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

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.