INTRODUCTION

Bibliometrics is defined as “the use of statistical methods in the analysis of a body of literature to reveal the historical development of subject fields and patterns of authorship, publication, and use.”¹ In the biomedical field, bibliometric studies are of great interest because they make it possible to gain an overview and to evaluate research and scientific activity by calculating bibliometric indicators, which provide information about the quantity of published research in a specific field. Commonly, the quality of articles is assessed by the number of times an article is cited in other publications. In addition, some indicators, such as the impact factor, the immediacy index, and lists of highly cited articles, can help one to assess the quality and popularity (within a field) of the journal that published the article.²

Scientific production in dentistry^3,4 and, more specifically, in orthodontics has undergone a progressive increase due to the increasing demand for scientific knowledge regarding procedures and tools. This has been accompanied by a growing number of scientific journals specializing in orthodontics included in the category of “Dentistry, Oral Surgery and Medicine” in the Journal Citation Reports (JCR; property of Clarivate Analytics, Philadelphia, Pa). Within the “Dentistry, Oral Surgery and Medicine” JCR category, only three journals specialized in orthodontics in 2008, while in 2016, the number had risen to eight, almost triple the previous number.

Numerous bibliometric studies have been conducted in different areas of biomedical research to determine the characteristics of the most highly cited articles and to register information about authors, topics, and journals.^5–10 Several bibliometric studies have analyzed the top 100 most-cited articles in dentistry, endodontics, and implant dentistry.^7–9 Each of these articles analyzed citations, authors, countries, and institutions, following different methodologies and searching in different databases according to the field under investigation. In orthodontics, only one bibliometric study has been published,¹⁰ but this only extracted data from three orthodontic journals and did not include bibliometric analysis, the lack of which could be a source of bias.¹¹ For this reason, the present study set out to identify and perform bibliometric analysis of the 100 most-cited articles in the field of orthodontics, indexed in the Web of Science Category of “Dental, Oral Surgery, and Medicine” from 1946 to 2016.

MATERIALS AND METHODS

The most-cited articles in orthodontics were identified by conducting a database search in the Science Citation Index Expanded of the Web of Science (property of Clarivate Analytics). The search term used was the truncated term “orthodon*,” entered in the topic field, with no restrictions regarding the year of publication. The types of retrieved documents were limited to original articles and reviews (article or review), discarding other texts, such as editorials, letters, proceedings, etc. The search was performed on October 16, 2017. The results were sorted according to the number of citations per article, and the first 150 works registered were selected and exported to a text file. This text file was imported into Microsoft Access to create a database. A manual review was then performed by two authors (BT and VPG). Whenever a disagreement arose, a third reviewer (AAA) made the definitive decision. Any articles not related to the field of orthodontics were discarded. After the manual review, only the top 100 most-cited articles were included, discarding the rest.

Subsequently, a manual edit of the Microsoft Access database was made in order to unify terms (authors, institutions) and to remove any typographic or indexing errors. First, author normalization was performed, and whenever it was not clear whether different authors shared the same name, their institutional affiliation was investigated. Similar criteria were applied to normalize institutional data; only macroinstitutions (universities, hospitals, etc) were registered, while departments, research units, etc, were eliminated. After the manual edit, the evaluators established the type (basic, clinical, or review) and the field of study. This analysis was based on information included in the abstract and, if necessary, the whole work.

The following variables were analyzed by two other authors (RLD and AVI): the journal in which the article was published, the year of publication, number of authors, organizations, country of origin, and topic. The abstract of each article was retrieved to manually check its pertinence. The type and field of study were determined according the method described by Fardi et al.⁹

RESULTS

The 100 most-cited articles in orthodontics are shown in Table 1, listed in order according to the total number of citations received and the number of citations per year. As shown in Figure 1, the 100 most-cited articles in orthodontics were published between 1946 and 2012, the first decade of the new millennium being the most productive (41 out of 100 articles). Table 2 shows the 21 scientific journals in which the 100 most-cited articles in orthodontics were published.

Table 1 The 100 Top-Cited Articles in Orthodontics, Ordered by Number of Citations (848–115)¹

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Table 2 The 21 Journals in Which the Top 100 Cited Articles Were Published (Orthodontic Journals Highlighted in Boldface)¹

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The number of authors participating in the 100 most-cited articles was 251; only 31 were responsible for two or more articles (Table 3). The authors of the 100 most-cited articles in orthodontics worked in institutions in 22 different countries (Table 4). The distribution of these countries by continent was as follows: 12 European countries, four American countries, four Asian, one African, and one Australian.

Table 3 The 31 Most Common Authors of the Top 100 Cited Articles in Orthodontics

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Table 4 The 22 Countries of Origin of the Authors of the Top 100 Cited Articles in Orthodontics

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Table 5 shows the 42 institutions that published two or more articles on the list of the 100 most-cited articles in orthodontics. Table 6 shows not only the type of article but also the field of study.

Table 5 The 42 Institutions of Origin With Two or More Top-Cited Articles

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Table 6 Categorization of Articles in Terms of Manuscript Type (Basic, Clinical, or Review) and Field of Study (27 Items)

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DISCUSSION

When an article appears on the list of the 100 most-cited articles in orthodontics, this supposes that the article has marked a milestone in the development of orthodontics. In theory, the quality of an article is reflected by its recognition by the scientific community and how it generated change in clinical practice, generated discussion, or triggered new directions in research.⁹

The aims of this bibliometric study were to identify and analyze the 100 most-cited articles in orthodontics indexed in the Web of Science Category of “Dental, Oral Surgery and Medicine” from 1946 to 2016. Bibliometric analysis is the only valid tool for this type of study, as it makes it possible to perform a reliable search in the Science Citation Index (SCI) database and include all potentially valid articles.

The only study¹⁰ published to date to have analyzed the 100 most-cited articles in orthodontics did not include adequate bibliometric analysis and may not have included all of the most-cited papers. The articles included in the earlier analysis were published in only three SCI dental journals.¹¹ Moreover, the study lacked a predefined search strategy, and its inclusion criteria were such that it may have omitted some articles.¹⁰ The present study adopted a more thorough methodology, using the topic word “orthodon*” to search in the Web of Science and then ranking the articles by the number of citations and applying detailed criteria to the selection process, as recommended by other authors.¹¹ This bibliometric process was more laborious but may be considered more reliable.

The 100 most-cited articles in orthodontics were cited between 115 and 848 times each (a mean of 169.93); the most cited article was published in 1983.¹² Of the 100 articles, 18 were cited over 200 times, which indicates their high scientific impact. In addition to considering the total number of citations, calculating the number of citations per year also provided useful information, as older works obviously may have received more citations simply because they have been available longer.¹² When the articles were placed in order according to the number of citations per year, a different article from the one receiving the most citations appeared in first place, an article published in 2009.¹³ The most-cited article overall received an average of 24.23 citations per year, while the latter received 31.66 citations per year.¹³

Regarding the number of citations per article, these varied between 115 and 399 citations, with the exception of the most-cited article, which received 848 citations. These numbers were similar to the most-cited articles in other fields, such as periodontics, in which the most frequently cited works had between 100 and 346 citations.¹⁴ However, in fields such as cardiology, the numbers of citations are eight or 10 times higher, ranging between 815 and 3932 citations. Such disciplines have higher numbers of scientific journals and more published articles, which may be an indicator of greater research activity in these fields.⁵

The 100 most-cited articles in orthodontics were published between 1946 and 2012, with almost half (44 articles) published since the year 2000. The fact that more recent articles received more citations could also be related to the fact that the total number of articles has increased since the year 2000. The one other similar study only included articles published after 1975, which made it difficult to compare results with the present work, as the two studies covered different periods. In the present study, it was the publication dates of the articles themselves that defined the period analyzed, rather than a decision by the authors.

The 100 most-cited articles were published in a total of 21 scientific journals. Not all of the most-cited articles were published in journals specializing in orthodontics, some being published in journals dealing with other dental specialties, such as surgery, periodontics, or dental materials. This reflects the increasingly multidisciplinary nature of treatment involving orthodontics. The number of scientific journals publishing orthodontic research included in the category of “Dentistry, Oral Surgery and Medicine” in the JCR (property of Clarivate Analytics) has almost tripled, from three in 2008 to eight in 2016. Nevertheless, the list compliled only included three journals specializing in orthodontics alone, which published 70 out of the 100 most-cited articles.

Most of the 251 authors who contributed to the 100 most-cited articles in orthodontics (87.65%, 220 authors) were involved in a single article, 7.97% (7 authors) had two articles, 3.19% (8 authors) three articles, and, lastly, 1.2% (3 authors) had four articles. Regarding the latter three authors, T Takano-Yamamoto (Tohoku University, Japan) had the largest number of citations (881), with an article about screws for orthodontic anchorage; T Deguchi (Ohio State University, Columbus, Ohio) had 648 citations, with two articles about orthodontic anchorage and one about dental movement; and BU Zachrisson (University of Oslo, Norway) received the third highest number of citations for three articles on different subjects.

The authors of the 100 most-cited articles in orthodontics came from 22 different countries. Almost half of the most-cited articles (49) were by authors based in the United States, although in terms of the average numbers of citations per article, authors from the UK received the highest mean number per article (471.66 citations).

Forty-two institutions published two or more articles on the list. The University of Oslo (10), The Ohio State University (9), and the University of North Carolina (9) were the three with the most articles that appeared.

Most of the articles cited were clinical studies (69), 18 were systematic literature reviews, and the remaining 13 were of some other (basic) type. The most frequent field of study was microscrews (18 articles), followed by biological and biomechanical studies (15). The topics of the 100 most-cited articles in orthodontics changed over time. Articles from the 1940s (two articles) dealt with topics such as cephalometry and mixed dentition treatment. The 100 most-cited articles did not include any articles published during the 1950s. Then, during the 1960s (two articles) and 1970s (seven articles), the field of study shifted to topics such as adhesion, biomechanics and biology, cleft lip and/or palate, injuries/discomfort, and orthognathic surgery. During the 1980s (23 articles) and 1990s (22 articles), the field of study expanded to include new topics such as assessment indices, asymmetries, Class II treatment, demineralization, facial esthetics, friction, genetics, implants, maxillary canines, root resorption, and the stability of orthodontic treatment. In the first decade of the new millennium (41 articles), new topics appeared, such as corticotomies, new technologies, temporomandibular joint, transplanted teeth, and numerous articles involving implants. Since 2010, only three articles appeared on the list; their topics were root resorption, new technologies, and assessment indices.

This study had several limitations that should be noted. First, by using the truncated search term “orthodon*,” it is possible that some articles may have escaped attention. It was thought that if an article did not include this truncated term in the “Topic” field (Title, Abstract and Keywords), there was only the tiniest possibility that the subject of the article could be related to orthodontics. Additional general terms could have been used in order to increase the reliability of the search, but it is doubtful that this would have identified more articles. The search identified several articles that did not deal with topics related to orthodontics but which included the truncated term in the abstract; these were later discarded. Second, self-citation can be a cause of potential bias in bibliometrics. However, among these 100 articles, the self-citation rate was only 0.6%, which is much lower than the rate reported for articles in general medicine (5.97%).⁵ This limitation has been noted by several authors,¹⁵ generating some controversy as to whether counting self-citation is valid or not, as self-citation is an easy means of increasing a journal's “impact” factor.^16,17

CONCLUSIONS

• This bibliometric citation analysis provided new, useful, and interesting information about scientific progress in the field of orthodontics.

• The 100 most-cited articles were published in a total of 21 scientific journals.

• Most of the articles cited were clinical studies, and the most frequent field of study was microscrews.