Review ArticleCausal assessment of occupational lifting and low back pain: results of a systematic review
Introduction
The etiology of low back pain (LBP) is not fully understood, but some cases have been theorized to arise from injury to the muscles, ligaments, tendons, intervertebral discs, nerves, or vertebrae of the lumbar spine [1]. There is a high incidence of LBP in the working-age population [2], [3], which is associated with high medical costs related to various diagnosis and treatment strategies [4]. More importantly, the indirect costs of LBP because of lost productivity and disability likely far exceed the substantial medical costs, and the overall economic burden of LBP in the United States appears to be increasing [5].
Given that LBP is a major occupational health concern, efforts have been made to reduce the incidence and impact of work-related LBP. An improved understanding of its etiology as it relates to modifiable risk factors may optimize the effectiveness of strategies aimed at prevention. Lifting is a physical activity that is common to many different types of occupations and that has previously been considered as a risk factor for LBP. Study designs that have previously assessed lifting as a potential cause of LBP include expert opinions [6], [7], [8], biomechanical and ergonomic assessments of loads to the spine [7], results from observational studies [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], and systematic reviews [34], [35], [36], [37], [38]. However, many of these studies were considered of low quality as they did not adjust their results for the numerous other known or suspected risk factors for LBP that may have confounded these reported associations.
Merely reporting an association between a suspected risk factor and an outcome is not sufficient to establish causality. To prevent making public health decisions on falsely causal associations, the epidemiologist Sir Bradford Hill proposed a number of criteria that should be considered before declaring that a causal association truly exists [39]. Because single studies are not able to fulfill some of these criteria (eg, consistency of association), numerous studies are typically required to make this determination. Synthesizing results from multiple studies requires careful consideration of specific research questions, study designs, study populations, study of methodological quality, and specific types of statistical analyses [40]; care must also be taken to consider all relevant studies.
A systematic review is an appropriate study design to help determine a potentially causal association between an isolated risk factor and LBP by summarizing all available evidence in light of the many criteria that have been proposed to determine causation [40], [41]. During this process, the methodological quality of studies can also be assessed to determine the importance that should be accorded to their results in light of their propensity for bias and confounding [40], [42], [43]. To date, no systematic review has explicitly applied these criteria to assess the evidence supporting a potentially causal relationship between occupational lifting and LBP. Conducting this assessment may help guide to establish guidelines related to exposure in occupational settings and also to provide guidance to stakeholders involved in the adjudication of work-related LBP claims.
The primary objective of this study was to conduct a systematic review of the scientific literature related to occupational lifting and LBP. Secondary objectives were to assess the methodological quality of studies and assess the levels of evidence supporting various Bradford-Hill criteria for causality [39] for LBP and occupational lifting overall, as well as within specific subgroups.
Section snippets
Literature search
An electronic search of Medline (1966 to November 2007; updated in August 2008), EMBASE (1980 to November 2007), and CINAHL (1982 to November 2007) was executed using a broad strategy with three main components: setting (ie, work related); etiology (ie, occupational lifting); and outcome (ie, LBP). Both indexed search terms and free text were used. (Note: search strategy and results are available on request.) A hand search was also performed of the three occupational health journals with the
Results
The search strategy yielded a total of 2,766 citations. A total of 275 were deemed potentially relevant at the first level of screening. On further review, 35 satisfied the eligibility criteria. These studies enrolled a total of 88,864 (mean: 2,539; SD: 4,531) participants for lifting. The mean prevalence of LBP in study participants was 37.2% (SD: 19.6). These studies were conducted in 16 different countries, including the United States (11 studies), Sweden (4 studies), and the Netherlands (4
Discussion
Previous studies [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33] and systematic reviews [34], [35], [36], [37], [38] have reported an association between lifting and LBP. However, many of these studies were hampered by methodological flaws that may have been biased toward a positive result. Moreover, the most of these studies failed to adjust for potential confounding factors, which may be
Conclusion
This systematic review failed to uncover high-quality studies that consistently supported any of the Bradford-Hill criteria to establish causality between occupational lifting and LBP. Although lifting is a complex dynamic activity, heterogeneous reporting across studies made pooling of different types of lifting difficult. There was some moderate evidence for association for specific types of lifting and LBP, and some consistent evidence for association between lifting greater than 25 to
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FDA device/drug status: not applicable.
Author disclosures: SD (consulting, Palladian Health; scientific advisory board, Palladian Health); BKK (consulting, Medtronic).
This study was funded by a peer-review grant provided to Drs Wai, Bishop, Kwon, and Dagenais by the Workers Compensation Board of British Columbia.