Mapping the 25 top-cited research papers in plantar fasciitis in runners

María Ayala-Gascón maria.ayala@ucv.es © Consejo General de Colegios Ofi ciales de Podólogos de España, 2018. Editorial: INSPIRA NETWORK GROUP S.L. Este es un artículo Open Access bajo la licencia CC BY-NC-ND (www.creativecommons.org/licenses/by-nc-nd). Mapping the 25 top-cited research papers in plantar fasciitis in runners Mapeo de los 25 artículos más citados sobre fascitis plantar en corredores


INTRODUCTION
As from the 1970s, running became the most popular recreational physical activity due to its clear health benefits, easyto-practice nature and low cost. The number of runners and running events has considerably raised ever since 2000. The disadvantage of this sport, however, is the high risk of lower limb injury, incidence ranging from 19 to 79 % [1][2][3] . Several studies have reported a 27 to 70 % injury rate in runners in their first running year as a result skeletal-muscle overload 4 .
Plantar fasciitis is one of the five most common skeletal muscle injuries in runners. This degenerative process occurs when the plantar fascia is exposed to repeated strain. The aetiology of plantar fasciitis is related to multiple factors that can be grouped into three general categories: training, anatomical/anthropometric factors, and biomechanical variables 1,4-7 . In runners, this injury has an incidence that ranges from 4.5 to 10 %, and a revalence between 5.2 % to 17.5 % 1 .
The bibliometric analysis of bibliographic references in scientific papers contributes to the reconstruction of the intellectual process that gives way to new knowledge, the identification of current research areas, the determination of the influence of papers on subsequent literature, and the evaluation of the reach of a scientific publication. Three methods are used in the identification and analysis of bibliographic references: direct citation, co-citation analysis, and bibliographic coupling 8 .
In a paper, a bibliographic citation acknowledges the work done in a specific area by another author. Direct citation is the most common method for the evaluation of the impact of a document on the scientific community.
The higher the number of citations of a paper, the more valuable the paper is in its field 9 . Co-citation quantifies the frequency of two documents being cited together in the literature, which allows identify the most influential papers and authors in a discipline. Bibliographic coupling identifies and quantifies the cases in which papers cite the same references. Two documents citing the same papers is indicative of the existence of links between them; the higher the number of shared references, the greater their thematic proximity 8 . Knowledge maps generated with this type of data are useful to better visualize this information 10 .
The Institute for Scientific Information (ISI) has carried out the most relevant bibliographic compilation of scientific papers ever since 1960. The Web of Science is one of the most important databases for the categorisation of data relative to citation and the academic impact of information 9,11,12 . This study aims to identify the 25 most cited running-related plantar fasciitis research papers and analyse their main characteristics by means of bibliometric analysis.

MATERIAL AND METHODS
Data was retrieved on 3 February 2016 by searching the Web of Science Core Collection (WOS-CC). This particular database was chosen for its characteristics: list of names of all paper authors; inclusion of all of the authors' workplace (this piece of information is key for scientific collaboration analysis); it specifies the number of citations received by each paper and includes the references cited by each paper 11,13,14 . The search equation was performed using the following English terms: [(plantar fasciitis)] AND [runn* OR athlet*] in the "Topic" field. Synonyms of the terms did not retrieve different records.
Once the search was completed, the 25 most cited research papers related to the study of this pathology in both professional and amateur runners were selected. Documents other than articles were ruled out. Records including some of the search terms but not showing a relationship with the subject were also eliminated.
Basic data were recorded and analysed, including abstract, title, authors, year of publication, paper type, journal, organisation, country, research area, number of citations, citation density (average number of citations/year), bibliographic references, impact factor, quartile, and WOS category.
The different denominations of authors and institutions were standardised, this being essential to ensuring more accurate results. Where two or more variants were found for the same name, coincidence in institutional affiliation for each of them was verified. To standardise institutions were identified in the original scientific document or in their respective websites.
The software used for co-occurrence (co-authored and co-citation) networks and for bibliographic coupling network was Bibexcel version 2014-03-25 and Pajek version 4.08 15,16 . For the geolocation of authors, the online tool GPS Visualizer was used. Table I    All along three decades, Figure 2 shows the distribution of publications and the cumulative number of articles, differentiating one-author articles from co-authored ones. The decade with more published papers is 2000-2009, with a total of 11. 84% of the most cited articles were written by more than one author.

RESULTS
Warren BL (n = 3) is the author with more articles. He is followed by Chandler TJ, Kibler WB, Pulisic M, and Riddle DL (n = 2) (Table I), who are the authors who collaborated in 2 articles each (Chandler TJ with Kibler WB, and Pulisic M with Riddle DL). The rest of authors publishing in cooperation only have one co-authorship with some of the authors.
The 25 most cited articles were published by researchers from 6 different countries. In terms of country of origin, the leading publishing nation is the US (n = 21), followed by Canada and Germany (n = 2), and Australia, England and Scotland (n = 1). 24 % of articles are the result of national collaboration. Only 2 articles involved international collaboration, the participating countries being USA with Scotland, and Australia with England.
The geolocation of the authors of the 25 most cited research papers and the collaboration links between their countries can be seen on the map in Figure 3. 32 institutions participate in the articles (Table II). Lexington Clinic, University of California Davis, University of New Orleans, and Virginia Commonwealth University have 2 high-impact articles each. Figure 4 shows the institutions' collaboration network. 28.1 % of the organisations collaborate with 2 institutions, and 31.2 % cooperate with one institution only. Collabora-tion between institutions from the same continent is more frequent than with institutions from a different continent.
The most cited articles were published in 11 journals (Table III). The journals publishing the most articles were the American Journal of Sports Medicine and Foot and Ankle International, 5 articles each, followed by Sports Medicine, 4 articles.
The journals where the 25 most cited articles are published have an impact factor between 17.333-0.650. The journal that published the most cited article was the Bristish Journal of Sports Medicine; its IF is 5.025, which makes it the second journal with the greatest impact in the Sport Science category of the Journal Citation Report Science Edition. The highest impact factor is that of Archives of Internal Medicine (IF = 17.333), the journal that published the fifth most-cited article in the rank.
Articles are classified into 7 different WOS categories. 44 % of them are published in journals classified in more than 1 category. The two research areas contributing to the knowledge of plantar fasciitis in runners to a greater extent are orthopaedics (n = 16) and sport science (n = 15).     Figure 5 shows the network of citations between the 25 most cited articles. 68% of articles were cited by some of the remaining 24, the article most cited between them being Functional biomechanical deficits in running athletes with plantar fasciitis by Kibler WB, 1991 (n = 9), followed by Plantar fascia release for chronic plantar fasciitis in runners de Snider MP, 1983 (n = 7) and Risk factors for plantar fasciitis: A matched case-control study by Riddle DL, 2003 (n = 5).
After a citation analysis, Figure 6 shows that the most influential authors/articles on the most cited authors are Kibler WB, 1991, V19, P66, Am J Sport Med -cited in 9 articles-, followed by Pfeffer G, 1999, V20, P214, Foot Ankle Int, Kwong PK, 1988, V7, P119, Clin Sport Med, and Snider MP, 1983, V11, P215, Am J Sport Med, each being cited in 7 articles. The figure also shows the co-citation network for the pairs of authors/articles jointly cited in 4 or more articles.
The bibliographic coupling analysis is illustrated in Fig     different research areas; yet, the specific study of running-related plantar fasciitis only accounts for 14 % of that literature.
In both cases, orthopaedics and sport science are the research areas contributing to the development of the discipline to a greater extent.
In spite of being the most prevalent injury in runners [19], the literature found in the WOS-CC on the same date shows a greater proportion of specialised publications for other more frequent lower-limb injuries caused by skeletal muscle overload in runners, such as Medial tibial stress syndrome, Patellar tendinopathy, Achilles tendinopathy, or Patellofemoral syndrome 1,2 , which account for 61.4 %, 37.7 %, 24.7 %, and 23.9 % respectively of the studies conducted on these pathologies in general.
The 25 most cited articles were published between 1983 and 2009. It is worth noting that the publication of arti-cles focused on plantar fasciitis in runners started with US researchers, following the 1980s boom -about 25 million Americans started running for the first time between November 1984 and November 1986 17 .
Although a total of 6 countries contribute to the top-25 list, 84% of articles come from the US. This could be attributed to the United States' large scientific community and sufficient funding 9,11,20 .
The decade starting in 2000 is the most productive one, paper-wise. The most cited articles are also from that period. This finding illustrates the researchers' growing interest in this injury, especially as of 2000, with the ever-increasing popularity of running, the organisation of running events and the subsequent injury incidence and the need to prevent it 3,21,22 .
Published in the British Journal of Sports Medicine in 2002, the article A retrospective case-control injuries analysis of 2002 running injuries is also the most cited of all articles on plantar fasciitis in general and could therefore be said to be the most influential for the scientific community studying this particular pathology.
The 25 most cited research papers were cited between 29 and 410 times. Citation density is a measure that shows the relative importance of a paper regardless of the time elapsed since publication. Although it is possible for papers with a higher citation density not to have been included in this study, the method considering the number of absolute citations helps identify the papers more frequently used by researchers as a basis for the own studies 11 . The citation rate gradually increased over the past decade, which points to a higher degree of consideration of this study field.
The articles were published in 11 journals. The most productive journals were: American Journal of Sports Medicine, with 5 of the most cited articles, followed by Foot and Ankle International and Sports Medicine, both of them with 4 articles. Previous studies found a journal's impact factor to be the strongest indicator for citation, and that most top-cited articles are published in journals with high impact factors 9,23 . Yet, our results suggest that citing the most cited articles is not conditioned by the impact factor of the journal.
The limitations of this study must be noted. Though a well-defined method was used in the identification of the 25 most cited articles, some reviews with a high number of citations were not included because they did not fall under the object of study; likewise, some relevant and influential papers with a lower number of citations were not included, as not enough years have elapsed for them to be further cited. It could be the case that some papers may have been more cited simply because their citation period was longer. In other words, this method favours the oldest papers, which have managed to have a greater number of citations over time. Additionally, a snowball effect might be present: some authors are more likely to cite certain papers only because they are highly cited and not because of what they are worth 24 .
In conclusion, this study provides detailed information on the characteristics of the 25 most cited research papers. It sheds some light on the history and development of scientific knowledge as regards the aetiology, diagnosis, prevention and treatment of plantar fasciitis in runners, and it gives an insight into the authors and institutions contributing to the development of this field to a greater extent, who have become world references. The study helps determine the importance of the articles and their impact on the scientific community.