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

Background context: Occupational low back pain (LBP) is a common musculoskeletal disorder that results in high healthcare use and a heavy societal burden from morbidity and medical costs. The etiology of LBP is unclear, although numerous physical activities in the workplace have been implicated in its development. Determining the causal relationship between LBP and specific occupational activities requires a rigorous methodological approach.

Purpose: To conduct a systematic review of the scientific literature focused on establishing a causal relationship between occupational carrying and LBP.

Study design: Systematic review of the literature was performed.

Sample: Studies reporting an association between occupational carrying and LBP.

Outcome measures: Numerical association between different levels of exposure to occupational carrying and the presence or severity of LBP.

Methods: A systematic review was performed to identify, evaluate, and summarize the literature related to establishing a causal relationship between occupational carrying and LBP by using the commonly used Bradford-Hill framework. The literature was searched using Medline, Embase, CINAHL, Cochrane Library, the Occupational Safety and Health (OSH-ROM) database, gray literature (eg, studies not published in peer-reviewed journals), hand-searching occupational health journals, reference lists of included studies, and content experts. Study quality was evaluated using a modified version of the Newcastle-Ottawa Scale. Levels of evidence supporting specific Bradford-Hill criteria were evaluated for different categories of carrying and types of LBP outcomes.

Results: This search yielded 2,766 citations. A total of nine high-quality studies reported on occupational carrying and LBP, including four case-control studies and five prospective cohort studies. These nine studies reported strong and consistent evidence against a statistical association between carrying and LBP. Three studies assessed dose-response, of which only one reported a dose-response trend that was not statistically significant. Five studies were able to assess temporality, but none reported results fulfilling this aspect of causality. The biological plausibility of carrying and LBP was not discussed in any of the nine studies. None of these studies attempted to evaluate the experiment criterion by devising studies in which the exposure to carrying and level of LBP could be measured before and after implementing a strategy aimed at reducing carrying in the workplace to determine its effect on LBP.

Conclusions: This review failed to identify high-quality studies that supported any of the Bradford-Hill criteria to establish causality between occupational carrying and LBP. Based on these results, it is unlikely that occupational carrying is independently causative of LBP in the populations of workers studied.