Elsevier

The Spine Journal

Volume 10, Issue 6, June 2010, Pages 554-566
The Spine Journal

Review Article
Causal assessment of occupational lifting and low back pain: results of a systematic review

https://doi.org/10.1016/j.spinee.2010.03.033Get rights and content

Abstract

Background Context

Low back pain (LBP) is a disorder that commonly affects the working population, resulting in disability, health-care utilization, and a heavy socioeconomic burden. Although the etiology of LBP remains uncertain, occupational activities have been implicated. Evaluating these potentially causal relationships requires a methodologically rigorous approach. Occupational repetitive and/or heavy lifting is widely thought to be a risk factor for the development of LBP.

Purpose

To conduct a systematic review of the scientific literature to evaluate the causal relationship between occupational lifting and LBP.

Study Design

Systematic review of the literature.

Sample

Studies reporting an association between occupational lifting and LBP.

Outcome Measures

Numerical association between different levels of exposure to occupational lifting and the presence or severity of LBP.

Methods

A search was conducted using Medline, EMBASE, CINAHL, Cochrane Library, OSH-ROM, gray literature (eg, reports not published in scientific journals), hand-searching occupational health journals, reference lists of included studies, and content experts. Evaluation of study quality was performed using a modified version of the Newcastle-Ottawa Scale. Levels of evidence were evaluated for specific Bradford-Hill criteria (association, dose-response, temporality, experiment, and biological plausibility).

Results

This search yielded 2,766 citations, of which 35 studies met eligibility criteria and 9 were considered high methodological quality studies, including four case-controls and five prospective cohorts. Among the high-quality studies, there was conflicting evidence for association with four studies reporting significant associations and five studies reporting nonsignificant results. Two of the three studies that assessed dose-response demonstrated a nonsignificant trend. There were no significant risk estimates that demonstrated temporality. No studies were identified that satisfied the experiment criterion. Subgroup analyses identified certain types of lifting and LBP that had statistically significant results, but there were none that satisfied more than two of the Bradford-Hill criteria.

Conclusions

This review uncovered several high-quality studies examining a relationship between occupational lifting and LBP, but these studies did not consistently support any of the Bradford-Hill criteria for causality. There was moderate evidence of an association for specific types of lifting and LBP. Based on these results, it is unlikely that occupational lifting is independently causative of LBP in the populations of workers studied. Further research in specific subcategories of lifting would further clarify the presence or absence of a causal relationship.

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

References (62)

  • L. Manchikanti

    Epidemiology of low back pain

    Pain Physician

    (2000)
  • M. Ghaffari et al.

    Low back pain among Iranian industrial workers

    Occup Med (Lond)

    (2006)
  • E.S. Nahit et al.

    Short term influence of mechanical factors on regional musculoskeletal pain: a study of new workers from 12 occupational groups

    Occup Environ Med

    (2001)
  • A.O. Sanya et al.

    Low back pain prevalence amongst industrial workers in the private sector in Oyo State, Nigeria

    Afr J Med Med Sci

    (2005)
  • L.R. Prado-Leon et al.

    Occupational lifting tasks as a risk factor in low back pain: a case-control study in a Mexican population

    Work

    (2005)
  • C. Peek-Asa et al.

    Incidence of acute low-back injury among older workers in a cohort of material handlers

    J Occup Environ Hyg

    (2004)
  • G. Byrns et al.

    Risk factors for work-related low back pain in registered nurses, and potential obstacles in using mechanical lifting devices

    J Occup Environ Hyg

    (2004)
  • E.F. Harkness et al.

    Risk factors for new-onset low back pain amongst cohorts of newly employed workers

    Rheumatology (Oxford)

    (2003)
  • Y.B. Yip

    A study of work stress, patient handling activities and the risk of low back pain among nurses in Hong Kong

    J Adv Nurs

    (2001)
  • P. Lee et al.

    Low back pain: prevalence and risk factors in an industrial setting

    J Rheumatol

    (2001)
  • H.O. Svensson et al.

    The relationship of low-back pain, work history, work environment, and stress. A retrospective cross-sectional study of 38- to 64-year-old women

    Spine

    (1989)
  • J.W. Frymoyer et al.

    Epidemiologic studies of low-back pain

    Spine

    (1980)
  • A. Magora

    Investigation of the relation between low back pain and occupation. 3. Physical requirements: sitting, standing and weight lifting

    Ind Med Surg

    (1972)
  • J.F. Kraus et al.

    Epidemiology of acute low back injury in employees of a large home improvement retail company

    Am J Epidemiol

    (1997)
  • F.G. Ory et al.

    Respiratory disorders, skin complaints, and low-back trouble among tannery workers in Kanpur, India

    Am Ind Hyg Assoc J

    (1997)
  • J.A. Engels et al.

    Work related risk factors for musculoskeletal complaints in the nursing profession: results of a questionnaire survey

    Occup Environ Med

    (1996)
  • I.A. Nuwayhid et al.

    Work activities and the onset of first-time low back pain among New York City fire fighters

    Am J Epidemiol

    (1993)
  • A. Burdorf et al.

    An epidemiological study of low-back pain in crane operators

    Ergonomics

    (1990)
  • S.J. Linton

    Risk factors for neck and back pain in a working population in Sweden

    Work Stress

    (1990)
  • H.H. Hamberg-van Reenen et al.

    Is an imbalance between physical capacity and exposure to work-related physical factors associated with low-back, neck or shoulder pain?

    Scand J Work Environ Health

    (2006)
  • J. Rosecrance et al.

    Low back pain and musculoskeletal symptoms among Kansas farmers

    Am J Ind Med

    (2006)
  • Cited by (134)

    View all citing articles on Scopus

    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.

    View full text