Knee joint is the largest compound joint in human body. Patellofemoral instability as a common disease in orthopedics and the main cause of anterior knee pain syndrome, especially in young people, often leads to patellar dislocation or subluxation, which brings great pain to patients (1). The multiply risk factors leading to patellar instability include the dysplasia of patellar and trochlear osseous structure, the integrity and balance of periarticular ligaments, the systemic conditions affecting connective tissue, and the overall muscle strength, which also complicate the diagnosis and analysis of patellar instability (2). Patellar dislocation leads to pain, decreased mobility, osteochondral fractures, and patellofemoral arthritis (3). The complexity of the pathogenesis also leads to the diversity of treatment methods, and there is no unified opinion on the choice of conservative or surgical treatment, as well as on the choice of surgical procedure (4).

In view of the above aspects, patellofemoral instability has attracted more and more attention of researchers, and a large number of relevant studies have been published. However, the rapidly increasing number of publications makes it increasingly difficult for researchers to keep up with the latest findings, even in their professional field (5). Although the emerging systematic evaluation and meta-analysis can provide researchers with more ideas, these articles only focus on some specific aspect (4, 6). Some meaningful information, such as the number of publications, the collaboration of countries, institutions and authors, and the scientific analysis of research hotspots, prediction of hotspots are not included (7).

Bibliometrics based on scientific statistical methods can reliably identify and analyze the above information, so as to identify global trends and build knowledge structure, which is not only beneficial to novice researchers, but also to the experts (8). Bibliometric analysis has been applied in medical field, exerting a great impact in Orthopedics (9), Psychiatry (10, 11), Urology (12) and other fields.

As shown in Figure 1, our analysis mainly contains five aspects. Firstly, we analyze the growth trend of the number of publications. Then we analyze the most productive countries, institutions and researchers, which helps researchers find the most suitable research institutions or scholars to cooperate. Then there is the analysis of journals, including journal co-citation analysis and the dual-map overlay visualization, which helps researchers find the most suitable journals for study and submission. The most prominent part of our research is the analysis of references and keywords. We conduct main path analysis and co-citation cluster analysis on references, which are helpful to clarify the development process and research progress in this field. The co-occurrence analysis of keywords identifies the relationship between various research directions in this field. In addition, we use multiple analysis software to comprehensively analyze keywords to identify the research frontiers and development directions in this field, which may inspire researchers and guide researchers to make more breakthroughs in these promising directions.

In order to avoid duplicated and unnecessary investment of research funds and time, the researchers independently and systematically reviewed the Clarivate Analytics Web of Science (WOS) database on the last day of 2021, that is, December 31, 2021. The WOS database is the selected collection of high-quality academic peer-reviewed literatures. We found that in the research field of patellar instability, no bibliometric articles have been published. Bibliometrics makes quantitative analysis of publications based on mathematical and statistical methods. It can scientifically make use of data extracted from articles to analyze and visualize the research hotspots and research frontiers in the identified research domains, which may greatly promote the development of the research field.

The Science Citation Index Expanded (SCI-Expanded) of the Clarivate Analytics Web of Science Core Collection (WoSCC) was the most commonly used scientific information source for bibliometric analysis (13). We had carefully formulated the retrieval plan and selected the last day of 2021 for information screening and extraction, so as to ensure the integrity of the latest research information in 2021 to the greatest extent and avoid the information bias caused by the daily update of the database. The specific search formula was as follows: TS = (“instable patella*” OR “patella* instability” OR “instability of patella*” OR “patella* displacement” OR “displacement of patella*” OR “dislocation of patella*” OR “patella* dislocation” OR “high-riding patella*” OR “patella* alta” OR “lateralized patella*” OR “patella* tilt” OR “subluxation of patella*” OR “patella* subluxation”). The search timespan was set as 2001–2021. We only included two types of literature: original articles and reviews, which had standard references and had been strictly reviewed by experts in the same field. Finally, 2,155 records were finally retrieved. Then we exported records in “plain text” format with “complete records and reference”.

In total, we mainly used 5 scientometric software and Microsoft Excel program to perform bibliometric analysis (Figure 1) (14).

R-bibliometrix software package based on R-Studio (version3.0.3, http://www.bibliometrix.org) (15), VOSviewer software (version1.6.16, https://www.vosviewer.com/download) (16), CiteSpace software (version5.7R5W, https://citespace.podia.com/courses/download) (17), Hiscite software (https://histcite.updatestar.com/) (18) and Pajek software (http://vlado.fmf.unilj.si/pub/networks/pajek/) (19) were employed by the researchers to perform bibliometric analysis.

The VOSviewer software developed by Professor van Eck and Waltman and the CiteSpace software developed by Professor Chen are highly reliable and practical bibliometric software mainly used to visualize and analyze the knowledge structure and the evolution trend of scientific literature in a certain field (20). Besides, Citespace and VOSviewer can extract sub-clusters from the overall structure of literature network through clustering analysis to identify research subdomains, namely research hotspots (21, 22). Nodes in the figures represent countries, institutions, authors, journals, citations or keywords, and links between nodes represent collaboration, co-occurrence or co-citation relationships. We set the nodes in the figures generated as the rainbow ring diagram pattern. The center color of each rainbow node represents the year the study published, and the outer colors represents the years the literature highly cited. The flow of knowledge can be seen from the change of colors in nodes, links and clusters. The warmer the color, the later the year.

In addition, we employed the R-bibliometrix, VOSviewer and CiteSpace software for overlapping analysis to identify the research frontier, which can to a great extent predict the research directions that may produce significant breakthroughs in the next few years. Hiscite and Pajek software are integrated to extract main research trajectories from the huge literature citation network (19, 23). Through in-depth review of the result, researchers can quickly grasp the knowledge base and development trend in the field, so as to improve their own research results and adjust their own research strategies.

As is shown in Table 1, the most productive countries and institutions are mainly located in North America, Europe and East Asia. The top 5 influential countries are the United States, Germany, the United Kingdom, Japan and France. In this field, the United States has a pivotal impact, yielding a total of 17,931 citations, far exceeding that of other countries. Figures 3A,B display the international cooperation among different countries worldwide. The Citespace setting parameters of Figure 3A were as follows: # Years Per Slice = 1, Top N% = 25, pruning algorithm was adopted. The thickness of the lines between the two countries indicates the cooperation strength. It can be found that extensive cooperative relations have been established among countries in North America, Western Europe and East Asia, but the cooperation among developing countries is still weak and needs to be further strengthened. In addition, it’s necessary for us to note that the average Chinese article citation is not high, whereas the average article citation of Finland is outstanding, suggesting that there is not only a need to improve the quantity but also to seek breakthroughs in the international influence of publications from emerging countries.

Most of these prolific institutions are world-renowned research institutions, with prominent positions in the history of orthopedics and sports medicine research. We can see from the Figure 3C that extensive cooperation between institutions has been established. The Citespace setting parameters of Figure 3C were as follows: # Years Per Slice = 1, Top N% = 5, pruning algorithm was adopted. For example, Professor Wang Fei from Hebei Medical University has formed a stable cooperative relationship with the Hospital for Special Surgery. In addition, from the color and size of the nodes, it can be seen that the top 4 prolific institutions are Special Surgery Hospital, Hebei Medical University, University of Minnesota and Harvard Medical School. Focusing on the research of these high-yield institutions and the collaboration can help researchers understand the most significant scientific advances worldwide.

Additionally, a total of 318 publications from the top 15 influential authors account for 14.76% of all publications in this field. The author with the highest total citations was Andrew A Amis with 1,586 citations, followed by Philip B Schöttle with 1,265 citations, Elizabeth A Arendt with 1,251 citations and David H Dejour with 1,120 citations. Tables 2, 3 shows the details of the top 15 active researchers, including their H-index (24, 25), total citations, and the relevant information of their representative articles. Collaborations among these authors and their productions over time were shown in Figures 4A,B, respectively. The Citespace setting parameters of Figure 4A were as follows: # Years Per Slice = 1, Top N% = 2, pruning algorithm was adopted. It can be found that there are four research clusters, which are radiated by one or two core authors, such as Fei Wang, Vicente Sanchisalfonso, Andrew J Cosgarea, Jack Farr, Elizabeth A Arendt and David Dejour. Generally speaking, the research cluster led by Fei Wang, Andrew J Cosgarea and Jack Farr has been comparatively active in recent years, while the research cluster led by Elizabeth a Arendt and David dejour had greater influence a few years ago.

It is worth noting that one of the most influential authors–Andrew A Amis is a professor from the Department of Mechanical Engineering in Imperial College London, focusing on biomechanics for orthopedics (26). The comprehensive research of medicine and engineering has gradually become the hotspots, which can make a major breakthrough that is difficult to be achieved through traditional clinical researches (27).

With David H Dejour as the representative, Lyon School stands out as one of the notable landmarks in the progress of the research of patellar instability. Through the research and summary of several generations of scholars, Lyon School had established the theory that the abnormality of bone structure is the basic cause of patellar instability, and they formulated quantitative standards for radiographic detection and surgical indications for osseous risk factors (6, 28). In the last 20 years, soft-tissue surgeries such as medial patellofemoral ligament reconstruction have gradually become the worldwide research hotspot (29), which led to the formation of the America School with soft-tissue reconstruction as their core concept. And it can be seen from Figure 4 that Chinese scholars played an increasingly significant role in this field. For example, Professor Fei Wang made outstanding contributions in both clinical and preclinical medicine research on patellar instability.

There are still some limitations in our current research.

Firstly, there are certain limitations in our study. Firstly, in our study, we only searched the Web of Science Core Collection (WoSCC) and did not incorporate other databases, such as PubMed, Scopus or Embase. However, it may be unscientific to merge and analyze the data from multiple databases, because different databases have different measurement of citation frequency counting and classification of publications (9, 86).

Secondly, only English publications were included, which may lead to the omission of a portion of high-quality articles published in other languages.

Finally, there existed two potential disadvantages: (1) there was no manual cleaning of the sample data before formal analysis; (2) because the analyses were completed by software, there might be some errors or biases in our results. For example, the name of the journals may be changed over a long period of time. In addition, two authors with the same name may be repeatedly accumulated.

This is the first comprehensive bibliometric analysis of patellar instability. Our results show that patellar instability has gradually attracted the attention of scholars, which can be seen from the increasing number of articles and citations year by year. So far, the United States has been in a leading position in this field. Hospital for special surgery and Andrew A Amis are the most prolific institutions and the most influential authors respectively. The American Journal of Sports Medicine and Knee Surgery Sports Traumatology Arthroscopy are the most influential and productive journals in the study of patellar instability, with the most citations and publications respectively. According to the analysis of references, we identified 11 research hotspots of patellar instability in chronological order, and identified the main research paths in this field. In addition, the comprehensive analysis of keywords identified mpfl (medial patellofemoral ligament) construction, clinical outcome, risk factors, prevalence, articular cartilage, tibial tuberosity and tibial tubercle-trochlear groove (TT-TG) distance as significant research directions in the future, which deserves the attention of researchers. In short, scholars, especially researchers newly entering this industry, can benefit from our research, which can enable them to clearly and quickly understand the global hotspots, trends and knowledge structure of this field, so that they can be inspired to a certain extent.