance in the field as it systematically analysed the developments, priorities, and trends in CA therapy. New research should carefully consider the most popular and recent clusters and read references applying high median centroids, citation frequencies, and citation burst values.