Clear aligners: A network and bibliometric analysis of 50 pivotal articles

Abstract

Background. A bibliometric analysis uses statistical and mathematical methods to analyze the published literature, extracting meaningful information that helps to identify trends, assess research impact, and visualize key data patterns through graphs and trendlines.

Objectives. The aim of the study was to list the top 50 papers on clear aligners, evaluate them critically and apply a bibliometric analysis to investigate the achievements and prospects in this field of study.

Material and methods. A computerized database search (Scopus) was conducted on September 24, 2023, to find articles on clear aligners published in the scientific literature between 2013 and 2023. The top 50 cited manuscripts were chosen. Author-based characteristics were obtained from the Scopus database. The dataset from Clarivate™ InCites Benchmarking & Analytics™ and Journal Citation Reports (JCR) was used to determine parameters based on journals and articles. To improve the visual analysis, the keywords were collected systematically from the chosen articles.

Results. The database search produced a directory of the 50 most often cited articles out of 1,405 papers. Of the 50 most frequently cited works on clear aligners, 35 (70.0%) were original research studies and 15 (30.0%) were reviews. The keyword-network analysis indicated ‘removable orthodontic appliance’ as the most important and repetitive keyword.

Conclusions. The results of the present bibliometric study indicated that clear aligner therapy (CAT)-related papers received increasing citations. The study determined the most influential articles by highlighting their authors and the journals in which the papers were published.

Keywords: bibliometric analysis, orthodontics, network analysis, clear aligner, clear aligner treatment

Introduction

Since Dr. H. Kesling developed a set of thermoplastic tooth positioners to gradually achieve teeth alignment in 1946, clear aligners have been used in orthodontics.

The introduction of new materials, dental attachments, tooth movement staging, interproximal enamel reduction, and the use of inter-arch elastics to handle several kinds of malocclusion have all contributed to a significant change in the handling over the last 15 years.^{1, 2, 3, 4, 5, 6}

Nowadays, clear aligner therapy (CAT) is an effective alternative to the conventional fixed appliances (FA) in orthodontics because of rapid advancement in technology and computer-aided design and manufacturing (CAD/CAM).^{7, 8, 9}

Adult patients prefer clear aligners for their comfort and esthetics. This is why the demand for aligners has greatly increased. All over the world, there are numerous brands of clear aligners, each with distinctive features and efficacy.^{3, 10, 11, 12}

As a consequence of the technological progress, there have been a lot of research projects and publications regarding clear aligners, which may make it more difficult for researchers to obtain accurate data.^{13, 14}

Citations indicate a relationship between authors, research teams, study subjects, or nations. They are a tool for measuring the influence and frequently the quality of a publication in a certain field. Furthermore, mapping bibliometric networks with a graphical analysis can offer a clear and thorough summary of a sizable dataset.¹⁵

A bibliometric analysis is a mathematical and statistical method of quantitative measurement to assess the quality of publications within a scientific research area. Several details on authors, journals and articles might be highlighted through this process; moreover, a visual network analysis might be provided for the keywords of these articles.¹⁶

As far as we know, network and bibliometric analyses have been used in different dentistry fields, although surveys on clear aligners have not been carried out recently.^{15, 17}

Thus, the goal of the present study was to carry out a bibliometric analysis of the 50 most often cited articles on clear aligners to allow a clearer view and to improve research on this topic.

Material and methods

Search strategy

On September 24, 2023, two authors independently conducted a database search to find papers on clear aligners published in the scientific literature considering the last 10 years (from 2013 to 2023), as shown in Figure 1.

The keywords used in the Scopus database (Elsevier, Amsterdam, the Netherlands) for the advanced search were: (“orthodontic” OR “orthodontic treatment” OR “orthognathic”) AND (“clear aligner” OR “attachments” OR “3d printed aligner” OR “digital set up” OR “digital planning” OR “customized aligner” OR “ thermoforming aligner” OR “digital models” OR “clear aligner treatment” OR “clear aligner therapy” OR “invisible orthodontics”). Scopus was chosen to narrow the publication time of papers and locate all possibly relevant research, independent of the kind of publishing. As reported in the literature, Scopus allows access to articles in all fields.¹⁸ The database is easy to search and covers more journals than PubMed® and the Web of Science, adding more precise information regarding citations and authors.

Each author downloaded the complete records of the publications retrieved from Scopus into a Microsoft Excel spreadsheet (Microsoft Corp., Redmond, USA). At this stage, the papers were independently screened by 2 reviewers (A.F and F.D.) according to titles and abstracts to determine the eligibility of the studies, excluding any papers that did not directly relate to clear aligners. In case of disagreement, a third reviewer contributed to reaching a consensus (V.G.). The 2 authors then carried out full-text screening. All the articles not concerning clear aligners were excluded. Therefore, out of the remaining papers, only the 50 papers on clear aligners most often cited in the literature were included in the bibliometric analysis.

The sample comprised papers on all types of clear aligners, regardless of the brand.

Data extraction

Article-based bibliometric parameters

The following article-based bibliometric data was extracted using the Scopus database: title; author(s); journal; keywords; publication year; study design; total number of citations; and the mean number of citations/year.

Journal-based bibliometric parameters

The journal-based bibliometric data listed below was gathered using Clarivate™ InCites Benchmarking & Analytics™ and Journal Citation Reports (JCR): the 2022 Journal Impact Factor (JIF), an indicator of the importance of a journal, derived from the data indexed in the Web of Science Core Collection (if available); quartile with regard to the category “Dentistry, Oral Surgery & Medicine”; Eigenfactor Score (EF), measuring the number of times the articles from the journal published in the past 5 years have been cited in the JCR year; Normalized Eigenfactor Score (nEF), using journal year rescaling; Article Influence Score (AI), which normalizes EF by determining the average influence of the journal’s articles over the first 5 years after publication; 5-Year Journal Impact Factor (5-JIF), considered as the average number of times the articles from the journal published in the past 5 years have been cited in the JCR year; and Immediacy Index, the count of citations in the current year to the journal that reference content in this same year.

Author-based bibliometric parameters

With the help of the Scopus database, the following author-based bibliometric parameters were extracted: name; affiliation; country; H-index; number of articles among the top 50 articles about clear aligners; and number of citations of the papers which made up the top 50 articles about clear aligners.

Results

Articles

From the research query, 2,235 papers were found. With regard to the years 2013–2023, 1,405 papers were found through applying a limiting filter; they were collected from the most cited to the least cited ones.

Table 1 details the top 50 articles on clear aligners.^{1, 2, 4, 5, 6, 7, 8, 9, 10, 13, 14, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57}

Of the 50 articles, 24 were about Invisalign® (Align Technology, San Jose, USA), while the remaining 26 were about unspecified clear aligners.

A total of 35 (70%) articles were original articles, and the remaining 15 (30%) were reviews. Among the 35 original papers, 14 were prospective studies, with 2 RCTs (28%), 12 were retrospective studies (24%), 6 were in vitro studies (12%), 2 were finite element analysis studies (4%), and 1 study had a cross-sectional design (2%). Among the reviews, 7 were systematic reviews (14%), 6 were reviews (12%), and only 2 were meta-analyses (4%). Further details are shown in Figure 2.

The oldest article was from 2013 and the most recent papers were from 2021. The year 2020 was the publication year for as many as 11 papers, whereas 2015 was the year with the greatest number of citations (Figure 3). The most often cited paper about clear aligners has collected 289 citations and was published by Angle Orthodontist in 2015; it is a systematic review titled: “Efficacy of clear aligners in controlling orthodontic tooth movement: A systematic review”.⁴ The second most often cited article is titled “Treatment outcome and efficacy of an aligner technique – regarding incisor torque, premolar derotation, and molar distalization”; it is a retrospective study published in 2014 by BMC Oral Health, which has been cited 144 times.¹⁴ The third article is a retrospective study reporting 113 citations; it is titled “Forces and moments generated by removable thermoplastic aligners: Incisor torque, premolar derotation, and molar distalization” and was published by American Journal of Orthodontics and Dentofacial Orthopedics in 2014.¹³

Further details are shown in Table 1.

Number of citations

The 50 articles obtained a total of 2,862 citations. As shown in Figure 3, 2015 was the year with the greatest number of citations, when an important systematic review headed “Efficacy of clear aligners in controlling orthodontic tooth movement: A systematic review” was published.⁴ The article with the greatest average number of citations per year (32.1) is the first on the list, published in 2015 and it is a systematic review.⁴ However, the article titled “Has Invisalign improved? A prospective follow-up study on the efficacy of tooth movement with Invisalign” was published in 2020 and had a mean of 24.75 citations per year.⁷

Journals

Table 2 shows that the top 50 referenced papers on clear aligners were published in 20 different scientific journals. Angle Orthodontist had the most citations of any scientific journal (n = 753; 26.31% of total citations). This journal also published the most manuscripts from among the 50 most referenced papers (n = 11; 22%).

Furthermore, Table 2 illustrates the range of the 2021 JIF from 24.9 (International Journal of Oral Science) to 1.0 (Australian Dental Journal) for the scientific journals which published the 50 most often cited articles on clear aligners in the Web of Science, while the 5-JIF varied between 13.7 (International Journal of Oral Science) and 1.16 (Seminars in Orthodontics). Out of the 20 scientific journals, 80% were in the category “Dentistry, Oral Surgery & Medicine”, according to the JCR dataset.

Further details are shown in Table 2.

Authors, institutions and countries

To identify the most productive authors on the topic, information about all authors (201) was downloaded from Scopus and imported into an Excel file. Among the most productive authors, G. Rossini, M. Simon, J. Hennessy, and S. Mummolo were listed as the first authors twice.

Keywords network analysis

Based on the cooccurrence map of keywords, concepts like “clear aligners”, “orthodontic tooth movement” and “removable orthodontic appliance” occupied the larger and centrally positioned nodes (Figure 4).

Discussion

This bibliometric analysis aimed to update the previous analysis,¹⁵ expanding the research to clear aligners in general. A sample size of 50 articles was chosen following other comparable publications¹⁵ to obtain an adequate amount of information and to create a graph with significant trendline data. The last 10 years have been chosen to analyze the most recent articles of greater interest for clinicians and researchers.

It is worth noting that the most cited type of article is a systematic review, a product that is at the apex of scientific evidence, and is therefore a reference for researchers and clinicians. The same type of article in the previous bibliometric analysis reached the 3^rd rank with 83 citations.¹⁵ The particular systematic review sums up the literature on the efficacy of clear aligners.⁴ At the 2^nd and 3^rd place, there are two retrospective clinical studies.^{13, 14} They focus on the predictability of critical movements with aligners.

It is important to observe that the 50 most cited articles do not include articles of great scientific interest, which, being recent, still have to reach many citations to fall into the first 50.^{58, 59, 60, 61} The year 2015 was the year with the greatest number of citations, although 2020 featured a high number of citations as well, almost equal to 2015, despite a short time span between 2020 and 2023, which was probably related to the high number of articles published. The results show that the pandemic period was very advantageous for scientific production.

Time may affect the citation ranking of articles. Thus, at the top of the list may be the oldest studies, while high-quality original articles published in recent years may be underrated. For this reason, the average number of citations per year was calculated.

There is a wide range of journals in which the articles were published (20). This shows that no journal has developed a specificity in the subject. On the one hand, the dispersion of scientific production is a limitation; on the other hand, it creates diversification in the advertising field.

It is challenging to figure out the function of each author who contributed to the top 50 publications mentioned, which showed an average of more than 4 authors per piece. We evaluated each author’s overall number of citations, the total number of papers they had published, their standing in the author list, and the connection between productivity and the researcher’s level of expertise. The bibliometric indicator under consideration is the H-index, which measures the effectiveness of a scientist’s studies while considering the publication volume and visibility.

Nearly all of the most often quoted pieces were from Central Europe, but there were also some from the USA and China. In most cases, citations come from institutions that are located in the author’s country of origin. The top 2 institutions with the most citations are both in Central Europe – in Germany (University Hospital Freiburg) and Italy (University of Turin). Neal D. Kravitz is renowned in the USA for garnering numerous awards from his private practice. Half of the top 10 authors on the list are Italian authors who are affiliated with the University of Turin.

Table 3 provides more information.

A keyword co-occurrence network (KCN) is a useful tool for mapping to investigate the connections between keywords in the literature, and comprehend the knowledge structure and components of a scientific area.^{16, 17} Keywords are a set of terms that allow correct indexing in computerized databases, facilitating research in the scientific literature.

The required minimum quantity of keyword occurrences in the analysis dataset was set at 7. The occurrences of each keyword and the overall strength of the link were tabulated. The keywords were mapped using VOSviewer (Centre for Science and Technology Studies, Leiden University, the Netherlands). The nodes of the generated network stand for keywords, while their edges indicate keyword associations. Similar keywords were grouped in clusters of the same color. The sizes of the nodes indicate how frequently the terms occur. The degree of connection between the nodes is indicated by the thickness and length of the lines.

Keywords like “clear aligners”, “orthodontic tooth movement” and “removable orthodontic appliance” occupied the larger and centrally positioned nodes. The keyword “removable orthodontic appliance” had significant weight in the keyword map. This orthodontic appliance has become widespread among adult patients, as it is comfortable, esthetic and allows more practical oral hygiene. This is why the keyword “periodontal disease” also carries significant weight. The term “orthodontic tooth movement” was mentioned in multiple research investigations that looked into the predictability of particular results. The term “clear aligner” was used frequently, indicating that there is increasing interest in testing this kind of device, while the presence of the term “Invisalign” was reduced, since the selected articles concerned treatment with any type of clear aligners (Figure 4).

Limitations

The main issue is that the bibliometric analysis is very variable over time. The results may change, depending on the month.

The articles have been sorted in descending order according to the number of citations, but this parameter is not sufficient to assess the value of an article.

Citations are affected by time, so high-quality but more recent articles may be underreported.

Also, there may be a bias due to self-citations.

Furthermore, another important limitation is the challenge of evaluating researchers’ contributions accurately. It would be essential to evaluate the actual contribution.

Conclusions

The results of the present bibliometric study indicated that CAT-related papers received increasing citations. The study determined the most influential articles by highlighting their authors and the journals in which the papers were published.

Ethics approval and consent to participate

Not applicable.

Data availability

The datasets supporting the findings of the current study are available from the corresponding author on reasonable request.

Consent for publication

Not applicable.

Use of AI and AI-assisted technologies

Not applicable.