Posterior cruciate ligament research output in asian countries from 2009 - 2019: A systematic review

Purpose This study aimed to determine the number of posterior cruciate ligament (PCL) publications performed in Asian countries and to identify factors associated with research output in this region. Materials and methods Searches of existing academic journal articles were performed using PubMed, Google Scholar, and the Cochrane Library from January 1, 2009 until December 31, 2019. Results A total of 265 articles were published in the last 10 years in Asian countries, with an increase in publications after 2010 and an average of 26 articles every year. More than half (70%) of the articles were published in journals with an impact factor (IF) ≥1. The majority of the publications were cohort studies (27%), followed by case reports (16%), systematic reviews/meta-analyses (2.6%), laboratory studies (1.8%), and case-control studies (1.5%). South Korea and China had the most PCL publications, and most authors were from South Korea. Conclusion The PCL research output in Asia is low in quantity but high in quality publications, and the majority of publications come from South Korea, China and Japan, with most being cohort studies and case reports.


Introduction
The posterior cruciate ligament (PCL), which consists of the anterolateral (AL) and posteromedial (PM) bundles, is the strongest ligament in the knee joint. Some initial reports have shown good functional results with nonoperative treatment for PCL injuries. Biomechanical and clinical studies suggest a less benign natural history of PCL deficiency that results in persistent symptoms and premature osteoarthritis. Longer-term follow-up studies have also described an increasing incidence of arthritis, declining knee function, and making PCL reconstruction more widely accepted, especially with the improvement of operative techniques. Although operative indications for these injuries remain controversial, there has remained a strong interest in the literature regarding the methods of reconstruction [1].
Some controversy still remains regarding the epidemiology, diagnosis, treatment, rehabilitation programs and outcomes [1,2]. An early and accurate diagnosis of PCL rupture should be based on the history of illness, a vascular examination to exclude vascular damage, a physical examination for the assessment of PCL and collateral ligament injuries, and a radiographic examination. Once the diagnosis is established, an appropriate treatment plan should be determined according to the time of injury, rupture pattern, presence of remnants, combined collateral ligament injuries, alignment, and tibial slope [1,2].
The number of publications regarding this ligament is increasing every year worldwide. There are no data that provide information on all PCL publications, especially in Asian countries. This study aims to consolidate the information on PCL research publications in Asian countries.

Search strategy
Electronic systematic searches were performed using PubMed, Google Scholar, and the Cochrane Library for existing academic journal articles and reviews between January 1, 2009 and December 31, 2019. The search terms included [posterior cruciate ligament] OR [PCL]. Author affiliations were subsequently screened, and all abstracts with at least one author from any of the Asian countries, according to the PRISMA guidelines for conducting the review, were followed [3]. We have registered our study in research registry with unique identifying number review registry 975.

Inclusion and exclusion criteria
An article was included if the study was performed in an Asian country and if the research topic was posterior cruciate ligament, PCL or PCL reconstruction. All original studies, including laboratory studies, animal studies, cadaveric studies, observational studies, interventional studies and qualitative studies, were included and published between January 1, 2009 and December 31, 2019. We also included papers in other languages when an English abstract was available. We excluded book sections, conference presentations, abstracts, guidelines, commentaries, personal views, and studies.

Data collection
All abstracts were screened independently by two reviewers to determine whether they met the eligibility criteria. The full texts of eligible studies were then reviewed for eligibility again, followed by data extraction and analysis.

Data extraction and analysis
The data of the eligible articles were abstracted into variables such as country, year of publication, journal, study design and topic. The study design was classified based on the level of evidence and included systematic reviews and meta-analyses, randomized controlled trials (RCTs), nonrandomized controlled trial (NRCTs), cohort studies, case-control studies, cross-sectional studies, case reports, cadaveric studies, animal studies, and laboratory studies.
The published journals were divided by impact factor (IF) into low-(IF below 1) and high-(IF of 1 or above) impact journals. Information regarding the country was collected, which included the number of PCL publications.

Study selection
The search strategy included articles between January 1, 2009 and December 31, 2019. The search results identified 865 records. After removing duplicates and after title selection, 593 articles were selected for full-text assessment. Three hundred and twenty-eight papers were excluded for the following reasons: (1) the research topic was not posterior cruciate ligament, PCL or PCL reconstruction, (2) the study was performed outside Asia, and (3) it was not an original study, e.g., expert opinions or article replies. A total of 265 articles met the eligibility criteria for data extraction (Fig. 1).  Table 1 shows the top 10 countries in Asia with the most publications in the past ten years. The country with the most publications was South Korea. South Korea published 89 articles, which constituted 33% of all articles published on this topic, followed by China and Japan (30% and 11%, respectively). Table 2 Korea  89  2  China  81  3  Japan  29  4  India  23  5  Taiwan  12  6  Iran  9  7  Turkey  8  8  Indonesia  4  9  Malaysia  4  10  Singapore  3  11  Thailand  2  12 Saudi Arabia 1 Table 3 shows the top 10 articles with the highest number of citations from 2009 to 2019. There were 9 articles reporting clinical research and 1 review article. Table 4 shows the top 10 first authors on publications of posterior cruciate ligament research. Seven were from South Korea, and 3 were from China. Table 5 shows the top 10 investigators in all positions on publications of the PCL. Most of them were from South Korea, and 1 person was from China. Trend of publications by year and type of studies on the posterior cruciate ligament. Table 6 shows the trend of publications by year and type of studies on the posterior cruciate ligament. The majority of the publications (27%) were cohort studies, followed by case reports (16%).

Discussion
The research output of PCL publications in Asian countries is low in quantity compared with that of ACL publications [4]. There have been only 265 articles in the last 10 years. This small number of publications is also suitable because the incidence of isolated posterior cruciate ligament (PCL) rupture is low among cases of knee injury [5]. South Korea and China were the countries with the most publications on the PCL (64%), followed by Japan, India and Taiwan (10%, 8% and 4%, respectively). Based on first-author and author data in all positions, South Korean authors dominated the publications on the PCL, followed by Chinese authors; additionally, Lee Yong Seuk from the Department of Orthopaedic Surgery, Gachon University School of Medicine, Gil Hospital, Incheon, South Korea is the author with the most PCL publications.
The majority of the publications (27%) were cohort studies, followed by case reports (16%), and few studies were case-control studies, laboratory studies, and systematic reviews/meta-analyses (1.5%, 1.8% and 2.6%, respectively). A subanalysis of the types of studies performed in each country showed that systematic reviews and meta-analyses were only performed in South Korea and China (1.5% and 1.1%, respectively). Laboratory studies were performed in Japan, China, and India (0.07%, 0.07%, and 0.04%, respectively). There were only a few casecontrol studies that were performed and published in 2 countries: Japan and China (0.07% and 0.07%, respectively). Zhongguo xiu fu chong jian wai ke za zhi = Zhongguo xiufu chongjian waike zazhi = Chinese journal of reparative and reconstructive surgery 11 5 The American journal of sports medicine 10 6 Knee surgery & related research 8 7 Arthroscopy techniques 6 8 BMC musculoskeletal disorders 6 9 Zhongguo gu shang = China journal of orthopaedics and traumatology 6 10 Acta orthopaedica et traumatologica turcica 5 Table 3 Top 10 articles with most-number of citations.
No. Article title Number of citation 1 A prospective randomized study comparing arthroscopic single-bundle and double-bundle posterior cruciate ligament reconstructions preserving remnant fibers [6].   Transtibial versus tibial inlay techniques for posterior cruciate ligament reconstruction: long-term follow-up study [12].  Table 4 Top 10 first author on Posterior Cruciate Ligament Research and Publications.

Quality of the journals
The published journals were divided by impact factor (IF) into low-(IF below 1) and high-(IF of 1 or above) impact journals. A total of 186 articles published in Asian countries in the past 10 years were from highimpact journals (70%). This showed that most journals had a high quality.
One may argue that the strong motivation for publication in high-IF journals is to raise the international ranking of the universities or education centers. The other motivations are to evaluate and share new knowledge or novelty and to document the work. Twenty-four articles (9%) were conducted in Asian countries and were published in local journals in native languages, such as Zhongguo xiu fu chong jian wai ke za zhi and Zhongguo gu shang Sichuan da xue bao. Yi xue ban is a Chinese language, as the readership of these journals is limited to Chinese people in these countries. This caused the impact factor to be low.
The article with the most citations (107 citations) was a cohort study with the following title: A prospective randomized study comparing arthroscopic single-bundle and double-bundle posterior cruciate ligament reconstructions preserving remnant fibers (doi:10.1177/ 0363546510382206). Among the top ten articles with the most citations, most were cohort studies (50%), followed by cross-sectional studies, surgical techniques and randomized control trials (20%, 20% and 10%, respectively) ( Table 3).
This study indicates the need for collaboration research in the region. To bridge the PCL research gap among Asian countries, international collaborations with guidance from more established research centers should be encouraged.
This systematic review only focused on studies performed in Asian countries. Many researchers in Asian countries may have contributed data in an overseas-based research project or participated in research when they resided overseas. The purpose of this exclusion is to depict a clearer picture of local research output and quality.
PCL research output in Asia is low in quantity but high in quality publications, and the majority of publications come from South Korea, China and Japan, with most of the studies being cohort studies and case reports.

Funding
The authors declare that this study had no funding resources.

Availability of data and material
Data will be provided upon request.

Provenance and peer review
Not commissioned, externally peer-reviewed.

Declaration of competing interest
No potential conflicts of interest relevant to this article were reported.