Bibliometric Analysis of Research Trends on Manual Therapy for Low Back Pain Over Past 2 Decades

Introduction

LBP is a common symptom¹ that encompasses various types of pain, such as nociceptive, neuropathic, or nociplastic pain. It can affect individuals of all ages, from children to the elderly.² The highest rates of LBP occur among manual workers, people with physical and mental disabilities, smokers, and obese individuals.¹ The pathoanatomical etiology of LBP remains unclear, and it is estimated that most people will experience at least one episode of LBP during their lifetime. Although acute LBP is usually self-limiting, studies indicate that over 60% of individuals with LBP experience recurrent episodes one year after onset, leading to chronic and persistent pain.³ Between 1990 and 2015, the number of people with disabilities caused by LBP increased by 54% worldwide,¹ with the fastest growth observed in low-income and middle-income countries.⁴ The dysfunction and emotional problems caused by chronic pain seriously affect patients’ quality of life. As a consequence, LBP has become a global public health problem that necessitates enhanced research and exploration to reduce the associated economic and medical burden.^5,6

Management guidelines advocate a multimodal approach to improve symptoms of LBP, incorporating educational reassurance, analgesics, and non-pharmacologic treatments.⁷ However, the prevalent issue of overusing opioids and opting for surgery persists, warranting attention.⁸ The treatment of LBP focuses on relieving pain and preventing its progression. The primary goal of LBP treatment is to alleviate pain and prevent its progression. Complementary and alternative therapies, such as acupuncture, physical factor therapy, and exercise therapy, are commonly employed to address LBP. Despite their recognized safety and efficacy, long-term adherence to these treatments can be challenging for patients, and noticeable effects may require time to manifest.

As a non-invasive non-pharmacological therapy, MT is widely used in the treatment of LBP due to its recognized safety and effectiveness.⁹ MT encompasses massage, bone setting, chiropractic manipulation, joint mobilization, among others. Many clinical guidelines recommend MT for managing acute LBP.¹⁰ Clinical trials have shown that MT can not only alleviate pain but also foster enhanced connectivity between brain regions responsible for pain perception and motor processing.^11–15 This phenomenon may be attributed to the manipulation’s ability to realign dislocated vertebrae, increase joint space, optimize biomechanical structures, and alleviate nerve root compression and irritation. Additionally, MT has been shown to reduce fear avoidance behaviors by modulating blood oxygen level-dependent signals in the brain circuits of LBP patients.¹⁶ Few bibliometric studies have been performed on research relating to MT of LBP, though many clinical trials were conducted in this field. The purpose of this study was to demonstrate the new trends and frontiers of MT for LBP by using bibliometric software.

Methods

Search Strategy

All relevant articles from 2000-01-01 to 2023-01-01 were obtained from WOSCC, which includes Social Science Citation (SCI)-EXPANDED, Social Science Citation Index (SSCI), Arts & Humanities Citation Index (A&HCI), Conference Proceedings Citation Index – Science (CPCI-S), and Conference Proceedings Citation Index -Social Science and Humanities (CPCI-SSH). WOSCC is the world’s largest and most influential citation-based database, encompassing comprehensive literature data. It is the preferred choice for bibliometric analysis due to its extensive coverage of scholarly publications.^17,18 To avoid bias due to the daily update of the database, we completed the retrieval process on 24 February 2023.

Searching keywords was established about Medical Subject Heading (MeSH) terms from PubMed. The retrieval strategy was as follows: TS= (low back pain) And (manual therapy). Document types were articles and reviews. The language was restricted to English. A total of 1643 publications met the inclusion criteria. The flow chart is shown in Figure 1.

Figure 1 Flow chart of this study.

Data Extraction

By the following exclusion criteria, records were excluded: (1) A record was extracted if “low back pain” and “manual therapy” was not found in its title, abstract, or keywords; (2) Publication time out 2000–2023; (3) Other document types: conference abstracts, letters, news, editorial materials, proceeding papers, book chapters, short reports, case studies; (4) Not in the English language.

The data that came from WOSCC were inputted into Microsoft Excel 2016 (Redmond, Washington, USA), CiteSpace (Drexel University, Philadelphia, USA), and VOSviewer (Leiden University, Leiden, the Netherlands).

Analysis Tools

VOSviewer is a bibliometric analysis software for drawing knowledge maps, which can be used for co-occurrence analysis, co-citation analysis, and literature coupling analysis.¹⁹ It can visually display research results and has unique advantages in clustering technology and map display.^20,21 CiteSpace is a literature visualization software developed in Java language.²² Through the processing of the literature in the existing database, the software can objectively analyze the research process of a subject field accurately and conveniently, predict the future research trend of the field, and provide new ideas for the following research direction.^23,24 Therefore, the application of this software has attracted the attention of many scholars and researchers.²⁵

In this study, VOSviewer (version 1.6.19) and CiteSpace (version 6.1.R6) software were used to visually analyze the literature related to the MT of LBP in the WOSCC database and draw the knowledge map, aiming to gain the research status, research hotspots, and development trends in this field and provide a reference for future research.

Parameter Settings

We used VOSviewer to perform citation analysis for authors, bibliographic coupling analysis for journals, co-occurrence analysis for countries and keywords, and co-citation for cited references.^18,20,26,27 Different types of analysis and units of analysis were selected according to the needs of the analysis content. The parameters were set as follows: Counting method selected “Full counting”, Minimum number of occurrences of a keyword selected “5”.

CiteSpace was used to carry out visual analysis from the aspects of countries, authors, keywords, and references. The period is set to 2000–2023. Years Slicing is set to 1. The threshold is set to “TOP N”, which is N = 50. For network clipping connection, select “Pathfinder” and “Pruning the sliced networks”. We chose institutions, keywords and references as nodes to conduct co-occurrence analysis and generate visual maps.

Results

Analysis of Publications

A total of 1643 publications were included in the analysis. As shown in Figure 2, the number of publications showed an overall rising trend from 2000 to 2022. It reached a peak in 2020, with 130 publications. From 2012 to 2022, the number of publications accounted for 60% of the total number of publications. The number of articles published in this research field increased approximately exponentially, and the prediction model formula was y = 36.75e^0.0495x. In this formula, Y represented the number of publications and X represented the year. R² is 0.7099, showing that the curve was well fitted. This indicates that the field has a particular research foundation and continues to receive attention from researchers.

Figure 2 Annual publication outputs and growth prediction from 2000 to 2022. The number of published documents increases exponentially, and conforms to the formula: y = 36.75e0.0495x, R2 is 0.7099, showing that the curve was well fitted.

Analysis of Countries/Regions

A total of 1643 articles from 61 countries or regions were published, as shown in Table 1. The United States had the largest number of papers in this field (714 publications, 43.45%), followed by Canada (212 publications, 12.90%), Australia (198 publications, 12.05%), the United Kingdom (167 publications, 10.16%), and China (97 publications, 5.90%). The size of each circle in the Figure 3A represents the number of papers published by the corresponding country/region. The United States ranked first in terms of publication, citation times and cooperation with other countries. The average publication time in the UK was the earliest, indicating that the UK was the first to carry out relevant research. The most frequently cited article on average was in Italy, suggesting that Italy may be a rising star. The comprehensive analysis of publications, links, citations and Avg.pub. Year indicated that the United States and the United Kingdom held the dominant positions in this field.

Table 1 Top 10 Countries That Published Articles on MT for LBP

Figure 3 The analysis of countries and institutions. (A) Cooperation map of countries in MT from 2000–2023. The circle represents the country, the higher the number of articles, the larger the circle; Connections represent cooperation and communication, and the more connections, the closer cooperation and communication. (B) Cooperation map of institutions from 2000–2023. The circle represents the research institution, and the higher the number of publications, the larger the circle. The connection represents the number of cooperation and exchanges between research units, and the more connections, the closer the cooperation and exchanges. The outermost purple circle represents centrality, and the higher the centrality, the more important the node.

Analysis of Institutions

A total of 603 institutions and 825 connections were generated, and the density of the network was 0.0045 in Figure 3B. The purple outer circle represents the centrality, with wider circle indicating higher centrality.²⁰ Nodes with high centrality are considered significant points in the field.²⁸ The lines between nodes represent the cooperation relationships,²⁹ with more lines indicating more cooperation. University of Pittsburgh, Canadian Memorial Chiropractic College, University of Alberta Centrality bigger than 0.1 indicated that these three institutions have a place in the field. In order of the number of articles published, the top five institutions were: Canadian Memorial Chiropractic College, the University of Sydney, the University of Alberta, Vrije University Amsterdam, and the University of Utah. The top three were all from Canada. The biological basis and MT of musculoskeletal system injuries was one of the five research areas of Canadian Memorial Chiropractic College. The college focused on macro-and micro-level changes in LBP treated by MT.^30,31 From the perspective of node connections, there were close cooperations between institutions. Especially University of Pittsburgh (33 publications) in the United States had the highest centrality, which means it has established good research cooperation with other institutions (Table 2). Analysis of publications and centrality indicated that Canadian Memorial Chiropractic College was the leading research institution.

Table 2 Top 5 Institutions That Published Articles on MT for LBP

Analysis of Authors

For better visualization, only 174 authors with at least 5 articles were included in the collaborative author network. Nodes were authors in the knowledge graph, and links were relations. Five different colors appeared in Figure 4, with the same color indicating a close collaboration among the authors.³² We selected the top 12 authors, most of whom were from the United States in Table 3. The author who collaborated most strongly with the others and published the most articles was Long, Cynthia R. She is from Palmer Center for Chiropractic Research (PCCR), Palmer College of Chiropractic and IA. PCCR is the most funded chiropractic research center in the United States. Her research focused on the efficacy analysis of manipulation therapy with different intensities in the intervention of LBP in veterans.^33,34 The articles of Maher, Christopher G. had the highest citations, but the total link strength was not high, which indicated that his articles were receiving attention and should strengthen collaborative communication with other researchers. He is from the George Institute for Global Health, Sydney Medical School, at University of Sydney. Maher, Christopher G. focused on non-drug therapy, MT, and exercise therapy for LBP.^35,36

Table 3 Top 12 Authors That Published Articles on MT for LBP

Figure 4 Cooperation map of authors from 2000 to 2023. Nodes represent authors, and the size of nodes is proportional to the number of publications. The connections represent the number of cooperative exchanges between authors, and the more connections, the closer the cooperation and exchanges.

Analysis of Journals

Academic journals serve as the medium through which researchers publish their results, and they reflect the quality of research to some extent. A total of 840 journals and 1643 articles were included in the study. As shown in Table 4, the total number of articles published by the top ten cited journals accounted for 36% of all literature. In terms of total citations, Spine had the highest one (6807), followed by Annals of Internal Medicine (5132) and European Spine Journal (4774). The Journal of Manipulative and Physiological Therapeutics (234) had the highest number of publications, accounting for 14.53% of the total number of publications. Most of the journals in the field of MT of LBP were published in the United States. The journal with the highest impact factor was Annals of Internal Medicine in Q1. Figure 5 showed three clusters of journals in this field. The green cluster mainly published on spine-related diseases, the blue cluster published MT and other complementary therapies, while the red cluster published pain-related content.

Table 4 Top 10 Journals According to Citation

Figure 5 Bibliographic coupling analysis highly cited journals, weighted by citations, visualized map. Nodes represent journals, and the larger the node, the greater the number of citations.

Analysis of Keywords and Strongest Burst Keywords

The 614 keywords were divided into 7 clusters by VOSviewer. Red labels represented the locations, characters, and types of LBP. In Figure 6A, green, blue, yellow, and purple represented the current treatments, epidemiology, risk factors, and research methods. Figure 6B showed the keywords cluster map, offering insights into the development trends and new directions of this discipline. CiteSpace was utilized to cluster keywords, employing the classical log-likelihood ratio (LLR) algorithm, which yielded a total of 12 clusters. In this figure, Q=0.3579, S=0.6777. The Q value > 0.3 and S value > 0.5, demonstrating that the network clustering was significant and the literature within each cluster topic was consistent.³⁷ The biggest cluster was the #0 systematic review, followed by auricular point acupuncture, laser therapy, and physical therapy. Figure 7 showed the top 25 keywords with the strongest citation bursts. The left endpoint of the red line represented the emergence time, and the right represented the end time. Recent emerging keywords highlighted current research hotspots, focusing on validity, chronic LBP, clinical treatment guidelines, depression, and musculoskeletal manipulation. The current field of MT of LBP attention is directed towards its effectiveness, as well as the psychological state of patients, such as depression and anxiety.³⁸

Figure 6 The analysis of keywords. (A) Co-occurrence analysis of keywords by overlay visualization. Nodes represent keywords, and the larger the node, the more times it appears. (B) Cluster analysis of keywords. The # in the figure represents clustering, and the keyword clustering map is generated according to LLR method. There are 12 clusters in total.

Figure 7 Top 25 Keywords with the Strongest Citation Bursts. The left end of the red line represents the time when the keyword appears, and the right end represents the time when it ends.

Analysis of References

Reference co-citation map can explore research topics closely related to the academic field.^39,40 A total of 44,897 cited literature were included, and 348 literature met the criteria when the minimum number of citations of a cited reference was set to be at least 20. In Figure 8, the red cluster mainly focused on epidemiology, diagnosis, and evaluation. The blue cluster focused on non-drug therapy, spinal manipulation, and so on. Green clustering mainly focused on the advantages of MT compared with other therapies. The yellow cluster was concentrated in randomized controlled trials and systematic reviews.

Figure 8 A network diagram of co-cited references. Nodes in the figure represent references, and the larger the number of citations, the higher. Wired representatives are cited in the same literature.

The citation frequency of a single article reflects its academic level and influence.⁴¹ The higher the citation frequency, the greater the attention received. Table 5 showed the top ten with the number of citation articles. Among them, the article⁴² titled “Survey of chronic pain in Europe: Prevalence, impact on daily life, and treatment” by Breivik, H. had the most citations (3157). The survey showed that the majority of patients with chronic pain received non-drug treatment, with one-third of them choosing massage therapy. Among these ten references, four were published in Annals of Internal Medicine,^43–46 one was published in Lancet,⁸ one was published in Manual therapy⁴⁷ and three in European Spine Journal.^48–50 Combined with Table 4, this indicated that these two journals were the core journals for the manipulation of LBP.

Table 5 Top 10 Cited References Related to MT for LBP

Top 25 References with the Strongest Citation Bursts in Figure 9, references with suddenly increasing citation bursts at different times represented current research hotspots.⁵¹ By analyzing these references, we can predict the future development trend of research. Among recent publications, the article with the strongest citation bursts was published in Annals of Internal Medicine, titled “Noninvasive Treatments for Acute, Subacute, and Chronic Low Back Pain: A Clinical Practice Guideline From the American College of Physicians”.⁴⁴ This article suggested that non-pharmacological treatment should be the first choice for both acute and chronic LBP, including exercise, acupuncture, MT, motor control training, progressive relaxation, and behavioral cognitive therapy. Moderate-quality evidence suggested that MT, compared with other therapies, can relieve pain and improve functional status in the short term.⁵²

Figure 9 Top 25 References with the Strongest Citation Bursts. The left end of the red line represents the time when the reference appears, and the right end represents the time when it ends.

Discussion

The first study on MT for LBP was published in 1985.⁵³ The study divided 109 patients into three groups and treated them with short-wave diathermy, spinal manipulation, and placebo. The results showed that more than half of the three treatment groups had relief of LBP. Since then, the number of researchers involved in this field has increased steadily. However, there have been no global study to summarize and analyze the current research status.

Bibliometrics enables us to systematically explore academic literature and research findings in relevant fields. Recently, there has been an abundance of bibliometric analyses on LBP, including bibliometric analyses of non-specific LBP,³⁷ exercise intervention for LBP,⁵⁴ and acupuncture treatment for LBP.⁵⁵ However, there have been few bibliometric studies conducted on research related to MT of LBP, despite the numerous clinical trials conducted in this field. Through bibliometric analysis, we can assess the quality of existing research from the available literature, understand the current research activity, and explore the evidence level of the effects and potential influencing factors.

This study represents the first global bibliometric analysis of MT for LBP. We used VOSviewer and CiteSpace to analyze 1643 articles included in the WOSCC database and studied the research hotspots and future trends over the past 23 years. Since 2000, the number of publications has shown a steady upward trend, reaching a peak in 2020. Based on the current publishing trend, we predict that the number of publications will continue to grow steadily in the future.

In the field of MT of LBP, the United States is the most productive country, and the Canadian Memorial Chiropractic College is the most published research unit, Long, Cynthia R, who has the highest number of publications and the close collaborations with other authors. From the current thematic classification of MT for LBP, the main research content includes rehabilitation,⁵⁶ orthopedics, complementary medicine,⁵⁷ health care science,³⁴ neurology,^58,59 sports science, and other disciplines. Only 10% of LBP has a clear cause, while the majority is categorized as non-specific pain with an unknown cause. Non-specific LBP encompasses both acute and chronic cases, with acute pain lasting less than six weeks and chronic pain lasting more than three months.⁶⁰ Non-steroidal anti-inflammatory drugs and muscle relaxants have been found effective in treating acute LBP, whereas strong evidence supports the efficacy of manual and exercise therapy in treating chronic LBP.⁶¹ Guidelines (supported by low to moderate strength evidence) recommend patient education, different types of exercise, and MT to reduce LBP within 20 weeks.⁶² The mechanical force applied during MT initiates a series of neurophysiological responses from the peripheral and central nervous systems.⁴⁷ However, the deeper mechanisms require further investigation.

By the visual analysis of keywords and highly cited literature, the hotspots in this field focused on evidence-based medicine-related research such as manual strength and safety research,^10,63,64 Moreover, there is a growing emphasis on investigating the psychological state and quality of life of patients with LBP.⁶⁵ Yao et al⁶⁶ demonstrated that MT could potentially delay intervertebral disc aging by reducing oxidative stress through the Silent mating type information regulation two homolog-1/forkhead box O1 pathway. This finding holds great significance in improving the physical function of elderly individuals through non-drug and non-invasive MT. It is important to pay attention to LBP in special populations. A retrospective study revealed that pregnant women experiencing LBP are more likely to develop postpartum depression compared to those without such symptoms.⁶⁷ Pregnant women often seek MT to alleviate pregnancy-related back pain, which have shown significant benefits in reducing pain, improving mobility, and sleep quality.⁶⁸ However, more high-quality evidence is still required to confirm the reliability of these findings.

Due to varying research quality, there are certain differences in the classification of guidelines for the treatment of LBP.⁶⁹ There remains a shortage of high-quality randomized controlled trials concerning MT for LBP,^70–72 prompting researchers to prioritize exploring high-quality evidence, such as randomized controlled studies, matched sample cross sectional studies and meta analysis. Future focus areas will further delve into the normalization of MT, the efficacy of different intensity techniques, and the neurophysiological mechanisms of MT for LBP.

Limitation

Several limitations need to be noted regarding this study. First, only the data in WOS was selected, and the literature in other databases was not included. Second, due to language and time constraints, non-English papers and literature that were not within the set period were lost. In addition, due to the limitations of the software, the format of the case and abbreviation cannot be modified, and the setting of the threshold and clipping method may cause some data to be cropped.

Conclusion

MT of LBP has garnered increasing attention from scholars. This study systematically summarized the literature in this field through bibliometrics. Current research primarily focuses on the effectiveness and safety of MT, the neurophysiological mechanism involved, and the impact of pain on psychological states. Although there exists a certain research foundation in this area, high-quality evidence-based medicine evidence is still lacking. In the future, significant efforts will be dedicated to enhancing the quality of research in the field of MT for LBP. This will involve conducting rigorous and methodologically sound studies, including randomized controlled trials, matched sample cross-sectional studies, and meta-analyses, all supported by evidence-based medicine principles. Through these endeavors, the aim is to delve deeper into the intensity and standardization of MT techniques and ultimately establish its effectiveness in treating LBP. By emphasizing evidence-based approaches, this research will contribute valuable insights to the field and pave the way for further advancements in the understanding and practice of MT for LBP.