Bibliometric evaluation of ultrasound in orthopaedics global publications: 10-year trends and hotspots

Background. The past decade has witnessed the development of ultrasound in orthopaedics, the growth rate of publications has been accelerating. In this study, analysis of the publications on ultrasound in orthopaedics over the past decade has been completed. We aim to provide inspiration for future research on ultrasound in orthopaedics through our work. Methods. All relevant publications between 2009 and 2020 from Web of Science were collected. Statistical Package for Social Science and GraphPad Prism 8 software were used to generate and analyse diagrams. Meanwhile, from the aspects of co-occurring keywords, VOSviewer software and CiteSpace were employed to visualize the research trends. were will published the papers Ozcakar, L had the papers, and a YM received the highest citation The keywords about "MRI", "complication", "female" and "male" were noted as new topics.

Using literature databases and literature metrology characteristics, bibliometrics is a viable way to quantitatively and qualitatively evaluate trends in research activity over time. It provides a convenient way to understand development trends in a certain eld and evaluates academic groups and individual researchers (11). Similarly, in terms of policy and decision making, bibliometrics can also provide supporting evidence (12). In many scienti c elds, including diabetes (13), cardiovascular disease (14), gastrointestinal diseases (15) and respiratory medicine (16), bibliometric research methods have been widely used. The present study was aimed at comprehensively analysing the research progress with respect to ultrasound in orthopaedics based on the Web of Science (WOS). We applied a bibliometric analysis for the purpose of grasping the research trends related to ultrasound in orthopaedics and predicting its possible future hotspots.
Although Julien Dartus et al. assessed the past two decades of orthopaedic publications in France using bibliometric science (17), Xiao Zhai et al. also used bibliometric methods to report the publication trends of ultrasound in spines from 1994 to 2015 (18). To date, no study has reported ultrasound research in orthopaedics. We will explore the comprehensive review of the publication trends and analyse future hotspots covering ultrasound in orthopaedics.

Data sources and search strategies
Studies published from 2009 to 2020 were used in this study. It has been widely accepted that the online database Science Citation Index-Expanded (SCI-E) of Thomson Reuters'WOS is one of the most suitable tools for collecting data, so all publications we studied were obtained from this database. To avoid omissions introduced by the rapid update of the database, we nished all of the search progresses in a single day, September 19, 2020. The search strategies were set as follows: TS = ((ultrasound) OR ultrasonography) OR (ultrasonic) OR (sonography)) AND Web of Science Categories = Orthopaedics AND Language = English. Only original articles and reviews with standard peer reviews were selected for our research, while other types of studies were excluded. Speci c processes of enrolment and selection are shown in Fig. 1.

Data collection
All types of data were extracted from the correlative publications by three authors (

Bibliometric analysis
Thomson Reuters' WOS had a large collection of studies, especially those focused on biomedicine, so we chose the WOS to obtain the characteristics of all incorporated publications. The term relative research interest (RRI) was de ned as the number of publications in a particular research eld divided by the total publications across all elds per year. The impact factor (IF) was obtained from the information provided by the journal citation reports (JCRs) published in 2020. It is widely accepted that the H-index plays an important role in evaluating the scienti c research impacts of a scholar or a country. Especially in medicine, the H-index of articles serves as a potential tool to measure academic productivity. The index of H means that a scholar or a country has published at least H papers, and each paper has been cited in other publications at least H times, which takes up a larger part than other indicators we collected (total citation count, citations per paper, and total paper count) in assessing scienti c achievement.
Citespace is a practical statistical software We used the links between nodes in the map to show the bibliometric characteristics, such as reference, institution, author and terms, and we also analysed the potential research trends of future research. Meanwhile, the keywords were extracted according to this software. Using the Java programming language, VOSviewer is a convenient science mapping software tool.
which is widely used for co-citation network analysis and visualization. The technique used for map construction is based on a co-occurrence matrix.

Results
In all, 1,677 articles dated from 2009 to 2020 met our inclusion criteria, with the United States ranking rst in the number of publications at 469 (27.9%), followed by England at 164 (9.8%) and Japan at 160 (9.5%). By comparing the number of papers published per year, we found that the largest number of publications occurred in 2019, with 194 publications (11.6%) ( Fig. 2A). When the numbers of all-eld publications were considered, the global interest of ultrasound in orthopaedics measured by the value of RRI started to increase since 2016 and increased to 0.045% in 2019; meanwhile, the peak of the curve also emerged in 2019 during this progress (Fig. 2B). We believe that the trend of growth will also accelerate in the future. The cooperative relations between countries can also be visualized (Fig. 3). As the country with the highest number of publications, the United States also had the closest ties to other countries.

Growth trends of publications
The global cumulative publication numbers and the top 6 countries, as well as the corresponding model tting curves, are shown in Fig. 4. Based on these growth curves, we found that the growth of publications of the entire world was on a fast curve, which was also applicable for several major countries, such as South Korea and China ( Fig. 4F and 4G). The number of papers published by those countries per year has grown rapidly in recent years, especially China, which will show an obviously faster growth curve in publications in this eld compared to other countries over the next twenty years (Fig. 4G). Although some countries still lead in the number of papers published now, their growth rate will decline over the next twenty years, such as the United States and Japan ( Fig. 4B and 4D).  Table 1. The impact factor was reported according to the journal citation reports (JCR) 2020

Institutions with research publications on ultrasound in orthopaedics
The Mayo Clin in the United States had the highest number of publications among institutions worldwide, with 33 papers documented by this a liation, which accounted for 2.0% of all publications. Within the list of the top 20 institutions in this eld, American institutions account for over half, three institutions were in China, two were Canada institutions, one was a Turkey institution, one was a Brazil institution, one was a Japan institution, and one was Korea (Table 2).

Analysis of keywords in publications of ultrasound in orthopaedics
We analysed the keywords extracted from 1,677 publications using VOSviewer. As presented in Fig. 6A, 66 keywords, de ned as terms that occurred more than 60 times within titles and abstracts in all papers during the analysis process, were identi ed and classi ed into three clusters, namely, diagnosis, treatment and epidemiology. Within the cluster of diagnosis, the following keywords were frequently mentioned: diagnosis

Research trends of ultrasound in orthopaedics
In terms of the publication volume of all countries, the United States leads the rst (Fig. 2), while China had the fastest growth rate (Fig. 4G). We speculate that the reason for this phenomenon is related to the index of the country's GDP ( Fig. 2A). We can also nd that the developed countries published more research, such as the United States, England and Japan ( Fig. 2A). This shows that countries leading in science and technology are exploring more of the uses of ultrasound. Improvements have emerged in ultrasonic technologies in developed countries, such as the invention of ultrasound and microbubble therapy (19), which can e ciently increase cell membrane permeability, resulting in enhanced tissue distribution and intracellular drug delivery of molecules, the popularity of small probes (20) and the combination of ultrasound, X-ray and MRI (21).
these new technologies often appear in developed countries.
Published articles with the highest citation frequency are associated with correlative academic impacts in a certain eld. Detailed information regarding the top ten most frequently cited publications on ultrasound in orthopaedics is provided in Table 4. The study published in The Journal of Arthroplasty in 2011 with Kwon, YM as the corresponding author provides the most common combination of ultrasound in orthopaedics, and the research indicates that in metal-on-metal hip resurfacing arthroplasty, ultrasound is essential as a diagnostic tool (22). The ranked second to fourth articles are all from the same journal, the American Journal of Sports Medicine, which describes the application of ultrasound diagnosis for the repair of chronic Achilles tendinopathy, large and massive rotator cuff tears and double-row rotator cuffs, respectively (23)(24)(25). For the top 10 cited articles about ultrasound in orthopaedics, their focus was an evaluation of the effectiveness of ultrasound diagnosis in the treatment of a disease. The results were similar to clusters such as diagnosis, treatment and statistics (Fig. 6A).  (Table. 3) In this eld, RRI displayed a quickly rising trend (Fig. 3). We analysed the reason for the increase in research interest from the special advantages of ultrasound. On the one hand, hospitals are increasingly using ultrasonic devices to reduce exposure to X-ray radiation during examinations with the improvement of people's health awareness (13), and in the diagnosis of some orthopaedic diseases, the diagnostic rate of ultrasound is not inferior to CT. For example, ultrasound is more accurate in assessing the early healing process of fractures (26); the last but not the least, ultrasound as a diagnostic and therapeutic method does have overwhelmingly taken its place in the scienti c arena (27), and the use of bedside ultrasound also provides ideas for rapid diagnosis in critical and emergency situations in the future. For example, the advent of portable ultrasound makes rapid diagnosis of soft tissue injury possible in the wild (28). Thus, the acceptance of ultrasound is improving, and the study of ultrasound in orthopaedics is also undergoing a development process.
Regarding the top 10 institutions and top 10 authors, half of the top 10 institutions are from the United States (Table. 2), and three of the top 10 authors are also from the United States (Table. 3). We predicted that these phenomena were related to the cooperation between countries. For example, the map based on WOS data reveals that the United States has connections with many countries in the eld (Fig. 3); however, other countries have few connections. Therefore, the quality of articles in other countries is lower than in the United States. This phenomenon has called on scientists from all over the world to break through boundaries and bring about deeper cooperation. Only in this way can we promote the development and application of ultrasound in the orthopaedics eld.

Research focused on ultrasound in orthopaedics
According to the map based on the bibliographic data from the analysis of all keywords (Figs. 6A, B), we found that the key words were divided into three clusters, namely, diagnosis research, treatment research and epidemiology research. The density of the keywords outlined the overall structure of the gure and draw attention to the most important areas of ultrasound in orthopaedics, revealing an even distribution among the three groups that was consistent among the three clusters. However, the clusters of diagnosis and epidemiology studies were relatively new with respect to the publication dates, and the potential reasons for this are as follows.
First, the research over the past decade indicates that the research direction has transitioned towards various aspects of diagnosis rather than just assessing the fracture. For example, doctors can use ultrasound to diagnose arthritis (29), dysplasia of the hip(30), subacromial pain syndrome(31), neuromuscular diseases(32) and so on. Furthermore, scientists from different countries have also tried to analyse ultrasound studies in orthopaedics using statistical methods in recent years. Thus, keywords such as correlation and reliability have emerged within the epidemiology research cluster. According to the results of the pictures (Fig. 6B), the research direction has shifted towards epidemiology research. For example, in the aspects of the prediction of tendon injury risk, assessment of tendon healing and provision of further insight into tendon physiology, the usefulness of ultrasound was con rmed by epidemiology methods (33). However, the application of ultrasound in osteology is in its infancy, and scientists must further strengthen their research efforts (Fig. 6A).
With respect to the latest research hotspots, MRI from the diagnosis research cluster is the most recent (cluster 1), which indicates that ultrasonic diagnosis has been used in combination with other diagnostic tools to improve diagnostic e ciency. For example, the assessment of fracture healing remains challenging due to a lack of consensus on imaging and clinical criteria as well as the lack of a true gold standard (26).
However, ultrasound signs of healing can be identi ed as early as 1-2 weeks post fracture. By attaching a position sensing device to the ultrasound probe, 3D reconstructed images can be generated, which can help in the interpretation of complex fracture pattern healing (34). In clinical practice, Aspelin P et al. also conducted ultrasonic diagnosis on 32 patients with lower limb soft tissue injury and found that the diagnosis of haematoma was good (35). In the diagnosis of deep tissue damage, MRI has been shown to play a signi cant role, especially in spinal cord injury (36) and disc herniation (37), compared with ultrasound and CT. Thus, investigating the combination of kinds of imaging rather than just ultrasound is likely to be a new hotspot in future research. For example, MRI performed better in the diagnosis of in ammation, but ultrasound was more valuable in the diagnosis of small bone lesions (38). Therefore, in the treatment of rheumatoid joints, the researchers assessed the disease in three ways: synovitis, osteitis, and erosions. They found that the combination of ultrasound and MRI improves the accuracy of diagnosis (38), which is a trend for future diagnosis. According to the epidemiology research cluster (cluster 3), key words such as female and male in the latest research appeared more frequently. Therefore, epidemiological factors such as gender and age have gradually drawn attention in a large number of clinical studies, and gender is one of the important in uencing factors in the diagnostic models and prognostic analysis models of some diseases. For example, in a study of combined ultrasound and nerve stimulator-guided deep nerve block, the research objects were also grouped by sex (40). Similarly, in a study of risk factors for osteoporosis and associated fractures, females have long been listed as a risk factor, and due to the secretion of female oestrogen, it has also become a point of consideration in the treatment of osteoporosis patients (41).
In Fig. 7, the words that catch people's attention and last the longest included bone, articular cartilage, and the new salient keywords included instability and fracture. Therefore, we speculated that the study of bone joints would be a trend. Over the last two decades, a number of technical advances have improved ultrasound imaging of joints and soft tissues, increasing accuracy in joint disease assessment (42). For example, ultrasound detection of synovial effusion and synovial hypertrophy in knees has obtained good results (43,44).
Clinical research is a research method to discuss a series of problems such as the diagnosis, treatment, prognosis, prevention of etiology and so on, while ultrasound as a clinical application device, research results can be quickly converted to clinical. Clinical randomized controlled trial is a highly reliable evidence in the eld of evidence-based medicine, nally, we searched Clinicaltrials.gov and found 7 documented clinical randomized controlled trial (Table 5), Ultrasound guided knee injections, as a research focus of bone and joint discipline, is also the focus of clinical randomized controlled studies, which is similar to the results in Fig. 7. What is more surprising is that most of the clinical studies we collected are related to nerve block under ultrasound guidance and achieve good therapeutic effect, which provides a direction for the application of ultrasound in treatment to a certain extent. since all the 7 clinical trials were therapeutic, it is reasonable to believe that ultrasound, as a traditional diagnostic tool, has become a trend to be widely used in clinical treatment.
This bibliometric analysis investigated the publications that were extracted from the WOS database. We try to keep the data objective and reliable. However, limitations are inevitable. Due to our inclusion criteria, only English studies have been collected, and some important but non-English studies related to ultrasound have been ignored. In addition, the database is still constantly updated, so our results may be slightly different from the actual results.

Conclusions
The United States was the most productive country for research on ultrasound in orthopaedics, and we predicted that China will surpass the United States in publication over the next two decades. The focus of keywords gradually shifted from treatment research to diagnosis research and epidemiology research. It was also recommended to focus on promising research hotspots, such as MRI, complications and gender. Although the current amount of research is not enough, we believe the growth trend in the future will be rapid. Our study provides profound insights into the research history and current status of ultrasound in orthopaedics, which may indicate its future trend. (Fig. 8 Availability of data and materials: Figure 1 Flow diagram of the inclusion process. The detailed process of screening and enrolment.