Association of Low Back Pain with Shift Work: A Meta-Analysis

Shift work (SW) is the main working schedule worldwide, and it may cause sleep disorders, breast cancer, and cardiovascular disease. Low back pain (LBP) is a common problem in the workplace; however, the association between LBP and SW remains unclear. Therefore, we conducted a meta-analysis to determine the association between SW and LBP. This study was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. The PubMed, Embase, and Web of Science databases using a set of associated keywords were queried. The inclusion criteria were as follows: (1) adult employees hired by a company or organization; (2) SW exposure; and (3) the outcome of LBP according to examination or assessment. A total of 40 studies were included that met the inclusion criteria for the meta-analysis. SW was significantly associated with LBP (odds ratio [OR]: 1.31, 95% confidence interval [CI]: 1.18–1.47, p < 0.00001). Furthermore, it was observed that LBP was significantly associated with night shift (NS) (OR: 1.49, 95% CI: 1.24–1.82, p < 0.0001) but not with rotating shift (RS) (OR: 0.96, 95% CI: 0.76–1.22, p = 0.49). Moreover, LBP was significantly associated with SW in health care workers (HCWs) (OR: 1.40, 95% CI: 1.20–1.63, p < 0.0001) but not in non-HCWs (OR: 1.19, 95% CI: 0.94–1.50, p = 0.14). SW was significantly associated with LBP. Furthermore, the subgroup analysis showed that NS, but not RS, was associated with LBP. Compared with SW in non-HCWs, SW in HCWs was significantly associated with LBP.


Introduction
To date, shift work (SW) is an important issue in occupational medicine. Approximately 20% of the full-time workforce in Taiwan comprises shift workers [1]; the rate is around 30% in the United States [2], while it is 21% in Europe. According to the International Labour Organization, SW is defined as "a method of organization of working time in which workers succeed one another at the workplace." Torquati et al. showed that SW increases the risk of poor mental health by 30% [3]. Furthermore, fatigue, insomnia, and various somatic diseases are common SW disorders [4].
Several cohort studies showed that SW at night was associated with the risk of coronary heart disease and incident atrial fibrillation [5], type 2 diabetes [5], ischemic stroke [6], breast cancer in females [7], non-alcoholic fatty liver disease [8], decreased brain functional connectivity [9], and obesity [10]. However, there was no significant association between SW and heart failure [11] or obstructive sleep apnea [12]. Additionally, quitting SW decreased coronary heart disease risk among women [13].
Low back pain (LBP) is a common disorder in humans. Globally, the age-standardized point prevalence of LBP in 21 regions was investigated and found to be around 5.6%, 13%, 9%, and 12% in Central Latin America, Australasia, Asia, and Europe, respectively, in 2017 [14]. Poor general health, physical and psychological stress, and characteristics of the person increase the risk of future episodes of LBP or sciatica [15].
Several studies have shown that SW is significantly associated with LBP. The rotating shift (RS), irregular shift, longer night shift (NS), and SW over 16 h were positively correlated to LBP [16,17]. However, NS over 16 h was associated with LBP, which was elevated when participants had sleep problems [16]. Many factors, such as SW, sleep disorders [18], poor mental health [3], and breast cancer [19], may cause LBP [17]. In contrast, other studies showed no significant association between SW and LBP [20,21]. Kawaguchi et al. showed no significant association between irregular SW and LBP (odds ratio [OR]: 1.1, 95% confidence interval [CI]: 0.6-1.9) [20]. A retrospective analysis using 13 years of occupational data from the National Longitudinal Survey of Youth, comprising approximately 11,000 American workers, showed no elevated risk of injury due to evening RS, night RS, or long-term working [21]. Given the inconsistent reports on the correlation between SW and LBP, this study aimed to investigate the relationship between SW and LBP.

Protocol and Registration
A systematic review and meta-analysis according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were conducted. This review protocol was registered at PROSPERO (registration number, CRD42022356707) and the Kaohsiung Medical University Hospital Institutional Review Board (KMUHIRB-EXEMPT(I)-20220009).

Data Sources and Search Terms
MEDLINE (PubMed), Embase, and Web of Science databases were queried on 1 September 2022, for related studies. There were no limitations to the publication dates and target keywords used to identify all articles. Two researchers (C-C Yang and H-M Chen) performed rudimentary searches using different keywords. The researchers separately proposed a set of key search words as follows for low back pain: " Appropriate modified search methods were used for EMBASE and the Web of Science databases.

Eligibility Criteria
The inclusion criteria were as follows: (1) SW exposure; (2) LBP, based on questionnaire assessment, lumbar spine computed tomography, or magnetic resonance imaging examination.

Study Selection Process
In the first screening, three investigators (H-M Chen, H-Y Chuang, and C-C Yang) individually assessed the abstracts of the preliminary articles included. Subsequently, in the second screening, two investigators (H-M Chen and P-Y Huang) performed full-text screening to identify articles that met the eligibility criteria and exclude those that were not eligible. The disagreements between H-M Chen and P-Y Huang regarding the eligibility of a study were resolved by three researchers (C-C Yang, C-L Wang, and P-J Huang) following a comprehensive evaluation.

Data Collection
In each eligible study, information regarding the study characteristics, SW and LBP cases, and the association between SW and LBP was obtained. Several attempts were made to contact the relevant authors to provide details in the event that the information was missing or inaccurate.

Study Characteristics
The study data was recorded with respect to the following variables: the country where the study was conducted; publication year; sampling framework (clinical-or communitybased); sample size; characteristics of participants; and the number of outcome events (i.e., the number of LBP events), as appropriate.

Low Back Pain
The classification of LBP was as follows: questionnaires for LBP assessment. LBP was defined based on the individual study criteria.

Statistical Analysis
A calculation of the overall pooled prevalence ORs for LBP was made according to SW and non-SW exposures. A standard error (SE) of 95% CI was used for the OR. In this meta-analysis, the prevalence of OR and SE was reported. The main prevalence ORs and SEs were combined using a random-effects model meta-analysis to calculate the pooled prevalence OR and 95% CI for the primary outcome. A random-effects model was used to assess the possibility of heterogeneity regarding whether the ORs of the included studies originated from their characteristics [26], while I 2 was used to report the heterogeneity among the enrolled studies. Moreover, separate subgroup meta-analyses for shift styles (NS and RS) and worker types (health care worker (HCW) and non-HCW) were performed. The Review Manager version 5.4 and R version 3.6.2 were used for all statistical analyses.

Meta-Analysis
A random-effects model meta-analysis revealed variations in the association between exposure to SW and LBP (ORs derived from 40 studies) [17, (Table 1, Figure 2). The pooled prevalence OR was significant. The random-effects model meta-analysis indicated a significant positive association between SW and LBP (OR: 1.31, 95% CI: 1.18-1.47, p < 0.00001).
A funnel plot of the log-transformed ORs of the association of LBP with exposure to SW as well as the SEs of the 51 ORs showed that an adequate number of studies had small SEs (i.e., larger sample sizes) and smaller ORs (Figure 3).

Discussion
This study is a meta-analysis based on the original studies. In the meta-analysis of the 40 original studies, we found that SW was significantly associated with LBP (OR: 1.31, 95% CI: 1.18-1.47, p < 0.00001). Another meta-analysis conducted by Gohar et al. showed a statistically significant association between nursing in SW and sickness absence between 1990 and 2019 (OR: 1.47, 95% CI: 1.23-1.77, p < 0.01) [66]. Sun et al. showed that non-specific chronic LBP was significantly associated with working NSs in nurses [67]. Further, Jegnie et al. showed that working hours and SW had a statistically significant association with LBP in Ethiopia [68]. On the contrary, Moscato et al. [47] and Yang et al. [60] showed no significant association between SW and LBP. It was observed that the population in the former one had lower body mass index and the latter one obese worker only take 26.9% in total worker much less than the average in their country based on the journal named Our World in Data [69]. Hence, obesity may be probably one of the risk factors of SW. However, how the body weight influence on the association between SW and LBP requires further investigation.
In addition, although we know that different job descriptions and surroundings lead to different risks of LBP [59,70], the focus of the study was on SW styles without restrictions on occupation and area. In our meta-analysis, some original studies showed a significant association [27,[29][30][31]34,37,44,45,56,58,59,63,64], whereas some studies revealed no significant association between SW and LBP. The reason for this difference may be the different study designs, study populations, and careers. Moreover, we found that some studies used different definitions for shift style, such as NS only, three-shift system, or occasionally SW. Similar to the study by Arsalani et al. [28], who categorized SW into morning shift, circulatory shift, NS, and others in their cross-sectional study of the Asian population. Beyen et al. used day shift, NS, and both as SW in a case-control study conducted in Europe [31]. Further, El-Soud et al. categorized SW into day shift and RS in a longitudinal study. The definition of SW seemed to vary between the West and East. Indeed, different work styles are required depending on the type of work. In this meta-analysis, we applied a broad definition of SW. It is difficult to collect information, which sometimes leads to less data, and a non-significant result is expected. Although a different study would have led to difficulty in collecting data, we still selected the most related data for analysis. In contrast, LBP results were not objective if only the questionnaire was used without professional identification. Work-related LBP [33], chronic LBP [63], and others have been reported. A more rigorous evaluation is needed in the future to understand the timing of SW that leads to LBP. Then, the result will be more convincing and allow employers to pay attention to the SW issue.
SW can cause many sleep problems, including reduced sleep quality, insomnia, and reduced sleep duration [71]. A previous study showed disruption of the circadian clock, especially due to NS and RS, leading to changes in melatonin and cortisol levels [72]. Morris et al. studied tissue physiology concerning circadian rhythms in the intervertebral disc and showed that changes in circadian rhythms cause harm to the intervertebral disc. [73]. In a mouse model, Dudek et al. showed that circadian rhythm disruptions lead to degenerative intervertebral disc disease [74]. These animal studies imply that SW leads to sleep interruption, which may cause circadian rhythm disruptions and LBP.
In some studies, we found that NS and RS led to health problems. According to a review by Feskanich et al., long-term NS increased the risk of hip and wrist fractures (OR: 1.37, 95% CI: 1.04-1.80) [77]. Additionally, Bukowska-Damska et al. revealed that NS workers had a lower mineral density of lumbar vertebral bones [78], while Quevedo et al. revealed that RS workers had a lower mineral density of lumbar vertebral bones [79]. According to these two studies, NS may be associated with a risk of fracture and low bone density, which may lead to LBP. However, how these potential confounding factors influence the association between SW and LBP require more study in the future.
In the subgroup analysis, we found that LBP was significantly associated with HCWs (OR: 1.40, 95% CI: 1.20-1.63). Stereotypically, HCWs are assumed to have more health information and knowledge, and fewer health problems than individuals in other occupations. However, a study by Kyle et al. study revealed that 69% (95% CI: 64.6-73.6) of Scottish nurses had obesity problems, especially in nurse groups, and non-health-related occupations (68.9%, 95% CI: 68.1-69.7) [80]. The reason for this result may be that HCWs need to shift patients, resulting in bend postures [81].
In this study, a positive relationship between SW and LBP is shown, resulting in many possible adverse health effects of SW, such as sleep disorder [18], poor mental health [3], and breast cancer [19]. The government and business organizations need to realize their responsibility concerning ways to decrease the occupational injury. Although some workers' tasks may be demanding, employers can modify the task content as well as provide reasonable break time and regular health check-ups to develop a comfortable environment for the employee. Healthy employees would create more worker power and decrease the burden of social welfare.

Conclusions
In conclusion, SW was significantly associated with LBP according to the meta-analysis of 40 studies. Compared with non-SW, NS showed a significant association with LBP, while RS was not significantly associated with LBP. Furthermore, HCWs showed a significant association with LBP. The possible mechanisms require further investigation.