Global publication trends
Number of global publications
In total, 2492 publications met the criteria. From 1991 to 2020. Though the RRI declined since 2018, there has been a steadily increasing trend of global publications annually. The number of publications increased from 2 (1991) to 250 (2020). Most research was published in 2019 (255, 10.23%) (Fig. 1A).
Contributions of countries and regions
A total of 65 countries and regions published articles in this domain. The United States published the most papers (938, 37.64%), followed by China (567, 22.75%), England (219, 8.79 %) and Germany (195, 7.83%). (Fig. 1B and C).
Global trends of publications
The logistic regression model was used to construct the time curve of the number of the publications, which could help to predict the future trend in the next few years. Figure 1D showed the model fitting curves of the growth trend. According to the time curve, the number of publications in this field was estimated to reach a platform.
Quality of the publications
Papers from the United States had the highest citation frequency (68,195). Then, China ranked second in total citation frequency (14,776), followed by England (12,132), Germany (5,657) and Japan (5,266) (Fig. 2A). Fig. 2B showed the average citation frequency of top 20 countries. Wales had the highest average citation frequency (197.33), followed by Norway (88.2) and Singapore (87.36). Besides, United States outranked other countries with H-index of 103, followed by China (57), England (50), Germany (40) and Japan (39) (Fig. 2C).
Analysis of global publication
The Journal of Orthopaedic Research published the most articles, with 100 publications. American Journal of Sports Medicine ranked second with 83 articles. Moreover, there were 77 articles in Tissue Engineering Part A and 63 articles in Biomaterials on the research of stem cells for tendon. The top 20 journals with most papers were presented in Fig. 3A.
Figure 3B showed the top 20 research orientations related to stem cells for tendon. The top 5 popular areas of research were cell biology (747 papers), orthopedics (514 papers), engineering (487 papers), materials science (443 papers) and research experimental medicine (258 papers), respectively.
The top 20 authors published a total of 637 papers, which accounted for 25.56% of all articles in this field (Fig. 3C). Ouyang HW from China published the most researches on stem cells for tendon with 47 papers, followed by Chen X with 41 papers and Yin Z with 40 papers. In this study, we included all authors into the analysis, regardless of the relative contributions of the authors to a single study.
The top 20 contributive institutions with the most articles were shown in Fig. 3D. The University of Pittsburgh published the most articles, with 96 papers. Zhejiang University ranked second (70 papers), followed by the Shanghai Jiao Tong University and Chinese University of Hong Kong (both 64 papers).
Bibliographic coupling analysis
Compared with other measurement of citation analysis, bibliographic coupling has advantages to construct a similarity relationship between documents. Bibliographic coupling will happen when two works cite a common third one in their bibliographies, indicating that the two works share a related theme.
We used VOS viewer to analyze the names of journals in total publications. As illustrated in Fig. 4A, 113 journals appeared in the bibliometric map (the minimum number of publications of each journal was over five). The top five journals with greatest total link strengths were as follows: Tissue Engineering Part A (13,345 times), Journal of Orthopaedic Research (13,320 times), Biomaterials (10,755 times), American Journal of Sports Medicine (9,011 times) and Stem Cells International (8,993 times).
There were 265 institutions included (the minimum number of publications of an institution was over five) and their publications were analyzed by VOS viewer (Fig. 4B). The top five institutions with greatest total link strengths were as follows: Chinese University of Hong Kong (17,189 times), University of Pittsburgh (16,529 times), Zhejiang University (15,872 times), Shanghai Jiao Tong University (10,824 times), and University of Connecticut (8,026 times).
Countries and regions
There were 32 countries and regions included (the minimum number of publications from a country or region was over five) and their publications were also analyzed by VOS viewer (Fig. 4C). The top five countries or regions with greatest total link strengths were as follows: United States (460,795 times), China (343,358 times), England (201,947 times), Germany (175,086 times), and Italy (153,702 times).
Co-authorship analysis presents the relationship of items which was built according to the number of coauthored documents, which acts as an effective role in evaluating leading scientists, countries, and organizations.
378 authors were identified (the minimum number of publications from an author was over five) and their publications were analyzed by VOS viewer (Fig. 5A). The top five authors with greatest total link strengths were as follows: Chen X (262 times), Yin Z (245 times), Heng BC (181 times), Shen WL (129 times), and Tang KL (122 times).
Studies (defined as minimum number of documents form an organization that were used more than five) identified in the 265 institutions were analyzed by VOS viewer (Fig.5B). The top five institutions with greatest total link strengths were as follows: Chinese University of Hong Kong (86 times), Zhejiang University (81 times), Harvard University (73 times), University of Pittsburgh (70 times) and Columbia university (66 times).
Countries and regions
The studies in 32 identified countries and regions (the minimum number of studies from a country or region was over five) were analyzed by VOS viewer (Fig. 5C). The top five countries and regions with greatest total link strengths were as follows: United States (337 times), China (210 times), England (152 times), Germany (140 times), and Italy (104 times).
Co-citation analysis indicates the relationship of items which are built based upon the number of times they were cited together, and it has abilities to avoid academic isolation and to accelerate knowledge integration for consistency of cross discipline.
The journal was included if the minimum number of citations from a source was over 20. There were 825 journals met the criteria (Fig. 5D). The top five journals with greatest total link strengths were as follows: Biomaterials (554,821 times), Journal of Orthopaedic Research (429,514 times), American Journal of Sports Medicine (385,081 times), Journal of Bone and Joint Surgery-American volume (282,060 times) and Tissue Engineering Part A (242,487 times).
A total of 808 publications (the minimum number of citations of a reference was over 20) were analyzed using VOS viewer (Fig. 5E). The top five publications with greatest total link strengths were as follows: Bi et al. (9936 times); Schweitzer et al. (5733 times); Pittenger et al. (4522 times); Young et al. (4356 times) and Awad et al.(4314 times).
Co-occurrence analysis identifies that the relationship of items is constructed based upon the number of publications when they occur together. It is a powerful tool to evaluate research areas as well as hot issues, and to track the academic progress. The keywords with minimum number of occurrences over five, were analyzed by VOS viewer. As illustrated in Fig. 6A, the 213 identified keywords could be divided into five clusters: ‘‘animal study’’, ‘‘tissue engineering’’, ‘‘clinical study’’, ‘‘mechanism research’’ and ‘‘stem cells research’’.
In the “animal study’’ cluster, the most used keywords were mesenchymal stem-cells, regeneration, anterior cruciate ligament and patellar tendon. In the cluster of ‘‘tissue engineering’’, the primary keywords were tendon, in-vitro, tissue engineering and tissue. In the ‘‘clinical study’’, the main keywords were repair, platelet-rich plasma, achilles tendon and tendinopathy. In the ‘‘mechanism research’’ cluster, the frequently used keywords were differentiation, stem cells, expression and extracellular-matrix. As for the cluster of ‘‘stem cells research’’, the main keywords were bone marrow, stromal cells, marrow stromal cells and mesenchymal stem cells. These results showed the research field distribution of publications related to the study of stem cells for tendon.
In Fig.6B, VOS viewer applied different colors to keywords based on when they appeared in literature for the average time. The purple color indicated that the keywords appeared earlier while keywords in yellow color appeared later. Generally, each of the 5 clusters had a trend of balanced development. Both of 5 clusters were undergoing different degrees of changes on the research hotspot, which meant a diversified developing trend. Some keywords, such as tenogenic differentiation belonged to the “tissue engineering’’ cluster, had potential to be studied more in future.