Introduction

Pain is a major health problem with high prevalence in all ages and has been determined as a core component of age-dependent chronic health conditions.1,2 Chronic pain has been defined as pain lasting more than three to six months' duration,3,4 with prevalence ranging from 2% to 40% amongst adults at the population and primary health care levels.5 In particular, military veterans are a group of people who experience chronic pain and related factors. Internationally, chronic pain experienced by veterans has been associated with psychiatric comorbidities.6,7 A broader characterization of the prevalence of musculoskeletal (MSK) chronic pain-in terms of potential covariates and occupational demands that link with chronic pain-is needed to support disability prevention. For instance, the nature of military occupations are different from other occupations given that they involve diverse levels of exposure to occupational hazards. Thus, persons involved in military occupations may be at greater risk of developing MSK conditions and acute and chronic pain due to the challenges involved in military duties, such as continuous training, operation of heavy machinery, safety sensitive work, and unpredictable environments.8

A recent systematic review showed that most of the pain research on military forces has focused on comorbidities with post-traumatic stress disorder and traumatic brain injuries. Amongst all the studies included in the analysis, the prevalence of chronic pain in military veteran occupations ranged from 25% to 72%, and for pain of three months' duration was 27%.9 However, this review did not include specifically the prevalence of MSK pain amongst active duty personnel. The Life After Service Survey (2010) in Canada showed that chronic physical health conditions were two to three times more common in veterans than in the Canadian general population, and 41% of veterans reported having chronic pain or discomfort.10

Several studies explored the rates of MSK pain in active and retired military personnel amongst other health problems, and showed that back pain was the most frequent issue.11,12 In the United States military, a group of authors determined that 63% of the non-deployed active duty soldiers in the study had at least one pain diagnosis, and 59% of them had a primary pain diagnosis.13 In this study, three categories for MSK primary diagnosis were the most common: back and neck pain (22% frequency), non-traumatic joint disorders (28%), and other MSK pain (30%).13 Moreover, in a cohort study characterizing pain intensity numeric rating scores, comorbidities and medical diagnoses in 5,237,763 US veterans, the most common MSK diagnoses were non-traumatic joint pain, back pain, and osteoarthritis, consistent with International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes.14 Additionally, this study found that 50% of all veterans who received care in veteran hospitals were diagnosed with an MSK condition.14 Another study showed that injuries were reported as the main cause of medical services utilization, and the main types of injury resulting in hospital admission were fractures (40%) and sprains.15 Recent evidence demonstrated that the risk of MSK pain was higher for active duty military personnel and enlisted female personnel and for those exposed to the operation of motor vehicles.16

An understanding of MSK disorders associated with chronic pain within the military population would help to determine the magnitude of the problem in terms of the economic costs and the subsequent treatment interventions needed.17 Also, it could help to point out the effects on general well-being, since chronic pain and MSK have been associated with low quality-of-life indices, long-term disabilities, and potential high health care utilization.13,18 Moreover, exploring the prevalence in the literature of MSK chronic pain in active personnel could help to understand the magnitude of this issue before their retirement. The comparison of the prevalence in active versus retired members could also help to assess the differences between chronic pain for both groups and to explore the potential baseline of disease amongst veterans after service exposures. A search of relevant databases was conducted (Embase, PsycINFO, and PubMed) and found no existing or in-progress protocols or systematic reviews comparing the prevalence in active versus retired members.

The objectives of this review are to explore the scientific literature related to the prevalence of MSK chronic pain among active and retired members of military personnel, and to characterize anatomic variation and potential factors that could influence the frequency of MSK pain.

Review question

What are the prevalence, anatomic variation, and related factors of chronic MSK pain amongst active and retired military personnel?

Inclusion criteria

Participants

Active and retired military members will be considered for inclusion, with no restriction on country, age, or length of service. Studies including participants with different occupational backgrounds or with a condition other than chronic MSK pain will be excluded.

Condition

This review will consider MSK chronic pain with a recurrence greater than three months with the involvement of bone, joint, muscle, or related soft tissues. The condition of MSK chronic pain is consistent with the current development and refinement of the definition in the literature. In general, chronic MSK pain is defined as a recurrent pain arising from an underlying disease process from bone, joints, muscles, or related soft tissues11,12; however, this classification was limited to nociceptive pain, excluding pain perceived in MSK tissues but not originating primarily from the referred structures.12 The International Association for the Study of Pain developed a new classification of pain first by etiology, second by the underlying pathophysiological mechanism and, finally, by body site.12 This new classification considered seven categories of chronic pain: primary pain, cancer pain, posttraumatic and postsurgical pain, neuropathic pain, headache and orofacial pain, visceral pain, and MSK pain.12

Chronic MSK pain was further classified into two categories: primary MSK pain and secondary MSK pain.4 Primary MSK pain affects muscles, bones, joints, and tendons, and is characterized by emotional distress or disability.19 This type of pain cannot be attributed to a known disease process and can be localized (cervical, low back, thoracic, and limb pain) or diffuse.19 Secondary MSK pain has been defined as chronic pain originating from an underlying disease classified elsewhere, such as persistent inflammation, structural change, and in association with the nervous system.4 Both primary and secondary MSK definitions will be considered conceptually in the search of scientific literature to narrow the scope of the search and to potentially reduce the heterogeneity of the prevalence. The studies that use any of the classifications listed will be eligible for inclusion in the review.

Context

Military-related occupations and associated activities in both active and retired personnel (eg, training, combat, operation of weapons systems vehicles and vessels) that could influence MSK chronic pain will be considered, not limited by any geographical location.

Types of studies

This review will consider original epidemiological (interventions, cross-sectional, or cohort) and experimental studies that evaluate and report the prevalence of MSK chronic pain in military personnel (active and retired). Analytical observational studies, such as retrospective-prospective (cohort studies) and cross-sectional studies will be considered for the prevalence component.

This systematic review will also consider descriptive observational study designs and descriptive cross-sectional studies for inclusion. Moreover, it will consider studies that measure MSK chronic pain and independent factors such as anatomic location and diagnosis for active and retired armed forces members.

Studies in French or English will be considered for inclusion. Qualitative studies will be excluded. Studies published prior to January 1, 2009 will be excluded in order to avoid potential variation in the definition of chronic pain.12

Methods

The proposed systematic review will be conducted in accordance with JBI methodology for systematic reviews of prevalence.20 This protocol was registered in PROSPERO: CRD42020153704.

Search strategy

The search strategy, created in consultation with Dalhousie University and Veterans Affairs librarians, will aim to locate both published and unpublished studies.

The initial search of Embase, PsycINFO, and PubMed was performed using simple and multiple conjunctions of the pre-specified keywords "military personnel," "prevalence," "military veterans," "chronic pain," "musculoskeletal pain," and "veteran health" (see Appendix I). The text words contained in the titles and abstracts of relevant articles, and the index terms used to describe the articles were used to develop a full search strategy for each database. The search strategy, including all identified keywords and index terms, will be adapted for each information source. The reference lists of all studies selected for critical appraisal will be screened for additional studies.

An R software (R Core Team, 2019) script will be used to automate and document the search process using the selected terms for PubMed through the appropriate R package easyPubMed (v2.13; Fantini 2019).

A search strategy will include Embase, PsycINFO, CINAHL, MEDLINE (PubMed), and Web of Science for published studies. Sources of unpublished studies and gray literature to be searched include ProQuest Dissertation and Theses, MedNar, Government of Canada publications, and other international government websites.

Study selection

Following the search, all duplicated references will be removed using Covidence software (Veritas Health Innovation, Melbourne, Australia) and all identified citations will be screened through assessment of the title, abstract, and subject terms by two independent reviewers. Any disagreements will be resolved through discussion or with a third reviewer. From each relevant article, the full text and their citation details will be imported into the JBI System for the Unified Management, Assessment and Review of Information (JBI SUMARI; JBI, Adelaide, Australia).21 All references will be managed using ZOTERO software Version 5.0.77 (Corporation for Digital Scholarship and Roy Rosenzweig Center for History and New Media, VA, USA).

The full text of selected citations will be assessed in detail against the inclusion criteria by two independent reviewers. Reasons for exclusion of full-text studies that do not meet the inclusion criteria will be recorded and reported in the systematic review. Any disagreements that arise between the reviewers at any stage of the study selection process will be resolved through discussion or with a third reviewer. The results of the search will be reported in full in the final systematic review and presented in the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) flow diagram.22

Assessment of methodological quality

Eligible studies will be critically appraised for methodological quality by two independent reviewers using the standardized critical appraisal instrument from JBI (Critical Appraisal Checklists for Studies Reporting Prevalence Data).20

Any disagreements that arise will be resolved through discussion or with a third reviewer. The results of critical appraisal will be reported in narrative form and summarized in a table. Following critical appraisal, studies that meet the nine items listed in the critical appraisal tool for prevalence data will be included. Excluded studies will be presented as an appendix in the final review.

Data extraction

Two independent reviewers will extract data from papers included in the review using the standardized data extraction tool for prevalence and incidence studies available in JBI SUMARI.20,21 The data extracted will include specific details about the condition, populations (number of subjects), study methods, and proportions of interest to the review question and specific objectives. Any disagreements that arise between the reviewers will be resolved through discussion or with a third reviewer. Authors of papers will be contacted to request missing or additional data where required.

Data synthesis

Where possible, data extracted from papers included in the analysis, will be pooled in a statistical meta-analysis using R statistical software using the mean difference of prevalence. Effect sizes will be expressed as a proportion with 95% confidence intervals around the summary estimate. Heterogeneity will be assessed using the Cochrane Q statistic to indicate the presence of heterogeneity and the I² test to assess the magnitude of between-study heterogeneity.23 Subgroup analyses (gender, active, retired personnel) and stratified analysis will be conducted using the predictors one at a time or, if there is enough information, a multivariable meta-regression model.24 A random effects meta-regression model will be selected because the true study effect may vary across studies.24 Moreover, the influential studies will be evaluated to determine if individual studies are influencing the summary estimate derived from the meta-analysis.24 A forest plot will be used for the graphic presentation of the results and a funnel plot for the evaluation of publication bias.24 Where statistical pooling is not possible, the findings will be presented in narrative form including tables and figures to aid in data presentation where appropriate.

Acknowledgments

Veterans Affairs Canada and the Dalhousie University Library services for their help in the databases searches.

Appendix I: Search strategy

PubMed

Search conducted September 30, 2020

References