Introduction

Spine surgery has dramatically evolved as a consequence of worldwide researches and ideas in recent years. To assess the research contributions around the world, biomedical research publication has been used as an index for scientific research productivity [1–7]. The quantity and quality of publications are measurements of scientific achievement [2, 3, 7–10]. Recently, many studies using bibliometric methods have evaluated the worldwide research productivity in several biomedical fields [2, 4, 5, 9–13]. Such articles also have been published in general orthopedics and its subspecialties [3, 14, 15].

However, as far as we know, bibliometric studies concerning the quantity and quality of articles published in spine journals worldwide are scarce. Therefore, the purpose of this study was to evaluate the research productivity in major spine journals using Scopus database over the 10-year period and to give an insight into the worldwide spine research for spine surgeons and researchers.

Materials and methods

The structure of this study was remodeled on previous publications [2, 3, 10, 12–14]. The collected data were based on the Scopus online database which was widely used in the similar studies [1, 2, 9, 10, 16]. This platform was chosen because it was the largest abstract and citation database of peer-reviewed literature and delivered a comprehensive overview of the world’s research output [17]. Scopus covers nearly 22,000 titles from 5000 publishers worldwide, and contains 55 million records and provides 100 % MEDLINE coverage [17, 18]. Scopus database was developed by Elsevier, combining the characteristics of both Web of Science and PubMed [2, 10, 18]. These characteristics allow for enhanced service for educational and academic needs, and medical literature research and bibliometric analysis [2, 10, 18].

A comprehensive online search was performed using the database of Scopus in November 20, 2014. A total of five spine journals, including Spine, European Spine Journal, The Spine Journal, Journal of Neurosurgery: Spine, and Journal of Spinal Disorders and Techniques were used as source journals in this study. The search terms using International Standard Serial Number (ISSN) of these 5 journals were as follows: “1529–9430 or 0940–6719 or 1432–0932 or 0362–2436 or 1528–1159 or 1547–5654 or 1547–5646 or 1536–0652 or 1539–2465”. Articles published in the 5 journals from January 2004 to December 2013 were identified. Only article and review were included. Letters, notes, and editorials were excluded.

Scientific output was evaluated based on a methodology developed and used in other bibliometric studies [2, 3, 10, 12]. The following information was collected and analyzed: total publications and trends of worldwide contributions in spine research between 2004 and 2013, countries’ contribution and h-index, authorship, subspecialty, funding source, journal patterns, institution and top 10 cited articles. According to previous publications [19], the academic degrees of the authors were classified as MD only (i.e., MD without an advanced research degree), clinical-scientist (i.e., MD, PhD and MD, MS), non-clinician-scientist (i.e., PhD and MS), and others (i.e., authors with neither an MD degree nor an advanced research degree). Funding source was classified as commercial funding, noncommercial funding, or no funding [20].

The primary outcome was the total number of articles published by different countries, which was considered as an index of research output. The h-index from Scopus was also collected. The h-index is a country’s number of articles (n) that have attracted at least n citations [2, 9, 10]. That is to say, a country with an h-index of 10 has published 10 articles, and each of them has received at least 10 citations. Publications with fewer than 10 citations are not calculated by the h-index. It providing a single-number innovative metric quantifies both country scientific productivity and scientific impact [2, 9, 10]. The assessments of bibliometric analysis (e.g., countries, authors, institutions, most cited articles) were collected from Scopus and converted to rank order [2, 9, 10]. Only the 10 top ranked were listed. If two or more measurements have the same ranking number, all of them were shown in this study.

Data collected from Scopus were exported to Microsoft Excel for further analysis. Regression analysis was used to determine significant changes in time trend between 2004 and 2013. Data analysis was performed using statistical software SPSS version 19.0 (SPSS Inc., Chicago, IL, USA), and p < 0.05 was considered statistically significant.

Results

A total number of 13,115 publications on spine were indexed in the Scopus database from 2004 to 2013. A significant increase was demonstrated in the worldwide number of annually publications during the study period (p = 0.000) (Fig. 1). Compared with 856 articles collected in 2004, a total of 1628 articles were identified in 2013, indicating a 1.9-fold increase in publications between 2004 and 2013.

Fig. 1
figure 1

Number of articles by publication year

Full size image

The articles that we identified were published by authors from 93 countries worldwide. The top 10 countries whose researchers published the largest number of articles in the field of spine are listed in Table 1. These top 10 countries published 88.32 % (11,583/13,115) of total spine articles. The largest number of publications in the field of spine surgery was from the United States (5137; 39.17 %), followed by Japan (1408; 10.74 %), and China (1131; 8.62 %). In addition, United States had the highest h-index (106), followed by Canada (60), and United Kingdom (54).

Table 1 Top 10 countries contributed to research publications in the field of spine surgery
Full size table

The proportion of countries’ contributions by year was further analyzed (Table 2). Among the top 10 countries, only China and South Korea showed significant increases (from 2.10 to 13.64 %, p = 0.000; from 1.17 to 7.00 %, p = 0.000) in the spine research output over the past 10 years. The other eight countries (p > 0.05) remained relatively static in research productivity in the field of spine surgery.

Table 2 Proportion of the top 10 countries' contributions to the field of spine surgery by year
Full size table

The number of authors was ranged from 1 to 38, with a median of 5. A total of 374 articles (2.85 %) were published by one author. The articles with five authors were the most common articles (2392; 18.24 %), followed by four authors (2134; 16.27 %), and six authors (2102; 16.03 %). Because the academic degrees held by the authors were not provided in the articles published in European Spine Journal, the other four spine journals were used to collect the author degrees. MD-only authors were the predominant author group, accounting for 58.63 % (31,417/53,582) of authors, followed by non-clinician-scientist (8130; 15.17 %), clinical-scientist (7903; 14.75 %) and others (6132; 11.44 %). In addition, 57 study groups participated in publishing the spine articles. The Spinal Deformity Study Group led the rankings with 22 publications, followed by The International Spine Study Group (17), and Harms Study Group (14).

Degenerative diseases were the most common subspecialty topic (3242; 24.72 %), followed by basic research (2453; 18.70 %), general spine (2045; 15.59 %), deformity (1627; 12.41 %), trauma (1175; 8.96 %), rehabilitation (892; 6.80 %), oncology (847; 6.46 %), and others (834; 6.36 %). Moreover, 3742 articles (28.53 %) were supported by funding source, while the majority of articles (9373; 71.47 %) received no funds. Among the funded studies (3742), the most articles (2745; 73.36 %) were supported by noncommercial funding. The other studies (997; 26.64 %) received at least one commercial funding.

Figure 2 shows the publication ranking of the 5 spine journals. The journal Spine published the largest number of spine research, with a total of 5960 articles published (45.44 %), followed by European Spine Journal (2811, 21.43 %), and Journal of Neurosurgery: Spine (1747, 13.32 %).

Fig. 2
figure 2

Articles published in the spine journals. ESJ, European Spine Journal; JNS, Journal of Neurosurgery: Spine; SJ, The Spine Journal; JSDT, Journal of Spinal Disorders and Techniques

Full size image

The top 10 productive authors in the field of spine surgery are shown in Table 3. There were 13 authors in the top 10 list because some authors published an equal number of articles. These authors published at least 74 publications. Vaccaro AR published the largest number of spine articles, with a total of 187 publications (1.43 %), followed by Lenke LG (147; 1.12 %) and Takahashi K (107; 0.82 %). Eight of these 13 authors (61.54 %) were from United States, 3 authors (23.08 %) from Japan, and 2 (15.38 %) from Canada.

Table 3 Top 10 productive authors in the field of spine surgery from 2004 to 2013
Full size table

Table 4 shows the top 10 most productive institutions in the field of spine surgery. These institutions published at least 127 publications. University of California, San Francisco published the largest number of articles (260; 1.98 %), followed by Thomas Jefferson University (211; 1.61 %), and University of Toronto (185; 1.41 %).

Table 4 Top 10 productive institutions in the field of spine from 2004 to 2013
Full size table

The top 10 cited articles between 2004 and 2013 are shown in Table 5. The number of citations for these articles ranged from 684 to 303. Six of them were published in Spine, followed by European Spine Journal (2) and The Spine Journal (2).

Table 5 Top 10 cited spine articles from 2004 to 2013
Full size table

Discussion

Spine surgery has a significant progress in recent years due to the contributions and ideas from all over the world. In fact, it should be inequality in scientific contribution for different countries. However, the study investigating the characteristics of different countries’ contributions to the field of spine surgery is lacking. Increasing bibliometric studies were widely used to evaluate the worldwide research productivity in many biomedical fields and also been reported in the field of orthopedics [1–5, 7, 9, 10, 13–16]. These bibliometric studies include a series of visual and quantitative procedures of the communication and utilization of literature to evaluate scientific publications. Bibliometric studies have been applied primarily to reveal the global trends of research within a given topic, field, institute, or country [1, 3, 7, 8, 14, 16]. In light of this, to the best of our knowledge, the present bibliometric study could provide a general picture of worldwide research productivity in the field of spine surgery for the first time.

According to basic bibliometric theory, an increase or decrease in the scientific publication indicates the speed of progress in science and technology [2, 3, 9, 10]. A rapid change in the number of publication likely indicates an important turning point of the development. The present study found that the number of spine articles that we identified significantly increased between 2004 and 2013, nearly two times, indicating that there is an important and rapid progress phase for the development of the spine research in recent years.

The characteristics of geographic distributions in scientific research productivity could reflect the research capabilities and technological developments of different countries [2, 3, 7–9]. Our study found that 88.32 % of total articles in the field of spine surgery published by the top 10 countries, indicating that the worldwide research output in spine surgery was concentrated in fact. A small number of countries published the majority of the spine research, which was similar with other medical filed [3–8]. In addition, as shown in this study, different countries had different scientific output. It may depend on population size, socioeconomic status or overall research capability of the country [2, 7–9]. Especially, United States accounted for nearly 40 % of the total research output in the field of spine surgery, far more than any other country. It is not surprising that United States leads the rankings, which has been proved in many biomedical fields [3, 4, 6–8, 14]. Most importantly, United States also have the highest h-index, much higher than any other country, which indicated that spine articles originating from United States had not only the largest quantity, but also the highest quality. Therefore, these findings further prove the overwhelming influence of United States in spine surgery.

Under the context of a rapid growth in the worldwide research output, the proportion of a country’s contribution to the world provides a measure of the relative contribution [3, 21]. Among the top 10 productive countries, only two countries including China and South Korea showed rapid increases in the publication year by year, indicating rapid progress and increasing contribution to the spine surgery in these countries. This result may mirror the high attention on spine research and increasing funding in these countries, especially in China [22, 23]. In addition, although United States showed a decline trend year by year in general orthopedics according to the previous study [3], which was also found in other medical field [21], this trend was not proved in the field of spine surgery according to the result of our study. United States still ranked first each year during the past 10 years, while its research output kept stable, no significant increase or decrease.

Regarding the top 10 productive authors in spine surgery, this study found that all these authors were from 3 countries including United States, Japan and Canada, and the United States has far more productive authors than the other two countries. Moreover, an analysis of the top research institutions found that the United States also had the largest number of top research institutions in spine surgery. These results may demonstrate that the United States has the strongest research power in the field of spine surgery.

Our study found that most of the articles (97.15 %) were published by multiple authors, and the articles with five authors (18.24 %) ranked the first. More than half (50.54 %) of the total articles had four to six authors. Among them, MD-only authors were the major contributors to the spinal publications, while MD with an advanced research degree, non-clinician-scientist, and study group also had special contributions to the field of spine. This phenomenon is similar with other medical fields [24, 25]. The main reasons may be the increased complexity of multidisciplinary research and the increase in large multicenter studies and collaborations [19, 24–26].

The present study demonstrated that more than two-thirds of the spine publications had no funding support. This result is similar with previous publications [27]. It is still needed to increase the financial research sources in the field of spine. Regarding the funded articles, multiple sources of funds were found in support of spinal researches. Most articles received financial support from noncommercial funding. However, the commercial funding, accounting for more than one quarter of the funded articles, is also very important for spine research. The commercial funding may have an advantage in conducting translational research and developing products for better patient care [28].

Spine and European Spine Journal published nearly seventy percent of total publications, indicating the great importance in the expansion of new knowledge in the field of spine surgery. In addition, these two journals also have high impact factor. These suggest that publication in Spine and European Spine Journal and had not only large quantity, but also high quality, suggesting the significant impact of these two journals in the field of spine surgery.

The number of citations was further used to identify hot spots and trend in the field of spine during the past 10 years in this study, as represented by the top 10 cited articles [2, 10, 12]. Interestingly, all the top 5 articles according to total citations discussed the low back pain. This result may indicate that low back pain is an important topic in the field of spine and attract more attention and researches. In addition, Spine and European Spine Journal published 8 of 10 top cited articles, which might indicate that articles published in these journals may likely be more influential.

There are several limitations in our study. A total of five main spine journals were included in this study as source journals. Therefore, some general orthopedics and basic research journals which may publish some articles related to spine surgery were not included in the present study. Nevertheless, these five subspecialty spine journals used in this study could represent the major publications contributed to the field of spine surgery.

Conclusions

As far as we know, the present study is the first bibliometric evaluation on the worldwide research productivity in the field of spine surgery. This study reveals that the number of publications in the field of spine surgery has been increasing at a rapid rate over the past 10 years. United States is the most productive country, not only in quantity, but also in quality. Indicating it`s special contributions to the body of spine publications. Moreover, China and South Korea show rapid progresses in spine research and increasing contributions to the field of spine surgery.